Mechanical and Tribological Requirements and Evaluation of Coating Composites

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1 26 Mechanical and Tribological Requirements and Evaluation of Coating Composites Sture Hogmark Uppsala University Staffan Jacobson Uppsala University Mats Larsson Balzers Sandvik Coating AB Urban Wiklund Uppsala University 26.1 Introduction Coating Composites Typical Mechanisms of Coating Composite Failure Tribological Coatings of Today 26.2 Design of Tribological Coatings Current Designs of Coating Structures Coatings of Tomorrow 26.3 Design of Coated Components General Design Considerations Design Considerations for Cutting Tools Design Considerations for Forming Tools and Machine Components 26.4 Evaluation of Coating Composites Important Parameters Adhesion to the Substrate Basic Coating Properties Intrinsic Mechanical Properties Tribological Properties Tribological Response of Coated Components Failure Analysis of Coating Composites 26.5 Visions and Conclusions 26.1 Introduction Coatings are increasingly being used to improve the tribological properties of mechanical components such as tools for metal cutting and forming, and machine elements such as sliding bearings, seals, and valves. This chapter presents tribological requirements and design considerations in the development of coating composites, and recommends methods and techniques for their evaluation. It is aimed both for those involved in development of coatings and for the practicing tribology engineer. Current concepts of coating composites used in cutting tools, forming tools, and machine elements are reviewed, and the respective requirements addressed within each category are summarized. Concepts of coatings designed for tools and machine elements are exemplified. Further, it is demonstrated how the tribological properties of coating composites are related to the fundamental coating properties (chemical, thermal, mechanical); the general coating characteristics such

2 FIGURE 26.1 Types of composite materials and the functional improvements addressed. as composition and microstructure, topography, and thickness; and different aspects of the substrate material. The important tribological phenomena discussed include: premature failures such as coating detachment, permanent surface deformation, cracking and spalling, occasional scratching, and material pickup gradual coating removal, including abrasive, erosive, and tribochemical wear. Techniques to evaluate some of the more important fundamental and tribological properties are presented, and several examples are given, mainly with thin physical vapor deposition (PVD) coatings. The evaluation techniques can be utilized both in coatings selection and development of future coatings. The chapter concludes with some recent trends and speculations about the future Coating Composites In the development of modern materials, the functionality is often improved by combining materials of different properties into composites. Many classes of composites exist, most of which address improved mechanical properties such as stiffness, strength, toughness, and resistance to fatigue. Coating composites (i.e., surface-engineered materials) are designed to specifically improve functions such as tribological, electrical, optical, electronic, chemical, and magnetic (Figure 26.1). It is thus natural to select the bulk of a component to meet the demands for stiffness, strength, formability, cost, etc., and then modify or add another material as a thin surface layer, which is the location of virtually all other functional properties. Application of coatings on tools and machine elements is therefore a very efficient way of improving their friction and wear resistance properties. The obvious aim of applying tribological coatings is to obtain increased lifetime. There are, however, several other positive effects, including: 1. The improved wear resistance of coated metal cutting tools is usually utilized to increase the feed rate and cutting speed and thereby the productivity, rather than to give prolonged tool life. 2. Reduced friction often means reduced energy consumption. In some cases, a lowered friction may permit the exclusion of lubrication or of cooling stages. 3. Increased or controlled friction can be a beneficial effect in applications such as brakes, bolted joints, and safety connectors.

3 4. Reduced tendency to sticking and material pickup from the countersurface is crucial for the performance of forming tools and in many sliding applications. Antisticking agents can be omitted in forming tool applications. 5. Components of reduced weight can be designed by application of coatings. Reduced weight means, for example, an increased ratio of power to weight of car engines, which in turn can give lower fuel consumption. There are also some limitations in thin coating application: 1. The coating process often involves significant heating of the substrate. This restricts the number of potential substrate materials. 2. There is always a risk of galvanic corrosion associated with applying a coating to a metallic substrate. 3. Good tribological performance often requires high levels of hardness and stiffness of the substrate material, because the thickness of PVD and CVD coatings is so limited that they normally need firm support. 4. The PVD process has a limited ability to coat complicated geometries because it, in principle, is a line-of-sight method. 5. Finally, it should be noted that premature coating failure due to poor adhesion to the substrate or cracking and spalling of the coating material can be disastrous because loose coating fragments in many applications can aggravate the wear. Examples of components that may be at high risk to these phenomena include journal bearings, ball bearings, gears, and piston/cylinder systems. The rapid development of new coating technologies thus has led to an accelerated increase in the use of coated tools and other components during the last couple of decades. This chapter is primarily restricted to thin (1 to 10 µm) physical vapor deposited (PVD) and chemical vapor deposited (CVD) coatings Typical Mechanisms of Coating Composite Failure It is important to realize that the mechanical contact between solids is localized to a number of microscale contact spots that together form the real area of contact. Irrespective of the nominal (or geometrical) contact area, a good estimate of the real contact area is obtained by dividing the normal load by the hardness of the softer of the two mating surfaces. In other words, the contact pressure in these tiny areas is of the order of their hardness. Bearing this in mind, it is not surprising that damage can occur, even in apparently very mildly loaded tribological contacts. One such example is the damage that occurs when sliding ceramic seals against each other despite the nominal contact pressure being 100,000 times lower than their hardness (Blomberg et al., 1994; Hogmark et al., 1992b). The wear resistance of a coated component is mainly determined by the coating as long as it covers the contact area. As soon as the coating is partly worn through, or the substrate is exposed due to adhesive failure or cracking and spalling, the wear resistance of the substrate material becomes important. An indepth description of the large number of wear mechanisms found in the applications of coated components is given in Holmberg and Matthews (1994). Characteristic of coating composites as to their modes of failure is the inhomogeneity of the surface layer due to the presence of the coating. There is often a large difference in mechanical, thermal, and chemical properties between the coating and substrate materials, and the interface between them often represents a steep discontinuity in these properties. There are principally two modes by which a coating composite may fail: 1. premature failure 2. failure due to gradual wear If the wear resistance of the coating is high and the coating wears down gradually, a considerable prolongation in life can be experienced as compared to that of an uncoated reference, even if the coating

4 FIGURE 26.2 Illustration showing how the life of a component can be prolonged by application of a thin coating. A hypothetical life-limiting wear depth is denoted by w c and the coating thickness by f c. A denotes an uncoated reference component; B denotes the same component supplied with a coating that experiences slow, gradual wear; and C refers to an undesired situation of premature coating failure. is thin (Figure 26.2). On the other hand, if the coating fails prematurely due to poor adhesion or cracking and spalling, the lifetime may even be shortened, because coating wear debris entrapped in the contact interface can aggravate the wear of the substrate material. It is clear from Figure 26.2 that increasing the coating thickness will prolong the lifetime as long as the wear rate is gradual and steady. However, as will be further elucidated in Section , the thickness of PVD and CVD coatings is restricted by their internal stresses, which build up in the coating during deposition. Therefore, the coating is often very thin compared with the tolerable wear depth. Consequently, in addition to displaying sufficient adhesion and resistance against cracking and spalling, the coating must be extremely wear resistant in order to considerably improve the tribological properties of the coating composite. Often, premature coating failure ends the life of well-functioning components after a period of gradual wear. In contrast to failure due to gradual coating wear, premature coating failure is very difficult to predict both experimentally and theoretically. Some of the more important mechanisms of coating failure are summarized in Figure Premature Failure Premature failure designates a situation where the full tribological potential of the coating is not gained (see Figures 26.2 and 26.4). Instead, the component fails due to one or several of the following reasons: 1. coating detachment 2. permanent deformation of the coating composite 3. cracking and spalling of the coating 4. pickup of material from the countersurface In general, tribological applications put higher demands on the coating adhesion than any other area of application, although the demands can differ substantially from one tribological situation to the other. It is instructive to distinguish between the actual adhesive forces (the strength of the physical adhesion or atomic bonding that acts between coating and substrate), and the practical adhesion by which is understood the ability of the coating composite to resist interfacial failure in its practical application. Naturally, the upper limit of the practical adhesion is correlated to the atomic bond strength, but this parameter cannot be directly measured (see Section ). Cracking and spalling of the coating may be the result of occasional or repeated excessive mechanical or thermal loading. A brittle coating on too soft a substrate can fracture and spall off, as exemplified in

