Unsteady Heat Conduction and Fracture Mechanics in Multilayer Engine Coatings

Koutsakis, G. Unsteady Heat Conduction and Fracture Mechanics in Multilayer Engine Coatings. University of Wisconsin-Madison, 2022.

Unsteady heat transfer and fracture mechanics analytical techniques were developed to allow optimization of multilayer coatings for reciprocating internal combustion engines. The analytical heat conduction solution handles time-varying heat flux and temperature boundary conditions. The mathematical formulation was derived using the matrix method coupled with complex analysis residue-calculus Laplace transform inversion techniques. As compared to finite difference heat conduction schemes, the analytical nature of the solution requires no spatial discretization and the accuracy is guaranteed. The method to predict the surface temperature of multilayer engine walls was integrated with a commercial system-level simulation software package. Full simulations were performed using experimental boundary condition and with calibration data for a production multi-cylinder diesel engine to demonstrate thermal efficiency benefits of coating over extended drive cycles.

The fracture-based framework was developed using well-established techniques. The method evaluated the transient elastic energy release rates assuming equi-biaxial stress followed by plane strain after delamination. The peak energy release rate location was predicted to lie within the coating, suggesting that the coating may begin to fail via spalling rather than pealing off the substrate. This analysis was validated against several coated pistons tested in a high-output, single-cylinder diesel engine. The model was found to provide good trendwise comparison with the post-run coating integrity observations. Coatings with maximum energy release rate that significantly exceeded the toughness failed.

A high-throughput computational optimization was performed to maximize efficiency by minimizing heat transfer while obeying a structural integrity constraint for a production diesel engine using a multilayer-coated piston. The output uncovered the optimum coating material, thickness and thermomechanical properties for a real-world driving scenario. Over eight hundred real materials were investigated, and the optimization required more than one million drive cycle evaluations. This elucidated the importance of including mechanical considerations in the design of thermal barrier coatings for improved engine performance.