Oxidation behavior of T92 steel with NiCoCrAlY coating by EB-PVD

Oxidation behavior of T92steel with NiCoCrAlY coating by EB-PVDDongbo Zhang ⁎,Juanbo Liu,Zhiyong Xue,Xueping MaoKey Laboratory of Condition Monitoring and Control for Power Plant Equipment Ministry of Education,North China Electric Power University,Beijing 102206,Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 28November 2013Accepted in revised form 26April 2014Available online 14May 2014Keywords:T92steelNiCoCrAlY coating EB-PVD OxidationOxidation behavior of NiCoCrAlY coating on T92steel was investigated in this study.NiCoCrAlY coating was fabricated on T92steel by electron beam physical vapor deposition (EB-PVD).Isothermal oxidation test was per-formed at 650°C for about 300h.X-ray diffraction (XRD)was employed to characterize the phases in the coating.Scanning electron microscopy (SEM)with energy dispersive spectrum (EDS)was adopted to analyze microstruc-tures and element distribution.Micro-hardness test and high temperature erosion wear test were also done for detecting the properties of T92steel and substrate with NiCoCrAlY coating.The results showed that the oxidation weight gain of T92steel and substrate with NiCoCrAlY coating were 1.119mg/cm 2and 0.325mg/cm 2,respective-ly.Fe 2O 3and Fe 3O 4were found on T92substrate surface,while Al 2O 3and Cr 2O 3formed on NiCoCrAlY coating sur-face after oxidation at 650°C.Elements between coating and substrate diffused during oxidation.The wear rates of T92substrate and coating of high temperature erosion wear test were respectively 27.82mg/cm 2and 1.51mg/cm 2.Micro-hardness of T92steel with coating was about 270HV.Depositing NiCoCrAlY coating on T92steel can effectively improve the oxidation resistance.©2014Elsevier B.V.All rights reserved.1.IntroductionIn order to improve the thermal ef ficiency of a power plant,the in-crease of the operating temperature and pressure of steam is usually the more effective method than others.In Ultra-supercritical unit (USC),the steam temperature and steam pressure are more than 593°C and 31MPa,respectively [1].The superheater and reheater are exposed to the highest steam temperature and pressure,therefore confronting a severe problem of high temperature oxidation and corro-sion [2,3].To content so strict work condition,the steels applied for superheater and reheater materials must have excellent mechanical properties such as superior toughness,good creep resistance,high oxidation resistance and corrosion resistance.Nowadays,T92steel has been extensively employed to manufacture superheater and reheater in the USC and is widely studied due to its outstanding mechanical properties.However,T92steel still lacks suf ficient oxidation resistance faced with the aggressive working condition of USC.A prior method to enhance the oxidation resistance of T92is to pre-pare a MCrAlY(M =Ni and/or Co)coating on the surface of T92steel.It not only improves the oxidation resistance and corrosion resistance of T92[4,5],but also maintains the excellent mechanical properties of T92substrate [6,7].In the MCrAlY(M =Ni and/or Co)coating,Ni and Co are the matrix elements of bond layer.Ni has the ability to mitigate the thermal stress while Co possesses superior oxidation resistance and hot corrosion resistance.According to research results,when concentration of Co in the coating is between 20wt.%and 26wt.%,thecoating holds the good toughness.Cr not only can guarantee the corro-sion resistance,but also can promote the formation of Al 2O 3.Al can form the Al 2O 3to improve the oxidation resistance,but a high concentration of Al will impair the coating strength,so that the concentration of Al should be reduced as low as possible on the premise of contenting the antioxidant requirement [8–11].The addition of Y in the coating pro-motes the chemical combination between oxidation layer and substrate,improving the cyclic oxidation resistance of the coating system [12].The antioxidant mechanism of NiCoCrAlY coating is that the protective oxidation products such as Al 2O 3and Cr 2O 3films are formed on the coating surface at high temperature and then the dense oxide layers iso-late the metal parts from the oxidation