ICMCTF2006 Session E1-1: Friction and Wear of Coatings I: Lubrication and Surface Effects
Time Period TuM Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF2006 Schedule
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8:30 AM |
E1-1-1 Tribological Properties and Characterization of Pulsed Laser Deposited MoSx-WSey Composite Films
J.J. Hu, J.S. Zabinski, J.E. Bultman, J.H. Sanders, A.A. Voevodin (Air Force Research Laboratory) Lubrication with MoS2-based materials is excellent in vacuum/dry environments, but degrades in wet environments, where an increase in friction coefficient and a decrease in wear life occur. By incorporating additives into MoS2, for example, inorganic sulfides/oxides and metals, increased load-carrying capacity, endurance and environmental adaptability can be achieved. In this study, MoSx-WSey composite films were deposited using pulsed laser ablation for tribological investigations. The chemistry and microstructure of the films were characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The films showed a dense granular morphology by SEM observations and a significant sulfur deficiency by XPS measurements. A predominant hexagonal MoSx phase was observed in the films using XRD, SAED diffractions and high-resolution TEM. Chemical analyses by EDS and XPS verified both S and Se bonding in the films. Hexagonal layer structures and crystallization of the film were studied in details. The MoSx-WSey composite film exhibited a larger expansion along the c-axis (spacing between basal planes) than the individual MoS2 and WSe2 films grown by PLD using pure targets. Significant curvature of (002) basal planes was observed with high-resolution TEM. The existence of S vacancies and Se replacement on S sites were proposed to explain the bending of S-Mo-S layers (i.e., basal planes). The tribological properties of the PLD MoSx-WSey composite films were measured in dry and wet conditions using a ball-on-disc tribometer. The reduced friction was correlated with the increased crystallinity and increased separation of basal planes in the films. |
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8:50 AM |
E1-1-2 Au-MoS2 Nanocomposite Coatings: Tribology, Nanostructure, and Electrical Properties
J.R. Lince, H.I. Kim, P.M. Adams (The Aerospace Corporation); D.J. Dickrell, M.T. Dugger (Sandia National Laboratories) Nanocomposite coatings formed by cosputtering metals with solid lubricants like MoS2 have been extensively studied. Recently, it was discovered that increasing the metal content above 50 at% gave coatings that could operate effectively at contact stresses far below that previously used (i.e., below 1 MPa).1 The ability to use these films at lower contact stresses opens up new application areas, including electrical contacts. Previous studies of these coatings have been extended to include atomic force microscopy (AFM), X-ray diffraction, and electrical resistance measurements. The MoS2 nanophase was shown to be amorphous in the as-grown films. However, contact AFM analysis showed that a thin crystalline MoS2 layer forms at the surface during sliding, in agreement with macroscale tribological testing and surface analysis via Auger Electron Spectroscopy. Based on the widely different electrical resistivities of the Au and MoS2 phases, contact resistance measurements were used to elucidate the nanostructure of the films. Analysis of the contact resistance variations with Au:MoS2 ratio indicates that the Au particles are separated from each other by MoS2 within the films. Overall, the results have better defined the compositions useful for electrical contact applications, and also increased our scientific understanding of sputter-deposited solid lubricant films by showing clear correlations between their nanostructure, tribological performance, and electrical properties. 1 J. R. Lince, "Tribology of Co-Sputtered Nanocomposite MoS2 Solid Lubricant Films over a Wide Contact Stress Range," Tribol. Lett., 17(3) (2004) 419-428. |
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9:10 AM |
E1-1-3 Synthesis and Tribological Properties of Al2O3/Au/DLC/MoS2 'Chameleon' Coatings
