It is generally known that the porosity of thermal barrier coatings is essential to guarantee a sufficiently high strain tolerance of the coating during thermal cycling. However, much less is known about the influence of the specific kind of porosity such as micro cracks and typically larger pores on the performance of the coatings. Both features are usually formed during plasma spraying of yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). In this investigation the influence of micro cracks on the thermal cycling behavior was studied. The amount of micro cracks within YSZ thermal barrier coatings was changed by changing the powder feed rate. However, only small changes of the total porosity were observed by changing powder feed rate. Mercury porosimetry served as a tool to investigate both the amount of micro cracks and pores in the coating. Additionally, micro crack densities were determined from metallographical investigations. A linear dependence between the amount of fine pores determined by Hg porosimetry and the crack density was obtained for one set of coatings. Thermal cycling specimens with thermal barrier coatings having different micro crack densities were produced and tested in a gas burner test facility. At high surface temperatures above 1300°C failure occurred in the ceramic close to the surface. Under these conditions the samples with increased horizontal micro crack densities showed a significant increase of thermal cycling life.

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