ICMCTF2006 Session E1-2: Friction and Wear of Coatings I: Lubrication and Surface Effects

Tuesday, May 2, 2006 1:30 PM in Room California

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		E1-2-1 Mechanical and Tribological Characterisation of nc-(Ti_1-xAl_x)C_y / a-C Coatings Deposited Using Magnetron Co-Sputtering. M. Carlsson, O. Wilhelmsson, E. Lewin, U. Jansson (Uppsala University, Sweden); U. Wiklund (Angstrom Laboratory, Sweden); S. Hogmark (Uppsala University, Sweden) Coatings providing functional surfaces are constantly gaining in interest. A functional surface in this case means suitable hardness and elastic properties that influences friction and wear. This can be achieved with design of composition and deposition parameters resulting in functional nanocomposites, nc-coatings. This paper characterises and evaluates nc-(Ti_1-xAl_x)C_y / a-C deposited using magnetron co-sputtering. The Me:C ratio was varied to give a carbon matrix with content ranging from 0 to 85% graphite, as measured by ESCA. The crystallite sizes ranged from 16 nm for pure nc-TiC to 2.5 nm for the nc-(Ti_1-xAl_x)C_y / a-C with the highest carbon content, as measured by XRD. As the Me:C ratio could be controlled, the coating hardness and Young's modulus could be tuned. This enabled tuning of coatings for specific elastic properties, i.e. adapted for different contact situations, friction levels and wear resistance. The amount of aluminium was shown to affect the level of graphitisation and the formation of a tribofilm in ball-on-disc testing, i.e. it affected the friction level and coating wear. For instance, it is possible to achieve hardness and friction coefficients ranging from 20 GPa and 0.28 for nc-TiC to 11 GPa and 0.06 for nc-(Ti_1-xAl_x)C_y / a-C with highest carbon content, respectively. FEG-SEM, Nanoindentation, RAMAN spectroscopy and light interference microscopy (WYKO) have also been used in this study.
1:50 PM		E1-2-2 Influence of Surface Finish of PVD Coatings on Tribological Performance in Sliding Contacts P. Harlin (Dalarna Univerisity, Sweden); P. Carlsson, U. Bexell, M. Olsson (Dalarna University, Sweden) The use of low friction PVD coatings to improve the tribological performance of components exposed to sliding contacts, e.g. forming tools and machine elements, has increased during the last years. In the present study the influence of surface finish on the tribological performance, i.e. friction and material transfer tendency, of two different commercial PVD coatings, TiN and WC/C, in sliding contact with ball bearing steel has been evaluated using two different types of sliding wear laboratory tests. Post test characterisation using SEM/EDS, AES and ToF-SIMS was used to evaluate the prevailing friction and wear mechanisms and to explain the friction data obtained. The results show that the surface topography of the coating is of outmost importance in order to control the material transfer tendency and thus the friction characteristics in a sliding contact. The initiation points for material pick-up, resulting in material transfer, were in most cases found to be scratches and small irregularities on the coating surface. Once initiated, the material transfer tendency will increase generating a tribofilm at the sliding interface. Hence, in tribosystems were material transfer must be avoided the substrate surface topography as well as the intrinsic topography of the PVD coating must be kept low. The possibility for post coating deposition surface treatment in order to improve the tribological performance of PVD coatings will be discussed.
2:10 PM		E1-2-3 Influence of Surface Morphology on Friction and Wear of Multilayer Coatings R. Mertens, O. Massler, D. Nilsson, M. Esselbach, M. Grischke, A. Ravagni (Balzers AG, Liechtenstein) It is known, that the friction coefficient of a sliding contact depends strongly on the surface morphology and chemistry. Besides coating type and lubrication situation, especially the profile of the surface plays a leading role for the friction. This paper shows the effect of various pretreatment methods of surfaces on surface profile and tribological behaviour of DLC-coated surfaces under dry and mixed friction conditions in sliding contacts.
2:30 PM	Invited	E1-2-4 Nanotribology of Ultra-Thin Carbon Layers P.N. Lemoine (University of Ulster, United Kingdom) Amorphous carbon protective coatings are used in tooling components, data storage systems, microelectronic devices and biomedical implants. For many of these applications, the carbon film is extremely thin (10 to 50nm) and hence its tribological characterisation is a challenging task which will be examined in this paper. Firstly, we developed nanoindentation protocols for measuring hardness and Young modulus of ultra-thin films of hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (t-aC). The study encountered tip blunting events along with hard film/soft substrate and soft film/hard substrate indenting scenarios. These effects dramatically change the H and E values. This highlighted the need for a technique to extract the intrinsic film hardness and this was shown with the developed analytical hardness model. Secondly, nano-scratch measurements, secondary ion mass spectroscopy (SIMS), X-ray reflectometry (XRR) and low kV energy dispersive X-ray analysis (EDX) all showed a larger interfacial intermixing of carbon and silicon for the ta-C samples as opposed to a-C:H samples. This agrees with literature but the simultaneous confirmation from four independent surface sensitive techniques is novel, to the best of our knowledge. Thirdly, combining these surface sensitive measurements, we found that, for both deposition processes, the thick films are intrinsically denser and harder than the thin ones. The result is discussed in term of the grow processes. Finally, this study also outlined that in the case of a-C:H, Raman analysis and nanoindentation test different properties, a finding not formulated in the literature.
