Tribological characterization of selected hard coatings

Transcription

1 Faculty of Technology and Science Department of Materials Engineering Patrik Karlsson Tribological characterization of selected hard coatings Master thesis of 2 credit points Date/Term: Supervisor: Pavel Krakhmalev Krister Svensson Examiner: Jens Bergström Serial Number: 29: 1 Karlstads universitet Karlstad Tfn Fax Information@kau.se

2 Acknowledgement Following persons deserve special thanks. Associated Professor Pavel Krakhmalev for great supervision throughout the thesis project, interesting discussions of the thesis project and material science, fast response of questions, and great help with scientific tools. Senior University lecturer Krister Svensson for great supervision, fast response, and guideline for AFM issues. Johan Nordström at the company of Oerlikon Balzers for supply of PVD coatings and material data. Odd Sandberg at the company of Uddeholm Tooling AB for supply of substrate for PVD coatings. The department of Materials Engineering at the University of Karlstad for letting me use the scientific tools needed to perform this thesis.

3 Abstract Hard coatings are often used for protection of tool surfaces due to coating properties like low friction and high wear resistance. Even though many of the hard coatings have been tested for wear, it is important to try new wear test setups to fully understand tribological mechanisms and the potential of hard coatings. Few experiments have been performed with dual-coated systems where the sliding contact surfaces are coated with the same, or different, hard coating. The dual-coated system could be the solution to many new technical devices and perhaps a further improvement of conventional coated systems. In this thesis, the wear tests of dual-coated systems were performed in dry reciprocating sliding mode at room temperature. This, quite off the ordinary, wear test setup was performed to study selected hard coatings and set focus on wear mechanisms in forthcoming future surface coating application areas like MEMS and orthopedic implants. Wear tests of four different PVD hard coatings, CrN, TiAlN, WC/C and diamond-like coating (DLC) were performed in a slider-on-flat-surface (SOFS) tribo-tester with reciprocation sliding mode at room temperature and dry sliding with TiAlN coated counter body. Wear mechanisms and the amount of wear were estimated, by investigation of the wear scars produced in SOFS, by means of scanning electron microscopy (SEM), atomic force microscopy (AFM) and optical profilometer (OP). Typical wear mechanisms found for coated surfaces in reciprocation sliding contact were crack formation, surface flattening for shorter sliding distance, elongation of surface defects, debris and thin film formation. Two types of film formation were found: tribo-oxidation film and formation of a self-lubrication film. The tribo-oxidation was the most evident for CrN and the formation of a self-lubrication film was revealed for DLC, where smearing of asperities were the initiation of the process. The DLC coatings showed lowest friction coefficient and worn volume of all the selected hard coatings. Adhesion measurements were performed for all coatings by AFM. Both the unworn and worn surface of each coating were investigated and two coatings, DLC and TiAlN, showed low adhesion forces, which indicated promising properties for small scale devices like MEMS and NEMS with coated, non-sticking, surfaces.

4 Contents 1. Introduction Surface coatings Deposition of hard coatings Properties of hard coatings Hard coating classifications Tribology of coatings Wear simulation Hertzian contact pressure Surface characterization methods Aims Experimental part Material properties Material preparation Substrate surface preparation procedure Preparation of sliding wheel Brushing of coatings Wear tests SOFS setup Surface characterization Atomic Force Microscopy (AFM) Tip calibration Scanning Electron Microscopy (SEM) Optical Profilometer (OP) Results CrN Original surface of CrN Friction of CrN Optical profilometer measurements of CrN Scanning electron microscope measurements of CrN Atomic force microscopy measurements of CrN DLC Original surface of DLC...38

5 5.2.2 Friction measurements of DLC Optical profilometer measurements of DLC Atomic force microscopy measurements of DLC Scanning electron microscope measurements of DLC TiAlN Original surface of TiAlN Friction measurements of TiAlN Optical profilometer measurements of TiAlN Scanning electron microscope measurements of TiAlN WC/C Original surface of WC/C Friction measurements of WC/C Optical profilometer measurements of WC/C Scanning electron measurements of WC/C Adhesion measurements Discussion Friction of selected hard coatings Tribological characterization of selected hard coatings Load condition Load condition Load condition Adhesion measurement correlation to worn volume Conclusions References...78

6 1. Introduction Longer service life, ability to tolerate greater loads, ease and low cost of maintenance, environmental gain in conservation of resources, improved response in kinetic systems, lower energy consumption, resistance to corrosion, low friction, use of low cost base material, etc are just a few good reasons for coating machine parts. Many industries have understood the advantages with coated systems, and that is why coated machine parts and tools like gears, bearings and cutting tools can be coated with DLC, WC/C, CrN and TiAlN. Even though tribological performance of many hard ceramic coatings is quite well investigated, there still are many tribological phenomena that can not yet be explained. The wear mechanisms at macro scale are believed to have reached a level where the understanding are somewhat established. The problem with further understanding arise when tribological studies at the micro scale and nano scale show that the macro level laws of friction can not be applied. Differences in scale proportions enhance some forces at smaller scales that play a minor part at macro scale tribological performance, for example adhesion forces. Many new small technical devices like MEMS are hindered in further development because of high adhesion forces rather than high friction forces. Contact surfaces may stick to each other due to adhesion forces and the device function breaks down. This problem with adhesion force might be solved with coated contact surfaces with low surface potential. In this thesis investigation of selected coatings: CrN, TiAlN, WC/C and DLC was performed at both macro and micro level by means of scanning electron microscopy (SEM), atomic force microscopy (AFM) and optical profilometer (OP). Wear tests were performed in a slider-on-flat-surface (SOFS) tribo-tester with three different load conditions: constant depth, constant contact pressure, and constant load. The load conditions were based on a model of Hertz contact pressure theory. The wear test setup was chosen with dual coated system, where TiAlN was the coating on the tool steel. The TiAlN was chosen since the coating was harder than the rest of the selected coatings. By creating dual coated systems, the investigations were presenting a wear test setup which was not as common as tribological research of hard coatings done by others. Wear tests were performed at macro/ engineering level and the wear tracks was investigated all the way down to the nano scale in an atomic force microscope, where adhesion measurements were performed. The investigations of the different coatings showed many interesting results, where investigations like adhesion measurements of the wear tracks turned out to be unique. 1

7 2. Surface coatings In order to improve surfaces properties of material, coatings have been used throughout history and improvements are achieved every year. The beginning of hard coating techniques can be dated back to 1643 when Evangelista Torricelli did his famous experiment with an upturned glass tube filled with mercury, in which he established the existence of vacuum. With the existence of vacuum, Michel Faraday developed the deposition technique in In the 196s the technique started to spread to industrial applications when cutting tools were coated with TiN for the first time in U.S. industry [1]. Surface coatings are used in a wide area of applications and the deposition techniques are basically divided into three groups: solid phase, liquid phase and vapor phase [2]. Hard coatings are one of many surface layer deposition methods, fig 1: Fig. 1. Illustrates the variety of surface coatings divided in manufacturing methods [1]. In the crystallization group, see fig 1, two major deposition techniques are used: physical vapor deposition (PVD) and chemical vapor deposition (CVD). Both methods produce surface coatings with superior tribological properties and they are used to deposit thin film of hard coatings on components to extend their performance and life under severe environmental conditions. The final coating properties depend on process parameters like temperature, pressure and deposition technique. By controlling process parameters, it is possible to design coatings for a 2

8 wide range of substrate materials and application areas. In this thesis hard surface coatings produced from the vapor phase, by physical vapor deposition, will be examined. 2.1 Deposition of hard coatings Physical vapor deposition (PVD) is widely used today as a manufacturing method for coating of substrate to enhance surface properties with superior resistance to tribological phenomena like wear, friction, oxidation etc. The deposition of the thin film takes place in a vacuum chamber. Normally the substrate is cleaned and dried with N 2 gas before entering the vacuum chamber. After insertion of substrate into chamber, vacuum is increased ( Torr) and the surface is de-gased by a high current density plasma by sputtering at elevated temperature (15-5 C) [3]. During the last step in PVD, a material is evaporated and deposited and forms a thin film on a substrate material. The temperature during deposition of PVD is low when compared with other methods like Chemical Vapor Deposition (CVD). One advantage of deposition at lower temperatures is that unwanted softening and geometrical changes of the substrate do not occur and subsequent heat treatment steps are not needed [4, 5]. Even though there exist several PVD methods, they can be divided into two major parts [6], evaporation PVD and sputtering PVD, fig 2. a) b) c) Fig. 2. Subdivided PVD methods are described in picture (a) [6]. Picture (b), (c) shows evaporation and sputtering deposition respectively [3]. The sputtering mechanism involves glowing discharge and momentum transfer process to produce a thin film on a substrate. High-energy particles cause atom or cluster of atoms to be knocked free from the surface of a target, which contain the coating material. The coated film on the substrate contains condensed atoms from ejected target material and particles that have reacted with gas that the PVD vacuum chamber contains. 3

9 The process starts with a glowing discharge that forms a flux of ions that are pointed to the target material and the ions start to sputter atoms from the target in dependence on surface. Via momentum transfer, the atoms are transferred and condensed on the substrate. As the process continues, a thin film is formed. Sputtering PVD can be subdivided to diode, magnetron, ion beam and triode sputtering. In PVD by evaporation, the target coating material is placed in a crucible that is heated to sublimation point in high vacuum environment. There are four main methods for evaporation: induction heating, resistance heating, arc and electron beam gun. By heating the crucible, the coating material is vaporized and condensed on the substrate forming a thin film Properties of hard coatings PVD coatings are popular to use in coating of machine parts and tools. Requirements of the coated parts are pushed forward as the development of new hard coatings continues. Mainly the requirements of coating material properties are [7]: low heat transfer coefficient low friction coefficient high wear resistance high hardness high toughness fine-grain, crystalline microstructure chemical inertness smooth surface morphology good adhesion to substrate Not many bulk materials can fulfill all these requirements and that is why the PVD coatings have gain high popularity in application areas in severe working environment. The properties of PVD coatings can often be explained by their prominent microstructure during growth. During deposition the coating gets high density of non-equilibrium built-in structural defects during the bombardment of particles against the substrate in the growth process of the coating. Since the sputtering process takes place with reactive gas like Ar + or N + the particles consists of back-scattered inert gas neutrals or ions accelerated towards the substrate via negative substrate bias potential [8]. In the arc evaporation the particles consists of metal ions 4

10 of multiple ionization states. The defects created during deposition, fig 3, act as obstacles for dislocation movement and are one of many other strengthening agents of PVD coatings. Other mechanisms of strengthening of hard thin films during deposition are second phase particles, solutes, internal boundaries (column, grain and phase boundaries), high density of point and line defects etc [9]. Fig. 3. Surface defects in PVD coatings due to ion-bombardment [6]. Typical microstructure for PVD coatings are the columnar structure. This columnar structure is formed during deposition, when a flow of atoms reaching the substrate with a limited range of directions. Even though the microstructure often has a columnar structure the surface morphology can have different appearance depending on deposition method, fig 4, [1]. Fig. 4. SEM pictures of CrN deposited by arc evaporation (a,c) and sputtering (b,d) on Si substrate. Picture (a) shows typical morphology of arc deposited PVD coating with droplets and picture (b) shows a finer morphology of sputtered PVD with insert picture of 4x magnified view of the same surface. Picture (c) and (d) shows fracture cross-section of arc and sputtered PVD respectively [1]. 5

11 As seen in fig 4, the PVD coatings deposited by arc evaporation are often rougher than that of PVD coatings deposited by sputtering. Droplets are formed during arc evaporation and contain material ejected from the source surface. This occur due to the rapidly melt of the source material by the arc [6], [9]. The droplets can be found both on the coating surface and in the body of arc deposited coatings [1], [11]. The columnar structure has many advantages e.g. great tolerance against erosion, stresses, thermoshock etc [12]. Along with many advantages of columnar structure there are some disadvantages too, which are more or less connected to error in process parameters. If the density of the columnar structure is not sufficient, pores can form between columns and form gaps that reach all the way through the coating down to the substrate. These kinds of pores may lead to bad corrosion resistance, since corrosive media can reach the substrate-coating interface [13]. PVD coatings have high strength due to its small grain size (or columnar width) in the size of 1-3 nm for single-phase coatings. Many, but not all, of these coatings follow the Hall- Petch relationship. k y σ ys = σ + (1) d Where σ ys is the yield stress, σ is the lattice resistance to dislocation movement, k y is the Hall-Petch factor which depends on the material and measure the relative hardening contribution of grain boundaries and d is the grain size [14]. The strengthening effect due to decreasing grain size can be illustrated by increasing of hardness of the coating, fig 5. Fig. 5. Correlation between hardness and grain size (a), hardness and biaxial residual stress in sputtered hard coatings (b). TiN and TiB 1.4 N.65 follow the Hall- Petch relation [15]. 6

