Microstructure Evolution Of YSZ Thermal Barrier Coatings Prepared By APS At High Temperature

Thermal barrier coatings(TBCs)are widely applied in aircraft engines and gas turbines to reduce the temperature of the metallic substrates thereby increasing the lifetime of the hot section components and the engine efficiency.On prolonged exposure at elevated temperatures,thermal barrier coatings will go to failure inevitably.The properties of the top ceramic coat will change obviously due to high temperature annealing,especially some key properties such as the thermal insulation properties and strain resistance.At the YSZ/NiCrAlY interface,the internal growth stress of the non-protective oxides greatly increases due to the high temperature oxidation,resulting in the formation of cracks and the spalling of the ceramic layer.The properties of materials depend on the composition and the microstructure.Therefore,it is necessary to study the microstructure evolution of thermal barrier coatings at high temperature in order to further understand the failure mechanisms of thermal barrier coatings.In this thesis,XRD,SEM,TEM are used to characterize the microstructure of the as-sprayed samples and the samples annealed at high temperatures,focusing on the microstructure evolution of the top ceramic coat and the YSZ/NiCrAlY interface oxidation at high temperature.The main results are as follows:Firstly,the microstructures of the top coat before and after annealing are characterized by XRD,SEM,and TEM.The results show that the phases of the top ceramic coat do not change after high temperature annealing.With the annealing time increasing,the growth of columnar grains in the ceramic splats causes the sintering necks formation between the grains and the intra-lamella pores tend to heal,while the inter-lamella pores develop into a row of isolated holes.After long exposure,the lamellar structure and the columnar grains disappear.In addition,the reason why "white point" vacancy cluster phenomenon is not observed after high temperature annealing of APS YSZ thermal barrier coating is analyzed.Secondly,the microstructures of the YSZ/NiCrAlY interface of the samples before and after the annealing are characterized by SEM and TEM.It is found that as the oxidation progresses,the thermally grown oxide(TGO)formed at the YSZ/NiCrAlY interface is shown to evolve from a bi-layered structure of the inner and outer layers of Al2O3 and Y2O3,to a three-layered structure of the bottom,middle,and upper layers of Al2O3,YAG(Y3AI5012)and spinel(NiCr2O4),then to the four-layered structure of the bottom,second,third,and upper layers of Al2O3,YAG,NiCr2O4 and NiO,respectively.Furthermore,the structural layers of TGO at the YSZ/NiCrAlY interface after high-temperature oxidation are discussed.Further,the mechanism for the YSZ/NiCrAlY interfacial oxidation is proposed,and the oxidation mechanism model is established.In addition,the effect of Y on the oxidation of YSZ/NiCrAlY interface in the APS thermal barrier coatings is analyzed.