Analysis Of Interface Failure Of Thermal Barrier Ceramic Coating Under Thermo-mechanical Loadings

Thermal Barrier Coating (TBC) is a kind of ceramic layers with excellenthigh temperature oxidation resistance and very low thermal conductivity,which is sprayed onto bond coat (NiCrAlY) deposited on the surface ofsubstrate. TBC can effectively reduce the service temperature of theprotected substrate and alleviate the thermal shock loading. Furthermore, itcan reduce the requirement for cooling air and improve the thermalefficiency of aircraft/engine. So TBC has been widely used in the fields ofspace flight, chemical industry, metallurgy and energy industry.The interface coalescence and spallation problems always are the keysubjects in thermal barrier coating investigations. Due to the mismatch inthe thermo-mechanical parameters of metals and ceramics, the ceramiccoating often detaches from the substrate under the coupled effects ofcompression, shear and tensile stresses, which would result in thedegradation and fracture of substrate. So the emphasis of this thesis is tofirstly analyze and predict the variations of thermal/residual stress fields inthermal barrier ceramic coating system during thermal cycles, which makesa contribution to the interface failure studies of the TBC in the followingsections. And then the experimental and theoretical investigations of theinterface failure mechanism of the TBC were analyzed and performed underthermo-mechanical loadings. The main content in this paper are listed asfollows,Firstly, the modeling of the residual stress fields of the TBC wasestablished and a analytical solution was deduced during cooling to ambientafter air plasma sprayed deposition. The residual stresses in the as-receivedsamples were successfully predicted by the plasma spraying parameters.Furthermore, a new two-dimensional analytical solution of thethermal/residual stress field has been obtained under the condition ofnon-linear coupled effects of temperature gradient, thermal fatigue,deposited residual stress, thermally grown oxide (TGO) thickening,elasto-plasticity deformation and creep deformation of TBC. At the sametime, the influence of bending moment and curvature on thermal/residualstress variations in TBC is also considered during thermal cycling. The calculated results of the stress field of the TBC system, which wereobtained by the above analytical solution, agreed well with the previousexperimental results. The relationships of the stress field evolutions,residual strain in TBC and thermal cycles were also obtained and discussed,which was of particular advantage to research the interface failure of theceramic coating.Secondly, thermal residual stress in TBC may directly cause adhesivefailure (delamination at interface) or cohesive failure (spalling ormicro-cracking within ceramic coating). The buckling delamination failuremechanism of thermal barrier ceramic coating was analyzed and reviewed inits operation condition. Under external thermal and mechanical loadings,the first order shear deformation theory model was introduced to analyzethe thermo-mechanical buckling characteristic of thermal barrier coatings(TBC) system with arbitrary across-the-width delamination. Characteristicequation and boundary conditions are derived on the basis of the potentialvariation principle and Euler equations. The closed-form solutions andequations governing buckling of the TBC system were obtained by the statespace scheme. The effects of the TBC system aspect ratio, relative thickness,delamination location and length, temperature gradient onthermo-mechanical buckling failure of TBC are discussed in detail. It wasfound that the evolution of initial interface delamination length played animportant role in predicting the buckling failure characteristic and itsdurability of the TBC system.Thirdly, in experimental investigations, the special samples with athrough-width delamination or surface crack were designed and prepared byair plasma spraying technique. And then the interface failure investigationsof the TBC system were performed under the different compressive, tensileor shear loading conditions by the combinations of the compressive tests,oxyacetylene torch heating equipment, observation and measurementequipments. The main contents are given in the following, (1) thedelamination buckling failure phenomena and critical buckling loadingconditions were obtained firstly by controlling the external mechanicalloading and temperature, which substantially indicated that thedelamination buckling failure existed in TBC service. The interface fracturetoughness, energy release rate and stress intensity factor were evaluated and calculated by Suo-Hutchinson model. The thermo-mechanical bucklingfailure mechanism of the TBC system was discussed in detail. (2) Twodifferent bending failure modes (â… andâ…¡) were observed and discussedduring thermo-mechanical bending loading. The bending failuremechanisms of the two modes in TBC system were analyzed by SEM andEDX observations. The interface fracture toughness, energy release rate andstress intensity factor in thermo-mechanical bending tests were alsoevaluated and calculated by Suo-Hutchinson model. (3) The entire processof the initiation, nucleation, propagation and coalescence of the multipletransverse surface cracks in the ceramic coating was clearly observed underthermo-mechanical tensile loadings. The relationship of the transversesurface cracks number and displacement was recorded and discussed indetail. In addition, the fracture location of each interface failure mode wasidentified by SEM and EDX. Finally, the interface fracture toughness,energy release rate and stress intensity factor in tensile tests werecalculated using Suo-Hutchinson model and shear lag model. In general, theinvestigations in this thesis have systematically analyzed the interfacefailure mechanism, which is of great advantage to provide the experimentaland theoretical basis of the TBC system in the future.