UC Santa Barbara

Efforts to improve gas turbine fuel efficiency face a significant obstacle in the degradation of thermal and environmental barrier coatings (T/EBC) made of refractory oxides. Although these coatings effectively protect against heat and water vapor, they inadequately address the challenges posed by molten silicates made of engine-ingested ash, sands, and dusts. Additionally, certain candidate coating materials are susceptible to microcracking driven by anisotropy, compromising their ability to prevent ingress of gas species that hasten component failure. This dissertation features new computational and experimental approaches to investigate and design T/EBC materials, offering fundamental insights into molten silicate attack and grain boundary microcracking.

The first aspect of this dissertation addresses the need to develop a protocol for selecting compositions of silicate deposits for the purpose of assessing performance of candidate coating materials. The crux of the study is the statistical analyses (i.e. principal component analysis and \textit{k}-means clustering) of a curated database to identify exemplary compositions. Potential chemical interactions between the exemplary silicates and several coating materials were explored using thermodynamic calculations. The computational framework is expected to inform future work on investigating potential attributes and deficiencies of new coating materials.

The second aspect of this dissertation explores the potential of HfO$_2$-based coating materials as barriers to molten silicates. (These materials are attractive candidates because of their high-temperature phase stability and resistance to water-vapor mediated volatilization.) Since pure HfO$_2$ is susceptible to extensive grain boundary microcracking, two alternatives were explored: one based on crack-free hafnia/hafnon composites, and one based on hafnate compounds, building on the understanding of corresponding zirconate compounds.

High-temperature exposures of the hafnia/hafnon composites to two exemplary silicates revealed that in all cases interactions include extensive grain boundary penetration, presumably driven by the lack of chemical equilibrium between the composite constituents and the silicate melts. The findings prompted a comparative study of the interactions of Gd-hafnate and Gd-zirconate with the exemplary silicate melts. As with the hafnia/hafnon composites, the melts readily penetrate the Gd-hafnate, but not the Gd-zirconate, owing to rapid co-formation of protective barrier layers of apatite and fluorite above the zirconate. Further insights on the underlying cause of these differences were gleaned from complementary studies in which short exposures (1-3 min) using Gd-lean versions of the compounds were used to quantify the kinetics of the interactions. The results showed that Hf$^{4+}$ diffuses in the melt more slowly than Zr$^{4+}$. Further, the findings implied that the dissolution processes are diffusion-controlled and that the slower kinetics of the processes as well as the subsequent crystallization necessary for protection of the hafnia-based systems may be the root cause of their poor resistance to silicate attack. More broadly, the work demonstrates how fundamental studies of the kinetics of dissolution, diffusion, and crystallization can inform coating material selection and assessment.

The third aspect of this dissertation responds to a longstanding need for high throughput assessments of the driving forces for grain boundary microcracking in brittle materials of high anisotropy. The framework is based on a finite element approach to computing the energy release rate (ERR) for intergranular cracking. Simulations of bicrystals and polycrystals comprising periodic arrays of hexagonal grains were performed for 35 materials covering all 7 crystal systems. The assessments reveal that, while crystal system is not a determinant of likelihood for cracking, materials with large thermal and elastic anisotropy are more sensitive to grain orientations. Moreover, ERR distributions for bicrystals and polycrystals of the same material are in reasonable agreement with one another, implying that the details of neighboring grains in the polycrystals are relatively unimportant with regard to the average crack driving forces. Therefore, future studies might avoid the computational cost of simulating large polycrystals by opting for bicrystals alone, with the recognition that the details of the tails of the ERR distributions may not be accurately depicted. From a broader perspective, the high throughput nature of the approach should find considerable utility in not only the design of monolithic and multi-phase coatings, but also patterned and textured materials that avoid microcracking.