C.C. Baker, J.J. Hu, A.A. Voevodin (Air Force Research Laboratory) Chameleon coatings are nanocomposite coating systems that adapt their tribological performance to changes in environmental conditions such as humidity and temperature. In this research we have investigated the tribological properties of a new nanocomposite 'chameleon' coating system that incorporates Al2O3 in an Au matrix with diamond like carbon (DLC) and MoS2 nanoparticle inclusions. The coating design approach included a formation of nanocrystalline hard oxide particles for wear resistance, their embedding onto an amorphous matrix for contact toughness enhancement, and inclusion of nanocrystalline and/or amorphous solid lubricants for friction adaptation to different environments. The coatings were produced using a combination of laser ablation and magnetron sputtering. Chemical and structural analysis of the coatings included x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy, and micro-Raman spectroscopy. Mechanical properties such as coating hardness and toughness were investigated using nanoindentation, scratch, and indentation adhesion tests. It was observed that Al2O3 is valuable in enhancing film toughness. The chemical analysis was used to ascertain a correlation between chemical bonding of species and frictional properties. Friction measurements were studied by cycling between humid air and dry nitrogen conditions, as well as tests at temperatures of 500oC. The graphitic carbon component imparted low friction in humid air, MoS2 was excellent for dry N2 conditions, and Au was valuable for low friction at elevated temperatures. This new coating system validates the universality of the 'chameleon' design approach. |
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9:30 AM |
E1-1-4 High Temperature Tribological Behavior of Self-Lubricating Mo-Se-C Sputtered Coatings
T. Polcar, M. Evaristo, A. Cavaleiro (ICEMS - University of Coimbra, Portugal) Transition metal dichalcogenides belong to the class of materials more developed for solid lubrication. However, the main limitation of these materials is the detrimental effect of air humidity causing increase in friction. In previous works, molybdenum diselenide has been shown as a promising coating retaining low friction even in very humid environment. In this work, Mo-Se-C films were deposited by sputtering from C target with pellets of MoSe2. Besides the evaluation of the chemical composition, the structure, the morphology, the hardness and the cohesion/adhesion, special attention was paid to the thermal behavior in both inert and oxidant atmospheres from the point of view of tribological characterization. The C content varied from 27 to 66 at.% which led to a progressive increase of Se/M ratio. As a typical trend, the hardness increases with increasing C content. The coatings were tested at temperatures up to 500°C on pin-on-disc tribometer. The friction coefficient, the wear rate and the dominant wear mechanisms were evaluated as a function of carbon content in the coatings. Tribological behavior was correlated to the structure and the mechanical properties of the coatings. |
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9:50 AM |
E1-1-5 High Temperature Tribological Behaviour of CrN-Ag Lubricating Hard Coatings
K. Kutschej (University of Leoben, Austria); C.P. Mulligan (US Army ARDEC-Benét Labs); D. Gall (Rensselaer Polytechnic Institute); C. Mitterer (University of Leoben, Austria); R. Kaindl (University of Innsbruck, Austria) The CrN-Ag composite coating system is a proper candidate to combine the hard, wear and corrosion resistant properties of CrN with a lubricating effect due to the incorporation of Ag. We have previously shown that Ag incorporation decreases the friction coefficient and increases the wear resistance of CrN coatings at room temperature, and that Ag segregates to and agglomerates on the coating surface at elevated temperatures. This paper shows the influence of this diffusion controlled mechanism on the high temperature tribological properties. The 5 µm thick coatings were prepared by magnetron co-sputtering of a Cr and Ag target in a pure N2 atmosphere, where the power of the Ag target was varied to incorporate different amounts of Ag (0, 12, 22 at.%) in the coating. High temperature ball-on-disc tests at 600°C showed a lowering of the friction coefficient from around 0.45 for the pure CrN coating to 0.3 for composite coatings. During sliding, Ag continuously diffuses to the coating surface and is removed in the wear track. However, the friction coefficient increases after a distinct sliding distance when depletion of Ag in the coating decreases its supply rate. SEM and Raman mapping show a Ag-deficient area around the wear tracks after a ball-on-disc sliding distance of 300 m. Moreover, XRD investigations show the formation of Cr-Ag-oxides on the coating surface which also reduce the lubricating effect of the Ag surface agglomerates. The surface roughness as well as the wear tracks after high temperature tests have been investigated by an optical profiler. It has been found, that the surface roughness increases whereas the wear track depth decreases with the incorporation of Ag. |
