Processing and Deformation of ZrB2

Abstract

Zirconium diboride, ZrB2, based materials have been proposed for structural applications at ultra-high temperatures (>2000 [degrees] C). However, their mechanical behaviour at such temperatures is only poorly documented. In this work, the processing and the deformation behaviour of ZrB2 at temperatures up to 2000 [degrees] C is investigated. Densification of zirconium diboride based materials is difficult and most reported routes use a combination of high pressures and high temperatures to obtain a high density. However, it had been reported that with the aid of carbon, boron carbide and silicon carbide, pressureless sintering of ZrB2 is possible. Further work in this thesis shows that the key factor to obtain successful sintering is to limit the oxidation of the raw materials. It is shown also that dense materials can be obtained from relatively coarse powders with only carbon as the sintering additive. Adding silicon carbide or boron carbide does allow the grain growth at the sintering temperature to be limited. Mechanical characterisation of these materials was performed firstly using small-scale hardness measurements by nano-indentation at moderate temperatures (25-300 [degrees] C). The indentations were carried out at strain rates in the range 10-4 and 10-1 s-1. An analysis to extract the Peierls stress (6.6 ± 0.7 GPa) and activation energy (2.56 ± 1:6 x 10-19 J) for lattice resistance controlled plastic flow is presented. Additional mechanical characterisation consisted in measuring the self-contact hardness at temperatures from 900-2000 [degrees] C. These measurements clarify that the initial rapid decrease in hardness at room temperature is followed by a region of more or less constant hardness before further decreases in hardness become apparent at the highest temperatures. A TEM investigation of the deformation mechanisms shows that near room temperature, extensive dislocation flow occurs underneath indentations, whereas at the highest temperatures measured in this work, dislocations either anneal out or do not partake in the deformation. The available data was then summarised through proposing a deformation mechanism map for ZrB2.