Analysis Of Tensile Failure And Mechanical Properties Of Thermal Barrier Coating Systems At High Temperatures By Using Digital Image Correlation Technique

Thermal barrier coatings(TBCs) have been widely applied in thermal protection field of high-temperature component in aviation turbine engine and industrial gas-turbine applications due to their well thermal insulation, wear resistance, and corrosion resistance. However, due to the complicated structure and serving environments, TBCs usually encounter failure in the form of surface coating cracking and interface spalling without warning. It is urgent to develop many advanced experimental techniques and equipment to characterize high-temperature mechanical behaviors of TBCs, further predict their service life. In this paper, we have designed and developed a high temperature test platform based on non-contact and non-destructive digital image correlation method. Tensile failure processes of air plasma sprayed TBCs systems under different service temperatures were studied. The main conclusions are summarized as follows,Firstly, the evolution of residual stress near the coating surface region of TBCs before and after heat treatments was evaluated by micro-Raman spectrum method. When heat treatment temperature increases from 800 oC to 1000 oC, residual stress in the coating changes from-29.61±1.98 MPa to-7.8±1.98 MPa, respectively. These results would be useful for discussing the fracture strength, interface shear strength of the coating system under tensile tests at room temperature.Secondly, tensile fracture process of bond coat/substrate system has been successfully monitored and analyzed under high temperatures by using our developed high-temperature mechanical test platform. All important experimental data was recorded by computers. A traditional double shear lag model was introduced to analyze fracture strength of bond coat layer. The analysis shows that the critical fracture strain of bond layer at room temperature, 800 oC, 900 oC, 1000 oC were monitored as 0.53%, 0.65%, 0.56% and 0.44%, respectively, the corresponding fracture strength was estimated as 1060.0 MPa, 942.5 MPa, 728.3 MPa and 528.4 MPa, respectively.Thirdly, tensile fracture process of air plasma sprayed thermal barrier coating system under different high temperatures has been successfully estimated by the same equipment. Similarly, the critical fracture strengths of the coating at different temperatures has been successfully estimated by using shear lag model. The value was evaluated about 94.4±8 MPa, 53.7±1.7 MPa, 21.9±1.5 MPa, 17.7±1.3 MPa and 11.5±1.1 MPa under room temperature, 800 oC, 900 oC, 1000 oC, 1100 oC, respectively. Based on strain contours by DIC technique and SEM observations, possible failure mechanisms of air plasma sprayed TBCs under different temperatures were discussed.