12 Grain size in hard coatings can reach down to the size of 2-3 nm in nanocolumnar coatings like TiN-TiB 2 so that high hardness is achieved (~42 GPa) [9]. Besides the Hall-Petch relationship, such high hardness value can be explained by the formation of almost perfect crystals during deposition. In TiN-TiB 2 the addition of B improves the cohesion of grain boundaries by affecting the local bonding at the interface, which in turn results in the high hardness of TiN-TiB 2. The achievement of small grain size in TiN-TiB 2 can be explained by processes that takes place during deposition, fig 6. By sputtering of the TiN-TiB 2 target, B, N and Ti atoms arrive to the substrate. This results in formation of nuclei, which consist of TiN, TiB 2 and TiN-TiB 2 in grain-boundary. Fig. 6. Nucleation during film growth of sputtering deposition of TiN-TiB 2 [9]. Boron segregates to surface and interface due to its low solubility in TiN. This leads to formation of disordered areas enriched with B. These areas cover TiN surface and inhibit mobility of boundary. The formation of disordered areas filled with B affects film growth and hinder grain coarsening. Similar process takes place for N when the B areas promote nucleation of TiB 2, which has low solubility for N. The broken up growth leads to formation of small grains and grain coarsening can not take place [9]. Another strengthening mechanism influencing properties of hard coatings is alloying. By replacing atoms in an original crystal structure with other atoms of different size, stresses in the lattice causes barrier to dislocation movement and the material gets stronger. As an example of alloying strengthening in ternary carbon nitride coatings, N atoms are replaced by the bigger C atoms in TiCN and smaller Al atoms partly replace Ti in TiAlN [16, 9]. Atomic bondings also play a key role and determine mechanical properties of coatings. In hard coatings three types of bonding are typical: metallic, covalent and ionic bonding, fig 7. 7

13 a) b) Fig. 7. Atomic bonding in PVD coatings. Metal to nonmetal structure (a) and atomic bonding (b) [6], [9]. Covalent bonding is found in the hardest coatings and is typical of high-energy bonds e.g. 4.5 ev for H-H. The bond is formed when atoms are sharing electrons, forming electron pairs. The covalent bond can be found in coatings like diamond, B 4 C, SiC, AlN and Si 3 N 4, fig 7. The covalent bond contributes to hardness and thermal stability but decrease the adhesion of the coating to the substrate. Metallic bond contributes to toughness and adhesion to the substrate for the coating but decrease hardness of the coating. The metallic bonding is formed in crystals of metals containing conducting electrons. Charged ions are positioned in lattice cites. These ions are in equilibrium with the conducting electrons, which fills the lattice space. The sea of electrons contains free electrons widespread to all metallic ions. Interactions between external electron shells form additional bond forces, which occur for transition metals. Metallic bond has high energy e.g ev for Fe, which belongs to the transient group of elements with incompletely filled electron shells. The ion bonding is formed when valence electron transition from a less electronegative atom to a more electropositive atom occurs. The ion bonding occurs between atoms of metal and non-metals, mainly oxides, and the bonding energy can be up to 7.9 ev e.g. NaCl. The ion bonding results in brittle behavior and high thermal expansion but increase the chemical stability of the coating, fig 7. 8

14 The different atomic bonding in hard coatings is often found as complex combination of interaction between different bond structures, table 1, [6]. Table 1. Typical characteristics of coatings with Covalent-, Metallic- and ion-bond [2] Hard coating classifications Hard coatings exist in various forms, complex and less complex mixtures. As mentioned before, the coatings are fabricated of nitrides, carbides and borides of transition metals, carbon based etc. Often the chemical composition is used when a specific coating is addressed, for example TiN, CrN, TiAlN, TiC and c-bn. Some of these coatings, with modification, have gained a name on the global coating market without using the chemical composition, for example Dymon-iCTM, Graphic-iCTM and MoSTTM [6]. This is though more of an exception than a rule. To classify coatings, it is common to break down the different coatings in subcategories: monolayer coatings and complex coatings, where the complex coatings are further subdivided into multi-component coatings, multi-phase coatings, composite coatings, multi-layer coatings and gradient coatings [2, 17], fig 8. f) g ) Fig. 8. Overview of coating structures. Multi-component (a), multi-phase (b), composite (c), multi-layered (d), gradient (d). (f), (g) shows mono-layered coating and multilayered coating respectively with increasing mechanical destruction: I, II, III [2]. Monolayer or single phase coatings consists of a metal e.g. Al, Cr, Mo, Au, Ag and Cu or a phase like TiN, TiC and CrN. The complex coatings consist of more than one material and the 9

15 variety of material distribution is high. The multi-component coatings base on carbides and nitrides with transition metals like TiN, VN, CrN, ZrN, NbN, HfN, TaN, WN, TiC, VC and ZrC. Solid solution of carbides and nitrides is usually formed in these coatings, by alloying with a third element. They usually form a substitution solid solution. The multi-component coatings usually have good tribological properties. Recent modern advanced multi-component coatings increase these properties further e.g. Ti(C,N), (Ti, Al)N, (Ti, B)N, (Ti, Zr)N, (Ti, Nb)N, (Ti, Al, V)N and (Ti, Al, V, Cr, Mo,)N [2,6]. Multi-phase coatings consist, as the name proposes, of several phases. Composite coatings are multi-phase where one phase is dispersed in another phase matrix like Ti/Al 2 O 3. More advanced composite coatings with three-dimensional nanostructure [9] and extremely small grain size have reached the open coating market. One of these coatings was mentioned earlier: TiN-TiB 2. With properties like high hardness, thermal stability and low friction coefficient, the advanced composite coatings are popular in tool applications like cutting tools. Multi-layer coatings, also known as micro laminates or sandwich-coatings consist of several different layers built on top of each other. The sandwich-coatings consist of layers with different mechanical properties. The area in between the layers consists of transition layers, which are basic layers partly diffused into each other. With this kind of coating structure, several different mechanical properties can be included in one coating e.g. good adhesion to the substrate for the first layer, good corrosion resistance for the second layer and good tribological properties for the top layer. The number of layer can vary a lot e.g. three-layer coatings: TiC/ Ti(C,N)/ TiN, TiC/ TiN/ Al 2 O 3, six-layer coating: TiN/ Ti(C,N)/ ZrN/ (Ti, Al)N/ HfN/ ZrN, eight-layer coating: TiN/ Ti(C,N)/ Al 2 O 3 / TiN/ Al 2 O 3 / TiN/ Al 2 O 3 / Ti(C, N) [2]. To reach good adhesion between the layers it is important to get the transition layers to form a coherent interface zone. This is often achieved by coupling materials with similar atomic bonding structure or materials that are mutually soluble. By constructing hard coatings in several layers, the coating gets better resistance to mechanical destruction compared to mono-layer coatings, fig 8. In monolayer coatings, propagation of cracks that usually forms in hard coatings, during mechanical destruction, goes through the whole coating section. In a multi-layer structured coating, the cracks that formed on the surface are inhibited to continue growing due to the next coating layer and interface. Instead, the crack stops or turns in another direction, which leads to destruction of small pieces of the coating instead of dramatic failure of the whole system as in the monolayer coating. By acting as a crack barrier, the multi-layered coating can withstand much higher 1

16 mechanical load compared to a single-layer coating. Gradient coatings e.g. TiN/ Ti(C, N)/ TiC are similar to multi-layer coatings with the exception that the different layers are not divided in steps. Instead the gradient coatings show a more continuous structure [6]. The development of coatings has gone from single layer or single phase coatings to layered structures of various kinds. Superior mechanical properties have been achieved by further development of hard coatings into nano-structured coatings like nano-composite and nanocolumnar coatings e.g. TiN-TiB 2 and TiB 2 [9]. These coatings consist of superior mechanical properties like high hardness and high melting point, compared to ordinary hard coating. Nano-structural coatings are a hot research object in material science and as the research trends towards nanoscience, the knowledge of the fundamental construction blocks of coatings, and the ability to manipulate them at the atomic level are increasing. 2.2 Tribology of coatings Several parameters are involved in the tribophysical and chemical process during contact of coatings in relative motion, fig9. That is why the phenomena connected to tribology of hard coatings are hard to distinguish. It is not often that one specific wear behavior occurs without involving several other wear phenomena. Fig. 9. Tribological processes that occur for coatings in mechanical contact [6]. One way of describe ordinary tribological scenarios for coating behavior in mechanical contact is to divide the situations, fig 1, into macromechanical, micromechanical, chemical and nanomechanical or nanophysical effects [7]. 11

17 Fig. 1. Tribological contact mechanisms [19]. The friction and wear phenomena s in macromechanical tribological mechanisms are described by the stress and strain distribution over the whole contact area, plastic and elastic deformation that stress and strain result in, and the total wear particle formation. There are four typical parameters, which control tribological contact behavior: the surface roughness, the hardness relationship between coating and substrate, the thickness of the coating, the size and hardness of the debris in the contact [18]. The correlation between the four parameters, fig 11, results in several different contact conditions characterized by specific tribological mechanisms [19]. Fig. 11. Macromechanical contact conditions and wear mechanisms for coatings [19]. The hardness ratio substrate/ coating are an important parameter. Reduction of friction can be achieved by using soft films. These films may reduce sliding-originated surface tensile stresses which lead to subsurface cracking, and eventually to severe wear. Hard coatings can 12

18 also reduce friction and wear by prevent ploughing, if it is applied on a softer substrate. A hard coating has built in compressive stresses which can prevent the occurrence of tensile forces. If the substrate is increased in hardness, further improvement of the system can be achieved e.g. ploughing and deflection due to counterpart is inhibited by better load support from the substrate. The thickness of the coating affects the ploughing component of friction in soft coatings. For rough soft coatings the degree of asperity penetration through the coating into the substrate is affected by the thickness also. Reduced contact area and lower friction can be achieved by applying a hard coating, which is thick enough to support a softer substrate in carrying the load [18]. Thin hard coatings on the other hand are not preferable on a soft substrate, since coating fracture occurs if substrate is deformed, fig 12. Fig. 12. Film fracture of thin hard coating film due to substrate deformation [18]. High surface roughness has negative influence of friction and wear. Scratching of hard asperities in the counter-face often occurs during sliding and the asperities may lead to abrasive or fatigue wear. The asperities also lead to reducing of the real-contact area with extremely high contact stress. The high stress at asperities may be subject to asperity interlocking and breaking, which contributes to higher friction. Even though loose particles or debris often are present during sliding contacts they should be avoided if possible. In some sliding conditions they may contribute very much to friction and wear e.g. by particle entrapping, embedding, hiding, crushing, etc. Stress and strain formation, particle formation and material liberation at asperity-asperity contact are described by micromechanical tribological mechanisms. At micromechanical level, typical of 1 µm to nanometers in size, the basic mechanisms for nucleation of cracks are shear and fracture. The cracks nucleate, propagate and lead to material liberation and formation of wear scar and wear particles. The fundamental understanding of micromechanical tribological phenomena is poor and more research has to be done in that area in order to gain further knowledge of tribological phenomena at the smaller scale [19]. The tribochemical reactions that take place on the coated surface during mechanical contact of sliding change the composition of the surface and, thereby, also its mechanical 13

19 properties: a new material pair is formed [6]. This affect the friction and wear of the coating since these tribological phenomena include surface-related mechanism e.g. shear, cracking, asperity ploughing etc. High local pressures and flash temperatures, which can reach up to 1 C during sliding, trigger the tribochemical reactions at spots where asperities smash together. The tribochemical reactions can be divided into two parts: formation of thin film on coating and oxidation of coating. The formation of a thin film is believed to form during sliding on coatings like Diamond- Like coating (DLC) with very low friction behavior [6, 2] down to µ = The formation of hydrocarbon-rich microfilm, or graphite, on the hard coating can be the explanation for the low friction coefficient. From a micro-scale viewpoint there is a soft coating on a hard substrate but now the coating acts as a hard substrate when the film forms, fig 13. The thin film also inhibits ploughing and thereby the friction is reduced. Fig. 13. Thin film formation of coatings [6]. Oxide layer is easily formed on metals in environment containing oxygen e.g. air. This applies to metals like copper, iron, aluminum, nickel, zinc, chromium to mention a few. The oxides that forms, influence the tribological behavior of the surface in different ways e.g. copper oxide is sheared more easily then the metal. Aluminum oxide, on the other hand, may form a very hard thin layer. The small dimension of the oxide particles compared to the surface roughness does not necessary mean that they contributes to an abrasive tribological effect. Sometimes the oxide particles assemble up to form layers, strong enough to carry the load [6]. Nanophysical changes in coatings, fig 1, are still under intensive research. With scientific tools like atomic force microscope (AFM) the possibility of study friction and wear on a molecular scale has been achieved. The aim is to find the origin of friction at the atomic scale and determinate the relationship of the friction laws at the microscale with the nanoscale friction. The latest research regarding friction suggest that friction arise from lattice vibrations due to sliding contact of two surfaces where their outer surface atoms moves in opposite direction. The mechanical energy needed to slide the surfaces onto each other is believed to 14

