Sintering Behavior And Anti-Sintering Design Of Plasma Sprayed Thermal Barrier Coatings

Thermal barrier coatings(TBCs)have been developed to protect hot end components against high-temperature heat fluxes,which are highly required for the development of highperformance gas turbines.Currently,the preparation of thermal barrier coatings by plasma spraying is one of the most widely used preparation methods.The plasma sprayed TBC(PSTBC)has a porous structure,which leads to a significant increase in thermal insulation performance and strain tolerance of the coating.However,under prolonged high-temperature service,the TBC will be densified by sintering,leading to spalling or failure.As a result,the hot-end components may be damaged,which causes huge safety hazards and economic losses.Therefore,the sintering of the TBCs is an important factor limiting the life of gas turbine.In this investigation,8YSZ coatings were prepared by plasma spraying and subjected to isothermal thermal treatment.The structure and property changes of the coating during thermal exposure was investigated by scanning electron microscopy(SEM),atomic force microscopy(AFM),transmission electron microscopy(TEM)and mechanical property testing.The structure evolution and the performance degradation caused by sintering were revealed.Subsequently,finite element simulation was used to analyze the effect of coating sintering and thickness on crack extension,which guide the design of the TBC structure.Finally,anti-sintering TBCs were designed by a multi-scale structure.The finite element simulation was used to investigate the effect of structural design on strain tolerance.Moreover,the mechanism to weak the degradation in strain tolerance by multi-scale structure was revealed.Theoretical guidance is provided for the development of new long-lifespan TBCs.The experimental results showed that the TBCs underwent significant structural and property changes after isothermal heat treatment.The sintering resulted in structural densification,which leads to increase in the stiffness of the coating.In addition,the stiffness increase exhibited a rapid rate at early exposure.The multiscale undulation of the originally smooth two-dimensional(2D)pore surface triggers multipoint contacts between the upper and lower inner surfaces,resulting in pore healing during thermal exposure.This is the main reason for the large increase in stiffness at the initial exposure period.The healing of 2D pores is the main structural characteristic change in TBCs after thermal exposure and the main reason for the stiffening and changes in mechanical properties.Simulation results showed that both sintering and thickening of the coating increase the stress concentration inside the coating and the crack expansion force,which will have a negative effect on the lifetime of the TBCs.It is found that the effect of sintering is greater than that of coating thickening.Therefore,reducing the sintering effect would be one of the effective ways to improve the lifetime of the TBCs.Simulation of multi-scale structured TBCs showed that implanting vertical cracks in the ceramic layer reduces the coating stiffness and the strain energy release rate by 87.90 % and 79.90 %,respectively.The degrees of reduction depend on the vertical crack density.Finally,the mechanism of the anti-sintering TBCs is analyzed and discussed.The mechanical properties of the designed TBCs were scale-sensitive.The macroscopic stiffening is much lower than the microscopic stiffening.Therefore,the macroscopic sintering effect of the coating is reduced during the thermal exposure,thus maintaining a high strain tolerance.