Spray parameters play an important role on the microstructure and properties of plasma sprayed coatings. Parameters such as spray distance, plasma gas flow and current, raster speed and spray angle all can be varied. In this paper, an integrated study to investigate the effects and influences of spray angle on properties of yttria-stabilized zirconia coatings was carried out with spray angles of 60°, 75° and 90° (to the substrate surface). In situ coating property sensor (ICP) based on beam curvature measurements was used to measure the evolving stress and elastic moduli of the resultant coatings and combined with other characterization tools for thermophysical property and microstructure analysis, such as laser flash and scanning electron microscopy (SEM). The results indicate that the coating with 60° spray angle had the lowest thermal conductivity and more compliant structure. This study seeks to understand the mechanism for this effect and will provide important insight into parametric sensitivities on complex spray parts.

The effects of powder loading on temperature distributions of yttria-stabilized zirconia particles during flight, which can be used as a melting status indicator, were investigated in atmospheric dc plasma spraying. Commercially available diagnostic systems were utilized to measure the state of in-flight particle parameters. As the powder feed rate was increased, the intensity of a peak related to the latent heat increased, suggesting the increase of semi molten particles. Interesting findings are that the deposition efficiency of the coating actually increased in some conditions at higher powder feed rates. This implies that higher molten degrees of particles do not always give higher deposition efficiencies. The loading effects also affected the result of diagnostics, which requires special care when the diagnostic condition is different from the actual spray conditions.

Thermal conductivity plays a critical role in the thermal transport of thermal sprayed coatings. In this paper, a combined image analysis and finite element method approach is developed to assess thermal conductivity from high-resolution scanning electron microscopy (SEM) images of the coating microstructure. Images are analyzed with a collection of image processing algorithms to reveal the microscopic coating morphology. The processed digital image is used to generate a two-dimensional finite element meshing in which pores, cracks and the bulk coating material are identified. The effective thermal conductivity is then simulated using a commercial finite element code. Results are presented for three coating material systems: yttria stabilized zirconia (YSZ), molybdenum and NiAl, and results are found to be in good agreement with experimental values, obtained using the laser flash method. YSZ coatings are also annealed and the analysis procedure repeated to determine if the technique could accurately assess changes in coating morphology.