Influence of isothermal aging conditions on APS TBC's interfacial fracture toughness

Abstract

Interfacial toughness is an important factor to address thermal barrier coating's (TBC) durability. In this paper, a promising method, based on interfacial indentation, is used to analyze apparent interfacial toughness of TBC deposited by Atmospheric Plasma Spray (APS). The specimens made by APS were treated using different oxidation holding times and temperatures, namely 1050°C-100h, 1100°C-100h, 1050°C-300h and 1100°C-300h, respectively. The morphology of the interface between the TBC and the underlying bond coat was analyzed by Scanning Electron Microscopy (SEM). Results have shown that the fracture toughness of the interface between the bond coat and the top coat decreased as the oxidation conditions become more severe that is the temperature and/or to a lesser extent the exposure time is increased. Simultaneously, the thickness of the thermally grown oxide (TGO) generated on top of the aluminum reservoir bond coat increases as well. The TGO is a “double layer” oxide successively composed of a first scale of Al2O3 close to the bond coat and a second scale of CoCrNiO close to top coat. In addition, it was also found that the possible thermally activated spallation of APS TBC's system occurs in the zone of the TGO layer, especially within the CoCrNiO oxide scale. Consequently damage of APS systems is shown to initiate at the interface through complex mechanism of delamination in relation with both the toughness and the microstructure of the interface. In order to inhibit the growth of the detrimental CoCrNiO oxide and in turn favors the development of a dense and stable alumina scale, a Supersonic Fine Particles Bombarding (SFPB) method was used to optimize the quality of APS coating.