3:10 PM		E1-2-6 Superlow Friction Diamond-Like Carbon Coatings: The Effect of Hydrogen Content N.J.M. Carvalho, J-M Jacquet (Bekaert SA, Belgium); A. Cavaleiro (ICEMS - University of Coimbra, Portugal) Effective lubrication systems are required to face the increasing pressure from government regulations to reduce fuel economy and exhaust emissions, and increase durability of automotive engines. One way to improve fuel efficiency is to reduce the mechanical friction losses in engine components, which are responsible for the major portion of the energy consumption within the internal combustion engine. The deposition of diamond-like carbon coatings (DLC), and in particular hydrogenated amorphous carbon (a-C:H) films on mating and/or reciprocating surfaces is a strong alternative to decrease the friction and improve the wear resistance of the tribological components. As a result of systematic studies and application of advanced chemical vapor deposition methods over the last years, our research team has developed a-C:H coatings for valve train components having friction coefficients as low as 0.05, under dry sliding against steel in air environment, in combination with high hardness and adhesion. This paper discuss the effect of hydrogen content in the coating on the tribological behaviour and oxidation kinetics. The relationship between the hydrogen content, surface properties, tribological behaviour under different dry ball-on-disc conditions, chemistry of the transfer film, and mechanical properties is investigated to contribute to the understanding of friction and wear phenomena of DLC coatings. The influence of hydrogen on the oxidation behaviour of the coatings was studied using thermogravimetric and differential thermal analyses. Their mechanical properties and structural modification were evaluated as a function of the annealing temperatures.
3:30 PM		E1-2-7 Tof SIMS and XPS Characterization of Worn DLC Surfaces Grown in Various Gas Plasma Mixtures in the Presence of Humidity and Oxygen O. Eryilmaz, A. Erdemir (Argonne National Laboratory); J. Lee, I. Petrov (University of Illinois at Urbana-Champaign) SIMS has been used to gauge the near surface chemistry of hard-disk drives and a few other tribological surfaces in the past. However, 3D imaging has not yet been employed in the characterization of tribological surfaces. In this study, we have attempted to use this capability to understand the relationship between surface chemistry and tribology of diamond like carbon (DLC) films grown in gas discharge plasmas with varying hydrogen-to-carbon (H/C) ratios. Specifically, these DLC films are grown in acetylene, methane and methane-hydrogen gas plasma mixtures having H/C ratios of 1 to 10. Using Raman, and x-ray photoelectron spectroscopy we attempted to analyze the near surface chemistry and microstructure of surfaces of sliding contacts. Finally we have correlated these findings with changes in friction and wear of those DLC films. Tribological tests were run in a ball-on-disk machine under a 2 N load in dry and moist nitrogen and oxygen environments. Based on the tribological and surface analytical findings, a mechanistic explanation is provided for the friction and wear of various DLC films in dry/humid nitrogen and oxygen environments.
3:50 PM		E1-2-8 Tribology of WC/C Coatings for Use in Oil-Less Piston-Type Compressors T.A. Solzak, A.A. Polycarpou (University of Illinois at Urbana-Champaign) Compressors have been operating at increasingly severe conditions in order to raise efficiency, necessitating them to function under less than fully lubricated conditions. With the use of "environmentally friendly" HFC refrigerants, which deteriorate natural protective surface "films," materials with enhanced tribological properties have become necessary. Extensive research has been conducted on the tribological performance of hard coatings, though very little in compressor simulated environments. In this study, controlled pin-on-disk experiments, imitating the wrist pin-connecting rod interface, were performed using a High Pressure Tribometer. The oscillating wear of the system was simulated through the use of a reciprocating disk in contact with a cylindrical pin at a constant load, oriented to achieve a line contact. Specimens used for these experiments included coated, 52100 steel wrist pins and uncoated, sintered iron disks. Coatings were provided by two leading manufacturers and consisted of single layer WC/C, multi-layer TiAlN + WC/C, and multi-layer WC/C + DLC. Dry and starved lubrication experiments were carried out without coatings to obtain a basis of comparison, while all experiments involving coatings remained dry. Those with coated pins were able to attain very high loads (contact pressures in excess of 1 GPa) while achieving friction coefficients below 0.1 without scuffing failures, but were subject to large plastic deformation of the sintered iron disks. The performance of the interfaces at several lower, constant loads in the presence of different refrigerants, including R134a, CO₂, and R600a, were compared with the use of nanomechanical property measurements, energy dispersive x-ray microanalysis, and surface profilometry. Based on the research presented in this work, it is concluded that coatings will be an essential component of oil-less compressors.
