Effects of deposition temperature and thermal cycling on residual stress state in zirconia-based thermal barrier coatings

Advanced ceramic multilayered coatings are commonly used as protective coat ings for engine metal components to improve performance, e.g. thermal barri er coatings (TBCs). Zirconia-based TBCs were produced by plasma spraying pr ocess and characterized in terms of microstructure, porosity, elastic modul us, adherence and residual stresses. In this contribution the residual stre sses in multilayered coatings applied on Ni based superalloys for use as th ermal barrier coatings were studied both by numerical modelling and experim ental stress measurement. The thermal residual stresses generated during th e spraying process of duplex TBCs were simulated by using an heat transfer finite element program and an elasto-plastic biaxial stress model. The TBC system was subjected to different thermal cycling conditions (maximum tempe rature, heating up and cooling down rates, dwell lime at maximum temperatur e, etc.). The stress distribution within the TBC was also modelled after th ermal cycling. The stress state in the as-deposited and in thermally cycled coatings was verified using an X-ray diffraction technique. The measuremen ts were in good agreement with the residual stress modelled calculations. I t was observed that the residual stresses were dependent on the thermal his tory of the TBC (as-deposited and thermally cycled). It is proposed that th ermal cycling allowed the stresses to relax by microcracking and creep mech anisms at high temperature such that on cooling down to room temperature, a n in-plane biaxial compressive stress will arise on the zirconia top coatin g due to the difference on the coefficients of thermal expansion between th e metallic substrate and ceramic coating material. (C) 1999 Published by El sevier Science S.A. All rights reserved.