Development of TBCs allowing higher combustion temperatures in gas turbines is of high commercial interest since it results in higher fuel efficiency and lower emissions. It is well known that TBCs produced by suspension plasma spraying (SPS) have lower thermal conductivity as compared to conventional systems due to their very fine porous microstructure. Moreover, columnar structured SPS TBCs are significantly cheaper to produce as compared to the conventionally used electron beam – physical vapour deposition (EB-PVD). However, SPS TBCs have not yet been commercialised due to low reliability and life expectancy of the coatings. Lifetime of a TBC system is significantly dependent on topcoat-bondcoat interface topography. The objective of this work was to study the effect of topcoat-bondcoat interface in SPS TBCs by changing bondcoat spray parameters and bondcoat surface heat treatment. High velocity air fuel (HVAF) spraying was used for bondcoat deposition while axial-SPS was used for topcoat deposition. Same topcoat spray parameters were used for all samples. Lifetime was examined by thermal cyclic fatigue and thermal shock testing. The influence of surface roughness on lifetime has been discussed. The results show that HVAF could be a suitable process for bondcoat deposition to achieve long lifetime SPS TBCs.

Development of thermal barrier coatings (TBCs) manufactured by suspension plasma spraying (SPS) is of high commercial interest as SPS has been shown capable to produce columnar microstructures similar to the conventionally used electron beam – physical vapour deposition (EB-PVD) process. Moreover, SPS is a significantly cheaper process and can also produce more porous coatings than EB-PVD. However, lifetime of SPS coatings needs to be improved further for them to be applicable in commercial applications. The bondcoat microstructure as well as topcoat-bondcoat interface topography affect the TBC lifetime significantly. The objective of this work was to investigate the feasibility of different bondcoat deposition process for SPS TBCs. In this work, a NiCoCrAlY bondcoat deposited by high velocity air fuel (HVAF) was compared to commercial NiCoCrAlY and PtAl bondcoats. All bondcoat variations were prepared with and without grit blasting the bondcoat surface. SPS was used to deposit the topcoats on all samples using the same spray parameters. Lifetime of these samples was examined by thermal cyclic fatigue and thermal shock testing. The effect of bondcoat deposition process and interface topography on lifetime in each case has been discussed. The results show that HVAF could be a suitable process for bondcoat deposition in SPS TBCs.

This study evaluates a method for producing Gd 2 Zr 2 O 7 /SrZrO 3 , a ceramic-matrix composite considered for use as a thermal barrier coating. GdZrO/SrZrO powders are synthesized by co-precipitation, then cold pressed and sintered to form the bulk composite material. Phase stability of the powder and bulk material is assessed by X-ray diffraction and several bulk material properties are determined, including microhardness, Young’s modulus, fracture toughness, thermal expansion coefficient, heat capacity, thermal diffusivity, and thermal conductivity. The results are presented and discussed.

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.

Thermal barrier coatings (TBCs) consisting of a MCrAlY bond coat and a YSZ topcoat were air plasma sprayed onto Hastelloy X substrates. Samples were thermally cycled between 100 °C and 1100 °C and thermal fatigue failures were investigated via microstructure analyses. Final fatigue failure was caused by the formation of interface-parallel cracks in the topcoat, which was found to strongly related to the oxidation behavior of the bond coat. The development of oxide layers was therefore studied in detail and a thermo-kinetic model was used to explore the role of elemental diffusion in oxide formation.

This study evaluates the effect of Ru and Ce on the oxidation behavior of NiCoCrAl coatings deposited by HVOF spraying. Isothermal oxidation tests were conducted at 900, 1000, and 1100 °C. Test samples were also cycled between 100 °C and 1100 °C, with a dwell time of 1 h at 1100 °C. In both cases, coatings with Ru had a lower oxidation rate than those with Ce additions. β-depletion due to interdiffusion was also found to be lower when Ru was present. To help explain the findings, simulation results are presented and discussed along with observations on the influence of oxidation time on microstructure evolution.

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.

Plasma spraying of thermal barrier coatings (TBCs) on gas turbine parts is widely used today either to enable higher turbine inlet temperatures with consequent improvement of combustion efficiency or to reduce the requirements for the cooling system and increase components life-time. Development of low conductivity TBCs, which allows us to further increase gas turbine efficiency and availability, is an ongoing challenge. In order to get low thermal conductivity values an experimental program was conducted. Two zirconia powders were used for coating deposition: yttria partial stabilised zirconia (YPSZ) and dysprosia partial stabilised zirconia (DyPSZ). Microstructure evaluations were performed to evaluate the influence of the spraying parameters on the coating morphology and porosity level. Two methods were utilised to evaluate the coatings thermal conductivity: Laser Flash (LF) and Transient Plane Source (TPS). A comparison between the two methods was made as well as a correlation study between coating microstructure/composition and thermal conductivity (TC).