Morphology,structure and formation mechanism of silicide coating by pack cementation process

Available online at www.sciencedirect.comsCIENCEs @oInEoT.Transactions of、Nonferrous MetalsSociety of ChinaScienceTrans. Nonferrous Met. Soc. China 16(2006)s239- -s244Presswww.csu.edu.cn/ysxb/Morphology, structure and formation mechanism ofsilicide coating by pack cementation processXIAO Lai-rong(肖来荣), CAI Zhi-gang(蔡志刚)，YI Dan-qing(易丹青), YINGLei(殷磊),LIU Hui-qun(刘会群), HUANG Dao-yuan(黄道远)School of Materials Science and Engineering, Central South University, Changsha 410083, ChinaReceived 10 April 2006; accepted 25 April 2006Abstract: The MoSi2 coating on C103 niobium based alloy was prepared by pack cementation method. The formative mechanism,morphology and structure of coating were investigated. The silicide coating was formed by reactive diffusion obeying parabolic ruleduring pack cementation process. It is found that the composite structural coating is composed of three inferior layers as follows. Themain layer is composed of MoSi2, the two phases' transitional layer consists of NbSi2 and a few NbsSiz and the diffuse layer iscomposed of NbsSis. The dense amorphous glass layer formed on the surface at high temperature oxidation circumstance caneffectively prevent the diffusion of oxygen into coating.Key words: niobium based alloy; pack cementation; silicide coating; formative mechanismoxidation were investigated to provide benefic references1 Introductionfor improving the properties of coating.Nb-based alloys have been used as important high2 Experimentaltemperature structural materials due to their high meltingpoint and good high temperature strength[1, 2]. However,C103 Nb alloy with composition of 89Nb- 10Hf-1Tithe poor high temperature oxidation resistance propertieswas cut into d 3 mmx40 mm sheets as specimens. Afterbecome an inevitable obstacle in application[3, 4]. As thegrinding, the specimens were treated by alkali solution,requirement of new two-component orbit attitudeultrasonic cleaning in ethanol, acid solution and distilledcontrolled engine, Nb-based components should bwater respectively. Then the treated specimens were leftendured at high temperature circumstance longer.in the drying chamber until the surface was dry.Therefore, the studies of corresponding high temperatureSlurry was produced by ball milling Mo powder ofoxidation resistance coating get more and more attractive.3.3 um in diameter with ethanol in stainless steel millingWith high melting point, moderate density and excellentmedium. C103 Nb alloy specimens were dipped into thehigh temperature oxidation resistance properties, MoSizslurry and sintered in vacuum atmosphere to form a Mobecomes one of important candidate materials which canlayer on surface. Then the final MoSiz coating samplesbe applied as protective coating on refractory alloys andwere prepared by pack cementation process.C/C composites [5, 6].Static oxidation testing were carriedat 1 600 C inRYOSUKE et al [6- -8] used melting salt, laser fusedair. Analysis balance was used to measure the liveweightand CVD method respectively to produce MoSi2 coating of coating samples at different time during packon molybdenum and alloyed steel substrates. In thecementation process. XD98 X-ray diffraction analysispresent work, MoSi2 coating was produced on C103 Nbwas used to identify the phases on surface of coating.alloy by preparing a Mo layer using slurry firingMorphologies of surface and cross-section of coatingtechnologywithsubsequentlypacked silicidewere observed by scanning electronic microscopy (SEM).cementation treatment. The formation mechanism o1Energy analysis"vestigate thecoating during silicification, characterizations of surfacedistribution of中国煤化工cross-sectionand crossed-section, transformation of structures afterof coating. TheTYHCNMHGtionsandstr-Corresponding author: XIAO Lai rong; Tel: +86-731-8830263; E-macaicsu@ 12.com; xiaolr368@sina.com.s240XIAO Lai rong, et al/Trans. Nonferrous Met. Soc. China 16(2006)uctures were compared between original and oxidizedMoSiz coating samples.| (a)●- Mo3 Results and discussion3.1 Morphology on surfaceSEM images and X-ray diffraction patterns oforiginal and oxidized MoSiz coating samples are shownin Fig.l and Fig.2 ，EDS data are listed in Table 1,respectively. As we can see from Fig.l(a)， the surfaceof Mo coating is relatively smooth after vacuumsintering except some tiny holes. The diameter of bigger20405080hole can reach 3 um. There are a few spherical particles20/(^ )composed of Fe, Ni and Cr on surface of Mo coating.These contaminations may be introduced by stainless| (b). :二Masi?Feiul」420/(°)10μm.