Creep and Oxidation of Hafnium Diboride Based Ultra High Temperature Ceramics at 1500C

Abstract

Ultra high temperature ceramics (UHTCs) are leading candidates for aerospace structural applications in high temperature environments, including the leading edges of hypersonic aircraft and thermal protection systems for atmospheric re-entry vehicles.Before UHTCs can be used in such applications, their structural integrity and environmental durability must be assured, which requires a thorough understanding and characterization of their creep and oxidation behavior at relevant service temperatures.Creep, or the progressive, time-dependent deformation of material under constant load, is a critical criterion in these applications, but not much is known with regard to UHTCs or whether there are interactions with oxidation processes. Thus, a facility for high temperature, mechanical testing in air was augmented for testing in argon. Then, the compressive creep of a popular UHTC, HfB 2 , was examined at 1500°C in argon and compared to results in air. HfB 2 specimens with 0, 10, 20, and 30% additions of SiC were tested, which enabled assessments of the effects of grain size and SiC content on creep behavior. Boundary mechanisms accommodated by diffusion through grains dominated the creep rates. The results also suggest that SiC formed a network of pointto-point contacts and increased creep resistance.A unique stressed oxidation test was devised in order to further investigate the interaction of creep and oxidation. The results indicate that up to 75 MPa of compressive stress, models of creep and oxidation in HfB 2 -based UHTCs can be decoupled.