ABSTRACT. Annex 4. Table of contents

Transcription

1 Ph.D. Marcin Kot Kraków, 15 March 2013 AGH University of Science and Technology Faculty of Mechanical Engineering and Robotics Assistant professor Annex 4 ABSTRACT Table of contents 1. Diplomas, degrees - the name, place and year of acquisition and the title of the doctoral dissertation Information on employment in research institutions The achievement being a base of initiation of the habilitation procedure Scientific career The period before receiving the doctoral degree The period after receiving the doctoral degree Discussion of the scientific aims and the received results Problems of contact mechanics and tribology of coating-substrate systems Tribological coatings with complex architecture..11 Ceramic/metal multilayer coatings Nanocomposite coatings The main scientific achievements Other scientific achievements Summary of scientific activity Publications Conferences Participation in grants and contracts conducted within cooperation with scientific centers and industrial partners Received awards Fellowships Didactic activity Organizational activity

2 1. Diplomas, degrees - the name, place and year of acquisition and the title of the doctoral dissertation 1993, graduated from the Technical School in Jędrzejów where, based on the thesis entitled Supply systems in two-stroke engines I received the professional title of automotive technician. 1998, graduated from the Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology in Cracow, where I received a Master's degree in mechanical engineering. Thesis: "Dilatation of polymer sensor composites" which was supervised by Professor Wiesław Rakowski. 2002, the Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology, I finished my doctoral thesis entitled: " Thermo-mechanical models of piezoresistive tribological layers." The supervisor was Prof. Wiesław Rakowski. I received a Ph.D title in the discipline of mechanics and machine engineering, scientific specialization: tribology. 2. Information on employment in research institutions professional experience (fourth and fifth year of study) at the Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology in Cracow assistant at the Department of Machine Design and Technology, the Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology in Cracow. Since assistant professor in the Department of Machine Design and Technology, the Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology in Cracow. 3. The archivement being a base of initiation of the habilitation procedure (art. 16 ust. 2 ustawy z dnia 14 marca 2003 o stopniach naukowych i tytule naukowym oraz o stopniach i tytule w zakresie sztuki Dz. U. nr 65, poz. 595 ze zm.) Mono-subject series of 11 publications in journals entitled: Tribology and contact mechanics of selected coating-substrate systems A1. M. Kot: Analiza właściwości mechanicznych układów powłoka-podłoże przy użyciu metody indentacji z wykorzystaniem wgłębników o różnej geometrii. Tribologia 236 (2011) A2. M. Kot (80%), P. Lacki : Contact mechanics of coating-substrate systems: I Methods of analysis and FEM modeling of nanoindentation tests. Journal of the Balkan Tribological Association, 18 (2012) , JCR, IF=0,158 2

3 A3. M. Kot: Contact mechanics of coating-substrate systems: II Nanoindentation experiments. Journal of the Balkan Tribological Association, 18 (2012) , JCR, IF=0,158 A4. M. Kot (70%), J.M. Lackner, Ł. Major, W. Rakowski: Analysis of spherical indentations of coating-substrate systems - experiments and FEM modeling. Materials and Design, 43 (2013) , JCR, IF=2,200 A5. M. Kot (70%), W. Rakowski, Ł. Major, J. Lackner: Load-bearing capacity of coatingsubstrate systems obtained from spherical indentation tests. Materials and Design, 46 (2013) , JCR, IF=2.200, A6. M. Kot (60%), W. Rakowski, R. Major, Ł. Major, J. Morgiel: Effect of bilayer period on properties of Cr/CrN multilayer coatings produced by laser ablation. Surface and Coatings Technology, 202 (2008) , JCR, IF=1,860 A7. M. Kot (70%), Ł. Major, J. Lackner, W. Rakowski: Enhancement of mechanical and tribological properties of Ti/TiN multilayers over TiN single layer. Journal of Balkan Tribological Association, 18 (2012) , JCR, IF=0,158 A8. M. Kot: Contact mechanics of coating-substrate systems: monolayer and multilayer coatings. Archives of Civil and Mechanical Engineering, 12 (2012) , JCR, IF=0,855 A9. M. Kot (60%), T. Moskalewicz, B. Wendler, W. Rakowski, A. Czyrska-Filemonowicz: Mechanical and tribological properties of nc-tic/a-c nanocomposite coatings. Solid State Phenomena, 177 (2011) A10. M. Kot (50%), E. Bełtowska-Lehman, A. Bigos, P. Indyka, J. Morgiel, W. Rakowski: Mechanical and tribological properties of electrodeposited Ni-Mo coatings. Inżynieria Materiałowa, 175/3 (2010) A11. E. Bełtowska-Lehman, P. Indyka, A. Bigos, M. Kot (20%), L. Tarkowski: Electrodeposition of nanocrystalline Ni-W coatings strengthened by ultrafine alumina particles. Surface and Coatings Technology, 211 (2012) 62-66, JCR, IF=1, Scientific career 4.1. The period before receiving the doctoral degree In 1993 I started my studies at the AGH University of Science and Technology in Cracow in the Faculty of Mechanical Engineering and Robotics. During the fourth and fifth years of my studies, I was a trainee assistant and this was the beginning of my scientific work. My Master s thesis: "Dilatation of polymer sensor composites" was supervised by Prof. Wiesław Rakowski and defended with a very good grade on 30 June The aim of this study was to analyze the temperature dilatation of polymer composites with conductive fillers, and its effect on the electrical conductivity of the composite. After that, on 1 November 1998, I started work as an assistant in the Department of Machine Design and Technology at AGH University of Science and Technology. My interest in the research of polymer sensor composites meant that I developed this topic within the doctoral dissertation. Then I took part in the development of polymer sensor composites based on epoxy and polyester-imide resins. Two groups of fillers: conductive particles, such as tin, indium and other particles and particles improving the tribological 3

