熔盐反应制备TiC涂层碳纤维

Preparation of a titanium carbide coating on carbon fibre using a molten salt methodXuanke Li a,b,*,Zhijun Dong a ,Aidan Westwood b ,Andy Brown b ,Shaowei Zhang a,c ,Rik Brydson b ,Nan Li a ,Brian Rand baThe Hubei Province Key Laboratory of Ceramics and Refractories,Wuhan University of Science and Technology,Wuhan,Hubei 430081,PR China bInstitute for Materials Research,The University of Leeds,Leeds LS29JT ,United Kingdom cEngineering Materials,The University of Sheffield,Western Bank,Sheffield S102TN,United KingdomA R T I C L E I N F O Article history:Received 14August 2007Accepted 20November 2007Available online 2January 2008A B S T R A C TA method for preparing protective titanium carbide (TiC)coatings on carbon fibres has been developed using a molten salt synthesis method.The TiC coatings were formed on the sur-face of carbon fibres in a reaction medium consisting of Ti powder in a mixture of molten LiCl–KCl–KF salts under an argon atmosphere at 900and 950°C.The structure and morphol-ogy of the TiC coatings were characterized by XRD,SEM and energy dispersive X-ray (EDX)analyses.The coatings consisted of homogeneous single phase cubic TiC with thicknesses in the range of 60–800nm.Variation of the synthesis time and reaction mixture was found to significantly affect the thickness and integrity of the TiC coating although variation of the reaction temperature had little effect.The coating thickness was closely related to the com-position of the molten salts and to the molar ratio between the carbon fibre and titanium.Ó2007Elsevier Ltd.All rights reserved.1.IntroductionDespite their uniquely superior properties,the interface compatibility problems between carbon fibre and most matri-ces have limited the industrial application of carbon fibre reinforced metallic and ceramic matrix composites.More-over,the applications of carbon fibres for composite rein-forcement are also limited since carbon reacts with many metallic and ceramic matrix materials.Much effort is there-fore being expended in attempts to provide a refractory coat-ing on the surface of the carbon to shield carbon materials from reactive environments at elevated temperatures and to improve the carbon fibre–matrix interfacial properties [1–5].This approach is only partially successful since the thickness,homogeneity and integrity of coating on the carbon fibre sur-face is difficult to control.Titanium carbide (TiC)is one of the most important refractory metal carbides used as advanced ceramics for wear resistance and aerospace applications.This is primarily due to its high melting temperature (3067°C),high Young’s modu-lus (410–450GPa),high chemical stability and high Vickers hardness (28–35GPa)[6,7],which allow its use in cemented carbides,such as cermets,and oxidation resistant materials.Thin protective titanium nitride and carbide layers have been formed on the surface of carbon fibres [3,5]using a chemical vapour deposition method.However,Kerridge and Polyakov reported [8]that Li–LiCl and Ca–CaCl 2ionic–electronic melts could be used as a medium for the reaction of a mixture of carbon black and transition metals to synthesis the carbides of titanium,zirconium,niobium and tantalum at 800–1000°C over 5–10h under an argon atmosphere.The mol-ten salt synthesis (MSS)method offers a route for synthesis of0008-6223/$-see front matter Ó2007Elsevier Ltd.All rights reserved.doi:10.1016/j.carbon.2007.11.020*Corresponding author:Address:The Hubei Province Key Laboratory of Ceramics and Refractories,Wuhan University of Science and Technology,Wuhan,Hubei 430081,PR China.Fax:+862786551274.E-mail address:xkli8524@ (X.Li).C A R B O N46(2008)305–309a v a i l ab l e a t w w w.sc i e n c ed i re c t.c omj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a te /c a r b o ntransition metal carbides at low temperatures in relatively short times,since the molten salt acts effectively as the reac-tion medium.This approach also provides the possibility for the controllable preparation of TiC products with various morphologies such as coatings,particles and nanofibres.Like the chemical vapour deposition studies[3,5],this work also describes the preparation