Yttrium silicates are among the candidates for protection of silicon-based ceramics in high temperature and moist environments due to chemical and mechanical compatibility with substrate, low volatility and moisture resistance. Here we reported on the development of yttrium silicate coatings by sol precursor plasma spraying. The use of a sol feedstock allowed easy composition flexibility. The microstructure and the structure of as-sprayed and heat-treated coatings were investigated. Finer microstructure was obtained compared to micrometric powder plasma spraying traditionally used to produce environmental barrier coatings (EBC). XRD analyses on as-sprayed coatings revealed amorphous or crystalline layers depending on plasma parameters. In EBC application, a volume change from crystallization or phase transformation was envisaged to be damaging due to induced stresses and fully crystalline phases are a key durability requirement for EBC from conventional plasma spraying. Yttrium silicates are characterized by an important polymorphism and the ability to form amorphous coatings. Therefore, special attention was so paid to the amorphous degree of the coatings.
This work deals with ZrB2-based coatings prepared by inert plasma spraying and their behavior under high heat flux in moist atmospheres. ZrB2 coatings with different compositions and microstructures were produced and subjected to high-temperature oxidation testing in order to identify the most oxidation-resistant sample. It is shown that coating microstructure can significantly influence oxidation kinetics and that uniformly dispersed nanoscale additives are particularly effective for slowing oxidation.
The aim of this study is to evaluate the thermal lifetime properties of yttria-stabilized zirconia (YSZ) coatings with a columnar microstructure. YSZ suspensions were sprayed under different conditions in order to obtain a sample lot with columnar microstructures varying from well-separated to closely spaced. Thermo-cyclic fatigue (TCF) tests were performed at 1100 °C and the results are presented. Coatings with well-separated columns reached 2150 cycles prior to failure compared to 1300 cycles in the case of coatings with compact columns. The apparent lower TCF resistance is attributed to a loss of thermal compliance inducing the development of sharp intercolumnar cracks. Failures seem to be linked to debonding at the TGO-substrate interface. The bond coat and substrate surface roughness also play a role in such failures and their impact is discussed.
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