4 properties of composites were introduced into polymer matrices. The new idea behind this work was the application of conductive composites as bearing materials. The plain bearings with thin polymer sensor layers deposited on steel substrates could be a structural element of a machine and, simultaneously, a part of the diagnostic system temperature and load sensor of a bearing. The criteria for optimization of the composite structure were sensor properties and the main goal was to obtain high values of the PCR and TCR coefficients (pressure and temperature coefficients of resistivity) and, at the same time, good tribological properties. The selected composites with the best properties contain up to several percent of the metal powder and a few percent of the graphite and molybdenum disulphide. A few technological problems were solved to ensure the repeatability of the sensor and tribological characteristics of the produced composites. I cooperated with other scientists and developed such materials within two KBN projects as the prime contractor [G1, G2 1 ]. In my doctoral thesis, I analyzed the effects of composite composition and the volume share of fillers, on the stiffness, dilatation and electrical conductivity of composites. I developed and presented the thermo-mechanical model based on the mechanics of composite materials and percolation theory. This model allows the quantitative analysis of electrical resistance changes in polymer sensor composite thin films under mechanical and thermal loads. The model was verified twofold by experimental studies of milli-bearings with sensor layers and finite element modeling. The topic of polymer sensor composites was further studied in cooperation with the Institute of Electron Technology in Cracow. A new group of composites based on acrylic-melamine resin was developed. The results of all studies on polymer sensor composites were presented at eight conferences in, among others, Japan, Austria, Egypt and in four publications in journals included in the list of the Ministry of Science and Higher Education [L1-L12 2 ]. My doctoral thesis entitled: "Thermomechanical models of piezoresistive tribological layers" was successfully presented on 10 November The main results were published in the paper in the journal from the JCR Master Journal List [L13] The period after receiving the doctoral degree In February 2003, I was hired as an assistant professor in the Department of Machine Design and Technology, in the Faculty of Mechanical Engineering and Robotics at AGH University of Science and Technology. This was a period of great expansion of the Tribology and Surface Engineering Laboratory where I work until now. My scientific field of interest shifted to new surface engineering technologies which reduce the unfavorable effects of friction between structural elements. Against the background of the latest literature analysis, I noticed a wide gap between the widespread application of thin, hard wear-resistant coatings, and research studies on their mechanical and tribological properties. Hence, a large part of my research activities has focused on the tribology, contact mechanics and failure mechanisms of coating-substrate systems. The studies of deformation, fracture, friction and wear of the coating-substrate systems were conducted by using the instrumental indentation 1 A list of research projects with their short description is presented in Annex 10 2 List of publications is presented in Annex 8 4

5 and scratch techniques, and a wide range of tribotesters. The analysis of failure mechanisms, characteristic areas of crack formation and their propagation were possible using microscopic techniques, like scanning (SEM) and transmission (TEM and HR-TEM) electron microscopy and others, such as XRD and EELS. Many of the microscopic analyses were conducted on cross-sections of trough indents and wear tracks. Thin foils prepared using the FIB technique were used. Knowledge of the interactions between the coating, substrate and contact element is an extremely important issue, not only from the perspective of an engineer who must select the coating for a particular application, but also for a scientist who is engaged in the design and deposition of new coatings Discussion of the scientific aims and the received results Nowadays, thin hard coatings are usually applied due to the fact that the surface of many elements plays an important role and allows users to fulfill the still rising demands for machine elements, tools and bio-implants etc. Therefore, depending on the application, coatings must be wear, fatigue and corrosion resistant, characterized by appropriate optical, magnetic, electrical and thermal properties, biocompatibility and others. Despite the growing area of coating applications, including those for coatings with sophisticated architecture, there are still no complex studies on the strength of coating-substrate systems that preclude the prediction of loads which cause the characteristic forms of destruction by yield or fracture of coating and substrate. That was the main reason I started studies in the field of contact mechanics and tribology coating-substrate systems Contact mechanics and tribology of coating-substrate systems Nowadays, coatings are deposited on elements in different scales, hence the load is carried through the contact with elements of different geometries from small radii in micromechanisms and asperity tops to larger radii of rolling bearings and gears. The results of my preliminary program of numerical analyses have shown that the different geometry of contacting elements causes different stress distribution and changes of areas subjected to maximum stress concentration. This problem is still rarely taken up in the literature, and the presented papers do not allow a complex assessment of the load bearing capacity of the coating-substrate systems, but are generally the analysis of a particular system. In addition, the main results of the tests are load value, practically impossible to compare even for the same coating, but with different thickness or deposited on the substrate with different properties. This problem is becoming more and more important since the coatings, including tribological applications, are applied on the substrates with dramatically different properties from cemented carbides, through steels and titanium alloys, to polymers. This causes the different interactions of coating and substrate and the different transfer mechanism of load from the cooperating element to the coating and afterwards to the substrate. Problems while analyzing mechanical properties of coated elements also result from measurement problems, where deformations are in a range from tens to hundreds of nanometers. 5

