Quantitative Failure Assessment For Surface Cracking And Interface Delamination Of Thermal Barrier Coatings By Acoustic Emission

Thermal barrier coatings （TBCs） have been widely used in aircraft turbineengines, due to the remarkable performances of heat protection, abrasion andanti-corrosion. TBCs is extremely easy to crack in coating surface and delaminationon interface, however, because of intricate structure of TBCs system and harshoperation conditions, and resulting in failure at last. It is difficult to predict time,location and level of cracking and delamination which are determined by manyfactors. It is obvious that the real-time detection of TBCs failure behaviors forprediction of TBCs reliability and life are significant using an advanced detectingtechniques. In this paper, acoustic emission （AE） method is used to monitor theprocesses of TBCs failure under axial loading, and qualitative and quantitativeanalysis are conducted for crack of coating surface and delamination on interface. Themain contents are stated as fallows:Firstly, Analytical solution for surface crack density where strain gradients andresidual stresses are considered in coating is developed. The interface crack length isalso used to evaluate the degree of damage as a key parameter for TBCs bucklingfailure. The theoretical result shows that interface crack energy release rate is afunction related to crack length. The relationships of surface crack density, interfacecrack length and their corresponding AE energy are found according to the linercorrelation between energy released by crack and AE energy.Secondly, The residual stresses, fracture strength and fracture toughness ofceramic coating for three type specimens are measured by indentation, and the shearstrength of ceramic/bond coating interface is determined by shear method. The resultsshow that the residual stresses, fracture strength, fracture toughness and shear strengthare range of59.8⁷4.7MPa,94.3¹23.3MPa,0.53⁰.69MPa·m1/2and10.2¹2.8MPa, respectively. The failure processes are monitored by AE method.Combining the analysis of strains evolution, the failure processes can be summarizedby surface crack initiation, multiplication and saturation following by interface crackinitiation, propagation and spalling. The frequency spectrum range for AE signals ofsurface and interface cracks are0.20⁰.24and0.26⁰.30MHz, respectively. Theexperimental results indicate that the saturation surface crack density are1.70,1.25and1.001/mm for three types specimen, which consists with theoretical, and associated with fracture and shear strength. The investigation also suggests that thesurface crack density linearly correlates to AE energy, and the slopes of linearrelationship depend on the micro-structure of materials, such as pores and defects inceramic coating.Thirdly, the buckling failure processes of coating are detected using AE methodsubjected to compression, which experience micro-buckling, buckling delamination,fracture of buckling coating and edge delamination. The interface crack ispredominately Mode Ⅰ at the onset of buckling, subsequently by increasing in theproportion of Mode Ⅱ to Mode Ⅰ, and the crack can be considered as pure Mode Ⅱ inthe processes of edge delamination. The fracture toughness of Mode Ⅰ is1.94J·m^-2,and the energy release rate of stable propagation is about70J·m^-2when buckling. Theresearch indicates that the frequency spectrum of interface crack AE signals areassociated with cracking mechanical mechanism. The range of frequency spectrum forMode Ⅰ and II interface cracks are0.40⁰.46and0.20⁰.24MHz, respectively. Theexperiment and theory indicate that there is a linear correlation between the halflength of bucking crack and AE energy.