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

The composition and structure of a plasma-spray coating produced from a 60ZrB(2) + 20SiC + 20AlN (wt.%) composite powder on a C/C-SiC substrate were examined. The coating 320-370 mu m thick is characterized by a heterophase structure, has no pores or cracks, and densely adheres 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 similar to 2000 degrees C in thermal cyclic mode (2 min heating, 10 min cooling, 15 cycles). The oxidation rate was <7 mu m/min. The main coating phases in the near-surface layers were zirconium dioxide m-ZrO2 and a mullite Al2SiO5 solid solution. Cross-sectional microstructural and elemental chemical analyses of the starting and oxidized coatings established the high-temperature oxidation mechanism. An Al2SiO5-based layer with inclusions of spheroidal m-ZrO2 grains 1-2 mu m in size was found to form. Thin SiO2-based amorphous films in the SiO2-Al2O3 system developed on the layer surface. The results testify that the coating is resistant to high-temperature oxidation to <= 2000 degrees C.