Delamination Failure on High-Output Diesel Engine Thermal Barrier Coatings

An analytical mechanics model was employed to predict the delamination of several thermal-barrier-coated pistons that had been previously tested in a high-output, single-cylinder diesel engine. Some of the coatings delaminated during engine operation. Results are presented for two thicknesses of the same coating material, and for two similar coatings with different levels of stiffness. All the coating thermomechanical properties such as thermal conductivity, density, volumetric heat capacity, thickness, elastic modulus, coefficient of thermal expansion, Poisson ratio and toughness, were measured prior to engine testing. Previous measurements of the piston transient heat flux, based on fast-response surface temperature data, in the same engine were used as an input to calculate the multilayer wall temperature distribution. A theoretical methodology was employed to evaluate and predict the coating durability. The method considers the release rate of elastic strain energy stored in the coating when it cracks as the main driving force for delamination; when the calculated energy release rate overcomes the toughness, coating crack failure is predicted to occur. The mechanics durability model was found to provide good trendwise comparison with the post-run coating integrity observations. When the energy release rate significantly exceeded the toughness, failure was indeed observed. Borderline cases, where the energy release rate was close to the material toughness, however, were more difficult to predict.