Oxidation of ultra-high temperature transition metal diboride ceramics

Abstract

The oxidation behaviour of transition metal diboride ceramics is reviewed with emphasis on the performance of zirconium diboride and hafnium diboride. First, the oxidation behaviour of nominally pure diborides is discussed, focusing on the transition to linear mass gain kinetics at temperatures above ∼1100°C. Next, the use of SiC and other additives that produce silica based scales when oxidised is reviewed. These additives improve oxidation protection due to the formation/stability of the outer layer of borosilicate glass that acts as a barrier to diffusion of oxygen to the substrate. However, elevated temperatures (>1650°C) and/or the combination of aerodynamic flow, high heat flux and reactive atmosphere associated with hypersonic flight destabilises the outer oxide and decreases oxidation protection. Other additives that affect the composition and structure of the crystalline oxide scale without forming an outer glassy layer are a promising approach to improving oxidation behaviour of diborides. These additives require further research to understand the mechanisms of improved protection and further optimise the protective behaviour. While the oxidation of ultra-high temperature diborides has been studied for many years, several possible areas for future research are identified.

Keywords:

• Zirconium diboride
• Hafnium diboride
• Oxidation
• Review

Research on UHTCs at Missouri University of Science and Technology has been supported by a number of grants over the years. Currently, UHTC research is supported through the US Air Force Office of Scientific Research/NASA National Hypersonic Science Center for Materials and Structures (grant no. FA9550-09-1-0477), the US National Science Foundation (grant no. DMR-0906584), and the US Air Force Office of Scientific Research (grant no. FA9550-09-1-0168). The authors wish to thank graduate student Ms Maryam Kazemzadeh Dehdashti, who provided the SEM images in Fig. 2. In addition, the authors are thankful for the contributions of Shi C. Zhang and former graduate students Alireza Rezaie and Adam Chamberlain.