Modern gas turbines rely on ceramic coatings to protect structural components along the hot gas path. These coatings are susceptible to accelerated degradation caused by silicate deposits formed when ingested environmental debris (dust, sand, ash) adheres to the coatings. This article reviews the current understanding of the deposit-induced failure mechanisms for zirconia-based thermal barrier coatings and silicate environmental barrier coatings. Details of the debris melting and crystallization behavior, the nature of the chemical reactions occurring between the deposits and coatings, and the implications for the thermocyclic durability of the coatings are described. Given the challenges posed in understanding how prospective coating materials and architectures will respond to a broad range of deposit compositions, it is proposed to develop an integrated framework linking thermochemical and thermomechanical models to predict coating durability. Initial progress toward developing this framework, and the requisite research needs, are discussed.

Multilayer ytterbium-hafnate/silicate coatings deposited by directed vapor deposition and designed to protect SiC-based ceramic matrix composites were assessed to determine their thermochemical stability and resistance to attack by molten silicate deposits (CMAS). The study revealed that reactions occurring at the interface between Yb 2 Si 2 O 7 and Yb 4 Hf 3 O 12 layers promote coating delamination following isothermal annealing for 100 h/1500°C while coating architectures involving Yb 2 SiO 5 in contact with Yb 4 Hf 3 O 12 do not experience similar degradation. The outer Yb 4 Hf 3 O 12 layers, segmented for compliance, were only moderately effective in mitigating CMAS infiltration at 1300°C and 1500°C. The results indicate that the reaction between the melt and coating forms large volumes of a silicate garnet phase at 1300°C, or a cuspidine-type aluminosilicate at 1500°C, in addition to the apatite and reprecipitated fluorite phases observed in related systems.

The phase equilibria between thermal barrier coating (TBC) materials and calcium-magnesiumaluminosilicate melts (CMAS, representing deposits formed when siliceous debris is ingested into modern turbine engines), were investigated at 1300ºC. The primary goal was to understand the influence of the deposit and thermal barrier oxide compositions on the melt solubility limits and reaction product constitution. Model deposit compositions with SiO 2 to CaO ratios from 1.4 to 4.7 and with or without MgO were reacted with yttria-and gadolinia-zirconia thermal barrier oxides ranging from pure ZrO 2 to the pure yttria and gadolinia. The reactions formed various crystalline silicate phases; rare earth-calcium-silicate apatite and zircon (ZrSiO 4) were observed most frequently. Following the reactions, the residual melts were depleted in SiO 2 and generally enriched in CaO, MgO, and Al 2 O 3. The implications of the anticipated changes in the melt viscosity and the cation partitioning between the melt and various solid solution phases on the efficacy of degradation-mitigating crystallization reactions are discussed.

No abstract

No abstract

No abstract

No abstract

No abstract