Chemical Interactions in Diboride‐Reinforced Oxide‐Matrix Composites

Thermochemical analyses of interfacial reactions in titanium, zirconium, and hafnium diboride reinforced oxidematrix composites have been carried out to evaluate the chemical compatibility. The chemical reactivity of these diborides with oxygen and the high volatility of B 2 O 3 ( l ) at reduced oxygen pressures are concerns during processing and operating conditions. The thermochemical stability and the vaporization behavior of B 2 O 3 ( l ) are discussed in terms of partial pressures of dominant gaseous species of the boron‐oxygen system at 1700 and 2300 K. The TiB 2 /ZrO 2 and TiB 2 /HfO 2 systems are thermodynamically stable in a limited oxygen pressure range. The TiB 2 /Al 2 O 3 system is stable, but the reactions in this system may apparently be accompanied by formation of gaseous products (B 2 O 3 , AlO, Al 2 O, and lower boron oxides) in the presence of elemental oxygen. These thermochemical considerations are very useful in evaluating the effectiveness of oxides as diffusion barrier coatings on diboride reinforcements.