Effect of in situ SiC and MoSi2 phases on the oxidation behavior of HfB2-based composites

Monolithic HfB2 and HfB2-15vol%SiC-15vol%MoSi2 composite samples were oxidized by a conventional electric furnace at 1700 degrees C for 5 h. Microstructural and phase analysis of the oxidized samples were performed by X-ray diffraction (XRD) analysis and field emission scanning electron microscope (FESEM) equipped with energydispersive spectroscopy (EDS). Besides, for analyzing the oxidation mechanism of the samples, thermodynamic calculations were also accomplished by HSC software. The changes in weight and thickness of the oxide scale were measured and the oxide growth rate of the oxidized samples was subsequently calculated. The results showed that HfB2-15vol%SiC-15vol%MoSi2 composite was much more resisted than that monolithic HfB2 due to the formation of a thin Si-rich glass layer on the surface of the composite sample. By acting to fill the porosities between HfO2 grains, Si-based glass phase enhanced the oxidation resistance of HfB2-15vol%SiC-15vol%MoSi2 composite. Conversely, the oxidized monolithic HfB2 had only a thick porous oxide layer (HfO2) which led to considerably lower oxidation resistance. On the other side, three layers containing HfO2 and Si-based glass phases were formed on the oxidized HfB2-15vol%SiC-15vol%MoSi2 composite. Moreover, no porosities and no porous layers were also detected on the oxidized composite sample. Consequently, HfB2-15vol%SiC-15vol% MoSi2 composite had a parabolic behavior owing to its diffusion-controlled oxidation under the isothermal oxidation process.