Coupled Thermal Mechanical Analysis Methodology for Thermal Performance Evaluation and Failure Mode Identification of Thermal Barrier Coatings for Heavy Duty Diesel Engines

Consumer demand and government regulation, as well as the Department of Energy’s SuperTruck program has motivated extraordinary effort to maximize the heavy-duty diesel engine’s thermal efficiency. Thermal barrier coatings (TBCs) provide an avenue for increasing the thermal efficiency of the mixing controlled combustion (MCC) process through the reduction of heat transfer losses. The myriad of previous studies presents conflicting conclusions, both in the merit of TBCs for cycle improvement, and also for coating durability and potential adverse effects from coating failure. While the most recent studies with thin thermal barrier coatings (~100-250 µm) have shown a majority of positive cycle impacts, the path to maximizing the improvements while also ensuring coating durability still remains uncertain. This work outlines a systematic approach for i) evaluating thermal barrier coatings both in their ability to reduce heat loss from the in-cylinder gas to the walls, and ii) quantifying material property effects on coating stresses, providing insights into pathways to achieving coating reliability. The key to reducing heat loss during combustion is to maximize the temperature of the coating in proximity to top dead center. To evaluate a coatings’ ability to generate a high amplitude surface temperature swing, and thereby reduce heat transfer losses, a 0D thermodynamic cycle solver coupled with a finite difference heat transfer partial differential equation (PDE) solver was used to correlate the efficacy of TBCs with key thermal properties. The results enable establishing a link between the thermophysical properties and the magnitude of heat transfer reduction, and it was determined that i) thermal effusivity provides critical directional guidance for maximizing the TBC effect, and ii) that reducing thermal conductivity provides higher heat transfer reductions compared to reducing the volumetric heat capacity of a coating. Secondarily, an asynchronous co-simulation methodology was developed to couple computational fluid dynamics (CFD) code with finite element analysis (FEA) to accurately model the highly variant spatial and temporal effects of diesel combustion on the temperature field developed in the coating, and the heat transfer pathways through the 3D TBC/piston assembly. The results indicate that the highly heterogenous combustion mode leads to large spatial variations of temperature swings, with some locations far exceeding the predicted temperature swings of the 1D solver, on the order of 600K. For the coating properties investigated, the localized heat transfer reduction in those areas of high heat flux was found to reduce the heat transfer losses by roughly 9% when only the piston is coated, and an additional 6-7% when the cylinder head/firedeck is coated as well. The predicted cycle indicated thermal efficiency increased by roughly 0.5-0.6 percentage points. Finally, to quantify material properties affecting coating reliability, validated mechanical models for residual stresses, elastic-plastic yield, and failure criterion were developed. It was postulated that the key mechanisms potentially leading to failure are the CTE mismatch between the coating and the substrate, and the temperature gradients throughout the thickness of the coating. The methodology for predicting failure relies on the new model to characterize the elastic-plastic deformation, the ability to determine the anisotropic/temperature dependent elastic moduli in FEA (ABAQUS), and the subsequent application of the William Warnke failure model adapted for use in technical ceramics. A sensitivity study for a simplified geometry was conducted to develop insights into coating property effects on stress states and potential failure mechanisms for TBCs in a heavy-duty diesel (HDD) engine application. Finally, the spatio-temporally resolved temperature field from the CFD-FEA Co-simulation methodology was subsequently applied to a 3D FEA model of the piston with validated ceramic material models and linked to the constitutive failure model to identify the primary mechanisms leading to failure. Results provide clear guidance regarding the preferred ranges of conductivity and CTE for the next generation advanced coating formulations. Finally, the insights are combined in order to provide a comprehensive view of the thermophysical property effects on heat transfer reduction and coating stress states in the context of pathways for achieving maximum heat loss reduction and thermal efficiency improvements, while also ensuring durability in a real-world heavy-duty diesel engine application