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

The previously unknown experimental HfO 2 -Ta 2 O 5 -temperature phase diagram has been elucidated up to 3000°C using a quadrupole lamp furnace and conical nozzle levitator system equipped with a CO 2 laser, in conjunction with synchrotron X-ray diffraction. These in-situ techniques allowed the determination of the following: (a) liquidus, solidus, and invariant transformation temperatures as a function of composition from thermal arrest experiments, (b) determination of equilibrium phases through testing of reversibility via in-situ X-ray diffraction, and (c) molar volume measurements as a function of temperature for equilibrium phases. From these, an experimental HfO 2 -Ta 2 O 5 -temperature phase diagram has been constructed which is consistent with the Gibbs Phase Rule.

Structure and thermodynamics of pure cubic ZrO2 and HfO2 were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their melting points (2710 and 2800 °C). Computations were performed using automated ab initio molecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 ± 0.09% for ZrO2 and 0.87 ± 0.08% for HfO2), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 ± 7 kJ/mol for ZrO2 and 61 ± 10 kJ/mol for HfO2, compared with 54 ± 2 and 52 ± 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO2 and HfO2 are similar and reach (4 ± 1)·10−5/K from experiment and (5 ± 1)·10−5/K from computation. An agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO2 and ZrO2, volume change on melting, and thermal expansion of the liquid to 3127 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO2 is an excellent oxygen conductor, perhaps even better than YSZ.

Design, calibration, and operation of a system for drop‐and‐catch (DnC) calorimetry on oxides from temperature above 1500°C are described. This system allows the measurement of heat contents and heats of fusion by drop calorimetry of small (100 mg or less) samples held by containerless levitation at high temperature and dropped in a calorimeter at room temperature. The spheroids, 2‐3 mm in diameter, prepared by laser melting of powders, are aerodynamically levitated in a splittable nozzle levitator and laser heated to the desired temperature monitored by radiation thermometry. The sample is dropped by splitting the nozzle and caught by splittable water‐cooled calorimetric plates at 25°C, which provide complete enclosure of the sample to avoid heat loss by radiation. The drop time is ~0.1 seconds, calorimeter equilibration time after the drop is ~15 minute. DnC experiments are automated with software‐controlled laser power and programmable delay between splitting the nozzle and catching the sample. The fusion enthalpy of Al2O3 measured by DnC calorimeter, 120 ± 10 kJ/mol, agrees well with previously reported values. The system can be used for measurements of fusion enthalpies of refractory oxides amenable to laser heating as well as for splat quenching of oxide melts.

Drop-n-catch" calorimetry was performed on Y 2 O 3 spheroids of 2-3 mm in diameter, prepared by laser melting of powders. Samples were aerodynamically levitated in a splittable nozzle levitator in air or argon flow, laser heated from ~2200 to 3000 °C and dropped into a calorimeter at 25 °C, thus measuring their enthalpy as a function of temperature. The fusion enthalpy of cubic Y 2 O 3 was derived from the step in the temperature-enthalpy curve as 119 ± 10 kJ/mol. Calculations performed using density functional theory and molecular dynamics techniques produced the value 127 ± 3 kJ/mol Y 2 O 3. This combined methodology enables accurate determination of the enthalpies of fusion and phase transition of refractory oxides, including those containing lanthanides and actinides.