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

Monday, April 27, 1998 1:30 PM in Room California

Monday Afternoon

Time Period MoA Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF1998 Schedule

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1:30 PM E1-2-1 Friction, Wear and Endurance of MoS2 Coatings
I.L. Singer (U.S. Naval Research Laboratory)

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.

2:10 PM E1-2-3 Tribological Properties of MoS2 Lubricant Coatings: A Film Chemistry Approach
P.D. Fleischauer (The Aerospace Corporation)
Significant advancements in the production of low friction, long wear life, sputter-deposited MoS2 lubricant coatings have been made in the last decade. The introduction of multi-layered coatings, the establishment of careful controls on doping during DC and magnetron sputter deposition, and the implementation of ion assisted deposition have resulted in lubricants with substantially longer wear lives (up to a factor of ten greater than in the early 1980s) and lower sliding friction coefficients. A major research effort, designed to improve the performance of solid lubricants, occurred during this time period, resulting in these major breakthroughs. Even with this concentrated effort, the typical investigation involved making an educated guess, based on previous experience, of the deposition conditions, target compositions, or post treatments that might be expected to provide improved performance of resulting coatings. In this paper I will attempt to step back and look at the chemistries of the primary and contaminant coating constituents and at the interfacial chemistries between coatings and substrates in order to explain the improvements that have been realized. A discussion of the relationship(s) among coating structure, chemistry, and tribological performance will be presented with emphasis on defect chemistry and multiple phase interactions. Speculations on the significance of coating properties and coating performance in applications such as in ball bearings will be presented.
3:30 PM E1-2-7 Ultra-low Wear Behavior of Pb-Mo-S Solid Lubricating Coatings
K.J. Wahl (US Naval Research Laboratory); D.N. Dunn (University of Virginia); I.L. Singer (US Naval Research Laboratory)

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.

3:50 PM E1-2-8 Structural Morphological, Chemical and Tribological Investigations of Sputter-Deposited MoSx/Metal Multilayer Coatings
M. Simmonds, H. Van Swygenhofen (PSI Paul Scherrer Institut, Switzerland); A. Savan (CSEM Centre Suisse d'Electronique et de Microtechnique SA, Switzerland); E. Pflueger (CSEM Centre Suisse d'Electronique et de Microtechnique SA, Switzerland); S. Mikhailov (CAFI - Centre d'Analyse par Faisceau Ionique, Le Locle, Switzerland)
Pure MoSx films, as well as MoSx/Pb and MoSx/Ti nanometer-scale multilayers with individual layer thickness varying over the range of 4 to 100 nm, have been prepared by magnetron sputtering at room temperature. The effects of changing the number of interfaces and the effects of changing the percentage of metal (i.e. metal layer thickness) have been investigated, using X-ray diffraction and reflection, AFM and XPS analysis. Some of these multilayer films showed significantly superior tribological performance compared to pure MoSx with pin-on-disk tribometer sliding wear tests in 50% relative humidity air. Both multilayer periodicity and the metal content of the films are shown to alter the MoSx microstructure and results are presented indicating how this may in turn influence the tribological performance of the films.
4:10 PM E1-2-9 Industrial Applications of Tribologically Improved MoS2 / Metal Composite Coatings Deposited by Closed Field Unbalanced Magnetron Sputtering.
V.C Fox, D.G Teer, J. Hampshire (Teer Coatings Ltd, United Kingdom)

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

4:30 PM E1-2-10 Low-friction TiN-MoS2 Coatings Produced by dc Magnetron Co-deposition
R. Gilmore, W. Gissler (Joint Research Centre of the Commission of the European Communities, Italy); M. Stoiber, P. Losbichler, C. Mitterer (The University of Leoben, Austria)
The feasibility of producing low-friction TiN-based coatings by co-deposition with MoS2 has been investigated. Samples were prepared by dc co-sputtering from a single unbalanced magnetron target composed of a TiN half and an MoS2 half. Coatings of varying composition were produced by placing stainless steel substrates at various positions between the two halves. Friction coefficients were measured using a pin-on-disk tribometer and hardness was determined by nanoindentation. Structural analysis was carried out using glancing-angle X-ray diffractometry, photoelectron spectroscopy and cross-sectional SEM, and composition was determined using glow-discharge optical emission spectroscopy. It was shown that coatings with a hardness of the order of 15 GPa and displaying friction coefficients below 0.2 could be obtained by choosing suitable composition and deposition parameters.
4:50 PM E1-2-11 In situ MoS2 Formation and Selective Transfer from MoDTP Films
C. Grossiord, J.M. Martin, Th. Le Mogne (Ecole Centrale de Lyon, France); Th. Palermo (Institut Français du Pétrole, France)

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.

Time Period MoA Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF1998 Schedule