(c)9Si2:-5103,B4(60100 um20/(° )Fig.2 XRD patterns of surface: (a) Mo layer; (b) Silicidedcoating; (c) Oxidated coatingTable 1 EDS data of different zones on coating surface(molefraction, %)_ZoneSiAlNaMo35.22.54.1061.938.144.6 _.4_0.48.7_0teel milling medium during milling process. Theeutectic phaslativel50kV 3o0Se Yo200introduced by中国煤化工n the surfaceFig.1 SEM images of Mo layer (a), Silicized layer (b) andof Mo coatingiMHC N M H G temperature.Fig.2(b) shows that the surface of silicified Mo coatingOxidated layer(c) at 1 600 C for 10 h.XIAO Lai -rong, et al/Trans. Nonferrous Met. Soc. China 16(2006)s241consists of MoSi2 and Al2O3 phases. EDS data show thatout-layer and forms the spherical shape low meltingthe half spherical particles are composed of Al and 0point eutectic phases on surface after cooling as shown inelements caused by Al2O3 particles in cemented powderFig.l(a).during high temperature processing. A small quantity ofThe morphology of coating's cross-section afterSi and Na are also found in Al2O3 as contaminations.silicification is shown in Fig.3 (b). The coating'sFig.2(c) shows the oxidized coating surface consists ofthickness increases to 125 μ m after pack cementation,MoSi, Mo,Sis and amorphous phases composed ofSi, 0 due to the silicates increased by reactive difusion. Someand Al elements. A dense oxidized film with smoothcavities caused by incompletely sealing of relatively bigsurface is formed on coating after oxidation (Fig. l(c)).cavities in Mo layer can be found in out-layer of coating.The low melting point eutectic phase is created by theFrom outside to inside, the coating can be divided intoreaction between Al2O3 and SiO2 from the oxidation ofmain layer, micro cavities zone and diffused layer. TheMoSi2[9]. In Fig.2(c), an amorphous bump can be foundmain layer composing of MoSi with about 75 um isin a low angle diffraction area. The oxidized sampleslense.The micro cavities zone with about 15 um inwere rapidly cooled to room temperature, causing themelting oxides rapid solidification into amorphous glass.thickness is formed by the aggregation of vacancy duringDue to the great gap of thermal expansion coefficientthe process of reactive diffusion. During the process ofbetween amorphous phases(a(SiO2) =2.4X 10 6/C) andSi atom diffuses through MoSiz layer and Nb substrate,MoSi2(a(MoSi2)=8.1X 10 6/"C), some micro cracks calvacancy source is generated and formed the dislocationbe found on the coating's surface after rapid coolingcreep deformations which are impelled by stress, whereform high temperature (Fig1().is created in new phase layer following its volumechanged [11]. In the early stage of reactive diffusion,3.2 Morphology of cross sectionnon-equilibrium vacancy distributes on the whole newAs shown in Fig.3 (a), Mo layer, whose thickness isthin layer.about 35 μ m，is dense except some cavities inside.With the proceeding of reactive diffusing， theThe transitional layer with deep color between Mo layerconcentration of vacancy increases gradually in interface.and substrate is composed of Nb, Fe and small quantitiesAfter diffusion, a low energy area appeares at theof Ni elements as listed in Table 2. The contaminations,interface between MoSi2 and NbSi2 layers to create manysuch as Fe and Ni, were come from the milling of Movacancy traps. The consequential vacancy zone is formedpowder. Compared to Mo, the sel-diffusion coefficientsfor the aggregations of vacancies. TORTORICI et al [12]of Fe and Ni are higher but the activated diffuse energyalso found the similar phenomenon between MosSis andis lower [10]. Consequently, the contaminated elementsNbsSis in diffusion couple of MoSiz/Nb. The formationin Mo layer diffus to substrate and out-layer and formof cavity zone can effectively release the thermal stress .two segregated areas. The Fe element diffuses to .inside the coating, and then decreases the possible creat-可、、B5Ew9ISEY中国煤化工6OEV S2o0HCNMH GFig3 Cross-section morphology: (a) Mo layer; (b) Silicide coating; (c) Micropore zone; (d) Interlayer of silicide coating.s242XIAO Lai -rong, et al/Trans. Nonferrous Met. Soc. China 16(2006)ion of inner micro crack. On the other hand, the cavitymain layer. The reverse flow of vacancy achieves, thezone is one of weaknesses for its relative lower strength.difusion of Si and also can be aggregated by vacancyThe mechanical properties of coating may be degradedtraps.under the flushing of high velocity and temperature flowBecause of the stronger appetency of oxygen withdue to the shear stress acting on cavity zone.Hf in substrate, the dispersed HfO in white color areAs shown in Fig.3(d), the bright layer closing toformed preferentially when the oxygen diffuses intosubstrate with a straight interface and2 μ m in thicknesscoating or substrate (Fig.4(b)).is NbsSiz. The dark area is NbSi2 phases with a fewNbsSis phases. After cooling to room temperature,NbsSi3 separates out and deposites from NbSi2 to formthe two phases' transitional layer, due to its solid solutiondecreasing. As we known from Nb-Si binary phasediagram, NbSiz and NbsSiz coexist in the range from37% to 67% in mole ratio. The similar phenomenon isalso found by CHIARA et al[13] in Nb/Si diffuse couple.The growing direction of needle shape NbsSis phases intwo phases' transitional layer is perpendicular to theDEYointerface of coating， corresponding to the diffusedirection of Si in Nb substrate.Table 2 EDS data of different zones in cross section(moleHfOfraction, 1%)Zoneie_MoNbA0100B52.97.1C63.436.6)[71.768.831.2i Topm43.057.0Fig.4 Cross-section morphologies of coating oxidated at 1 600°C for 10 h (a) and