Mechanism of steric hindrance reduction in improving tribological performance of methylphosphonate as a lubricant additive on steel-steel contact surfaces

While phosphates are widely adopted in industrial systems due to their oxidation stability, their limited tribological performance persists as a critical challenge. This study demonstrates that controlled reduction of steric hindrance through methyl group substitution at phosphorus centers significantly enhances the tribological property of methylphosphonate. Tribological evaluation in PAO10 base oil revealed that dicresyl methylphosphonate (DCMP) with optimized spatial configuration outperforms conventional tricresyl phosphate (TCP) on steel-steel contact surfaces in the whole friction system. Specifically, DCMP exhibited a 21.96 % reduction in wear volume, a 137.42 % increase in the last non-seizure load (P_B), and a 27.02 % increase in weld points (P_D). Additionally, XPS, FIB-TEM, and EDS analyses confirmed that DCMP formed a tribo-film with a higher phosphorus content, which was directly correlated with spatial structure modification. Crucially, DFT calculations confirmed that the introduction of methyl groups effectively exposed active phosphorus atoms and P=O oxygen atoms. This promotes the cleavage of P-O bonds via hydroxylation and adsorption-induced mechanisms. The reduced spatial constraints promote efficient adsorption and subsequent tribo-film generation. These findings propose a novel molecular design strategy for developing advanced phosphate-based extreme pressure additives.