Bulk ultrahard composites in the eutectic TiB2-TiC system by SHS under high gravity

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Abstract
Eutectic TiB2-TiC composite ceramics were prepared by combustion synthesis under high gravity. XRD, SEM, and EDS results showed that TiB2-TiC composites were mainly composed of the eutectic microstructures of a TiC matrix in which a large number of fine TiB2platelet grains were dispersed uniformly; meanwhile, at the boundaries of the eutectic microstructures, discontinuously dispersed ɛ-carbides enriched in Ti atoms, and a few isolated irregular α-Al2O3 grains and Al2O3-ZrO2 colonies were observed. Because high-temperature chemical reaction results in fully liquid products, the application of high gravity induces the Stocks flow in the melts, which leads to the formation of layered melts consisting of liquid Ti-Cr-C-B melt and liquid oxides. Therefore, it is considered that TiB2-TiC composites grow through eutectic transformation far away from the equilibrium state. The results of properties measurements indicate that, with increasing mass fraction of B4C + Ti + C in combustion systems, the relative density and fracture toughness of TiB2-TiC composites are all among 97–99% and 6.5–7.1 MPa m1/2, respectively, and the Vickers hardness and flexural strength are increased gradually to the maximum values of 28.6 GPa and 615 MPa, respectively. The achievement of full-density TiB2-TiC composites benefited from the design of fully liquid SHS products and application of high-gravity field, a high hardness of the composite ceramics resulted from the absence of intermediate borides, the achievement of stoichiometric TiC phases is due to rapid solidification, whereas a high flexural strength of the composite ceramics benefited from the homogenization and refinement of the microstructures due to the rapid separation of the liquid oxides and the rapid coupled growth of TiB2-TiC.

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