Physical Analysis of Pitting in Rolling and Sliding Discs

Abstract

Experiments are described in which two different twin discs machines and two different medicinal white oils were used to study the effects of an antioxidant and of sulphur as an extreme pressure additive on pitting performance. It was found that at 30°C the use of sulphur as an additive was detrimental to pitting but at 100°C, its use was beneficial. Tribochemical reactions at these temperatures were studied (using X-ray photoelectron spectroscopy and X-ray diffraction technique) and their effects were related to the pitting performance of the lubricants. Other experiments are described using calcium carbonate and using a Wöhler fatigue tester to support the findings of the disc machine tests. The wear debris was examined using ferrography, optical and scanning electron microscopes. These two forms of microscopies were also used to follow the sequence of events leading to pitting. It was found that the formation of surface cracks was preceded by two stages of topographical change on the disc-surface. A hypothesis of their appearance and role in the process of pitting is presented. Further disc machine experiments were carried out to investigate the effect of D-ratio on pitting. A strong correlation was found between fatigue life and the D-ratio, thus confirming the work of Dawson. It was also found that for the sulphurised oil at 30°C, different tribochemical reactions occurred under thick and thin (lubricant) films conditions. The effect of relative hardnesses on rolling contact fatigue was also investigated. Two equations were proposed to relate different combinations of hardnesses to increases in the contact areas.