interface between NbSi2/NbsSi3 (b)stiffness can prevent the further extension of cracks asillustrated in Fig.3(d). The precipitation of NbsSis3.3 Formation process of coatingimproves the strength of NbSi2 layer which also canPack cementation method to prepare the MoSizresist the oxidation after the inactivation of main layercoating on Nb substrate includes two independent[14]. NbsSi; phases, which distributes near the interfaceprocesses. The first procedure is to prepare Mo layer onbetween transitional layer and diffuse layer, improves theNb substrate by immersing or spraying the Mo slury,thermal shock resistance properties of coating wherebyand then followed with vacuum sintering. The secondstrengthens the combination of interface.process is pack cementation to silicify. Halide activatedThe whole thickness of coating increases to 180 μpack cementation method is the combination of highm after oxidation, and the dense oxidative film which istemperature chemical vapor deposition and reactiveformed on the surface of coating reaches to 20 umdifusion. The reactive procedures as follows[15, 16]. 1)(Fig.4(a)). From EDS data and XRD patterns, oxidativeNaF reacts with silicon powder to form volatile SiF2; 2)film is composed of SiO2 and a small quantity of Al2O3.'he gaseous halide diffuses to the surface of Mo layerAfter the formation of oxidative film, the process ofthrough porous pack driven by chemical potentialoxidation is controlled by the diffusion of oxygengradients; 3) MoSi2 layer is formed by reactive diffusionthrough the oxidation. The diffuse rate of oxygen in SiO2of Si and Mo; 4) The chemical potential gradient ofand Al2O3 is so slow that the oxidation resistance is gascous SiF2 is kept by the consume of depositionimproved by this dense oxidative film[14]. The cavityelements due to the growth of MoSi2 layer, subsequentlylayer still exists between the main layer and transitionalthe process of vapor diffusion is carried on; 5) Thezone, but the micro cavities have come into bigger holes.thickness of MoSi2 layer increases with the processing ofThe thickness of NbsSi3 increases from 2 to 46 um afterSi deposition a中国煤化工ich is drivenby chemical potprocedures ofoxidation, due to the Si in Si rich layer diffusing tovapor depositionTHCN MH G'substrate driven by chemical potential gradient. At thesame time, Si diffuses to outer layer to remedy the loss in2NaF+Si白SiF2(g)+2Na(1).XIAO Lai -rong, et al/Trans. Nonferrous Met. Soc. China 16(2006)s2432SiF2(g)口SiF4(g)+Si(2by the diffuse rate of Si through the MoSiz layer. The5Mo+3Si> MosSisfront of MosSi3 layer is moving to Mo layer as theMosSi;+7Si--- > 5MoSi2(4interface of reactive diffusion. After the whole Mo layerAs known from the analysis above, the growth oftransforms into MoSiz phases, the Si begins to diffusecoating is controlled by two processes: one is the gasinto the substrate. The diffusion of Si is achieved by theflux of halide reaching on surface of Mo layer; the otherreverse flow of vacancy, and subsequent micro cavityis the growth rate of MoSi2 during the reactive diffuselayer is formed by the aggregation of vacancy betweenthe MoSi2 layer and substrate. Because of the formativebetween Si and Mo.The gas flux of halide reaching on the surface ofenthalpy of NbsSis(- 516.8 kJ/mol) is lower than that ofNbSiz's(-161 kJ/mol)[18], the NbsSis layer forms firstlyMo layer can be denoted asby reaction of Nb and Si which diffuses through MoSi2.(5As the proceeding of reactive diffusion, Si reacts withRT' 0XNbsSis to form NbSi2, and the front of NbsSis phase layerwhere D; is the coefficient of gaseous diffusion; SP/OXmoves to substrate as reactive interface. The diffuse rateis the partial pressure gradient on substrate's surface.of Si in substrate is lower than before because of it isWith fixed D and sintering processing parameter, and thecontrolled by diffuse rate of Si in both MoSiz and NbSi2.Si which is used to form gaseous SiF2 in pack powder isFurthermore, the diffuse path prolongs with the increasealways enough, then the gas flux of halide reaching onof coating's thickness, as well as the time for diffusion ofsurface of Mo layer is a fixed value under the invariableSi reaches the reactive interface. Effected by the decreasepartial pressure gradient [17].of reactive diffuse rate, the growth of coating getsDuring the practicable pack cementation process,slower.the reactions (1) and (2) are controlled by the equilibriumpartial pressure of SiF2 and SiF4 which relate to the4 Conclusionsconsumption rate of Si or the growth rate of coating.Fig.5 shows the relative curve between gained mass on1) The silicide coating prepared by pack cermentati-coating surface and time of silicification. As the time ofnmethod is formed by reactive difusion. The process ofpack cementation increases, the mass gained in unit areasilicification obeys the parabolic rule.of coating increases, but the rate of mass gained decrease2) The coating has a typical composite structure asobeys the parabolic rule, which shows that the formationfollows. The main layer composes of MoSi. 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