Interfacial Fracture Toughness and Adhesion of Brittle Coatings on Ductile Substrates

Ceramic Thermal Barrier Coatings are widely used in different industrial applications e.g. vanes for gas blade turbines to protect hot sections in service at high temperatures, e.g. the gas inlet temperature is of 1400°C-1600°C. The in service failure of TBC coatings occurs often by delamination at the interface between metal and coating and leads further to local spallation of the TBC. The thermal and mechanical loading in service changes the microstructure and properties of the materials of the TBC system. This may reduce the resistance against initiation and propagation of delamination cracks (interfacial fracture toughness). Despite the diversity of test methods, there are still practical problems in using one or the other test method for the case of the TBC systems and in understanding the results of the tests. The aim of the present work was to use different fracture mechanics tests to evaluate the change in adherence after heat treatment of industrially used EB-PVD TBC systems and to contribute to a better understanding of the damage mechanisms in the TBC systems. The core test method was the sharp indentation with the following variations: Vickers indentation at the interface ceramic/metal and Rockwell indentation tests perpendicular to the TBC. Theoretical models for calculating interfacial fracture toughness need material properties. Therefore, they have been also experimentally determined. Young’s modulus (E) of the component materials of the TBC system has been determined by means of the Impulse Excitation Technique method. For the case of the TBC material, a general trend of increase of the Young’s modulus after heat treatment has been obtained. The measured value of the Young’s modulus of the substrate material IN625 did not change after heat treatment. The measured value of E of the BC material after heat treatment did not change significantly. Interfacial Vickers indentation tests gave reasonable results. After 24h exposure at 1000°C, a decrease of the apparent interfacial fracture toughness for all TBC thicknesses has been obtained. After 100h at 1000°C, a small increase of the interfacial fracture toughness in comparison with the values after 24h heat treatment has been observed, but the values are still smaller than the values in the as coated condition. Rockwell indentation tests on specimens with anisotropic substrates resulted in butterfly shaped delamination cracks. Measuring the crack lengths in cross section by SEM, for specimens with anisotropic substrates, it has been found that the crack length increases with the increase of the load. By means of SEM investigations, it has been observed that for a TBC system having a ceramic top coat with a thickness of about 280µm, the substrate was slightly deformed, when the TBC was substantial damaged. That means that the assumption of Vasinonta and Beuth [7], for the case of TBC system with a thickness of the top coat of 100µm, that the elasto-plastic deformations of the substrate and BC are the driving forces of the delamination is not correct for thicker TBC and the interaction TBC/indenter cannot be neglected. Instrumented Rockwell indentation test on the as coated TBC with anisotropic substrate, resulted in a butterfly shaped delamination crack of about 4mm span width that has been measured, compared to a span width of the delamination crack of about 2mm in the heat treated specimen, 500h at 1000°C in air. It implies that interfacial fracture toughness of the TBC has increased after heat treatment. A decrease of the crack length after heat treatment has been also observed after instrumented Rockwell indentation test on TBC specimens with isotropic substrate. These results are in contradiction to results obtained in [7] and [78], for EB-PVD TBC systems with a thickness of 100µm. Shorter cracks with the increase of the duration of the heat treatment implies consequently that the resistance of the TBC against crack propagation has increased