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10:10 AM |
E1-1-6 Thermal Stability of Reactive Sputtered Tungsten Oxide Coatings
N.M.G. Parreira, T. Polcar, A. Cavaleiro (ICEMS - University of Coimbra, Portugal) Recently the oxide materials have become more interesting for tribological applications because of their oxidation stability and low tribo-oxidation sensitivity. Tungsten oxide coatings are considered as promising candidates for tribological application at elevated temperature due to their ability to form oxygen deficient Magnéli phases leading to significant decrease of friction coefficient. To verify these theoretical assumptions, tungsten oxide coatings with different stoichiometry prepared by d.c. reactive magnetron sputtering were tested in situ at high temperatures. W-O coatings were deposited on the steel substrates. The evolution of the structure with the temperature was studied in situ and after annealing by X-ray diffraction (XRD). The mechanical properties were investigated by depth sensing indentation before and after annealing of the coatings. Morphology of coating surface, ball scars, wear tracks and wear debris were examined by scanning electron microscopy (SEM); the chemical composition was obtained by electron probe microanalysis (EPMA). Wear testing was done using a high temperature tribometer (pin-on-disc); the maximum testing temperature was 800°C. Evaluation of friction coefficient was measured at different temperatures and the wear rates of coatings and balls were determined. Three different materials were used as counter-parts: 100Cr6 bearing steel, Si3N4 and Al2O3 balls. The XRD results showed a strong dependence of the structure on the temperature and the environment. In the air atmosphere, the oxidation of W and β-W3O revealed at 600 and 800°C, respectively; only an increase in the crystallinity was observed in case of annealing of WO3. The friction coefficient was higher than expected, particularly at room temperature. The dominant wear mechanisms were described using analysis of the wear tracks and the shape and distribution of the wear debris particles. |
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10:30 AM |
E1-1-7 Lubricious Oxide Formation on AlCrVN Coatings by Vanadium Diffusion and Oxidation
R. Franz (University of Leoben, Austria); S. Dreer-Wilhartitz, B. Sartory (University of Innsbruck, Austria); P. Wilhartitz (Plansee AG, Reutte, Austria); V.H. Derflinger (Balzers AG, Liechtenstein); H. Hutter (Vienna University of Technology, Austria); C. Mitterer (University of Leoben, Austria) It has been shown that the incorporation of V into state-of-the-art coatings, namely TiAlN and AlCrN, can significantly improve their friction behaviour at high temperatures. This is due to the formation of so-called Magnéli phase oxides on the surface of the coating. These oxygen deficient phases with easy crystallographic shear planes may act as solid lubricants in the tribological contact. For example, the coefficient of friction of AlCrN at 700°C could be reduced from values of app. 0.8 down to 0.2 in ball-on-disc tests by alloying of AlCrN with V. Here, another property of the formed vanadium oxides, in particular V2O5, comes into play. The rather low bulk melting temperature of V2O5 of 685°C results in a liquid surface layer, causing a transition from a solid to a liquid lubrication regime. However, to provide an adequate supply of V from the coating bulk to the coating surface, the mechanisms of V out-diffusion and subsequent oxidation has to be studied in detail. Therefore Al0.68Cr0.10V0.22N samples were prepared by cathodic arc-evaporation and annealed in air at temperatures ranging from 550-700°C. The depth-resolved composition of the oxidised coating surface was than analysed by secondary ion mass spectrometry. It revealed, that with increasing annealing temperature the diffusion of V from the coating to the surface is enhanced resulting in an increased thickness of the oxidised layer. Furthermore, it is indicated that the resulting V vacancies in the bulk of the coating are in turn filled by Fe atoms diffusing out of the substrate leading to a weakening of the coating. The results obtained on V-diffusion and oxidation are in good agreement with the observed friction and wear properties of the coatings in the relevant temperature range. |
