Microstructure and Thermal Cycling Behaviour of Atmospheric Plasma Sprayed Thermal Barrier Coatings with High Segmentation Crack Densities

A high strain tolerance of thermal barrier coatings (TBCs) is favourable for the application of such coatings in hot gas sections of turbines. In order to improve strain tolerance of atmospheric plasma sprayed (APS) TBCs, coatings with a thickness of 500µm and very high segmentation crack densities produced with fused and crushed (f&c) yttria stabilized zirconia (YSZ) have been developed. Using a Triplex II plasma gun and an optimized spraying process, we obtained coatings with segmentation crack densities up to 8.5 cracks/mm, while the porosity of the samples was lower than 6%, which is typical for segmented coatings. The density of branching cracks was quite low which is inevitable for a good interlamellar bonding.
Thermal cycling tests yielded promising strain tolerance behaviour for the manufactured coatings. Samples with high segmentation crack densities showed a promising lifetime in burner rig tests at rather high surface (1350°C) and bondcoat temperatures (1050°C). Coatings with lower crack densities had a reduced burner rig performance. Microstructure investigations showed that the segmentation crack network was stable during thermal shock testing for different crack densities. The main failure mechanism was delamination and horizontal cracking in the vicinity of the interface between the thermal grown oxides (TGO) and TBC.