Tribological Properties of Sputter-Deposited MoS 2 Films Containing Titanium
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0890-Y09-02.1
Tribological Properties of Sputter-Deposited MoS2 Films Containing Titanium James E. Krzanowski and Dyumani Nunna Mechanical Engineering Dept. and Materials Science Program University of New Hampshire Durham, NH 03824, U.S.A ABSTRACT The tribological properties of sputter-deposited MoS2 and MoS2-Ti films were investigated in this study. The deposited films were characterized using microprobe analysis for composition and x-ray diffraction (XRD) for structure. The frictional properties of the films were examined using a pin-on-disk (POD) with counterfaces of 440C steel, aluminum, tungsten carbide and alumina. The tests were run under low (25%), medium (50%) and high (70%) humidity levels. MoS2 films without Ti were first examined under cyclic humidity conditions between 25 and 50% R/H. The results showed that for steel, WC and alumina counterfaces, the effect of the higher humidity was to increase the friction, but lower friction could be recovered when the humidity was reduced back to 25%. For films containing Ti, the best results were obtained at a concentration of 20 at. % Ti. These films performed well for steel and WC counterfaces, but poorly against aluminum. The effect of deposition temperature (up to 450oC) was examined for MoS2 and MoS2-5% Ti films. Higher temperatures yielded more crystalline films, but the addition of Ti partially countered this effect. The POD test showed that at medium humidity levels the friction decreased with temperature, but increased slightly when tested under low humidity. In all cases, the 5% Ti-containing films had a fiction coefficient of about 0.1 below that for films without Ti. INTRODUCTION Molybdenum disulphide (MoS2) is a naturally lubricious material due a crystal structure that results in the stacking of S-Mo-S planes along the c-axis direction [1]. MoS2 has been deposited as a coating material using a number of methods, including sputtering [2] and pulsed laser deposition [3]. These coatings are widely used as solid lubricants for applications where the operating atmosphere is vacuum or an inert gas. The use of MoS2 as a solid lubricant in humid air environments is limited due to its poor tribological performance at humidity levels near 50% and above. In order to reduce the humid-air sensitivity of MoS2, research has been conducted on compositional modifications to MoS2; element or compound additives investigated include Au [4], PbO [5], and Fe, Ni and Sb203 [6]. Some improvements in frictional properties under humid environments have been reported, possibly due to a higher degree of basal plan orientation or to protection of MoS2 edge planes by the dopant elements. The addition of Ti to MoS2 has received considerable attention [7,8], and several studies indicate significant improvement in tribological properties and the ability to use MoS2-Ti coatings on cutting tools for dry machining. Despite these studies, much still needs to be learned about the role of additives in MoS2, including the effects on crystal structure, the environmental and operating limit
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