20 be converted into elastic energy, or phonons, which eventually is converted into heat [19, 21, 22]. Further research will perhaps show other result or confirm and develop the statement, and new theories will explain the origin of tribological phenomena at the nanoscale [19] Wear simulation Hard coatings, as mentioned earlier, are used in many machine applications and technical devices. The interaction between surfaces in relative motion can be of various kinds, fig 14. Fig. 14. Interacting surfaces in relative motion [6]. Since the variety of contact situation of real applications is high, it is obvious that the wear simulation models for real applications also vary a lot. Some of the wear simulation models are standardized, fig 15. They are pin-on-flat, pin-on-disc and block-on-ring. [6]. Fig. 15. Standardized wear test methods [6]. The pin-on-flat equipment are used for tribological coating evaluation regarding wear rate, coefficient of friction etc in both dry and lubricated reciprocating sliding conditions. This is also true for the pin-on-disc equipment with the exception that larger specimens must be used and the sliding is continuous, not reciprocating. The block-on-ring equipment is often used for determination of adhesive wear rate and the tests are performed under lubricated wear conditions [6] Hertzian contact pressure When materials are tested, the questions of contact pressure often arise. Materials with different mechanical properties will experience different contact pressure when applying the 15

21 same load. In real applications, due to relatively high contact areas, plasticity of the coating and the substrate is never achieved. Therefore mathematical analysis of stresses is a good approximation. One way of estimating the contact pressure is to apply the Hertzian contact theory formulas [23]. In the Hertzian contact theory, the contact pressure distribution, p ( x, y), over the contact area is expressed as: 2 2 x y p ( x, y) = p 1 (2) a b Where p is the maximum contact pressure, a and b are the major and minor axes in an elliptical contact. For the elliptical contact situation between a wheel and a flat surface, the major and minor axes can be expressed as: b a R = R / 3 Where the R 1 and R 2 are the principal radii of curvature of the wheel. The maximum contact pressure for elliptical contact then can be expressed as: p *2 3P 6PE = = 3 2πab π R1R 2 1/ 3 R F 1 R 1 2 Where F is a shape factor and E * is the effective elastic modulus, which can be expressed as: (3) (4) E = 2 * 1 ν 1 1 E 1 + ν E (5) Where E 1 and E2 are the elastic modulus for the wheel and the sample respectively. ν 1 and ν 2 are the Poisson number for the wheel and the sample respectively. The distance between two distance points when compressing the wheel to the flat sample can be expressed as: 1/ 3 2 ( ) 9P R 1 δ = F 2 *2 2 (6) 16 R 1/ 1R2 E R2 The Hertzian theory of contact pressure is based on some assumptions: The surfaces are continuous and non-conforming The strains are small Each solid can be considered as an elastic half-space The surfaces are frictionless 16

22 2.3 Surface characterization methods Among many surface characterization methods of hard coatings, three methods are more common for tribological characterization of coatings. These methods are scanning electron microscopy (SEM), optical profilometer (OP) and atomic force microscopy (AFM). The optical profilometer uses phase-shifting/ vertical-sensing interference technology combined with an optical microscope. This surface characterization method provide a noncontact 3D method of measuring the roughness of surfaces with sub-nanometer height resolution capability for smooth surfaces and approximately 3 nm height resolution for rougher surfaces [7]. The profilometer works with an interchangeable magnification objective, fig 16. The objective is coupled with a beam splitter and reference mirror, which together acts as an interferometer unit. White light is split in the interferometer, where part of the light travels to a spot on the sample of interest and the rest of the light travels to a reference mirror. When the two parts recombines, interference fringes are produced at the point of focus. By measuring the resulting interference pattern irradiance from sequential shifting of the phase of one light beam of the interferometer relative to the other light beam by known amount, the surface height of the sample of interest are determined. The optical profilometer measure the sample surface by using a piezoelectric transducer, which moves the objective in the vertical direction in steps of 5 to 1 nm. The irradiance signal from the profilometer measurements is detected by using a CCD array, and the recorded data can be converted and displayed as topography information on a computer screen. a) b) Fig. 16. The optical profilometer (a) with close up on the microscope objective (b) [7]. The scanning electron microscopy (SEM) method uses a high-energy electron beam to emit electrons from the surface of the sample of interest. The electron beam, with energy ranging from a few hundred ev to 4 kev, is focused by condenser lenses and can be moved in the x and y axes directions in a raster by scanning coils or deflection plates. The spot size of the electron beam is typical of.4 nm to 5 nm in size, and the resolution of SEM is approximately 1 nm to 2 nm [17]. Particles with different energy levels are detected after the 17

23 electron beam has interacted with the sample, which takes place in a vacuum environment. These particles are secondary electrons, backscattered electrons and x-rays, fig 17. Fig. 17. Electron bombardment of sample surface in SEM [17]. The secondary electrons have low energies and are typical ejected from within a few nanometers from the sample surface by inelastic scattering interaction with the beam incident electrons. The secondary electron detections are very sensitive to the topography of the sample, and this is also why the secondary detectors are used for topography investigations of surfaces. Secondary electron detectors collect scattered secondary electrons, which are sent to a computer for graphical display as the SEM moves in a raster. Planes which are in line with the detectors collects more secondary electrons and these parts appears more bright on the computer screen compared to other places on the sample, which are tilted away from the detectors. The intensity difference, in the secondary electron detector input signal, appears as a topographic picture of the sample on the computer screen. Backscattered electrons are collected from elastic collisions with constituent atoms of the sample. The intensity of the backscattered electron signal gets higher as the atomic number increases (higher atomic density) for elements belonging to the sample of interest. The backscattered electrons can, therefore, reveal information of the atomic composition of the sample of interest. X-rays are generated by atoms following the emission of a secondary electron. Energy dispersive spectrometry (EDS) or wavelength dispersive spectrometry (WDS) can be used for analyzing the x-ray energy emitted, which is characteristic of the atom. The x-ray analysis can reveal chemical composition of the sample of interest. The atomic force microscopy (AFM) method belongs to the methods of scanning probe microscopy (SPM). The AFM uses a small cantilever with a sharp tip (probe), which is used to scan the surface of interest, fig 18. The tip radius of curvature are typical on the order of nanometer, which gives a vertical resolution of less than.1 nm and lateral resolution of about.2 nm [7]. By mounting a sample on a piezoelectric tube, the sample surface can be scanned by letting the piezoelectric tube keep a constant force against the AFM tip and then 18

24 continuously move in the x and y direction during scanning. The sample surface differences in height and friction are recorded during scanning by using a laser beam and detector that register the cantilever vertical movements (height) and torsion (friction). The laser beam is pointed to the cantilever and reflected to the detector, which consists of four quadrant sensors (split-diode photodetector), fig 18. The signals from the detector are sent to a computer, which calculate the difference between the four quadrants and then present the data as friction map and height map, which can present surface roughness and topography in 3D. There are two primary scanning modes of AFM: contact and non-contact mode. The contact mode was described earlier, when the piezoelectric tube keeps a constant force against the cantilever tip in the z-direction. In the contact mode, the tip is in contact with the sample surface throughout the scanning procedure. The non-contact mode, or tapping mode, uses a piezo mounted over the cantilever, which keeps the cantilever oscillating at the resonant frequency of the cantilever. The amplitude of the oscillation is kept large enough to avoid the tip getting stuck to the sample due to adhesion forces. Non-contact mode is often used in roughness measurements when the sample is soft or effects of lateral forces must be minimized [7]. The AFM can also measure adhesion force of a sample surface. This is performed by letting the piezoelectric tube move in the z-direction, moving the tip against the sample until it finally snaps on to the surface by weak attraction forces like Van der Whaals forces. The piezo continues to press the tip to the sample surface until the cantilever bends and the deflection signal reaches a setpoint force value. When the setpoint value is reached, the tip is slowly moved away from the sample until the tip finally snaps off from the sample surface. By using the deflection measurements of the cantilever and Hooke s law, the adhesion force can be calculated. The adhesion force is then the applied force needed for the tip to snap off from the sample surface. a) b) Fig. 18. Principles of AFM, with close up on the tip and cantilever (a) and laser beam with reflection to the split-diode photodetector (b). The photodetector detects the AFM tip height movement (AFMsignal) and friction (FFM-signal) against the sample surface during scanning [7]. 19

25 3. Aims By coating two surfaces that are going to slide against each other, several positive factors could occur. The friction could be reduced, which is positive for engineering applications at macro level. If the adhesion force would be reduced in the dual coated system, the application at smaller scale would open up new possibilities to new technical devises like MEMS with non-sticking surfaces. In this thesis, four different coatings, CrN, TiAlN, WC/C and DLC were selected for tribological characterization. By creating wear scars on each coating in a SOFS-machine with reciprocating sliding at room temperature, wear mechanisms were investigated after each wear test, on each wear scar, by means of scanning electron microscopy, atomic force microscopy and optical profilometer. By varying load and sliding length, the progress of wear and wear mechanisms were studied. This was performed to develop knowledge on general performance of hard coatings, wear mechanisms, and ideas of coating selection for advanced applications. In general, the wear mechanisms were expected to be different between the nitrides and the carbon coatings but with the dual-coated system, the effects on friction and wear mechanisms with the TiAlN coated tool were uncertain due to the poor research of this type of wear test material setup. The friction was expected to vary between the selected coatings, and the friction coefficients were to be studied for comparison after the wear tests. Adhesion measurements was performed by atomic force microscope on each wear track to investigate if any of the selected coatings had potentials, with non-sticking surfaces, for coating of parts used in small devices like MEMS. 4. Experimental part 4.1 Material properties Mechanical properties for materials that were used in this thesis are following, table2: Table 2. Mechanical properties of materials. 2

26 Vancron 4, PM tool steel, alloyed with N was used as a substrate. The substrate was supplied by Uddeholm Tooling AB. The substrate arrived in the form of heat treated circular pieces with dimensions of 18 mm diameter and 5 mm thickness and a hardness of 63.3 HRC. Hardness of the substrate was chosen to be high enough to avoid plastic deformation of the substrate during sliding test of the coated test samples [7]. PVD-coatings used in this thesis were supplied by Oerlikon Balzers and the material properties and chemical composition data was supplied by the manufacturer. 4.2 Material preparation Before sending substrate materials to Oerlikon Balzers for coating, the substrate surface was prepared for mirror-like surface roughness. When the coatings process was complete and the samples arrived the final polishing and brushing was performed Substrate surface preparation procedure The substrate preparation was divided into following steps: Mounting of test samples in Bakelite. Rough manual grinding on paper, Struers 15, to remove scratches introduced into the samples when the test samples were cut into specified thickness by Uddeholm Tooling. Ultrasonic cleaning in alcohol for 2 min followed by drying in warm air for 2 min. Automatic grinding for 6 minutes, 25 N load, Allegro plus diamond grinding wheel with 6 µm diamond spray and blue lubricant. Ultrasonic cleaning in alcohol for 2 min followed by drying in warm air for 2 min. Automatic polishing for 7 minutes, 25 N load, polishing wheel with 3 µm diamond paste and green lubricant. Ultrasonic cleaning in alcohol for 2 min followed by drying in warm air for 2 min. Removal of Bakelite. Ultrasonic cleaning in alcohol for 2 min followed by drying in warm air for 2 min. The substrate preparation procedure described above was developed by slowly increase time in steps for both grinding and polishing. The time for grinding and polishing was increased until the result of mirror-like surface was achieved when the surface was examined by using light optical microscope and SEM. By slowly increase time for substrate preparation scratches was removed and no new surface defects, introduced to the surface by too long polishing time [24], was found. 21

27 4.2.2 Preparation of sliding wheel The sliding wheel used for wear test in SOFS was prepared in a procedure similar to the substrate for test samples. Grinding and polishing were done manually in a turning machine. Both grinding and polishing were performed in 2 minutes steps with breaks for surface investigation in light optical microscope. The steps were divided into: Grinding with paper, Struer 5. Ultrasonic cleaning in alcohol for 2 min followed by drying in warm air for 2 min. Grinding with paper, Struer 8. Ultrasonic cleaning in alcohol for 2 min followed by drying in warm air for 2 min. Grinding with paper, Struer 1. Ultrasonic cleaning in alcohol for 2 min followed by drying in warm air for 2 min. Polishing with polishing paper, green lubrication and 3 µm diamond paste. The sliding wheel was polished until a mirror-like surface with no scratches was achieved Brushing of coatings When the coatings arrived they lacked a final common industrial procedure: brushing. This final stage of the coating process is done to remove most of the droplets that can form during the PVD process. The process is not like polishing, when a mirror-like surface is sought. Brushing is often done with small hard particles that are firmly moved over the coating in order to remove droplets without destroying the coating surface itself. With lack of proper tools a method was developed to achieve similar brushing result with slightly different tools: Ultrasonic cleaning of samples in alcohol for 2 min followed by drying in warm air for 2 min. 15 seconds manually brushing of samples coated surface using a polishing wheel,.25 µm diamond paste and water as lubrication. The samples were turned 9 every third second. Ultrasonic cleaning of samples in alcohol for 2 min followed by drying in warm air for 2 min. 4.3 Wear tests Wear tracks were produced by SOFS under reciprocating sliding test mode. Three different conditions for reciprocating sliding in SOFS were used: constant wheel penetration depth, constant Hertzian contact pressure and constant load. The constant condition values were 22