environment [13].Therefore,MCrAlY coating was used to protect the substrate at a high temperature condition.Preparing MCrAlY coating on the T92substrate can improve the oxidation resistance and mechanical properties of T92steel.A lot of methods,such as air plasma spray (APS)[14],vacuum plas-ma spray (VPS)[15],high velocity oxygen fuel spray (HVOF)[16],and magnetron sputtering [17],have been used to prepare MCrAlY coating.While coating prepared by electron beam physical vapor deposition (EB-PVD)has a good mechanical property,which can form a columnar microstructure [18].The columnar microstructure can not only promote the high temperature inter-diffusion between the MCrAlY coating and the substrate,but also offer a superior strain and thermal shock tolerant behavior to the coating [18,19].In addition,coating prepared by EB-PVD is metallurgical bonding,and interface between the coating and sub-strate possesses a good adhesion strength,which could protect coating from spallation.The objective of this paper was to investigate the oxida-tion behavior of NiCoCrAlY coating on T92substrate prepared by EB-PVD.Micro-hardness change of coating and element diffusion duringSurface &Coatings Technology 252(2014)179–185⁎Corresponding author.E-mail address:zhangdb@ (D.Zhang)./10.1016/j.surfcoat.2014.04.0640257-8972/©2014Elsevier B.V.All rightsreserved.Contents lists available at ScienceDirectSurface &Coatings Technologyj ou r n a l h o m e p a ge :w ww.e l s e v i e r.c o m /l oc a t e /s u r f c oa toxidation between substrate and coating were studied.In addition,high temperature erosion wear test of coating and substrate were also per-formed in this study.2.Experimental 2.1.Preparation of coatingIn this study commercial ASME SA213T92steel was used as substrate material and its nominal chemical composition was given in Table 1.The T92steel was cut into the cylindrical specimens with the dimension of Ф6mm ×8mm.NiCoCrAlY coating was deposited on T92substrate by EB-PVD (UE205,Ukraine Barton Welding Institute).During deposition,the substrate temperature was 850°C and back-ground vacuum was about 5×10−3Pa.The deposition rate was about 0.3μm/min.The coating thickness was about 20μm after deposition and the NiCoCrAlY coating in this study had a nominal composition (wt.%):16–20Cr,18–22Co,8–10Al,0.5–1.5Y,and Ni as balance.2.2.Microstructures and oxidation characterizationTo study the oxidation behavior,samples with and without coating underwent a isothermal oxidation test in a tubular furnace (Sk2-6-12)at 650°C for about 300h.Testing the weight gain was commonly an effective method to evaluate oxidation resistance and the weight change was measured by an electronic balance (MS204S/01,Mettler Toledo)with a sensitivity of 1×10−3g.After oxidation,the specimens with coating and without coating were characterized by an X-ray diffraction (XRD,D/max2550HB+/PC)to con firm the surface phase.Scanning electron microscopy (SEM,Hitachi-S4800)with energy dispersive spectrum (EDS,Horiba-7593-H)was used to detect the mor-phologies and element distribution at the cross-section and the surface.FM-300hardness tester with 25g loading and 5s loading time was employed to test the micro-hardness of the coating and substrate.In addition,high temperature erosion wear test (GW/CS-MS)was accom-plished to observe the wear resistance of T92substrate and coating.The erosion wear test was performed at 600°C with 36mesh quartz sand.The erosion time was 1min.,erosion angle was 45°,erosion distance was 60cm and abrasive flow rate was 125g/min.3.Results and discussion 3.1.XRD resultsFigs.1and 2were a set of XRD spectrum of specimens at different states.It can be seen from Fig.1that the surface phase of T92substrate was NiCrFe.NiCrFe phase possessed excellent toughness and stable me-chanical strength [20].Fe 2O 3and Fe 3O 4formed on the surface of T92substrate after oxidation at 650°C.In the spectrum,the Fe 3O 4diffrac-tion peak has shifted 1–2°from standard position to high-angle direc-tion.As atomic radius of Cr was smaller than that of Fe,the solution of some Cr atoms in the Fe 3O 4made lattice constant diminish.This phe-nomenon characterized by XRD was that the diffraction peak shifted to high-angle position.Therefore,it can be speculated that minority (Fe,Cr)3O 4probably formed besides main composition phases of Fe 2O 3and Fe 3O 4.Fig.2was the XRD result of coating.