Eco-friendly tribofilm formation from bio-oil on ferrous surfaces: Flash-catalysis versus flash thermolysis

Plant-based triacylglycerol (TAG) lubricants, such as palm oil (PO), that can form amorphous carbon (a-C) tribofilms, offer a promising route toward low-carbon manufacturing. However, the pathway by which TAGs transform into these protective layers remains unclear. In particular, the role of substrate catalysis is not well defined. Here, we use Raman spectrokinetics to track PO reactions on various surfaces and show that a-C forms on metallic iron-bearing substrates (iron (Fe), stainless steel (SS), and Inconel 718) at lower temperatures than on Al, glass, Cr 2 O 3 , or Fe 2 O 3 , with kinetics consistent with a surface-limited catalytic process. X-ray photoelectron spectroscopy (XPS) with depth profiling reveals a nanoscale passive oxide layer on iron and ferrous alloys. However, the higher activation temperature on metal oxides suggests that tribofilm formation most likely occurs via catalysis on metallic iron (Fe⁰) sites rather than on the oxide layer. This would limit the catalytic surface area unless the metal beneath can be made accessible. We further propose a tribological extrapolation in which, under friction-induced mechanical motion, asperity shear may transiently disrupt the native oxide, exposing nascent Fe⁰ sites with catalytic activity. This would expose nascent Fe⁰ with catalytic activity, reducing the activation barrier for TAG dehydrogenation and polymerization. Simultaneously, flash heating at this point could provide the thermal energy needed to drive the reaction. This thermo-mechanochemical catalytic model based on Fe⁰ catalysis explains the compatibility of ferrous alloys with biolubricants and offers practical design rules for promoting protective tribofilm formation through alloy selection.