28 approximated by using Hertzian contact theory for an elliptical counter body pressed against a flat surface. With R1 = 5mm and R2 = 25mm the shape factors was estimated to F 1 = 1. 7 and F 2 =. 96 [23]. The Hertz theory formulas were simulated in Matlab with load range from 1 N to 3 N and step size of 1 N, fig 19. Max contact pressure (MPa) a) Load vs Max Contact Pressure Substrate/ Substrate TiAlN/ TiAlN TiAlN/ WC-C TiAlN/ CrN TiAlN/ DLC Load (N) b) Approach of distance points vs Load pressure Substrate/ Substrate TiAlN/ TiAlN TiAlN/ WC-C TiAlN/ CrN TiAlN/ DLC Load (N) Approach (mm) x 1-3 Fig. 19. Results from Matlab simulation with max Hertzian contact pressure (a) and depth (b). Broken lines and arrows in (a) and (b) were the corresponding load for the pre-determined load condition e.g. contact pressure of 1865 MPa (a) and depth of 2 µm (b). In the simulation the material properties described earlier were used. Young s Modulus of TiAlN was used for the wheel and the samples elastic modulus was set to the coating of investigation. This setup was used for all simulations except one where the elastic modulus of the substrate was used for both the wheel and the sample. Data from supplier indicated that the thickness range of the coatings was 1-4 µm. In order to avoid penetration down to the substrate during tests, the penetration depth was set to 2 µm and corresponding values of load were selected, fig 19. The constant contact pressure was set 23

29 to 1865 MPa which is a value in between service conditions for coated machine elements [25]. For the constant load conditions the load was set to 6 N during sliding in SOFS. The starting point for sliding with constant load was the set with the highest number of slidings. After sliding in SOFS the friction curve was analyzed and by pinpointing a change in the friction curve the next set for number of slidings in SOFS was chosen: labeled x in the table 3. Table 3. SOFS sliding setup SOFS setup The sliding mode during sliding in SOFS was reciprocating sliding with a speed of,17 m/s and sliding length of 15 mm. The sliding wheel (counter body) was coated with TiAlN, fig 2. Before sliding the samples and the sliding wheel was cleaned in alcohol for 2 min and dried in warm air for 3 min. After the cleaning process, the samples were attached to a magnetic fixture and locked in position. During sliding, data was collected continuous by computer software. After sliding, the samples were cleaned and placed in vacuum chamber and the data from software were collected, examined and presented in friction graphs by using Matlab. Fig. 2. Coated wheel and sample 4.4 Surface characterization Three different scientific tools used for tribological investigation of the selected coatings were used before and after the wear simulation: AFM, SEM and OP. 24

30 4.4.1 Atomic Force Microscopy (AFM) Veeco Innova atomic force microscope was used to investigate the worn samples. Before the investigation was performed the samples were cleaned in alcohol and dried in warm air Tip calibration SiN (NFC18) AFM-tip from µ-mash was used throughout the investigations in AFM. Before every investigation-session the tip was calibrated for stiffness using a pre-calibrated tip made by µ-mash. The calibration procedure started with voltage-signal calibration of the software. By doing a point spectroscopy (adhesion measurement) on a hard surface it was possible to attach two points on the force-distance curve and let the software calibrate the voltage sensitivity by measuring the slope (V/ µm) between the two attached points, fig 21 (b). When the point spectroscopy was performed, the tip approached the sample and eventually jumped to contact. The tip was pushed to the sample until a setpoint was reached and after that, the tip started to retract from the sample. The last step when the tip snapped off the sample surface was also the measurement of an adhesion force, fig 21 (a). When the sensitivity was calibrated, the distance to distance - slope (µm/ µm) was checked to see that the software calibration was performed right i.e. the slope equals unity. a) b) c d e Fig. 21. AFM force distance graphs are shown in (a) and (b), where the two calibration points mentioned earlier can be seen in (b) as two crosses. The brighter cantilever in (c), (d) and (e) is the calibrated cantilever used for the calibration of the cantilever in use (the darker one in (c), (d) and (e)). In (c) the cantilevers are in setup position and in (d), (e) point spectroscopy is done on hard surface and reference cantilever respectively. 25

31 When the sensitivity was calibrated the tip was moved to the end of the pre-calibrated tip and another point spectroscopy was done. This time the two points were again attached to the force-distance curve, and the distance to distance slope was measured. By knowing the stiffness of the calibrated cantilever, the new cantilever with unknown stiffness could be calculated by using the following equation: d hard k = k 1 ref (5) d soft Where k and k ref are the calculated stiffness of the cantilever in use and the stiffness of the reference, i.e. calibrated, cantilever respectively. d hard and d soft are the point spectroscopy (µm/ µm) measured on a hard surface and the reference cantilever respectively, where equals to 1 if the sensitivity calibration is correct. Several point spectroscopies were d hard performed on the reference cantilever and an average stiffness value was calculated for use in the AFM investigations. Adhesion measurements were performed on every sliding track and original surfaces of the hard coatings. This was done by first scanning an area of 4 x 4 µm in the middle of a sliding track. In the selected area, 9 different points were selected for adhesion test. The raw data from the adhesion tests was collected from the AFM and plotted by using Matlab and Microsoft Excel. For error estimation of the adhesion test, 9 point spectroscopies were performed on one spot, and the difference in adhesion force was examined. The raw data from the scanned areas was collected for examination of surface roughness and topography by using SPM Lab Analysis. The tip shape was examined before every new sample was mounted for investigation in the AFM. By using a tip shape calibration sample, (PA1) from µ-mash, it was possible to see if the tip shape was sharp enough to register the small patterns that the calibration surface was made of. The tip was changed to a new one when the pattern indicated blunting of the tip. When a new tip was mounted, the stiffness calibration of the new tip was performed again Scanning Electron Microscopy (SEM) Surface topography and overview pictures were collected in the scanning electron microscope. Secondary electron were used for topographic investigations of the surface and back scattered electrons were used when difference in elements present at the surface was to be distinguished. 26

32 4.4.3 Optical Profilometer (OP) The optical profilometer was used for investigation of surface roughness of the original surfaces and sliding tracks of the samples. The sliding tracks depths were also investigated by using the profilometer. This was done by selecting points along a track, fig 22. Sample 1 2 wear track 3 Fig. 22. Profilometer investigations of the wear tracks produced in SOFS. Depth, width and surface roughness of the track were then measured at the points within the wear track. Average values were calculated for surface roughness, depth and width of the sliding tracks produced by SOFS. Worn volume was estimated by using the optical profilometer software, Vision 32. By scanning the surface, with the wear track included, it was possible to choose the worn area only and use a built-in function of worn volume calculation in the software. Optical profilometer scans were also performed for contact points on the coated wheel. The surface scans of the wheel were performed for comparison of wear performance with the worn samples e.g. the optical profilometer scans of the wheel were supposed to indicate if the wheel was worn or not after the wear tests. 27

33 5. Results 5.1 CrN Original surface of CrN AFM, Optical Profilometer and SEM were used to study the surface morphology of CrN. SEM revealed a surface morphology typical for arc deposited coatings: droplets and holes, fig 23 (d) (f). AFM showed a height of ~2 µm for the largest droplets on the CrN surface, fig 23 (a), and ~.4 µm for smaller droplets, fig 23 (b). The larger holes and droplets diameter was about 6 1 µm. The Optical Profilometer 4X6 µm scans indicated a surface roughness (Ra) of 54 nm +/- 14 nm µm [µm] 4 [µm] a) b) [µm] [µm]. µm 4 c) d) e) f) Fig. 23. Overview pictures of CrN collected by AFM (a), (b) (with one common height color bar), optical profilometer (b) and scanning electron microscope (d), (e) and (f). 28

34 5.1.2 Friction of CrN Friction data from SOFS software was collected during sliding of samples. The data was analyzed and presented in the graphs, fig. 24. According to the model of Hertzian contact theory for elliptical contact the CrN was tested under contact pressure of 1865 MPa at 83N load and 1674 MPa at 6N load test conditions. The friction curves showed similar behavior for both loads: higher initial friction coefficient of.7, which decreased and stabilized to.6 +/-.5 after 5 mm sliding distance (333 slidings) CrN 83N load, 64 slidings in SOFS a) Friction coefficient Distance (mm) CrN 6N load, 64 slidings in SOFS b) Friction coefficient Distance ( mm) Fig. 24. Friction graphs of CrN after 64 slidings in SOFS with 83N load (a) and 6N load (b). 29

35 5.1.3 Optical profilometer measurements of CrN Optical profilometer scans of the wear tracks CrN were used for investigation of wear mechanisms and measurements of the wear tracks revealing wear depths, widths etc. The investigations showed flattening and some abrasive scratches for short sliding distance, fig 25 (a), where the sample and wheel scan were collected with 1x and 5x magnification lens respectively. With increasing sliding distance the coating was more worn and surface defects like holes was elongated. Optical profilometer scans were also done for the wheel to see if the wheel was worn or not. The wheel scans were filtered for better vision of the wear, which showed the scans as flat surfaces, fig 25 (b), (d) and (f). The scans revealed that the wheel was scratched for shorter sliding distance and for longer slidings the wheel was worn and the coating destroyed after 16 slidings. The surface roughness values (Ra) in the flat parts of the wear tracks, measured at 3 points along the wear track, decreased as sliding distance increased: nm +/- 3.91nm, /- 2.9 nm and /- 2. nm after 4, 16 and 64 sliding respectively. a) b) c) d) e) f) Fig. 25. Optical profilometer scans showing CrN wear tracks of sample after 4, 16 and 64 slidings (a), (c), (e) and corresponding wear of wheel (b), (d), (f) at 83N load. Arrows indicate sliding direction. 3

36 The surface roughness measurements were also done by scanning an area, which included the wear track, of 92 µm X 12 µm, at three points along the wear track. After selecting the worn area manually in software Vision 32 the surface roughness (Ra) showed an increasing surface roughness with increasing sliding distance: nm +/- 5.2 nm, 384 nm +/ nm and nm +/ nm after 4, 16 and 64 slidings respectively at 83N load. For lower load and shorter sliding distance, the CrN showed adhered material, possible from tribo oxidation, and flattening, fig 26 (a). After longer slidings, fig 23 (c), (d), both the coating and the wheel were worn. The surface roughness (Ra) of the tracks was nm +/- 1.2 and nm +/ after 16 and 64 slidings respectively. The wheel showed a round piece of worn coating with a region looking like delamination, fig 26 (d). a) b) c) d) Fig. 26. Optical profilometer scans where figure (a) and (c) shows wear of sample after 16 and 64 slidings respectively with corresponding wear of wheel (b) and (d) at 6N load. The arrows indicate the sliding direction.. The wear track profiles were collected by using the profilometer with line scan direction across the wear track. The data from the line scans was exported to Matlab software where the data was interpret and presented in graphs, fig 27. The graphs represented an average value of the width and depth from the middle section of the wear tracks for CrN. Numerical average values, measured at three points along each track, showed following: with 83N load the average max wear depth was.65 µm +/-.33 µm, 1.45 µm +/-.66 µm, and 2.7 µm +/-.91 µm after 4, 16 and 64 slidings respectively. Along with the increasing average wear depth the average wear width also increased from µm +/ µm, µm +/ µm to µm +/ µm after 4, 16 and 64 slidings respectively. 31

37 .5 CrN 83N load a) b).5 CrN 6N load Depth (micro meter) Depth (micro meter) slidings 16 slidings 16 slidings 64 slidings 64 slidings Scan length (micro meter) Scan length (micro meter) Fig. 27. Wear track profiles for CrN at a load of 83N (a) and 6N (b). At 6N load the average wear depth were.4 µm +/-.25 µm, 2.17 µm +/ µm and average wear width were µm +/ µm, 53. µm +/ µm after 16 and 64 slidings respectively. The wear profile graphs, fig 27, together with average worn volume for the whole track, fig 28, shows that the wear was higher with increasing load for CrN. This was true since the wear profile for higher applied load was wider compared to lower load conditions. Worn Volume for CrN Worn volume (mm^3) 4,E-3 3,5E-3 3,E-3 2,5E-3 2,E-3 1,5E-3 1,E-3 5,E-4,E+ 5 1 Sliding distance (mm) CrN 83N CrN 6N Fig. 28. Worn volume for CrN Scanning electron microscope measurements of CrN Scanning electron microscope was used for investigation the worn surface of CrN after wear test in SOFS. CrN showed a flatted surface with some abrasive wear after 4 slidings, fig 29 32

