ICMCTF2007 Session E1-2: Friction and Wear of Coatings: Lubrication, Surface Effects and Modelling

Tuesday, April 24, 2007 8:00 AM in Room California

Tuesday Morning

Start	Invited?	Item
8:00 AM		E1-2-1 The Inclined Impact Test, an Efficient Method to Characterize Cohesion and Adhesion Properties of Coatings, Deposited on Bearing Rings K.-D. Bouzakis, A. Asimakopoulos, M. Batsiolas (Aristoteles University of Thessaloniki, Greece); G. Erkens (Metaplas Ionon, Germany) The impact test is applied for the quantitative assessment of fatigue, cohesive and adhesive properties of thin hard coatings, deposited on machine elements, cutting tools etc. A recent variation of this test is the inclined one, whereas the coated surface is loaded simultaneously with normal and tangential loads, allowing for the quick characterization of the film cohesion and adhesion. Up to now, the impact test is mainly used on coated specimens with simple geometries, as for example cutting inserts, coated plates etc. In the described investigations, inclined impact tests were conducted on PVD coated bearing rings. The tests were performed on the internal cylindrical surface of the outer bearing ring, as well as on the external one of the inner ring. The rings were fixed during the impact tests with the aid of appropriate holders. Through a developed FEM simulation of the contact between the indenter ball and the cylindrical ring surfaces during the inclined impact test, critical film stresses were determined, for predicting the film performance under real operational conditions of the bearings. The mechanical properties of the applied thin films and substrates, used in the FEM calculations, were detected by nanoindentations and FEM-based evaluation of the obtained results. Furthermore, the film fatigue, cohesive and adhesive behavior were investigated in dry, as well as in a lubricated contact environment.
8:20 AM	Invited	E1-2-4 Friction and Wear of Coatings - a Review and View into Future K. Holmberg (VTT Technical Research Centre of Finland) A thin film on a surface changes the surface properties and influences considerably on the friction and wear behaviour of a coated surface. This can and has been used with great success in tools, machine components, devices, optics, electronics, MEMS, bioimplants etc. Still there is a lack of good methods for proper surface design with coated components. The basic friction and wear mechanisms with coated surfaces are analysed on macro, micro and nano level. It involves the influence of mechanical properties of the surfaces, the dynamics of the chemistry on top of the moving surfaces and macro and micro geometrical effects. The basic friction mechanisms are adhesive and ploughing friction. Similarly the basic wear mechanisms are adhesive and abrasive wear. The influencing parameters are analysed and it is shown how surface deformation, fatigue and tribochemical wear are more related to changes in surface properties. The tribomechanisms of diamond and DLC coated surfaces are analysed and it is shown how scale effects can be used to explain the variation in friction and wear behaviour of the surfaces. The analysis is based on a large review of published data of the related coatings. It is further shown how FEM modelling and computer simulations can be used as a systematic approach to optimal surface design of coated components.
9:00 AM		E1-2-6 Fretting Wear of TiN and VC PVD Coatings Under Variable Environmental Conditions R. Rybiak, S. Fouvry (Ecole Centrale de Lyon, France); B. Wendler (Technical University of Lodz, Poland) TiN and VC hard coatings manufactured by a PVD method were selected and tested against a smooth polycrystalline alumina ball. The series of fretting tests with reciprocating sliding at a frequency of 5 Hz, 100 µm displacement amplitude, 100 N normal force and at wide ranges of relative humidity (10-90%) as well as of temperature (23-400°C) was carried out. The intensity of the wear process is well-known to be significantly dependent on the environmental conditions. In practice, most of the mechanical systems operate under variable environmental conditions. Hence, in this work a new methodology of the tests, resembling those real situations is proposed. A dissipated energy approach was applied to quantify the wear rates of the hard coatings. The approach gives stable prediction of wear kinetics only under constant relative humidity or temperature. A change of environmental parameters induces a modification of the third body rheology. This phenomenon was characterized by the evolution of wear kinetics associated with a significant variation of the corresponding energy wear coefficient. In presented work a composite wear law, integrating the wear energy coefficients as a function of humidity or temperature, is introduced. It permits a prediction of the wear response under variable environmental conditions. The stability of this approach is demonstrated by comparing various variable relative humidity and temperature sequences. Scanning electron microscope, transmission electron microscope as well as X-ray photoelectron spectroscopy analyses were performed in order to obtain information regarding evolution of the third body structures with environmental parameters.
