Development of residual stress and damage in thermal barrier coatings

Photo-luminescence piezo-spectroscopy through the electron beam deposited (EBPVD) yttria-stabilised zirconia (YSZ) thermal barrier coating has been used to study the evolution of residual stress due to the alumina thermally grown oxide (TGO) formed on Pt-Al bond coats on CMSX-4 substrates. The coated specimens were thermally cycled between room temperature and 1150 °C, holding for 1 h at the top temperature. The experiments explored the effects of two different bond coat surface finishes (normal and polished) and thickness of YSZ from 70 to 270 μm.In most cases the mean TGO stress was found to gradually decrease with thermal cycling in agreement with previously published results. This is mainly due to rumpling increasing the waviness of the TGO and hence the ability to relieve stress by bending. For the same reason, the TGO residual stress in specimens with polished bond coats was higher initially than in those with a normal surface finish. Stress mapping showed that there are regions, approximately 100 μm in size, where the stress is particularly low (< 1 GPa compressive) and these were attributed to local TGO damage or delamination. The incidence of these low stress “events” increased markedly just before coating spallation. The failure was in the TGO, or in the YSZ close to the TGO, probably as a result of weakening in this region by the damage induced by rumpling and driven by the stored energy in the YSZ.