$(b) (e): Premature failures due to coating detachment (b), cracking and spalling (c), coating and substrate deformation (d), and coating and substrate deformation including coating fracture (e).$ (f) Pickup of material from the countersurface. (g) Gradual coating wear. (h) Initial gradual wear followed by premature coating detachment; see C in Figure 26

(f) Pickup of material from the countersurface. (g) Gradual coating wear. (h) Initial gradual wear followed by premature coating detachment; see C in Figure 26

5 FIGURE 26.3 Common failure mechanisms of coating composites. (a) Initial state. (b) (e): Premature failures due to coating detachment (b), cracking and spalling (c), coating and substrate deformation (d), and coating and substrate deformation including coating fracture (e). (f) Pickup of material from the countersurface. (g) Gradual coating wear. (h) Initial gradual wear followed by premature coating detachment; see C in Figure (i) Coating detachment followed by accelerated wear of the substrate material. (a) (b) (c) FIGURE 26.4 (a) Excessive mechanical loads have caused cracking and spalling of a brittle electroless nickel coating applied to a gear wheel made of quenched and tempered steel. (b) Topography of a TiN-coated, high-speed steel milling tool for which the coating has fractured. (c) Cross-section of the tool in (b), revealing thermal softening of the substrate due to severe cutting conditions. Figure 26.4(a). Frictional heat may soften the substrate, making it unable to support a hard and brittle coating (see Figure 26.4b). Once the coating has fractured, coating fragments are peeled off, typically by the shearing action of the countersurface. When the coating is partially removed, the rate of detachment often increases due to excessive contact pressure at the edge of the scar. Critical parameters to resist coating cracking and spalling include coating thickness and toughness, and substrate hardness. Applying thin, hard coatings to rolling element bearings always puts the detachment, cracking, and spalling mechanisms at high risk.

6 Permanent deformation of the coating composite basically involves permanent changes in component geometry and/or in surface topography. Decisive parameters for a change in geometry are Young s modulus and the hardness of coating and substrate, and coating toughness (see Section ). Tiny scratches or cracks may disqualify a forming tool used to, for example, press compact disks. Coating hardness is the crucial parameter for scratch resistance, and coating toughness or fracture resistance for the resistance to surface cracking. Work material locally adhered to, for example, the surface of a sheet forming tool used in the automotive industry will inevitably produce indentations or scratches in the surface of the product. Material transfer between the contact surfaces of sliding machine elements is a similar problem, often named galling, scuffing, or seizure. Pickup of material from the countersurface is typical of many situations of sliding contact, and by no means unique to coating composites. It is generally reduced or avoided by giving the surface a smooth topography and making sure that the chemical affinity to the countersurface is low. This is often accomplished by applying a proper coating. Tribochemical layers may form when, for example, machining certain work materials at high cutting speeds. They are the result of mechanical smearing of or chemical reactions with constituents in the work material, and may have the positive effect of protecting the coating from excessive damage (Nordgren and Melander, 1994; Palmai, 1984) Failure Due to Gradual Wear This category of failure includes regular wear; that is, gradual material loss mainly determined by the intrinsic coating properties. When thin PVD or CVD coatings are involved, gradual wear often means mild wear due to abrasion, erosion, chemical dissolution, etc., and does not, in principle, deviate from the mechanisms causing wear of homogeneous materials. Suitable evaluation techniques are given in Section Because the wear rate of thin coatings in most applications is extremely low, oxidation and other types of chemical degradations often play a significant role. In typical applications of bulk materials, the wear mechanisms are normally characterized as mechanical. The resistance against abrasive wear of coatings increases with coating hardness, which preferably should be higher than that of the counter material (at the relevant contact temperature) (Khruschov, 1974). High Young s modulus and hardness of both coating and substrate, combined with a sufficient coating thickness, are also important in avoiding cracking caused by deformation of the substrate (see Section 26.3). Resistance to particle erosion (i.e., wear due to mechanical attack by liquid or gas-borne particles) requires a coating that combines high hardness, fracture toughness, and corrosion resistance. The toughness is usually the most decisive parameter if the erosion results in a mechanically dominated material removal mechanism. Adhesive wear of today s coating materials rarely occurs unless the strength of the coating material is weakened by thermal softening and/or chemical attack from the counter material or the environment. A high contact temperature facilitates chemical degradation and dissolution. In some instances, the selection of coating material has to be performed in consideration of the ability of the counter material (e.g., work material in a forming operation) or environment (e.g., lubricant of a machine component) to react (or not react) chemically with the coating. Sometimes, protective layers are formed by such reactions; but more often, they promote the gradual coating wear Tribological Coatings of Today General Overview Application of surface coatings for tribological uses may require deposition temperatures ranging from room temperature up to more than 1000 C (Figure 26.5). The coating thickness ranges from microns to several millimeters. Typically, the atomistic methods produce the thinnest coatings. For some methods,

7 Atomistic deposition Particular deposition Full-thickness deposition Process temperature[ C] PVD CVD Electrochemical plating Chemical (electroless) plating Thermal spraying Overlay welding 0, Layer thickness [µm] FIGURE 26.5 Typical values of coating thickness and process temperature (temperature at the substrate surface) of today s tribological coating methods. Coating deposition WC /C MoS2 DLC CrN PV D (Ti,Al)N Ti(C,N) TiN CV D Diam ond SiC Al TiC TiN Ap plication Com ponents Form ing tools Metal Cutting tools Substrate ma terial Al alloys 1% C steel Cu alloys QT - steel Mg alloys Carbon steel Cold work steel "D1" HSS "H13" Hot work steel CC Al SiC Si 3 N 4 RT Tem perature C FIGURE 26.6 Limited heat resistance often excludes materials from different applications and coating processes. This figure illustrates typical temperature limits of potential substrate materials compared with typical working temperatures of applications and deposition temperatures of PVD and CVD coating processes. high deposition temperatures can cause undesired phase transformations, softening, or shape changes of the coated component. An important benefit of PVD and CVD processes is the high flexibility as to composition and structure of the coating. These coatings are today successfully utilized to coat a large variety of mechanical components (Figure 26.6). Most of today s PVD and CVD coating materials consist of nitrides (TiN, CrN, etc.), carbides (TiC, CrC, W 2 C, WC/C, etc.), oxides (e.g., alumina), or combinations of these. Important exceptions are molybdenum disulfide (MoS 2 ), diamond-like carbon (DLC), and diamond. MoS 2, WC/C, and DLC can be classified as low-friction coatings because they often display friction coefficients ranging from 0.05 to 0.25 in dry sliding (Holmberg and Matthews, 1994; Hirvonen et al., 1996). Their wear resistance is

8 FIGURE 26.7 (a) Comparative friction values recorded during dry sliding in a ball-on-disk geometry (Hollman et al., 1997a). A water-lubricated diamond/al value is added for comparison. (b) Intrinsic abrasive wear resistance of diamond and TiC coatings obtained by micro abrasion against diamond abrasives (see Section ). The wear resistance of cemented carbide (CC) and high-speed steel (HSS) is also shown. (From Hollman, P. (1997a), Microand Nanocrystalline Diamond Coatings with Extreme Wear Resistance and Ultra Low Friction, Acta Universitatis Upsaliensis Dissertation in Science and Technology 325, ISBN With permission.) generally inferior to that of nitrides, carbides, and oxides. On the other hand, nitrides, carbides, and oxides normally give friction values between 0.4 and 0.9 in dry sliding. In this context, they are referred to as wear-resistant coatings. A very important exception to this simple classification is the CVD diamond coating, which in many applications combines an ultralow friction with very high wear resistance (Figure 26.7). Friction values below 0.05 have been recorded for diamond in nonconformal dry sliding. A further reduction in friction down to 0.02 can be obtained by water lubrication. This makes diamond a very strong coating candidate, in particular when environmental considerations have to be met (Hogmark et al., 1996). Figure 26.6 indicates that the deposition temperature and heat resistance of the substrate materials strongly limit the number of possible coating/substrate combinations. All types of PVD and CVD coatings can be applied to most ceramic materials and cemented carbides (CC). The most heat-resistant steels, such as high-speed steel (HSS) and some of the forming tools, can be coated by all types of PVD and some low-temperature CVD processes; whereas progress in PVD or CVD coating of low-alloy steel, copper-based alloys, and light metals like aluminum and magnesium remains very limited. In addition to the restricted number of coating candidates, this last category of substrate materials has a relatively low hardness in comparison with ceramics, CCs, and tool steels. This fact must be considered when designing coating composites, as further discussed in Section Materials, Properties, and Applications of Some Common Coatings The number of commercially successful tribological CVD and PVD coatings is still rather limited. They are briefly summarized in Table 26.1, together with some of their most important tribological properties, substrate materials, and applications. The four categories of tribological components metal cutting tools, hot forming tools, cold forming tools, and machine elements all put specific demands on the coating composite, as generalized in Table From Tables 26.1 and 26.2 general conclusions can be drawn on how to design coated components for specified applications (see Section 26.3) Design of Tribological Coatings A tribological coating composite is primarily designed to offer two types of functions: 1. a specified friction behavior (including low, high, or just stable friction level) 2. a high wear resistance

