Papers

ICMCTF1999 Session E1: Reduction of Friction Through Coating/Surface Modification

Wednesday, April 14, 1999 1:30 PM in Room Council/Chamber/Cabinet

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		E1-1 The Deposition of TiN-MoS₂ and TiAlN-MoS₂ Hard Coatings of Low Friction Coefficient by a Combined Cathodic Arc/Magnetron Sputter Process R. Goller, P. Torri, R. Gilmore, W. Gissler (Joint Research Centre of the European Commission, Italy) To deposit multicomponent coatings of the type TiN-MoS₂ and TiAlN-MoS₂ a combined process involving reactive cathodic arc deposition for the hard component and magnetron sputter deposition for the lubricating component was applied. The Mo and S content of the coatings was in the range 10 to 20 at. %. The coatings displayed a f.c.c. structure typical for TiN and TiAlN with slightly enlarged lattice constants. By suitable process parameter control, coatings of high adhesion on hard metal and HSS substrates were produced. Hardness values of up to 40 GPa and friction coefficients of less than 0.2 were obtained. To test the suitability of such coatings for use in dry machining, coated drills were tested in unlubricated conditions and displayed increased lifetimes with respect to coatings which were deposited without MoS₂.
1:50 PM		E1-2 Friction and Electrical Contact Transitions of MoS₂ Coatings: Effect of Environment. M. Belin (Ecole Centrale de Lyon, France); K.J. Wahl, I.L. Singer (Naval Research Laboratory) Triboscopic studies are performed on duplex layers of 300 nm MoS₂ on 35 nm TiN, prepared by ion-beam assisted deposition onto a steel substrate. Spatially-resolved friction and electrical contact resistance measurements are obtained at high contact pressure (P=1 GPa), in three different atmospheres: dry nitrogen (RH<<1%), dry air (RH<<1%) and ambient air (RH>40%). Tests are performed in each atmosphere and also by switching from dry to ambient air after selected numbers of test cycles. Results show that humidity has a major and rapid effect on friction and electrical resistance: the friction coefficient rose from 0.02 in dry nitrogen and dry air to typically 0.10 in ambient air and the electrical resistance dropped by more than an order of magnitude, both within a few cycles of adding humidity. The durability of the layer was also strongly affected by humidity: for RH>40%, catastrophic breakdown occurred by about 1500 cycles. A simple model of contact is considered, based on assumptions concerning the real contact area and electrical conduction. Results are then discussed in terms of the role of water on the electrical resistivity of MoS₂.
2:10 PM		E1-3 Effects on the Morphology, Orientation and Structure of MoS₂ Co-Sputtered Composites A. Savan (Centre Suisse D'Electronique et de Microtechnique S.A., Switzerland); M.C. Simmonds (Paul Scherrer Institute, Switzerland); E. Pflüger (Centre Suisse D'Electronique et de Microtechnique S.A., Switzerland); H. Van Swygenhoven (Paul Scherrer Institute, Switzerland) Co-sputtered films of MoS₂ with respectively Au, Ti, Pb, Ni, or WSe₂ have been deposited in order to investigate the relationships between microstructure, dopant and tribological behavior. The alloying constituents were selected based on possible chemical effects, for example substitution in the MoS₂ structure, intercalation, edge-bond passivation, and with and without simultaneous oxygen gettering of the sputtering environment. The mechanical properties of the composite films were investigated by pin-on-disk sliding wear testing in 50% relative humidity air, microscratch and indent adhesion characterization, and nanoindentation hardness as a function of the dopant element(s) and concentration. X-ray diffraction was used to determine the influence of dopant composition and volume fraction on the relative amount of basal- or edge-oriented crystals, or "amorphous" phase of the MoS₂ composite. These results indicate similar behavior to MoS₂-metal nanoscale multilayer film structures that have demonstrated, for example, the suppression of columnar microstructures. Co-sputtered films also show the same trend towards lower and more stable friction coefficients and longer sliding lifetimes, as compared to pure MoS₂ films in humid atmospheric test conditions.