38 (a). Cracks were also visible at the edge of the wear track, fig 29 (b). Material defects that were found at the original surface, were pressed down and pushed out in the sliding direction. a) b) Fig. 29. Scanning electron microscope images from investigation of CrN after 4 slidings at 83N load. Figure (b) shows close up of upper edge from wear track in (a). Arrows indicate sliding direction. The pressed down droplets, found after short sliding distance, fig 3 (a), (b), were also observed after longer slidings with the exception for more elongated structure of the surface defects, fig 3 (c), (d). a) b) c) d) Fig. 3. Scanning electron microscope images from investigation of CrN. Picture (a), and (b) present wear mechanisms found in the middle of the track for CrN with 83N load and 4 slidings. Picture (c) (f) summaries wear after 16 slidings. Arrows indicate sliding direction. 33

39 After the longest sliding distance, the wear track of CrN showed elongated holes with crack formation in between the holes, fig 31 (b). a) b) Fig. 31. CrN 83N load and 64 slidings. Figure (b) showed a close up image of middle area of wear track in (a). Arrows indicate sliding direction. Scanning electron investigation indicated a thin film formation in the wear tracks. The discovery was first detected at the edge of the wear track, fig 31 (a) with a close up (b). Once the film was detected, scanning electron microscope investigations focused on areas where the scanning electron microscope detectors could distinguish between the original coating and the film formed. At the edge of the wear track, it was easier to see the difference between untouched area and the thin film on the worn surface, fig 31 (a), (b). The pictures in fig 31 were collected by tilting the specimen for better view of the topographical changes. Due to the tilted specimen the determination of the film thickness was difficult to be made, but the thickness was roughly less than 1 µm. a) b) Fig. 32. SEM investigation of CrN 83N load, 64 slidings. Picture (a) and (b) showed edge of wear track where (b) showed a close up on the thin film formation. The arrows indicate the sliding direction 34

40 The thin film was further investigated using different detectors in scanning electron microscope. This was performed to investigate if the film was formed from elements belonging to the coated wheel or the coated sample. The detectors used in the investigation were Inlense detector, fig 33 (b), secondary electron detector, fig 33 (c), and back scattered electron detector, fig 33 (d). Both secondary electron detector and back scattered electron detector showed that smeared parts of the formed film had different density, compared to material defects within the coating and surrounding worn area of the smeared film. The difference in density indicated that the film was formed from a mixture of elements, where some of the elements did not originate from the coating. There were also cracks present in the thin film, which revealed its presence better by using the back scattered electron detector and the secondary electron detector in scanning electron microscope. The thin film was smeared out in the sliding direction. a) b) c) d) Fig. 33. SEM investigation of CrN 83N load, 64 slidings. Picture (b) (d) showed close up pictures from circular area in (c) with different SEM detectors in use: Inlens (b), Se2 (c) and BSE (d). Arrows indicate sliding direction. 35

41 By using EDX, the chemical composition of the film formation was roughly estimated. A line scan was performed over an area of deposited film, fig 34 (a), (b). The investigation showed that the film contained a high concentration of O and smaller amount of Ti and Al. Cr and N were also detected, but decreased in parts of the line scan where the film was present. a) b) c) d) e) f) g) Fig. 34. EDX line scan (b) at edge of wear track (a) for CrN after 64 slidings at 83N load. EDX shows the chemical composition (c) (g) of scanned path (b). 36

42 5.1.5 Atomic force microscopy measurements of CrN One selected debris particle, fig 35 (a) (e), was investigated by AFM for friction and adhesion. A setpoint of.4 µn and scan rate of µm/ s was used. Since the lateral force scan showed differences in friction and adhesion, a test was performed for investigation of the connection between friction and adhesion. The investigation showed that, in this case, high friction corresponded to high adhesion and vice verse. The particles showed a height of ~.2 µm and were found in wear tracks after a short sliding distance at 83N load. a) b) c) d) e).3.2 Force-Distance curve f) g).3.2 Force-Distance curve deflection (micro Newton) deflection (micro Newton) Z position (micro meter) Z position (micro meter) Fig. 35. AFM investigation of CrN after 4 slidings at 83N load. Lateral force, deflection and height mode are shown in (a), (b) and (c) respectively. A line scan over a particle is showed in (d) with corresponding height graph (e). Force-distance graphs from adhesion tests are shown in (f) and (g) where (f) corresponds to adhesion of the particle and (g) corresponds to adhesion of the surroundings of the particle. 37

43 5.2 DLC Original surface of DLC SEM, AFM and Optical Profilometer were used to study the topography of the DLC surface before wear tests. SEM investigations, fig 36 (c), showed rounded columnar asperities, placed mostly in clusters. The diameter of the rounded parts was about 1 µm. The surface beneath the round structure was dense with a small amount of holes with a diameter of.5 1 µm. The AFM verified the morphology seen by SEM and the AFM height scans showed round asperities with heights reaching close to 1 µm, fig 36 (a), (b), (c). The optical profilometer with 4 X 6 µm scans indicated a surface roughness (Ra) of nm +/ nm [µm] [µm] 4 1 a) b) µm µm [µm] [µm] c) d) Fig. 36. Overview pictures of DLC from AFM (a), (b), SEM (c) and Optical Profilometer (d) Friction measurements of DLC Friction data from SOFS software was collected during sliding of samples. The data was analyzed and presented in graphs, fig. 37, by using Matlab. The first contact of wheel to 38

44 coating surface showed a friction of.6 which decreased quickly and stabilized to.9 +/-.3 after a few slides, fig 37 (a). The distance to change was 21 mm (14 slidings) for both 52N (1161 MPa) and 6N (1211 MPa) load. For 216N (1865 MPa) load, fig 37 (b), the initial friction coefficient was.7, which decreased down to.15 after 7 mm sliding distance (47 slidings) and stabilized at.1 +/-.15 after 27 mm sliding distance (18 slidings)..7.6 a) DLC 52N load, 64 slidings in SOFS Friction coefficient Distance (mm) DLC 216N load, 64 slidings in SOFS.9 b).8.7 Friction coefficient Distance (mm) Fig. 37. Friction graphs of DLC after 64 slidings at 52N (a) and 216N (b) Optical profilometer measurements of DLC Optical profilometer scans of the wear tracks were used for investigation of wear depth, width, wear mechanisms etc. The sample and wheel scan were collected with 1x and 5x magnification lens respectively where the wheel scans were filtered for curvature and the scans were used to see if the wheel was worn or not after wear tests. The investigations of surface topography at lower load (52N) showed scratching of wheel contact point, fig 37 (b), and the sample surface showed adhered material in the wear track, fig 37 (a), with increasing 39

45 homogenous structure for longer sliding distance. The surface roughness (Ra) of the wear tracks decreased as sliding distance increased: nm +/- 1.3 nm, / nm and nm +/ nm after 4, 16 and 64 slidings respectively. a) b) c) d) e) f) Fig. 38. Scanned pictures from optical profilometer showing DLC wear tracks of sample after 4, 16 and 64 slidings (a), (c), (e) and corresponding wear of wheel (b), (d), (f) at 52N load. The arrows indicate the sliding direction. The surface roughness values (Ra) were collected by first scanning the sample, including the wear track, with 5x magnification lenses (92 µm X 12 µm). When the raw data from the scan was saved, it was possible to manually select the worn area in Vision 32 and collect the surface roughness from the wear track only, and not including the surrounding surface of unworn coating surface. The values were collected at three points along the wear track and the error estimation was calculated by standard deviation of the three measurements. This was performed for all wear tracks of DLC. The scratching of the wheel was visible with a very short sliding distance at 6N load, fig 39 (b) and the sample surface showed a wear track with smearing of asperities or adhered material in the middle section of the wear track. With increasing sliding distance at 6N load 4

46 the wear track showed similar surface topography as for the lower load of 52N, fig 38 (e), with the difference that material was pushed to the side of the wear track, fig 39 (c) and evidence of ploughing at the end contact points of the wheel, fig 39 (d). The surface roughness of the wear tracks, from the profilometer scans, were nm +/ nm and 96 nm +/ nm after 8 and 64 slidings respectively at an applied load of 6N. a) b) c) d) Fig. 39. Scanned pictures from optical profilometer showing DLC wear tracks of sample after 8 and 64 slidings (a), (c) and corresponding wear of wheel (b), (d), at 6N load. The arrows indicate the sliding direction. At increased load (216N), flattening was observed on the wear track surface and the surface roughness decreased with increasing sliding distance. The surface roughness measurements from the optical profilometer scans showed a surface roughness of nm +/ nm, nm +/ nm and 18.5nm +/ nm after 4, 16 and 64 slidings respectively. The wheel showed built up material at end points of contact region, fig 4 (b), (d), (f). The wheel contact point showed a flat area more than a worn surface. The wear tracks also showed formation of built up material. This built up material was placed at the side of the wear track. The optical profilometer scans of DLC wear tracks also showed a small amount of abrasive wear for longer sliding distance, fig 4 (c), (e). 41

47 a) b) c) d) e) f) Fig. 4. Scanned pictures from optical profilometer showing DLC wear tracks of sample after 4, 16 and 64 slidings (a), (c), (e) and corresponding wear of wheel (b), (d), (f) at 216N load. The arrows indicate the sliding direction. Despite the larger applied load, the DLC coating did not show evidence of severe wear, but much wider tracks were found, compared to track width at lower load. At 52N load the average wear widths were µm +/ µm, µm +/ µm and µm +/ after 4, 16 and 64 slidings respectively. Similar results were found for 6N load. For 216N load the average widths were µm +/ µm, µm +/ µm and µm +/ µm after 4, 16 and 64 slidings respectively. Wear track profiles were collected, as complement to scanned wear tracks, by using the profilometer with line scan across the wear track. The data from the line scan was exported to Matlab software where the data was interpret and presented in graphs, fig 41. The graphs represented an average value of the width and depth from the middle section of the wear tracks for DLC. Numerical average values from all the tracks, measured at three points along the track, showed following: at 52N load, the average max wear depths were +.2 µm +/-.74 µm, +.3 µm +/-.36 µm, and µm +/-.34µm after 4, 16 and 64 slidings respectively. With increasing load of 216N, the average max wear depths were.13 µm +/- 42

48 .29 µm,.21 µm +/-.11 µm, and.27 µm +/-.24 µm after 4, 16 and 64 slidings respectively. With an applied load of 6N the average max wear depths were µm +/-.67 µm and +.3 µm +/-.5 µm after 8 and 64 slidings respectively. The profiles were similar to lower loads, where the profile scans showed adhered material a) DLC 52N load 4 slidings 16 slidings 64 slidings Depth (micro meter) Scan length (micro meter) b) DLC 216N load 4 slidings 16 slidings 64 slidings Depth (micro meter) Scan length (micro meter).5.4 c) DLC 6N load 8 slidings 64 slidings.3 Depth (micro meter) Scan length (micro meter) Fig. 41. Wear track profiles of DLC from optical profilometer at load 52N (a), 216N (b) and 6N (c). The wear tracks profiles was collected from the middle section of the wear track 43

49 The average worn volume, interpret by software Vision 32, increased for increasing sliding distance for DLC, fig 42. Worn volume (mm^3) Worn Volume for DLC 5,E-4 DLC 52N DLC 216N 4,E-4 DLC 6N 3,E-4 2,E-4 1,E-4,E+ 5 1 Sliding distance (mm) Fig. 42. Worn volume for DLC coating Atomic force microscopy measurements of DLC AFM scans, fig 43, were performed for deeper understanding of the wear behavior of DLC. 1 µm X 1 µm scans were performed in the middle of the wear track and the raw data was then interpret in SPM lab analysis software. The height mode scan, fig 43 (a)-(c), showed deformed and smeared out asperities after short sliding distance, covering valleys between asperities found on the original coating surface. After increased sliding distance, the smearing leveled off, fig 43 (c). 5. nm [µm] [µm] [µm] a) b) c) [µm] [µm] [µm] 8 1 d) e) f). nm Fig. 43. AFM height scans for DLC after 16 (b), (e) and 64 (c), (f) slidings at 52N load. 3D solid texture view with 45º tilted plane around the y-axis for DLC is shown in (d)-(f). Height scans of original surface (a), (d) shows unworn original surface of DLC. The arrow indicate sliding direction for (a)-(f). 44

50 By measuring the peak sharpness and height of asperities, in the software SPM lab analysis, the peak sharpness and height decreased with increasing slidings. The measurements were performed by using 2D line scans, comparing the original surface with a worn surface, fig 44 (b), (d). [nm] a) b) [nm] [nm] c) d) [nm] Fig D line scan of single protrusions showed cut off top for longer slidings (c), (d) compared to untouched surface (a), (b). Circular area with line scan (a) and (b) corresponds to height graph (b) and (d) respectively. Figure (a) and (b) are 1 µm X 1 µm scans from fig 43. The AFM height scans for DLC at higher applied load during sliding in SOFS showed flattening for longer sliding distance, fig 45 (b), (c) and some smearing of asperities after shorter sliding distance, fig 45 (a). Some cracks were observed after 16 slidings at applied load of 216 N, fig 45 (b) [µm] [µm] [µm] 4 4 a) 35 b) 35 c) [µm] [µm] Fig. 45. AFM height scan for DLC after 4 (a), 16 (b) and 64 (c) slidings at 216N load with indication arrow of sliding direction for (a)-(c). Circular area (b) indicates cracks. 2 [µm] nm. nm 45

