Ceramics based on group IV-V transition metal borides and carbides possess melting points above 3000 °C, are ablation resistant and are, therefore, candidates for the design of components of next generation space vehicles, rocket nozzle inserts, and nose cones or leading edges for hypersonic aerospace vehicles. As such, they will have to bear high thermo-mechanical loads, which makes strength at high temperature of great importance. While testing of these materials above 2000 °C is necessary to prove their capabilities at anticipated operating temperatures, literature reports are quite limited. Reported strength values for zirconium diboride (ZrB2) ceramics can exceed 1 GPa at room temperature, but these values rapidly decrease, with all previously reported strengths being less than 340 MPa at 1500 °C or above. Here, we show how the strength of ZrB2 ceramics can be increased to more than 800 MPa at temperatures in the range of 1500–2100 °C. These exceptional strengths are due to a core-shell microstructure, which leads to in-situ toughening and sub-grain refinement at elevated temperatures. Our findings promise to open a new avenue to designing materials that are super-strong at ultra-high temperatures.

Bioinspired "brick-and-mortar" alumina ceramics containing a nickel compliant phase are synthesized by coextrusion of alumina and nickel oxide. Results show that these structures are coarser yet exhibit exceptional resistance-curve behavior with a fracture toughness three or more times higher than that of alumina, consistent with significant extrinsic toughening, from crack bridging and "brick" pull-out, in the image of natural nacre.

Flexural strength, elastic modulus, and hardness were used to characterize the mechanical properties of ZrB 2 -SiC composites that contained varying SiC particle sizes. Dense ZrB 2 -SiC composites consisting of 70 vol% ZrB 2 and 30 vol% a-SiC were produced via hot pressing. This series of composites had maximum SiC particle sizes that ranged from 4.4 to 18 lm. The mechanical properties scaled with the maximum SiC particle size, not with ZrB 2 grain size. Flexural strength decreased as the maximum size of SiC particles increased from 1150 MPa at 4.4 lm to 245 MPa at 18 lm with an abrupt decrease in strength at~11.5 lm. Elastic modulus remained constant at~530 GPa for compositions containing SiC particles smaller than 11.5 lm, but exhibited a decrease with larger SiC particle sizes. Vickers and Knoop hardness were 21.4 and 17.2 GPa, respectively, for ceramics with SiC particle sizes <11.5 lm, but hardness decreased for larger SiC particle sizes. The decreases in strength, elastic modulus, and hardness with SiC particles larger than 11.5 lm were coincident with stressinduced microcracking in the composites. R. Cutler-contributing editor

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