ICMCTF2003 Session E1-1: Friction and Wear of Coatings

Thursday, May 1, 2003 8:30 AM in Room California

Thursday Morning

Start	Invited?	Item
8:30 AM		E1-1-1 Effect of Crystallographic Orientation on Wear Properties of MoS₂-Ti Coatings by Pulsed-dc in Nitrogen and Humid Air. I. Efeoglu, F. Bulbul, Ayhan Celik (Ataturk University, Turkey) A closed field unbalanced magnetron sputter ion plating (Teer-CFUBMSIP/550) system has been used for the deposition of MoS₂-Ti composite coatings on different types of steel substrates with using pulsed-dc. The mechanical and tribological properties of MoS₂-Ti coated composites onto selected steel substrates were characterized using microhardness test, friction-wear test under atmospheric conditions and dry nitrogen. Structural analysis was pointed out using X-ray diffraction and SEM-EMPA. MoS₂-Ti films showed a very dense structure with pronounced basal plane (002) orientation which is better for friction with more dense structure and good adhesion than the radom oriented or amorph MoS₂-Ti composite coatings. Oxide formation has been occurred rapidly due to the highest S/Mo ratio and the lowest Ti contect. The coefficient of friction may be attributed to the oxidation of MoS₂ at the wear scar to form MoO₃ that is known to cause a serious abrasive wear.
8:50 AM		E1-1-2 Investigation of the Structural Evolution of the Top Surface of Graphit-iC^TC and MoST^TM Coatings after Multi-pass Rubbing Tests. D. Mercs, N.M. Renevier, D.G. Teer (Teer Coatings Ltd, United Kingdom) Graphit-iC^TC and MoST^TM coatings are two amorphous solid lubricant coatings which exhibit very high load bearing capacity as well as exceptional friction and wear properties. After a short running in duration, during pin on disc or multi-pass tests, the friction coefficients against hard metallic or ceramic counterparts are respectively as low as 0.05 and 0.02 for Graphit-iC^TC and MoST^TM coatings. This behaviour is believed to come from a modification of the structure of the top surface of the coatings during the rubbing tests. An original unidirectional multi-pass procedure has been set in order to verify if the low friction coefficients recorded for both MoST^TM and Graphit-iC^TC coatings were coming from reorientation of their top surfaces. RHEED analysis of the coatings, deposited on Si (100) substrates, were performed before and after multi-pass test with a normal load of 20 N and confirmed the top surface reorientation of MoST^TM coatings after rubbing. As no clear evidences of top surface reorientation of Graphit-iC^TC coatings has been found after the rubbing tests, the wear debris has been analysed using transmission electron diffraction (TED) and were found to be polycrystalline graphite.
9:10 AM		E1-1-3 Tribology of Cosputtered Au-MoS₂ Composite Films at Varying Contact Stresses J.R. Lince (The Aerospace Corporation) Solid lubricant coatings for sliding electrical contact applications like slip ring assemblies have very different requirements from typical applications like ball bearings and cutting tools: they have significantly lower contact stresses and sliding speeds. Cosputtered metal-MoS₂ films have shown superior properties for high contact stress applications. However, low contact stresses have not been well-studied. We are optimizing the performance of Au-MoS₂ films, concentrating on films with Au contents greater than 50 at%, in order to maximize the conductivity of the material (typical films generally have less than about 20 at% metal). We evaluated film performance at widely different contact stresses, i.e., in pin-on-disk tests (~730 MPa mean contact stress) and in disk-on-disk tests (~0.3 MPa). In the pin-on-disk tests, Au-MoS₂ films with higher Au content (i.e., ~70 at% Au) performed poorly, exhibiting an average coefficient of friction (COF) of about 0.2 and premature failure. The low Au-MoS₂ films (i.e., ~50 at% Au) performed well, with a COF of 0.015, and exhibiting significantly longer wear life. In contrast, in the disk-on-disk tests, the high Au-MoS₂ films performed well, with a COF of 0.04, outperforming the low Au-MoS₂ films, whose friction rose to about 0.2 by the end of the test. This switch in tribological performance at different contact stresses will be explored using results of Auger Nanoprobe analysis comparing the unworn and worn films, as well as the transfer films.