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10:50 AM |
E1-1-8 Effects of Relative Humidity on Tribological Properties of Boron Carbide Coating Against Steel
P.D. Cuong (Korea Institute of Science and Technology, Korea); H.S. Ahn (Seoul National University of Technology, Korea); E.-S. Yoon, K.H. Shin (Korea Institute of Science and Technology, Korea) Boron carbide (B4C) coatings of 100 nm thick were synthesized on silicon substrate by DC magnetron sputtering using B4C target with a mixture of Ar and methane (CH4 at 1.2 vol. %) as processing gas. In this paper, friction and wear properties of the coating were studied in relation to the effects of relative humidity (RH). Reciprocating wear tests using 3 mm diameter steel balls as counterpart were carried out at three different humidity of ambient environment. It revealed that the relative humidity significantly affected the tribological properties of boron carbide coating. The coefficient of friction was decreased approximately from 0.42 to 0.11, and 0.09 by the tests running at 5, 45 and 85 % RH, respectively. We have used a confocal microscopy to observe worn surfaces of the coating and scars on steel balls. Both the coating and the balls were significantly worn out even though boron carbide is much harder than the steel. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses were performed to characterize the chemical composition of the coating and worn surfaces. After 2000 sliding cycles, the test under RH of 5% showed the complete removal of the coating, whereas at 45 and 85% RH coating underwent 5000 cycles was remarkably remained. The presence of boron and carbon elements in some particular areas in wear scar of the balls denotes that there was a transferring material from coating to the counterpart steel ball during sliding. In high humidity environment, tribochemical reactions occurred between sliding surfaces forming boric acid on worn surface of coating and a soft layer on the ball surface. We suggest that both, the boric acid and the layer that formed on steel ball grew up with the increase of relative humidity may contribute to the low friction and resulted in the less wear of both coating and the ball. |
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11:10 AM |
E1-1-9 Tribological Behaviour of TiSiCN Coatings Tested in Air and Coolant
H. Xu, X. Nie (University of Windsor, Canada); E. Langa, R. Wei (Southwest Research Institute) Two TiSiCN coatings and one TiN coating as a comparison basis were investigated in this paper. The coatings were deposited on stainless steel substrates using a Plasma Enhanced Magnetron Sputtering (PEMS) process, a variation of the physical vapour deposition (PVD) technique. X-ray diffraction (XRD) was used to investigate the coating microstructures. The Hysitron Ubi 1 nanomechnical test instrument was used to test the hardness of the coatings. The sliding wear behaviour of these coatings was studied in air and coolant (Hangsterfer's S-500) by using a ball-on-disc tribometer. Alumina and aluminum balls were used as counterparts to evaluate the wear behaviour of the coatings. Scanning electron microscopy (SEM) with energy-dispersive x-ray (EDX) analysis was used to examine the wear tracks on the discs and the wear scars on the pins. Compared to the TiN coating, both TiSiCN coatings had lower coefficients of friction in the air. In the coolant environment, one TiSiCN coating showed both good wear resistance and low friction. It was observed that in the coolant the coating wear was reduced by 98% and the coefficient of friction decreased from 0.45 to 0.1 for the aluminum counterparts. The wear mechanism contributed to different behaviours was discussed. |
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11:30 AM |
E1-1-10 Interlayer Thickness Influence in the Tribological Response of Dual-Layer Coatings
G.A. Fontalvo (University of Leoben, Austria); J. Keckes (Materials Center Leoben, Austria); R. Daniel, C. Mitterer (University of Leoben, Austria) Multilayer coatings based upon single layers with different elastic properties often show better tribological and mechanical properties than monolayer coatings. The reasons for this behavior might be an increase in coating/substrate adhesion, an improvement of load support, a reduction of surface stresses or an improvement of the resistance against crack propagation. To throw more light on these effects, the present work deals with the influence of coating thickness and residual stresses in the tribological response of dual-layer model coatings