AN INDENTATION BASED NON-DESTRUCTIVE EVALUATION TECHNIQUE FOR THERMAL BARRIER COATING SPALLATION PREDICTION

摘要

A load-based micro-indentation method for NDE of TBCs exposed to thermal loads has been developed. TBC thermal cyclic exposure tests were performed where after a defined thermal cycling period, surface stiffness response was measured to assess damage accumulation and identify macroscopic debonding failure sites. Microstructural analyses were conducted correlating YSZ adherence to TGO/BC and evidence of internal crack formations with the measured surface stiffness responses. Finite element analyses indicate that high YSZ/BC interfacial rumpling leads to the development of both in-plane and out-of-plane residual stresses upon cooling. Additional rumpling of this interface as a result of non-uniform oxide growth leads to enhanced residual stresses. Average stress levels within the TBC have been found to decrease with accumulated thermal exposure, yet the variance of these values was found to increase primarily due to enhanced residual stress variation across the coupon which can be identified by our micro-indentation technique. As a result, areas producing relative increases in surface stiffness response enable early detection of initial TBC spallation locations. In addition, finite element analyses of YSZ/TGO/BC interfacial stresses generated upon cooling provide an explanation for the experimentally observed micro-cracking and failure patterns.