9:30 AM		E1-1-4 Effects of Load and Humidity on the Friction Coefficient of Ti-doped MoS₂ Coatings. I.L. Singer, R.N. Bolster (US Naval Research Laboratory) The sliding friction behavior of Ti-doped MoS₂ coatings (MoST_TM from Teer Coatings Ltd.) was investigated as a function of load and relative humidity. Friction tests were performed on a ball-against-disk machine in dry (<2%RH) and humidified (40% and 90% RH) air. A stationary hardened steel ball was dead-weight loaded against the rotating coating; weights spanned 0.1N to 2.4N. In each test, the friction coefficient was allowed to stabilize, then the load was stepped from lowest to highest then back to lowest value; this schedule was repeated 6 to 8 times. In all cases, the friction coefficient decreased as load increased. Friction coefficients in humid air were always greater than those in dry air, at a given load. Friction data were analyzed with a Hertzian friction model and the friction coefficient, µ, fitted to µ = S₀/P + α, where S₀ is the pressure-independent shear strength, P is the calculated Hertzian pressure and α is the first order pressure dependent coefficient of S. S₀ values were 12 -18 MPa in dry air and 30 - 69 MPa in humid air; α values were 0.01 - 0.02 in dry air and 0.04 - 0.05 in humid air. Reasons for the Hertzian behavior and interpretation of the shear strength values will be discussed.
9:50 AM		E1-1-5 Atomic-scale Investigations of Friction at Amorphous Carbon Interfaces via Molecula Dynamics Simulations G. Gao, G.M. Chateauneuf, P.T. Mikulski, J.A. Harrison (US Naval Academy) Classical molecular dynamics simulations have been conducted using both the REBO and AIREBO potentials to investigate the atomic-scale friction and wear associated with the sliding contact of a hydrogen-terminated diamond (111) surface and amorphous carbon films. Numerous varieties of the films, with different thickness, different micro-structure (sp²-to-sp³) ratio), and different hydrogen content, have been generated with these potentials. Effects of the film thickness, film structure, hydrogen content, tribo-chemical reaction, and long-ranged molecular interaction, on mechanical and tribological properties of the system have been investigated. Information obtained from these simulation studies should be invaluable in explaining phenomena observed in the atomic-scale tribological experiments.
10:10 AM		E1-1-6 Low-friction Pulsed Laser Deposited TiAlN and TiAlCN Thin Films for Wear Protection J.M. Lackner, W. Waldhauser (Laser Center Leoben, Austria); R. Ebner (Materials Center Leoben, Austria); W. Lenz (Laser Center Leoben, Austria) Titanium-aluminium (TiAl) based hard coating systems show excellent tribological behaviour at room and elevated temperatures in air atmospheres. Compared to titanium nitride (TiN) lower wear rates are found for titanium-aluminium nitride (TiAlN) coating systems in metal cutting and polymer forming contacts at temperatures around 200 to 400°C. Further improvement of the friction and wear behaviour can be reached with titanium-aluminium carbonitride (TiAlCN) coatings at room temperature. In the present work TiAlN and TiAlCN coatings were deposited by employing the pulsed laser deposition (PLD) method. A pulsed Nd:YAG laser of 1064 nm wavelength was used for vaporization of TiAl targets in low-pressure N₂ or N₂ / C₂H₂ containing atmospheres at room temperature. The highly ionized metal vapour was then deposited onto the substrates (polished high-speed tool steels (AISI M2)). The coatings were characterized by light-microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and hardness tests. The results show that strong textured coatings with very smooth surfaces, very good adhesion to the substrate's surface and high hardness can be achieved by the PLD method. To demonstrate the industrial applicability of these coatings pin-on-disc tests at various temperatures were performed, pointing out the excellent friction and wear behaviour of TiAlCN coatings at room temperature and of TiAlN coatings at elevated temperatures.
10:30 AM	Invited	E1-1-7 Phenomena in Microwear Experiments on DLC and Me-DLC Coatings: Friction, Wear Fatigue, and Plastic Deformation K.I. Schiffmann (Fraunhofer Institute für Schicht- und Oberflächentechnik, Germany) AFM-based microwear experiments allow deeper understanding of mechanisms of wear on the microscopic scale due to a well defined single asperity contact, elimination of roughness effects, and the high lateral and vertical resolution in the micro- and nanometer range. In this talk results of linear oscillating microwear experiments on metal containing DLC- coatings, thin pure DLC-coatings, Si- and Si:O-doped DLC-coatings will be presented. The experiments have been performed using either a diamond tipped cantilever-based system or an electrostatic transducer system (Hysitron Inc.) in conjunction with a standard AFM. Diamond tip radii of less than or equal to 1µm and loads in the range of some millinewtons lead to contact areas of only 0.1-0.2 µm₂ and contact pressures in the range of 2 to 20 GPa. Under these conditions, for metal-DLC coatings (e.g. W-DLC) material fatigue on a nanometer scale can directly be observed and identified as an important wear mechanism. Furthermore, the columnar growth structure of the film and percolation of the metallic nano-particles inside the film, i.e. metal content and metal type, strongly influence the fatigue and wear resistance of the coatings. Oscillating microwear experiments on thin DLC, Si-DLC and Si:O-DLC coatings on glass substrates are analysed with regard to friction, wear and plastic deformation. The friction coefficient µ can be understood in terms of a combination of Hertzian elastic contact and an additional ploughing term µ = c₁F^-1/3 + c₂Fn (F= load) were n strongly changes during the first wear cycles. Comparison of residual wear depth and residual indentation depth shows that the wear volume may completely be due to plastic deformation and not to a real material loss. In other cases material loss and plastic deformation both contribute to the observed wear volume. Therefore, evaluating only residual wear marks may lead to a misinterpretation of nanowear results.
