Super-strong ZrB2-based UHTCs at High Temperatures

Reference	Presenter	Authors (Institution)	Abstract
09-121	Guo Jun Zhang	Zhang, G.J.(Donghua University); Zou, J.(University of Birmingham); Ma, H.B.(Donghua University);	ZrB2-based ultra-high temperature ceramics (UHTCs) with chemical and physical stability at high temperatures (e.g., above 2000?C) and in reactive atmospheres (e.g., monatomic oxygen) are key materials for applications in liquid/solid rocket engines and thermal protective systems of hypersonic flight (nose cones and leading edges). Strong UHTCs are generally desired for their service at high temperatures. About six years’ ago, strong ZrB2-SiC-WC ceramics with strength higher than 650 MPa at 1600?C were developed in my group. It is one of the few reports on the ceramics without any strength degradation up to 1600?C in inert atmosphere at that time. By optimizing the types and amounts of the sintering additives, super-strong ZrB2-WC ceramics with strength values exceeding 1GPa have been recently realized at 1800?C. In this presentation, several state-of-the-art technologies were combined to investigate the bonding character, morphology and composition of oxygen contamination in diboride powders and ceramics. The trajectories of these impurities during sintering were analyzed, their removal by WC and getter by WB were confirmed from experimental observations and theoretical calculations. The segregation of W in the grain boundary of ZrB2 was also identified. The combination of these features makes the obtaining of “clean” ZrB2 ceramics with excellent high temperature mechanical property feasible. In addition, the residual thermal stresses in different samples were evaluated by both Raman spectroscopy (RS) and X-ray diffraction (XRD). The temperatures at which the stresses began to accumulate were calculated. The relationship between the high-temperature bending strength and the residual stress as well as the related mechanisms were discussed, which provides a train of thought and method for designing of materials with high temperature strength.
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