Investigations On Fracture Toughness And Residual Stress Of Thermal Barrier Coatings By Vickers Indentation Technique

Thermal barrier coatings （TBCs） have been widely applied in nuclear technology and aircraftengine due to their excellent wear resistance, corrosion resistance, and thermal insulation asengine components. During engine in service, many factors contribute to the cracks which happenin surface and interfacial of TBCs, the spallation of ceramic coating greatly makes the fullapplication field narrowed. In this paper, the fracture toughness and residual stress were studiedwith thermal cycles by Vickers indentation technique. The digital image correlation （DIC） andsingle edged notched beam （SENB） and Raman spectroscopy were presented to investigate thefracture strength and the fracture toughness and residual stress. The main research contents aresummarized as follows,Firstly, the fracture toughness of freestanding8YSZ coating under thermal cycles wasmeasured by SENB method. The DIC technique has been firstly used to monitor the evolutions offull and local field strain in freestanding8YSZ coating during bending process, and obtain the dataof fracture strain, fracture time and the crack growth path. In this paper the method of statisticalanalysis of weibull modulus based on Vickers indentation fracture test was adopted becauseceramic materials was a kind of anisotropic and uneven material. Based on the analysis of straindistribution, the failure mechanism of air plasma sprayed TBCs under bending was discussed. Theresults show that fracture time and load with in-situ DIC method are larger than those by tensiletesting machine test, the fracture strength of freestanding8YSZ coating changes from28±2MPato77.78±6.48MPa, the corresponding fracture toughness ranges from1.04±0.12MPa·m^1/2to2.23±0.22MPa·m^1/2by DIC method compared to1.12±0.12MPa·m^1/2to2.45±0.2MPa·m^1/2bytraditional SENB method.Secondly, Raman shift and piezo-spectroscopic coefficient （PSC） were measured by Ramanspectroscopy. Based on the evolution of young’s modulus, the residual stresses of thermal barriercoating of vacuum plasma spraying during thermal cycles were estimated by Raman spectroscopymethod. The thickness and morphology of TGO under thermal cycles were analyzed though SEM,and the failure mode of TBCs under thermal cycles was also studied. Considering the influence ofthe maximum indentation load cracks and residual stress, the fracture mechanics theoretical modelwas used to analysis the experimental data. The results show that the residual stress changes from29.66±5MPa to190.62±10MPa by Raman spectroscopy. The results consist well with thereported data by the X-ray diffraction （XRD）. Thirdly, the surface, interfacial, and lateral region of the fracture toughness and residualstress of air plasma spraying thermal barrier coating were estimated by modified Vickersindentation technique method. Considering the influence of indentation trace, maximumindentation depth and residual stress, the surface and interfacial indentation fracture model havebeen modified. The Young’s modulus and hardness of surface and lateral of thermal barriercoatings were measured by nanoindentation technique. The surface fracture toughness of8YSZTBCs by air plasma spray method changes from0.64±0.2MPa·m^1/2to3.67±0.2MPa·m^1/2, thecorresponding residual stress varies from26.8±5MPa to234±10MPa, the surface residualstress8YSZ TBCs by vacuum plasma spray method varies from79.82±15MPa to169.62±12MPa by Vickers indentation technique, and the corresponding fracture toughness changes from1.5±0.2MPa·m^1/2to4.11±0.2MPa·m^1/2. The results consist well with that obtained by theRaman spectroscopy. The interfacial fracture toughness varies from0.11±0.05MPa·m^1/2to0.81±0.05MPa·m^1/2, the corresponding residual stress varies from5±1MPa to30±3.5MPa, theaverage lateral region of fracture toughness changes from0.37MPa·m^1/2to1.14MPa·m^1/2, and thecorresponding residual stress varies from21.1MPa to98.37MPa, respectively. The indentationfailure process of thermal barrier coatings was analyzed.