Microstructure and oxidation behavior of NiCoCrAl-YSZ microlaminates produced by EB-PVD

Microstructure and oxidation behavior of NiCoCrAl/YSZ microlaminates produced by EB –PVDGuodong Shi a ,⁎,Zhi Wang a ,Mincong Liu a ,Jun Liang b ,Zhanjun Wu aa State Key Laboratory of Structural Analysis for Industrial Equipment,Dalian University of Technology,Dalian 116024,China bCenter for Composite Materials,Harbin Institute of Technology,Harbin,150001,ChinaA R T I C L E D A T AA B S T R A C TArticle history:Received 24May 2011Received in revised form 21July 2011Accepted 30July 2011Two NiCoCrAl/ZrO 2–Y 2O 3microlaminates (A and B)were fabricated by electron beam physical vapor deposition,which were different in layer number and metal-layer thickness.The layer number was 20and 26,respectively.And the metal-layer thickness was 35and 14μm,respectively.The microstructure and isothermal oxidation behavior were investigated.During the exposure in air at 1000°C for 100h,the t to m phase transformation occurred in the ceramic layers,and oxide scales formed at the surfaces of not only the outer metal-layers but also the internal metal-layers for the microlaminates.The oxidation rate of microlaminate B was greater than that of microlaminate A.Their overall mass gains were significantly dependent on the number and thickness of the metal-layers.The oxidation products were also influenced by metal-layer thickness.Oxide scales of the 35μm thick metal-layer microlaminate (A)consisted mainly of α-Al 2O 3and θ-Al 2O 3,while the oxidation products of the 14μm thick metal-layer microlaminate (B)were the mixture of α-Al 2O 3,θ-Al 2O 3and Cr 2O 3.It was also found that the growth of the oxide scale adjacent to the top YSZ layer was controlled by the oxygen diffusion,and that the growth of the oxide scale adjacent to the internal YSZ layer was controlled by the metal ionic diffusion.©2011Elsevier Inc.All rights reserved.Keywords:Microstructure OxidationMetal/ceramic laminates Physical vapor deposition1.IntroductionLaminated structures are increasingly being considered for various applications because of the improving capability to tailor the fabrication of these structures to meet specific property needs [1].Especially for the microlaminate with micron layer thickness,the small size scales lead to increases in the material strength due to various strengthening mechanisms [2].Metal/ceramic microlaminates possess superior high tem-perature creep resistance compared to metals;simultaneously they have better toughness and structural integrity compared to monolithic ceramics [3].Therefore,metal/ceramic microlami-nates have been investigated as potential structural materials for high temperature applications in the aerospace industry [1].In the applications these materials would be exposed not only to severe mechanical stresses,but also to aggressive gasses resulting in oxidation and corrosion.Many researchers have reported the mechanical properties of the ductile/brittle micro-laminates,such as deformation and fracture process [4–8].However,a detailed investigation into the high-temperature oxidation behavior of these materials has not been publicly reported.In the present work,the isothermal oxidation of two NiCoCrAl/ZrO 2–Y 2O 3(YSZ)microlaminates was carried out at 1000°C for 100h.The effects of the number and thickness as well as position of the metal layers on the oxidation behavior were discussed.Furthermore,the oxidation mechanism of the microlaminates was investigated.2.ExperimentalTwo microlaminates (microlaminate A and B)with alternating layers of NiCoCrAl and YSZ were produced by electron beam physical vapor deposition (EB –PVD).The ingots of ZrO 2–8Y 2O 3M A T E R I A L S C H A R A C T E R I Z A T I O N 62(2011)1066–1070⁎Corresponding author.Tel./fax:+8641184706791.E-mail address:gdshi@ (G.Shi).1044-5803/$–see front matter ©2011Elsevier Inc.All rights reserved.doi:10.1016/j.matchar.2011.07.018a v a i l ab l e a t w w w.sc i e n c ed i re c t.c o mw w w.e l s e v i e r.c o m /l o c a t e /m a t c h a r(mol%)and Ni –20Co –12Cr –4Al (wt.