Numerical simulation of a load-based microindentation technique for nde on thermal barrier coating systems

Abstract

The durability and life-cycle of thermal barrier coatings (TBC) applied to gas turbine blades and combustor components limits 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 materials used in turbines, Yttria stabilized zirconia (YSZ) is widely used for TBCs are most prevalent, due to its low thermal conductivity, high thermal expansion coefficient and superior mechanical strength. However, thermally grown oxide (TGO) and thermal expansion coefficient mismatch are the major causes of large residual stress and will cause interfacial rumpling instability leading to large scale 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, a load-based micro-indentation method for NDE of TBCs exposed to thermal loads is investigated. The surface stiffness response is measured to assess damage accumulation and identify macroscopic debonding failure sites. 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 TGO growth leads to enhanced residual stresses. Finite element analyses of YSZ/TGO/BC interfacial stresses generated upon cooling provide an explanation for the experimentally observed micro-cracking and failure patterns. The associated coating degradation is evaluated using a non-destructive load based multiple partial unloading micro-indentation procedure. The results show that the proposed 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.