6 My studies on the contact mechanics of coating-substrate systems initially focused on the effect of contact geometry on the deformation and characteristic failure forms of such complex systems. I conducted experiments using the indentation technique with continuous measurement of load and penetration depth. In order to determine the allowable load of coating-substrate systems for various geometry of contacting elements, I suggested and performed indentation tests using indenters with different geometries [A1 3, L18]. I noticed that one of the main parameters that determines the occurrence of specific forms of failure is the ratio of coating thickness to indenter radius t/r i. However, the results of indentation tests in the form of typical force-penetration depth curves do not allow users to evaluate the stress level at certain load conditions and contact with the element of defined geometry. Thus, I have used a procedure of indentation test analysis of coating-substrate systems based on the transformation of the typical force-penetration depth curves into stressstrain curves, which represents the changes of mean pressure in the contact zone as a function of increasing penetration depth. A crucial problem necessary to determine the contact area between indenter and coating is to link the contact depth h c with penetration depth h measured during the test. Usually, in the literature a simplification was used assuming that the ratio h c /h is constant and equal 0.5, which substantially overestimates the calculated pressure values compared to the real stress level. In my studies, I defined the changes of this ratio vs. increasing deformation of the system based on the results of indentation tests conducted under a wide load range and the finite element method modeling results (FEM). I conducted the research program and the results interpretation for typical tribological coatings like TiN, CrN and a-c:h deposited on steel substrates. For the analysis of test results I used the procedure of transformation indentation curves described previously, which allowed the determination of the allowable loads of coating-substrate systems at different contact geometries. I have shown that, using conventional, commercially available, diamond indenters, the various states of deformation of such systems can be induced. For a sharp Vickers indenter (t/r i >5) the maximum pressure in the contact zone is equal to coating hardness, because only the coating is plastically deformed at low loads. At higher loads the plastically deformed zone reaches the substrate, which corresponds to a drop in pressure. However, for indentation with a spherical indenter R i =20 μm (0.05<t/R i <0.5), the mean pressure does not reach such a high value as for the Vickers indenter. Initially, the pressure in the contact area increases linearly and reaches a maximum at the level of several GPa, considerably lower than coating hardness. Hence, the conclusion is that the substrate is plastically deformed earlier than the coating. The results showed that for more rigid coatings the maximum pressure reaches higher values. This means that, at the initial state of deformation, rigid coatings carry mechanical loads and prevent the plastic deformation of the substrate. Further increase of the indenter tip radius (t/r i <0.005) leads to a reduction of mean pressure in the contact zone to a level of 1 2 GPa for the studied systems. However, at MPa pressure I noticed a distinctive change in the stress-strain curve slope resulting from the development of the yielded zone in the substrate. 3 List of publications being the basis of initiation of the habilitation procedure Point 3 and Annex 5 6

7 The matrix of critical load values causes substrate yield, and coating fracture for different relative coating thickness was used to create failure maps of coating substrate systems. I plotted such maps in t/r P K /R 2 (film thickness of the reduced radius of contact - critical force to the square of the radius) coordinates, which releases the results from the influence of indenter geometry. The yield and fracture curves on these maps, by approximation to the specific geometry of contacting elements, allow the prediction of the load limit for particular friction nodes or the proper selection of a coating and its thickness. The results of these studies meant that I set an aim for myself - the precise analysis of deformation, stress distribution and the specific failure forms of coating-substrate systems for different contact geometry and material properties of the coating and the substrate. This was also due to the necessity to analyze the mechanical properties of the coating-substrate systems at the micro and nano scale and literature studies that, in many cases, are difficult to interpret because of the lack of a full description of the mechanical response of the coating-substrate system. My studies in the field of contact mechanics of coating-substrate systems were conducted using techniques such as indentation with continuous measurement of load and penetration depth, scratch tests and tribotesters operating at low load ranges. I have also carried out numerical experiments through modeling of coated elements using the finite element method (FEM). This has allowed me to quantify the critical loads that cause the destruction of coating-substrate systems and to develop the analytical relationships. In modeling the contact geometry, material properties of the coating and the substrate were varied in wide ranges, therefore the results of modeling can be approximated to real systems. The tests that I have carried out range from the elastic regime of deformation up to significant plastic deformation of the coating and substrate, and for hard coatings to their fracture. The first area of analysis was the contact of coating-substrate systems against elements with a small tip radius of R i =50 200nm, i.e. typical geometry of Berkovich and Vickers indenters used to determine the micro/nanohardness and modulus of elasticity of coatings. For such indenters and typical thickness of a tribological coating the t/r i ratio is in the range Despite the fact that the mechanical testing techniques I have used have been known for over 20 years, scientists are still encountering a lot of problems during the testing of coated elements, especially with the interpretation of results. This is particularly evident for the widely-used nanoindentation technique. This is due to a smaller and smaller thickness of currently applied coatings, their complex architecture and deposition on the substrates with a wide range of mechanical properties. In the literature, results of the indentation tests of selected coating-substrate systems are usually presented without critical discussion on whether the measured value is actually the value representing the coating properties. The generally accepted 1/10th rule says that the penetration depth h max should not exceed 10% of coating thickness in order to avoid the impact of the substrate on the obtained results. In my first publication on the field of mechanical testing of coating-substrate systems using the instrumented indentation technique, I presented the main sources of measurement errors and a review of the methods of result analysis [L14]. All the models of hardness changes of coating-substrate systems presented in the literature facilitate the interpretation of nanoindentation test results for coatings even thinner than 1μm. However, they require 7