γ-Ni 3Al and NiAl 2O 4phases were mainly detected in the surface of as-deposited coating.The appearance of O element was perhaps attributed to the processing of NiCoCrAlY coating with pact Al 2O 3and Cr 2O 3oxide films formed on the coating surface after oxidation for about 300h at 650°C.The dense oxide layers would isolate the metal parts from the oxidation environment,which would prevent further oxidation.Table 1Nominal chemical composition of T92steel (wt.%).C Mn P S Si Cr Mo V Nb N Al Ni 0.110.340.0150.0030.398.790.910.240.0710.050.0410.26Fig.1.The X-ray diffraction spectrum of T92substrate at differentstates.Fig.2.The X-ray diffraction spectrum of NiCoCrAlY coating at differentstates.Fig.3.The isothermal oxidation kinetics curve of T92substrate and NiCoCrAlY coating at 650°C for about 300h.180D.Zhang et al./Surface &Coatings Technology 252(2014)179–185Fig.4.The SEM images of blank T92substrate and as-deposited NiCoCrAlY coating.(a)Blank T92substrate surface.(b)As-deposited NiCoCrAlY coating surface.(c)The cross-section of as-deposited NiCoCrAlY coating.Table2As-deposited NiCoCrAlY coating element distribution(wt.%).Ni Co Cr Al Y Fe148.2622.1313.7411.37 1.12 3.38 252.6618.199.3714.360.96 4.46342.0024.7817.847.39 1.076.92Fig.5.The SEM image of T92substrate surface after oxidation at650°C.(a)After oxidation for30h.(b)After oxidation for100h.(c)After oxidation for about300h.181D.Zhang et al./Surface&Coatings Technology252(2014)179–1853.2.Isothermal oxidation behaviorIn order to exhibit the oxidation resistance characteristic,the iso-thermal oxidation test was performed at 650°C for about 300h.For comparison,the substrate without coating was also oxidized at the same condition.Fig.3was the weight gain curves of T92with coating and without coating.It can be seen that the oxidation kinetics curve of T92substrate was basically in accord with parabolic rate law.The weight gain increased extensively at the first 24h and then kept a par-abolic increasing tendency,which was in agreement with the published literature about oxidation law of T92steel [21].At the beginning of oxidation,massive Fe was quickly oxidized to form oxides,consequent-ly,the initial oxidation possessed a large oxidation rate.XRD results showed that Fe 2O 3and Fe 3O 4oxide films formed on the T92surface after oxidation.With the oxide film formation,the oxidation rate gradually slowed down.As the Fe 2O 3and Fe 3O 4oxide films were not compact and continuous enough,the weight gain was still increasing.While the oxidation kinetics increasing tendency of NiCoCrAlY coating was basically similar to the T92substrate,which kept a fast increasingtendency at the beginning stage of the oxidation and then possessed a slight rise which finally tended to remain stable.At the beginning stage of oxidation,a high oxidation rate attributed to the discontinuousness of Al 2O 3and Cr 2O 3.With the increase of oxidation time,Al 2O 3and Cr 2O 3oxide films gradually became compact and continuous,which iso-lated the substrate from oxidation environment at high temperature.Consequently,the oxidation kinetics of NiCoCrAlY coating displayed a slight rise and then remained stable during oxidation.However,the total weight gain data of T92substrate was 1.119mg/cm 2for about 300h oxidation,while that of NiCoCrAlY coating was merely 0.325mg/cm 2.The weight gain data of T92substrate was extensively larger than that of T92substrate with NiCoCrAlY coating,which indicat-ed that the oxidation resistance of coating was much better than that of substrate.Therefore,the result of oxidation kinetics meant that it was an effective method to improve oxidation resistance of T92steel by depositing NiCoCrAlY coating with EB-PVD.3.3.Microstructures and element composition distributionTo analyze the microstructures and element composition distribu-tion of specimens,SEM with EDS was employed to characterize the sur-face and cross-section.Fig.4showed the SEM images of T92substrate and NiCoCrAlY coating before oxidation.It can be seen