Detection And Failure Of Micro-cracks In 8YSZ/NiCoCrAlY Thermal Barrier Coatings Under Thermal Cycling Conditions

MCrAlY coating(M is Ni,Co and other elements)and 8wt.%yttria partially stabilized zirconia coating were prepared on the surface of nickel-based single crystal superalloy by atmospheric plasma spraying(APS).Scanning electron microscopy(SEM)and X-ray diffraction(XRD)were used to study the TGO morphology and the microstructure evolution of the coating interface during thermal cycling.According to Empyrean X-ray diffraction(XRD)and Raman spectroscopy(RFS)tests the internal residual stress of the thermal barrier coating,discusses the failure mechanism of the thermal barrier coating under thermal cycling,and establishes the failure model.In addition,the immersion ultrasonic nondestructive testing technology was used to detect the thermal barrier coatings of different thermal cycles,and the distribution of defects such as internal pores and microcracks and the variation of defect content were analyzed.The results show:Due to the interdiffusion of elements between the coating and the substrate and preferential selective internal oxidation,the MCrAlY coating forms a thermal growth oxide layer(TGO)between the ceramic coating and the bonding layer after high temperature and constant oxidation at 1050?.TGO consists of a dense oxide layer containing ?-Al₂O₃ as a main component and a non-dense mixed oxide layer containing Cr₂O₃ and CoO as main components.At the initial stage of oxidation,m-ZrO₂?t-ZrO₂ phase transition occurs in the ceramic layer.In the middle stage of oxidation,small cavities in the ceramic layer are connected by stress to form large-sized voids.In the later stage of oxidation,due to the reaction of Al₂O₃ and Cr₂O₃ with NiO to form a spinel phase intermetallic compound such as Ni(Al,Cr)₂O₄,the increasing hard and brittle phase of spinel becomes the main component of the gray TGO layer.Since the particles of the bonding layer material are cooled and contracted before the spraying,So that the surface of the ceramic layer of the original test piece is subject to the compressive stress of-120MPa,and the bonding layer connected with the ceramic layer is affected by the tensile stress of 130MPa.During the thermal cycle,the ceramic layer undergoes a phase change,which causes the tensile stress to decrease.After 10 thermal cycles,the tensile stress changes to the compressive stress.The compressive stress reached a maximum at 100 cycles of thermal cycling.After 150 cycles of thermal cycling,it is converted from compressive stress to tensile stress.After that,the tensile stress is gradually increased due to the thermal mismatch between the ceramic layer and the TGO layer.The stress concentration is present at the hard and brittle phase of the gray TGO spinel,and microcracks are generated when the tensile stress reaches a certain value.Under the tensile stress,the microcracks extend longitudinally or perpendicularly along the interface between the ceramic layer and the TGO layer,which eventually leads to the overall failure of the thermal barrier coating.The failure of the coating during the thermal cycle is characterized by the gradual growth of the thickness of the TGO layer,the combination of the increase of the spinel phase content and the gradual increase of the tensile stress in the gray layer,which in turn leads to the formation of interfacial cracks,growth and coating stripping.The immersion ultrasonic nondestructive testing technology was first applied to the failure detection of thermal barrier coatings.Without destroying the thermal barrier coating test piece,it can accurately reflect that the defects in the bonding layer are mainly distributed at the edge position and the central region of the thermal barrier coating test piece,which is consistent with the scanning electron microscope detection results.