Experimental and analytical investigation of transient thermal behaviour in mullite and YSZ thermal barrier coatings for IC engines

The transient thermal behaviour of atmospheric plasma-sprayed (APS) mullite and 8 wt% yttria-stabilised zirconia (8YSZ) thermal barrier coatings (TBCs) for low-heat-rejection internal combustion engine applications was investigated. The study aimed to evaluate the influence of coating material and thickness on transient thermal response under cyclic thermal loading through a combined experimental and analytical approach. Mullite and YSZ coatings of two thickness regimes (∼350–400 µm and ∼940–990 µm) were deposited on HVOF-sprayed NiCrAlY bond coats. Thermal swing behaviour was evaluated using an indigenously developed cyclic oxy-LPG flame rig under constant surface temperature and constant incident heating, with transient temperatures recorded at 1 ms temporal resolution. Mullite coatings exhibited higher porosity (5–7%), lower thermal conductivity, and lower thermal effusivity than YSZ coatings, resulting in greater thermal swing and improved thermal insulation behaviour. Thin mullite coatings demonstrated the highest thermal swing response, with a gradual transient, whereas YSZ coatings exhibited a rapid initial temperature rise followed by a slower thermal response. Increasing coating thickness reduced the differences in thermal behaviour between the two coating systems. A one-dimensional analytical model was developed to predict the transient temperature distribution within the coating and showed good agreement with the experimental measurements. The results demonstrate that coating thickness, material composition, porosity, and coating microstructure collectively govern the transient thermal behaviour of APS TBCs. The proposed methodology provides a practical framework for evaluating transient thermal performance and supports the design and optimization of thermal barrier coatings for advanced low-heat-rejection internal combustion engine applications.