Plasma electrolytic oxidation coatings on additively manufactured aluminum–silicon alloys with superior tribological performance

Abstract

Due to the increasing demand for lightweight structures with complex geometries, additively manufactured (AM) aluminum–silicon (AlSi) alloys have aroused considerable attention. However, their insufficient hardness and poor tribological performance remain a critical issue for several applications requiring high contact pressures and wear resistance. Herein, current-controlled plasma electrolytic oxidation (PEO) coatings were developed on AM AlSi alloys in alkali electrolytes. To obtain PEO coatings with excellent mechanical properties, the electrolyte system was modulated and optimized by adjusting the concentration of the silicate and hexametaphosphate. The corrosion behavior of AM AlSi alloys in various electrolytes were evaluated via potentiodynamic polarization tests. The influence of silicate and hexametaphosphate on the structure, composition, thickness, roughness, and hardness of the prepared PEO coatings were determined using scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy system, microprofilometer, and microhardness tester. The tribological behavior of the PEO coatings were tested using a ball-disc-tribometer without lubrication. Silicate benefits the PEO process under the constant current mode; however, a mass of mullite and noncrystalline materials form. Hexametaphosphate adversely affects the formation of the initial passive film on AM AlSi alloys, and many defects form on the obtained coating. But it can decrease the roughness of the coating and accelerate the dissolution of the matrix, promoting the formation of alumina and inhibiting the formation of mullite and SiO2. The PEO coatings, prepared in the mixed electrolytes (with a concentration ratio of silicate and hexametaphosphate of ~0.7–1), have the fewest defects and highest hardness (~1300 HV), and possess superior tribological performance; the wear rates of the coatings were only ~1/150th that of the AM AlSi10Mg substrate. Both the synergy of silicate and hexametaphosphate and the modified growth mechanisms of PEO coatings on AM AlSi alloys have been proposed.