Layer thickness measurement of ceramic systems with a numerical model for flash thermography

Abstract

Thermography is a non-destructive testing technique which is non-contacting as well as imaging and therefore advantageous for delicate materials. It is more widely used for qualitative evaluation but can also provide data for a quantitative analysis when the heat source is actively controlled. With flash thermography, fast diffusion or conduction processes can be monitored, providing an effective method for the inspection of metals and thin coatings. Ceramic layered systems are used as thermal barrier coatings, for example on turbine blades. Gas turbines with such blades can be operated at higher temperatures, which leads to an increase in efficiency. To fully exploit this advantage, the properties and thickness of the coating must be within a predefined parameter range. Due to the challenging and hard to monitor manufacturing process, the coating must be examined afterwards. With the help of a numerical model using a finite difference solution, multiple material parameters such as thermal conductivity and layer thickness can be determined simultaneously with nonlinear regression fitting. During the research presented herein, a four-stepped sample of ceramic coatings on an Inconel steel basis is examined and the thermographic data analysed. After the thermographic measurements, the sample was cut, embedded in resin, and polished. The scanning electron microscopy images and measurements serve as the reference values to compare the extracted parameters to.