The Tribological Behavior of MoS 2 -Cr Films Sliding Against an Aluminum Counterface
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The Tribological Behavior of MoS2-Cr Films Sliding Against an Aluminum Counterface James E. Krzanowski, Department of Mechanical Engineering and Materials Science Program, University of New Hampshire, Durham, NH 03824, U.S.A. ABSTRACT The tribological performance of MoS2-Cr films in a pin-on-disk test sliding against an aluminum ball has been examined in this study. The MoS2-Cr films were deposited by pulsed laser deposition (MoS2) and simultaneous sputter deposition of Cr, giving a Cr content in the film of 10 mol. %. The results were also compared with films of MoS2 alone. The frictional behavior of MoS2-Cr films was not improved compared to the MoS2 alone, so SEM/EDS studies of the ball and flat were conducted to determine the nature of the transfer films and examine any interface reactions that occurred during the pin-on-disk (POD) test. In the early stages of the POD test (500 cycles) on the MoS2-Cr film, Al-oxide particles formed and caused cratering and scratching of the wear track, and the coefficient of friction neared 0.7. At later stages (9000 cycles), a thick oxide-based transfer film formed on the ball, but on the flat the track composition was closer to that of the original coating. For the films without Cr, after 104 cycles a smooth wear track was observed, and a thin transfer film of MoS2 was found within grooves on the ball wear scar along with Al oxide, which resulted in superior tribological performance. INTRODUCTION The natural lubricious properties of MoS2 have led to numerous investigations aimed at depositing it as a coating material. The early work of Spalvins [1] used sputtering methods to deposit films of MoS2. Since then, d.c., r.f., magnetron and reactive sputtering methods have been used [2-4], in addition to ion beam enhanced deposition [5], pulsed laser deposition [6] and chemical vapor deposition [7]. The primary issues investigated were the effects of chemical purity, crystallographic texture, operating environment, and types of dopant elements on tribological properties. The application MoS2 in air environments has been limited by the degradation of the frictional properties of MoS2 in humid environments. This has led to studies on the effect of oxygen on the structure properties and structure of MoS2 films. Lince [8] found that oxygen can form solid solutions with MoS2 by substituting for S atoms, resulting in the formation of MoS2-xOx compounds. The effects of oxygen were reported as enhancing edge growth and reducing wear life. However, it has also been suggested that the presence of oxygen in MoS2-xOx can reduce friction compared to pure MoS2, and films with 5-15% oxygen often exhibit very low friction coefficients (in the 10-2 range) [9]. In order to improve the tribological properties of MoS2 in humid environment applications, research has been conducted on compositional modifications to MoS2. Elements and compounds added to MoS2 include Au [10], PbO [11], and Fe, Ni and
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Sb203 [12]. Improvements in frictional properties in humid environments have been reporte
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