9 TABLE 26.1 Commercially Successful Combinations of PVD and CVD Coatings and Substrate Materials, Common Tribological Applications and Typical Ranges of Deposition Temperature, Hardness Levels, and Friction Coefficients Against Steel Coating (Deposition Technique) Substrate Material Deposition Temperature ( C) Hardness (GPa) Dry Friction vs. Steel Applications TiC (CVD) CC, HSS a Tools for metal cutting and forming Al 2 O 3 (CVD) CC Metal cutting tools TiN (CVD) CC Metal cutting tools TiN (PVD) CC, HSS Metal cutting tools CrN (PVD) HSS, forming tool steels Tools for metal cutting and forming CrC (PVD) Tool steels Forming tools, machine elements Ti(CN) HSS, CC Metal cutting tools (PVD) TiAlN b HSS, CC Metal cutting tools (PVD) DLC (CVD) CC, tool steels Machine elements DLC (PVD) Tool steel and lowalloyed steels RT Machine elements, magnetic hard disks W 2 C (CVD) CC, tool steels a Forming tools WC/C HSS, tool steels Machine elements (PVD) MoS 2 (PVD) HSS, tool steels Metal cutting tools, machine elements Diamond (CVD) CC, SiC Cutting tools for aluminum alloys Duplex c Forming tool steels Given by the coating To fulfill these functional demands, a sufficient adhesion between coating and substrate, plus a sufficient load-carrying capacity, are both necessary prerequisites. The load-carrying capacity is the ability of the composite to resist tribological loads without premature failure. Usually, premature failure is characterized by cracking or delamination of the coating or by subsurface plastic deformation Current Designs of Coating Structures Given by the coating Forming tools, machine elements a Heat treatment is often performed subsequent to coating deposition by quenching from the process temperature. b Processes combining the chemistry and physics of PVD and CVD are normally used. c Duplex treatment combines ion nitriding with PVD coating in one process. Successful top coatings applied in duplex treatment include TiN and CrN. Today, three types of coating structures are frequently found: single-layer coatings, sandwich coatings, and graded coatings. One of the first and today one of the most common thin coatings to be used in tribological applications is hard chromium (Cr). This coating is electrolytically deposited, for example, to improve the wear resistance of piston rings, hydraulic pistons, bearing shafts, chain saw teeth, etc. Most commercial PVD and CVD coatings consist of one single layer, often containing one structure phase. Among the most common are TiC (commercially introduced by Sandvik Coromant [1969] on cemented carbide metal-cutting tools); TiN (introduced in 1978 on high-speed steel metal-cutting tools [Figure 26.8]), CrN; alumina (Al 2 O 3 ), and diamond-like carbon (DLC). All these coatings are usually applied directly to the surface of a homogeneous substrate material. Consequently, to ensure a high load-carrying capacity of the coating composite, the substrate must possess high hardness and Young s modulus.

10 TABLE 26.2 Four Important Coating Application Areas, and the Prevailing Contact Conditions and Desired Properties of the Substrate and Coating Material Component Conditions of Application Desired Properties of Substrate Material Desired Properties of Coating Material Metal cutting tools Hot forming tools Cold forming tools Machine elements High static and dynamic (intermittent cutting) mechanical loading, high surface temperatures ( C), high shear stresses, chemically reactive work material often including abrading particles, and often presence of cutting fluid. High contact temperatures ( C), chemically reactive work material, often including abrasive particles, thermal cycling due to water cooling, presence of antisticking agent High contact pressure, presence of abrasive particles, high shear stresses, often presence of lubricant Friction and wear properties of both mating surfaces often of equal interest; the contact temperatures and often also pressures are moderate; often presence of lubricant and sometimes also abrasive particles High levels of hot hardness, fracture toughness, wear, and fatigue resistance; the substrate material is supposed to work reasonably well also if the coating is locally removed High Young s modulus, yield stress, and hot hardness to resist macroscopic deformation, high resistance to thermal fatigue, and chemical reactions High Young s modulus and yield stress to resist macroscopic deformation, high fracture toughness, and high hardness to support the coating Sufficiently high Young s modulus, yield stress, and, in rolling contact and lubricated sliding, a good fatigue strength to resist the cyclic loads; high hardness to support the coating (hardened low- and mediumgrade steels dominate) High chemical and thermal resistance, high hardness and toughness at the high contact temperature; very good adhesion to the substrate and low solubility in the work material High chemical and thermal resistance, including resistance to thermal fatigue (heat checking) and low tendency to stick to the work material (low chemical potential between coating and work material); low friction, high hot hardness and toughness to resist abrasive wear Low chemical potential between coating and work material give low friction and avoids work material pickup; high hardness and toughness to resist abrasive wear Low friction properties, combined with good resistance to surface damage; a certain amount of running in wear is sometimes desirable to conform the sliding surfaces and reduce the asperity contact stress peaks; an ability to accommodate permanent substrate deformation, combined with a relatively high hardness to resist any abrasives are other desired properties (a) (b) FIGURE 26.8 (a) Typical morphology of a PVD-TiN coating, and (b) transmission electron microscopy image revealing the initial Ti layer normally used to enhance the adhesion of the TiN coating. Thin coatings of alumina have been manufactured for more than 20 years using CVD; and today, CVD alumina on cemented carbide inserts represents one of the largest industrial applications. Like CVD TiN, alumina is normally deposited on an intermediate layer of TiC.

11 FIGURE 26.9 Sandwich coating composed of TiC, Al 2 O 3, and Ti(CN) on a CC metal cutting insert intended for machining austenitic stainless steel. The Ti(CN) coating consists of several layers with different carbon:nitrogen ratios. (Courtesy of Seco Tools.) Several successive layers of different composition are often applied to form a sandwich coating for demanding applications. Metal cutting inserts for machining austenitic stainless steel are typically CVD coated with three layers, namely, TiC-Al 2 O 3 -TiN starting from the substrate (Figure 26.9). TiC accounts for good bonding to the CC material; Al 2 O 3 provides good wear resistance at elevated temperatures; and, in addition to its attractive color, TiN gives a clear visual indication of which edges of the insert have been used. In addition, TiN often reduces the problem with sticking of the work material. Another example of a sandwich layered coating is found in the reader head in computers (see Hedenqvist et al., 1992). A graded coating composition or structure improves the load-carrying capacity by offering smoother transitions in mechanical properties from those of the hard and stiff coating to those of the softer and more flexible substrate. In this way, the contact load can be distributed over larger areas, which reduces the maximum contact stresses and the stress at the coating-substrate interface. Successful graded surface layers on steel have been produced by nitriding or carburizing for many decades, and the examples of forming tools and machine components taking advantage of these treatments are abundant (Bell, 1975; Dingremont et al., 1995a,b). Today, graded compositions are also utilized in commercial PVD coatings such as Ti(CN) (Bergmann et al., 1990) Coatings of Tomorrow Materials and Strengthening Structures There seems to be an almost universal relationship between hardness and toughness of all materials, as illustrated in Figure Improving the hardness of a material will almost always be at the expense of reducing the toughness (Courtney, 1990; Allen and Thomas, 1998). There are two important exceptions, however. By reducing the grain size or by introducing a fiber structure, both hardness and toughness can be raised. The fracture toughness of state-of-the-art bulk ceramics is typically within the range of 3 to 8 MPam 1/2. PVD or CVD coatings of the same materials usually display much lower toughness, in the range of 0.1 to 1.0 MPam 1/2. Consequently, much of today s development in tribological coatings focuses on improving their toughness. Several phases and layers can be combined into sandwich coatings, graded coatings, duplex coatings, multilayer, superlattice, nanocrystalline, and multicomponent coatings, etc. (Figure 26.11) (Holmberg

Hardness Deformation Solid solution Hard phase Grain refinement (Hall-Petch) Composite structure Pure state Toughness FIGURE 26.10 The general hardness vs.

