Abstract
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.
Highlights
IntroductionThermal barrier coatings (TBCs) are widely employed on gas turbine engines to achieve higher efficiency due to the insulation properties of the coatings allowing higher combustion temperatures [1,2]
Thermal barrier coatings (TBCs) are widely employed on gas turbine engines to achieve higher efficiency due to the insulation properties of the coatings allowing higher combustion temperatures [1,2].This feature made the TBCs interesting for other applications such as internal combustion engines used in the automotive industry [3,4,5,6,7,8,9].The atmospheric plasma spray (APS) is one of the most used thermal spraying techniques in which liquid/molten droplets of a powder feedstock impact the surface of the part to be coated and solidify in structures identified as splats
The feedstocks were decided based on the applicability on the combustion chamber of diesel engines to achieve higher engine efficiency and on the previous research carried on both spraying methods
Summary
Thermal barrier coatings (TBCs) are widely employed on gas turbine engines to achieve higher efficiency due to the insulation properties of the coatings allowing higher combustion temperatures [1,2]. Due to the smaller size of the droplet in SPS, increased influence of the spray distance and spray angle can be noticed as compared to APS, affecting the droplet’s temperature and velocity at impact more significantly [16,17,18,19]. The substrate shape interferes with the spraying due to complex shapes, in the SPS coating This is expected because it influences the plasma drag that impacts the trajectory and velocity of the droplet. Three different shapes with different roughness were sprayed, and different microstructures were observed, from thicker dense coatings in the center of the substrate to thin columnar coatings in the edges [22] This variation was due to the substrate shape characteristics, e.g., with different curvature radius, there were effects on the deposition rate and the column growth. The resulting coating had the microstructure and porosity evaluated via image analysis and the thermal properties evaluated with laser flash analysis and finite element modeling
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