Mass spectrometric measurements of the silica activity in the Yb2O3-SiO2 system and implications to assess the degradation of silicate-based coatings in combustion environments

The Yb2O3-SiO2 system is a promising coating material for silicon-based ceramics and composites in combustion environments due to the low silica activity in this silicate system. These activities lead to lower reactivity with the water vapor component of a combustion environment and hence less formation of Si(OH)4(g). In this study the silica activities in the Yb2O3-SiO2 system are measured via a vapor pressure technique. Reducing agents are used to increase the vapor pressure of SiO(g) so that it is measureable in the temperature range of interest. The measured SiO(g) pressures are then used to calculated the silica activities. Activities are reported as a function of temperature for the Yb2O3 + Yb2SiO5 and Yb2SiO5 + Yb2Si2O7 biphasic fields. Combining the results of this work with those reported earlier, it was found that the flux of silicon tetra-hydroxide and the thermodynamic activity of silica are lower in the biphasic fields of yttrium silicates and yttrium oxide than the biphasic field consisting of ytterbium silicates and ytterbium oxide. This difference has been attributed to the smaller ionic potential of yttrium and higher optical basicity of yttrium silicates when compared to the bigger ionic potential of ytterbium and smaller optical basicity of ytterbium silicates in these systems. The measured activities are then used to calculate some representative Si(OH)4(g) fluxes from Yb2O3-SiO2 compounds in a typical laboratory test furnace and compared to literature values.