%)were evaporated alter-nately to produce the microlaminates and the layer-thicknesses were controlled by their deposition time.Following deposition the microlaminates were removed from the substrates.Micro-laminate A had 10NiCoCrAl layers with thickness of 35μm and 10YSZ layers.Among the YSZ ceramic layers,the top YSZ layer was 6μm thick,and the other 9internal YSZ layers were 1μm thick.Microlaminate B consisted of 13NiCoCrAl layers with thickness of 14μm,12internal YSZ layers having thickness of 1μm and a top YSZ layer.But the top YSZ layer of microlaminate B was only 0.3μm thick due to an accidental trouble of the EB –PVD equipment.Rectangular coupons of 10×10mm 2were cut by linear cutting machine.The isothermal oxidation tests were performed in an electrical resistance furnace at 1000°C in dry air up to 100h.After isothermal oxidation,the specimens were left to cool down out of the furnace and,then,weighed.Character-ization of the composition and structure of the oxide scales was performed by scanning electron microscopy equipped with an energy dispersive X-ray microanalysis system.Phase identifi-cation of the oxidation products was performed by X-ray diffraction.3.ResultsFig.1shows the oxidation kinetics curves for the microlami-nates at 1000°C.The mass gain of microlaminate B was higher than that of microlaminate A.And the gap of the mass gain between the microlaminates increased with the increasing oxidation time.After 100h of oxidation,the oxidation mass gain of microlaminate B was 4times higher than that of microlaminate A.The thinner top YSZ layer having the lower diffusion resistance of oxygen was one of the factors resulting in the higher mass gain of microlaminate B.Fig.2shows the X-ray diffraction patterns for the as-deposited microlaminate samples and the samples suffering 100h isothermal exposure in air at 1000°C.For the as-deposited microlaminates,the YSZ layers were mostly composed of metastable tetragonal phase.After 100h exposure in air at 1000°C,the t to m transformation in the YSZ layers was observed.According to the formula of Toraya et al.[9],the volume fraction of the m -ZrO 2(V m )was calculated by measuringthe intensities of (111)and 111ÀÁreflections of the monoclinic phase and of the (111)reflection of the tetragonal phase:V m =1:311X m 1+0:311X mð1ÞX m =I m 111ðÞ+I m 111ÀÁI m 111ðÞ+I m ÀÁ+I t 111ðÞð2Þwhere X m is the integrated intensity ratio,I m and I t are the peak intensities of the m -ZrO 2and t -ZrO 2,respectively.The volume fractions of m phase were 0.30and 0.36for the microlaminates A and B,respectively.It was reported that the t to m transforma-tion was affected not only by the component,but also by the grain size and the residual stress for YSZ layers [10,11].Since the grain size and the residual stress depended mostly on the layer thickness of the microlaminates,the difference in the m phase fraction should be attributed to the difference in the layer thickness between the two microlaminates.For the NiCoCrAl layers,the different oxidation products were formed during 100h exposure in air at 1000°C,which were related to the layer-thickness,as shown in Fig.2.Oxide scales of the 35μm thick metal-layer microlaminate (microlaminate A)consisted mainly of α-Al 2O 3andθ-Al 2O 3,while the oxidation products of the 14μm thick metal-layer microlaminate (microlaminate B)wereFig.1–Oxidation kinetics curves of the microlaminates at 1000°C.Fig.2–XRD patterns for:a)microlaminate A and b)microlaminate B.1067M A T E R I A L S C H A R A C T E R I Z A T I O N 62(2011)1066–1070the mixture of α-Al 2O 3,θ-Al 2O 3and Cr 2O 3.This indicated that the metal-layer thickness of the microlaminates had an effect on their selective oxidation behavior.Fig.3shows cross-sectional SEM micrographs of the microlaminates suffering 100h isothermal exposure in air at 1000°C.The several microns thick oxide scales,the