12 Hardness Deformation Solid solution Hard phase Grain refinement (Hall-Petch) Composite structure Pure state Toughness FIGURE The general hardness vs. toughness relationship of materials, and the effect on this relationship from the most commonly used metallographic strengthening mechanisms. FIGURE Possible structures of tribological coatings. and Matthews, 1994; Sproul, 1994). Obviously, the strengthening mechanisms known to metallurgists for many decades are now being introduced to tribological coatings, which also offer numerous new combination possibilities New Coating Materials Diamond coatings have recently been introduced on inserts for aluminum cutting (Figure 26.12) (Karner et al., 1996). Diamond offers a unique combination of high hardness, high wear resistance, low friction properties, high thermal conductivity, and environmental friendliness. The latter applies both to processing and application. Diamond will probably become one of the most versatile coating materials once it can be deposited at a more moderate temperature and ways of improving its toughness have been established. Up to now, the PVD techniques have not allowed deposition of tribological quality alumina coatings. However, recent advances in process technology have made reactive deposition of high-quality alumina coatings possible (Sproul et al., 1995). Cubic boron nitride (CBN) cutting edges are currently produced by conventional hot isostatic pressing and brazed onto the tip of cemented carbide cutting tools. CBN is second to diamond, the hardest material, 5200 HV, and it is very effective in cutting hardened steels and other difficult-to-machine alloys. Applying CBN directly to the tool in the form of a coating would of course be very attractive. The current restriction is that CBN coatings produced by PVD normally exhibit excessively high compressive stresses (Sproul, 1996a). Carbon nitride (C 3 N 4 ) would theoretically be harder than diamond if it could be given the same structure as Si 3 N 4. Although there have been reports of producing crystalline carbon nitride coatings, to date no one has presented fully crystalline C 3 N 4 coatings. The carbon nitrides produced thus far, often denoted C x N y, have shown extreme elastic properties combined with relatively high hardness values (15 to

substrate. Note that the partially removed coating is significantly smoother than the substrate revealed in the lower part of the micrograph. (From Hogmark, S., Hollman, P., Alahelisten, A.

13 (a) (b) FIGURE Characteristic topographies of diamond coatings: (a) top view of a 10-µm CVD diamond coating (R a = 430 nm); (b) smooth (R a = 25 nm) diamond coating as deposited with 400 V negative bias on the substrate. Note that the partially removed coating is significantly smoother than the substrate revealed in the lower part of the micrograph. (From Hogmark, S., Hollman, P., Alahelisten, A., and Hedenqvist, P. (1996), Direct current bias applied to hot flame diamond deposition produces smooth low friction coatings, Wear, 200, With permission.) (a) (b) FIGURE (a) Process layout, and (b) microstructure of PVD-TiC. (From Wiklund, U. and Larsson, M. (1999a), Low friction PVD titanium-carbon coatings, Wear, 241, With permission.) 60 GPa) (Sjöström et al., 1996). A limitation of C x N y is its low thermal stability. It loses nitrogen and softens at 300 C, (Hellgren 1999; Sproul, 1996b). Recently, TiC coatings have been produced by argon plasma-activated PVD, comprising simultaneous electron beam evaporation of Ti and sputtering of carbon. The substrates are rotated at a sufficient speed above the two sources to allow for the deposited constituents to react chemically (Figure 26.13) (Wiklund and Larsson, 1999a) Duplex Coatings Because many of the wear-resistant PVD and CVD coatings are relatively brittle, they can be successfully applied only to hard and stiff substrate materials such as hardened steel, cemented carbides, or structural ceramics. On softer substrates, an intermediate layer acting as a mechanical support for the coating is usually required (see Section ). For steel and titanium alloys, this support is preferably achieved by nitriding (Dingremont et al., 1995a,b; Johansson, 1993). If nitriding and PVD coating are performed in sequence within one coating system, the process is called duplex treatment Multilayered Coatings Multilayered coatings are distinguished from sandwich layers by their periodically repeated structure of lamellae of two or more materials. The lamella thickness can vary from a few nanometers to a few tenths of a micrometer. Coatings made of multilayered structures have proven harder and significantly tougher than homogeneous coatings of the same materials. One possible mechanism for this improvement would

FIGURE 26.14 The superior toughness of a multilayered Ti/TiN coating (a) compared with a homogeneous TiN coating (b), as demonstrated by 500 gf Vickers indents.

14 FIGURE The superior toughness of a multilayered Ti/TiN coating (a) compared with a homogeneous TiN coating (b), as demonstrated by 500 gf Vickers indents. The corresponding coating structures are shown in (c) and (d). be that the lamellae structure obstructs dislocation glide and also crack propagation (Figure and Section ) (Gunnars, 1997; Sugimura et al., 1995) Superlattice Coatings Multilayered coatings of materials with similar crystal structures tend to form columnar crystals that extend through the entire coating, provided that the thickness of the individual lamellae is sufficiently thin, typically 5 to 25 nm. Such coatings are referred to as superlattice coatings. Some of the first examples of superlattice coatings were obtained by combining TiN/VN and TiN/NbN (Todorova et al., 1987; Chu et al., 1993; Shinn et al., 1992; Larsson et al., 1996). Several authors have shown that this type of multilayered coating structure can improve the hardness as well as the toughness, compared to single layers of the same materials (see Sections and ) (Chu et al., 1993; Nordin et al., 1998). Superlattice strengthening is well-known from classical metallurgy, where it is sometimes referred to as substructure strengthening or order hardening (Courtney, 1990; Allen and Thomas, 1998). By selecting a suitable combination of materials for the multilayered structure, it is possible to improve resistance against wear, corrosion, oxidation, high friction, etc. For example, superlattice coatings with thin lamellae of TiN and TaN (Figure 26.15), have shown very good results in cutting of stainless steel. This is believed to be a result of a very good toughness, combined with the low affinity of TaN to the work material (Selinder et al., 1998) Nanocrystalline Coatings The yield strength, hardness, and toughness of polycrystalline materials all generally improve with decreasing grain size, in accordance with the well-known Hall-Petch relation. A similar phenomenon appears to be valid for thin coatings down to nanometer-sized grains. In addition to improved mechanical properties, nanocrystalline materials can exhibit higher thermal expansion, lower thermal conductivity, unique optical, magnetic and electronic properties, etc. (Suryanarayana, 1995; Sproul, 1996b). Currently, nanocrystalline structures (of bulk materials as well as thin coatings) are being explored for tribological applications (Voevodin et al., 1997; Veprek and Reprich, 1995) Multicomponent Coatings Multicomponent coatings are composed of two or more constituents in the form of grains, particles, or fibers. Although many of today s single-layer coatings have a multicomponent structure, this is still a rather unexplored means for coating strengthening.

(a) (b) FIGURE 26.15 A cross-section of a TiN/TaN polycrystalline superlattice coating on HSS: (a) fine-grained columnar structure revealed by SEM; (b) the superlattice structure revealed by TEM.

