Pressure-Induced Transitions in the Friction and Wear of Self-Lubricating PEO-Graphite Coatings for Magnesium-Polymer Sliding

Abstract

This study investigates the influence of nominal contact pressure on the tribological behaviour of oxide coatings produced by plasma electrolytic oxidation (PEO) on AZ31B magnesium alloy and subsequently modified with graphite. While the effects of composition are well documented, the systematic link between unit pressure and the efficiency of self-lubricating PEO coatings remains a critical knowledge gap. Reciprocating pin-on-plate tests were performed at nominal unit pressures of 1.0, 1.5 and 2.0 MPa using a PEEK-HPV counterface to simulate engineering contacts in pneumatic actuators or polymer bearings. Results show that graphite modification significantly improves wear resistance, reducing pin mass loss by up to 51% at 1.0 MPa (from 1.45 mg for the reference to 0.7 mg for sample C). Graphite modification tended to stabilise the friction coefficient µ and promoted the formation of a uniform lubricating tribofilm, particularly under moderate loads. However, at the highest pressure of 2.0 MPa, a transition in the wear mechanism was observed, leading to increased surface degradation, although modified samples still outperformed the reference by approximately 36% in terms of polymer wear. Statistical analysis (Friedman test and 2 k factorial design) confirmed that while processing parameters influence coating thickness (up to 8.22 µ m), the contact pressure is the dominant factor governing the stability of the self-lubricating regime. These findings provide direct engineering guidelines for the operating limits of modified PEO coatings in lightweight mechanical assemblies.