2:30 PM		E1-4 Chemical Characterization of Solid Lubricant Films Using a New Raman Tribometer S.D. Dvorak, K.J. Wahl, I.L. Singer (US Naval Research Laboratory) We have developed a new tribometer that allows us to simultaneously measure friction, perform optical microscopy, and obtain Raman spectra of a contact during reciprocating sliding. The apparatus incorporates computer-controlled data acquisition to measure friction force and position data, which are used to obtain spatially resolved friction measurements along a wear track. The use of an optically transparent, hemispherical slider enables us to visually monitor the buildup and breakdown of transfer films in the contact area during sliding. A unique feature of this instrument is the ability to perform Raman spectroscopy through the transparent slider in order to monitor in vivo chemical changes of transfer films throughout their life cycle. We are currently performing studies on solid lubricant thin films, including MoS₂ and Pb-Mo-S, which have been chosen for their Raman-active chemistry^1,2. Chemical and morphological changes in the contact area during sliding are correlated with the tribological performance of the coatings. These results are compared to existing wear models for these solid lubricants, which are currently based on ex situ optical microscopy and chemical analysis of worn surfaces and transfer films. ¹ N.T. McDevitt, M.S. Donley, and J.S. Zabinski, Wear, 166 (1993) 65-72. ² K.J. Wahl, L.E. Seitzman, R.N. Bolster, I.L. Singer, Surf. Coat. Technol., 73 (1995) 152-159.
2:50 PM		E1-5 Coating Characteristics and Tribological Properties of Sputter-Deposited MoS₂/Metal Composite Coatings Deposited by Closed Field Unbalanced Magnetron Sputter Ion Plating. N.M. Renevier, V.C. Fox, D.G. Teer, J. Hampshire (Teer Coatings Ltd., United Kingdom) As previously reported, [1] the properties of of MoS ₂ coatings can be improved by the co-deposition of a small amount of titanium. These MoS ₂/Ti coatings known as MoST@superTM@, were harder, much more wear resistant and also less sensitive to atmosphere water vapour during tribological testing. This coating has given excellent industrial results for a wide range of cutting and forming applications. Other metal additions as Cr, W, Zr… were studied of a varying composition and substrates. Friction coefficients were measured using pin on disc and reciprocating wear tests. While coating hardness was determined by nanoindentation. Structural analysis was carried out using X-ray diffraction, scanning electron microscopy and transmission electron microscopy of cross section through the coating. Coatings composition was also determined. [1] V.C. Fox, N.M. Renevier, D.G. Teer and J. Hampshire, The Structure of Tribologically Improved MoS@sub2/Metal Composited Coatings and Their Industrial Applications, Proceedings of The PSE Conference in Garmisch Partenkirchen, 14-18 September 1998, Germany.
3:30 PM		E1-7 The Surface Chemistry of Extreme Pressure Lubricant Additives P.V. Kotvis (Benz Oil, Inc); J. Lara (University of Wiscosnin-Milwaukee); W.T. Tysoe (University of Wisconsin-Milwaukee) Chlorinated hydrocarbons are commonly added to the base fluid to synthesize lubricants used under extreme pressure (EP) conditions. It has been demonstrated that the interfacial temperature in the EP regime varies linearly with the applied load and that temperatures in excess of 1000 K can be attained. At these temperatures, both microbalance experiments carried out under high pressures as well as molecular beam experiments performed in ultrahigh vacuum are used to measure film growth kinetics. Analysis of the resulting films reveals that chlorinated hydrocarbons thermally decompose forming a layer that consists of iron chloride (FeCl₂) which can also incorporate small (˜ 50 Å diameter) carbon particles. These particles may affect the coefficient of friction of the film. The lubricants fail and the interface seizes when the film is removed significantly rapidly for metal-metal contact to take place so that EP lubrication is described as a dynamic phenomenon where the reactive formation of the film is balanced by its removal and these ideas can be used to successfully describe the extreme-pressure behavior of chlorinated hydrocarbon lubricant additives. Under appropriate circumstances, sufficient carbon can be incorporated into the iron substrate that it becomes a carbide. In this case, seizure is prevented even when the halide layer is removed because of the hardness and high melting temperature of this material. These ideas have also been applied to sulfur-containing additives and reveal that both dimethydisulfide and carbon disulfide thermally decompose to form an anti-seizure layer that consists of FeS and this chemistry can also be successfully used to describe the extreme-pressure activity of sulfur containing molecules.
