Interface Rumpling And Failure Caused By Stress In Oxide Layer And Bond Coat In Thermal Barrier Coating Systems

Thermal barrier coating(TBC)is a key material and technology in the manufacture of aviation engines and ground gas turbines.At present,thermal barrier coating system is generally composed of ceramic thermal insulation layer,oxide layer(TGO),bond coat and superalloy substrate.The key to its application is to ensure that the coating does not spall during service.Generally speaking,the failure of thermal barrier coating in service is mostly related to the interface rumpling of TGO/bond coat.Therefore,the study of interface rumpling mechanism is the basis of understanding the failure of thermal barrier coatings.Preliminary studies have shown that the stress of the bond coat induced by phase transition at high temperature,together with the high-temperature stress of the TGO control the formation of interface rumpling.It can be inferred that the residual stress of bond coat and TGO will also affect the deformation and failure of TBC interface at room temperature.In this paper,the research is carried out on two common problems existing in the application of thermal barrier coating: 1)the influence of processing residual stress in bond coat induced by grit blasting during the preparation process on the interface rumpling and failure of thermal barrier coating;2)the spontaneous TBC failure formed after long-term placement at room temperature after service,which has not been reasonably explained and brings great risks to the TBC safety and service.In this paper,TBC samples and experiments are designed for these two problems.Moreover,numerical simulation is carried out for in-depth research.The main research contents and results of this paper are as follows:The effect of bond coat stress on interface rumpling and coating failure was studied by using the popular Ni Co Cr Al Y bond coat and superalloy matrix(Chapter 3).Bond coats with different residual stresses were prepared by regulating grit blasting parameters.Microstructure analysis was used to study the evolution of the coatings with different residual stress.The effect of residual stress on the TGO growth,interface rumpling and spalling behavior were studied by use of light interference,fluorescence spectroscopy and X-ray diffraction(XRD)characterization methods.The results show that the compressive stress relaxation in the bond coat at the initial oxidation stage causes the interface rumpling,which leads to the rapid growth of spinel oxide and the formation of interfacial cracks,and has a negative effect on the interfacial bonding.The mechanism of spontaneous failure of TBC at room temperature was studied by using Pt-?/?' bond coat which had uniform growth of TGO(Chapter 4).The evolution of the surface morphology of the coating was studied by means of microscopic image analysis.The time dependence of TGO spalling failure at room temperature was studied by the measurement of surface roughness and residual stress in situ.The results show that the time-dependent spalling of TGO at room temperature may be closely related to interfacial deformation and crack growth.At room temperature,the TGO spalling shows a typical time-dependent behavior.The surface roughness of the oxide layer shows a certain variation rule at room temperature(decrease-increase-decrease-stability).The increase of the roughness is caused by the interfacial deformation at room temperature,while the decrease of the roughness is caused by the local spalling of the oxide layer.In order to overcome the limitation of experimental observation,finite element modeling was used to simulate the effect of oxide stress on interface rumpling and the growth of interface cracks(chapter 5).The stress state of TGO and bond coat were analyzed.Moreover,by means of interfacial cohesion model,the effects of interfacial morphology on interfacial crack growth were investigated.The study shows that the residual stress in the coating is large enough to cause plastic deformation on the surface of the bond coat.When the rumpling amplitude of the interface exceeds the critical value,the interfacial crack nucleation and expansion will occur.Therefore,the TGO spalling at room temperature is time-dependent and region-selective.The coating with more interfacial deformation and nonuniform growth of the TGO is easier to spall and fail.In conclusion,it was proposed that the residual stress of bond coat can affect the interface rumpling of thermal barrier coating.The effect of the residual stress of bond coat on interface rumpling and failure of thermal barrier coating was revealed through the contrast experiments by means of grit blasting.The spontaneous failure of thermal barrier coating at room temperature after service was studied.The effect of TGO stress on interface rumpling and TBC failure at room temperature was clarified through in-situ experiments.The interface failure between TGO and bond coat was simulated and analyzed using the interface cohesion model.The critical amplitude of the interface rumpling causing the interface crack nucleation was determined,which provided a research orientation for improving the anti-spalling performance of the thermal barrier coating.