Microstructure and properties of plasma electrolytic oxidation coating on 55% SiCp/Al matrix composites

Protective coating was successfully prepared on two types of SiCp/Al matrix composites by plasma electrolytic oxidation (PEO) method, where the SiCp/Al matrix composites contains high volume fraction of SiC particles with sizes of 60 μm and 40 μm, respectively. Current-time curves were recorded during the PEO process in a constant voltage mode. The surface and cross-section morphologies, composition and adhesive force of the coatings were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), and coating adhesion automatic scratch tester, respectively. The electrochemical corrosion behavior of the substrate and coated composites was investigated in 3.5% NaCl solution by potentiodynamic polarization tests and EIS. The results show that the current change trend and coating morphology features of the two type of composites were consistent. SiC particles at high volume fraction could hinder the formation of the film and the ejection of the melt, resulting in rough, uneven pores with variable sizes, coarse molten particles and micro-cracks on the coatings. There is ablation on 60 μm-SiC/Al coating. The PEO coating was mainly composed of mullite, α-Al2O3, γ-Al2O3 and amorphous phases. The adhesive force of coating on 40 μm-SiC/Al matrix composite (68.1 N) is better than that of 60 μm-SiC/Al matrix composite (22.0 N). The corrosion resistance of SiCp/Al matrix composite was improved by plasma electrolytic oxidation treatment. •The influence of SiC particles size on plasma electrolytic oxidation is analyzed.•SiC particles hinder the formation of the film and the ejection of the melt.•The large size of SiC particles causes the coating ablation.•SiC particles affect the properties of plasma electrolytic oxidation coating.