51 5.2.5 Scanning electron microscope measurements of DLC Scanning electron microscopy was performed to confirm the wear mechanisms found for DLC in optical profilometer and AFM investigations. The SEM investigations for wear mechanisms of DLC did confirm the smearing of asperities at lower load and short sliding distance, fig 46 (a), (b). Material was placed in valleys between worn asperities, fig 46 (b). The material did not show any difference in chemical composition compared to its surroundings. For higher load, material pushed to the side could be visible at the edge of a wear track, fig 46 (c)-(f). The middle of the wear tracks also revealed significant flattening compared to lower applied load after wear test in SOFS, fig 46 (f). a) b) c) d) e) f) Fig. 46. SEM investigation of DLC after 4 slidings (a), with close up (b), at 52N load. Picture (c) shows edge of wear track after 4 slidings at 216N load. Figure (d) (f) corresponds to zoomed areas (circular) of (c), beginning from left (d) which was unworn area and moving in to center (f) which was the most worn area. 46

52 5.3 TiAlN Original surface of TiAlN SEM, AFM and Optical Profilometer were used to study the morphology of the TiAlN coating surface before wear tests. SEM investigations, fig 47 (c), showed a coating morphology with droplets and holes from PVD arc deposition. The diameter of the droplets was about 1 µm with some exceptional larger droplets of about 4 µm diameter in size, fig 44 (c). SEM investigations of TAlN coating surface also revealed holes with 1 µm in size with some larger holes in the size of 6 µm in size, fig 44 (d). AFM verified the topography seen by SEM and the AFM height scans showed particles reaching a height of ~2 µm, fig 47 (a), (b). The optical profilometer with 4 X 6 µm scans indicated a surface roughness (Ra) of 2.97 nm +/ nm [µm] 2. µm a) b) µm [µm] c) d) Fig. 47. Overview pictures of TiAlN surface before wear test. Picture from AFM (a) showed height from height mode scans similar to optical profilometer scans (b). Scanning electron microscope showed droplets and holes (d) and (c) - close up pictures of a droplet. 47

53 5.3.2 Friction measurements of TiAlN Friction data from SOFS software was collected during sliding of samples. The data was analyzed and presented in the graphs by using Matlab software. According to the model of Hertzian contact theory for elliptical contact the TiAlN had a contact pressure of 1865 MPa at 59N load, 1871 MPa at 6N and 2219 MPa at 1N load. The friction coefficient for low load increased initially to.75 after 1 mm (67 slidings) and stabilized after a value of.7 +/-.9 after 6 mm sliding distance (4 slidings). For increased load, the friction coefficient raised initially to.8 after 12 mm (8 slidings) and continue to change throughout the total sliding distance of 96 mm (64 slidings) ended with a friction coefficient of.8 +/.12, fig a) TiAlN 1N load, 64 slidings in SOFS.8 Friction coefficient Distance (mm) 1 b) TiAlN 6N load, 64 slidings in SOFS.8 Friction coefficient Distance (mm) Fig. 48. Friction graphs from SOFS data showing friction coefficient for TiAlN at 1N load (a) and 6N load (b). 48

54 5.3.3 Optical profilometer measurements of TiAlN Optical profilometer scans of the wear tracks TiAlN were used for investigation of wear tracks regarding wear depth, width and wear mechanisms. The sample and wheel scanned at 1x and 5x magnification lens respectively. The wheel scans was filtered, in software Vision 32, for curvature and the scans were used to see if the wheel was worn or not during wear tests. The investigations of wear at higher load (1N) showed that the wheel was worn, fig 49 (b), (d), (f) after 16 slidings. The wear tracks showed mostly flattening and small adherence of material for short sliding distance, fig 49 (a). For longer sliding distance, fig 49 (b), (c), the wear tracks showed a mixture of flattening and abrasive wear. The surface roughness (Ra) of the wear tracks, with applied load of 1N, increased as sliding distance increased: nm +/- 8.1 nm, / nm and nm +/ nm for 4, 16 and 64 sliding respectively. The surface roughness measurements for all TiAlN wear tracks were done by scanning an area, which included the wear track, of 92 µm X 12 µm, at three points along the whole wear track. The worn area was selected manually in software Vision 32 and the surface roughness was recorded and calculated for average value and standard error estimation. a) b) c) d) e) f) Fig. 49. Wear tracks of TiAlN sample after 4 (a), 16 (c) and 64 (e) slidings at 1N load. Corresponding wear of wheel is showed for (b), (d) and (f) for 4, 16 and 64 slidings. Arrows shows sliding direction. 49

55 For lower applied load, the wheel was worn after 64 slidings, fig 5 (f), with worn material pushed to the side of the contact point. For shorter sliding distance the wheel contact point was flattened fig 5 (b), (d). The wear tracks showed similar behaviour as for higher applied load with a combination of flattening and a small amount of abrasive wear, fig 5 ( a), (c), (e), (g). a) b) c) d) e) f) g) h) Fig. 5. Scanned pictures from optical profilometer showing TiAlN wear tracks of sample after 4, 16 and 64 slidings (a), (c), (e) and corresponding wear of wheel (b), (d), (f) at 59N load. Figure (g) shows wear of sample after 22 slidings with corresponding wear of wheel (h) at 6N load. The arrows indicate the sliding direction. 5

56 The surface roughness of the wear tracks, with applied load of 59N, increased as sliding distance increased: nm +/ nm, / nm and nm +/ nm after 4, 16 and 64 slidings respectively. At an applied load of 6N, the surface roughness was 24.7 nm +/- 2.25nm and nm +/ nm after 22 and 64 slidings respectively. Wear track profiles were collected, as complement to scanned wear tracks, by using the profilometer with line scan direction across the wear track. The data from the line scan was exported to Matlab where the data was interpret and presented in graphs, fig 51 and fig 52. The graphs represented an average value of the width and depth from the middle section of the wear tracks for TiAlN. Numerical average values from all the tracks, measured at three points along the track, showed following: at 1N load the average max wear depths were.7 µm +/-.8 µm,.26 µm +/-.31 µm, and.3 µm +/-.1 µm after 4, 16 and 64 slidings respectively. The average wear tracks widths were µm +/ µm, 443. µm +/ µm and µm +/ µm after 4, 16 and 64 slidings respectively..4 TiAlN 1N load.3.2 Depth (micro meter) slidings slidings 64 slidings Scan length (micro meter) Fig. 51. Wear scar profiles from profilometer scan across wear tracks for TiAlN at 1N load. At lower applied load both the wear depth and wear widths were slightly lower compared to higher applied load for TiAlN. The average wear depths for 59N load were.8 µm +/-.57 µm,.17 µm +/-.37 µm and.26 µm +/-.62 µm after 4, 16 and 64 slidings respectively. The average wear widths after sliding at 59N load were µm +/- 4.3 µm, µm +/ µm and µm +/ µm after 4, 16 and 64 slidings respectively. The average max wear depth for 6N load was similar to the wear depth at 59N after 64 slidings. After 22 slidings the average wear depth was.4 µm +/-.17 µm with an average wear width of µm +/ µm, fig

57 b) TiAlN 59N load Depth (micro meter) slidings slidings 64 slidings Scan length (micro meter) TiAlN 6N load.4.3 c).2 Depth (micro meter) slidings 64 slidings Scan length (micro meter) Fig. 52. Wear scar profiles for TiAlN at 59N and 6N load. The average worn volume, interpreted by software Vision 32, increased for increasing sliding distance for DLC, fig 53 Worn volume (mm^3) Worn Volume for TiAlN 7,E-4 TiAlN 1N 6,E-4 TiAlN 59NN 5,E-4 TiAlN 6N 4,E-4 3,E-4 2,E-4 1,E-4,E+ 5 1 Sliding distance (mm) Fig. 53. Worn volume for TiAlN 52

58 5.3.4 Scanning electron microscope measurements of TiAlN Scanning electron microscope investigations was performed to conclude the wear mechanisms that acted during dry reciprocating sliding of the TiAlN coated surface. The investigations for wear mechanisms at the worn coated surface of TiAlN were concentrated on wear after the highest applied load, where the wear was most pronounced. The scanning electron microscope revealed flattening of surface at the worn area of TiAlN after short sliding distance, fig 54 (a). At an applied load of 1N, the wear track also showed outdrawn debris particles after short sliding distance, fig 54 (b). For longer sliding distance the wear track showed crack formation, fig 54 (c), (d), and formation of tribolayer fig 54 (e), (f). a) b) c) d) e) f) Fig. 54. SEM investigation of TiAlN wear tracks at 1N load. Overview of wear track after 16 slidings is seen in (a) with close up on wear debris in (b) taken with Inlens (left) and Se2 (right) detector. Wear track after 64 slidings is seen in (c) (f) with comparison of Inlens (e) and Se2 (f) detector. 53

59 5.4 WC/C Original surface of WC/C SEM, AFM and Optical Profilometer were used to study the morphology of the WC/C surface before wear tests. SEM investigations of WC/C showed a surface topography with rounded columnar particles with a broad variation in size, fig 55 (c). The diameter of the larger rounded parts was about 3 µm, but smaller rounded parts were also present. The smaller rounded parts had a diameter of about 1 µm. The surface beneath the round structure was dense with a small amount of holes. The AFM verified the morphology of WC/C seen by SEM. The AFM height scans, fig 55 (a), (b) showed round structures with some particles reaching a height of ~1.8 µm. The optical profilometer with 4 X 6 µm scans indicated a surface roughness (Ra) of 27. nm +/ nm [µm] [µm] 2. µm 4 1 a) b) [µm] [µm] µm c) d) Fig. 55. Original surface morphology of WC/C measured by AFM (a)- (b), SEM (c) and Optical profilometer (d). 54

60 5.4.2 Friction measurements of WC/C Friction data from SOFS software was collected during sliding of samples. The data was analyzed and presented in graphs by using Matlab, fig. 56. For lower loads of 44N (988 MPa) and 6N (196 MPa) the friction coefficient was.6 initially and decreased quickly down to.23 after 2 mm sliding distance (13 slidings), and stabilized after 2 mm (133 slidings) with a friction coefficient of.22 +/ a) WC/C 44N load, 64 slidings in SOFS Friction coefficient Distance (mm) b) WC/C 6N load, 64 slidings in SOFS Friction coefficient Distance (mm) Fig. 56. Friction graphs, interped from rawdata collected in SOFS during wear test at applied load of 4N (a) and (6N) (b) with 96 mm sliding distance (64 slidings). 55

61 For a higher applied load of 295N (1865 MPa) the friction coefficient was.8 initially, fig 57, and decreased quickly down to.23 after 44 mm sliding distance (3 slidings), and stabilized after 27 mm (18 slidings) ending with a friction coefficient of.2 +/ WC/C 295N load, 64 slidings in SOFS.8.7 Friction coefficient Distance (mm) Fig. 57. Friction graphs, interped from rawdata collected in SOFS during wear test at applied load of 295N with 96 mm sliding distance (64 slidings) Optical profilometer measurements of WC/C Optical profilometer scans of the wear tracks WC/C were used for investigation of wear depth, width and wear mechanisms. The sample and wheel scans were collected of 1x and 5x magnification respectively, where the wheel scans were filtered for curvature and the scans were used to see if the wheel was worn or not during wear tests. For lower loads and short sliding distance WC/C showed very little wear, and flattening of the coated surface was the main wear mechanism for both the sample and the wheel, fig 58 (a), (b). For longer sliding the optical profilometer investigations showed the same result for both a) b) Fig. 58. Optical profilometer scans of sample (a) and wheel (b) after 5 slidings in SOFS wear test at 6N load. Arrows indicate sliding direction. 56

62 44N and 6N load: a small amount of material pushed to the side of the wear track and more surface flattening in the centre of the wear track, fig 59 (c), (e). The wheel showed small amount of adhered material at the end points parallel to the sliding direction, fig 59 (d), (f). Surface roughness values (Ra) of WC/C wear tracks were collected, in the same way as for all other coatings, by first scanning the sample, including the wear track, with 5x magnification lens (92 µm X 12 µm). The worn area was selected manually in software Vision 32, at three points along each wear track, and the surface roughness was recorded. The surface roughness values of the wear tracks were nm +/ nm, 33.4 nm +/- 4.8 nm and nm +/ nm after 4, 16 and 64 slidings respectively at an applied load of 44N. Surface roughness values for 6N load were estimated, by Vision 32, to 4.14 nm +/ nm and nm +/ nm after 5 and 64 slidings respectively. a) b) c) d) e) f) Fig. 59. Optical profilometer scanning of WC/C wear tracks after 4, 16 and 64 slidings (a), (c), (e) and corresponding wear of wheel (b), (d), (f) at 44N load.. The arrows indicate the sliding direction. 57

