LINGAPPA RANGARAJ, SIRIYARA J. SURESHA, CANCHI DIVAKAR, and VIKRAM JAYARAM Dense ZrB 2 -ZrC and ZrB 2 -ZrC x~0.67 composites have been produced by reactive hot pressing (RHP) of stoichiometric and nonstoichiometric mixtures of Zr and B 4 C powders at 40 MPa and temperatures up to 1600°C for 30 minutes. The role of Ni addition on reaction kinetics and densification of the composites has been studied. Composites of~97 pct relative density (RD) have been produced with the stoichiometric mixture at 1600°C, while the composite with 99 pct RD has been obtained with excess Zr at 1200°C, suggesting the formation of carbon deficient ZrC x that significantly aids densification by plastic flow and vacancy diffusion mechanism. Stoichiometric and nonstoichiometric composites have a hardness of~20 GPa. The grain sizes of ZrB 2 and ZrC x~0.67 are~0.6 and 0.4 lm, respectively, which are finer than those reported in the literature.

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Synthesis and densification of monolithic zirconium carbide (ZrC) has been carried out by reactive hot pressing of zirconium (Zr) and graphite (C) powders in the molar ratios 1:1, 1.25:1, 1.5:1, and 2:1 at 40 MPa, 12001-16001C. Monolithic ZrC could be synthesized with a C/Zr ratio B0.5-1.0 and the post heat-treated samples have the lattice parameter in the range 4.665 to 4.698 Å . Densification improves with an increasing deviation from the stoichiometry. Fine-grained (B1 lm) and nearly fully dense material (99% RD) could be obtained at a temperature as low as 12001C with C/Zr B0.67. Microstructural and XRD observations suggest that densification occurred at low temperatures with nonstoichiometric Zr-C powder mixtures.

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