ICMCTF1999 Session E2-2: Wear Resistance of Ceramic, Metallic and Composite Coatings

Thursday, April 15, 1999 1:30 PM in Room California

Thursday Afternoon

Start	Invited?	Item
1:30 PM		E2-2-1 Study of the Wear and Friction Behaviour of Hard Sputtered Carbon Coatings S. Yang, D. Camino (Teer Coatings Ltd., United Kingdom); A.H.S. Jones (Teer Coatings, Ltd., United Kingdom); D.G. Teer (Teer Coatings Ltd., United Kingdom) The exceptional tribological properties of a low deposition temperature PVD carbon coating, Graphit-C, have been recently reported. This paper describes the latest development of this coating and particularly how, by an optimisation of the deposition parameters, it has been possible to obtain very hard coatings with extremely low specific wear rates. The coatings have been deposited in a Closed Field Un-Balanced Magnetron Sputter Ion Plating (CFUBMSIP) installation. An extensive study of the deposition parameters (Ion current Density, working pressure, substrates bias, deposition rate) is reported. By finding the appropriate conditions of deposition, very hard carbon coatings (plastic hardness up to 4000 kg mm^-2) can be routinely deposited. Preliminary analytical results are presented in order to characterise such hard coatings: HRTEM, SEM, XRD and Raman analysis are some of the different techniques used for this work. Finally, some of the applications requiring such a hard, low wear and low friction surface treatment are reported and tribological test results which demonstrate that this coating will outperform the treatments currently used.
1:50 PM		E2-2-2 Tribological Evaluation of CN_x Thin Films for Space Applications M.Y. Chen, A.A. Voevodin, J.H. Sanders, J.S. Zabinski (Air Force Research Laboratory) Reactive magnetron sputtering with and without the assistance of an electron cyclotron resonance source was used to deposit CN_x thin films. The films had nitrogen content ranging from 0.3-0.9 (25 to 48 at.% nitrogen) with a wide range of hardness (1-16 GPa). Hard monolithic and hard/soft bilayer CN_x coatings were subjected to lubricated tribotests in vacuum to explore their potentials for space applications. Ball-on-disc friction tests were performed between 440 °C stainless steel balls and CN_x-coated 440C substrates. Micro-Raman spectroscopy was used to analyze the structure of the coatings, the transfer films, and the wear debris. The effects of electron cyclotron resonance assistance and bilayering on the tribological properties were examined. Comparison of these properties among CN_x, DLC, and Ti-doped DLC films will be presented.
2:10 PM		E2-2-3 Influence of Soft Coatings on Chip Formation and Cutting Performance H.K. Toenshoff, B. Karpuschewski, A. Mohlfeld, H. Seegers (University of Hannover, Germany) Today, special research activities are concentrated on optimizing coating properties for improving the performance of tools. (TiAl)N-based hard coatings are well known for their high microhardness even at elevated temperatures as well as for their good oxidation and wear resistance. Further improvements concerning chip removal can achieved by reducing friction in cutting. In this paper the cutting performance of different coated carbide tools are investigated in turning and drilling of tempered steel. Influences of coating composition and roughness of coated tools are considered. Highlighted are effects of MoS₂- and WC/C-coatings on chip formation and contact conditions. Besides, influences of cutting as well as of cooling conditions are examined. Cutting forces and contact conditions are determined experimentally. Chip formation is analyzed by using a special device to interrupt turning processes. Wear phenomena are characterized by SEM and EDX. Variations of contact and load conditions are correlated with the determined wear mechanisms. Effects of soft coatings on cutting performance in turning and drilling are discussed.
2:30 PM		E2-2-4 Enhanced Mechanical Seal Performance Through CVD Diamond Films P.J. Kelly, R.D. Arnell (University of Salford, United Kingdom); M.D. Hudson, A.E.J. Wilson (GEC-Marconi Materials Technology Ltd., United Kingdom); G. Jones (Gledco Engineering Materials Ltd., United Kingdom) Mechanical seals are the predominant type of seals found on a wide range of pumps, mixers, compressors and other similar machines within the fluid handling industries. The performance of the seal, and therefore in turn, the performance of the particular machine, is determined by the sealing interface and the interface materials used. Indeed, the bulk and tribological properties of the mating materials are fundamental to the performance of a mechanical seal. The present 'state-of-the-art' seal face combination is a high duty carbon-graphite material mated against a reaction bonded, or alpha sintered silicon carbide. Whilst this combination allows high duty specifications to be met, there are increasing demands from new applications for improvements in performance and reliability beyond that which can be achieved with current materials. Consequently, seal designers have identified the need to utilise surface engineering techniques to meet future requiremnts. CVD diamond coatings possess excellent tribological properties. This material offers the potential to enhance mechanical seal performance in terms of reduced wear and friction and increased reliability and life. Polycrystalline diamond coatings have, therefore, been deposited by microwave plasma-assisted CVD onto various grades of silicon carbide. The coating morphologies and substrate properties were varied systematically using the Taguchi Method. A Taguchi array has been carried out in which the performance of the coatings was assessed against a range of counterface materials in a thrust washer test rig. The results of these tess are reported here. Consideration is also given to the role of microasperities in determining the performance of a seal. This work was carried out under a project funded under the DTI EPSRC LINK Surface Enigeering Programme.
