Thermal degradation of n-hexadecane base oil and its impact on boundary friction and surface adsorption

The chemical and physical properties of lubricants can alter during use. High temperatures may cause thermal autooxidation of the base oil, which could impact the performance of lubricant additives. Here, the effect of high temperature on the properties of n-hexadecane base oil is investigated. n-Hexadecane undergoes an irreversible transition from high to low boundary friction at 122–134 °C when heated in air. FTIR, UV–vis and NMR spectroscopy indicate the presence of carbonyl- and hydroxyl-containing oxidation products (carboxylic acids, alcohols, esters, ketones and aldehydes). ATR-FTIR shows that iron carboxylates form exclusively inside and around the rubbed friction surface. QCM-D is used to investigate the adsorption of degradation products onto an iron(III) oxide surface and reveals that almost half the adsorbed mass is effectively irreversibly adsorbed. •Thermal degradation of n-hexadecane base oil in air occurs at 122–134 °C.•Degraded oil exhibits decreased boundary friction and reduced wear.•Polar species such as carboxylic acids, alcohols and ketones form in degraded oil.•Adsorption of degradation products onto iron oxide is partly irreversible by QCM-D.•Tribochemical reaction of degradation products yields film of iron carboxylates.