Papers

ICMCTF2010 Session E1-1: Friction and Wear of Coatings: Lubrication, Surface Effects and Modeling

Thursday, April 29, 2010 1:30 PM in Room Pacific Salon 1

Thursday Afternoon

Start	Invited?	Item
1:30 PM		E1-1-1 Identification of Hard Turning Signature of Coated Tools on Cut Surface Anna Zawada-Tomkiewicz (Koszalin University of Technology, Poland); Mohamed El Mansori (Arts et Métiers ParisTech, France) This paper reports on the use of wavelets to identify the machining signature of coated tools. Both coated and uncoated CBN tools were experienced in hard turning (CBN tools).The idea is to assess the correlation between the changes in tool geometry (i.e. the quality of coating) and the induced surface integrity by hard turning. Indeed, the cut surface in hard turning process is different from conventional machining because of high strength of hardened steel and brittleness of CBN tools. The machined surface was then modeled as a set of non-stationary signals generated by uneven material removal process. The digital wavelet transform was hence developed to extract the features of the quality of tool coating. Different qualities of tool coatings (tool wear, tool chipping, tool breakage) were identified which exhibit different characteristics in signal components of the machined surface. The surface roughness increased indeed gradually with the increase of tool wear. The results show, in general, that the surface modification in term of integrity was reflected in both the static component (approximation component) and the dynamic one. The surface roughness changed suddenly when tool chipping or breakage was observed. Different form and size of tool coating change was exhibited in different dynamic components and created specific signature of the coating on the machined surface.
1:50 PM		E1-1-2 On the Early Stages of Friction of Tetrahedral Amorphous Carbon Thin Films Deposited by Femtosecond Laser Ablation Pierdomenico Paolino (Ecole Centrale de Lyon, France); Aurélien Sikora (Université Jean Monnet, France); Christelle Guerret-Piécourt, Julien Fontaine (Ecole Centrale de Lyon, France); Florence Garrélie, Christophe Donnet, Anne-Sophie Loir (Université Jean Monnet, France); Michel Belin (Ecole Centrale de Lyon, France) Tetrahedral amorphous carbon (ta-C) coatings are known for providing high hardness and good wear resistance, together with low friction coefficients in ambient air. Such paradox – high hardness, meaning difficult to shear, and low friction, meaning easy to shear – can be explained by the growth of a "tribofilm" or "third-body" on the sliding counterface. However, the role of surface layers on these interfacial materials is still unclear, especially the influence of sp²-rich top layer reported in the literature for some ta-C films. In this study, we are considering ta-C coatings deposited on 52100 bearing steel coupons by femtosecond laser ablation of a graphite target. Their tribological behaviour has been investigated with a reciprocating pin-on-flat tribometer, not only measuring friction coefficient but also electrical contact resistance. The ta-C surface, both inside and outside wear tracks, has been then characterized by electrically conductive Atomic Force Microscopy. Additionnal analysis of the wear tracks have been carried out by Scanning Electron Microscopy with Energy Dispersive X-Ray spectrometry, abd by X-Ray Photoelectron Spectroscopy. During friction experiments, a "running-in" period is systematically observed for few tens of cycles, with concomittant decrease of friction coefficient and increase of electrical contact resistance. The relatively low initial electrical contact resistance is explained by the presence of tiny "droplets" that appears to be more conductive on AFM electrical images than the smooth surrounding surface of the films. However, even a single sliding cycle affects these spots, making them much more resistive. In light of these results, the surface modification phenomena involved in such early stages of friction will be discussed, in order to account for the observed tribological behaviour, and for the growth of interfacial materials.
2:10 PM		E1-1-3 Recent Developments on DLC Films and Tribomechanical Characterizations Vladimir Trava-Airoldi, Polyana Radi, Fernanda Marciano, Lúcia Santos, Deiler Lima-Oliveira, Evaldo Corat (Instituto Nacional de Pesquisas Espaciais, Brazil) The ressearch on diamond-like carbon (DLC) films are growing and developing and is a completelly opened area to development of news composites with new properties. The challenge is to obtain films with very high hardness and adhrence and with high restistance to corrosion and wear. We developed in our ressearch group, at the first time, DLC films with extremally high hardness and very resistant to corrosion by incoporating nanocrystalline diamond (NCD) particles during the deposition process. Another ressearch challenge is on the characterization of these films, the usual experimental approach on tribomechanical characterization is to vary one parameter at time while keeping the others constant, thus measuring the influence of each variable. This approach requires more experimentation, ignores the interactions between the parameters and can lead to wrong conclusions. We develop, also at the fist time, a methodology that combine statistical and tribomechanical analysis in order to improve the film characterization. This paper presents the experimental/statistical tribomechanical studies on NCD-DLC composites. The factorial design was used to describe the overall effect on the friction and wear of the NCD particle size and test conditions such as load and sliding speed. The response surface methodology was used to develop a mathematical modeling of friction and wear of these films, by using the experimental results, in order to identify the parameters that control friction and wear and to obtain the equation that describes these parameters to construct the tribologic maps. Nano hardness was also measured as a part of the whole characterization.
