Fretting wear rate of sulphur deficient MoS x coatings based on dissipated energy
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auwerens Institute for Materials Research, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium, and Center for Scientific and Research in Metal Manufacturing, B-3590 Diepenbeek, Belgium
L. Stals Institute for Materials Research, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium
Jiawen He State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, 710049 Xi’an, People’s Republic of China
J-P. Celis Department Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium (Received 29 January 2001; accepted 29 September 2001)
The fretting wear of sulphur-deficient MoSx coatings with different crystallographic orientations has been investigated in ambient air of controlled relative humidity. The coefficient of friction and the wear rate of MoSx coatings sliding against corundum depend not only on fretting parameters like contact stress, fretting frequency, and relative humidity, but also strongly on the crystallographic orientation of the coatings. For randomly oriented MoSx coatings, the coefficient of friction and the wear rate increased significantly with increasing relative humidity. In contrast, basal-oriented MoSx coatings were less sensitive to relative humidity. The coefficient of friction of both types of MoSx coatings decreased on sliding against corundum with increasing contact stress and decreasing fretting frequency. A correlation between dissipated energy and wear volume is proposed. This approach allows detection in a simple way of differences in fretting wear resistance between random- and basal-oriented MoSx coatings tested in ambient air of different relative humidity.
I. INTRODUCTION
MoS2 is known for its excellent solid lubrication properties in reducing friction, wear, and energy loss. MoS2 coatings have already been successfully used in high vacuum and aerospace environments.1 The main obstacle to the use of MoS2 under ambient condition is its high sensitivity to moisture.2,3 Different methods have been tried out to improve the humidity resistance of MoSx coatings in air of high relative humidity, such as codeposition with other materials,4–6 producing multilayers,7 and postdeposition processes, e.g., ion beam bombardment8 and laser beam treatment.9 Until now, the tribological properties of MoS2 coatings reported were mainly related to pin-on-disk wear tests.10–14 However, coatings a)
Address all correspondence to this author. J. Mater. Res., Vol. 16, No. 12, Dec 2001
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are frequently subjected to oscillating contact conditions. Friction and wear data for MoS2 coatings under such bi-directional contact conditions are not yet available. It is widely accepted that the coefficient of friction of MoS2 coatings decreases with increasing contact stress. However, the effects of sliding speed, as well as the combined influence of load and speed on the coefficient of friction of MoS2 coatings, have not been deeply investigated. Most literature deals with the coefficient of fric
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