| Thermal barrier coatings（TBCs）have characteristics of low thermal conductivity,high temperature resistance,and corrosion resistance.It is widely used in aerospace,energy systems,and civil technology to protect the alloy matrix.However,the multimaterial structure and the demanding operating conditions can lead to a premature failure of the TBC system,such as debonding and cracking.Therefore,the interfacial adhesion properties of thermal barrier coatings have become an overriding concern.In this dissertation,based on the traditional three-point bending experiment,a modified three-point bending method is proposed to study the interface bonding properties of thermal barrier coatings.The experimental method,numerical and theoretical models are established.The main research contents are as follows:（1）A modified three-point bending method for the characterization of interface fracture strength of TBCs has been proposed.The bond-coat and the top-coat（ceramic）are sprayed on half span of a beam,and then a hard steel sheet is bonded on the top of the ceramic layer to prevent its vertical surface cracks.The interface fracture path is between the ceramic layer and the adhesive layer.An optical microscope is used to observe the interface cracking process,and the interface crack length is determined by micro-scale digital image correlation（DIC）method.（2）A cohesive zone finite element model of the interface fracture has been established.The interface crack has been simulated by the cohesive force model embedded in ABAQUS software.When the interfacial adhesion energy reaches a critical value,cracks begin to propagate.The interfacial interface adhesion energy is 140-160 J/m2,which is inversely calculated from the interface crack length measured with the loading displacement.（3）A method of calculating the critical interface adhesion energy based on the change of compliance variation is established.According to the load-displacement curve,the flexibility（compliance）of the thermal barrier coating system during cracking is calculated,and the critical interface adhesion energy is calculated by the relationship between the flexibility and the crack length.The results show that the energy release rate of the interfacial cracking is consistent with the simulation results as well as the related literature.（4）A method has been developed to calculate the interfacial stress intensity factor using the bimaterial interface fracture theory combined with digital image correlation technique.A three-point bending test specimen with a pre-crack is prepared.The crack opening displacement has been obtained by the micro-scale digital image correlation method.According to the bimaterial interface fracture theory,composite stress intensity factors have been obtained being as KⅠ＝12.85MPa·m1/2 and KⅡ=4.58MPa·m1/2.Moreover,the loading phase angle is Ψ= 16.04°. |
