Low-Temperature Processing of ZrB 2 -ZrC Composites by Reactive Hot Pressing
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REFRACTORY transition metal borides and carbides, such as ZrB2, ZrC, HfB2, HfC, and TaC, have been called ultra-high-temperature ceramics (UHTCs), because of their high melting points (>3000 C), better oxidation and corrosion resistance. The UHTCs have been investigated for applications that include thermal protection materials for advanced re-entry vehicles, molten metal crucibles, and high-temperature electrodes.[1–3] The high melting point, strong covalent bonding and low self-diffusion coefficient of ZrB2-based composites typically require pressure-assisted sintering at very high temperatures (>1900 C). The ZrC0.9-ZrB2 composite has been produced by sintering and directional solidification in argon atmosphere at 2300 C.[4,5] The hardness and fracture toughness of the composites were pffiffiffiffi 24 GPa and 5.44 MPa m, respectively. Composites of ZrB2-ZrC have been fabricated by infiltration of Zr metal into the B4C preform at 1850 C to 2000 C for 1 to 2 hours.[6–10] These composites showed the formation of ZrB2-ZrCx with free Zr metal. The self-propagating hightemperature synthesis of mechanically alloyed Zr-B-C powder (MA-SHS) was used to produce the ZrB2-ZrC powder composite in air followed by spark plasma sintering (SPS). The relative density (RD), hardness LINGAPPA RANGARAJ, Scientist ‘‘E1’’, and CANCHI DIVAKAR, Scientist ‘‘F’’, are with the Materials Science Division, National Aerospace Laboratories, Bangalore-560 017, India. SIRIYARA J. SURESHA, Materials Research Centre, and VIKRAM JAYARAM, Professor, Department of Materials Engineering, are with the Indian Institute of Science, Bangalore-560 012, India. Contact e-mail: [email protected] This article is based on a presentation given in the symposium entitled ‘‘Materials Behavior: Far from Equilibrium’’ as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred during December 15–16, 2006 in Mumbai, India. Article published online March 28, 2008 1496—VOLUME 39A, JULY 2008
and fracture pffiffiffiffi toughness were 98 pct, 17.8 GPa, and 3.4 MPa m, respectively.[11–12] High-density ZrB2-based materials have also been obtained by hot pressing at temperatures lower than 2000 C with the addition of sintering aids such as Ni[13] and pressureless sintering at temperature ~1850 C with the addition of B4C.[14] SPS of ZrB2-ZrC composites with the addition of lanthanum (1 to 5 wt pct) enhanced the RD to 99 pct and reduced the densification temperature to 1800 C.[15] Reactive hot pressing (RHP) of Zr and B4C powder mixture at 30 MPa and 1900 C for 60 minutes resulted in 98.4 pct RD with a bending strength of ~450 MPa.[16] The two-stage transient plastic phase processing of three different molar ratios of Zr to B4C (3:1, 3.5:1, and 4:1) at a final temperature of 1600 C for 4 hours yielded ZrB2-ZrC composites with full density.[17] It has been recently shown that high energy milling of elemental Zr and B can be used to produce ZrB2 at temperatures as low as 600 C, and dense ZrB2-SiC composites can be obtained at 1700 C.[18] Becau
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