8 the performance of multiple measurements at a wide range of load and deformation. The second step is to fit the curves of various function forms (depending on the model), usually with many unknown coefficients, to the experimental results. Such analyses are rather time consuming. I compared such a method with the procedure based on transformation of indentation curves. I presented this method of nanoindentation result analysis in articles [A2-A3, L15-L17]. I tested the tribological coatings ZrN, CrN and a-c:h with different thickness, deposited on soft and hard steels and silicon substrates. I also analyzed changes in contact pressure, deformations and stress distributions in the coating and substrate using FEM modeling. The combination of the developed procedure with modeling results allowed the determination of the effect of hardness, as well as the modulus of elasticity of the coating and substrate and coating thickness on the deformation of the whole system. It is extremely important for the measurement methodology to define the so-called critical penetration depth at which the substrate starts to yield. Above this penetration, the substrate affects the measurement results. The results of FEM modeling showed that the ratio of coating to substrate hardness of H C /H S has the greatest impact on this parameter, while the elastic properties ratio E C /E S and coating thickness do not affect the indentation results as strongly. Tests performed for the selected coating-substrate systems with different properties have shown that the critical depth of penetration compared to the coating thickness t, in many cases must be significantly lower than the commonly accepted 10% t rule. This rule is effective until the ratio of coating to substrate hardness H C /H S is lower than 5. This creates the inevitable impact of substrate properties on the indentation results (hardness and elasticity modulus) of thin coatings with thicknesses up to 1μm when they are deposited on soft substrates, like untreated titanium alloys, soft steels and polymers, where H C /H S is significantly greater than 5. Therefore, it can be concluded that the applied loads should be lower and lower to reduce penetration depth below the critical limit. However, as I presented in [A2-A3] extremely low loads during the testing of hard coatings cannot induce the fully plastic regime of deformation, and so the results do not match the hardness of the coating. The results showed that, for hard ceramic and carbon coatings, penetration depth should be at least in the tens of nanometers range. Additionally, a significant scatter of low-load nanoindentation test results is also caused by even a small surface roughness. Hence, an effective tool for analyzing test results is to link them with the results of FEM modeling according to the procedure I developed. The next aim of the research that I developed was the contact of coated elements with spherical indenters when relative coating thickness is in the range t/r i =0,005 0,25. So far, the results of spherical indentations usually presented in the literature were load values which caused fracture or delamination of the coating determined from characteristic pop-ins on indentation curves. However, the load values do not allow the generalization of results to other coating-substrate systems and should not be treated as the characteristic strength parameters. An interesting issue, however, is how to determine the actual state of stress leading to some specific forms of destruction. The main problem is how to relate the parameters measured during the indentation test - load and penetration depth, with corresponding strain and stress levels. I analyzed this issue by comparing the indentation and FEM results for a wide range of material parameters of coatings and substrates and for different contact geometry. 8

