A critical review of the non-destructive evaluation of thermal barrier coatings prior to failure

Abstract

A critical review of literature on thermal barrier coatings (TBC) with an emphasis on the stochastic nature of TBC failure data is undertaken. The Characteristic Life Equation, first proposed by Professor John R Nicholls in modelling the influence thermally grown oxides have on TBC spallation is devised. The works of A.G Evans and co-workers on the mechanics of failure of thin, brittle layers on ductile substrates during thermal cycling was applied to the thermally grown oxide layer within a TBC system where plate theory is used to analytically derive forces acting on initial defects allowing to analytically determine the critical height required for an induced delamination to cause buckling leading to spallation failure. The probabilistic models proposed as part of the ‘Characteristic Life Equation’ allows for the coefficients of Weibull Stochastic models of TBC spallation failure to be related in strain-energy and material property terms through modelling the mechanics of crack propagation within the thermally grown oxide of a TBC system. X-Ray Diffraction (XRD), Raman spectroscopy, Digital Image Correlation (DIC) and THz radiation in the form of a reflection based polariscope were reviewed to investigate whether these NDE techniques have the potential to evaluate TBC ‘performance’ using the characteristic life equation. A preliminary sensitivity analysis of the characteristic life equation was carried out. DIC was assessed for its suitability to non-destructively evaluate both in and out-of-plane displacements on a TBCs surface. The observed range in DIC data for the range of speckle patterns and system parameter settings explored was found to be able to reliably resolve out of-plane measurements of 0.03069mm with an associated error of 2.87𝑥10−7𝑚𝑚. XRD, Raman, DIC and THz radiation, potentially in combination with other non-destructive evaluation techniques as well as the characteristic life equation may ultimately aid develop a methodology by which the remaining life of a TBC may be non-destructively assessed. Should this goal be achieved - probing the Characteristic Life Equation non-destructively may prove invaluable as a tool in the improvement and development of TBCs.