INVESTIGATION OF BORIDE COMPOUNDS FOR VERY HIGH TEMPERATURE APPLICATIONS, Part II

Abstract : The prediction that metal rich diboride compounds would exhibit superior oxidation resistance was verified. Studies were made of high pressure hot pressed Hf and Zr diborides, the most oxidation resistant diborides, at B/ metal ratios between 1.7 and 2.1. Measurements between 1200 and 2200 K at partial pressures of 7 to 40 torr oxygen and flowrates of 100 to 200 cu cm/min were performed. The parabolic rate constants for HfB2 oxidation were about 10 times smaller than the corresponding ZrB2 rate constants. Si additions improved oxidation resistance below 1600 K. Measurements of vapor deposited ZrB sub 1.85 and Boride Z were performed. The best 'pure' diboride is HfB sub 1.7. Sintering studies on ZrB2 indicated that densification proceeds by grain boundary diffusion and that ZrB sub 1.89 can be sintered to 96% theoretical density at 2100-2200 C without discontinuous grain growth. Additions of Zr to ZrB sub 1.7 permitted densification at 1800 C. Measurements were made of the thermal conductivity and emissivity of TiB2, ZrB2, HfB2, and TaB2.