Fracture Toughness And Damage Evolution Characterization Of Thermal Barrier Coatings Using Acoustic Emission Technique

Thermal barrier coatings （TBCs） have been the key thermal protection material of the enginedue to their excellent thermal insulation, wear resistance, and corrosion resistance. However, dueto the complex structure and the poor operation circumstance, the coating fracture and spallingoften appears leading to coating failure and even catastrophic accidents. Therefore, the study ofthe TBC failure mode and critical mechanical parameters-surface fracture toughness and interfacefracture toughness can provide the basis of the life prediction and reliability assessment, and it hasbecome a concernful issue of research. In this paper, we use non-destructive testing methods（NDT） for monitoring the coating damage of TBC during three-point bend test and indentation test,to gain the critical mechanical parameter-fracture toughness. The main contents of this paper areas follows:Firstly, the expression of TBC’s surface fracture toughness under bending load was deduced.Then the TBCs’ injury process during bend test has been real-time monitored by acoustic emissiontechnique （AE） and digital speckle correlation method （DSCM）. The air plasma sprayed （APS）TBCs failure mode has been discussed under the bending load, and the critical thresholds weredetermined through the inflection points of the AE parameter and the strain. Inserting the criticalthresholds into the theoretical model, the surface fracture toughness of as-received and oxidizedTBCs were estimated as1.005MPa·m^1/2and3.531MPa·m^1/2, respectively. The quantitativeanalysis of surface cracks was performed by acoustic emission parameter. The impact of oxidationon TBC surface failure was analyzed.Secondly, the theoretical models of indentation total cracks length method and acousticemission parameter method to solve the surface fracture toughness of TBCs were introduced. Theindentation tests of as-received and oxidized TBCs were monitored by AE technique to real-time.The different AE signals generated by different injury were distinguished, and the linearrelationship between the total crack length and total AE energy was obtained. The surface fracturetoughness of as-received TBCs was estimated by indentation total crack length method as1.0MPa·m^1/2. The surface fracture toughness of as-received and oxidized TBCs were estimated by AEparameter method as1.645MPa·m^1/2and3.846MPa·m^1/2, respectively. Comparing the TBCs’results with different test method, the reliability of the indentation-AE method to obtained TBCs’surface fracture toughness was verified.Finally, according to the Suo-Huthinson model, the expression of TBCs’ interface fracturetoughness was derived. With the critical thresholds of Chapter2, the interface fracture toughness of as-received and oxidized TBCs were calculated as2.13MPa·m^1/2and2.39MPa·m^1/2,respectively. The AE signals generated by as-received and oxidized TBCs were filtered using thewavelet analysis method. The quantitative analysis of interface cracks was performed by acousticemission parameter. The impact of oxidation on TBC interface failure was analyzed.In general, with the systematic application of AE technique, the critical mechanicalparameters–fracture toughness of TBCs under bending and indentation loads were obtained. Theresults will provide important experimental methods and reference for TBC reliability and lifeprediction, and also expend the application areas of AE technique.