Physical Analysis of Pitting Failure with an Extreme-Pressure Additive in Rolling and Sliding Steel Discs

Abstract

Surface contact fatigue or pitting failure, a phenomenon associated with high, compressively stressed point and line contacts, terminates the useful operating life of machine elements such as gears and ball and roller bearings. In this investigation, an Amsler disc-on-disc machine was used to obtain the life to first pit for (i) two medicinal white oils, and (ii) the same oils plus a sulphur-phosphorus type of extreme pressure e.p.) additive. The number of cycles to first pit was found to give a strong correlation with the surface roughness to lubricant film thickness ratio, D. It was also found that, for D ratios less than about 1.5, the presence of the e.p. additive tended to shorten the fatigue life, with respect to the plain mineral oils, whereas, above this ratio, the converse was true. However, at high D ratios (D>10) the additive had the adverse effect of promoting surface wear. Other experiments are described in which a similar trend was observed, using a Unisteel rolling contact fatigue machine. The additive was also found to reduce fatigue life in experiments using a Wohler rotating cantilever fatigue tester. A magnetic method of crack detection was developed in conjunction with the disc tests which enabled initial surface fatigue cracks to be automatically detected. The results obtained showed that the technique has considerable potential for aiding a better understanding of the complex nature of pitting failure. Based on the results of analyses using Auger electron spectroscopy (AES), a highly sensitive technique for examining the elemental composition of the first few atomic layers of a surface; the magnetic crack detector and scanning electron microscopy (SEM), possible mechanisms are put forward to explain the action of the e.p. additive.