This study compares the deposition and oxidation behavior of two oxide-dispersed CoNiCrAlY powders, one commercially obtained, the other prepared in a high-energy attrition ball mill using CoNiCrAlY and nanosize α-alumina powders. The custom powder was deposited by HVOF spraying using two sets of parameters, one optimized for CoNiCrAlY powder, the other for fine alumina. Coatings produced under the latter conditions were found to be porous, which can be attributed to a low degree of melting in the dispersed alumina. Isothermal oxidation testing at 1373 K for up to 1000 h in air caused oxidation not only at the surface, but also inside the coatings due to the movement of oxygen through the pores. The coatings deposited under the other set of parameters, i.e., at higher power levels, were free of pores. Isothermal oxidation tests were also carried out on coatings produced from the commercial powder, in this case, by HVOF and as well as vacuum plasma spraying. The coatings obtained by HVOF spraying were found to have a thinner thermally grown oxide layer than not only the VPS coatings, but also conventional metallic bond coats. Internal oxidation in the HVOF coatings is due to insufficient cohesion of the spray particles. Furnace cycling tests were conducted on specimens with an additional ceramic thermal barrier coating. Specimens with VPS bond coats produced from commercial oxide-dispersed powder achieved almost same number of cycles to delamination as specimens with conventional metal bond coats.

This study investigates the influence of segmentation crack density on the strain tolerance of thermal barrier coatings produced by atmospheric plasma spraying. A Triplex II plasma gun is used to spray fused and crushed yttria-stabilized zirconia, forming thick deposits with high segmentation crack densities, low porosity, and low branching crack density, which is necessary for good interlamellar bonding. Thermal cycling and burner rig tests yield promising results in terms of lifetime and strain tolerance behavior and microstructural analysis shows that the segmentation crack network was stable during thermal shock testing. The main failure mechanism was delamination and horizontal cracking in the vicinity of the TBC-TGO (thermally grown oxide) interface.

Segmentation cracked coatings are promising candidates for thermal barrier coatings (TBC) because of their high strain tolerances. By atmospheric plasma spraying (APS), cylindrical stand-alone YSZ coatings with and without segmentation cracks were fabricated. The diameter of the tubes was 10 mm, and the thickness of the coatings approximately 1 mm. The segmentation crack densities were in the range up to 3.5 cracks/mm, while the porosity amounted for all coatings between 5% and 7%. Further details of the spraying process and the obtained microstructures will be given and discussed. The samples were tested in an uniaxial high temperature compression rig to measure the stress relaxation. The comparison of the stress relaxation measurements yielded a different performance of segmented and non-segmented coatings with advantages for the cracked ones. It was observed that the cracks increase the resistance to stress relaxation.