63 For higher applied load (295N), the investigations in the optical profilometer showed signs of abrasive wear for the sample and flattening of contact point of wheel after short sliding distance, fig 6 (a), (b). With increasing sliding distance the wheel showed signs of abrasive wear, fig 6 (d), (f), and the sample showed a mixture of abrasive wear, fig 6 (c), and delaminating of the coating in the centre of the wear track, fig 6 (e). The surface roughness measurements showed 62.2 nm +/ nm, 78.5 nm +/ nm and nm +/ nm after 4, 16 and 64 slidings respectively. a) b) c) d) e) f) Fig. 6. Scanned pictures from optical profilometer showing WC/C wear tracks of sample after 4, 16 and 64 slidings (a), (c), (e) and corresponding wear of wheel (b), (d), (f) at 295N load. The arrows indicate the sliding direction. Wear track profiles were collected, as complement to scanned wear tracks, by using the profilometer with line scan across the wear track. The data from the line scan was exported to Matlab, where the data was interpret and presented in graphs. The graphs represented an average value of the width and depth from the middle section of the wear tracks for WC/C. By continue the line scan at three points along the whole track an average max value of width and depth could be estimated. 58

64 The average max depth scans for WC/C at 44N applied load, fig 61 (a), were.9 µm +/-.89 µm,.9 µm +/-.42 µm and.14 µm +/-.39 µm after 4, 16 and 64 sliding respectively. The max average wear widths were 324. µm +/ µm, µm +/ µm and µm +/ µm after 4, 16 and 64 slidings respectively. The average max depth scans for WC/C at 6N applied load, fig 61 (b), were.3 µm +/-.15 µm and.17 µm +/-.1 µm after 5 and 64 sliding respectively. The max average wear widths were µm +/ µm and µm +/ µm after 5 and 64 slidings respectively..5.4 a) WC/C 44N load.3 Depth (micro meter) slidings 16 slidings 64 slidings Scan length (micro meter) b) WC/C 6N load Depth (micro meter) slidings 64 slidings Scan length (micro meter) Fig. 61. Wear track profiles, measured across the wear track of WC/C after wear test at applied load 44N (a) and 6N (b) 59

65 The average max depth scans for WC/C at 295N applied load, fig 62, were.26 µm +/-.14 µm,.33 µm +/-.7 µm and.62 µm +/-.26 µm after 4, 16 and 64 sliding respectively. The max average wear widths were µm +/ µm, µm +/ µm and µm +/ µm after 4, 16 and 64 slidings respectively. WC/C 295N load.4.2 Depth (micro meter) slidings 16 slidings 64 slidings Scan length (micro meter) Fig. 62. Wear track profiles, measured across the wear track of WC/C after wear test at applied load of 295N. An average worn volume for WC/C was estimated by software Vision 32. The worn volume increased with increasing sliding distance, fig 63. Worn volume (mm^3) Worn Volume for WC/C 1,E-3 WC/C 44N 8,E-4 WC/C 295N WC/C 6N 6,E-4 4,E-4 2,E-4,E+ 5 1 Sliding distance (mm) Fig. 63. Average worn volume for WC/C Scanning electron measurements of WC/C Scanning electron microscope investigations were performed for WC/C wear tracks. The investigations were focused on wear mechanisms at wear tracks tested at highest applied load, since the optical profilometer showed most wear for higher applied load test conditions. 6

66 For short sliding distance the scanning electron investigations of WC/C showed flattening of the wear track surface, fig 64 (a), with pressed down droplets, fig 64 (b) and enlarged surface defects like holes fig 64 (a). The droplet s surrounding worn areas were outdrawn in the sliding direction, fig 64 (b). Pressed down droplets was also found in the wear track after 16 sliding, fig 64 (f), with the difference that the holes, found at the original surface and the wear track after 4 slidings, were larger, fig 64 (e). a) b) c) d) e f) Fig. 64. Scanned pictures from SEM investigations showed WC/C wear track after 4 slidings at 295N load, focusing on droplets (a), (b) and surface fatigue (c), (d). Similar wear mechanisms were shown for 16 sliding at 295N load (e), (f). The arrows indicate the sliding direction. 61

67 First evidences of fatigue wear were found at elongated pattern after shorter sliding distance of 6 mm (4 sidings), fig 64 (c), (d). For the longest sliding distance of 96 mm (64 slidings) the scanning electron investigations of WC/C wear tracks showed a wear track surface with evidence of delaminating coating and surface fatigue, fig 65 (a). The delaminating of the coating was found at holes, fig 65 (c), (d), where the remained material in the holes showed a cracked surface, fig 65 (d). a) b) c) d) Fig. 65. SEM investigation of WC/C at 295N load. Figure (a), (b) showed wear track after 64 slidings with close up on surface craters, (b), (c). Arrows indicate sliding direction. 5.5 Adhesion measurements Adhesion measurements were performed for each wear track of all coatings. The adhesion measurements were performed in atomic force microscopy (AFM) with a setpoint of.4 µn. The measurements were performed by scanning a 4 µm X 4 µm area. In the scanned area, nine points of each wear track were chosen for adhesion tests. The chosen adhesion points were at the flat region of the wear tracks. After adhesion measurements of each sample the tip 62

68 was checked for sharpness by scanning on a reference sample. If the tip was found worn in the tip shape-test, it was replaced and calibrated by using the reference cantilever. Adhesion Force (µn) The adhesion data was collected and an average value was calculated and presented in graphs, fig 66. The highest adhesion values were found for CrN and WC/C. This was also true for the original coating surfaces before wear tests, fig 66 (d).,2,18,16,14,12,1,8,6,4,2 Adhesion measurements - all samples,2 a),18 b) CrN 83N,16 DLC 52N,14,12 TiAlN 1N,1 WC/C 44N,8,6,4, Slidings (N) Adhesion Force (µn) Adhesion measurements - all samples CrN 83N DLC 216N TiAlN 59N WC/C 295N Slidings (N) Adhesion Force (µn),2,18,16,14,12,1,8,6,4,2 Adhesion measurements - all samples CrN 6N DLC 6N TiAlN 6N WC/C 6N Slidings (N) Adhesion Force (µn),2,18,16,14,12,1,8,6,4,2 c) d) Adhesion measurements - all samples 1) CrN 2) DLC 3) TiAlN 4) WC/C Index of Original coated surface Fig. 66. Adhesion measurements of all hard coatings with load setup 1 (a), 2 (b) and 3 (c) respectively. For comparison, adhesion test of original coating surface was performed (d). 6. Discussion The wear test conditions used in this thesis were chosen to be in a regime where no severe damage of the coatings occurred. By choosing a low to intermediate load conditions [25], indicated by the Hertzian contact theory model, the severe wear situations were avoided. It should be mentioned that the model of Hertzian contact pressure is a rough estimation of the contact pressure and the values that are given by the Matlab simulation are only valid for an initial state. Once the model starts sliding, and the contact points get larger due to wear, the values obtained in the simulation starts to differ from the original calculated equations. The 63

69 specific material used in the setup seems to be unique, and a validation of the theoretical model used in Matlab would require a more advanced simulation with a fully integrated geometrical CAD part to estimate the precision of the used model. 6.1 Friction of selected hard coatings The friction graphs showed an initial high friction values for all coatings, fig Friction of selected hard coatings CrN DLC TiAlN WC/C Friction coefficient Sliding distance (mm) Fig. 67. Representative friction graphs of selected hard coatings. Hard coatings are known to be wear resistant by preventing wear mechanisms, which increase friction in sliding contacts: ploughing [6, 7, 19]. No indication of ploughing was found at the contact point of the wheel for the shortest sliding distances, and thus ploughing was not the reason for the initial high friction of the selected hard coatings. The asperity deformation and high shear force introduced at the contact interface, by the wheel sliding against the sample, are more realistic reasons for the initial increasing friction for the hard coatings [7]. High shear force introduced at the contact interface between the wheel and the samples, which increase friction, can be reduced if a microfilm of transferred material is formed in between the contact surfaces [6, 7]. For CrN a micro film or tribo-oxidation film was formed for longer sliding distance of 96 mm, as was shown with scanning electron microscope investigations. The thin film formation, together with flattening of surface, could be an explanation for the decrease in friction for CrN after longer sliding distance of 96 mm (64 slidings), fig 67. The friction coefficient value of CrN indicated a value of ~.6, which was comparable to other similar tribological investigations of CrN sliding against Al and steel [3, 31, 32] (~.5 for Al and ~.7 for steel). 64

70 The decrease in friction for TiAlN is harder to explain, and flattening of surface is probably the main cause for decrease in friction for longer sliding distance. Even though the thin film formation was indicated to start forming at longer sliding distance for TiAlN, no evidence could be produced by means of EDX when the material was the same for both the wheel and the sample. TiAlN showed a friction of ~.7-.8, fig 67. The same values were found in an article where two coatings, AlCrN and TiAlN, were compared for tribological performance [27]. It should be mentioned though that there was some difference with the wear test setup used in the article compared to the setup used in this thesis. In the article [27], SiN was used as the counter body, the load was only 5N and the coatings were structured as mono layered, not nano structured as TiAlN used in this thesis. The friction coefficient obtained from sliding TiAlN against SiN showed a value of ~.7 and wear debris formation was assumed to be the reason for an unstable friction curve. The friction curve for TiAlN used in this thesis shows the same behavior and it is possible that the friction did not stabilize due to formation of wear debris particles. Friction measurements of WC/C showed a decreasing friction after a few slidings, and the friction coefficient stabilized at ~.2, fig 67. The same value was found for WC/C sliding against Zn [26] and steel [28] at applied load of 2N and 3N respectively. The friction curve of DLC showed the same behavior as WC/C with high initial friction which decreased after a few sliding, and this is probably because of the WC/C coatings belonging to the group of DLC coatings, with multilayer structure [26, 34]. The DLC coating did though show a friction coefficient of ~.1 which was lower than the WC/C coating, fig 67. The same value was found for DLC sliding against SiN [35]. The decreasing friction for WC/C and DLC occurred due to flattening of surface and thin film formation, which was observed with scientific tools like SEM, AFM and optical profilometer. 6.2 Tribological characterization of selected hard coatings The surface roughness values of the original surfaces where measured after the brushing procedure for all the coatings, table 4. Table 4. Surface roughness values of selected hard coatings. 65

71 The surface roughness values for the coatings used in this thesis were slightly lower than values found in scientific reports, [26, 27, 28]. The small difference in surface roughness was probably because of the brushing procedure, which removed some surface defects. Investigations of the original surface of the selected hard coatings by means of SEM, AFM and profilometer showed a typical morphology of arc deposited coatings, fig 68. a) b) c) d) Fig. 68. Morphology of CrN (a), TiAlN (b), WC/C (c) and DLC (d). Arrows indicates surface defects. The morphology of CrN, fig 68 (a), was in agreement with other reports of arc deposited CrN coatings [1]. Even though the brushing procedure removed many droplets, there were still typical material defects of arc deposited methods left at the surface after brushing of the coating. This was true for TiAlN also [16], even though the surface roughness indicated smaller amount, or smaller size, of surface defects for TiAlN compared to CrN, table 4. WC/C and DLC showed surface morphology with more rounded asperities, fig 68 (c) (d), compared to CrN and TiAlN. The DLC morphology showed smaller amount of holes compared to WC/C. The same type of surface morphology was found in scientific articles for DLC [35] and WC/C [34], where the reference article of WC/C [34] showed surface preparation defects, which could not be found for WC/C used in this thesis. CrN showed similar wear damage for all three load conditions, and this was true for TiAlN also. The only measurable differences in wear damage were wear track depth, width and worn volume which were increasing with higher applied load. The difference in applied load did not exceed 23N for CrN and 4N for TiAlN. The small difference in applied load probably caused CrN and TiAlN respectively to show similar wear damage for all three load conditions, compared to WC/C and DLC which had a higher applied load in the load condition 2, constant contact pressure, table 5. 66

72 Table 5. Load conditions for selected hard coatings. Load 1 (N) setup for constant penetration depth of 2 µm Sliding set Load 2 (N) setup for constant contact pressure at 1865 Mpa Load 3 (N) setup for constant load Sample Sliding set Sliding set TiAlN 1 4, 16, , 16, , 22 CrN 83 4, , , 16 WC/C 44 4, 16, , 16, , 5 DLC 52 4, 16, , 16, , 8 Load condition 1 and 3 showed similar wear damage for DLC, and this was true for WC/C also. Once again, the explanation for this behaviour could be that the applied load did not differ enough, between load condition 1 and 3 for DLC and WC/C, to show any significant difference in wear damage other than measurable differences in wear depth, width and worn volume which increased with applied load Load condition 1 CrN, with applied load of 83N, showed a flatten surface with small amount of abrasive wear for a short sliding distance of 6 mm, fig 69. a) b) Abrasive wear Fig. 69. CrN with focus on abrasive wear area, broken lines in (a) with close up of the same area in (b). For longer sliding distance of 96 mm, CrN showed crushed droplets, thin film formation, cracks and partly delamination of the coating from the sample, fig 7, and the wheel. a) b) Delaminated areas Fig. 7. CrN with focus on delamination, area with broken lines in (a) and close up in (b). 67

