STRUCTURE–PHASE TRANSFORMATIONS IN ZrB₂–SiC–AlN PLASMA COATINGS ON A C/C–SiC SUBSTRATE AFTER HIGH-TEMPERATURE THERMAL CYCLIC HEATING

Abstract

The composition and structure of a plasma-spray coating produced from a 60ZrB₂ + 20SiC + 20AlN (wt.%) composite powder on a C/C–SiC substrate were examined. The coating 320–370 μm in thickness is characterized by a heterophase structure, has no pores or cracks, and is densely bonded to the substrate. The coating phase composition corresponds to that of the starting powder. A coated sample was oxidized in a supersonic oxygen–propane–butane flow at ~2000 °С in thermal cycle mode (2 min heating, 10 min cooling, 15 cycles). The oxidation rate was <7 μm/min. The main coating phases in the coating near-surface layers were zirconium dioxide m-ZrO₂ and Al₂SiO₅ mullite solid solution. Cross-sectional microstructural and elemental chemical analyses of the starting and oxidized coatings established a high-temperature oxidation mechanism. An Al₂SiO₅-based layer with inclusions of spheroidal m-ZrO₂ grains 1–2 μm in size was found to form. Thin SiO₂-based amorphous films in the SiO₂–Al₂O₃ system developed on its surface. The results testify that the coating is resistant to high-temperature oxidation up to ≤2000 °С.