Oxidation Behavior And Failure Analysis Of Plasma Sprayed Thermal Barrier Coatings

Thermal barrier coating (TBC) system is usually used to thermally protect turbine blades andvanes from the hot gases in gas turbine engines. In this thesis, a bond coat (CoNiCrAlY) has beendeposited on a Ni-base superalloy by high velocity oxygen fuel (HVOF), and then7wt.%Y2O3-ZrO2(YSZ) has been deposited by atmospheric plasma spraying (APS) as a top coat.The bonding strength and thermal shock of the TBC system have been studied to determine theappropriate heat treatment process of coatings.9M,7700-SG100and7700-F4plasma equipmentsare used to get three different microscopic morphologys ofceramic layers. Isothermaloxidation andcyclic oxidation have been studied at950℃. They show that thermally grown oxide (TGO) arelayered distribution. The failure began in TGO/ceramic coating interface and ceramic surface.Analyze the causes of thermal barrier coatings failure and the influence of ceramic layer on thefailure. The present investigation may provide insight for better understanding the plasma coatingoxidation, the failure mechanismofAPS TBC and practicalproduction processofthe blades.A top coat has been deposited on a bond coat after1120℃,2h and850℃,24h vacuum heattreatment. The TBC has best tensile and thermal shock resistance.Early oxidation a large number of-Al2O3oxide are generated in the adhesive layer interface,as well as small amountsofCr2O3、Co3O4. After100h isothermaloxidation, the content ofthe oxideare in a sharp increase,-Al2O3continuously transform to-Al2O3and the entire scale is highlynon-uniform. After400h isothermal oxidation, TGO layer consists of two layers: an inner-Al2O3layer and a thick oxide layer of complex oxides (Cr2O3, Co3O4, NiO,(Ni,Co)(Al,Cr)2O4, etc) withlots of big gaps. After1100h isothermal oxidation, there are many cracks in TGO layer, partialoxidation membrane peeling. During the oxidation,-NiAl transforms to-Ni3Al and phasebecause of the formation of Al2O3. Below the surface of the adhesive layer, a poor aluminum stripis expending. During oxidation, the groupofC coatingsdeposited by7700-F4are first failure: thereare axial cracks on the ceramic layer surface and ceramic layer flaking from interface ofTGO/ceramic layer. Failure process is: longitudinal cracks on the ceramic layer surface expend tointernalcausing surface cracking and expanding; longitudinalcrack nucleation and grow in bulge ofTGO and then extend to the surface of ceramic layer. Transverse cracks at TGO/ceramic interface and ceramic layer internal from the surface of1/3are extension forming larger cracks, then largertransverse cracks and longitudinal cracks are intersection. Flaking and cracks cause coating failure.Cracks initiate and propagate due to the stress resulted from the growth of TGO, the difference ofthermalexpansioncoefficient and thetransformation ofphase.