9 I presented such analyses for TiN coatings deposited on steel substrates in [A4, L23]. Combining indentation and modeling results allowed the description of the analytical relationships for load values that cause the substrate yield F pl /R i 2 and, extremely importantly in further analysis, the coefficient of the stress concentration binding the mean stress in the contact zone with the maximum tensile stress in the coating. I defined the load value at which the first crack is formed in a coating by searching for specific discontinuities on the stress-strain curves and using SEM and TEM microscopy. This allowed me to identify the specific areas in a coating prone to fracture. For thin coatings (t/r i 0,01) cracks occur most often on the coating surface just outside the contact zone with the indenter, while for thicker coatings (t/r i 0,2) crack initiation takes place in the coating-substrate interface around the symmetry axis of the indenter. However, it should be noted that the area of maximum tensile stress concentration varies depending on the relative penetration depth (h max /t). At low penetration depth h max /t<0,03, the tensile stress peaks occur on the coating surface. Then, up to h max /t 0,5 most vulnerable to cracking are areas at the coating-substrate interface, and above this h max /t value the tensile stress is again higher on the surface. Of course, the given values of relative penetration depth are approximate, since they strongly depend on the elastic properties of the coating and substrate E C /E S and the extent of the plastically deformed zone in the substrate. However, the load level and area subjected to fracture is highly influenced by the state of residual stress in the coating after the deposition process. The microscopic examination of cross-sections through the indentation imprints confirmed the modeling results, namely the characteristic areas of crack formation in the coatings. Thus, comparing the mean pressures in the contact zone calculated according to the previously described procedure with the modeling results, and taking into account the state of residual stresses measured by X-ray diffraction, I defined the level of tensile stresses leading to the fracture of tested coatings. The value of this stress is practically constant, independent of coating thickness, for a specific material. Hence, it can be treated as a strength parameter of coating material, limited of course to a specific coating microstructure. The results of such studies and their experimental verification performed for TiN coatings with a μm thickness range deposited on various substrates were presented in [A4, L23]. This paper also presents the failure maps for selected coatingsubstrate systems. These maps determine the elastic and plastic deformation regime and loads that cause coating fracture. They can be used to predict the permissible load in a particular friction node of known geometry of interacting elements. In my studies I also investigated the problem of the impact of coating microstructure on the characteristic areas of crack formation and directions of their propagation. I analyzed and modeled coatings with different microstructures from the columnar (TiN) to amorphous (a-c:h). For ceramic coatings characterized by a columnar microstructure, cracks appeared mainly outside the contact zone and propagated through the column borders, in other words, the areas with the lowest strength, leading to energy dissipation by a mechanism called intercolumnar sliding. In amorphous coatings, cracks initially propagate perpendicular to the surface and then, in the middle of the coating thickness are deflected parallel to the surface direction, outside the contact axis. The results of FEM modelling showed that in this area normal stress decreases to zero while the shear stress reaches its maximum and this causes crack deflection. Analysis of this mechanism was presented in [L21, L22]. 9

10 Analysis of deformations and fracture in the coating-substrate systems raised another question. Can the deformation maps which are the result of indentation tests be used as a criterion for tribological contact of the friction pairs, where one element is coated. How do the abrasive wear resistance and wear character of coating-substrate systems for varying the deformation range change? I expected the lowest wear in the elastic deformation regime. An increase of load up to substrate yield or coating fracture causes the deterioration of the tribological properties and an increase in wear. Therefore, to obtain the low wear rate in a friction node with hard coating, the condition of small deformation of the system must be satisfied. Many parameters affect the deformation, like relations between the mechanical properties of the coating and the substrate, load, contact geometry and a state of residual stress. All of them affect the wear mechanism of the system. The indentation results and deformation maps for TiN coatings presented in [A4] were used to determine the loads applied in the ball-on-disc tribological tests. I carried out tests at applied loads below and above the permissible level of substrate yield [A5, L19, L20]. For each system and load, the wear index was calculated. It was found that the wear is at a constant level until load exceeds the value where the substrate does not provide sufficient support for the coating. FEM modeling results showed that this occurs when the radius of the yielded zone r pl is greater than the contact radius r C of the coating and contacting partner. It means that only load-caused local yield of small volumes of the substrate are not dangerous for the whole system. TEM analysis carried out on cross-sections of wear tracks exhibited that under the elastic regime of deformation (even at low substrate yield), the abrasive wear mechanism with small spallation of the grains on the coating surface dominated. The wear index was several times smaller than for load-caused substrate yield (above critical load). At higher load conditions, the catastrophic failure of the coating was observed with a trough coating fracture, delamination and substrate exposure. The debris with hard particles remaining in the friction zone further intensified wear of the system. The different deformation character of the systems at elastic and plastic deformation of the substrate was seen on the wear track profiles [A5]. Plastically deformed substrates led to the formation of pile-ups on the sides of the track. The analysis I conducted in the field of contact mechanics and tribology of coatingsubstrate systems on different groups of coatings deposited by various methods, like magnetron sputtering, arc, glow discharge and laser techniques, was possible due to my activity in several projects [G3, G4, G6, G9, G11 and G12] and cooperation with research centers dealing with the deposition of wear resistant coatings. Cooperation with research groups specializing in microstructure studies using microscopic techniques: SEM, TEM, XRD, EELS, and others, was extremely helpful in the identification of deformation, fracture and wear mechanisms, depending on the architecture and microstructure of the coating. I also studied the mechanical and tribological properties of coatings designed for biomedical application within the projects [G11-G13]. In this area, besides the biocompatibility, wear resistance and high fracture toughness of coatings are required. In the Polish Artificial Heart (PSS) project I dealt with the analysis of the mechanical and tribological properties of coating-substrate systems for artificial heart elements, such as membranes and pumps supporting its work. In the case of membrane analysis, they allowed 10