2:50 PM		E2-2-5 Pitting Failure Mode in Thin Film Coated Surfaces O.O. Ajayi (Caterpillar ,Inc.); L.E. Seitzman, M.H. Haselkorn, P.H. McCluskey (Caterpillar, Inc.) Coated steel substrates were evaluated under two different contact conditions. In both cases the failure mode was pitting, defined as localized volumetric loss of material from interacting surfaces. However, closer examination of the pitted areas revealed that the cause of pitting is different. In unlubricated pure sliding, pitting (in the substrate) occurred as a result of cracking of the coating which provided a pathway for ambient chemistry to the steel substrate. In lubricated rolling/sliding contact, pitting was due to contact fatigue. In both cases, the thin film coating increased the pitting resistance compared to uncoated surfaces, even though the mechanism of pitting in the two cases is different. Pitting life will be discussed as it relates to these coated tribological systems.
3:30 PM		E2-2-7 Induction Surface Hardening of Hard Coated Steels K. Pantleon, O. Kessler, F. Hoffmann, P. Mayr (Stiftung Institut f. Werkstofftechnik, Germany) The deposition of hard coatings with CVD-processes is commonly used to improve the wear resistance e.g. of tool steels in forming. The advantages of CVD are undisputed (high deposition rates with simple equipment, excellent coating properties). Nevertheless, the disadvantage of the CVD-process is the high deposition temperature, consequently the properties of steel substrates are negatively influenced. Therefore, a subsequent heat treatment of the coated steels is necessary to restore the properties of steels ready for operation. Induction surface hardening is used as a method of heat treatment after the deposition of TiN hard coatings on steel substrates. Influences of both the coating properties and the substrate properties are discussed in dependence on the parameters of induction heating. Thereby the heating time, heating atmosphere and the power input into the specimen are changed. The effect of induction surface hardening on the properties of the coating-substrate-systems is mainly characterized using investigations of microstructure and chemical composition as well as measurements of hardness and residual stresses in dependence on the distance from the surface. Furthermore, the scratch test is used to estimate critical loads for cohesive and adhesive failure of the coatings. Additionally, distortion measurements are carried out. The results emphasize the advantage of induction surface hardening as a method of subsequent heat treatment of CVD-coated steels.
3:50 PM		E2-2-8 Enhanced Tribological Properties of TiN/CN_x Multilayer Coatings with an Equiaxed Microstructure D. Epstein, I. Polonsky, L.M. Keer, M. Graham, M.M. Lacerda, Y.-H. Chen, Y.-W. Chung (Northwestern University) Hard tribological coating materials, such as titanium nitride, can in theory prolong fatigue life of gears and bearings. Coatings, if sufficiently thick, can protect substrates from high stresses induced by asperities. TiN coatings develop a coarse columnar structure at thicknesses above a few hundred nm. Failure of these coatings is often cohesive, viz. along grain boundaries. This work seeks to develop thick TiN coatings with a fine equiaxed microstructure. In this case, we interrupted the TiN growth at regular intervals by depositing a thin layer of carbon nitride (CN_x), which suppresses column formation. These TiN/CN_x coatings were deposited by unbalanced DC magnetron sputtering with different TiN/CN_x thickness ratios onto different substrates. Rolling contact fatigue (RCF) lives of coatings deposited on steels were measured. We showed that optimum microstructure, hardness, and significantly enhanced RCF lives can be obtained at certain TiN crystalline texture and TiN/CN_x thickness ratios.
4:10 PM		E2-2-9 A Simulation Test for the Selection of Coatings and Surface Treatments for Plastics Injection Moulding Machines S.J. Bull, R.I. Davidson (University of Newcastle, United Kingdom); A.M. Jones, A.R. McCabe (AEA Technology plc., United Kingdom) Glass-filled polymers are known to produce considerable wear on the screws and barrels of injection moulding machines and several coatings and surface treatments have been used to combat this ranging from chromium plating to High Velocity Oxy Fue l (HVOF) WC/Co. However, quantification of the benefits of such coatings and treatments has proceeded on an ad hoc basis and there is little data available on which to aid selection. In this study we have developed a novel wear tester to simulate the cond tions of wear which occur in the barrel of an injection moulding machine and used it to rank the coatings and surface treatments which are often used in this application. The tester concept is similar to that of the ASTM rubber wheel abrasion test except that the rubber wheel is replaced by a steel wheel heated to a fixed temperature (up to 200°C) by hot air and the sand is replaced by pelletised plastic. The plastic sticks to the hot wheel and is softened and dragged past the sample which is also heated to the same temperature. The sample is pressed against the plastic coated wheel with a small load and a wear scar is produced at the contact point by a combination of abrasive wear from the glass filler and sliding wear from the plastic. In the case of glass filled nylon the best surface treatment was the combination of nitriding and high intensity ion implantation closely followed by HVOF WC/Co. Some treatments (e.g. laser hardening and air plasma sprayed WC/Co) performed worse than uncoated steel. This paper discusses these results and suggests some improvements to the test rig design which should make it more widely applicable.
