Degradation of Thermal-Barrier Coatings at Very High Temperatures
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to the alloy substrate and to accommodate the mechanical strains arising because of differences in thermal expansion coefficients and elastic moduli between the ceramic and the substrate. The need for increased performance and fuel efficiency in turbine engines has led to demands for higher and higher operating temperatures. This has resulted in the development of single-crystal blade technology, more advanced corrosion-resistant coatings, and complex blade-cooling techniques. Undoubtedly, thermal-barrier coatings will form a key element in future engine designs with predicted gas inlet temperatures approaching 2000°C, leading to temperatures on the order of 1600°C on the surface of the coating. The ideal system for a hot-gas path component would consist of a strain-tolerant thermal-barrier ceramic layer deposited onto a bond coat which exhibits good corrosion resistance and closely matched thermal expansion coefficients. Within the bond coat, the composition can be graded to provide the necessary corrosion resistance, mechanical properties, and diffusion barrier at the interface with the substrate.7 Developments in thermal-barrier coating technology include optimization of the composition of the coating, particularly with respect to the performance of metastable phases, and optimization of the deposition process. For instance, electron beam physical vapor deposition (EBPVD) coatings deposited onto conventional bond coats have shown great improvements in thermalshock resistance, as compared with similar plasma-sprayed coatings.8 Under service conditions, gas-turbine components can be susceptible to various modes of damage, including erosion, oxi-
dation, and hot-salt corrosion. Any coating system needs to be able to resist such damage and remain well-bonded to the substrate throughout the required service life. As temperatures of operation are increased, the problems of oxidation and hot corrosion are likely to change. For instance, bond-coat alloys such as M-Cr-Al-Y, where M is Ni or Co, rely on the development of an A12C>3 scale for oxidation resistance, with the reactive element Y promoting good adhesion of the scale. For kinetic reasons, however, such alloys generally perform better at temperatures above 950°C, probably due to difficulties in establishing the A12O3 scale at lower temperatures. Conventional hot-corrosion problems occur because of salt contaminants, such as Na2SO4/ NaCl mixtures, which deposit on the components, melt, and dissolve the protective scale. Corrosion occurs by an initiation and propagation process and can result in catastrophic rates of damage. Such problems tend to occur over a limited temperature range, viz., above the melting point and be-. low the dewpoint of the salts (a maximum range of about 650 to 950°C for turbine conditions) and may be less important for components where the surface temperatures are well above 1000°C. Although conventional hot-salt corrosion may not cause major damage to future turbine components operating at such high temperatures, it is necessary to consi
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