Atmospheric plasma sprayed thick thermal barrier coatings with high segmentation crack density

Atmospheric plasma sprayed thick thermal barrier coatings (TBCs), comprising of 1.5 mm thickness yttria stabilized zirconia (YSZ) coating, have been developed for increasing thermal protection of combustor applications. Different segmentation crack densities of the YSZ coating were created by controlling the deposition conditions. It was found that the substrate temperature played a dominant role in determining the segmentation crack density. The density was found to increase with the increase of substrate temperature and liquid splat temperature. High passage thickness, if achieved by low plasma gun speed, contributed to improve the segmentation crack density. The density could not be effectively improved by increasing the powder feed rate, although a high passage thickness was obtained in this case. The pores, mainly consisting of delaminations with the radii range between 0.02 and 1 μm, prevent the segmentation cracks from propagation. The coatings sprayed at high substrate temperature show an excellent intersplat bonding and often even continuous columnar grains through several splats. Additionally, horizontally branching cracks were also formed together with the segmentation cracks when a high lamellar thickness was used during the spraying. Thermal cycling tests showed that the coating with a segmentation crack density of approximately 3.6 mm−1 had a lifetime of more than 1700 cycles at 1238 °C (surface)/938 °C (bond coat), indicative of excellent thermal shock resistance. Failure of the TBCs occurred by chipping in the surface layer of YSZ coating, which is different from the traditional interface delamination failure occurred with thinner TBCs.