thermally grown oxides (TGO),were observed at the in-terfaces between the top YSZ layers and the NiCoCrAl layers.But the oxide scales were not obvious at the interfaces between the internal YSZ/NiCoCrAl layers.However,EDX results suggested that the surfaces of the internal metal-layers of the microlaminates were also oxidized,as shown in Fig.4.This indicated that the oxidation rate was a function of the layer-interface number of the microlaminates.At the same time,EDX results further confirmed the oxidation products identified by XRD.For specimen A,only Al participated in oxidation reaction during 100h oxidation.For specimen B,Cr was also oxidized besides Al after 5h oxidation,but on other elements except Al and Cr partici-pated in oxidation reaction during 100h oxidation.After 100h oxidation,Al contents in the internal unox-idized zones of the metal-layers of microlaminates A and B were determined by EDX.The Al content for microlaminate B was about 0.36wt.%,much less than the one (2.70wt.%)for microlaminate A.This indicated that Al in the metal layers of microlaminate B was more easily depleted than that of microlaminate A during oxidation due to the relatively low total amount of Al element in the relatively thin individual metal layer.4.DiscussionIn general,the oxidation of bulk materials occurs on the surfaces.However,it was found from the present work that oxidation also occurred at the interfaces between the internal NiCoCrAl/YSZ layers for the microlaminates at 1000°C.This indicated that oxygen could easily diffuse through the ceramic layers to the surfaces of the internal metal layers.Two mechanisms have been proposed for transferring oxygen through zirconia coat-ings:ionic diffusion from the crystalline structure of ZrO 2and gas penetration through short circuit paths like grain bound-aries,pores and microcracks,which the latter plays a major role [12–14].The YSZ layers produced by EB –PVD had segmented columnar structures,as shown in Fig.5.As a result,oxygen could easily diffuse into the YSZ layers and penetrate along the intercolumnar gaps up to the ceramic/metal interfaces,oxidiz-ing the metal layers.Oxygen diffused not only into the top YSZ layers from the edges and surfaces of the specimens,but also into the internal YSZ-layers from the edges of the specimens.Therefore,the total oxidized surface areas and oxidation rates of the specimens increased with the increasing layer number.This was one of the reasons why the oxidation rate of the microlaminate B was higher than that of microlaminate A.As shown in Fig.4a,the Al peaks spanned the internal ceramic layers,indicating the existence of oxidation products of metal layers in the internal YSZ layers.However,the metal oxidation products were not found in the top YSZ layers.This was believed to be a result of the different growing direction of the oxide scales between the outer metal-layers and the internal metal-layers during the high temperature oxidation.In general,the growth rate of TGO is dominated by the diffusion of the negative and positive ions.When TGO grew outward under the condition of thermodynamic equilibrium,the diffusion law for the metal ions can be described by [15]:C W Mdh o=D M C ′M −C W M ð3Þwhere D M is the diffusion coefficient of the metal ions in theTGO,h is the thickness of TGO,t is the time of TGO growth,C ′Mand C ″Mare the concentration of the metal ions at oxide –metal interface and gas –oxide interface,respectively.Let k o =D MC ′M −C W M C WMð4ÞThus,the outward growth rate of the TGO can be expressed as:dh o dt =k ohð5Þwhere k o is the outward growth rate constant for the TGO.Likewise,the inward growth rate of the TGO can be expressed as:dh i dt =ki h ð6Þk i =D OC W O −C ′O C′Oð7Þwhere k i is the inward growth rate constant for the TGO,D O isthe diffusion coefficient of the oxygen ions in the TGO,C ′Oand Fig.3–Cross-sectional micrographs of a)microlaminate A andb)microlaminate B after exposure in air at 1000°C for 100h.1068M