15 (a) (b) FIGURE A cross-section of a TiN/TaN polycrystalline superlattice coating on HSS: (a) fine-grained columnar structure revealed by SEM; (b) the superlattice structure revealed by TEM. (From Nordin, M., Larsson, M., and Hogmark, S. (1998), Mechanical and tribological properties of multilayered PVD TiN/CrN, TiN/NbN, TiN/MoN and TiN/TaN, Surf. Coat. Technol., 106, With permission.) 26.3 Design of Coated Components General Design Considerations As mentioned, the load-carrying capacity is the ability of the composite to resist tribological loads without premature failure due to subsurface plastic deformation, cracking, or delamination of the coating. The primary consideration in the design of coated components should be to avoid such premature failure by achieving a sufficient load-carrying capacity. Second, when this is achieved, attention is turned to increasing the intrinsic wear resistance of the coating in the application. The tools in the toolbox for design of coated components include coating thickness, coating material, coating microstructure, state of residual stress, and roughness. All these parameters are determined by the choice of deposition process and process parameters, but they are also often influenced by the material and topography of the substrate (see Section ). The choice of design settles the intrinsic coating properties, including resistance to different wear mechanisms, hardness, modulus, toughness (improved by residual compressive stresses), high-temperature properties, and friction performance Stresses in Tribology of Coatings At least two types of contact stress patterns must be considered in the design of coatings: 1. Hertzian contact stresses related to the macroscopic contact geometry 2. stresses associated with the microscopic asperity contacts In static contact, rolling contact, or sliding contact with very low friction, the maximum shear stresses occur at a depth roughly equal to half the Hertzian contact radius. For a ball bearing with 10-mm balls, this typically corresponds to 0.1 to 0.3 mm. The group of components that may fail by sub-surface fatigue due to excessively high cyclic Hertzian stresses at a depth below the surface includes ball bearings, gears, and other machine elements with nonconforming contact surfaces. Here, the maximum shear stress typically appears well below the thickness of a PVD or CVD coating. Consequently, applying a thin coating is seldom effective in reducing these problems. In sliding contact between conformal surfaces, such as those of journal bearings, face seals, piston/cylinders, and cutting and forming tools, the load is distributed over a large nominal contact area. The detrimental contact stresses are localized to the areas of asperity contact. In these microscopical contact zones, the highest stresses usually appear within a few microns of the contact interface (Figure 26.16). Here, the prognosis of improving the friction and wear properties by applying a thin coating is significantly better.

FIGURE 26.16 Normal stress distribution at different levels below the contact surface.

16 FIGURE Normal stress distribution at different levels below the contact surface. The discrete, localized elastic and plastic normal load distributions associated with asperity contacts are gradually smoothed below the surface. The pressure peaks at the coating interface can lead to local plastic deformation of the substrate, despite the average stress level being rather low. In all sliding contacts, the location of the maximum shear stress approaches the surface when the friction increases. For a friction coefficient of about 0.3 or higher, it is confined to the contact surface (Mao et al., 1994) and, consequently, thin coatings could be efficient in reducing wear and improving the load-carrying capacity of the composite Topography of Coated Components Several aspects have to be considered when designing for the optimum topography of thin, hard coatings. To minimize the maximum contact stresses on the asperity level, the coating surface should be as smooth as possible (see Figure 26.16). Because most thin coatings inherit the substrate topography, the final step in substrate surface preparation should be a careful polishing or a very mild blasting; this is also recommended to increase the practical adhesion of coatings with high residual stresses (see Section ). However, all coating processes introduce some surface irregularities, and it may also be necessary to polish the coating or use a superficial layer with good running-in properties (see Section ). In most tooling applications, the surface finish of the tool is replicated on the product, and the requirement on the coating topography is related to that of the product. In many metal cutting operations, however, constituents in the work material can form protective tribochemical layers on the tool (Palmai, 1984; Nordgren and Melander, 1988). This mechanism can be facilitated by a reasonably rough topography. A certain roughness can also be beneficial for oil retention of sliding bearings, etc Primary Design Goal: Improving the Load-Carrying Capacity The primary goal of improving the load-carrying capacity can be reached by a number of design choices. At least three coating approaches lead to a reduced risk for sub-surface plastic deformation (Figure 26.17). 1. A hard coating can relieve the substrate by hosting the maximum shear stress, provided it is thicker than the depth to the location of maximum stress. 2. A thin, high modulus coating can spread the load over a larger area on the substrate by a drum skin effect. 3. A soft coating yields and thus gives a flatter, less concentrated load distribution over the substrate.

(a) (b) (c) (d) FIGURE 26.17 Mechanisms of improving the load-carrying capacity of a coating composite by load spreading.

17 (a) (b) (c) (d) FIGURE Mechanisms of improving the load-carrying capacity of a coating composite by load spreading. The concentrated elastic load distribution on an uncoated substrate (a) can be spread out by using a harder coating thick enough to accommodate the location of maximum shear stress (b), by using a thin, high-modulus coating that widens the load bearing area of the substrate by a drum skin effect (c), and by using a softer coating that yields and thus flattens out the stress distribution on the substrate (d). Of course, the substrate properties are also decisive in avoiding plastic deformation: Substrates with a high hardness resist plastic deformation up to higher levels of stress. Substrates with a high hot hardness retain their deformation resistance up to high temperatures. Substrates with a high thermal conductivity reduce the risk of thermal softening induced by friction heating. Substrates with a low modulus get large nominal contact areas and thus reduce the level of subsurface stress (the modulus of the substrate is normally not a design criterion because it cannot be varied by modifying the microstructure, but is practically a constant for the chosen material). The risk for premature failure by cracking and delimitation of the coating can be reduced by proper design choices, including: Tough coatings can accommodate more deformation without fracturing. The toughness (measured as critical component strain to coating fracture) is improved by high compressive residual stresses in the coating (see Section ). Smooth coatings avoid detrimental contact stress concentrations, thus reducing the risk for both cracking and plastic deformation. For coating processes yielding high residual stresses, the risk for delamination is lower for thin coatings because the load exerted on the interface from the stress in the coating is proportional to the coating thickness (see Section ) Secondary Design Goal: Improving the Wear Resistance of the Coating The intrinsic wear resistance of a coating generally does not differ from that of the same materials in bulk form. Thus, the same design criteria as for bulk materials apply. Some of the exceptions were mentioned in Section

26.3.2 Design Considerations for Cutting Tools Cutting tools will probably continue to be a leading application of modern tribological coatings.

18 Design Considerations for Cutting Tools Cutting tools will probably continue to be a leading application of modern tribological coatings. They are relatively small so that batch coating can keep the cost within reasonable limits, even for high-tech coatings of, for example, nanocrystalline and nanolayered multicomponent layers. Today, there are two obvious trends in cutting tool developments. Dry machining is desirable to avoid the extra costs and environmental problems associated with cutting fluids. High-speed machining of hardened steel has the potential of giving sufficiently high quality of the machined surface to make finishing operations such as grinding or polishing unnecessary. In both cases, the heat generation along the tool surfaces will be even more intense than with today s contact conditions and, consequently, the tools must possess further improved hot hardness, thermal and chemical stability, etc Design Considerations for Forming Tools and Machine Components Forming tools and machine components constitute much larger industrial sectors than do cutting tools, and application of thin tribological coatings on forming tools and machine components thus has enormous potential. There are several reasons why the use of coatings in these applications remains relatively scarce. Many forming tools and machine elements are too large to be economically coated by today s processes. Further, the substrate materials of most forming tools and machine elements cannot resist the currently used deposition temperatures. In addition, these components often have complicated and narrow sections that are difficult or even impossible to coat. Finally, the high tool cost often makes the user restrictive against applying new, unexplored coatings. The automotive industry encourages research on new concepts of surface engineering, with the general aim to substitute traditional steel components with components made of lighter materials, typically aluminum, titanium, and magnesium alloys. The ultimate aim is to reduce fuel consumption. The application of thin, wear-resistant, and/or low-friction coatings on inherently soft materials requires a supporting, intermediate layer to achieve a sufficient load-carrying capacity. Electroless nickel, hard chrome, laser cladded or thermally sprayed hard coatings, and nitrided and carburized layers, are all candidates for providing this property. The primary design parameters of the supporting layer are thickness and the Young s modulus, and the aim is to avoid plastic deformation and to minimize elastic deformation of the substrate. An additional, softer low-friction coating may have to be applied on top of the wear-resistant coating (Figure 26.18). This layer serves to spread the load, thereby reducing the stress peaks indicated in Figure 26.16, and simultaneously ensuring a mild running-in wear. Future coatings for forming tools and machine components are thus expected to be multilevel composites. The base material of, for example, a hardened steel is typically a particle composite; the base material plus coating constitute the coating composite; and finally, the coating is of sandwich type, and one or more of its layers may possess various substructures. FIGURE materials. Suggested structure and materials of a multilevel composite coating designed for soft substrate

