Numerical and Experimental Investigation of Thermal Barrier Effects of CNT-Reinforced Fly Ash/Alumina Coatings in Diesel Engine Pistons

Fly ash (FA), an industrial byproduct from coal combustion, presents significant disposal challenges, especially in developing nations. Given its mineralogical properties, FA shows potential in thermal spray coatings. This study evaluates FA-based coatings for pistons to improve thermal management in internal combustion engines through numerical simulations, analyzing their effects on the temperature distribution, thermal stress, and combustion efficiency. FA coatings were also applied to marine-grade steel with additives (50 wt % Al₂O₃ and 0-2 wt % CNT) to assess high-temperature performance. Microstructural analysis revealed that 2 wt % CNT-reinforced (2CAF) coatings showed agglomeration, reducing microhardness by ∼9.27% compared to 1 wt % CNT-reinforced (1CAF) coatings. The XRD analysis of 1CAF indicated ∼56.51% transformation of corundum to γ-alumina, lowering thermal conductivity by ∼15.40% compared to alumina/FA (AF) coatings, while 2CAF coatings showed increased conductivity due to CNT inhomogeneity. For piston applications, simulations showed an ∼24.59% rise in maximum surface temperature, from 241.39 to 300.76 °C, and an ∼62.06% reduction in heat flux, indicating enhanced durability and reduced cold-start emissions. Thermal cycling demonstrated that 1CAF coatings outlasted AF and 2CAF, suggesting FA-based TBCs as sustainable and economical options for enhanced engine performance and waste valorization.