9:20 AM		E1-2-7 Tribological and Mechanical Properties of CrN / TaN Heterostructure Thin Film Y.J. Kim, H.Y. Lee, T.J. Byun, K.S. Kim, J.G. Han, Y.H. Shin, Y.Z. Lee (SungKyunKwan University, Korea) While single layer coatings such as TiN and CrN have already found widespread use in tool industry to improve tool performance and to extend tool life time, they have some disadvantages such as low hardness, low thermal stability and corrosion resistance. Attempts to improve the coating properties include multilayered structure developed for superhardness, duplex properties combined with high wear and lubrication as well as long lasting wear and lubrication performance. However the basic understanding of tribological properties of multilayered coatings is still not established yet. In this study, the CrN / TaN heterostructure thin films which were superhard coating with lamination of hard transition metal nitrides were synthesized by Closed Field Unbalanced Magnetron Sputtering (CFUBMS). All the laminated layers were controlled the bilayer thickness (λ) ranging from 4.3 to 43 nm. The high hardness, high resistance to plastic deformation and the low value of friction coefficient were obtained by the multilayer with the bilayer period of 5.4 nm.
9:40 AM		E1-2-8 The Influence of Bias Voltage on Structure and Mechanical/Tribological Properties of Arc Evaporated Ti-Al-V-N M. Pfeiler (Materials Center Leoben Forschung G.m.b.H., Austria); K. Kutschej (Montanuniversität Leoben, Austria); M. Penoy, C. Michotte (CERATIZIT Luxembourg S.á.r.l., Mamer, Germany); C. Mitterer (University of Leoben, Austria); M. Kathrein (CERATIZIT Austria G.m.b.H., Austria) Titanium aluminium vanadium nitride has recently been described as a possible candidate for further development of titanium aluminium nitride based coatings.The aim of the present work was to investigate the influence of the bias voltage on structure, chemical composition, residual stresses and mechanical and tribological behaviour, in order to illuminate the possibility of tailoring the coating properties mentioned above. An industrial scale cathodic arc evaporation facility was used to deposit the coatings from powder metallurgically produced Ti_16.5Al₆₇V_16.5 targets. Coating structure and chemical composition have been studied using X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GDOES) and energy as well as wave length dispersive X-ray analysis (EDS, WDS). Coating hardness and tribological behaviour was determined using a nanoindenter, a high-temperature ball-on-disc tribometer as well as an optical white light profilometer. The residual stresses were evaluated by laser-assisted curvature measurements. With increasing bias voltage the structure changed from a dual-phase fcc+hcp to a single-phase fcc structure, resulting in higher hardness and compressive stress values increasing from 0.4 GPa to 1.3 GPa. Abrasive wear at room temperature increased with bias voltage from 0.37 to 1.08 x 10^-14 m³/Nm. At 500°C abrasive wear was low for 40 V and 80 V bias voltage. For the higher bias voltages, a tribologically formed layer was observed, which totally prevented abrasive wear. The results show a significant influence of the bias voltage on the structure developed during growth resulting in altered coating properties.