11 users to substitute the technology of manual graphite rubbing on the inner surfaces of the membrane by BN/TiN coatings deposited by the PVD technique[l49-l50]. Coatings are also deposited on polyurethane pipes connecting the cardiovascular system with the artificial heart chamber. For coatings in this application where protection against the contact of blood with polymeric material is crucial, I have focused mainly on adhesion to the substrate and the fracture resistance of coatings which prevent blood clotting. For this application I analyzed the mechanical properties of Ti, TiN, Ti(CN), TiO x, a-c:h and Si-a-C:H coatings. The results of mechanical and biological tests have shown that the best properties are exhibited by silicon dotted hydrogenated carbon Si-a-C:H coatings. These coatings are deposited on PU pipes and the studies are in the stage of biological tests in an artificial patient system. Within this project, I also analyzed the mechanical and tribological properties of multilayer coatings (description in Chapter 4.3.2) that could be applied on components of the drive system supporting the work of the artificial heart chamber. The [G10] project was conducted to produce coated elements on vascular prostheses. I performed the analysis with the results of biological tests that were helpful when selecting an optimal coating for artificial heart chambers. I have provided a summary of the work carried out in the field of the biomedical applications of coatings in a chapter of the monograph [L52] presenting the main results of the CardioBioMat project [G10]. The realized research program concerning the mechanical and tribological properties of hard coating-substrate systems presented in [A1-A5] papers allowed me to carry out the complex analysis of the impact of the most significant factors, such as mechanical properties, loads and contact geometry on deformations, failure and wear of such systems. Experimental studies carried out in the micro-and nanoscale and the modeling results enable the determination of the critical loads below the wear level characteristic for the applied system which could be expected, without catastrophic failure forms like cracking and delamination. The research methodology I presented, based on creating a specific deformation map of coating-substrate systems, allows the prediction of the critical loads for systems with different coating thickness, but with similar mechanical properties. This makes it possible to predict the lifetime of coated components and increase their reliability. The analysis of the deformation and wear of coatings which I have presented may be helpful in the suitable selection of the coating and its thickness, and thus reduce the amount of experimental tests Analysis of the mechanical and tribological properties of coatings with complex architecture The results of the analyses described in the previous section showed that the constant searching for tribological coatings characterized by high hardness is not appropriate, which is visible today in science and industry. Unfortunately, the coating hardness of 50 to 80 GPa corresponds to high stiffness (Young's modulus up to 600 GPa or more) and low fracture resistance. However, as I have previously stated, the high mismatch of coating and substrate elasticity modulus, in the case of the application of such coatings on steel and titanium alloys, leads to a significant stress concentration at the coating-substrate interface. On the other hand, 11

12 high hardness of the coatings usually indicates their potential high wear resistance. Thus, the best solution from a tribological point of view is a hard and flexible coating. The continuous development of technology and increasing demands for tools and machine elements have created a huge interest in coatings with further improved mechanical, chemical and tribological properties. In many cases a single-layer coating cannot meet all requirements. Hence, scientists are developing new kinds of coatings, such as multilayers and nanocomposite coatings. The first of them are composed of a stack consisting of a few to hundreds of thin layers with a nanometer thickness range, usually built on two alternately deposited materials. The typical material compositions are Ti/TiN, Cr/CrN, Zr/ZrN. While the nc-mx/a-mtr nanocomposite coatings are built of a soft, amorphous, matrix a-si 3 N 4, a-c, a-c:h are built with hard nanocrystalline, ceramic particles, usually carbides or nitrides of transition metals: Ti, W, Hf, Cr. Significant enhancement of mechanical properties could be obtained by the introduction of hard nanoparticles into electrochemically deposited metallic materials like in Ni-Mo+Al 2 O 3 i Ni-W+Al 2 O 3 coatings. Ceramic/metal multilayer coatings I started my studies into the mechanical and tribological properties of ceramic/metal multilayer coatings in 2006 in collaboration with scientists from the Institute of Metallurgy and Materials Science at the Polish Academy of Sciences, Cracow, and Joanneum Research Center in Leoben, Austria. The first aim was to develop the optimum architecture of the coating that exhibited the best mechanical properties and wear resistance. Two coating compositions, Ti/TiN and Cr/CrN, were investigated. I have analyzed the effect of the bilayer period (sum of the thickness of two neighboring layers), microstructure and crystallite size. The results and analysis for Cr/CrN coatings were presented in [A6, L24-L29]. The characteristic factor was the improvement of the mechanical and tribological properties with a decreasing multilayer period. Significant enhancement of mechanical properties was observed mainly on fracture toughness and abrasive wear resistance. The hardness of Cr/CrN multilayers rises from 17 to 23 GPa with a reducing bilayer period from 1000 do 250nm. The maximal hardness of multilayers was slightly lower than the hardness of a single ceramic CrN coating 26GPa. A significant increase in fracture toughness observed by scratch testing and wear resistance was found for multilayer coatings. The first cracks were observed at a 5N load for a single CrN coating, while the best multilayers cracked at an 8N load. The further reduction of the bilayer period led to a deterioration in mechanical properties, particularly evident for the coatings with a 30nm thickness of layers. This can be explained by many defects on interfaces and the loss of a characteristic multilayer structure. The optimal properties were exhibited by a multilayer with a 125nm bilayer period. Further period reduction to check how the results of mechanical and tribological properties can be transferred to other material combinations of ceramic/metal multilayer coatings and to help the design of such coatings for mechanical applications in the future, I tested the group of Ti/TiN multilayers and published the results in [A7, L29-L31]. TEM analysis carried out in the IMiM PAN in Cracow confirmed the multilayered architecture of coatings with a columnar microstructure of Ti and TiN layers and the established thickness of successive layers. The characteristic changes in mechanical properties with a bilayer period were similar 12

