Abstract
Suspension plasma spraying has become an emerging technology for the production of thermal barrier coatings for the gas turbine industry. Presently, though commercial systems for coating production are available, coatings remain in the development stage. Suitable suspension parameters for coating production remain an outstanding question and the influence of suspension properties on the final coatings is not well known. For this study, a number of suspensions were produced with varied solid loadings, powder size distributions and solvents. Suspensions were sprayed onto superalloy substrates coated with high velocity air fuel (HVAF) -sprayed bond coats. Plasma spray parameters were selected to generate columnar structures based on previous experiments and were maintained at constant to discover the influence of the suspension behavior on coating microstructures. Testing of the produced thermal barrier coating (TBC) systems has included thermal cyclic fatigue testing and thermal conductivity analysis. Pore size distribution has been characterized by mercury infiltration porosimetry. Results show a strong influence of suspension viscosity and surface tension on the microstructure of the produced coatings.
Highlights
Suspension plasma spraying (SPS) of yttria stabilized zirconia (YSZ) coatings have demonstrated a number of favorable properties when compared to air plasma spray (APS) or electron beam-physical vapor deposition (EB-PVD) coatings in terms of sintering resistance, low thermal conductivity, and cyclic lifetime [1]
Within the four samples tested for each coating, the measurements exhibited a standard deviation on the order of 0.01 W·m−1K−1; this level of precision results in all but the T2 and C2 coating samples displaying statistically different values for thermal conductivity
This trend is expected since coating density, which is inversely related to coating porosity, is multiplied by the thermal diffusivity and specific heat capacity to calculate thermal conductivity
Summary
Suspension plasma spraying (SPS) of yttria stabilized zirconia (YSZ) coatings have demonstrated a number of favorable properties when compared to air plasma spray (APS) or electron beam-physical vapor deposition (EB-PVD) coatings in terms of sintering resistance, low thermal conductivity, and cyclic lifetime [1]. The particles are influenced by the drag of the plasma stream in the boundary layer close to the substrate causing them to change trajectory; impacting the substrate in the molten or semi-molten state at a shallow angle The behavior of such small particles can be understood using the Stokes Equation: St ρ p d p vp μ g lBL (1). Fazilleau et al suggested a droplet size formula based on surface tension that includes influences from plasma properties of density and velocity [7]. In order to demonstrate the difference in suspension factors on coating microstructure, a number of suspensions have been manufactured that will show differences in density, viscosity and surface tension
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