Effect of applied pressure on densification of monolithic ZrC x ceramic by reactive hot pressing
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Tamoghna Chakrabarti Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, Karnataka, India
Rajaguru Kannan Materials Science Division, CSIR-National Aerospace Laboratories, Bangalore 560017, Karnataka, India
Vikram Jayaram Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, Karnataka, India (Received 28 October 2015; accepted 15 January 2016)
The effect of applied pressure on reactive hot pressing (RHP) of zirconium (Zr):graphite (C) in molar ratios of 1:0.5, 1:0.67, 1:0.8, and 1:1 was studied at 1200 °C for 60 min. The relative density achievable increased with increasing pressure and ranged from 99% at 4 MPa for ZrC0.5 to 93% for stoichiometric ZrC at 100 MPa. The diminishing influence of pressure on the final density with increasing stoichiometry is attributed to two causes: the decreasing initial volume fraction of the plastically deforming Zr metal which leads to the earlier formation of a contiguous, stress shielding carbide skeleton and the larger molar volume shrinkage during reaction which leads to pore formation in the final stages. A numerical model of the creep densification of a dynamically evolving microstructure predicts densities that are consistent with observations and confirm that the availability of a soft metal is primarily responsible for the achievement of such elevated densification during RHP. The ability to densify nonstoichiometric compositions like ZrC0.5 at pressures as low as 4 MPa offers an alternate route to fabricating dense nonstoichiometric carbides.
I. INTRODUCTION
Transition metal carbides (TMCs) including TiC, ZrC, HfC, and TaC have high melting temperature ($3000 °C), high hardness (20–30 GPa), high modulus ($400 GPa), good strength, and good wear-erosion properties.1,2 Generally, TMCs have a wide range of stoichiometry with the carbon:metal ratio ranging from 0.5 to 1.0.3,4 Out of all TMCs, ZrC as a monolith and also as a composite with ZrB2 has many potential applications including metal melting crucibles, nuclear reactors, and also structural components in re-entry vehicles.5,6 One of the problems encountered in processing of dense ZrC ceramics is the high temperature required. Early work indicated that hot pressing (HP) is required to produce reasonably dense ZrC at 2000–2300 °C.7 Nonisothermal sintering of ZrC at ;2300–2900 °C led to reasonably dense samples with considerable grain growth.8 Recently, it was shown that reactive hot pressing (RHP) of different molar ratios of carbon to zirconium powder mixtures at
Contributing Editor: Yanchun Zhou a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.30 506
J. Mater. Res., Vol. 31, No. 4, Feb 29, 2016
40 MPa can yield high relative density (RD) of the carbide phase at temperatures as low as 1200 °C, depending on the stoichiometry.9 The minimum limit of carbon that can be accommodated in the rock-salt phase has been shown to be as low as 33 mol%, which is an extension of the phase range reported earlier in binary Zr–C phase
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