Spectrum loading effects on the running-in of lubricated bronze and surface-treated titanium against alloy steel

Lubricated bearing surfaces like those in engine connecting rods commonly experience transient loading in service. These transients can affect friction, wear rate, and regime of lubrication. It was therefore of interest to investigate the effects of periodically-varying loads on candidate bearing materials under non-steady-state conditions and running-in. Light weight titanium alloys could be used for bearing components in fuel-efficient vehicles if their sliding friction and wear characteristics were improved. Candidate surface treatments and coatings for Ti-6Al-4V alloy were tribo-tested under diesel oil lubrication with time-varying loads. A variable load bearing test (VLBT) apparatus was built for that purpose. Baseline tests of a bearing bronze (CDA 932) against alloy steel (AISI 8620) showed that wear during a variable load test was greater than that for the same duration test under a constant load equal to the average of the spectrum loads. Spectrum loading effects on the running-in and steady-state friction and wear behavior of bronze and several Ti alloy surface treatments and coatings are discussed. Results indicate that the progression of observable wear and surface damage may be difficult to quantify under the nearly full-film lubrication regimes as are found in engine bearings. Implications for screening candidate materials and coatings for lightweight engine bearings are discussed. (C) 2012 Elsevier B.V. All rights reserved.