NDE of Gas Turbine Thermal Barrier Coatings

Thermal barrier coatings are increasingly being used in advanced power plant and industrial gas turbines to protect metal components from the hot gas stream. Their use improves the durability of turbine parts and increases efficiency of operation. However, premature failure remains a critical concern. It is very important to have a non-destructive evaluation technique for quality control and to detect early damage of thermal barrier coatings during use. Current inspection methods are either destructive or do not provide sufficient evaluation of the coating. An optical, nondestructive inspection method for thermal barrier coatings based on a technology known as optical coherence tomography is being developed. This technology will provide unprecedented imaging of the thermal barrier coating layers and structure and will be capable of detecting damaged coatings and conditions indicative of degradation of the coating. In Phase I, a high-speed instrument capable of nondestructively imaging subsurface structures in thermal barrier coatings was constructed and tested. The dependence of imaging performance on various instrument components and parameters were investigated. The feasibility of this approach was confirmed and a design for a Phase II prototype was developed. In Phase II, an optimized instrument will be constructed and tested. Nondestructive inspection of thermal barrier coating samples subjected to different degrees of thermal aging will be performed. An algorithm for predicting the remaining service life of thermal barrier coatings based on optical coherence tomography data and other nondestructive testing will be developed and validated. Commercial Applications and Other Benefits: Development of this technology will allow better quality control inspection of thermal barrier coating coated turbine parts at manufacturing and maintenance facilities, will allow identification of coatings that are close to failure. These capabilities will lead to cost savings for turbine parts manufacturers since destructive testing for quality control will be eliminated. Substantial operations savings can also be expected if this technology can help maximize the useful life of components or indicate when failure is about to occur. Unplanned shutdowns are extremely costly to the power industry and its customers.