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 pTiO(g)=0.1 kPa compared with SiO(g) with pSiO(g)=1.3 × 103 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.
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Document Type: Research Article
Affiliations: Department of Mechanical Engineering, University of Texas at El Paso, El Paso, Texas 79968
Publication date: May 1, 2008