Residual Stresses Analysis on Thermal Barrier Coatings—NDT Tool for Condition Assessment

Improvement in the engine efficiency follows reduction in fuel consumption which is possible by increasing the engine combustion temperature. Coating the piston of diesel engine with a high temperature-resistant material, known as thermal barrier coating, generally 6–8% Y₂O₃ stabilized ZrO_2, is a popular method to reduce the temperature it experiences in service and to increase engine efficiency. Whether bare or coated component, fabrication and different thermal expansion coefficients of the ceramic coating and piston metal cause generation of residual stresses in them. These hidden residual stresses (tensile or compressive) play a significant role in governing the failure mechanism of the different sections of the components and their important role (also developed in service) is mostly neglected in engineering practices. Residual stresses analysis of components in service may throw light on the condition of the components without destroying them. In this work, portable X-ray residual stress analyzer was used to evaluate the condition of Al–Si alloys piston flat plates that were coated with 250-µm-thick 6–8% Y₂O₃ stabilized ZrO₂ and subjected to thermal treatments. The analysis revealed (a) residual stress-free pattern for uncoated Al–Si substrate, (b) compressive residual stress at the substrate (Al–Si)–coating (TBC) interface and (c) tensile residual stress at the substrate (Al–Si)–coating (TBC) interface of a thermal shocked coated substrate. The analysis method exhibited good possibility for using this as a tool for non-destructive testing for predicting the onset of failure at the coating substrate interface, without destroying the component in service.