Microstructure and High-Temperature Oxidation Behavior of ZrB2–ZrO2–MoSi2–Al Coatings for the Protection of Carbon/Carbon Composites

The ZrB2–ZrO2–MoSi2–Al composite coatings were prepared on the carbon/carbon composite using a new multi-chamber detonation accelerator by thermal spraying methods from micropowders. The initial powders ZrB2-xMoSi2 (x = 24, 35, 45 wt%) with 5wt% Y2O3 and 20wt% Al. Y2O3 were successfully used as a stabilizer of the high-temperature tetragonal and cubic modification of zirconia. Aluminum was added as an oxidizing agent during spraying and creating “plastic” lamellae and nano-dispersed inclusions and relieves internal stresses. In the present work, the microstructure, elemental and phase composition of coating were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) before and after annealing at 1500 ℃ in air for 1 h. Described coatings are characterized by the lower than 1% porosity and good adhesion to the substrate. The uniform and dense lamellar-like microstructure of the ZrB2–ZrO2–MoSi2–Al coatings contributed to its good oxidation-resistant at high temperatures. As-sprayed coating consisted of the t–ZrO2, h–ZrB2, m–ZrO2, t–MoSi2 and fcc–Al. A significant content of aluminum in the initial powder mixtures during high-temperature oxidation leads to formation of a new phase—mullite (3Al2O3 · 2SiO2). The ZrO2 spherical particles in 3Al2O3 · 2SiO2 matrix were formed at 1500 ℃. Evolution of the microstructure of a mullite formed during high-temperature annealing in air, depending on the content of molybdenum silicide in the initial powders, was clearly described and discussed in the current work.