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

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-and-catch 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 Yb2Si2O7 and CaYb4Si3O13 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.