19 FIGURE Illustration of how the coating deposition parameters influence the tribological response of coating composites through the coating characteristics, which in turn determine the basic tribological properties. The influence from the substrate is also indicated. Currently, plasma-assisted PVD processes are being evaluated for production of duplex coatings (see Section ). The functional PVD coating is typically CrN or TiN or a wear-resistant, low-friction coating (e.g., DLC or WC/C) (Farges et al., 1989; Bell, 1997) Evaluation of Coating Composites Important Parameters A general theory of the relations illustrated in Figure 26.19, starting from the coating deposition parameters and predicting the tribological response of coated components, is still far from being realized (Hogmark et al., 1997b). Such a theory would involve the relationships between the deposition parameters (substrate temperature, plasma characteristics, etching time, gas pressure, etc.); the substrate characteristics (e.g., composition, microstructure, topography); and the coating characteristics (thickness, chemical composition, microstructure, topography, etc.). The influence from the substrate is primarily related to the nucleation and growth of the coating, and to the coating topography. Consequently, the substrate material and surface preparation prior to deposition are crucial to the coating adhesion, and, in turn, to the performance of the coating composite. Further, the coating characteristics consequently govern the basic coating properties (chemical, thermal, mechanical, etc.) (Hogmark et al., 1997c). By tribological properties, one can understand resistance against deformation, abrasion, erosion, adhesive contact, repeated impact, and material pickup of the coating composite. These properties are determined by the basic properties of the coating and, assuming that the coating is thin, also directly by the basic properties of the substrate (see Figures 26.1 and 26.3). Finally, the tribological response of a coated component in operation is estimated from the tribological properties necessary to sustain the actual tribological situation, that is, conditions such as geometry, contact pressure, sliding velocity, temperature, lubrication, etc. Those involved in coatings development and production usually assess some of the relevant coating characteristics and basic properties, whereas end users pay direct attention to the tribological response of the coated component in the actual application. Knowledge of the entire chain of properties is important for the general understanding of the behavior of coating composites, even if the correlation

20 illustrated by the arrows in Figure can only be given qualitatively. Because different applications put different demands on the coating composites, the set of most decisive parameters will vary Adhesion to the Substrate Obviously, a good adhesion of the coating to the substrate is a crucial property of most applications of coated components. However, any adhesion test will, inevitably, superimpose a stress field over the coating-substrate interface. This stress field will depend on the type of tribological loading (indentation, scratching, sliding, abrasion, impact, etc.), as well as on the elastic and plastic parameters of the coating and substrate. Important parameters include the nature, magnitude, and homogeneity of coating residual stresses, the shape, flatness, and roughness of the interface, etc. Thus, any test value of adhesion will only be representative of the particular test from which it has been obtained. Because the situation in the test most likely deviates significantly from that of the intended application, the result must be handled with caution. In fact, the relationships between the above parameters are so complicated that a general theory to predict practical adhesion does not exist. One has to be very careful when trying to correlate results from indentation or scratching performed with, for example, spherical diamond tips with the situation of sliding contact in actual components like tools and machine elements. On the other hand, scratch tests can be very useful for obtaining qualitative measures of the mechanical strength (adhesion, cohesion, toughness, etc.) of coating composites (see Section ). To evaluate the practical adhesion of coated components, the advice is to use, if not field tests, tribological tests with the closest possible resemblance to the actual situation Basic Coating Properties Basic coating properties such as thickness, composition, structure, morphology, and topography are best studied by modern imaging and analytical techniques (see Table 26.3) (Hogmark et al., 1997b). Coating thickness and morphology of thin PVD or CVD coatings are often easy to reveal from fractured cross-sections (see Figures 26.8, 26.12, and 26.14). Compositional depth profiles of coatings 1 µm in thickness or less can be obtained by XPS or AES. These techniques combine alternating ion etching and analysis. Due to a larger information depth, EDS gives the average coating composition, and often also signals from the substrate material. Thicker coatings can be profiled without prior sample preparation by GDOES (Figure 26.20). Alternatively, EDS or AES can give the corresponding information by analyzing polished cross-sections or tapered sections Intrinsic Mechanical Properties Some of the intrinsic mechanical properties are of particular interest. These are the Young s modulus and residual stress, that is, properties related to elastic deformation, hardness (i.e., property related to plastic deformation), and toughness or fracture resistance (i.e., properties related to fracture) Young s Modulus The intrinsic Young s modulus (elastic modulus) of the coating (E c ) is a useful parameter in the measurements and calculations of the stress state and the cracking and delamination behavior of coating composites. It is possible to obtain E c through a number of techniques, but the uniaxial tensile test is the most straightforward (Hollman et al., 1997b). The Young s modulus of a coating can be obtained from ( ) E = k E t w k t w c s s g c (26.1) where k is the slope of the tensile curve, k g is the slope of force vs. strain for a strain gage glued to the coated sample (determined separately), E s is the Young s modulus of the substrate material (known), w is the width of the coated sample, and t c and t s are the thicknesses of coating and substrate, respectively.

21 TABLE 26.3 Common Techniques Used to Assess Basic Coating Properties Technique Type of Information Lateral Resolution (µm) Information Depth a (µm) Scanning electron microscopy (SEM) Transmission electron microscopy (TEM) Energy dispersive X-ray spectroscopy (EDS) X-ray diffraction (XRD) X-ray photo-electron spectroscopy (XPS) Auger electron spectroscopy (AES) Rutherford backscattering (RBS) Glow discharge optical emission spectroscopy (GDOES) Secondary ion mass spectrometry (SIMS) Surface topography and defects, coating thickness, porosity and structure morphology, fractography, etc. Grain size, morphology and defects by imaging, phase composition and orientation by electron diffraction, elemental composition by electron energy loss spectroscopy (EELS) Elemental composition with good relative quantitative accuracy (0.5 to 5%); sensitivity 0.1% Phase structure, texture and composition, chemical modulation period of multilayers, strain in surface layers Elemental and chemical composition through the nature of the chemical bonds; depth profiling down to 1 µm; relative quantitative accuracy (10 to 30%) Elemental composition, especially the light elements C, N, O; some chemical information; depth profiling down to 1 µm; sensitivity 0.2%, relative quantitative accuracy (10 to 20%) Composition, including all elements (except H); depth profiling down to 50 µm Composition, including all elements (even H); depth profiling down to 50 µm depth; sensitivity down to 0.01% Composition, including all elements (even H); depth profiling down to 10 µm; sensitivity 0.1 ppm!; relative quantitative accuracy (10 to 50%) b Raman spectroscopy Often used to analyze the proportion of sp 2 (graphite) and sp (diamond) bonds of carbon coatings Atomic force microscopy Quantified surface topography and friction recordings down to c (AFM) atomic scale Profilometry c Quantified surface topography d a The information always represents an average composition of a surface layer, the thickness of which depends on the depth of penetration of the activating radiation and the absorption of the measured signal. b Used in SEM, the resolution is about 1 µm. In TEM, a lateral resolution down to about µm can be obtained due to a limited specimen thickness (1 to 20 nm). c Includes stylus and optical profilometry. d Refers to the maximum surface profile height/depth. FIGURE GDOES elemental depth profile of a coating aimed for machine elements. A superficial DLC layer has been deposited on top of a Ti/TiN multilayer coating.

The intrinsic elastic modulus of thin coatings can also be determined by nanoindentation (see Section 26.4.4.3), vibrating reed tests, bulge tests, beam bending tests, ultrasonic wave propagation, etc.

22 The intrinsic elastic modulus of thin coatings can also be determined by nanoindentation (see Section ), vibrating reed tests, bulge tests, beam bending tests, ultrasonic wave propagation, etc. (Brown et al., 1993; Rouzaud et al., 1995; Blakely, 1964; Hoffman, 1989; Schultrich et al., 1984) Residual Stresses Residual stresses (σ res ) are usually generated in PVD and CVD coatings during deposition and subsequent cooling from the deposition temperature. During deposition, structural misfits in epitactic coating nucleation and growth, and simultaneous ion bombardment, can generate stresses of tensile or compressive nature. On top of these, stresses due to mismatch in thermal contraction between coating and substrate materials, and possible phase transformations during cooling, are superimposed (Nix, 1989). The actual stress during application (σ) is given by: σ= σ + σ res app (26.2) where σ app denotes any stress field induced by external forces during application, including the consequence of frictional heating and thermal mismatch. Excessively high compressive stresses can result in spontaneous coating detachment (e.g., during cooling from the process temperature) or detachment when the component is externally loaded (see Figure 26.21(a)) (Hogmark et al., 1997a). The risk for detachment depends on the geometry and topography of the coating-substrate interface, as well as the interfacial stresses induced by the tribological loading, as described by Wiklund et al. (1999b). The magnitude of the interfacial normal or shear stress generated by the residual stress in the coating can, for unfavorable shapes, exceed 50% of the residual stress level. For coatings on a rough substrate, they can amount to 25% of the residual stress, if the roughness is of the same order as the coating thickness (see Figure 26.21(b)). Even if the adhesion is optimized, there remains a high risk that the induced interfacial stresses will reach the yield stress of the substrate material. Because most wear-resistant PVD and CVD coatings are quite brittle, a compressive σ res will usually improve their apparent cohesion and toughness. During PVD deposition, the compressive stress state can be controlled by varying the bias voltage applied to the substrate. The common techniques used for residual stress measurements are based either on measurements of the elastic strains in the film (using X-ray diffraction), or on the deflection of thin coated substrates. The two approaches yield comparable accuracy. X-ray techniques can yield information of all strain components in the coating (Rickerby et al., 1987; Sue, 1992), and also give information about the strain distribution through the thickness of the coating (a) (b) FIGURE A compressive residual stress of 4 GPa in a TiN coating on a rough HSS substrate generates normal stresses across the interface in convex areas. A tensile residual stress state would give similar stresses of opposite sign. (a) Coating detachment along a sharp ridge as a result of combined excessively high residual stress and external loading by scratching; (b) maximum tensile stress max σ n across the interface vs. radius R of surface ridges for four coating thicknesses.