from Fig.4(c)that the coating thickness was about 20μm.There were some small holes in the interface between substrate and coating,which may have resulted from high deposition temperature of EB-PVD process.Table 2was the corresponding EDS analysis of Fig.4(c).The EDS results showed that a bit of Fe element appeared in the coating.The concentration of Fe increased from substrate to coating surface.As the substrate tempera-ture was up to 850°C,Fe element diffused from the substrate to coating during the coating preparation.Fig.5was a set of SEM images of T92substrate surface after oxi-dation for 30h at 650°C.Table 3was the corresponding EDS element distribution.With the oxidation duration increasing,the T92substrate surface became coarse and uneven.There was a crack on the surface after oxidation for about 300h,as was shown in Fig.5(c),which wasTable 3The element distribution on the surface of T92substrate after oxidation for (a)30h,(b)100h and (c)300h (wt.%).OCr Mn Fe (a)30h (wt.%)126.44 2.390.4770.70233.59 2.380.2963.74329.013.510.3467.14(b)100h (wt.%)129.72 3.390.2266.67224.91 4.670.3170.05331.82 4.710.4463.03(c)300h (wt.%)131.72 5.29–62.99228.58 6.67–64.75330.945.510.2463.31Fig.6.The SEM image of NiCoCrAlY coating surface at 650°C.(a)After oxidation for 30h.(b)After oxidation for 100h.(c)After oxidation for about 300h.182 D.Zhang et al./Surface &Coatings Technology 252(2014)179–185commonly considered to be the difference of thermal expansion coef fi-cient (TEC)between oxide scales and substrates.The difference led to a thermal stress.O,Cr and Fe elements were mainly detected in the surface,which was in accord with the XRD results that the surface phases were Fe 2O 3and Fe 3O 4and some solid solute Cr.Existing element Mn on substrate surface was probably the results of diffusion.Fig.6was a set of SEM images of NiCoCrAlY coating surface after ox-idation for 100h at 650°C.Table 4was the corresponding EDS element distribution.Obviously,with the oxidation duration increasing,the oxide scale morphologies became more compact.The oxide scales should be the Al 2O 3and Cr 2O 3according to XRD and EDS results.The oxide scales were not continuous after oxidation 30h,which led to the oxidation rate remaining relatively large.The result was in accord with the oxidation kinetics as was shown in Fig.3.The oxide films grad-ually became continuous and compact during oxidation duration from 30h to 300h at 650°C,consequently,the oxidation kinetics displayed a slight rise and finally remained stable.Fig.7was a set of cross-section SEM images of NiCoCrAlY coating after oxidation for about 300h at 650°C.Table 5was the correspondingEDS element distribution.It can be seen that after about 300h oxidation,the bond layer was still compact,which indicated that the coating and substrate possess strength pared with as-deposited coat-ing,the concentration of Al increased and Cr decreased from substrate to coating,where it can be speculated that the Al diffused from coating to substrate while the Cr diffused in the opposite direction during oxida-tion.In addition,the concentration of Fe decreased from substrate to coating,consequently,Fe element diffused from substrate to coating.3.4.Micro-hardness test and high temperature erosion wear testMicro-hardness was accomplished by FM-300hardness tester with 25g loading and 5s loading time.The result showed that the average hardness of coating was about 270HV,while the average hardness of T92substrate was about 235HV.Coating hardness was higher than that of T92substrate,which can improve the performance of a substrate at a high temperature.During the oxidation at 650°C for about 300h,coating hardness was generallystable.Fig.7.The cross-section SEM image of coating at 650°C.(a)After oxidation for 30h.(b)After oxidation for 100h.(c)After oxidation for about 300h.Table 4(a)The element distribution on the surface of NiCoCrAlY coating after oxidation for (a)30h,(b)100h and (c)300h (wt.%).ONi Co Cr Al Y Fe (a)30h (wt.%)112.6341.7917.909.8213.670.90 3.27212.6142.5318.6411.079.58 1.23 4.34314.1246.8115.038.2811.93–3.83(a)100h (wt.%)114.1234.2917.0312.8113.92–7.83215.5542.4813.299.9011.63 1.91 5.24317.2540.939.0311.5912.34 1.477.39(a)300h (wt.