A T E R I A L S C H A R A C T E R I Z A T I O N 62(2011)1066–1070C ″Oare the oxygen concentration at oxide –metal interface and gas –oxide interface,respectively.When k i was larger much than k o ,the inward growth was dominant for the TGO.When k i was smaller much than k o ,the TGO grew outward mainly.For the outer metal-layer and the internal metal-layer,initial concentration of the metal ions was equal;however,the concentration of oxygen arriving at the gas –oxide interfaces was different.Due to the larger cross sectional area and smaller length of oxygen diffusion path,the oxygen diffused more easily in the top YSZ layer.As a result,the oxygen concentration was greater at the interface between the top YSZ layer and the adjacent TGO,and k i was larger.Thus,the TGO adjacent to the top YSZ layer grew mainly toward the NiCoCrAl layer.However,the oxygen concentra-tion was lower at the interfaces between the internal YSZ layers and the adjacent TGO layers,and k i was smaller,because of the smaller cross sectional area and larger length of oxygen diffusion paths in the internal YSZ layers.As a result,the TGO layers adjacent to the internal YSZ layers grew mainly toward the YSZ layers.The results suggested that the oxide growth was controlled by the oxygen diffusion for TGO adjacent to the top YSZ layer,and was controlled by themetal ionic diffusion for TGO adjacent to the internal YSZ layer.Thus it was reasonable to deduce that the oxidation rate of the outer metal-layer adjacent to the top YSZ layer was greater than that of the internal metal-layer.Furthermore,the oxidation products were different for the two microlaminates.The Al 2O 3layer was easiest to form during oxidation of the NiCoCrAl alloy due to selective oxidation.The selective oxidation of Al in NiCoCrAl alloys could be described by Wanger's theory [16].In order to keep Al 2O 3layers stable,the following condition must be satisfied:J Al ≥J ′A 1ð8Þwhere J Al is the flux of aluminum ions transported from theNiCoCrAl alloy to the interface of oxide scale/alloy;and J ′Alis the flux of aluminum ions transported outward in the oxide scale.If there is no sufficient aluminum transported to theinterface of oxide scale/alloy (namely J Al <J ′Al),Al 2O 3layer will be not protective,and other elements will be oxidized in the NiCoCrAl alloy.When the metal-layers of the microlaminates were thin-ner,Al in the metal layers was more easily depleted,aluminum transported to the interface of oxide scale/alloy was easier to become insufficient,and the single and dense Al 2O 3layers were harder to form during high-temperature oxidation.This agreed with the results obtained from the present work:microlaminate A with relatively thicker metal-layers formed only Al 2O 3oxide scales,while microlaminate B with relatively thinner metal-layers formed the oxide scales consisting of Al 2O 3and Cr 2O 3.In general,Al 2O 3layers were very dense and able to prevent oxygen ion and metal ion from diffusing,decreasing the oxidation rates of the alloys.While the oxide layers of Cr or Ni (Co)were relatively loose and easy to crack,resulting in the decreasing oxidation resistance of the alloys [17].Therefore,the unstable Al 2O 3layers and the oxidation of Cr element arising from the lower thickness of the NiCoCrAl layers was also one of the reasons causing the greater oxidation rate for microlaminateB.Fig.4–Element distribution of cross-sections of a)microlaminate A after exposure in air at 1000°C for 100h,of b)and c)microlaminate B after exposure in air at 1000°C for 5h and for 100h,respectively.Fig.5–Cross-sectional microstructure of the EB –PVD YSZ layer.1069M A T E R I A L S C H A R A C T E R I Z A T I O N 62(2011)1066–10705.ConclusionDuring exposure in air at1000°C,the t to m phase transforma-tion occurred in the YSZ layers.After100h high-temperature exposure,the volume fractions of m phase reached0.30and0.36 for the microlaminates A and B,respectively.The segmented columnar structures