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The melting and crystallization behaviors of model calciummagnesium-alumino-silicate (CMAS) compositions relevant to the degradation of thermal barrier coatings (TBCs) were investigated. A primary goal was to establish a baseline for studies on CMAS reactions with TBC materials, reported separately, and their potential to mitigate degradation. Ternary calcium alumino-silicate (CAS) compositions investigated melt below their equilibrium solidus owing to their metastable phase constitution. Additions of MgO or FeO x have significant effects on the melting behavior, depending on the C:A:S proportions. Amorphization on cooling is commonplace, with MgO, AlO 1.5 , and especially FeO x promoting crystallization. The behaviors of amorphous and crystalline versions of the same CMAS are different and depend on heating/cooling rates, with attendant implications for their interaction with TBCs. II. Selection of Model CMAS CompositionsThe composition is critical to the infiltration kinetics through its effects on (1) the onset and range of melting, (2) the viscosity of the melt, g, and its dependence on temperature, (3) the intrinsic crystallization kinetics, and (4) the chemical interactions with candidate TBCs that serve as a foundation for the mitigation strategy. Figure 1 shows the viscosities at 1300°C for a wide range of CMAS compositions reported in the literature (circles), 5 calculated using the heuristic Giordano, Russell, and Dingwell (GRD) model for magmatic melts. 12 The trend in log g is reasonably linear when plotted against the Si:O ratio for compositions within the range defined by a melt with nominally no bridging oxygens, (SiO 4 ) 4À , to a fully connected network (SiO 2 ). The viscosity D. Butt-contributing editor Manuscript No. 34200.

The delamination tendency for bilayer gadolinium zirconate/yttria-stabilized zirconia (GZO/YSZ) thermal barrier coatings (TBCs) was investigated using a continuous laser-based thermal gradient test and a computational model for the analysis of multilayer structures. TBCs with different architectures were exposed to molten silicate compositions, representative of aeroengine deposits, and subjected to thermal cycles prescribed to impart specified levels of strain energy in the coating. The exposed surface of the intercolumnar gaps in the outer part of the GZO layer was found to rapidly dissolve into the intruding molten silicate and precipitate reaction products that seal the flow paths, limiting the penetration depth and the ensuing stiffening of the TBC. Nevertheless, the stiffened layer magnifies the thermal stresses in the coating upon thermal cycling. The influence of the thermal history and multilayer structure on the driving force for delamination was modeled and compared with the experimental results. The effects that the substrate coefficient of thermal expansion, the temperature gradient, and the TBC thickness have on the driving force for delamination were analyzed and the critical amount of stored elastic strain energy for failure under different scenarios was assessed.

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