4:10 PM		E1-2-9 Tribological Behavior of CrN/WN Multilayer Coatings Grown by Ion-Beam Assisted Deposition Y.-Z. Tsai, J.G. Duh (National Tsing Hua Univerisity, Taiwan) CrN monolayer coating and CrN/WN multilayer coatings were deposited on the silicon (100) substrate by ion-beam assisted deposition process. The bilayer period of these coatings was controlled in the range between 3nm to 30nm. The wear resistance of CrN/WN multilayer coatings and CrN monolayer coating was investigated using a pin-on-disc tribometer. The surface roughness of the coatings was evaluated by atomic force microscopy (AFM). The difference of wear morphology and microstructural characterization between the CrN/WN multilayer coating and the CrN single layer coating was investigated by optical microscope (OM) and scanning electron microscopy (SEM), respectively. Owing to the dense structure and smooth surface roughness, the CrN/WN multilayer coatings exhibited better wear resistance in friction coefficient and wear rate. The wear mechanism of CrN/WN multilayer coatings and CrN monolayer coating was proposed and discussed.
4:30 PM		E1-2-10 Characterisation and Erosion Behaviour of NiCrAlY Coating Produced by Plasma Spray Method on Two Different Ni-Based Superalloys S.B. Mishra, S. Prakash, K. Chandra (Indian Institute of Technology Roorkee, India) Degradation of materials due to solid particle erosion, either at room temperature or at elevated temperatures is encountered in a large variety of engineering industries. In such environments protective coatings are frequently used. Nickel-based coatings are used in applications where wear resistance, combined with oxidation or hot-corrosion resistance is required. In the present investigation, Ni-22Cr-10Al-1Y coating was deposited on Superni 75 and Superni 600 (Ni-based Superalloys) by a shrouded plasma spray process. The coatings have been characterized for porosity, microhardness, microstructure, surface roughness and X-Ray Diffraction (XRD). An electron probe micro analyzer (EPMA) was used to obtain X-ray elemental mappings of the coatings along the cross-section. Erosion studies were conducted on uncoated and plasma-spray coated superalloy specimens using an air-jet erosion test rig at a velocity of 40 ms^-1. Scanning electron microscope (SEM) with EDAX attachment and optical profilometer were used to analyze the eroded surfaces. Ti, Fe and Mn have been noticed in small but uniformly distributed concentrations in the coatings, indicating the probable diffusion of these elements from the substrate. The NiCrAlY coating on Superni 600 gave the lowest erosion rate. The erosion was found greater during the transition period prior to achieving a steady-state erosion conditions for both the coatings. The possible erosion mechanisms are discussed.
4:50 PM		E1-2-11 Tribology of HVOF and APS Sprayed Nanostructured WC-Co Based Plasma Sprayed Coatings A.K. Basak, S. Achanta, J.-P. Celis (Katholieke Universiteit Leuven, Belgium); M. Vardavoulias (Pyrogenesis S.A., Belgium); P. Matteazzi (CSGI, Italy) The development of super hard materials where hardness increases dramatically by a nanosized composite structure dominate the wear resistance coating's market. Though different surface modification techniques are available for the synthesis of such metallurgical coatings and thin films,the use of thermal spraying technique seems to be more efficient and economical in term of coating deposition onto engineering components. The feasibility of building up thick (300 µm) nanostructured WC-Co based coatings by thermal spraying, mainly high velocity oxygen fuel (HVOF) and atmospheric air plasma (APS), from engineered nanostructured powders has been demonstrated in this work. Metallic and ceramic particles with crystallite sizes smaller than 20 nm were used as spraying materials, which were produced through Mechanomade process. An optimisation of the spraying parameters resulted in the deposition of dense and well-adherent coatings on different types of substrate materials with crystal sizes of around 20-30 nm with less than 1% porosity and 60-80 MPa bond strength with the substrate. Tribological properties of nanostructured coatings have been reported in literature to some extent but the understanding of the improved friction and wear properties of such coatings is not yet fully understood. Experimental data on friction, sliding wear resistance, resistance to abrasion, and corrosion-wear resistance of the thermal sprayed nanostructured coatings of interest in this work, were collected. These tests confirmed that the nanostructured coatings possess interesting tribological characteristics in comparison with commonly used industrial wear resistant materials. This investigation has clearly demonstrated that the proper selection of spraying parameters is the key factor to achieve a homogeneous distribution of phases in the coating which in turn results in an excellent friction and wear resistance.