Wear behavior of Pb–Mo–S solid lubricating coatings

doi:10.1016/S0043-1648(99)00100-3

Wear

Volume 230, Issue 2, May 1999, Pages 175-183

https://doi.org/10.1016/S0043-1648(99)00100-3 Get rights and content

Abstract

Amorphous Pb–Mo–S coatings 200 to 510 nm thick were deposited by dual ion-beam deposition (IBD) onto steel and Si substrates. Coating wear studies were performed using ball-on-flat reciprocating sliding with steel ball counterfaces in dry air. Tests were run between 1 and 100,000 sliding cycles, and wear depths measured by interference microscopy. Morphology and chemistry of the as-deposited coatings and worn surfaces were investigated with optical microscopy, micro-Raman spectroscopy and cross-section high resolution transmission electron microscopy (HRTEM). Pb–Mo–S coatings were found to be quite wear resistant; no more than 25% of the coating thickness was removed by 10,000 sliding cycles. Two wear mechanisms were identified. 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. At the micrometer scale, wear occurred by plowing. The long endurance of Pb–Mo–S coatings was attributed to slow wear of the coatings, with lubricant redistribution processes playing a minor role.

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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¹: Present address: Department of Materials Science, University of Virginia, Charlottesville, VA 22903, USA.

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