Investigations On Fracture Toughness And Residual Stress Of Thermal Barrier Coatings During Thermal Cycling

Thermal barrier coatings (TBCs) have been wide applied in stationary turbines of power plants and aircraft turbines to protect basic advanced superalloys materials from high temperature and corrosion, and thereby prolong the lives of these components. However, the crack nucleation, propagation and spallation of ceramic coating greatly limit the application and durability of TBCs in service. In this paper, the change characteristics of partial material properties (e.g. Young's modulus, hardenss and PSC) of 8YSZ with thermal cycles were studied by nanoindentation method and Raman spectroscopy. And then the fracture toughness and residual stress of TBCs were measured. The related conclusions are summarized as follows:Firstly, the Young's modulus and hardness in the top coat of 8 wt.% Y2O3-stabilized ZrO2 (8YSZ) TBCs have been measured by nano-indentation. The evolution of Young's modulus and hardness exhibits two regimes of stiffening in the increasing stage: an initially rapid rise, followed by a more progressive increase due to the sintering effect. And the results also indicate that the Young's modulus and hardness are anisotropic. The Weibull analysis has been applied to solve the scatter of experimental data due to the brittle characteristic of the ceramic coating.Secondly, the fracture toughness of 8YSZ TBCs is evaluated by Vickers indentation method. Two fracture mechanics theoretical models based on the half-penny crack system were used to analysis the experimental data, respectively. One is the traditional fracture mechanics theoretical model, and the other is the Lawn-Evans-Marshall (LEM) fracture mechanics theoretical model which has considered the influence of residual stress in the coating. The results by the traditional fracture mechanics theoretical model indicate that the surface fracture toughness of ceramic coating changes from 1.19MPa·m1/2to 4.4MPa·m1/2. And the interface fracture toughness of the coating varies from 0.54MPa·m1/2to 2.8MPa·m1/2 with thermal cycles under the load of 98N. However, the results by the LEM fracture mechanics theoretical model show that, the surface fracture toughness of ceramic coating changes from 0.52MPa·m1/2to 3.06MPa·m1/2 with thermal cycles with different indent loads. The corresponding surface residual stress varies -41MPa to -268MPa. The fracture toughness also presents anisotropic. It is found that the value of fracture toughness on the coating surface are generally large than that near the interface location.Thirdly, the evolution of Piezo-spectroscopic Coefficients (PSCs) of freestanding 8YSZ specimens with thermal cycles has been calibrated by Raman spectroscopy. It is shown that the PSC of 8YSZ is an exponential function of thermal cycle N for a given thermal cycling process, and it is opposite to the evolution of Young's modulus of 8YSZ with thermal cycles. Fourth, the effect of Young's modulus evolution of 8YSZ on residual stress measurement by X-ray diffraction (XRD) has been studied. When the evolution of Young's modulus of 8YSZ material is considered, the residual stress derived from the XRD data is more accurate. With the PSCs evolution above, the actual residual stress of 8YSZ TBCs has been determined by the Raman spectroscopy method. The results consist well with that obtained by the XRD.In general, the evolutions of mechanical properties and residual stress of 8YSZ TBCs with thermal cycles have been studied. The results would be useful in evaluating the reliability and life of the 8YSZ TBCs in future.