Can TBC porosity be estimated by non-destructive infrared techniques? A theoretical and experimental analysis

Abstract

Since thermal properties of solids such as thermal conductivity, thermal diffusivity and specific heat are related by equations containing the density of the material, by measuring two of the aforementioned parameters it would be possible to estimate the density and thus the porosity, if the theoretical density of the material it is known.Most of the techniques used for measuring thermal properties of TBCs are the contactless photothermal techniques. Among all these techniques, those able to measure thermal properties of TBCs deposited onto a metallic substrates offer the great advantage to be potentially applicable to real coated components without the need of any machining. This occurrence is typically related to the possibility to adopt a reflection experimental configuration, where the thermal stimulation and the detection of the sample response are measured on the same face of the component.A theoretical and experimental analysis of real capabilities of infrared techniques to estimate the porosity content of porous ceramic materials such as thermal barrier coatings (TBCs) is carried out.Experimental and theoretical analysis confirmed that, independently from the specific microstructural features of the TBC samples, the contactless photothermal techniques allow to non-destructively estimate the porosity content with an overall uncertainty estimated theoretically as small as ±5%. This uncertainty could result compatible with the requirements, especially for large porosity TBCs. Besides the uncertainty directly connected with the measurement, the uncertainty of the substrate effusivity, TBC specific heat and thickness contribute to the porosity estimation accuracy as well.