Analysis of residual stresses in thermal barrier coatings

Abstract

Stabilized zirconia coatings are used as thermal barrier coatings (TBCs) because of their ability to improve the performance and efficiency of gas turbine engines by allowing higher turbine inlet temperature and reduced cooling air flow. Yttria stabilized zirconia plasma sprayed coatings (PS top coating) were applied on high temperature Ni-based alloys pre-coated with a thin, dense stabilized zirconia coating produced by Physical Vapour Deposition (PVD bond coating). This contribution concerns the residual stress analysis of a TBC coating system. The coating residual stress was studied experimentally (by X-ray diffraction, micro-Raman techniques and micro-displacement laser transducer) and numerically in the thermal barrier multi-layered system for the as-deposited coatings after different thermal cycling processes. The PVD bond coating has an in-plane compressive residual stress after deposition (about −320 MPa). The plasma sprayed top coating has a compressive stress near the interface and presents low tensile (or compressive) stress on the surface after plasma spraying deposition (−20 to 30 MPa, depending on the substrate temperature and coating thickness). After thermal cycling, the stress changes to −760 MPa for the PVD bond coat and the PS coating stress changes to a compressive value of about −300 MPa. The stress within the Cr 2O 3 scale was −1680 MPa, as determined by micro-Raman spectroscopy. The residual stresses within the TBC during thermal cycling were modelled, the numerical results being in good agreement with the experimental measurements.