Thomas Alan Taylor

Plasma-sprayed yttria-stabilized zirconia coatings : structure-property relationships

Abstract A number of plasma-sprayed yttria-stabilized zirconia coatings were prepared representing a combination of significantly different starting powders and different plasma torch designs. The coatings had densities in the range 85%–91% of the theoretical. Coating hardness, particle erosion resistance and cohesive strength were proportional to density. The thermal fatigue resistance of zirconia coatings, 1.3 mm (0.05 in) thick, increased with increasing density, but their thermal insulation value was inversely proportional to density. In general, the coatings exhibited a number of properties related to density without separate regard to process.

Development of alloyed and dispersion-strengthened MCrAlY coatings

Abstract An on-going research program has investigated the effects of the separate and combined additions of Ta, Pt and an oxide dispersion to basic MCrAlY overlay coatings. The goals were improved oxidation and hot corrosion resistance, greater diffusional stability, and coating thermal expansion reduction in order to be closer to nickel base turbine blade alloys. It was found in cyclic burner rig testing at 1050°C that each individual addition improved resistance of the baseline compositions. Oxide and Pt additions reduced thermal expansion. Creep resistance was greatly increased by oxide additions, which also strongly increased yield strength. Yield strength was modestly increased by Pt at low temperature and significantly increased by Ta at low to moderate temperatures.

Surface roughening of metallic substrates by high pressure pure waterjet

The dependence of critical process parameters was found for the use of a high pressure jet of pure water to controllably erode superalloy substrates with a minimal thickness loss. Adequate roughness was obtained for subsequent thermal spray coating of an MCrAlY overlay.

Thermal properties and microstructure of two thermal barrier coatings

Abstract Two plasma sprayed yttria stabilized zirconia coatings are described in terms of their microstructure. The structures differ widely as a result of controlling the coating density, and in one producing a vertical macrocrack pattern that segments the structure. The thermal properties, including thermal expansion, thermal diffusivity and specific heat, were measured from room temperature to 1200°C. The thermal conductivity was calculated from these properties. It was found that a small increase in thermal conductivity occurs as a result of the first heating cycle, and this effect was examined in relation to microstructure.

Waterjet roughened surface analysis and bond strength

Abstract Surface features of superalloys roughened by a pure high-pressure waterjet in preparation for thermal spraying have been investigated using interferometric profilometry. Waterjet roughening was performed on Inconel 718 and Mar-M 509, and the surface morphology resulting from this process was compared to conventional grit blasting. Results confirm earlier reports showing that waterjet roughening yields a unique surface structure having some advantages over grit blast roughening. The surface structure of waterjet roughened superalloys is shown to have a high degree of microroughness and a high negative skewness of the surface height distribution. Tensile bond strength of an MCrAlY coating is shown to be enhanced by waterjet roughening of the superalloy substrate. Bond testing of the plasma sprayed MCrAlY on waterjet surfaces showed consistent high strength bonding regardless of coating thickness or superalloy type.

Dispersion-strengthened modified MCrAlY coatings produced by reactive deposition

The detonation gun coating process was used to change controllably the microstructure of MCrAlY powders to produce either oxide- or carbide-dispersed overlay coatings. Based on analyzed oxygen and carbon contents, and the starting powder composition, the equilibrium phases of the coatings were calculated. From this, the desired MCrAlY matrix composition was calculated for various hypothetical powder compositions. In one example, the matrix aluminium was designed to be about 14wt.%, by compensating for the alumina formed by the reactive coating process. In another case, a strong carbide former was added to compensate for some of the Cr carbide formation, based on the equilibrium calculation. The powders and coatings were made, and the coatings heat treated for phase analysis and oxidation testing. Results of this investigation demonstrate the benefits associated with oxide or carbide dispersions in MCrAlY coatings processed by reactive deposition.