Thermal Swing Evaluation of Thermal Spray Coatings for Internal Combustion Engines

Abstract

The efficiency of internal combustion engines is gaining increased interest due to the impact of fuel consumption on greenhouse gas emissions and the goals of countries to minimize emissions. Thermal barrier coatings (TBCs) have shown great potential in improving the efficiency of internal combustion engines. The TBCs, applied on the surface of the piston, apart from thermal isolation, should also follow the surface temperature variations in the combustion chamber, reducing the energy loss and not affecting volumetric efficiency, and thus accomplish a raise in fuel efficiency. This characteristic of the TBC can be associated with the thermal properties, but the best performance test for TBCs is the single cylinder engine test. The single cylinder engine test is an expensive and time demanding procedure, making it not easily accessible. The purpose of this work was to develop a thermal swing test method to evaluate the applicability of TBCs in the combustion chamber of an internal combustion engine. This was carried out by measuring the temperature variation on the surface of the coating (thermal swing response) exposed to heat pulses from a high velocity air fuel (HVAF) spray torch. The TBCs were tested as sprayed (AS) and after grinding them to reduce roughness (RR) in order to ensure similar thickness and roughness along the different TBCs. Characterization of the coating microstructure was carried by scanning electron microscopy (SEM) together with image analysis techniques, and the thermal properties were measured by laser flash analysis (LFA). By correlating the thermal swing response with the microstructure and thermal properties of the coatings, it was determined that the coatings with large open pores exhibited the highest thermal swing response, which was as high as 200 °C.