Research on the Shakedown Theory and the Stability of Thermal Barrier Coating System

Author:Sun Zuo

Supervisor:xu ying qiang


Degree Year:2018





Thermal barrier coating technology which improves the heat capacity and thermal efficiency of aviation power equipment effectively is an important surface engineering technology.But because of the thermal mismatch and interfacial oxidation of multilayer structure under thermal cycle,coating premature spalling failure is still very common in the actual application and experimental study.Structural shakedown analysis which is devoted to determine the stability limit load and shakedown characteristic of structure under complex cyclic loading can reflect the essence of the structure condition and actual safety degree and maximize the use of material plastic potential.It is one of the effective methods to enhance the structure performance and security of thermal barrier coatings.However,the classical shakedown theory is based on many assumptions,combining with the structural characteristics and the working conditions,the classical shakedown theory extension needs to be done before application.In this study,taking the TBCs as the main object,the shakedown theory and its expansion were used as the main line of the research.Shakedown theory related to the thermal cyclic loading,multilayer structure and interfacial oxidation-diffusion were developed,then,the stability limit load and shakedown characteristic of thermal barrier coating system were analyzed.The main research contents and results of this dissertation were as follows:1)Shakedown theory extention for temperature dependent material and multilayer structure,respectively.The structural stability criterion under temperature loading was established based on thermal elastic-plastic constitutive,cyclic stress-strain relationship and classic kinematic shakedown theorem,which expands the application scope of shakedown analysis.Using the feature of kinematic hardening,the existence conditions of static shakedown was established by considering the back stress into the yield criterion,and the existence format of kinematic shakedown was established with the considering the work by back stress in the plastic dissipation,which extends the application prospect for shakedown analysis of kinematic hardening material.The stability analysis for multilayer beam structure of elastic-plastic material and its numerical solution scheme was given,which extends the application of shakedown analysis.2)Shakedown theory extention for multilayer cylinder structure.Starting from the basic heat transfer and thermal stress theory,the temperature and thermal stress distribution was obtained.With the classical kinematic shakedown theorem,Tresca yield criterion and incremental failure criterion,a stability limit analysis method for multilayer structure was estabilished.The variation between material yield strength and temperature was simplified as bilinear relationship,and the stability limit of typical thermal barrier coating system was analyzed by using the method of compensation transformation to simplify the solving process.The results show that the stability limit can be obtained facility by shakedown analysis using the multilayer cylinder model.Stability limit of TBCs was significantly higher than the elastic limit,and the local stability limit in convex area was higher than concave area around the interface,which indicates that failure prior to occur in concave area.The larger the radius of curvature of the substrate and the thickness of TBCs,the higher the stability limit.3)Stability analysis method of structure with interfacial oxidation and diffusion,and the stability analysis for thermal barrier coating system.Considering the viscoelastic plastic constitutive,thermodynamic laws,fick diffusion and oxidation balance equation,the stability evaluation method for structure with interfacial oxidation and diffusion was estabilished based on the classical kinematic theorem.Based on the growing law of oxide from experiment,the material transform method was used to simulate the oxidate growth,which avoids the limitations of the traditional methods.The effect of oxidate growth on the stability of thermal barrier coating system was analyzed through the simulation of structure with the semicircle interface from the angle of the stress-strain and energy.It is concluded that,the method of combining the material transform and oxidation growth rule law from experiment to realize the thermally growth oxide layer thickening was simple,effective and helpful for engineering application.Oxide growth influences the residual stress of thermal barrier coatings significantly,and creep release stress obviously.Spalling failure may take place in the middle area of TC/TGO interface and convex peak area of BC/TGO interface.Estamited stability from the stress-strain evolution behavior,the interface stability of TGO/BC and TC/TGO interface reduce with the oxide growth.And stability of thermal barrier coating system reduces with TGO thickening assessed from energy.The stability results show agreement with limit results in previous analysis.4)Verification of stability results of multilayer structural TBCs.8YSZ thermal barrier coatings were prepared using plasma spraying method.With the thermal shock tests,taking the coating spalling as failure index,the TBCs and local interface stabilities were analyzed.The effectiveness of the TBCs stability results and the the stability limit extension methods for multilayer structure were validated.The results show that interface morphology changes in amplitude and TGO thickening induced by cylic thermal loading were significant factors that influence the interfacial stress-strain state and the stability of TBCs.Increasing the interfacial amplitude and thickening the TGO layer would decrease the stability of TBCs and cause the displacement instability at interface,which verified the main conclusions about TBCs stability in this study.For the preparation coating sample,TBCs had good stability performance when thermal shock temperature were 850 and 950℃ ℃,the structure would be considered in stable state.the coating sample stability decreased remarkably at 1050℃,so that the stability limit of TBCs would between 950 to 1℃ 050℃.The model used to analyze the stability limit load for TBCs were verified.