The Formation And Growth Kinetics Of Thermally Grown Oxides In Plasma Sprayed 8YSZ Thermal Barrier Coatings

Thermal barrier coatings(TBCs)applied in aircraft engines and gas turbine usually comprise a metallic bond coat(BC)and a ceramic top coat(TC),the formation of thermally grown oxides(TGO)is ascribed to the oxidation of bond coats during exposure process,which is a crucial factor to the degradation of thermal barrier coatings.In this work,8YSZ/CoNiCrAlY TBCs and pure BCs are deposited onto Inconel 718 substrate by air plasma spraying(APS),the morphology and microstructure of TGO in TBCs were systematically investigated,utilizing X-ray diffraction(XRD),scanning electron microscopy(SEM)as well as energy dispersive spectroscopy(EDS)means,the influence of temperature on microstructure of TGO is also investigated,then the mechanism of TGO formation is obtained.Besides,the growth kinetics of CoNiCrAlY in TBCs after isothermal oxidation at 1100 ? for various times were analyzed,and the influence of ceramic coat on oxidation rate of bond coat is also investigated.The model of TGO growth kinetics is then got on the basis of experimental results.The results of XRD analysis show that the as-sprayed bond coat is composed of y??'?? and trace ?-Al2O3 phase.The TGO layer is nearly composed of ?-Al2O3 after isothermal oxidation at 1100 0C of 2 h,with the oxided time increasing,Cr2O3,spinel oxides as well as a small amount of CoO and NiO except ?-Al2O3 are detected in TGO.In the later oxidation stage,CoO nearly disappears,contents of ?-Al2O3 reduce while that of NiO,Cr2O3 and spinel oxides increase.Ceramic coat mainly consists of stable t-8YSZ and C-8YSZ phase during heat treatment at 1100?.The results of morphology and microstructure of TGO show that the thickness of TGO in valley is more thicker then that in crest after isothermal oxidation at 1100 ? for the same time,the volatility of TGO is more obvious and the transformation of TGO from uniform a-Al2O3 to other mixed oxides arose as the increase of oxidation time.Cracks appear but failure of TBCs don't happen after isothermal oxidation at 1100 0C for 350 h.TGO eventually consists of a sub-Al2O3 layer with columnar micro structure and the upper porous Cr2O3,spinel oxides and NiO(abbreviated as CSN)clusters.Moreover,temperature plays a great role on the formation of TGO.The morphology of TGO is flat and TGO nearly consists of A12O3 after oxidation at 900 ? of 50 h,while TGO shows up obvious volatility and NiO?Cr2O3 and spinels appesr after oxidation at 1000 ? of 50 h,the contents of mixed oxides,especially NiO,obviously increase after oxidation at 1100? of 50h.Growth kinetics of TGO is analyzed by method of statistical calculation.The results show that the kinetics of TGO growth in APS TBCs and pure BCs follow the parabolic law of ?eq=Kp.t1/2+a,and their corresponding oxidation rate constant are 0.344 ?m/h0.5 and 0.354 ?m/h0.5,respectivly,which illustrates that ceramic coats have little effect on the rate constant of bond coats in TBCs.