2:50 PM		E1-1-5 Fretting Contact Study of Ti-6Al-4V/Carbon Couples in a Dry Shaft/Bearing Contact with Trust: Influence of a Plasma Nitridation of the Titanium Alloy Manuel Sylvestre, Hamid Zaidi, Jean-Paul Rivière, Dominique Eyidi (Université de Poitiers, France); Franck Doyen (Carbone Lorraine Composant, France) Fretting wear or wear by small displacements is defined when two contacting surfaces (“first-bodies”) are subjected to reciprocating motion of small amplitude in micron order. This phenomenon is observed in many mechanical assemblies and that can significantly reduce the life of the mechanisms This paper reports on the influence of a nitridation treatment of the titanium alloy on the fretting tribological behaviour of carbon/ Ti-6Al-4V couples in a dry shaft/bearing contact with trust. Two contact geometries are investigated: cylinder-in-cylinder and plan/plan. Both of them are subjected to low-amplitude oscillatory movements, with temperature reaching 270°C . The treatment of nitridation for the Ti-6Al-4V is a N₂-H₂ plasma nitridation at low temperature ( 700°C ), during 12 hours. In these conditions, the top surface hardness was improved by a factor three. In this study, the mechanism of transfer and wear of carbon against Ti-6Al-4V nitrided or not, have been studied with the scanning electronic microscope, the optical microscope and the interferential microscope. The morphological and profile analyses performed on rubbing surfaces showed various aspects of wear: prints, craters, transfer… This permits each degradation to be linked its location, development and origin. The friction couples recorded during the test have showed differences between the tests with the shafts with or without nitridation and especially at rise temperature. We discuss the experimental results and we suggest many possibilities in order to understand some specific tribological behaviours: impact of the third body, of the abrasion, etc.
3:10 PM		E1-1-7 Nanoscale Deformation Mechanism in the Sliding Contact of TMD-C Nanocomposite Coatings Tomas Polcar (Czech Technical University in Prague and University of Coimbra, Portugal); Albano Cavaleiro (University of Coimbra, Portugal) The reduction of friction continues to be a hot-topic in mechanical engineering. Lower friction between mechanical parts in contact diminishes the energy consumption, vibrations, noise, contact temperature, and wear. Among many ways to fight the friction, solid lubricants in the form of thin films deposited by physical vapor deposition methods are widely used in industrial applications. Transition metal dichalcogenides (TMD) are suitable as solid lubricants due to their anisotropic layered structure, where the adjacent lamellae with strong covalent bonding interact through relatively weak van der Waals forces. Pure sputtered TMD films are almost friction-less at ultra-high vacuum sliding conditions; however, the films are sensitive to environmental attack, particularly in the presence of oxygen and water, which limits their mechanical properties and wear resistance. Alloying of TMD with other element has improved their properties, such as adhesion, hardness and load bearing capacity. Nevertheless, the high sensitivity to environmental attacks still remains the main restriction for full the industrial use of TMD-based coatings as self-lubricants. In this talk, we will show the tribological behavior of different TMD coatings alloyed with carbon (WSC, WSeC, MoSC and MoSeC systems) deposited by magnetron sputtering. Three TMD microstructures have been prepared: i) randomly oriented platelets in amorphous carbon matrix, ii) nanograins of TMD in amorphous carbon matrix and iii) mixture of carbide and TMD nanograins embedded into the carbon matrix. Special attention has been paid to the analysis of the frictional and wear mechanisms under different operating conditions, such as contact pressure, air humidity or temperature. Nanoscale analysis of the wear track revealed the formation of a thin tribolayer exclusively consisting of TMD platelets oriented to exhibit the lowest friction. In some cases, the depth reorientation of the originally randomly oriented TMD platelets as a reaction to the sliding process has been observed. Such self-adaptation explains the extremely low friction coefficient together with a high load-bearing capacity; moreover, the films are much less sensitive to environmental conditions compared to pure TMD.
3:50 PM		E1-1-9 Growth, Structure and Tribological Properties of Atomic Layer Deposited Lubricious Oxide Nanolaminates Benedict Mensah, Hamidreza Mohseni, Thomas Scharf (University of North Texas) Oxides have been studied lately as good potential candidates for solid lubricants because they are thermodynamically stable and environmentally robust. Oxide surfaces are generally inert and typically do not form strong adhesive bonds like metals in tribological contacts. To this end, nanolaminate ZnO/Al₂O₃/ZrO₂ films were grown via atomic layer deposition (ALD) on high temperature M50 bearing steels. The lubricious, nanocrystalline ZnO layer was structurally-engineered to achieve low surface energy {0002}-orientated grains, which provided low sliding friction coefficients (0.2 to 0.3) and wear factors (1-3 x 10^-7 mm³/Nm). The amorphous Al₂O₃ layer played an important role in achieving this preferred orientation, while {101}-orientated tetragonal ZrO₂ acted as a high toughness/load bearing layer. It was determined that the ZnO defective structure (oxygen sub-stoichiometric with growth stacking faults) aided in shear accommodation by reorientating the nanocrystalline grains where they realigned to create new friction-reducing surfaces. Specifically, high resolution transmission electron microscopy (HRTEM) inside the wear tracks revealed the presence of basal stacking faults and partial dislocations which aided the shear accommodation during the interfacial sliding between the film and the ball counterface. This velocity accommodation mode was intrafilm shear where this friction-induced subsurface aided in shear accommodation and prevented brittle fracture. The solid lubrication mechanisms of these reorientated, plastically sheared ZnO grains will be discussed along with how ex situ annealing to 400°C affects structure-tribological property relationships. Environmentally-robust, l ubricious ALD ZnO/Al₂O₃/ZrO₂ nanolaminates are good candidates for providing low friction interfaces in moving mechanical assembles, such as fully assembled bearings, that require thin (~10-200 nm), uniform and conformal films.
