ICMCTF2009 Session E2-2: Mechanical Properties and Adhesion

Monday, April 27, 2009 1:30 PM in Room California

Monday Afternoon

Start	Invited?	Item
1:30 PM		E2-2-2 Parametric Study on the Behavior of a Film/Substrate System to Limit Crack Propagation by the Finite Element Method N.K. Fukumasu, R.M. Souza (University of São Paulo, Brazil); M. Ignat (SIMAP Grenoble INP, France) Coated systems under high tensile loads tend to present cracks that nucleate at the film surface and propagate perpendicular to the film/substrate interface. From experimental tests, it is possible to observe that some substrate materials have the capacity to prevent the propagation of film cracks into the substrate and along the film/substrate interface, due to high plastic deformation at the crack tip. In this work, this behavior was investigated by a parametric finite element analysis on some of the mechanical properties of the substrate and the thin film (Young’s modulus, yield stress and fracture toughness). Systems were composed of an elastic thin film onto a perfectly plastic substrate and the numerical analyses considered that both were perfectly bonded. Tensile loads were applied perpendicular to the crack path and crack propagation was included in the analysis. Results allowed correlating some of the parameters that were studied with the ability of the substrate to limit the tendency for crack propagation along the film/substrate interface.
1:50 PM		E2-2-3 Nano-Impact – a Reliable Tool to Predict Coating Performance B. Beake (Micro Materials Ltd, United Kingdom) An advanced nanomechanical test technique, nano-impact testing, can simulate the interrupted contact (and cyclic loading) conditions occurring in severe contact applications and evaluate the fatigue fracture resistance of coated components. Essentially a fast repetitive nanoindentation technique, nano-impact was developed to test coating properties at high strain rates and to investigate surface fatigue and fracture due to repetitive contact with the aim of using results to optimise the coating properties for improved durability. A commercial nanoindentation system [NanoTest system, Micro Materials Ltd] is utilised to perform high cycle and low cycle fatigue testing by nano-impact. In contrast to conventional impact testing at the macro-scale, or instrumented micro-impact, nano-impact testing offers some distinct potential advantages for testing thin coatings, in throughput, automation, surface sensitivity. The traditional wear test method of periodically stopping the test to remove the sample and ex-situ assessment of any degradation limits mechanistic study. There is a growing realisation that the next generation of wear test instruments should be developed to enable the wear process to be studied in situ reflecting a shift in focus towards designing coatings for wear protection based on mechanistic understanding. Counter-intuitively, the nano-scale impact test can be more severe than instrumented micro-impact testing. The contact strains are higher with the smaller and sharper indenters and higher contact pressures are possible, which can result in more accelerated wear behaviour. Shorter tests become feasible, with the possibility of performing repetitive tests across samples and obtaining a more complete statistical picture of the impact response. Clear correlations are observed with actual performance testing. In particular the rapid, automated, laboratory nano-impact tests correlate well with the longer, more laborious and expensive tests of (1) cutting tool life, ranking coating performance in terms of tool life in end milling and simulating the evolution of the tool wear (2) coating wear in high performance engine applications. As its usage grows, the laboratory nano-impact test will become a successful way to speed up the pace of new coating development for interrupted contact applications. The principles and applications of the nano-impact technique are reviewed in this presentation.
2:10 PM		E2-2-4 Adhesion of Diamond Like Carbon (DLC) Coatings on Metalic Biomedical Implants C.V. Falub, R. Hauert, G. Thorwath, U. Müller, M. Parlinska-Wojtan, C. Affolter, P. Schmutz, J. Michler (Empa, Switzerland); M. Tobler (IonBond AG); C. Voisard (Synthes GmbH) Owing to their excellent mechanical properties, e.g. high hardness, low friction coefficient, low wear rate, diamond like carbon (DLC) coatings are excellent candidates for surface engineering, such as protective films for metalic biomedical implants. However, interface chemistry and relatively high intrinsic stresses (~Gpa) can determine unpredictable in-vivo delamination of these layers, which may lead to the total failure of the implant. The speed of delamination, which in fact influences the lifetime of the coated implant, can range from ~ mm/day to below µm/year. While fast delamination of the order of hundreds of µm/day can easily be detected, very slow delaminations of the order of a few µm/year are very difficult to observe. Nevertheless, several delaminating microscopic spots growing with a speed of a few tens of µm/year may cause the failure of the coated implant after several years. Therefore, in order to estimate the lifetime of a coated implant in the human body it is extremely important to have a reliable quantitative method for determining the speed of coating delamination. Although qualitative studies of the mechanisms involved in DLC coated implant failure exist, to our knowledge no quantitative analysis of coating adhesion lifetime on biomedical implants has ever been done and that is the aim of this work. The discussion will tackle both fundamental physical and chemical aspects at the DLC/implant interface, but also fracture mechanics analysis using finite element modelling (FEM). In-vivo stability of different interlayers will be discussed in conjunction with electrochemistry experiments as well as mechanical failure tests.