4:10 PM		E1-9 Sputter-Deposited MoS₂ Films for Angular Contact Bearings in Space Applications J.R. Lince (The Aerospace Corporation) Sputter-deposited MoS₂ solid lubricant films exhibit advantages over traditional bonded solid lubricant films for spacecraft, including smaller thicknesses and higher load-carrying ability. Sputter-deposited MoS₂ films are being used increasingly for lubricating angular contact bearings on spacecraft in niche applications. Current applications include solar array drive bearings. Also, these films will be useful for gimbal bearings for precision pointing and tracking of infrared sensors used at cryogenic temperatures. Such temperatures preclude the use of liquids, and traditional solid lubricants cause significant torque noise. We are investigating ways to achieve lifetime and performance goals by studying film properties such as fracture toughness, adhesion to bearing surfaces, friction (both average and time variation) and robustness to storage in humid air. Results will be presented on torque and lifetime testing of angular contact bearings, using races coated with MoS₂ in a versatile sputter-deposition system. In this part of the study, we are concentrating on the tribological behavior of the ball-race interface by using nonlubricating cages (retainers) to preclude the formation of a transfer film on the race surface. We will present data concerning the use of Cr interlayers to improve adhesion, and cosputtering with metallic species to control the density and storage robustness of the films. Torque/lifetime results will be correlated with film morphology and surface chemistry during wear and after failure.
4:50 PM		E1-11 Chemical Vapour Deposition of MoS₂ Coatings Using the Pecursors MoCl₅ and H₂S I. Endler, A. Leonhardt (Institut für Festkörper- und Werkstofforschung Dresden, Germany); U. König, H. van den Berg (Widia GmbH Essen, Germany); W. Pitschke (Institut für Festkörper- und Werkstofforschung Dresden, Germany); V. Sottke (Widia GmbH Essen, Germany) MoS₂ lubricant coatings were synthesized by low pressure chemical vapour deposition (LPCVD) using a gas mixture of MoCl₅, H₂S and Ar. A thermodynamic analysis shows that the deposition of pure MoS₂ can be expected over a wide range of temperature. The codeposition of molybdenum chlorides MoCl₃ and MoCl₂ can be avoided. Deposition experiments were performed in a CVD apparatus equipped with separate gas inputs for argon/MoCl₅ gas mixture and H₂S. As substrates cemented carbide tools and silicon were used. The influence of process parameters on the deposition rate, layer composition and structure of the molybdenum sulphide coatings was investigated. Pure MoS₂ layers can be prepared at substrate temperatures of 500 and 600 °C. With lowering of the substrate temperature the chlorine content increases remarkably caused by the codeposition of ternary MoS_xCl_y compounds under definite conditions. Mo₂S₃ is codeposited. The deposition rate shows a maximal value of 1.6 mg/( cm²h) at 450 °C. With rising pressure it decreases strongly. A substrate influence on the deposition rate is also observed, which is higher on cemented carbide than on silicon. Layers deposited at 300 °C were amorphous and have a smooth surface. Between 400 °C and 600 °C crystalline layers of different morphology were obtained. The morphology changes from a network of bean-shaped to plate-like and rosette-shaped crystals at 400 °C, 500 °C and 600 °C, respectively. Texture also changes. XRD pattern show a variation in the (002) peak intensity which is stronger at the higher substrate temperatures. TEM investigations show that a preferred orientation of the (002) basal planes exists only for the first 100 nm of MoS₂ film.