Formation of self-lubricating PEO coating via in-situ incorporation of PTFE particles

Self-lubricating plasma electrolytic oxidation (PEO) coatings have been fabricated on AZ91 magnesium alloy via in-situ incorporation of PTFE particles. Surfactants are used to improve dispersion stability of the particles in aqueous solution and relatively low energy input is applied to ensure the particles are inertly incorporated into the coating. It was found that the porous layer has been sealed significantly and PTFE-enriched ridge-like protrusions can be obtained on the coating surface after sufficient treatment time. 3D surface micro-topography was reconstructed to disclose the uptake of the particles and wear mechanisms of the coatings. The particle containing protrusions can act as lubricant reservoirs, leading to ultra-low and stable friction coefficient between the coating and its counterpart during entire dry sliding wear test. The formation of the ridge-like protrusions is related to micro-sized gas bubbles adherence to the coating surface due to high viscosity of the electrolyte. As a consequence, the intensity of electric field at the edge of the bubbles increases, which could facilitate rapid sintering and accumulation of PTFE particles at the electrolyte/gas/coating interface.