of the YSZ layers resulted in that oxygen could easily diffuse into the YSZ layers from the edges of the microlaminate specimens and could penetrate along the ceramic layers.As a result,oxide scales formed at the surfaces of not only the outer metal-layers but also the internal metal-layers.The oxidation products of the two microlaminates were different,which was related to the thickness of the NiCoCrAl layer.The oxide scales of microlaminate A with35μm thick metal-layers consisted mainly ofα-Al2O3andθ-Al2O3,while the oxidation products of microlaminate B with14μm thick metal-layers were the mixture ofα-Al2O3,θ-Al2O3and Cr2O3.Therefore, the number and thickness of metal-layers were the important factors influencing the oxidation behavior for the microlami-nates.And the microlaminate having more layers and less metal-layer thickness had the greater oxidation rate.Further-more,it was also found that the growth of the oxide scale adjacent to the top YSZ layer was controlled by the oxygen diffusion,and that the growth of the oxide scale adjacent to the internal YSZ layer was controlled by the metal ionic diffusion.AcknowledgmentsFinancial support was from the National Natural Science Foundation of China(NSFC,project grant51002019)and the Fundamental Research Funds for the Central Universities.R E F E R E N C E S[1]Was GS,Foecke T.Deformation and fracture inmicrolaminates.Thin Solid Films1996;286:1–31.[2]Heathcote J,Odette GR,Lucas GE,Rowe RG,Skelly DW.On themicromechanics of low temperature strength and toughness of intermetallic/metallic microlaminate composites.ActaMater1996;44(2):4289–99.[3]Agrawa P,Sun CT.Fracture in metal–ceramic composites.Compos Sci Technol2004;64:1167–78.[4]Huang Y,Zhang HW.The role of metal plasticity andinterfacial strength in the cracking of metal/ceramiclaminates.Acta Metall Mater1995;43(4):1523–30.[5]Shaw MC,Clyne TW,Cocks ACF,Fleck NA,Pateras SK.Cracking patterns in metal–ceramic laminates:effects ofplasticity.J Mech Phys Solids1996;44(5):801–21.[6]Price RD,Jiang FC,Kulin RM,Vecchio KS.Effects of ductilephase volume fraction on the mechanical properties ofTi–Al3Ti metal-intermetallic laminate(MIL)composites.Mater Sci Eng A2011;528:3134–46.[7]Markaki AE,Clyne TW.Energy absorption during failure oflayered metal foam/ceramic composites.Mater Sci Eng A2002;323:260–9.[8]Fiscina JE,Gonzalez-Oliver CJR,Esparza DA.The sinteringkinetics of Ag–Y1Ba2Cu3O7-δlaminates and the interfacialmetal–ceramic reaction.Appl Supercond1995;3(5):277–87. [9]Toraya H,Yoshimura M,Somiya S.Calibration curve forquantitative analysis of the monoclinic-tetragonal ZrO2system by X-ray diffraction.J Am Ceram Soc1984;67:111–9.[10]Lange FF.Transformation toughening;part3:experimentalobservations in the ZrO2–Y2O3system.J Mater Sci1982;17:240–6.[11]Matsui M,Soma T,Oda I.Stress induced transformation andplastic deformation of Y2O3-containing tetragonal zirconiapolycrystals.Adv Ceram1984;12:198–202.[12]Mohsen S,Abbas A,Akira K.Microstructural analysis of YSZand YSZ/Al2O3plasma sprayed thermal barrier coatings after high temperature oxidation.Surf Coat Technol2008;202:3233–8.[13]Park JH,Kim JS,Lee KH,Song YS,Kang MC.Effects of the lasertreatment and thermal oxidation behavior ofCoNiCrAlY/ZrO2–8wt.%Y2O3thermal barrier coating.J Mater Process Technol2008;201:331–5.[14]Thomas KGS,Hertter M.Improved oxidation resistance ofthermal barrier coatings.Surf Coat Technol1999;120–121:84–8.[15]Stott FH.The protective action of oxide scales in gaseousenvironments at high temperatures.Rep Prog Phys1987;50: 861–913.[16]Schulz U,Menzebach M,Leyens C,Yang YQ.Influence ofsubstrate material on oxidation behavior and cyclic lifetime of EB–PVD TBC systems.Surf Coat Technol2001;146–147:117–23.[17]Lee EY,Biedrman RR,Sisson RD.Diffusional interactions andreactions between a partially stabilized zirconia thermalbarrier coating and the NiCrAlY bond coat.Mater Sci Eng A 1989;121:467–73.1070M A T E R I A L S C H A R A C T E R I Z A T I O N62(2011)1066–1070。