13 to those found for Cr/CrN coatings. The Ti/TiN coating with a 64nm bilayer has the best properties. The hardness of this coating of H=23.8 GPa is only slightly smaller than the hardness of the single TiN coating of H=25.6 GPa. In addition, for this material combination, the reduction of the period to 32 nm resulted in a drastic deterioration in mechanical properties. It is important to note that the hardness of the best multilayers is close to a single ceramic coating, while the elasticity modulus of all multilayers (E= GPa) has intermediate values between the values of the ceramic (E=378 GPa) and metallic (E=163 GPa) layers, and these values are slightly higher than for steel substrates. This is very advantageous because of the strength of the coating-substrate interface. This was confirmed by scratch tests and the much higher percentage values of critical loads for multilayers than for single TiN coatings. Unfortunately, the wear resistance of multilayer coatings was only at the same level as for TiN. The results of indentation tests of all multilayer coatings showed a significant hardness increase over hardness calculated from the rule-of-mixtures (the average hardness values of both layers in the multilayer). The results of my tests and literature studies show that the strengthening of multilayers is mainly caused by the existence of many parallel interfaces between layers. These interfaces are barriers to dislocation motion and lead to the hardening of metallic layers and thus the whole system. For the Ti/TiN system, a 0.3 GPa hardness increase due to one interface was found [L33, L36]. Detailed analysis of the role that both ceramic and metal layers play in the multilayer coatings, and the conclusions drawn from mechanical and tribological tests, showed the potential for possible further improvement of their properties by changing the thickness ratio of the ceramic to metallic layers. Then I suggested coatings with a greater share of ceramic layers. The studies were carried out for Ti/TiN coatings with an optimum 125nm period, but with a modified Ti:TiN layer thickness ratio from the previously tested 1:1 to 1:2 and 1:4. The hardness of these coatings is even higher than the single TiN coating, in spite of the soft metallic layers in the multilayer, and the critical load measured by scratch testing increased from 13N for TiN to 27N for the 8xTi/TiN ratio 1:4 multilayer. Most importantly, the wear index of the 8xTi/TiN ratio 1:2 coating was three times lower than for the 8xTi/TiN ratio 1:1 [A7]. Surprisingly, no further increase of wear resistance was found for coatings with a higher amount of the ceramic phase 8xTi/TiN ratio 1:4 [L31-L34]. Why has the coating with a 1:4 ratio not got better properties than the coating with a 1:2 ratio through a higher amount of hard ceramic phase? The answer was given by the results of experiments carried out according to a research program designed to analyze the deformation and failure mechanisms of multilayers. TEM analyses of thin foils performed through imprints after spherical indentations showed that the deformation mechanism of Ti/TiN multilayers was followed by a sequence of fracture of ceramic layers and yield of metallic ones. When a crack propagating in the ceramic layer is closed in a metal layer, the new crack must be formed on the other side of the Ti layer, but these mechanisms demand a higher amount of energy than for crack propagation in a continuous brittle, ceramic coating. However, these cracks may also propagate in the Ti layers to a limited length (30-40nm). Hence, if Ti layers are thinner, cracks can propagate through their whole thickness, which was confirmed by the TEM analysis presented 13