2:30 PM	Invited	E2-2-5 A Micro-FEM Modelling Based Fracture Mechanics Approach to Wear Resistance Assessment of Thin Hard Coatings K. Holmberg (VTT Technical Research Centre of Finland) Thin hard coating has been implemented successfully in several industrial applications. However, there even larger use is still limited by surface fracture, wear and the difficulty of lifetime prediction and control. A new approach is presented that is based on finite element modelling of the coated surface on micro level, simulation of critical stresses and strains and calculation of fracture toughness of the surface. TiN, MoS₂ and DLC coatings on steel substrate with and without bond layers have been investigated in a sliding ball against plane contact geometry both empirically and by modelling. The influence of residual stresses on the surface cracking is shown and the crack growth both vertically through the coating as well as horizontally as interface cracking between the coating and the substrate resulting in coating delamination is analysed. A parametric analysis of the influencing parameters has been carried out and gives guidelines for improved control of the cracking and wear process.
3:10 PM		E2-2-13 Structural and Mechanical Properties of Graded and Multilayered Al_xTi_1-xN/CrN Coatings Synthesized by a Cathodic-Arc Deposition Process Y.-Y. Chang, C.-P. Chang, C.-Y. Hsiao, D.-Y. Wang (Mingdao University, Taiwan) Graded Al_xTi_1-xN and multilayered Al_xTi_1-xN/CrN coatings were synthesized by cathodic-arc evaporation with plasma enhanced duct equipment. Chromium and TiAl alloy cathodes were used for the deposition of Al_xTi_1-xN/CrN coatings. During the coating process of graded Al_xTi_1-xN, an Al_0.63Ti_0.37N top layer was deposited on an interlayer of Al_xTi_1-xN/CrN, which was obtained by regulation of cathode power. With different cathode current ratios (Al_0.67Ti_0.33N/Cr) of 0.75, 1.0, and 1.25, the deposited multilayered Al_xTi_1-xN/CrN coatings possessed different chemical contents and periodic thicknesses. The nanolayer thickness and alloy content of the deposited coating were correlated with the emission rate of alloy cathode materials. In this study, field emission scanning electron microscope (FESEM), and X-ray diffraction using Bragg-Brentano and glancing angle parallel beam geometries were used to characterize the microstructure and the residual stress of the deposited films. High resolution transmission electron microscope (HRTEM) and scanning transmission electron microscope (STEM) were used for nanolayered structure analyses of the multilayered Al_xTi1-xN/CrN coatings. The composition of deposited graded Al_xTi_1-xN and multilayered Al_xTi_1-xN/CrN coatings were evaluated by a wavelength dispersive X-ray spectrometer (WDS). Hardness, Young’s modulus and fracture toughness of the deposited coatings were determined by nano-indentation and Vickers indentation methods. The effect of alloy content (Al, Ti, and Cr) on the microstructure and mechanical properties of Al_xTi_1-xN/CrN coatings were studied.
3:30 PM		E2-2-9 Effect of Aging on Adhesion of Black Anodic Coatings for Space Applications Y. Goueffon (CNES, France); C. Mabru (Universite de Toulouse, ISAE, France); L. Arurault (Universite de Toulouse, CIRIMAT-Institut Carnot, France); C. Tonon (Astrium, France); P. Guigue (CNES, France) Due to the space vacuum, thermal regulation of spacecrafts is passively managed by radiative exchanges between its external surfaces and the environment. Black inorganic anodized aluminium alloys are often used for their thermo-optical properties (αs≥0.93; εn≥0.9). However, many cases of flaking have been observed after thermal cycles carried out to simulate the space environment. In orbit, such particles could contaminate instruments of the satellite. The aim of this study is to evaluate the influence of the ageing on such films adhesion. The process used is a sulphuric anodizing, followed by an inorganic colouring and a sealing. It has been shown that the porosity and the thickness of the anodic film have a major influence on the mechanical behaviour during adhesion measurement performed by scratch tests and 4-point bending tests. Black anodized samples with different porosities have been submitted to thermal cycles. The influence of the pressure and temperature on the adhesion has been observed. Two main mechanisms have been highlighted: the difference of thermal expansion coefficient between film and substrate and the dehydration of the film. Finite element modelling illustrated the mechanical state at the interface between the substrate and its cracked or non cracked anodic film during thermal loads (process and cycling). Particularly, the presence of cracks can either be detrimental or beneficial on the interface loading depending on the mechanical properties of the film which change with the porosity.
