The Evaluation Of Interaction Between Local Phase Transformation Of Ceramic Topcoat And Nonlinear Crack Propagation In Thermal Barrier Coatings

Thermal barrier coatings(TBCs)play an important role in gas turbine industry due to their excellent thermal insulation performance.However,the rapid nonlinear propagation of cracks in the ceramic coating(YSZ)during long-term high temperature service induces catastrophic failure of the coating,which is a bottleneck problem hindering the development of TBCs for gas turbines.It is found that YSZ phase transformation is one of the factors affecting the accelerated crack propagation.Therefore,in this paper,the relationship between YSZ phase transformation and crack propagation is studied by combining experiment and simulation,which provides the basis for clarifying the nonlinear crack propagation.X-ray diffraction(XRD),Raman spectroscopy(Raman),scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS)were used to study the spatial/ temporal variation and crack propagation behavior of YSZ above the crack during the gradient thermal cycle.Combined with the evolution of the temperature field of the coating during the crack propagation,the interaction between the local accelerated phase transformation of YSZ and the accelerated crack propagation was clarified.The results show that due to the time/space property of YSZ temperature,it increases unevenly with the increase of thermal cycles,resulting in different phase transformation and sintering degrees due to the time / space property,which induces the increase of uneven phase transformation stress and lateral stress,and leads to the nonlinear propagation of cracks.Among them,the longitudinal crack depth increases logarithmically with the length of delamination crack;The longitudinal crack width is approximately linear with its depth.At the same time,when cracks appear inside the YSZ,the temperature field in the YSZ region above the YSZ has a significant mutation due to the thermal resistance effect,and it increases unevenly with the increase of crack size,resulting in the aggravation of the uneven phase transformation of YSZ.The temperature and monoclinic phase(M)content in each region of YSZ show a radioactive decreasing trend from the YSZ surface above the delamination crack center to the surrounding.The critical M phase content of coating failure is about 40 %.When the content exceeds 35 %,the nonlinear propagation rate of debonding crack increases significantly.The variation of porosity in the corresponding region is opposite to that of M phase content.The interaction between the two increases the crack propagation rate,and eventually leads to coating failure.