%)118.8328.9715.799.5415.52 1.1910.16217.3928.5317.4710.5317.03–9.05319.3132.1814.788.6214.39–10.72Table 5The element distribution on the cross-section of NiCoCrAlY coating after oxidation for (a)30h,(b)100h and (c)300h (wt.%).O Ni Co Cr Al Y Fe (a)30h (wt.%)113.5036.0322.9510.5612.22 1.01 3.73212.6633.7723.5512.7410.760.78 5.74313.5532.2121.0515.699.080.837.59(b)100h (wt.%)115.1028.9421.6911.4713.030.679.10217.9725.5016.5213.7212.01 1.0413.24314.6124.2520.2614.659.820.8215.59(a)300h (wt.%)118.1338.2211.618.2814.230.958.58215.6936.6918.749.299.92–9.67315.1428.1920.6913.238.550.3313.87183D.Zhang et al./Surface &Coatings Technology 252(2014)179–185Fig.8was the SEM surface morphologies of wear scar of T92sub-strate and NiCoCrAlY coating.It can be seen that there were lots of spall-ations on the surface of T92substrate,while the coating surface was still uniform and compact,which just displayed a slight wear scar.According to the magni fied SEM images of Fig.8(c)and (d),T92substrate surface presented grooves and the depth and length of grooves were greater than those of the coating surface.Table 6was the EDS analysis corre-sponding to Fig.8(b).Compared with the element composition on the surface of as-deposited coating,there were few changes after the ero-sion wear test,which demonstrated that NiCoCrAlY coating did not peel off.As the coating possessed good mechanical properties,thereby the spallations on the surface of coating signi ficantly decrease.Moreover,the EDS results showed that the coating surface contained O element,in-dicating that a slight oxidation occurred during the erosion wear process.Furthermore,the wear rates of the T92substrate and coating were re-spectively 27.82mg/cm 2and 1.51mg/cm 2.The wear rate of the coating was only 5.43%than that of the T92substrate,which was in good agree-ment with the SEM morphologies that the coating surface has few spall-ations.Based on the above analysis,we can draw a conclusion reasonably that fabricating NiCoCrAlY coating on T92steel can improve its wear resistance.4.ConclusionT92steel has been extensively employed to manufacture superheater and reheater in the USC.However,T92steel lacked suf ficient oxidationresistance faced with the aggressive working condition of USC.In this paper,NiCoCrAlY coating was deposited on the surface of T92substrate by EB-PVD to improve its oxidation resistance.The NiCoCrAlY coating thickness was about 20μm after deposition and the interface between the coating and substrate had a good adhesion strength.The isothermal oxidation behavior was performed at 650°C for about 300h.In the isothermal oxidation test,the weight gain data of T92steel and the sub-strate with NiCoCrAlY coating were respectively 1.119mg/cm 2and 0.325mg/cm 2.After about 300h of oxidation,Fe 2O 3and Fe 3O 4formed on the surface of T92substrate,while the Al 2O 3and Cr 2O 3formed on the surface of NiCoCrAlY coating.The formation of compact Al 2O 3and Cr 2O 3can effectively prevent T92substrate from further oxidation.High temperature erosion test was carried out at 600°C.The SEM morphol-ogies and EDS analysis of wear erosion test demonstrated that the T92substrate surface presented lots of grooves and the depth and length of grooves were greater than those of the coating surface,while the coating was still compact and did not peel off.The wear rates of the T92substrate and coating were respectively 27.82mg/cm 2and 1.51mg/cm 2,and the wear rate of the coating was only 5.43%than that of T92substrate.All re-sults indicated that the oxidation resistance and erosion wear resistance of T92steel can be effectively improved by fabricating NiCoCrAlY coating with EB-PVD.Con flict of interestThere is no con flict of interest.AcknowledgmentThis work is supported by the Fundamental Research Funds for the Central Universities.References[1] D.Q.Zhang,J.J.Xu,G.Q.Zhao,Chin.J.Mater.Res.22(6)(2008)599.Fig.8.The SEM images of T92substrate and NiCoCrAlY coating surface after erosion wear test at 600°C for 1min.(a)T92substrate surface.(b)NiCoCrAlY coating surface.(c)The mag-ni fication of T92substrate surface.(d)The magni fication of NiCoCrAlY 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