3:50 PM		E2-2-10 Effect of Hydrogen on the Mechanical and Structural Properties of SiC-Derived Carbon Films D.-S. Lim, H.-J. Choi (Korea University, Korea); M.J. McNallan (University of Illinois at Chicago); Y.H. Sohn (University of Central Florida) SiC-derived carbon films have been produced by etching silicon carbide in mixed gas environment containing chlorine and hydrogen^1-2. Hydrogen plays a crucial role in the SiC derived carbon films. But the detailed role of hydrogen on the carbon formation and its structure and properties is still not well understood. In this study, the effect of hydrogen content on the structure and mechanical properties of carbon films modified from sintered SiC by mixtures of chlorine–hydrogen gas at 1000 °C for 20 hours was investigated. Based on the structural analysis of carbon films produced, conversion from silicon carbide proceeded faster with increasing chlorine content at a high temperature. The rate of carbon formation determined by SEM observation was decreased with increasing hydrogen content. The hardness and elastic modulus of the carbon films tended to decrease with increasing hydrogen content. By varying the concentration of hydrogen, the ratio between the crystalline graphite and amorphous carbon was changed. The different bonding types and crystallinity of the carbon films directly correlated with their mechanical properties. The possible mechanism for the lowering mechanical properties has been discussed based on the analysis of HREM and Raman spectroscopy observation. ¹A. Nikitin and Y. Gogotsi, “Nanostructureed Carbide-Derived Carbon,” Encyclopidia of nanoscience and nanotechnology, Vol. 7 p 553-573 (2004), ² Hyun-Ju Choi, Jeon-Kook Lee, and Dae-Soon Lim, “Tribology of Carbon Layers Fabricated from SiC under the Different H2/Cl2 Gas Mixtures,” Accepted for Publication in Journal of Ceramic Processing Research (2008).
4:10 PM		E2-2-11 Tensile Testing of Amorphous Diamond-Like Carbon and Crystalline Diamond Coatings J. Schaufler, K. Durst, K. Kellermann, S. Rosiwall, M. Göken (University Erlangen Nürnberg, Germany) The research efforts on carbon coatings have strongly increased in the last decade. Many investigations, especially in the field of improvements in the deposition processes and the tribological behavior of the coatings under high loading conditions have been done. But up till now, a clear and broad understanding of the basic deformation and failure mechanism under elementary loading conditions has not been achieved. Of great importance is the interaction between the coating and the substrates with regard to the occurring failure mechanism. In this work the failure and delamination behavior of diamond-like carbon coatings and crystalline diamond coatings, both deposited to a thickness of 2 µm on steel substrates, were investigated under tensile loading. In both systems a chromium-chromiumcarbide transition layer was used to enhance the adhesion between the coating and the steel substrate. The two coating systems were deposited with two different deposition processes (DL C system: combined PVD-PECVD, diamond system: Hot filament CVD), therefore the width of the adhesion layers varies between 500 nm (DLC system) and 20 µm (diamond system). The coated specimens were deformed under tensile loading conditions with a micro tensile testing equipment. In-situ tensile tests in a SEM allow the analysis of the damage evolution in the coated systems. By comparison with the stress strain behavior of the uncoated steel substrates, the mechanical stresses within the coatings are derived allowing a better understanding of the failure behavior of the coated systems The investigated coating systems show a complete different failure behavior under tensile loading. In the DLC coating cracks occur after strain values of around 1.7%. These brittle cracks grow perpendicular through the adhesion layer to the surface of the steel substrate. Even after strains of more than 20%, the DLC coating still adheres to the substrate. Furthermore, after high deformation of th e specimens a formation of shear bands in the adhesion layer were observed with a TEM. In contrast, the crystalline diamond coating showed a spallation after strain values of 1% and large regions of the coating delaminated at the interface chromiumcarbide-diamond.
4:30 PM		E2-2-12 The Mechanical Strength of Micro-to-Nanoscale Porous Ag Coatings A. Jankowski, H. Ahmed (Texas Tech University) The use of porous metal coatings is ever increasing in renewable-energy system applications as solar cells and hydrogen fuel cells. In particular, the scale of porosity in metal coatings is particularly important to their catalytic performance. Potentially just as important is the mechanical stability of the porous coating in these devices. A series of rate-dependent deformation tests are now conducted to better understand operative deformation mechanisms in the evaluation of strength as the porous support structure changes across multiple length scales, i.e. from the micro-to-nano. Tensile testing is used to evaluate commercially available, free-standing silver membranes with constituent micron-to-submicron porosity. Scratch testing of porous silver-coated substrates permits evaluation of nanoscale porous structures. Preliminary findings indicate that the strain-rate sensitivity of tensile tested specimens is found to increase as length scale decreases. The trends are similar to those experimental results reported for bulk nanocrystalline metals. Underlying structural features that can contribute to this mechanical behavior include pore size, filament or strut size, and the grain size within. These features of length scale are evaluated, and then assessed for the scratch hardness behavior of nano-porous silver coatings. This work is supported through the J.W. Wright Endowment in Mechanical Engineering at Texas Tech University.