This study investigates the corrosion resistance Gd 2 Zr 2 O 7 /YSZ coatings and a YSZ layer of similar thickness. All coatings were produced by suspension plasma spraying, resulting in a columnar structure. Corrosion tests conducted at 900 °C for 8 h in a molten salt bath show that Gd 2 Zr 2 O 7 is not as corrosion resistant as YSZ. Molten salts react with Gd 2 Zr 2 O 7 producing GdVO 4 along the surface as well as between the columns of the coating. The formation of GdVO 4 between the columns, in combination with the low fracture toughness of Gd 2 Zr 2 O 7 , is likely responsible for the lower corrosion resistance. Furthermore, the presence of another layer of Gd 2 Zr 2 O 7 on top of the Gd 2 Zr 2 O 7 /YSZ coating, to prevent salt infiltration, did not improve corrosion resistance.

In this study, the current industry standard topcoat for thermal barrier coatings, 8YSZ, is deposited by suspension plasma spraying and its room-temperature erosion resistance is compared with that of SPS sprayed gadolinium zirconate/YSZ and triple-layered GZ dense /GZ/YSZ. A columnar microstructure was observed in both the single- and multi-layered TBCs. Single-layer 8YSZ had a higher erosion resistance than multi-layered GZ/YSZ despite of its higher porosity among the as-sprayed coatings. In the case of the triple-layer coating, the denser top layer helped to slightly improve erosion resistance over that of the GZ/YSZ double-layer TBC.

Many applications of thermally sprayed coatings call for increased fatigue resistance of coated parts. Despite the intensive research in this area, the influence of coating on fatigue is still not completely understood. In this paper, the spatiotemporal localization of crack initiation and the dynamics of crack propagation are studied. The resonance bending fatigue test is employed to test flat specimens with both sides coated. Hastelloy-X substrates coated with classical TBC YSZ/NiCoCrAlY composites were tested. The strain distribution on the coating surface is evaluated by the digital image correlation method (DIC) through the whole duration of the fatigue test. Localization of crack initiation sites and the mode of crack propagation in the coated specimen are related to the observed resonance frequency. The individual phases of specimen degradation, i.e. the changes of material properties, crack initiation, and crack propagation are identified. The tested coatings strongly influenced the first two phases, the influence on the crack propagation was less significant.

Dysprosia stabilized zirconia coatings with large globular pores have good potential as TBC topcoats. In previous work, such coatings have been produced by air plasma spraying with the aid of a polymer pore former. The aim of this work is to optimize the spraying parameters. A design of experiments approach was used to create a two-level full factorial test matrix based on spray distance, powder feed rate, and hydrogen flow. An agglomerated and sintered dysprosia stabilized zirconia (DySZ) powder mixed with polymer particles was sprayed on Hastelloy X substrates that had been prepared with NiCoCrAlY bond coats. The coatings obtained were evaluated based on thermal conductivity, thermocyclic fatigue life, and morphology, which are shown to correlate with spray parameters and in-flight particle properties.

The fatigue performance of conventional structural steel with an applied thermal barrier coating (TBC) was evaluated via cyclic bending. Tests were carried out for as-received and grit-blasted substrates as well as for samples with thermally sprayed bond coats and topcoats. Failure mechanisms were identified and changes in fatigue resistance were assessed based on results obtained for different loading amplitudes supplemented by fractographic analysis.

Dysprosia stabilized zirconia (DySZ) is a promising candidate to replace yttria stabilized zirconia (YSZ) as a thermal barrier coating due to its lower inherent thermal conductivity. It is also suggested in studies that DySZ may show greater stability to high temperature phase changes compared to YSZ, possibly allowing for coatings with extended lifetimes. Separately, the impurity content of YSZ powders has been shown to influence high-temperature sintering behavior. By lowering the impurity oxides within the spray powder, a coating more resistant to sintering can be produced. This study evaluates high purity and standard purity dysprosia and yttria stabilized zirconia coatings and their performance after extended heat treatment. Coatings were produced using powders with the same morphology and grain size; only the dopant and impurity content were varied. Samples were heat treated for up to 400 hours at 1150 °C and thermal conductivity measurements were plotted to show the evolution of thermal properties with respect to time. Thermal conductivity is compared to coating microstructure and porosity in order to track structural changes due to sintering.

Fundamental understanding of relationships between coating microstructure and thermal conductivity is important to be able to understand the influence of coating defects, such as delaminations and pores, on heat insulation in thermal barrier coatings. Object-Oriented Finite element analysis (OOF) has recently been shown as an effective tool for evaluating thermo-mechanical material behaviour, because of this method’s capability to incorporate the inherent material microstructure as an input to the model. In this work, this method was combined with multi-variate statistical modelling. The statistical model was used for screening and tentative relationship building and the finite element model was thereafter used for verification of the statistical modelling results. Characterisation of the coatings included microstructure, porosity and crack content and thermal conductivity measurements. A range of coating architectures was investigated including High purity Yttria stabilised Zirconia, Dysprosia stabilised Zirconia and Dysprosia stabilised Zirconia with porosity former. Evaluation of the thermal conductivity was conducted using the Laser Flash Technique. The microstructures were examined both on as-sprayed samples as well as on heat treated samples. The feasibility of the combined two modelling approaches, including their capability to establish relationships between coating microstructure and thermal conductivity, is discussed.

The aim of the study presented in this paper was to develop the next generation of production ready air plasma sprayed thermal barrier coating with a low conductivity and long lifetime. In order to achieve these goals; a number of coating architectures were produced using commercially available plasma spray guns. Modifications were made to powder chemistry including; high purity powders for sintering resistance, Dysprosia stabilised Zirconia powders and powders containing porosity formers. Agglomerated & Sintered (A&S) and Hollow Oven Spherical Powder (HOSP) morphologies were used to attain beneficial microstructures. Finally, dual layer coatings were produced using the different powder morphologies. Evaluation of the thermal conductivity of the coating systems from room temperature to 1200°C was conducted using laser flash technique. Tests were done on as-sprayed samples and samples heat treated for 100 hours at 1150°C in order to evaluate the first stage sintering resistance of the coating systems. Thermal conductivity results were correlated to coating microstructure using image analysis of porosity and crack content. The results show the influence of beneficial porosity on reducing the thermal conductivity of the produced coatings.