Fracture behaviour of thermal barrier coatings after high temperature exposure in air

Four point bending tests have been carried out on thermal barrier coating (TBC) specimens. The system studied consists of a plasma sprayed top coat (TC) of ZrO2 6-8 wt.% Y2O3 (YSZ), an FeNiCrAlY bond coat (BC) and a mild steel substrate. To study the influence of temperature on this system, two heat treatments were adopted. The specimens were held in a furnace for 2 h at 580 and 900 °C, and cooled inside the furnace to room temperature (RT). In addition, microstructural analyses were carried out after each treatment by using different techniques energy dispersive X-ray analysis (EDX); X-ray diffraction (XRD); Raman spectroscopy, and scanning electron microscopy (SEM)). Four point bending tests were carried out on heat-treated specimens and the results were compared with those for the as-sprayed condition. In situ observation of crack propagation events were carried out during the tests and it was found that cracks propagated through the TC-BC interface in specimens heat-treated at 580 °C heat treatment and also in the as-sprayed reference specimens. In contrast, after the 900 °C heat treatment they propagated across the BC-substrate interface. The drop in BC-substrate adhesion due to heat treatment at 900 °C prevailed over the mechanical weakness induced at the TC-BC interface.