of a TiC coating on car-bonfibre but,in this case,a medium of molten salts is used for reaction of titanium powder with carbonfibre,derived from polyacrylonitrile(PAN),at900–950°C under an argon atmosphere.This approach avoids some of the problems with current syntheses of TiC,which often require very high tem-peratures,rigorous reagent handling or yield non-stoichiom-etric products,and enables formation of a TiC coating with high purity and controllable thickness on the surface of car-bonfibre.2.Experimental2.1.Preparation of a TiC coating on carbonfibresPAN-based Tenax carbonfibre bundles,dried after immersion in acetone for several hours to remove the sizing agent,were used as the carbon source for the synthesis of a TiC coating by reaction with titanium metal powder(99.5wt%purity,200 mesh size ex Sigma–Aldrich)in a molten salt mixture com-posed of KCl,LiCl and KF.The diameter of this carbonfibre is around6–8l m.The molar ratio of titanium to carbon in the molten salts was chosen to be either Ti/C=1/1.5or1/2.5.The carbonfibre bundle was placed in an alumina crucible and covered by the mixture of titanium and the salts,heat-treated at900or950°C for1–5h under aflowing argon atmosphere and then allowed to cool to room temperature.After cooling, the product in the crucible was boiled repeatedly in distilled water to dissolve the salts in order to retrieve the TiC-coated carbonfibres by decanting off the supernatant solution.The coated carbonfibres were then dried at100°C for2h.2.2.The characterization of TiC-coated carbonfibresThe phases present in the TiC-coated carbonfibres after mol-ten salt treatment were determined by XRD using Cu K a radi-ation.The surface morphology of the coated carbonfibres was investigated using a Carl Zeiss LEO1530field emission scanning electron microscope(FESEM).Their elemental com-positions were obtained using the energy dispersive X-ray (EDX)analyzer linked to this instrument.Thermogravimetric analysis(TGA)was performed on90mg samples of uncoated and coated carbonfibres produced from$1cm lengths of short carbonfibres.Samples were heated at5°C/min to 880°C and held for1h underflowing air at a rate of250ml/ min.3.Results and discussionAfter reaction in the molten salts using a molar ratio of Ti/ C=1/1.5,a golden colouredfibre product was obtained. Fig.1a illustrates the XRD pattern for thefibreproductFig.1–(a)XRD pattern of golden colouredfibres prepared from Tenax carbonfibre using a Ti/C molar ratio of1/1.5in molten salts at950°C for5h,and(b)an SEM image and corresponding EDX elemental maps of(c)titanium and(d)carbon in thefibre cross sections.306C A R B O N46(2008)305–309prepared using this molar ratio in the molten salts at 950°C for 5h.The five sharp peaks at 2h =35.91°,41.71°,60.45°,72.36°and 76.13°correspond to diffraction from the 111,200,220,311and 222crystal planes of a face-centered cubic TiC phase,respectively.The relative intensities of these five peaks are also consistent with those expected for bulk TiC.A broad peak is also observed at 2h =ca.26°(and possibly ca.43°also),which can be attributed to the PAN-based carbon fibre,indi-cating that the PAN-based fibres have not been completely transformed into TiC fibres.This implies that TiC forms a coating only on the surface of carbon fibres during the molten salt treatment.The typical cross-sectional morphology of these golden coloured carbon fibres is shown in Fig.1b.This reveals that the product consists of fibres and each fibre is composed of a shell and core.In order to determine the elemental compo-sition of the fibre,the cross section of coated fibre was char-acterized by EDX analysis.Fig.1c and d shows maps of the titanium (Ti K a )and carbon (C K a )elemental distributions,respectively,corresponding to the SEM image shown in Fig.1b.This reveals that the fibres contain both titanium and carbon which are mainly located in the shell and the core of fibres,respectively.This confirms that a TiC coating has been formed on the surface of the carbon fibre.The thickness of this TiC coating on the carbon fibres is estimated from SEM to be about 800nm.The coatings are homogeneous with a densified structure and a smooth