In lubricated sliding contacts, components wear out and the lubricating oil ages with time. The present work explores the interactive influence between lubricant aging and component wear. The flat face of a steel pin is slid against a rotating steel disk under near isothermal conditions while the contact is immersed in a reservoir of lubricant (hexadecane). The chemical changes in the oil with time are measured by vibrational spectroscopy and gas chromatography. The corresponding chemistry of the pin surface is recorded using X-ray photoelectron spectroscopy while the morphology of the worn pins; surface and subsurface, are observed using a combination of focused ion beam milling and scanning electron microscopy. When compared to thermal auto-oxidation of the lubricant alone, steel on steel friction and wear are found to accentuate the decomposition of oil and to reduce the beneficial impact of antioxidants. The catalytic action of nascent iron, an outcome of pin wear and disk wear, is shown to contribute to this detrimental effect. Over long periods of sliding, the decomposition products of lubricant aging on their own, as well as in conjunction with their products of reaction with iron, generate a thick tribofilm that is highly protective in terms of friction and wear.

An industrial base oil, a blend of different paraffin fractions, is heated to 130 °C (1) in the ambient and (2) for use as a lubricant in a steel pin on a steel disk sliding experiment. The base oil was tested with and without test antioxidants: dimethyl disulfide (DMDS) and alkylated diphenylamine (ADPA). Primary and secondary oxidation products were monitored continuously by FTIR over a 100 h period. In addition, friction and wear of the steel pin were monitored over the same period and the chemical transformation of the pin surface was monitored by XPS. The objective of this work is to observe the catalytic action of the steel components on the oil aging process and the efficacy of the antioxidant to reduce oxidation of oil used in tribology as a lubricant. Possible mechanistic explanations of the aging process as well as its impact on friction and wear are discussed.