ICMCTF1998 Session E1-2: Reduction of Friction Through Coatings/Surface Modification (2)

The lubricating properties of MoS₂ have been studied and exploited for more than 50 years. The past decade, however, has seen great progress in development of new types of MoS₂ -like coatings and better understanding of their lubrication mechanisms (mainly in humidity-free environments). Driven by new processing techniques, surface characterization tools and, importantly, funding for space applications, researchers have developed new ways to control the structure and composition of MoS₂ -like coatings. Today, one can produce coatings that are dense and basally-oriented, nearly contaminant-free, and that give friction coefficients less than 0.01.

This presentation will mainly address three questions: 1) What are the mechanisms responsible for ultra low friction coefficients (typically around 0.01-0.02)? 2) What structure(s) or composition(s) promote wear resistance? and 3) What role do third body processes play in the endurance of the coatings? We will show that friction behavior is associated with a MoS₂ transfer film on the uncoated counterface during the early stages of sliding; that coating wear can occur by several well known wear modes, but is very non-linear; and that endurance is not a simple function of the wear rate of the coating but rather determined by the flux of third body particles in and out of the contact.

Ion-beam deposited Pb-Mo-S solid lubricating coatings can provide improved sliding endurance over unalloyed, ion-beam assist deposited (IBAD) MoS₂ coatings without sacrificing low friction coefficient ¹. While low friction was accounted for by formation of crystalline MoS₂ on sliding surfaces, the enhanced endurance was not explained. In this study, we examine the wear behavior of Pb-Mo-S coatings under reciprocating sliding conditions in dry air. Optical microscopy and interferometry, energy dispersive x-ray and Auger electron spectroscopies, as well as transmission electron microscopy were used to characterize the coating and the wear surfaces. The relationship between endurance and wear performance of Pb-Mo-S coatings will be discussed and contrasted with that of IBAD MoS₂ coatings ². Advantages of using metal alloyed coatings in place of IBAD MoS₂ will be shown.

¹¹ K.J. Wahl, L.E. Seitzman, R.N. Bolster, and I.L. Singer, "Low friction, high-endurance, ion-beam-deposited Pb-Mo-S coatings," Surf. Coat. Technol. 73 (1995) 152-159. ²²K.J. Wahl and I.L. Singer, "Quantification of a lubricant transfer process that enhances the sliding life of a MoS₂ coating," Tribology Letters 1 (1995) 59-66.

It has been reported [1] that the properties of MoS₂ coatings can be improved by the co-deposition of small amounts of metal. These MoS₂ / metal composite coatings were harder, much more wear resistant and also less sensitive to water vapour in the testing atmosphere, giving excellent wear resistance at loads as high as 140N on the reciprocating wear tester using a 6mm diameter WC pin. The coefficient of friction was as low as 0.02 at a humidity of 40% and adhesion scratch tests indicated critical loads in excess of 120N.

Optimisation of the coating parameters have resulted in a MoS₂/metal coating which shows higher wear resistance than that originally developed. An emphasis has been placed on testing and evaluation of the coatings in a variety of industrial applications. This new MoS₂ composite coating has given excellent results for a wide range of cutting and forming applications. Industrial testing of coated tools has been performed and the results are presented.

In addition the composite coatings have been studied using micro hardness testing, scratch adhesion testing, pin on disc and reciprocating friction and wear tests and the results are presented. The structure of the coatings have been studied by optical microscopy and SEM of normal sections and taper sections produced by ball cratering techniques. paragraph References

[1] The Tribological Properties of MoS2 / Metal Composite Coatings Deposited by Closed Field Magnetron Sputtering. D G Teer, J Hampshire, V Fox, V Bellido-Gonzalez. Metallurgical Coatings and Thin Films 1997, Proceeding of the 23rd International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, USA, April 1997

For many years, metal dithiophosphates have been used as antiwear additives in motor oils. Their action during friction is to create a tribochemical film known to be the formation of a solid transition metal phosphate glass on steel surfaces ¹. Among them, molybdenum dithiophosphate (MoDTP) is recognised to have antifriction properties. We are studying the formation mechanisms of the films and the chemical reactions induced by friction in vacuum.

The MoDTP films are generated in a Cameron-Plint friction machine. At 60 degrees C, a steel cylinder rubs on a steel flat (AISI 52100) under 350N and for 1 hour in base oil containing 1 weight per cent of MoDTP. The tribochemical films are studied in a ultrahigh vacuum (UHV) reciprocating pin-on-flat tribometer ². Before friction, the chemical composition of the wear surfaces are characterised by in situ surface analysis tools as Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). After friction tests in vacuum, the wear scars on the steel pin and the wear tracks on the flat are analysed by AES. AES depth profiles on transfer films are recorded to determine their thickness and composition.

The friction coefficient curve recorded in UHV presents three stages. At the beginning and for few cycles, the friction coefficient is near 0.3 (A). Then it decreases to a very low value near 0.01 (B). Finally the film is broken and the friction coefficient rises to 0.8 (C). AES spectra are recorded at each point (A, B and C) on the wear scar of the pin and the wear track of the flat. Very low friction seems to be correlated to the presence of sulphur, molybdenum, carbon and iron whereas in point A and C, the transfer film contains also phosphorous and oxygen. Depth profiles of the transfer films on the pin reveal different chemical compositions from the top surface to steel. We will discuss the results which indicate chemical transformations of initial films during friction in vacuum.

¹ M. Belin, J.-M. Martin, J.-L. Mansot, STLE Trans, 32(1989), 410. ² J.-M. Martin, Th. Le Mogne, C. Grossiord and Th. Palermo, Tribology Letters, 3(1997), 87-94.