Effect Of Pre-Oxidation Treatment On The Service Life Of Thermal Barrier Coating

Thermal barrier coating (TBC), which consists of a NiCoCrAlY bond coat and a ZrO2-8Y₂O₃ (wt. %) topcoat (TC), is fabricated on the nickel-base superalloy by air plasma spray (APS). The as-sprayed TBC is treated by pre-oxidation treatment technique. The morphology, phase composition, element distribution and residual stress are characterized by scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS), X-ray diffractometer (XRD), ruby fluorescence spectroscopy (RFS). The influence of pre-oxidation treatment technique on the service life of thermal barrier coating is studied.The results show that: pre-oxidation treatment can homogenize highly non-uniform microstructures resulting from thermal spraying of CoNiCrAlY powders in air and improve the density of coating. Pre-oxidation treatment makes O element only react with Al element by controlling the oxygen partial pressure in the vacuum furnace. After thermal exposure for 1 h at 1050℃, a continuous, uniform, and single-ingredient (Al₂O₃) TGO can quickly form at the ceramic topcoat/bond coat interface. The formation of Al₂O₃ layer can effectively suppress the formation of other non-protective oxides, and reduce the oxidation rate. TGO quickly accesses to the steady-state growth stage. After thermal exposure for 120 h at 1050℃, lamination does not appear within the TGO. TGO is still in the slow growth phase. Therefore, pre-oxidation treatment the high temperature oxidation resistance of TBC is improved.Pre-oxidation treatment can eliminate the residual stress produced in the as-sprayed deposition process. Homogeneous microstructures of bond coat reduce the probability of complex stress caused by convex and depression. Meanwhile, Pre-oxidation treatment intervenes with the growth process of TGO, reduces the growth rate of TGO, and reduces the stress concentration in TGO layer. RFS results show that: residual stress within the two kinds of coating both increases with the increase of thermal cycling times. However, residual stress within TBC treated by Pre-oxidation treatment rise more slowly than as-sprayed TBC. The increase of stress is relatively stable. Therefore, pre-oxidation treatment improves the stress state in TGO layer during thermal cycling process. After 400 cycles, the coating still remains well. At that time, the stress value is 492.5 MPa. Thermal cycle life of TBC is improved.In conclusion, Pre-oxidation treatment can homogenize microstructures of bond coat, eliminate the residual stress produced in the as-sprayed deposition process, and promote the formation of a continuous, uniform, single ingredient of Al₂O₃ layer. It would effectively suppress the formation of the detrimental oxides, and improve the stress state of coating, thus helping to improve the service life of TBCs.