surface composed of many polycrystalline grains.In order to reveal the influence of reaction time and Ti/C molar ratio on the thickness of the TiC coating formed on the surface of the carbon fibres,FESEM images of the product fibres produced at 950°C are shown in Fig.2a and b,after 5h reaction and in Fig.2c and d after 1h reaction.This shows that,after 5h and 1h,the TiC coating thicknesses on the product fibres are about 220and 65nm,respectively,confirm-ing that the thickness of the coating increases with reaction time.Very similar results were obtained following the corre-sponding reactions at 900°C and this suggests that there is no clear influence of reaction temperature in the range 900–950°C on the thickness of the coating.The thinly TiC-coated carbon fibres produced during the reactions for 1h show a smooth and crack-free surface and possess flexibility almost as high as that of the original uncoated carbon fibre.The 220nm coating thicknesses on the fibres shown in Fig.2a–b,produced with a Ti/C ratio of 1/2.5over 5h at 950°C,are still smaller than the 800nm thickness of the coating on the fibres shown in Fig.1b,and produced with a Ti/C ratio of 1/1.5over 5h at 950°C.This demonstrates that the thickness of the coating (as determined by SEM)increases not only with reac-tion time,but also with the molar ratio of titanium to carbon at 950°C.Both of these TiC-coated carbon fibre products are stiffer than the carbon fibre starting material and this can be attributed to the thickening effect of TiC coating formation.The dissolution of metals in molten salt is not well under-stood,but it has been suggested that transport reactions are allowed because metals dissociate to mobile cations and delo-calised electrons,a state which is considered to be intermedi-ate between ionic and metallic [8–10].Li et al.calculated [9]that Gibbs free energy change D G o ðT Þfor the reaction of Ti with graphite in the range of 298–1200K is givenbyFig.2–T ypical SEM images of the cross sections of TiC-coated carbon fibres prepared from Tenax carbon fibres using the molten salt method with a Ti/C molar ratio of 1/2.5at 950°C for (a,b)5h and (c,d)1h.C A R B O N46(2008)305–309307C ðgraphite ÞþTi ¼TiC ;D G o ðT Þ¼À184116:4þ10:60445ÂT ðJ =mol ÞThis result shows that titanium metal can react directly with graphite and form TiC,but due to some kinetic consider-ations,such as the contact area between titanium metal pow-der and carbon,it is difficult to form a densified TiC coating on the carbon surface.The molten salt mixture is believed to facilitate the dissolution and transport of the titanium and hence the formation of the TiC coating through diffusion of titanium cations from the molten salt to the surface of the carbon fibres with subsequent reaction.Therefore,it would seem that the concentration of titanium cations in the molten salt is important in determining the thickness of TiC coating.However,the exact mechanism of coating formation is still unclear.According to Pan et al.[10],the inclusion of K 2TiF 6in the molten salt mixture will be advantageous to the formation of titanium cations.Therefore,K 2TiF 6was added to the mol-ten salt mixture of KCl,LiCl and KF .The morphology of the TiC coating produced using the K 2TiF 6additive,with a Ti/C ra-tio of 1/2.5over 5h at 950°C,is shown in Fig.3a and b.In com-parison to the corresponding coating prepared without K 2TiF 6,shown in Fig.2a,the thickness of the TiC coating in-creases from about 220–480nm owing to the addition of K 2TiF 6.This suggests that the addition of K 2TiF 6does indeed increase the titanium cation concentration in molten salt,as expected.