Oxidation protection of commercial-purity titanium by Na2O–CaO–SiO2 and Na2O–CaO–Al2O3–SiO2 glass–ceramic coatings

Na2O–CaO–SiO2 and Na2O–CaO–Al2O3–SiO2 glass–ceramic coatings were first prepared on commercial-purity titanium TA1 to improve its high temperature oxidation resistance. The Na2O–CaO–Al2O3–SiO2 glass–ceramic coating showed better sintering behavior than the Na2O–CaO–SiO2 glass–ceramic coating. Both glass–ceramic coatings clearly decreased the oxidation rate of TA1 at 800 °C. After high temperature oxidation, both glass–ceramic coatings showed a good adhesion to the TA1 substrate. Between the Na2O–CaO–SiO2 glass–ceramic coating and the TA1 substrate, a Ti5Si3 interface layer was observed as a result of the reaction between SiO2 in the glass and Ti. While for the Na2O–CaO–Al2O3–SiO2 glass–ceramic coating, diffusion of Al to the TA1 could also be seen under the Ti5Si3 interface layer. Cross-sectional EDS line scanning images of the glass–ceramic coatings showed that diffusion of the elements such as O, Ca and Na into TA1 was negligible, suggesting that both glass–ceramic coatings were suitable for protection of TA1 against high temperature oxidation.