consisting of CrN with a chromium interlayer with varying CrN/Cr-thickness ratio. Ball-on-disc and reciprocating sliding experiments were carried out in ambient air at room temperature using coated high-speed steel and silicon samples, and alumina balls as counterfaces. The mechanical stresses in the both layers generated during the tests were calculated with the software package Elastica, and the residual stresses were measured using both the curvature method and x-ray diffraction. Wear tracks on the samples were characterized using both scanning electron microscopy and an optical profiler. The results show that residual stresses play a determinant role in the tribological response of the coatings provided that the CrN layer thickness exceeds a critical value. |
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11:50 AM |
E1-1-11 Micro-Abrasive Wear of DC and Pulsed DC Titanium Nitride Thin Films with Different Levels of Film Residual Stresses
R.C. Cozza, D.K. Tanaka, R.M. Souza (University of Sao Paulo, Brazil) In this work, six specimens with titanium nitride (TiN) thin films and cemented carbide (WC-Co) substrates were analyzed in terms of their micro-abrasive behavior. These specimens were obtained in a previous work, in which film depositions were run varying parameters such as bias (0, -50 or -100 V), type of target power (DC or pulsed DC) and, in the cases were substrate bias was zero, substrate condition (ground or floating). As a result, the level of film residual stresses varied from specimen to specimen, in the range from 4 to 11 GPa (compressive). In this work, micro-abrasive tests were run on these six specimens, using balls of 52100 steel and an abrasive slurry with distilled water and silicon carbide particles with average particle size of 5 µm. Two levels of applied normal load and rotation speed were selected during the experiments. Results were analyzed in terms of the wear mechanisms observed at the worn surface and also in terms of the wear resistance, characterized by the diameter of the worn crater. It was observed that the level of film residual stresses affected the wear resistance of the specimens, which was also affected by other characteristics of the films, dictated by the differences in deposition parameters. |
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12:10 PM |
E1-1-12 The Influence of the Boron Content on the Tribological Performance of Ti-N-B Coatings Prepared by Thermal CVD
J. Wagner, D. Hochauer (Materials Center Leoben, Austria); C. Mitterer (University of Leoben, Austria); M. Penoy, C. Michotte (CERATIZIT Luxembourg S.à.r.l., Luxembourg); W. Wallgram, M. Kathrein (CERATIZIT Austria GmbH, Austria) The deposition of ternary hard coatings within the system Ti-N-B by thermal CVD is well-established in cutting industry. Nevertheless, only little work has been published and a comprehensive study on the influence of deposition parameters on structure and tribological properties is still missing. In this work Ti-N-B coatings were deposited on cemented carbide between 850 and 1000°C by thermal CVD using BCl3, TiCl4, Ar, N2, and H2 gas mixtures at atmospheric pressure with varying BCl3 partial pressures. The boron content in the coatings increased with increasing BCl3 partial pressure and decreasing deposition temperature. Contents up to 35 at.% B were measured by glow discharge optical emission spectrometry (GDOES). The microstructure was investigated by scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and transmission electron microscopy (TEM). Hardness values between 20 GPa for TiN up to 45 GPa for the highest boron contents were measured by micro-indentation tests. The tribological properties were investigated by dry sliding experiments against alumina balls between 25 and 600°C at ambient air using a high temperature tribometer. The wear tracks after ball-on-disc tests were investigated using an optical profiler, SEM, Raman spectroscopy as well as infrared (IR) spectroscopy. At room temperature, the friction coefficient increases from 0.75 for pure TiN to about 1.1 for boron contents of 4 at.%, while low friction values down to about 0.3 were found for the highest boron contents. The friction coefficient increases from 0.6 to 1.0 at 500°C and from about 0.5 to 0.9 at 600°C with increasing B content. At room temperature, the lowest wear rates were found for the highest boron contents. The opposite trend was observed at 500 and 600°C, where the presence of cracks and the consequent oxidation throughout the coating favors the wear at higher temperatures. |