10:00 AM		E1-2-9 Adhesion of AA 5182 Aluminum Sheet to Nitrided and DLC Coatings at 420°C A.R. Riahi, A.T. Alpas (University of Windsor, Canada) Hot forming of aluminum alloy sheets is among the modern forming processes for industries such as automotive, aerospace, and packaging, due to low cost, high productivity, flexibility and ability to make complex shapes. A critical aspect, however; is the interaction of tool and aluminum surfaces. The gradual adhesion and accumulation of aluminum on the tool surfaces decreases the surface quality of the products. In this study the adhesions of aluminum to uncoated steel, non-hydrogenated DLC and ZrN coatings were investigated. The contact conditions of tool and aluminum sheet was simulated using a ball on disc configuration. The uncoated AISI 52100 Bearing Steel balls, DLC , and nitrided coatings on the balls were used to slide against AA5182 aluminum alloy discs under 0.2 N load and the sliding speed of 2 mm/s. The experiments were conducted at room temperature, 300°C and 420°C for a sliding distance of 2 m and the coefficient of frictions (COF) were recorded. Prior to each high temperature test, aluminum discs were heated to 300°C or 420°C and the coated balls were hold at 100°C to prevent the coatings oxidation at high temperatures. The contact surfaces and their cross sections were studied after each test, using Optical Surface Profilometer WYKO, SEM, EDS, and FIB. The COF of DLC coating compared to those of nitrided coatings and uncoated steel was less than 1/3 at room temperature, 2/3 at 300°C and equal at 420°C. The amount of material transfer to the contact surface of DLC, however, was so low that did not show significant effect on the surface roughness of the coating at all temperatures, while significant amount of Al was transferred to the nitrided and uncoated steel. The possible mechanisms behind the behaviours of the coatings will be discussed in the paper.
10:20 AM		E1-2-10 Influence of Contact Conditions on Tribological Behaviour of DLC Coatings B. Podgornik, M. Sedlacek, J. Vizintin (University of Ljubljana, Slovenia) By giving excellent frictional properties and greatly improving wear resistance of contact surfaces, diamond like carbon coatings (DLC) provide a great opportunity for improving performance and durability, and reducing frictional losses of mechanical systems. However, for the successful application of coated machine components, coatings have to perform adequately under dry and oil-lubricated conditions, with the majority of machine components still being oil-lubricated and operating under diverse contact conditions. In the case of metallic surfaces action of extreme-pressure (EP) and anti-wear (AW) additives, used to reduce friction and wear of contact surfaces, is well understood and described in detail. However, this is not the case for DLC coated surfaces, especially when it comes to the influence of contact conditions. The aim of the present investigation was to determine the influence of contact conditions on the tribological behaviour of boundary lubricated DLC coatings, when using additivated oils. Tests were performed under reciprocating sliding motion using ball on flat test configuration, with ball-bearing steel ball being loaded against a-C:H and WC/a-C:H coated discs, respectively. DLC coatings were tested in the contact pressure range of 1.0 to 3.0 GPa, sliding speed range of 0.01 to 0.15 m/s and oil temperature range of 20 to 200°C. Lubricants included in the investigation comprised pure PAO, and PAO mixed with commercial EP or AW additive. While phosphorous-based AW additives have only minor effect, sulphur-based EP additives were found to reduce friction and wear of W-containing DLC coatings through WS2-containing tribofilm formation. Furthermore, increase in contact pressure or temperature accelerates the process of friction reduction and results in lower friction while increase in sliding speed has the opposite effect due to more effective removal of generated frictional heat.
10:40 AM		E1-2-11 Influence of the Normal Load on the Tribological Behaviour of Graphite Powders M. Schmitt, K. Jradi, S. Bistac (Institut de Chimie des Surfaces et Interfaces, France) Owing to its good resistance, hardness and electrical conductibility in the basal plans, graphite is found in various tribological fields such as seals, bearings or electrical motor brushes. It was then essential to better understand the tribological behaviour of this material in order to optimise its use. In this context, different graphite powders (natural and synthetic) were tested in order to identify promising tribomaterials for braking use. The aim of this work is the determining of the mechanisms which are involved in the initiation and in the development of the transfer that occurs during the dry friction of the graphite/steel couple. Experiments were realized on a classical rotating pin-on-disc tribometer, in the ambiant, under various applied normal loads. Both the graphite pins and the film transferred on the steel discs were observed by Scanning Electron Microscopy and analysed by Raman and XPS spectroscopies. Tribological measurements indicated that the friction coefficient increased with the normal load for the lowest applied ones ; however, for the highest loads, a reversal of the friction behaviour as well as a modification of the transfer are noticed at a specific load. XPS measurements on graphite pins, after friction, showed that this observed decrease induced by the application of the critical load can be correlated with a lowest adsorption of oxygen and water vapour. Friction tests during only one lap were also carried out to study the very first steps of the transfer formation ; Raman spectroscopy analyses proved that the disorder of the graphitic strucuture appeared even during the very first moments of friction.