Effects of SiC or MoSi2 second phase on the oxide layers structure of HfB2-based composites

Monolithic HfB2, HfB2-30 vol% SiC and HfB2-10 vol% MoSi2 composites were prepared by SPS and oxidized in stagnant air at 1500 °C for 70 min. The microstructure of the oxide layer cross-sections showed that the oxidation extents were as follow: monolithic HfB2 > HfB2-30 vol%SiC > HfB2-10 vol% MoSi2.According to the EDS Line-scan, only one porous oxide layer containing a minor amount of B2O3was found on the HfB2 oxidized surface whereas a thick silicate glass layer and a porous oxide layer below that existed on the surface of HfB2-30 vol% SiC. After oxidation, the surface of HfB2-10 vol% MoSi2 had a narrow silicate-oxide compact layer covered by a very thin glass layer. X-ray diffraction patterns of the oxidized surfaces showed the monolithic HfB2,the HfB2-30 vol% SiC and HfB2-10 vol% MoSi2composites contain, upon oxidation, only m-HfO2 phase, mainly m-HfO2 with a minor amount of HfSiO4 and mainly HfSiO4 with a minor amount of m-HfO2 phases, respectively. Based on the observations in this study, it is suggested that the elimination of the porous layer and subsequent increase of the HfSiO4 phase are the main reasons for the better oxidation resistance of HfB2-10 vol% MoSi2.