Effect of Al2O3 content on microstructure and oxidation behavior of silicate enamel coatings on a Ni-based superalloy at 1000 °C

Abstract

Enamel coating, which is of low thermal conductivity and high chemical stability, has been employed for the protection of many hot parts against oxidation. It is composed of multiple oxides, and each oxide component plays a certain role on thermophysical properties. In this paper, a type of silicate enamel coating with different contents of oxide component Al 2 O 3 was prepared on K444 superalloy. Its microstructural evolution and cyclic oxidation behavior at 1000 °C were investigated. Results indicated that the A0 enamel coating, which was free of Al 2 O 3 in composition, suffered from serious crystallization at high temperature. The formation of quartz, cristobalite and SrSiO 3 crystals resulted in significant mass loss during cyclic oxidation. However, precipitation of these crystals was almost completely prohibited in the A10 and A15 coatings with 10 and 15 wt% Al 2 O 3 , respectively. The A10 coating had no mass loss after cyclic oxidation. But much high content of Al 2 O 3 resulted in the formation of abundant long strip crystals of SrAl 2 Si 2 O 8 in the A15 coating, which led to minor spallation. Furthermore, enamel coatings with Al 2 O 3 (above 6 wt%) promoted the formation of a continuous alumina scale at the enamel/alloy interface, which improved oxidation resistance and increased bonding strength. The well grown alumina scale at interface as well as the mild crystallization ensured high spallation resistance of the enamel coating A10.