High-purity nanocrystallized YSZ powders were used to manufacture thermal barrier coatings by air plasma spraying. After spraying, the coating samples were aged at temperatures of 1200, 1300, and 1400 °C. Coating samples made from ordinary YSZ powders were aged at the same temperatures. XRD analysis shows significant tetragonal-to-monoclinic phase transformation in the reference coatings after 100 h at 1400 °C in contrast to the phase stability exhibited by high-purity YSZ. The sintering behavior of the YSZ coatings was also examined along with the influence of MCrAlY oxidation.

This study assesses the thermal stability of YSZ coatings produced from nanostructured feedstock by means of atmospheric and suspension plasma spraying. Free-standing YSZ coatings were isothermally treated for 24 h at different temperatures (1200-1600 °C) and at 1550 °C for 20 to 100 h. Afterwards, the coatings were examined to determine the effect of heating on phase composition, microstructure, morphology, and hardness. No evidence of tetragonal-monoclinic phase transformation was detected in the coatings that had been treated for 24 h, even at 1600 °C, but in coatings treated for different periods of time at 1550 °C, a phase transformation occurred after 40 h. Overall, the suspension plasma sprayed coating showed the greatest degree of change due to thermal aging.

Different thermal spray technologies were used to apply CoNiCrAlY bond coats to Inconel substrates. Powder compositions were the same in all cases and particle size recommendations were followed for each torch. YSZ topcoats were deposited via APS on bond coat samples selected based on roughness, porosity, residual stress, oxidation, and isothermal TGO growth. The TBCs were furnace cycle tested for 10-1400 cycles as well as to failure and changes in bond strength and TGO thickness were recorded. It was observed that bond strength values, which are relatively stable during thermal cycling, decrease significantly just before failure brought on by topcoat spall off.

This study shows that surface preheating is required to avoid delamination of flame sprayed coating glazes. In the experiments, preheating parameters are determined from heat flux measurements and potential substrate degradation is characterized and controlled by optimizing spray parameters. Coating adhesion is determined by pull-out tests and remains constant even after freeze-thaw cycling. Although gas tightness was not characterized, aging tests show that no water percolates through pore networks in the coatings.

In order to improve the efficiency of gas turbines, thermal barrier coatings (TBCs) have been applied to components in the hot sections of advanced gas turbines. During service, thermally grown oxide (TGO), which consists of an Al 2 O 3 layer and a mixed oxide layer, forms at the interface between the top coating and bond coating. It is supposed that the reason for failures of TBCs, such as cracking, delamination or spalling, is due to decreased bond strength caused by TGO growth or due to the formation of stress concentration sites caused by porosities in the mixed oxide. In this study, to inhibit the growth of TGO, plasma sprayed CoNiCrAlY bond coating was remelted with a YAG laser prior to spraying the top coating. A thin Al 2 O 3 layer formed at the top coating/bond coating interface, and the formation of porous mixed oxide during thermal aging tests was inhibited. Four-point bending tests showed that the bond strength of TBC with remelted CoNiCrAlY was superior to standard TBC.