An Evaluation of Vaporizing Rates of SiO ₂ and TiO ₂ As Protective Coatings for Ultrahigh Temperature Ceramic Composites

Abstract

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 × 10 3 kPa at the Ti/TiO 2 and Si/SiO 2 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., TiSi x ). In addition, at 2500 K the TiO 2 layer would vaporize at a rate of 0.23 mm/s compared with the SiO 2 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.