73 These wear mechanisms are typical for reciprocating sliding when particles are kept in the wear track during wear test, [32], [33]. The same result was found in an article where tribological tests were performed for CrB 2, TiN and CrN sliding against alumina and steel in reciprocating mode [33]. In this article, crack formation and local coating delamination have been explained by fatigue cracks due to cyclic shear forces. An initial increasing friction has been explained, as mentioned earlier, by initial film formation of Al on the coatings. CrN had higher ability to form a thin film of Al compared to CrB 2 and TiN stated in the article. This might be comparable to the coatings used in this thesis where observations of CrN, by means of SEM, showed more pronounced film formation compared to TiAlN. The chemical analysis of the thin film for CrN in this thesis showed that the film contained elements of Ti, Al, N and O. This indicated that the film was produced from debris formation or tribo oxidation of the counterbody, which was coated with TiAlN. Cr was also present in the EDX analysis, but the amount of Cr was lower at the film formation compared to the original CrN coating surface. Text books of tribology contributed to a good explanation of crack formation and delamination of coatings in sliding contact [6]: Asperities mainly deform plastically when the sliding action takes place at the top of the asperities, although the general contact stress may be smaller than the yield stress of the material because the local stresses at the small asperity areas are much higher. Due to the high stresses, asperities will generate dislocations, dislocation pile-ups and crack nucleation at only some ten micrometers beneath the surface. Before a loose particle can form, several cracks must be nucleated because of the small plastic deformation of the surface. The delamination process is then described by the process of wear particle formation by shear formation of voids in the material nearby the surface. As the wear process continues, the voids grow until final failure of the coated surface. It was interesting to see that the surface roughness of CrN increased as sliding distance increased, when measuring the whole wear track. The increased surface roughness of the wear track of CrN were caused by delaminated areas of the coating as the sliding continued, compared to the measurements performed on the smaller flat parts which seemed to get finer as the sliding distance increased. The flat parts of the wear tracks, together with film formation, most likely caused the observed decreased friction with longer sliding distance. The small particles that were found in AFM after shorter sliding distance, fig 35, could be the initial stage of the thin film formation. This would then indicate that the transferred film had lower friction than the original coating and the CrN coating suffered high wear rate before, or during, forming of the wear protective film. The CrN coating showed the deepest wear track, highest worn volume and thereby the most worn coating, compared to the other coatings used 68

74 in this thesis. The coating structure was single layer, which might contribute to worse wear resistance. Other contribution to the poor wear resistance is surface defects like craters and droplets, which was found at the surface or CrN. Exactly how much the surface defects contributed to wear in this thesis is difficult to say, without using a reference coating of the same material with less surface defects. The TiAlN coating showed less worn surface, for all load conditions, used in this thesis. TiAlN was a nanostructured coating. The exact meaning of nanostructured in the TiAlN used throughout this thesis case is still unclear, but as mentioned earlier nano structured coatings can have small grain size. The coating can also be nano structured by means of nano structured layers, which enhance the tribological performance of the coating. Hardness is often high in nanostructured coatings, and so was TiAlN compared to the other coatings. The coating structure, with increased hardness might be an explanation to the excellent wear resistance. The wear scar surface showed a flatted surface, fig 71, with short sliding distance of 6 mm and an applied load of 1N, even though the profilometer line scan did show a small amount of abrasive wear. a) b) Worn surface (flatted) Original surface Fig. 71. TiAlN with focus on flatted surface. Edge of wear track area in broken lines (a) and close up in (b), where the edge or wear track is marked with filled line. For longer sliding distance, of 96 mm, the wear tracks of TiAlN were found to have wear debris, fig 72. Wear tracks of TiAlN also had flattening and small amount of cracks at the wear tracks. Observations of the wheel contact point showed a worn surface. a) b) Debris particles Fig. 72. TiAlN with focus on debris particles, area with broken lines in (a) and close up on the same area in (b). 69

75 In the article [27], which was mentioned before, similar wear mechanisms were found for SiN sliding against TiAlN. The wear mechanisms described for TiAlN in the article were oxidative and abrasive wear accompanying with the formation of debris. This is probably true for TiAlN used in this thesis too, when the unsteady friction graph and wear track profiles with increased surface roughness with increasing sliding distance indicated continuous formation of debris particles. One interesting thing mentioned in the article was that the tribochemical products formed in the wear track for AlCrN contained CrO 3 and Al 2 O 3. This result might be compared with CrN and TiAlN coated sample and wheel used in this thesis. The Ti, Al, N and O found for thin film formation of CrN with EDX analysis might be the result of a tribo-chemical reaction. In the article the tribo-chemical products of TiAlN was not determined. In this thesis the chemical products were not determined either due to two similar materials, TiAlN against TiAlN, sliding against each other. TiAlN are known to produce an oxide layer containing Al 2 O 3. This is actually a way of protect the coating from further oxidation and that is why TiAlN is excellent for cutting tool applications [6], [7], [29]. The layer formation has though proven to be a disadvantage if debris particles are formed, as for TiAlN used in this thesis. It is hard to say which sliding surface contributed more in the formation of debris particles, but the optical profilometer investigation of the sample and the wheel indicated a worn surface of the wheel for longer sliding distance, fig 49. The WC/C coating was not much affected by wear under an applied load of 44N. The wear track surface had a flatted surface throughout the wear test, and the wheel had material transferred to it after a sliding distance of 96 mm, fig 73. a) b) Adhered material Fig. 73. WC/C with focus on the wheel contact point, area with broken lines in (a) and close up view in (b), which showed adhered material. The transferred material could have occurred due to an initial thin film formation or tribooxidation. Further chemical investigations with EDX should be performed to be able to say if 7

76 the smeared out material came from the wheel or the sample. At loads of 44N the WC/C coating had great wear resistance, and this might be connected to its coating structure. The coating structure of WC/C used in this thesis was multilayer, where the layer consisted of amorphous WC and C. This structure, with its enhanced tribological properties, was mentioned earlier in this thesis, fig 8, is common for WC/C coatings as can be found in articles like [34]. The DLC coating had smearing and film formation with short sliding distance of 6 mm and an applied load of 52N, which was best shown in AFM and SEM investigations, fig 74. a) b) The optical profilometer did though give signs of this behavior of film formation with increased height of the wear track instead of a wear depth, fig 38, fig 41. The thin film formation seemed to occur when asperities was deformed by the sliding contact of the wheel. Material from cut-off asperities was placed in between droplets of original surface, and the formation of deformed material together with still existing droplets constituted a flat surface, which contributed to the decreased friction coefficient. The AFM investigation of DLC wear tracks showed the lubrication effect of the smearing phenomena really well, where cut of asperities and deformed material was mainly taking place with short sliding distance and leveled off at longer sliding distance of 96 mm, fig 44. It is hard to say how much the material transformation leveled off, since only a small area of the wear track was investigated by the AFM. The investigations did though show that the contact pressure at asperities was lowered by deformed material from cut-off asperities, which was placed in between droplets. If the material transformation leveled off or not does not really matter since the cut-off asperities themselves was, after deformation, broaden enough to lower the initial contact pressure and thereby also lowered friction. Deformed asperities Film formation Fig. 74. Film formation for DLC (area with broken lines in (a), and close up view of the same area in (b)). Both WC/C and DLC coatings belong to a special group of coatings: self lubricating coatings. In this thesis, the coatings were structured as single layer coating for DLC and multilayer for WC/C, where the layer consisted of amorphous WC and C [34]. Textbooks of tribology describe the low friction for this kind of coatings with a thin film formation [6], 71

77 which was described in the beginning of this thesis, fig 13. The thin film that forms in sliding contact with WC/C and DLC is a low shear film that consists of carbon with sp 2 (graphite) and sp 3 (diamond) bonds. The hardness of the coatings is high due to covalent bonds and the amount of sp 3 bonding [6]. When material is transferred and the thin film is formed, the two materials in contact form a new material pair, where the very thin low-shear film prevents ploughing and thereby lower the friction between the sliding contact surfaces. The film formation is known to form in sliding contact with WC/C and DLC, but few articles and books have put focus on the load support combination with pushed down deformed material in between droplets and cut off asperities, which was shown in this thesis. Some articles do though show the initial smearing effect [35, 36]. Perhaps it is obvious that the film formation, together with still existing cut-off asperities, act as a load support for the coating, but often the articles put focus on the film only [37, 38]. If it is not obvious, then the results from tribological investigations of DLC by means of SEM, optical profilometer and AFM in this thesis covered much more of the film formation mechanism, compared to articles found for tribological investigation of DLC Load condition 2 In this load condition of constant pressure, observations of CrN and TiAlN showed the same wear damage as for load condition 1. WC/C and DLC had a higher applied load in this load condition and the wear damage was somewhat different compared to the lower applied load in load condition 1 and 3, table 5. At an applied load of 295N, observations of the WC/C sample showed similar fatigue wear signs as CrN, where partly delamination and crack formation within the coating occurred, fig 75. a) b) Delaminated areas Fig. 75. WC/C with focus on delamination, area with broken lines in (a) and close up on the same area in (b). As for CrN, observations of the WC/C wear track showed crushed down and drawn out droplets and surface defects e.g. craters were also visible in the wear track, fig 64. The difference between CrN and WC/C fatigue wear was that WC/C also had signs of asperities 72

78 smearing, fig 64 (f). Observations of the sliding contact points of the TiAlN coated wheel, sliding against WC/C, at higher load showed evidence of abrasive wear and material transferred to it, possible evidence of ploughing, fig 63. The only reference found for WC/C fatigue wear e.g. outdrawn craters and delamination of coating, was found in an article with WC/C a:ch deposited on M2 steel substrates [36]. WC/C a:ch and a: CH, both a type of DLC coatings, were tested for engine applications in a special build cam-tappet rig with peak loads of 24N. In the wear test the WC/C a:ch coating had similar wear behavior as WC/C used in this thesis: initial flattening, delamination, pitting and abrasive wear. It was interesting to see an article, which tries to deal with real applications where the peak loads reaches close to the highest loads used in this thesis. With an applied load of 216N, observations of the DLC coating showed a wear track with instant flattening, material pushed to the side, fig 76, crack formation fig 45 (b) and material transferred to the wheel was more prominent, fig 4 (b), (d), (f). a) b) Flatted surface Material pushed to the edge of wear track Fig. 76. DLC with focus on the edge of the wear track, area with broken lines in a). Close up view of area in (a) showed material pushed to the edge of the wear track (left side in (b)), and flatted surface in the middle of the wear track (right side in (b)). Investigation of the DLC coating did not show signs of fatigue wear due to cyclic shear forces, as the WC/C coating did. The reason for that might be that the DLC had less surface defects, e.g. craters, than WC/C, and thereby, the wear resistance was better for DLC, compared to WC/C. Even though no references were found for tribological properties of DLC at higher loads, compared to the one used in this thesis, the coating had good tribological properties for high-load applications Load condition 3 In load condition 3, as mentioned earlier, the wear mechanisms for all coatings were similar to load condition 1, since the load did not differ enough to involve new wear phenomena. When the wheel contact point was studied after just a few slidings, the initial wear behaviour was somewhat different between the nitrides and the carbon coatings. The nitrides, for example 73

79 CrN, showed more scratching of the wheel contact point compared to the carbon coatings like WC/C, fig 77. a) b) Scratched surface c) d) Wheel contact point Fig. 77. Wheel contact point of CrN (a)-(b) and WC/C c)-d) after 16 and 5 slides respectively, at an applied load of 6N. Scratches of wheel contact point were observed for TiAlN sliding against CrN (area of broken lines in (a), with close up view in (b)). Scratches were not found at the contact point of TiAlN, when sliding against WC/C ((a), with close up view in (b)). This might indicate that the droplets of the nitrides played a major part at the initial state of wear, with hard asperities that scratched the wheel. The contact at the carbon coatings, though, might indicate smearing or low shear film formation as the initial mechanism of wear. It is hard to say if the film formation was the reason for less scratching of the wheel contact point, since the carbon coatings were softer than the nitride coatings used in this thesis. The reason for less scratching could simply has occurred due to easier deformation of asperities, and flattening of surface for the softer carbon coatings, compared to the harder nitride coatings. To truly confirm the film formation of the wheel contact point, a close up investigation of the wheel contact point surface in SEM, with chemical analysis (EDX), should be performed. 74