Investigations On Fracture Toughness And Residual Stress Of Thermal Barrier Coatings In High Temperature By In-situ Indentation Technique

Thermal barrier coatings(TBCs)have been widely used as a key thermal insulation material in defense-related science,technology and aircraft engine due to their excellent wear resistance,thermostability,corrosion resistance,and thermal insulation as engine components.During the working situation,many factors such as parameter mismatch,interfacial defects,temperature variations,residual stress and external forces will contribute to the cracks which happen in surface and interfacial of TBCs.In this paper,the Vickers hardness,fracture toughness and residual stress of the surface coating and interfacial were in-situ studied with temperature variations by a high-temperature Vickers indentation instrument.The main research contents are summarized as follows,Firstly,the conventional Vickers indentation instrument was modified to integrate the loading system,in-situ observation system,image acquisition system,vacuum and cooling water circulation system and other assistant systems.The new-style high-temperature Vickers indentation instrument can offer the in-situ service of evaluating the hardness,fracture toughness and residual stress of coatings within the temperature of 1100 ?.Secondly,Vickers hardness,fracture toughness and residual stress of surface coatings of the freestanding 8YSZ with temperature variations was in-situ tested by the modified Vickers indentation instrument.And the results show that micro-hardness on the top coating surface gradually decreases from about 2.96 GPa to1.06 GPa within the temperature varies from 20 ? to 1100 ?.On the top coating surface,fracture toughness changes in the range of 0.71 MPa·m1/2 to 0.54 MPa·m1/2initial stage,and then it gradually increases to 1.03 MPa·m1/2.The corresponding residual stress varies from-66.4 MPa to-15.38 MPa,and then it gradually increases to 12.92 MPa.It shows a compprssive stress initial and then transform to a tension stress in the region of 900~1000 ?.Thirdly,the hardness,fracture toughness and residual stress of interfacial were in-situ studied with temperature variations by a modified model with the high-temperature Vickers indentation instrument.The results show that micro-hardness gradually decreases from the as received 3.28 GPa to 1.23 GPa in thelateral region of the coating,and decreases from the as received 3.06 GPa to 2.13 GPa in the bond coat with the temperature varies from 20 to 1100 ?.When the residual stress was consideration,fracture toughness changes in the range of 0.25 MPa·m1/2 to0.48 MPa·m1/2,and then it gradually increases to 0.1 MPa·m1/2.The corresponding residual stress varies from-33.04 MPa to-50.96 MPa within the temperature varying,and then it gradually changes to 7.63 MPa.It shows a compprssive stress initial and then transform to a tension stress close to 1000 ?.