Effect of HT Cycling in Air on Microstructure of Detonation Gun Sprayed YSZ/NiCr Coating

Yttrium-stabilized zirconium (YSZ) thermal barrier coatings (TBCs) are applied in gas turbines due to their ability to shield metallic parts from exposure to high temperatures. The progress in this area is sought out, among others, by bond coat (BC) engineering providing a better stress relief at ceramic/metal boundary and excellent both oxidation and corrosion protection of the metallic substrate. The present BC based on NiAl intermetallic, being exposed to oxidizing atmosphere at high temperature, secure formation of Al 2 O 3 layer fulfilling both these requirements. The NiCr alloys are not only capable to produce a dense Cr 2 O 3 layer at the ceramic/BC boundary, but it may additionally offer closer coefficient of thermal expansion (CTE) to that of the metallic substrate. The aim of this work was a detail characterization of the microstructure, phase and local chemical composition of the TBC consisting of YSZ (ZrO 2 with 8 wt.% of Y 2 O 3 ) top coat (TC) and Ni 50 wt.% Cr bond coat. They were deposited over 321 s stainless steel substrates using a detonation gun (DG) technique. The investigations were performed with scanning and transmission electron microscopy backed with energy-dispersive x-ray spectroscopy (SEM/TEM/EDS). The high-temperature (HT) cycling of YSZ/Ni50Cr set was performed in air and comprised of 5 cycles, which consisted of heating the samples up to 900 °C at 10 °C/min., holding them at that temperature for 10 h, followed by their cooling down for ~ 30 min. The microstructure observations showed that DG allowed to produced dense YSZ coating of low porosity, while Ni50Cr bond coat differed in local chemical composition and contained numerous amorphous chromium oxide pockets. During the HT cycling, the oxygen from the air was still able to penetrate through the YSZ layer supporting formation of Cr 2 O 3 , while the amorphous chromium oxide transformed to R-CrO 2 (rutile type). Only small amount of NiO was noted, being dispersed within Cr 2 O 3 . Simultaneously, a significant inter-diffusion of Ni into steel substrate and Fe and Cr into the bond coat were documented. The lack of oxide formation in the substrate proves that the DG YSZ/Ni50Cr set could serve as a good protection over parts subjected to HT cycling.