A high performance NiCoCrAlY bond coat with dense dendritic microstructure was fabricated using laser powder deposition (LPD) technique. The thermally grown oxides (TGO) formed on the coating deposited by laser powder deposition is predominantly alumina instead of the mix oxides usually formed on the coatings prepared by air plasma spray and high velocity air fuel. Isothermal oxidation tests performed at 1150 °C reveal that the LPD bond coat shows significantly better spallation resistance and lower TGO growth rate. The superior spallation resistance of the TGO is further discussed in relation to the unique microstructure of the LPD bond coat.
An approach to make air plasma sprayed (APS) thermal barrier coatings (TBCs) with the enhanced strain and damage tolerance was reported, using a novel hollow spheres produced by electro‐spraying (ESP) technique. Compared with agglomerated & sintered (A&S) and hollow spherical (HOSP) yttria‐stabilized zirconia (YSZ) powders, the ESP powder showed a unique network microstructure and the TBCs exhibited a 2‐3 times longer thermal cycling lifetime. The splat morphology and the top coats microstructure were investigated. Some semi‐melted ESP particles were observed in the as‐sprayed top coat. The indentation coupled with the Raman mapping technique was employed to evaluate the strain and damage tolerance of the TBCs. The coatings deposited by the ESP powder show a lower in‐plane stiffness determined by three‐point bending tests. It is proposed that the superior performance is attributed to the lower amount of the short microcracks (0.5‐4 μm) with low angle (<45°) and the semi‐melted ESP particles remained in the YSZ top coat.
The effect of ambient oxygen concentration on thermographic phosphor thermometry was investigated to uncover the "oxygen quenching" mechanism of Eu-based phosphors using a lifetime-based measurement system. The phosphors being studied were the Eu doped yttria stabilized zirconias (YSZs) with Y 3+ concentrations of 0, 8, 12 and 57 mol%, having monoclinic, tetragonal, cubic and δ crystalline lattice, respectively. To vary the ambient oxygen concentration, three different gas phases (air, oxygen and nitrogen with methane) were used. It was found that the phosphorescent lifetimes and intensities of all four phosphors were sensitive to the ambient oxygen concentration, while the reference phosphor, Y2SiO5:Eu, was not influenced. The "oxygen quenching" phenomenon observed in Eu-doped YSZs was attributed to the oxygen vacancies in the phosphors. In addition, the four YSZ:Eu phosphors exhibited different sensitivity to ambient oxygen concentration. The oxygen sensitivity was found to be closely related to the site symmetry of Eu 3+ , which was previously determined by
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