Effects of Na 2WO 4 and Na 2SiO 3 additives in electrolytes on microstructure and properties of PEO coatings on Q235 carbon steel

Abstract

Ceramic coatings were achieved on Q235 carbon steel by plasma electrolytic oxidation in aluminate system with and without Na 2WO 4 and Na 2SiO 3 additives in electrolyte. Influence of Na 2WO 4 and Na 2SiO 3 on surface morphology, phase and elemental composition of PEO coatings were examined by means of scanning electron microscope (SEM), thin-film X-ray diffraction (TF-XRD) and energy dispersive X-ray spectroscopy (EDS). Effects of the two additives on the properties of the coatings including surface roughness, surface micro hardness and friction coefficient were studied. The results showed that W from Na 2WO 4 and Si from Na 2SiO 3 in electrolytes entered into the coatings. Na 2WO 4 additive had no evident effect on phase composition of the coating, while Na 2SiO 3 additive resulted in the coating changing from crystalline state to amorphous state and increased the content of P in the coating. Both additives reduced the surface roughness of the coatings. With Na 2WO 4 or Na 2SiO 3 into the electrolytes, the surface micro hardness of the coating was enhanced to 1433 and 1478, respectively, and the friction coefficients were also decreased to below 0.1.