Methods for characterizing the interfacial adhesion in thermal barrier coating systems

Thermal barrier coatings (TBC) are widely used to protect the hot section structural components, such as aircraft gas turbine, against hot corrosion and large thermal gradients during the service life. The resistance to interfacial debonding (adhesion) is one of the key factors for predicting the TBC durability during operation. The goal of the study is analysis of the existing approaches to quantification of the interfacial adhesion for multi-layered thermal barrier coating systems and thermally-grown oxide on Ni-based superalloys and discussion of the problem regarding the reasons for a large spread of the adhesion characteristics. Theoretical prerequisites for determining the adhesion characteristics are considered to provide a theoretical background for quantification of the interfacial adhesion. The absence of national standard tests applicable to TBC is marked along with a significant scatter of the values obtained using international test-regulating documents and proprietary technologies. The values of the intensity of the strain energy release are found to vary from 0.3 to 230 J/m2 depending on the method for measuring adhesion, experimental details and type of the system considered. The factors that determine a large spread of the interfacial adhesion values were divided into two groups: 1 — intrinsic, attributed to the features of TBC layer formation and temperature affecting the value and distribution of residual strains and stresses and direction of the failure initiation; 2 — extrinsic, which are related to test conditions and technique of experimental data processing. A complex approach involving both intrinsic and extrinsic parameters is required to address the problem of interfacial adhesion quantification.