D. Dini, K. Kim, D. Nowell, A. M. Korsunsky, University of Oxford, OXFORD, United Kingdom

Summary:

Surface coatings are at present widely used to control friction and wear in all kinds of sliding contacts. The substrate material can be designed for strength and toughness while the coating protects the contacted surface from wear. Moreover, a low friction and wear resistant coating allows for lower energy dissipation in the friction pair. On the other hand, selecting an appropriate low friction coating for a specific industrial application is still generally a challenging task.

Non-permanent assembly, loaded and submitted to vibration, may cause material damage consisting of crack formation and propagation or of particle detachment originated by microstructural transformation. In most industrial cases, the observed degradation is induced by the combination of an external static load and a relative motion, of low amplitude, between the contacting surfaces. This may be simulated by imposing small displacements between a specimen and a pad, maintained in contact under constant normal load (conditions generally referred to as “fretting”).

The purpose of this work was to investigate on the behaviour of different kinds of low friction coatings applied to Ti6Al4V substrate, in particular focusing our attention on the value of coefficient of friction (COF) provided by each coating as a function of some prescribed test conditions and following the evolution during the observed life time (Fig.1). A number of combinations of low friction coefficient coatings were investigated. A direct comparison between the values of COF and endurance life of the coatings was carried out.

We review the experimental procedures used for coating performance characterisation and the interpretation of experimental results. Particular attention is focused on the identification of suitable parameters allowing cross-correlation between different types of tests, and with the prototype; the mathematical description of the evolution of friction coefficient during fretting wear; the mechanisms of coating degradation; and the identification of candidate coatings for improved performance. We also touch upon the subject of surface treatment (peening), and the effect that it has on the specimen performance under fretting conditions.