11:10 AM		E1-1-9 New Concepts of Graded Zirconium Carbide Coatings for Components K. Bobzin (Materials Science Institute (WW), Germany); E. Lugscheider (Aachen University, Germany); O. Knotek, M. Maes (Materials Science Institute, Germany) For components like bearings or gear weels the usage of PVD-Coatings is still in an initial stage. The requirements for these types of coatings and the related processes are low friction, low deposition temperatures, realization on complex shapes and fatique resistance. This paper concentrates on new concepts of graded coating structures. Graded zirconium carbide coatings (ZrC-g) have a specific course of hardness with a hardness maximum in the middle of the coating. The above area is characterized by superstochiometric carbon content for low friction and acting as solid lubricant under tribological load. The area beneath the maximum hardness offers coating stiffness and realizes good bonding between coating and substrate. Furthermore this part of the coating can be tailored to the substrate characteristics. The characteristics of the coatings are discussed concerning adhesion quality, mechanical properties and structure, fatique behaviour measured by impact tests.
11:30 AM		E1-1-10 Friction and Heat Transfer Effects of Self Lubricating Coatings on Rapidly Engineered Forging Dies K. Agarwal, D. Mathur, R. Shivpuri (The Ohio State University); J. Lembo (ExtrudeHone Corporation); S.J. Dixit, R.S. Bhattacharya (UES, Inc.) Direct rapid tooling techniques like the 3D Printing, SLS and LENS have led to a decrease in the lead time for manufacture of dies for short run production. But due to the layer by layer manufacturing technique, the interface tribology and heat transfer plays a very important part in the metal forming applications. The high friction at the interface leads to premature die failure. To improve the tribology and heat transfer in such dies, the application of self lubrication coating was tried. The coating tried was a Ti/TiC multilayer coating with a thickness of 3 microns. The hardness of the coating was 2800 Vickers. This paper studies the effect of such a coating on the friction in the die during aluminum forgings. Hot and cold ring compression tests are performed with both coated and uncoated dies to compare and contrast. Finally, the self lubricated coating is applied on a scale down die which represents the typical precision aluminum forging and forgings are done on them. The microstructure and strength changes in the forgings obtained due the change in friction and heat transfer is evaluated. The study is unique in that the coatings are applied on rapidly engineered dies for forging applications in an environment with high sliding speeds, high temperatures and high wear. This kind of a surface modification can lead to an improvement in the die life in rapidly manufactured dies so that they can be used for short run productions.
11:50 AM		E1-1-11 Structure and Composition Optimization of Carbide-derived Carbon Films for Tribological Applications Y. Gogotsi (Drexel University); A. Erdemir (Argonne National Laboratory); M. McNallan (University of Illinois at Chicago); A. Kovalchenko (Argonne National Laboratory); B. Carroll (Drexel University) Tribological characterization of carbon-derived carbon (CDC) films at different loading and environmental (humid and dry air, nitrogen, vacuum) conditions as a function of the structure and chemical composition of the films was the goal of this study. The optimization of CDC films to combine high strength with low friction coefficient and high wear resistance is the motivation for the work. CDC coatings were synthesized by chlorination of silicon carbide at 1000-1100°C and various compositions of the Cl₂-H₂-Ar reaction gas. The structure of the CDC films includes nanocrystalline diamond, carbon onions and graphite. Pin-on-disk testing against Si₃N₄ counterbodies was used to characterize the tribological performance of the carbon layers. The CDC coatings show excellent tribological behavior in room air and exceptional behavior in dry environments, with friction coefficients less than 0.05. The performance of CDC in vacuum and the effect of high-temperature annealing and hydrogenation will be discussed. The effect of CDC surface treatment on tribological behavior also will be shown. Raman microspectroscopy, nanoindentation, and electron microscopy were used to characterize the relationship between the tribological behavior and the film structure.