Formation of WSi2 coating on tungsten and its short-term cyclic oxidation performance in air

Microstructural aspects of WSi2 coating on pure W and its short-term oxidation performance under cyclic heating and cooling conditions in air at 1100 and 1300 °C have been studied. Cyclic oxidation performance of this coating has been compared with its performance under isothermal oxidation. The coating was applied by using a pack siliconization method. The as-formed coating consisted of an outer WSi2 layer and an inner W5Si3 layer. The WSi2 layer had a columnar structure and had several through-thickness cracks generated due to the mismatch of coefficient of thermal expansion between the coating and the substrate. Based on the coating microstructure, the mechanism of coating growth during siliconizing has been suggested. Weight change data obtained under cyclic oxidation in air at 1100 and 1300 °C suggested that the above coating can provide protection to W substrate against oxidation for about 2 h. The oxide scale that formed on the coating during oxidation exposure consisted of SiO2 and WO3 at 1100 °C and only SiO2 at 1300 °C. The protective silica layer underwent spallation during thermal cycling, leading to a diminishing of the protective capability of the coating. More importantly, localized oxidation of the W substrate through discontinuities present in the coating at sharp corners caused severe damage to the coated samples. Isothermal oxidation exposure of the coating, in comparison, resulted in a much lower degree of damage and the coating provided protection for a much longer duration (up to 10 h) at the above temperatures. In this study, apart from reporting a hitherto unreported oxide scale morphology, the microstructural degradation of the coating during oxidation has been linked to the columnar structure of the WSi2 layer.