Performance and Degradation Behaviour of Thermal Sprayed Coatings for Internal Combustion Engines

Abstract

Thermal barrier coatings are pivotal in improving the efficiency of IC engines. The present work investigates the effect of powder morphology and APS gun traverse speed on the microstructure of mullite coatings. The powders and coatings were thoroughly characterised using XRD, FESEM, TEM, DSC, TGA, LFA, 3D noncontact profilometer, and micro-Vickers indentation. Experimental findings and analysis indicate that coatings deposited from spray-dried powders exhibited better microstructure, mechanical and thermal properties than fused-crushed powders. Optimisation of variable gun traverse speed of plasma spraying revealed that coatings deposited at lower traverse speed (400 mm/s) exhibited microstructure with fine and uniformly distributed pores and showed a higher compressive residual stress compared to that of coating produced at higher gun speed (600 mm/s). The surface of the coated substrates was subjected to high-frequency heating and cooling cycles simulating an IC engine cycle using a custom-built thermal rig with the provision of measuring the temperature at coating and substrate surfaces. The thermal swing behaviour of mullite and YSZ coatings of various thicknesses were measured and compared. Thicker coatings exhibited comparable thermal swing values, while thin mullite coatings outperformed their YSZ counterparts in thermal swing characteristics. An analytical model was employed to understand spatio-temporal distribution within the coatings during the cycling. The model could predict the transient behaviour of the coatings, demonstrating its efficacy in estimating thermal swing values based on steady-state heat input. Further, the hot corrosion behaviour of mullite coatings was evaluated under marine IC engine conditions. The coated specimens were exposed to a salt mixture of Na2SO4 and V2O5 at 700 °C for up to 300 hours. Post-hot corrosion assessments revealed that coatings remained intact without spalling. However, they reacted with the corrosive salts, thereby increasing porosity. Notably, Na2SO4 reacted with mullite to form nosean, a phase analogous to nepheline. Vanadium pentoxide acted as a catalyst and diffusion enhancer, facilitating the reaction between Na2SO4 and mullite coatings.