4:10 PM		E1-1-10 Sophisticated New Analyzing and Simulation Tools for Scratch, Impact, and Wear Tests Marcus Fuchs, Nick Bierwisch (Saxonian Institute of Surface Mechanics, Germany) In the work it will be presented how the extended Hertzian theory and the Oliver&Pharr method extended for layered materials¹ can be applied to scratch tests, indentation measurements, and impact experiments in order to extract more physical information from them. The methods also work perfectly for multilayer and gradient coatings and in principle allow the extraction of the mechanical parameters of any of the constituents of even very complex coating structures. By taking into account, that with lateral forces additional boundary effects like indenter tilting and mixed loads of sticking-sliding areas are coming into play, a much more profound simulation and modelling of even the classical scratch test is possible. But it will be shown, that one can particularly benefit from such a more comprehensive and sophisticated analysis when using a new multi-path scratch method combined with additional classical nanoindentation measurements² as it can extract a lot more mechanical properties from such experiments. It will be shown how the true initial failure can be detected and clearly defined critical stress values can be assigned to corresponding constituents of the coating. Additionally, it will be shown how wear simulations based on an extended Archad's law and by means of both methods mentioned above can be used not only to predict the wear progress for contact situations of complex material structures, but also to extract wear parameters of any surface material. The method can also be used to quickly simulate wear behavior. Hence, on the one hand it enables one to determine the lifetime of the sample according to its supposed application and, on the other hand, to find out which modifications of the coating structure are necessary to enhance the surface and, therefore, its lifetime. These new methods will be demonstrated on a variety of examples in combination with new analyzing modules woven into the software package FilmDoctor³. ¹ N. Schwarzer: "The extended Hertzian theory and its uses in analysing indentation experiments", Phil. Mag. 86(33-35) 21 Nov - 11 Dec 2006 5153 – 5767 ² N. Schwarzer: "Advanced scan procedures – Poly-Scan Scratches", www.siomec.de/docs/2008/001 ³ FilmDoctor: Special version for "next generation surface tester analysis", www.siomec.de/FilmDoctor
4:30 PM		E1-1-12 Tribological Properties of Water Confined by Self-Assembled Monolayers Michael Chandross (Sandia National Laboratories); Christian Lorenz (King's College London, United Kingdom); Matt Lane, Gary Grest (Sandia National Laboratories) We report the results of large-scale Molecular Dynamics (MD) simulations of water confined to sub-nanometer thicknesses between both hydrophobic and hydrophilic self-assembled monolayers (SAMS). We vary the amount of water and the applied pressure to examine the effects on the structure and dynamics of the confined water. Calculations of two dimensional diffusion constants indicate that the water remains liquid-like in all cases. The water confined between the hydrophilic SAMS is subjected to shear to measure the viscosity and microscopic friction. We find that while the viscosity increases by as much as a factor of 60 for low coverage and high loads, there is still no evidence of ice-like layers being formed. Friction coefficients can only be calculated at high shear velocities due to the low viscosity of the water and are found to decrease with increasing amounts of water, similar to experimental results.
4:50 PM		E1-1-11 Multi-Scale Contact Modelling of Textured Surfaces Under Dry Sliding Conditions Ola Rashwan, Vesselin Stoilov (University of Windsor, Canada) Contact between rough surfaces has been drawing a lot of researchers’ attention since 1960’s since many engineering applications involving normal and siding contact, especially in the automotive components and metal forming dies. The effect of the different surface textures and surface topography on the friction and adhesion involved in the normal and sliding contact is the main concern. In this paper a multiscale model of textured rough surfaces was developed using a modulated two-variable Weierstrass-Mandelbrot function with fractal parameters determined from real surface images. A finite element model was used to analyze the contact between the textured surfaces of specified geometries, such as square and sine wave patterns with the surface roughness imposed on the top using the fractal geometry. The model yields insight into the evolution of elastic, elastic-plastic, and fully plastic deformation at the contact interface in terms of the maximum local surface interference. The obtained results showed strong dependence of the macro static friction coefficient on the minimum fractal length, emphasizing the intrinsic multiscale nature of the friction phenomena. In addition, it has been shown a significant effect of the modulated surface topology on the macro level adhesion and friction. Thus this approach provides a viable tool for exploring the macro-micro scale dynamics of adhesion and friction and the possibly for their control through surface patterning.