Wear behavior of Pb–Mo–S solid lubricating coatings
Introduction
Advances in solid lubrication by MoS2 have been made by investigating the relationships between processing, microstructure, friction, wear and endurance. Studies of sputter-deposited MoS2 showed that coating microstructure plays a large role in both wear behavior and coating lifetime. For example, early sputter-deposited MoS2 coatings formed platelets having a columnar, edge-oriented (basal plane perpendicular to substrate), porous microstructure [1]. Because of this structure, the coatings lost platelets through fracture during sliding [2]as well as deformed by tilting or bending of columnar platelets 3, 4, 5, as observed by scanning electron microscopy. Fleischauer and Bauer [4]and Hilton and Fleischauer [5]used X-ray diffraction (XRD) to confirm and quantify reorientation after sliding. Hilton et al. [6]later showed that the degree of reorientation is dependent on the initial microstructure. The sputtered MoS2 coatings, like earlier burnished MoS2 films, wore rapidly in sliding and rolling contact 4, 6, 7, 8, 9, 10. More recently, coatings with dense, basal-oriented microstructures have been produced via rf-sputtering [11], ion beam-assisted deposition (IBAD) 12, 13, 14and pulsed laser deposition (PLD) 15, 16. However, wear studies of IBAD MoS2 coatings 17, 18determined that they, too, wore rapidly early in sliding life, despite their high endurance.
While processing advances enhanced MoS2 coating microstructure and endurance, a number of other improvements in sputter-deposited MoS2 coatings were achieved through co-deposition with a variety of metals 7, 19, 20, laser alloying with Au [21], and multilayer deposition (alternating metals with MoS2) 22, 23. Increased crystallite size and density [19], lower friction 7, 20, 21, and retarded growth of edge-oriented crystallites [22]were all observed. More recently, several groups have demonstrated that coating endurance could be further improved by co-deposition of PbO 20, 24, Pb [25], or Ti [26]with MoS2 using PLD, IBAD, and unbalanced magnetron processing, respectively. X-ray diffraction and Raman studies 24, 25, 27showed that PbO and Pb alloying produced amorphous coatings, but that sliding transformed the surface to MoS2. This was demonstrated conclusively by Dunn et al. using cross-section high resolution transmission electron microscopy (HRTEM) [28].
At present, it is not understood why the amorphous films perform as well as (or even better than) the crystalline films. In this paper, we investigate the wear behavior of amorphous Pb–Mo–S coatings. Optical microscopy and interferometry are used to characterize the worn surfaces' morphology and wear. Raman spectroscopy and cross-section HRTEM are used to examine the coating structure and wear track surfaces. Results are discussed in terms of wear mechanisms at the nanometer and micron size scales and compared to previous results from unalloyed, crystalline MoS2 coatings.
Section snippets
Experimental
Pb–Mo–S coatings were deposited by dual ion-beam deposition (IBD) in a vacuum chamber equipped with a Kaufman argon ion source; a full description of the IBD deposition process for these coatings is given elsewhere [25]. This process is considerably simpler than the IBAD counterpart used to produce MoS2 coatings: no assist gun was used, and the deposition stage was not heated (a small temperature rise to ∼60°C was observed during deposition). The coatings were deposited on various polished
Coating friction and wear depths
Friction coefficients were high during the first several passes (>0.1), but dropped below 0.05 by 10 to 100 cycles (depending on the particular coating) and were similar to those we reported previously [25]. Fig. 2a shows wear track depths as a function of reciprocating sliding cycle from tests performed on Pb–Mo–S coatings with four different thicknesses; to more clearly display changes in wear depths before 10,000 cycles, a semi-log plot of the wear data is shown in Fig. 2b. No measurable
Discussion
Wear scar analysis of steel sliding against Pb–Mo–S coatings in dry air revealed both nanometer scale and micrometer scale processes associated with wear. HRTEM studies of the wear tracks gave direct evidence that low speed, high stress sliding—a tribomechanical process—can convert amorphous Pb–Mo–S into crystalline MoS2. The transformation, previously inferred from Raman spectroscopy of wear scars on Pb–O–Mo–S [24]and Pb–Mo–S [25]alloys, has now been verified. Moreover, HRTEM shows that the
Conclusions
Pb–Mo–S coatings were found to be remarkably wear resistant. Two wear mechanisms were identified: layer-by-layer removal of MoS2 and plowing. At the nanometer scale, wear proceeded in a two-part process: transformation of the coating surface to MoS2, then layer-by-layer removal of MoS2. This process was observed by as few as nine sliding cycles, where Raman microscopy detected MoS2 in an optically transparent transfer film. Wear of the Pb–Mo–S coatings also occurred, on a microscopic scale, by
Acknowledgements
The authors thank R.N. Bolster for coating deposition, A. Kyriakopoulos and J.C. Wegand for contributing to the wear testing, as well as L.E. Seitzman and A. Erdemir for useful discussions. Work was performed while D.N.D. was supported by an ONR/ASEE post-doctoral associateship. The work was supported by the Office of Naval Research.
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Present address: Department of Materials Science, University of Virginia, Charlottesville, VA 22903, USA.