Effects of Atmosphere on the Pressureless Sintering Behavior of ZrB₂ Ceramics

The effects of atmosphere on the pressureless sintering behavior, microstructure development, and thermal properties of ZrB2 ceramics were studied. ZrB2 powders were sintered in a carbon free, refractory metal furnace under controlled atmospheres including helium, hydrogen, nitrogen, mild vacuum (~ 5 Pa) and high vacuum (~5 x 10-3 Pa). The sintering temperatures were varied from 1900⁰C to 2400⁰C with hold times ranging from 1 to 4 hours. Sintering in hydrogen resulted in higher weight loss than sintering in helium. Thermodynamic equilibrium calculations indicated that formation of ZrO(g) and H2O(g) were favorable in the presence of excess H2(g) at temperatures above 1800⁰C. These vapor species increased in concentration above 2000⁰C and were removed by the flowing H2(g), which resulted in the observed mass loss increase. No increase in density was observed between the mild vacuum and the high vacuum. Other effects of atmospheric conditions will be discussed such as those on thermal conductivity and microstructure development