Microstructural Changes in Suspension Plasma-Sprayed TBCs Deposited on Complex Geometry Substrates

Thermal barrier coatings (TBCs) are considered a promising solution for improving the efficiency of internal combustion engines. Among the thermal spray processes, the relatively newly developed suspension plasma spray (SPS) is an attractive candidate due to its unique microstructural features that have already demonstrated increased performance in gas turbine applications. To achieve these features, thermal spray conditions play an essential role. In specific uses, such as piston of diesel engines, parameters as spray angle and spray distance pose challenges to keep them constant during the whole spray process due to the complex geometry of the piston. To understand the effect of the spray distance and spray angle, a comprehensive investigation of the produced thermal spray microstructure on the piston geometry was conducted. Flat and complex geometry surfaces were coated using the same plasma parameters while the spray angle and distance were changed. Characterization was performed using scanning electron microscopy (SEM) combined with the image analysis technique to perceive the variation of the thickness and microstructures features such as pores, cracks, column density, and column orientation. The results showed that the changes in spray angles and spray distances due to the complex shape of the substrate have a significant influence on the microstructure and thermal properties (thermal conductivity and thermal effusivity) of the coatings. The thermal conductivity and thermal effusivity were calculated by modeling for the different regions of the piston and measured by laser flash analysis combined with modeling for the flat-surfaced coupon. It was shown that the modeling approach is an effective tool to predict the thermal properties and thus to understand the influence of the parameters on the coating properties. Connecting the observations of the work on the microstructural and thermal properties, the complex geometry’s influence on the produced coatings could be diminished by tailoring the process and generating the most desirable TBC for the internal combustion engines in future applications.