The higher operating temperatures in gas-turbine engines enabled by thermal barrier coatings (TBCs) engender new materials issues, viz silicate particles (sand, volcanic ash, fly ash) ingested by the engine melt on the hot TBC surfaces and form calcium-magnesium-alumino-silicate (CMAS) glass deposits. The molten CMAS glass degrades TBCs, leading to their premature failure. In this context, we have used the concept of optical basicity (OB) to provide a quantitative chemical basis for the screening of CMAS-resistant TBC compositions, which could also be extended to environmental barrier coatings (EBCs). By applying OB difference considerations to various major TBC compositions and two types of important CMASs -desert sand and fly ash-the 2ZrO 2 ÁY 2 O 3 solid solution (ss) TBC composition, with the potential for high CMAS-resistance, is chosen for this study. Here, we also demonstrate the feasibility of processing of 2ZrO 2 ÁY 2 O 3 (ss) air-plasma sprayed (APS) TBC using commercially developed powders. The resulting TBCs with typical APS microstructures are found to be single-phase cubic fluorite, having a thermal conductivity <0.9 WÁ(mÁK) -1 at elevated temperatures. The accompanying Part II paper presents results from experiments and analyses of high-temperature interactions between 2ZrO 2 ÁY 2 O 3 (ss) APS TBC and the two types of CMASs.

Fracture toughness of thermal barrier coatings (TBCs) has gained significant interest in recent years as one of the dominant design parameters dictating selection of materials and assessing durability. Much progress has been made in characterizing and understanding fracture toughness of relevant TBC compositions in their bulk form, but it is also apparent that the toughness is significantly affected by process‐induced microstructural defects. In this investigation, a systematic study of the influence of coating microstructure on the fracture toughness of atmospheric plasma‐sprayed TBCs has been carried out. Yttria partially stabilized zirconia (YSZ) coatings were fabricated under different process conditions inducing different levels of porosity and defect densities. Fracture toughness was measured on free‐standing coatings in as‐processed and thermally aged conditions using the double torsion technique. Results indicate significant variance in fracture toughness among coatings with different microstructures including changes induced by thermal aging. Comparative studies were also conducted on an alternative composition, Gd2Zr2O7 which, as anticipated, shows significantly lower fracture toughness compared to YSZ. The results not only point toward a need for process and microstructure optimization for enhancing TBC performance, but also a framework for establishing performance metrics for promising new TBC compositions.

Thermal barrier coatings (TBCs) used in gas-turbine engines experience severe degradation by calcia-magnesia-alumino-silicate (CMAS) deposits during high-temperature operation. The present study identified and evaluated the chemical and microstructural changes in air plasma-sprayed (APS) 7 wt.% Y 2 O 3 stabilized ZrO 2 (7YSZ) TBCs caused by CMAS attack under isothermal conditions at 1340 ˚C. Additionally, a 'model' experimental study was conducted by characterizing 7YSZ ceramic powders immersed in molten CMAS glass at 1300 ˚C for different exposure times. The combined results from both studies highlight the importance of local CMAS glass composition on the 7YSZ/CMAS interaction. Specifically, low Y-content in the glass, caused by a relatively large glass 'sink,' produces Y-depleted ZrO 2 grains that undergo tetragonal (t)  monoclinic (m) phase transformation upon cooling. Alternatively, small pockets of Y-enriched glass induce the formation of t"-ZrO 2 , a phase characterized by its high stabilizer content. After prolonged high-temperature exposure, solution-reprecipitation induces the formation of both m-ZrO 2 and t"-ZrO 2 throughout the APS 7YSZ TBC in accordance with the phase diagram. Using a thermomechanical model it is shown that the strain associated with the martensitic tm phase transformation plays an important role in the delamination failure of TBCs attacked by CMAS.

Thermally sprayed ceramics, when infiltrated with polymer, exhibit synergistic increases in strength and toughness. The structure of such composites-a dense, brick-mortar arrangement-is strikingly similar to that of nacre, as are the mechanisms underlying the robust mechanical behavior. This industrial-scale process thus presents an exciting tool for bio-mimetic exploration.

Thermal barrier coatings (TBCs) are increasingly playing a vital role in enhancing efficiency and performance of gas turbine engines. As engine operating temperatures rise, yttria-stabilized zirconia (YSZ), the currently principal TBC material, reaches its operational limits. Gadolinium Zirconate (GDZ)-based pyrochlore oxides are now emerging contenders, not only due to their lower thermal conductivity, but also their ability to resist attack by silicate deposits. However, GDZ cannot be directly substituted for YSZ due to its incompatibility with the thermally grown alumina layer, therefore requiring to be a component of multilayer system. Although industry has already adopted these materials in various applications, a number of fundamental issues arise with respect to layered-coating design, their properties and processing dependence. In this study several multilayer architectures, based on the YSZ-GDZ system, have been developed and tested for durability under furnace thermal cycling conditions. Coating designs considered optimization of microstructure and properties of individual layers based on their location within the top-coat thickness to address competing interests of thermal conductivity, compliance, and resistance to silicates. A large variance in durability was observed in coatings made with different multilayer designs. The results and the associated failure mechanisms are rationalized through preliminary evaluation of elastic energies at the failure locations and corresponding energy release rates. The results point to new strategies in the design and manufacturing of optimal multilayer coatings.