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ZrB2-SiC ultra-high temperature ceramics (UHTCs) were pressureless sintered with Y2O3-Al2O3 as the sintering additives. The effects of sintering additive and crystallization annealing on the microstructure and properties of ZrB2-SiC UHTCs were investigated. Sintering was activated by producing liquid phase of Y2O3 and Al2O3. The relative density of sintered ZrB2-20wt%SiC ceramic could reach 96% when the content of sintering additive was 6% and the sintering temperature was 1750ºC and its bending strength, Vickers hardness, and fracture toughness were 412 MPa, 13 GPa, and 6.0 MPa•m1/2, respectively. The crystallization annealing can result in YAG phase from grain boundary and enhance the high temperature properties of the UHTCs. The UHTCs have excellent ablation resistance at ultra-high temperatures, and a very low ablation rate of 0.0006 mm/s after ablation for 900s at 2800ºC.

An oxidation kinetics model of hot-pressed ZrB2-SiC composites was established and then proved by experiments. The relationship between oxidation rate and temperature was attained by reaction activation energy and frequency factor at the different oxidation stages. Results showed that the relations between the oxidation gain of unit area and the oxidation time are linear, conic and parabolic, respectively, during the stage of the oxidation prophase, midterm and anaphase.