23 (Venkatraman et al., 1992). To obtain the residual stress using X-rays, the elastic constants of the coating must be known. The substrate deflection techniques determine the coating residual stress by measuring the curvature of the thin coating composite, caused by the stressed coating (Townsend and Barnett, 1987; Röll, 1976; Stoney, 1909). The deflection is often measured using laser scanning techniques or profilometry (Nix, 1989). Coating to substrate thickness ratios in the range of 1:100 to 1:1000 are required for best accuracy, which means that substantial thinning is often necessary prior to deflection measurement of actual components. Deflection techniques can also be used in situ during coating deposition, if sufficiently thin substrates are used. Usually, the residual stress is obtained by the thin-film approximation technique (the coating is much thinner than the substrate). This technique has the advantage of not requiring data of the elastic properties of the coating material to calculate the residual stress. This is done by applying the so-called Stoney equation (Stoney, 1909): σ res 2 Es t s 1 1 = ( νs) t r c a r 61 b (26.3) where E s /(1 ν s ) is the biaxial modulus of the substrate; t s and t c are the substrate and coating thickness, respectively; r a is the radius of curvature after coating deposition on an originally flat substrate; and r b is the radius of curvature for the substrate prior to coating. The deflection technique assumes that the residual stresses are homogeneously distributed throughout the coating. The accuracy of the X-ray and deflection techniques is comparable. The nature of the residual stress of PVD coatings is generally compressive due to the ion bombardment during deposition. The nature and magnitude of the residual stresses in CVD coatings are primarily a result of the difference in thermal contraction between coating and substrate material Hardness To assess coating quality and to predict the coating performance in various applications, coating developers often use hardness measurements. However, the importance of a high intrinsic coating hardness should not be exaggerated. It is only in pure two-body abrasive wear that the wear resistance is very closely coupled to hardness. This also requires that the abrasives or abrasive surface is harder than the wearing surface. Almost all abrasive materials except for coated components are softer than 20 GPa, a value that is exceeded by many of today s PVD and CVD coatings (see Table 26.1). When the hardness of the coating exceeds that of the wearing, surface properties such as toughness, chemical stability, and fatigue resistance become more important. The intrinsic hardness of thin coatings can be directly measured by conventional microhardness testing if the indentation depth does not exceed 10% of the coating thickness. Consequently, conventional Vickers indentation is restricted to coatings thicker than about 5 µm. It is possible to use microhardness values obtained from thinner coatings if models that take the substrate deformation into account are applied (Jönsson and Hogmark, 1984; Vingsbo et al., 1986; Burnett and Rickerby, 1987). During the last decade, however, nanoindentation has become the most common technique to obtain intrinsic mechanical properties of thin coatings. In nanoindentation, the applied load is typically 0.01 to 5 g, as compared to 5 to 1000 g for microhardness testing. In nanoindentation, the load and tip displacement are continuously recorded (Figure 26.22). The hardness is obtained from load/displacement curves using different theoretical approaches (e.g., as proposed by Oliver and Pharr, 1992) Toughness Coating cracking or fracture often precedes severe damage of PVD and CVD coatings. Thus, the ability of the coating composite to accommodate deformation in tension or compression without crack nucleation and propagation is crucial. Critical situations are found in applications of nonconforming sliding or rolling (see Sections and ).

FIGURE 26.22 Nanoindentation curves for three different PVD coatings: TiB 2, TiN, and CrN.

24 FIGURE Nanoindentation curves for three different PVD coatings: TiB 2, TiN, and CrN. The obtained hardness values and Young s moduli [GPa] were 53/566 (TiB 2 ), 30/450 (TiN), and 25/330 (CrN), respectively. (a) (b) (c) FIGURE (a) Photograph of a four-point, beam-bending device suitable for operation in an SEM. A indicates the coated test specimen, B the load cell, and C the acoustic detector. (b) A representative plot of crack density (o) and acoustic emission (line) vs. coating and composite strain, respectively, for a 4-µm PVD TiN-coating on ASP2030 HSS. The insert shows a top view of a coating stressed beyond the fracture limit. (c) Critical strains of multilayered TiN/TaN coatings compared with those of the single TiN and TaN layers. The light bars represent the composite strain, while the dark bars represent the coating strain for which the residual stress has been subtracted. (From Wiklund, U., Hedenqvist, P., and Hogmark, S. (1997), Multilayer cracking resistance in bending, Surf. Coat. Technol., 97, With permission.) Several investigators have used beam bending to assess the deformability of coatings and to obtain numerical estimates of their fracture resistance and toughness (Ramsey et al., 1991; Oettel and Wiedemann, 1995; Wiklund et al., 1997). In the device of Figure 26.23(a), the bending load is continuously

$Multilayered coatings generally exhibit a higher critical strain to fracture than do homogeneous coatings (see Figure 26.23(c)).$

25 increased and the critical strain to initiation of the first crack can be recorded acoustically or in situ in the SEM (see Figure 26.23(b)). Multilayered coatings generally exhibit a higher critical strain to fracture than do homogeneous coatings (see Figure 26.23(c)). Because cracking is initiated by tensile stresses, any compressive residual stress must be overcome before cracking will commence. Consequently, high compressive residual stresses in the coating increase the critical strain of the component to coating fracture. The critical composite strain is thus a more important parameter than the critical intrinsic tensile strain of the coating. Note also that the true fracture strain of the coating materials is very low compared to that of homogeneous bulk ceramics, which usually is around 1%. This indicates a great potential of improving the coating toughness once the weak columnar structure can be avoided Tribological Properties Tribological properties here refer to the general tribological properties of the coating composite, for thin coatings involving influence from both coating and substrate. With relevant information on the general friction and wear properties, and supporting knowledge of basic coating and substrate properties, it is possible to recommend applications for given coating composites. As discussed in Section , the end user may, however, not be satisfied until a confirming field test has been performed. Below, five tribological properties are identified and simple tests for their assessment are demonstrated Scratch Resistance Scratch testing has, together with hardness measurements, become the most common way of assessing the mechanical quality of coating composites (Steinmann and Hintermann, 1989). Most often, the scratch test utilizes a spherical diamond tip of Rockwell C geometry. The tip loading is increased either stepwise or continuously, and critical loads for coating failure are determined by friction and acoustic emission (AE) recording. The critical load can be defined as the load for first crack appearance or first induced cohesive or interfacial fracture. Optical microscopy or SEM should be used to confirm the results (Steinmann et al., 1987), and can give detailed information about various modes of coating failure (Figure 26.24). A general rule of thumb says that a critical load of 30 N (using a diamond tip of 25-µm radius) is sufficient for sliding contact applications. Critical loads of 60 to 70 N are frequently recorded for PVD coatings on hardened HSS. It should be pointed out that the critical load usually increases with substrate hardness and coating thickness, and decreases with increasing surface roughness (Larsson, 1996a). Originally, the scratch test was developed for evaluation of adhesion, and adhesive failures are often associated with a sudden increase in friction force and acoustic emission. However, many of today s PVD and CVD coatings either deform to accommodate to the substrate deformation or fail by cohesive rather than interfacial fracture Resistance to Abrasive Wear In situations of mild abrasion, the coating material can determine the wear resistance of a coating composite. Standard abrasive wear tests are usually not capable of generating the intrinsic wear resistance of thin coatings. (a) (b) FIGURE Typical information revealed by SEM of scratch tested coating composites: (a) local cohesive failure at the edge of a scratch in a TiN coating on HSS; and (b) extensive coating failure along the scratch in a multilayered TiN/NbN coating on HSS.

