High-temperature oxidation of ZrB₂–SiC and ZrB₂–SiC–ZrSi₂ ceramics up to 1700°C in air

The high-temperature oxidation of ZrB₂–SiC and ZrB₂–SiC–ZrSi₂ ceramics up to 1700°C in air has shown their high corrosion resistance. A five-stage nonisothermal oxidation mechanism is established for the samples when heated at a rate of 20°C/min: 1) oxygen adsorption–desorption; 2) formation of ZrO₂ and B₂O₃ oxides; 3) formation of α-SiO₂ cristobalite and ZrSiO₄ zircon; 4) formation of amorphous SiO₂; and 5) formation of upper protective borosilicate film with ZrSiO₄ inclusions at less than 40 wt.% SiC. It is shown that small ZrSi₂ admixtures (<6–8 wt.%) substantially slow down the oxidation of the ceramics in initial heating stages due to diffusion-hindered formation of ZrO₂ and B₂O₃. The 67.3 wt.% ZrB₂–26 wt.% SiC–6.7 wt.% ZrSi₂ ceramics are characterized by the highest corrosion resistance since their upper scale layer represents borosilicate glass stabilized with ZrO₂ (tetragonal) inclusions.