Thermodynamics of reaction between gas‐turbine ceramic coatings and ingested CMAS corrodents

Abstract

The thermodynamic stability of ceramic coatings with respect to their reaction products is crucial to develop more durable coating materials for gas-turbine engines. Here, we report direct measurements using high-temperature solution calorimetry of the enthalpies of reaction between some relevant ceramic coatings and a corrosive molten silicate. We also report the enthalpy of mixing between the coatings and molten silicate after combining the results measured by high-temperature solution calorimetry with enthalpies of fusion measured by drop-andcatch calorimetry and differential thermal analysis. The enthalpies of solution of selected silicate and zirconia-based coatings and apatite reaction products are moderately positive except for 7YSZ, yttria-stabilized zirconia. Apatite formation is only favorable over coating dissolution in terms of enthalpy for 7YSZ. The enthalpies of mixing between the coatings and the molten silicate are less exothermic for Yb 2 Si 2 O 7 and CaYb 4 Si 3 O 13 than for 7YSZ, indicating lower energetic stability of the latter against molten silicate corrosion. The thermochemical results explain and support the very corrosive nature of CMAS melts in contact with ceramic coatings. K E Y W O R D Scorrosion/corrosion resistance, environmental barrier coatings, thermal barrier coatings

Rare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for resistance against attack by molten calcia–magnesia–aluminosilicate (CMAS) glass

“…Thus, at the Lewis acid-base chemistry level, the reactivity between a crystalline oxide ceramic and an oxide glass is expected to decrease with the decreasing difference between their OBs [16]. Based on this criterion, rare-earth pyrosilicates (RE = rare earth, RE 2 Si 2 O 7 ) such as γ-Y 2 Si 2 O 7 and β-Yb 2 Si 2 O 7 have been identified as promising CMAS-resistant EBC ceramics [16], which have also been studied by others [15,[17][18][19][20][21][22][23][24][25][26][27]. It should be emphasized that the OB difference analysis provides a rough screening criterion based purely on chemical considerations.…”

“…Although there have been several studies focused on the interactions between RE pyrosilicates and CMAS [15,[17][18][19][20][21][22][23][24][25][26][27], there is little known about CMAS interactions with pyrosilicate solid solutions. Fig.…”

High-Temperature Thermodynamics of Cerium Silicates, A-Ce2Si2O7, and Ce4.67(SiO4)3O

“…2,3 However, during the take-off and landing of aircraft, various rare-earth oxyapatites (M 2+ RE 4 (SiO 4 ) 3 O) are known to form as corrosion products between the rare-earth disilicates EBC and molten aerosol mineral dust, composed primarily of CaO-MgO-Al 2 O 3 -SiO 2 (CMAS). 10,11 Such rare-earth oxyapatite corrosion products are thermodynamically stable and can serve as an effective passivating layer to reduce or prevent further pitting and erosion of EBC/CMC on the engines of the aircraft. 10,12,13 Conversely, the potential mismatch of CTEs of the rareearth oxyapatite and the EBC/CMC could also cause increased internal stresses to build promoting mechanical failure.…”

“…10,11 Such rare-earth oxyapatite corrosion products are thermodynamically stable and can serve as an effective passivating layer to reduce or prevent further pitting and erosion of EBC/CMC on the engines of the aircraft. 10,12,13 Conversely, the potential mismatch of CTEs of the rareearth oxyapatite and the EBC/CMC could also cause increased internal stresses to build promoting mechanical failure.…”

Thermochemistry of calcium rare‐earth silicate oxyapatites

“…The formation of these oxyapatite phases is potentially desirable as they have been shown to crystalize quickly to arrest the melt infiltration and destabilize the melt to increase viscosity . The silicate debris mainly composed of CaO–MgO–Al 2 O 3 –SiO 2 (CMAS) are usually ingested by aircraft engines during and after take‐off, sticking to their hot surfaces and resulting in the formation of calcium RE silicate oxyapatites . Outside of their relevance to EBC application, calcium RE oxyapatites and their analogs are of interest in many areas including medical and biomedical sciences, geochemistry, environmental and nuclear sciences, and catalysis among others.…”