(In contrast,the effectiveness of the incorpora-tion of potassium fluoride in the molten salt mixture in order to enhance TiC formation,whether by promoting dissolution and transport of Ti metal or by improving the surface state of carbon fibres,is not clear).Fig.3a and b also shows some cracks,coating fragments and missing areas of coating,which suggest that the cracks are formed in the coating and that the coating then peels off the surface of the carbon fibre.As coating thickness in-creases,the coated carbon fibres become stiff and fragile with reduced flexibility.If the coating is too thick there is a possi-bility that cracks will develop at the coating-fibre interface,presumably due to the mismatch in thermal expansion coef-ficients between coating and core.Therefore,the control of TiC coating thickness and the influence of the coating thick-ness on the mechanical properties of the coated carbon fibre require investigation in further work.Finally,Fig.4shows the TGA curves of both uncoated and TiC-coated carbon fibres.The weight loss of the uncoated car-bon fibre sample commences at around 425°C,and the rate of weight loss increases markedly in the range 500–780°C.The weight loss of TiC-coated carbon fibre begins at around 645°C and in comparison to uncoated carbon fibres,this weight loss initiation temperature is increased by around 220°C.However,the rate of weight loss during subsequent oxidation is similar to that of the uncoated fibres.It can also be seen from Fig.4that the TiC-coated fibres show a slight weight gain at about 580°C,which can be attributed to the oxidation of the TiC coating.The relationship between the thickness of the TiC coating and mechanical properties and oxidation resistance is currently being investigated for TiC-coated carbon fibres.4.ConclusionsTiC-coated carbon fibres were successfully produced using a relatively low temperature molten salt method.A high qual-ity,crystalline TiC coating was produced on the surface of PAN-based carbon fibres.Analysis reveals that a thin,homo-geneous and crack-free TiC coating is formed on thesurfaceFig.3–T ypical SEM images of:(a)the surface and (b)the cross section of TiC-coated carbon fibres prepared from Tenax carbon fibres using the molten salt method at 950°C for 5h with K 2TiF 6additive and with a Ti/C molar ratio of1/2.5.308C A R B O N46(2008)305–309of carbonfibres.It is observed that the molten salt components,titanium/carbon molar ratio and the reaction time can all significantly affect the coating thickness and integrity of the TiC coating.However,variation of the reaction temperature between900and950°C had no clear effect on these parameters.Coated carbonfibres with a thickness of about65nm display goodflexibility but,as TiC coating thick-ness increases,the coatedfibres become stiff and fragile.This method offers possibilities for the synthesis of various transi-tion metal carbide coatings using MSS methods,and hence the use of carbide-coatedfibres for the reinforcement of metallic and ceramic composites.AcknowledgementsThe authors acknowledge thefinancial support of the Na-tional Natural Science Foundation of China(Grant No. 50672070)and Royal Society KC Wong Education Foundation.R E F E R E N C E S[1]Wang YQ,Zhou BL,Wang ZM.Oxidation protection of carbonfibers by coatings.Carbon1995;33(4):427–33.[2]Vincent H,Vincent C,Berthet MP,Bouix J,Gonzalez G.Boroncarbide formation from BCl3–CH4–H2mixtures on carbonsubstrates and in a carbon-fibre reinforced Al composite.Carbon1996;34(9):1041–55.[3]Hackl G,Gerhard H,Popovska N.Coating of carbon shortfibers with thin ceramic layers by chemical vapourdeposition.Thin Solid Films2006;513(1–2):217–22.[4]Sebo P,Stefanik P,Kavecky S,Ivan J.Influence of carbideforming elements(Ti,Zr)on the interface of coppermatrix-carbonfibre composite.Kovove Mater1999;37(6):367–76.[5]Vincent H,Vincent C,Berthet MP,Mourichoux H,Bouix J.Mechanical and chemical characteristics of carbon-fibersprotected with a carbide coating formed by reactivechemical-deposition.J Less Common Met1991;175(1):37–58.[6]Storms EK.The refractory carbides,refractory materialsseries,vol.2.New York:Academic Press;1967.p.82.[7]Pierson HO.Handbook of refractory carbides andnitrides.Westwood:Noyes Publications;1996.p.71.[8]Kerridge DH,Polyakov EG.Refractory metals in molten salts:their chemistry,electrochemistry,and technology.Dordrecht, Boston:Kluwer Academic;1998.p.81–6.[9]Li CH,Lu HB,Xiong WH,Chen X.Diamond and graphitecoated with polyalloys by an immersion method.Surf Coat Tech2003;150:163–9.[10]Pan W,Huang QL,Chen J,Cai J,Huang Y.Mechanism oftitanium deposition on Al2O3ceramic surface by molten salt reaction.Mater Lett1997;31:317–20.C A R B O N46(2008)305–309309。