14 in [L35, L36]. An extremely important issue for the application of multilayers is their higher fracture resistance than single ceramic layers. Generally, the parameter that represents the fracture toughness of coatings is K C, which binds the length of the frame of radial cracks at indent corners with applied load, assuming that cracks are formed through the whole thickness of the coating. For the multilayer coatings, such a method is not possible to apply, due to the inability to determine how many ceramic layers cracked, and due to the closing of the cracks in the metal layers. Therefore, I suggested the quantitative assessment of critical loads leading to coating fracture for single layer and multilayer coatings based on the results of indentation tests and FEM modeling. The issues related to the contact mechanics of single and multilayer coatings were presented in [A8, L37]. The main aim was to compare the concentration of tensile stresses leading to the fracture of a TiN single coating and Ti/TiN multilayer. The indentation tests indicated the distinctly higher susceptibility of multilayers. For the analyzed coating-substrate systems the same load caused higher deformations of systems with multilayers than those coated by a single ceramic layer. The result is a greater contact area of the indenter and coating, and therefore much lower mean pressures in the contact zone. To compare the load leading to coating fracture, I analyzed the maximum tensile stresses in ceramic layers in multilayer and single coatings using FEM modeling. I showed that the coefficient of stress concentration C Top at the same deformation ratio is greater for the outer ceramic layer in a multilayer than than for a single coating. This is caused by the easier bending of a multilayer coating. However, the reverse situation is at the coating-substrate interface where the C Bott coefficient is greater in a system with a single layer. There is a visible effect of the mismatch of coating and substrate mechanical properties, which was also shown in [A4] for systems with E C /E S =1 3. Increased stress concentration on the surface of the outer ceramic layer in the multilayer and lower mean pressure in the contact area means that the first cracks are formed at deformations similar to a single ceramic coating. If such a crack appears in a single coating, characterized by low fracture toughness, it propagates through its whole thickness. Meanwhile, such a crack in a multilayer propagates only in the outer ceramic layer, and then is sealed, closed in the metal layer as a result of plastic deformation. This crack can be formed in the next ceramic layer but the load must greatly increase, because at the moment of outer layer fracture the tensile stress in the second ceramic layer reaches 50 to 60% of the outer one. Analyzing the properties of ceramic/metal multilayer coatings, I also drew attention to the effect of metal layer plasticity on stress reduction in the ceramic layers [A8]. I conducted analyses by comparing the results of the corresponding FEM models with perfectly elastic or elastic-plastic properties of metal layers, assuming the value of yield stress measured for a single Ti coating. The local plasticity of metal layers enables small relative motion of ceramic layers. The deformation mechanism called "interlayer sliding", which limits the bending of ceramic layers, reduces tensile stress by about 10 to 20%, and therefore increases critical load caused multilayer fracture. These results were confirmed by SEM analysis of TiN and 8xTi/TiN surfaces after indentation tests. The research program on ceramic/metal multilayer coatings allows me to know their mechanical and tribological properties, which is a base for development of a new generation of coatings with pre-planned properties. Multilayer coatings that exhibit optimal properties have a bilayer period in the range of tens of nanometers and a higher amount of ceramic phase 14

15 than metallic ones e.g. 1:2. On the other hand, it is extremely important that the cracks closing in metal layers cannot be thinner than 30-40nm. The developed multilayer coatings resulted in attempts to apply them in biomedical elements within the projects [G9-G11]. These results also allow further studies into new groups of multilayer coatings, like ceramic/carbon (TiN/a-C:H), within the ongoing project [G11]. Nanocomposite coatings The second group of modern tribological coatings, which can meet high hardness and low elasticity modulus, are nanocomposite nc-mx/a-mtr coatings. The nc-mx usually means nanocrystalline particles of carbides or nitrides of transition metals (WC, TiC, TiN), and the a-mtr means the amorphous matrix (a-c, a-c:h, a-si3n4). The high hardness of such coatings, resulting from the severe fragmentation of the ceramic phase (5-10nm), is accompanied by significant crack resistance caused by the separation of solid particles in the soft matrix. Such a combination of mechanical properties results in high wear resistance. The conditions for improving the properties of nanocomposite coatings are small particle size up to 10nm, their separation by the amorphous matrix and the strong connection between the particles and the matrix. I performed research and analysis of the mechanical properties of nanocomposite coatings within the KomCerMet project [G7]. Its main aim was to develop a group of nanocomposite coatings based on carbon a-c and a-c:h matrices. Coatings were deposited by magnetron sputtering at the Technical University of Łódź in a laboratory led by Prof. B. Wendler. Coatings containing WC, TiC, CrC or TiN nanograins, introduced into carbon matrices, were deposited by the application of different targets. Microstructure studies using SEM, TEM, XRD, and other techniques were conducted in the Laboratory of Microscopic Examination, the Department of Materials Engineering and Industrial Computer Science, AGH, led by the group of Professor A. Czyrska- Filemonowicz. The studies I conducted helped to determine the effect of the microstructure of coatings, especially the material of nanoparticles and their amount on the mechanical and tribological properties of coatings. A wide range of tested material combinations allowed the selection of two coatings with optimum properties, nc-wc/a-c:h and nc-tic/a-c:h. These coatings are characterized by an average hardness of 15 to 20 GPa and an elasticity modulus of GPa. Taking into consideration the potential application of these coatings, the main advantage is the low value of the second parameter elasticity modulus, similar to steel and titanium alloys. In [L38, L41] I presented the microstructure and mechanical properties of nc-wc/a-c and nc-wc/a-c:h coatings. Microscopic analysis revealed the existence of 2-6nm size WC, β-wc 1-x and W 2 C nanoparticles, 6-14% volume fraction, embedded in carbon matrices. Introduction of these nanoparticles caused a slight hardness increase from 17.1 GPa for a pure carbon coating, a-c, to 18.4 GPa for an nc-wc/ac coating and improved adhesion to the substrate, the critical load was two times higher for the nanocomposite coating. The most advantageous aspect was the several times higher wear resistance. The extremely low wear index of 0, [mm 3 /Nm] evaluated by ball-on-disc testing is rarely presented in the literature. These coatings are also characterized by a low μ=0,05 0,07 coefficient of friction, which also indicates their 15