Silica Activity Measurements in the Y₂O₃–SiO₂ System and Applications to Modeling of Coating Volatility

Abstract

Rare‐earth silicate compounds, such as those in the Y2O3–SiO2 system, are promising candidates for coatings of SiC‐based ceramics and ceramic matrix composites in combustion environments. The predicted lower activity of silica in these silicates will lead to less reactivity with the water vapor combustion products. A procedure for measuring silica activities in this system is discussed. Knudsen effusion mass spectrometry is used and the measured vapor pressure of SiO(g) is correlated to activity. Due to the low vapor pressure of SiO(g) in the temperature range of interest, a reducing agent is utilized to boost this vapor pressure without altering the solid‐state composition. In addition, corrections are made for nonequilibrium vaporization. The measured silica activities are: Y2O3 + Y2O3·SiO2 two phase field: log[a(SiO2)] = −5200.26(1/T) + 0.0567 (1532 < T(K) < 1670) Y2O3·SiO2 + Y2O3·2SiO2 two phase field: log[a(SiO2)] = 4.2252(1/T)−0.5531 (1628 < T(K) < 1747) It is shown how these results can be used to predict reduced volatilization rates of the coatings.

Reactions of molten silicate deposits with yttrium monosilicate

“…Disilicates are of interest because of their closely‐matched thermal expansion coefficients (CTE) with the composite and because they are in equilibrium with the SiO 2 scale that invariably forms on the underlying Si‐based bond coat. Although the volatilization resistance of disilicate coatings is superior to that of SiC, further improvements are obtained by coating the disilicate with an outer layer of an RE monosilicate because the silica activities (and thereby volatilization rate) of monosilicates are an order of magnitude lower than those of the corresponding disilicates . The monosilicates, however, exhibit significant CTE mismatch with the composite, making design of robust coatings particularly challenging.…”

Stability of the Y2O3–SiO2 system in high‐temperature, high‐velocity water vapor

“…The phase diagram for the Y 2 O 3 ‐SiO 2 system in Figure shows four line compounds, Y 2 O 3 , Y 2 SiO 5 , Y 2 Si 2 O 7 , and SiO 2 . The Y 2 O 3 and SiO 2 activities across the compositional space of this system have been experimentally determined at 1650°C, with the results presented in Figure . The activity of SiO 2 varies from 1 in the SiO 2 –Y 2 Si 2 O 7 phase field to 0.28 in the Y 2 Si 2 O 7 –Y 2 SiO 5 phase field and to 1.6 × 10 −3 in the Y 2 SiO 5 ‐Y 2 O 3 phase field.…”

“…Experimentally determined activities of Y 2 O 3 and SiO 2 at 1650°C corresponding to the phase fields in Figure (Ref. ) [Color figure can be viewed at wileyonlinelibrary.com]…”

Matrix Development for Water Vapor Resistant SiC-Based Ceramic Matrix Composites

“…oxides or have high silica activities can be accurately assessed using the steam-jet furnace. At temperatures of 1300°C and 1400°C the partial pressure of Si(OH)4 (g) in the water vapor from the fused quartz capillary is high enough that it approaches equilibrium values saturating the gas stream and limiting SiO2 volatility from reduced SiO2 activity materials such as rare-earth silicates 72,73 ( Figure 3.4). Therefore, a capillary made of a material that does not react with high temperature water vapor (e.g.…”

“…To enable steam-jet testing of materials that have reduced SiO2 activity, such as rareearth silicates 72,73 , the fused quartz capillary must be replaced with a material that does not react with high-temperature water vapor. The success of the Pt/Rh wire used inside the fused quartz capillary to assist in the heating of the liquid water suggests that this material may be suitable for use as a non-reactive capillary.…”