Oxidative degradation of mineral oil under tribocontact and ineffectiveness of inhibitors

Oxidative degradation of mineral oil under tribocontact is investigated in comparison with static oxidation. A rolling four-ball apparatus lubricated with a small amount of sample oil (approximately 10 mg) was selected as the method for degrading oil samples. Applications of very small amounts of sample oils made it possible to detect small amounts of degradation product after friction tests. An FTIR analysis and gel permeation chromatography were used to characterize the degraded oils. It was found that mineral oil oxidatively degraded during tribo-operation at nominally moderate temperatures. The effect of oxidation inhibitors on the tribodegradation of mineral oil was investigated. A hindered phenol inhibitor, which acts as a radical scavenger, and a sulfur compound inhibitor, which acts as a hydroperoxide decomposer, were used. No inhibition was observed in the tribodegradation of mineral oils with either of two inhibitors, though they were effective for the static oxidation. It was also found that the sulfur compound inhibitor deteriorated triboconditions by forming considerable amounts of wear debris even under the wiling contacts.