Elevated temperature mechanical properties of zirconium diboride based ceramics

Research presented in this dissertation focused on the mechanical behavior of ZrB₂ based ceramic at elevated temperatures. Flexure strength, fracture toughness, and elastic modulus were measured at temperatures up to 2300ºC for three compositions: monolithic ZrB₂ (Z); ZrB₂ - 30 vol% SiC - 2 vol% B₄ C (ZS); and ZrB₂ - 10 vol% ZrC (ZC). In argon, Z, ZS, and ZC had strengths of 210 (at 2300ºC), 260 (at 2200ºC), and 295 MPa (at 2300ºC), the highest temperatures tested for each composition. Fractography was used extensively to characterize the strength limiting flaws as a function of temperature. Strength of ZS in argon was controlled by the SiC cluster size up to 1800ºC, and the formation of B-O-C-N phases that bridged SiC clusters above 2000ºC. For ZC, surface flaws introduced during specimen preparation were the source of critical flaws in the material up to 1400ºC, sub-critical crack growth of surface flaws between 1600 and 2000ºC, and microvoid coalescence above 2000ºC. It was also shown that thermal annealing at either 1400, 1500, or 1600ºC improves the strength and modulus of ZS at temperatures between 800ºC and 1600ºC. Heat treatment at 1400ºC for 10 hours produced the largest improvement in strength, 430 MPa at 1600ºC versus 380 MPa for the as processed material. As a whole, the research pointed to several key microstructural features currently limiting the mechanical properties at the highest temperatures. In particular, removal of unfavorable secondary phases, and improved control over microstructure, should be promising methods to improve the elevated temperature properties of ZrB₂ ceramics. --Abstract, page iv