Degradation of lubricating oils using rolling four-ball tribometer and analysis of degradation products

The effect of sliding on the degradation of lubricating oils was studied and tests for degradation of lubricating oils were performed using a rolling four-ball tribometer for the purpose of clarifying points of difference with data obtained by static oxidation tests. The application of the rolling four-ball tribometer allowed the carrying out of long-term stable friction tests by using very small amounts of test oils and to track the state of degradation of test oils after sliding tests by use of the infrared absorption spectrum and measurements of the molecular weight distribution by GPC, using small amounts of test oils. It was found that a grease-like substance formed during tests under high load conditions that consisted of numerous fine wear particles formed during sliding, but the degradation of test oils was not notably advanced. The tracking of the state of test oils during sliding under lower load conditions showed the formation of tribodegradation products characterized by the presence of iron soap, which was attributed to the formation of metal soap by the oxidation of test oils on the sliding surfaces.