Non-Destructive Thermal Barrier Coating Spallation Prediction by a Load-Based Micro-Indentation Technique

Abstract

Currently, the durability and life cycle of thermal barrier coatings (TBC) applied to gas turbine blades and combustor components are limiting the maximum temperature and subsequent efficiency at which gas turbine engines operate. The development of new materials, coating technologies and evaluation techniques is required if enhanced efficiency is to be achieved. Of the current ceramic coating materials used in gas turbine engines, yttria stabilized zirconia (YSZ) is most prevalent, its low thermal conductivity, high thermal expansion coefficient and outstanding mechanical strength make it ideal for use in TBC systems. However, residual stresses caused by coefficients of thermal expansion mismatches within the TBC system and unstable thermally grown oxides are considered the primary causes for its premature and erratic spallation failure. Through finite element simulations, it is shown that the residual stresses generated within the thermally grown oxide (TGO), bond coat (BC), YSZ and their interfaces create slight variations in indentation unloading surface stiffness response prior to spallation failure. In this research, seven air plasma sprayed and one electron beam physical vapor deposition yttria partially stabilized zirconia TBCs were subjected to isothermal and cyclic loadings at 1100°C. The associated coating degradation was evaluated using a non-destructive multiple partial unloading micro-indentation procedure. The results show that the proposed non-destructive micro-indentation evaluation technique can be an effective and specimen-independent TBC failure prediction tool capable of determining the location of initial spallation failure prior to its actual occurrence.

Development and Assessment of Coatings for Future Power Generation Turbines

“…Durability testing at room temperature was conducted during FY10 on commercially TBC coated René N5 coupons that were subjected to extended high temperature exposure and subsequent micro-indentation characterization to address residual stiffness of the coating, as well as to project potential coating spallation. Similarly during FY10, the residual stresses generated within the TBC/bond coat interface due to thermal cyclic loadings were correlated to measured indentation stiffness response of TBCs for predicting coating life (i.e., spallation failure) [7][8][9][10][11][12][13][14][15][16][17]. The unloading surface stiffness responses obtained from micro-indentation of TBC coupons subjected to various thermal loadings has proven to be a viable criterion for predicting TBC spallation failure location.…”

Progression in Non-Destructive Spallation Prediction and Elevated Temperature Mechanical Property Evaluation of Thermal Barrier Coating Systems by Use of a Spherical Micro-Indentation Method

“…In addition, at periodic intervals (~100 hour), these coupons will be removed from the furnace and examined for damage/spallation followed by further thermal loading until failure is reached. With this information it is believed that more sophisticated and accurate numerical TBC failure prediction models will be developed [46].…”