FIGURE 26.25 (a) Schematic view of the dimple grinder. A grinding wheel rotating about horizontal axes is loaded against the horizontally positioned specimen, which is also rotated.

26 FIGURE (a) Schematic view of the dimple grinder. A grinding wheel rotating about horizontal axes is loaded against the horizontally positioned specimen, which is also rotated. In this way, a spherical crater is obtained in the specimen surface. Any suitable abrasive medium can be used. (b) Representative V c vs. (SL V s /κ s ) plot (TiN on HSS). (From Gåhlin, R., Larsson, M., Hedenqvist, P., Jacobson, S., and Hogmark, S. (1997), The crater grinder method as a means for coating wear evaluation an update, Surf. Coat. Technol., 90, With permission.) (c) Typical plot of intrinsic abrasive wear resistance of some PVD coatings. The particle erosion wear is also indicated. (From Nordin, M., Larsson, M., and Hogmark, S. (1999), Evaluation of the wear resistance of multilayered PVD TiN/TaN coatings, accepted for publication in Surface and Coatings Technology.) (d) Localized adhesive failure detected by SEM of a wear crater made by dimple grinding. However, thin coatings can be evaluated in the micro-abrasion test originally proposed by Kassman et al. (1991) and further developed by Rutherford and Hutchings (1996, 1997), and Gåhlin et al. (1997). This technique makes it possible to distinguish the abrasive wear resistance of a thin coating material from that of the substrate, and also in situations where the coating is worn through. A small grinding wheel (dimple grinding) or ball (ball cratering) is used to produce a spherical crater in the surface of the coating composite (Figure 26.25(a)). The contact area is immersed in an abrasive medium. The test is interrupted at regular intervals; and the crater volume is estimated either from measuring the diameter, or directly by using three-dimensional surface profilometry. By assuming Archard s law to be valid for both coating and substrate, one arrives at: SL V V c s = + κ κ c s (26.4) where S is the sliding distance, L is the applied load, V c and V s are the wear volumes, and κ c and κ s are the specific wear rates of the coating and substrate, respectively (Figure 26.25(b)). Intrinsic abrasive wear resistance of some PVD coatings obtained as 1/κ c are given in Figure 26.25(c). Not only does the micro-abrasion test supply information about the coating and substrate wear resistance, but inspection of the test craters in the SEM reveals any content of coating defects such as

27 pores and cracks. A poor adhesion can be detected by the presence of spallings in the coating substrate interface (see Figure 26.25(d)). Micro-abrasion test results must be handled with caution. Two-body abrasive wear of the coated surface dominates if the abrasives stick to the surface of the grinding wheel or ball. This is likely to be the case if the wheel or ball is softer than the tested material. If any constituent in the tested material is softer, the abrasives preferentially stick to the test material surface and wear the rotating ball or wheel. In a situation where both surfaces are of equal hardness, the particles may roll rather than slide, and the wear is dominated by three-body abrasion (Axén et al., 1994). Other parameters that must be kept under control are size distribution and volume fraction of the abrasive particles, and viscosity and wetting angle of the liquid medium Resistance to Particle Erosion Surface damage caused by impinging hard particles is usually referred to as particle erosion. For a thin coating to be effective in erosion protection, the strain fields of the individual impacts must not penetrate into the substrate material. Particle erosion can also be used as a means to test intrinsic coating properties primarily the toughness, provided that the particle size, velocity, and angle of impact are chosen to limit the deformation energy transfer involved when the particle hits the surface. Resistance to particle erosion is rewarded by a combination of hardness and toughness, with the toughness being the dominant parameter Resistance to Sliding Wear Sliding wear is here referred to as wear in a tribological system where the coated component slides against a relatively smooth countersurface, free from hard particles or hard asperities. Naturally, it constitutes a very large group of tribological situations. After a sliding wear test, the mass loss of typical PVD or CVD coatings is too small to be resolved by weighing. The situation has, however, recently been improved by the introduction of accurate surface profilometers and by atomic force microscopes (AFM), by which very small wear volumes can be detected, mapped, or measured (Gåhlin and Jacobson, 1998). Apart from this, there is no principal difference in evaluating the sliding wear resistance of thin coatings and bulk materials. A new test for evaluation of friction and load-carrying capacity of coating composites has recently been suggested (Hogmark et al., 1998). Two elongated specimens slide axially against each other in a way similar to that of the contact between the edges of a pair of scissors. If the load is gradually increased, as in the scratch test, each contact spot along the wear track will experience a unique load. Unidirectional as well as multipass sliding can be applied, and critical loads for coating failure can be obtained as for the scratch test. The main advantage over the scratch test is that the contact situation is very much closer to practical applications of sliding contact. In addition, the counter material, as well as the contact geometry (radius of contacting rods), can be selected to represent the intended application Resistance to Wear in Rolling Contact The stress distribution in rolling contact between smooth surfaces can be estimated according to Hertz (Figure 26.17). Because the maximum shear stress is generated below the contact surface, the intrinsic wear resistance of the surface material is usually not the main concern. Strong adhesion and ability to deform elastically with the substrate are the two most important coating requirements in rolling contact (Figure 26.26). If a small, permanent deformation is accumulated in each contact event (ratchetting), the fracture limit of the coating will sooner or later be achieved. Rolling element bearings are currently being considered for application of coatings. Use in railway wheels is a larger-scale application which, of course, requires relatively thick coatings for wear protection. Pure rolling usually gives negligible wear. A small proportion of sliding in dry or boundary lubricated rolling contacts may give a mild wear. In slowly rotating roller bearings, such mild wear may gradually concentrate the contact pressure to the unworn regions of pure rolling and, eventually, cause catastrophic surface fatigue (Andersson, 1999). A wear-resistant coating can solve this problem.

FIGURE 26.26 (a) Test designed to simulate the tribological situations in rolling contact. The cylindrical test pin is rotated against two normally loaded disks.

28 FIGURE (a) Test designed to simulate the tribological situations in rolling contact. The cylindrical test pin is rotated against two normally loaded disks. (b) Typical coating wear after rolling contact simulation of TiN-coated HSS. The original grinding topography is seen at the left in the figure. Initially, the surface is smoothed due to a very mild wear mechanism. After a critical number of revolutions, the coating starts to spall. The stress distribution in sliding between nonconforming surfaces resembles that of rolling contact, if the friction coefficient is below 0.3. Practical examples include the cam and tappet contact of a car engine, or the contact between gear teeth, both of which are designed to operate in the boundary lubrication regime. This means that the friction coefficient typically amounts to 0.10 to 0.15, and the maximum shear stress is not confined to the contact surface. Consequently, good adhesion and the ability to deform elastically with the substrate are required for coatings in this kind of application, as it is for rolling element bearings Tribological Response of Coated Components The end users of coated components are recommended to make the final evaluation of the tribological response in field tests or in component tests (i.e., tests where the actual component is evaluated under realistic conditions). Any simplified laboratory test can deviate from the actual situation as to nominal and real contact pressure, sliding speed, heat conductivity and capacity, ambient cooling, etc., and correlation to the real case is hazardous. Examples of components that can be field-tested relatively easily are cutting tools and other relatively small components with limited life. Large and expensive forming tools, for example, may have to be evaluated in simplified tests, preferably through smaller model tools used in a pilot plant. It is sometimes possible to design a large component incorporating small inserts that can be easily exchanged and act as test surfaces. In carefully designed tests, these inserts will experience the same tribological conditions as the actual component Failure Analysis of Coating Composites Premature failure of coating composites almost always involves local or extensive coating detachment. Given the task to analyze the cause of a premature failure of a coated component and suggesting means to improve it, the following questions should be addressed: 1. Substrate condition: Is the hardness, chemical composition, etc., of the substrate that which is desired? 2. Substrate surface preparation: Is the substrate too rough? Has the grinding been too severe, thus generating, for example, untempered martensite, decarburization, or thermal softening? Are there any residual impurities in the interface? 3. Type of coating failure: Has the failure been initiated mechanically, thermally, or chemically?