Effects of TaB2 and TiB2 on the grain growth behavior and kinetics of HfB2 ceramics during pressureless sintering

HfB2, Hf0.95Ta0.05B2, and Hf0.95Ti0.05B2 powders were self-synthesized and the grain sizes were 2.04, 0.36, and 1.15 mu m, respectively. The three powders were pressureless sintered from 1700celcius to 2000celcius to study and compare effects of the introduction of TaB2 and TiB2 on the grain growth behavior and kinetics of HfB2. The results revealed that HfB2 showed moderately slow grain growth in the whole process. However, significant grain growth consistently happened in Hf0.95Ti0.05B2 and Hf0.95Ta0.05B2 at 1900celcius or above. Eventually, the grain size of Hf0.95Ti0.05B2 increased to almost the same as HfB2, but Hf0.95Ta0.05B2 still possessed smaller grains due to the finest original powders. The grain growth exponent was determined to be similar to 3, and the dominant growth rate-controlling mechanism was volume diffusion. The average activation energy of HfB2, Hf0.95Ta0.05B2, and Hf0.95Ti0.05B2 for grain growth at 1700celcius-2000celcius was 191 +/- 34, 678 +/- 73, and 321 +/- 61 kJ/mol, respectively.