An evaluation of vaporizing rates of SiO2 and TiO2 as protective coatings for ultrahigh temperature ceramic composites

For temperatures > 1973 K, the thermodynamic and kinetic analyses of the major gaseous species for a liquid titanate layer would vaporize significantly less than a silicate layer, when considering these layers as a protective barrier for ultrahigh temperature ceramic composites. At 2500 K, the major species is TiO(g) with p(TiO(g))=0.1 kPa compared with SiO(g) with p(SiO(g))=1.3 x 10(3) kPa at the Ti/TiO2 and Si/SiO2 equilibrium, respectively. The SiO(g) attains a partial pressure greater than ambient pressure at 2500 K even with a thermodynamic activity of 0.01 considering equilibration with a silicide (e.g., TiSix). In addition, at 2500 K the TiO2 layer would vaporize at a rate of 0.23 mm/s compared with the SiO2 layer's loss rate of 207 mm/s. Although the oxygen diffusivity and permeability through titanate solutions must be further analyzed, the thermodynamic and kinetic analyses for vaporization indicate a longer duration for a liquid titanate than for a liquid silicate layer at ultrahigh temperatures.