4:30 PM		E2-2-10 Development of TiC-Coated M62 Balls for Hybrid Bearings with Reduced Sphericity Distortion W. Park, M.R. Hilton (The Aerospace Corporation); P.C. Ward (MPB Corporation); D.W. Smith (Honeywell Space Systems) In recent years, powder metallurgy tool steel M62 has emerged as a promising bearing steel for its high hardness and rolling contact fatigue life. In this paper, we report on the fabrication of bearings made of M62 steel raceways with TiC-coated balls of the same steel, including aspects of heat treatment processing to maintain adequate sphericity of the balls. TiC-coated 440C steel balls have been used in precision bearings for spacecraft and missiles. However, the use of coated balls has been generally restricted to small ball sizes because of dimensional distortions of balls that develop from the high temperature CVD coating process and subsequent re-hardening heat treatment. We minimized the sphericity distortion of TiC-coated M62 balls of 1.19 cm diameter through series of process modifications that will be explained in detail. The performance of these TiC-coated M62 balls in bearing tests will be reviewed.}
4:50 PM		E2-2-11 Relative Significance of Elastic and Thermal Property Mismatches for Wear Protection with Hard Coats in Tribological Systems Z. Shi, S. Ramalingam (University of Minnesota) Hard-coats are in wide use to extend wear-life in open tribo-systems such as those encountered in cutting and forming tools and for erosion protection of turbine blades. Since coating failure during operation can have catastrophic consequences, use of hard coats in closed tribo-systems is less common. Quantitative coating-substrate adhesion strength and knowledge of the stresses in the film, the substrate and their interface during operation are necessary before a product designer will consider use of hard coats in closed tribo-systems. Methods to calculate the stress fields for prescribed boundary tractions and for specified loads for a given contact geometry are already known. But, in most tribo-contacts, there is significant frictional dissipation which leads to thermal stresses. Hard coat design must hence account for thermal stresses as well as mechanical stresses. In this paper, methods for calculating the stress fields due to mechanical loading as well as frictional heating are developed. Ball-on-disk geometry widely used to evaluate tribological properties of coated samples is considered. Calculated stress fields as a function of contact geometry, applied load, friction coefficient, sliding speed and film thickness are presented to establish the relative importance of elastic and thermal property (coefficient of thermal expansion and thermal diffusivity) mismatches unavoidable in coated contacts. Film flexure strength and coating-substrate bond strength necessary for reliable use of hard coats for wear protection in closed tribo-systems are determined. Importance of elastic and thermal property mismatches between the film and the substrate are discussed.
5:10 PM		E2-2-12 Wear and Friction Behaviour of Duplex Treated AISI 4140 Steel B. Podgornik, J. Izintin (University of Ljubljana, Slovenia); O. Wanstrand (Uppsala University, Angstrom Labboratory, Sweden); S. Hogmark (Uppsala University, Sweden) Nowadays mechanical systems operate in increasingly severe conditions, such as intensive loads, high speeds and harsh environments. Therefore designers are continually stretching the limits of the existing materials. To extend the use of available materials beyond their conventional limits' manufacturers are turning to different surface treatments and to hard protective coatings. The main limitation to more extended use of hard ceramic coatings is the load bearing capacity of the coating-substrate system. Due to very thin nature of the hard ceramic coatings, the substrate must bear the majority of the applied load. If the substrate has insufficient strength to bear the contact loading and thus support the coating, plastic deformation will occur, leading to premature failure of the coating. The challenge to improve the properties of hard ceramic coatings by thermochemical pre-treatment of the substrate has gained much attention in recent years leading to a new method called duplex treatment. Duplex treatment is an interesting method not only for increasing load bearing capacity but also for improving fatigue strength, temperature resistance and wear behaviour of mechanical components, particularly under high loads. In our study samples made of AISI 4140 steel pre-treated by plasma nitriding and coated by two different PVD coatings (TiN and TiAlN) were investigated with respect to the microhardness, surface roughness, residual stress, scratch adhesion and the dry sliding wear resistance. Wear tests in which duplex treated pins were mated to hardened ball bearing steel discs were performed on pin-on-disc machine. To examine the influence of nitrided zone on the performance of coating-substrate composite coatings were deposited on hardened as well as on plasma and pulse plasma nitrided samples nitrided under two different nitriding conditions; in conventional 25%N₂ and in N₂ poor gas mixture.