Microstructures and properties of TiN reinforced Co-based composite coatings modified with Y2O3by laser cladding on Ti e6Al e4V alloy Fei Weng a,c,Huijun Yu b,c,*,Chuanzhong Chen a,c,*,Jianli Liu a,c,Longjie Zhao a,ca Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials(Ministry of Education),Department of Materials Science and Engineering,Shandong University,Ji'nan250061,PR Chinab Key Laboratory of High-efficiency and Clean Mechanical Manufacture(Ministry of Education),School of Mechanical Engineering,Shandong University, Ji'nan250061,PR Chinac Shandong University,Suzhou Institute,Suzhou215123,PR Chinaa r t i c l e i n f oArticle history:Received28May2015 Received in revised form29July2015Accepted31July2015 Available online4August2015Keywords:Laser claddingTitanium alloyComposite coating Microstructure a b s t r a c tIn this study,TiN reinforced composite coatings were fabricated on Ti e6Al e4V substrate by laser cladding with Co42self-fluxing alloy,TiN and Y2O3mixed powders.Microstructures and wear resis-tance of the cladding coatings with and without Y2O3addition were investigated comparatively.Re-sults showed that the coatings were mainly comprised of g-Co/Ni,TiN,CoTi,CoTi2,NiTi,TiC,Cr7C3,TiB, Ti5Si3and TiC0.3N0.7phases.The coatings showed metallurgical bonding free of pores and cracks with the pared with the Ti e6Al e4V substrate,the microhardness and wear resistance of the coatings was enhanced by3e4times and9.5e11.9times,respectively.With1.0wt.%Y2O3addition,the microstructure of the coating was refined significantly,and the microhardness and dry sliding wear resistance were enhanced further.The effects of Y2O3were attributed to the residual Y2O3and decomposed Y atoms.©2015Elsevier B.V.All rights reserved.1.IntroductionTitanium alloys have been extensively used in industries in virtue of the characteristics of high specific strength and excellent corrosion resistance[1e3].While,their application under friction condition is restricted owing to the low hardness and poor wear resistance[4].In recent decades,laser cladding has been widely studied as an effective surface modification technique to enhance the surface properties of metals and their alloys[5].Compared with other surface modification techniques,such as thermal spraying, physical vapour deposition(PVD),and chemical vapour deposition (CVD),laser cladding coatings have the advantages of dense microstructure,strong metallurgical bonding with the substrates and notable improvement in properties[6].In order to fabricate cladding coatings with excellent proper-ties,proper cladding materials should be selected[6].As a series of alloys exhibiting low melting point and good self-shield effect, self-fluxing alloy powders have been widely introduced into laser cladding,such as the Ni-based alloy[7],Fe-based alloy[8]and Co-based alloy[9].Laser cladding Co-based self-fluxing alloys on steel [10],copper alloy[11]and titanium alloy[9]was proved effective in enhancing the microhardness and wear resistance of the sub-strate materials.On one hand,the solid solution of Co matrix acts as a binding phase between the reinforced phases and the sub-strate;on the other hand,the in situ formed intermatallics(e.g. CoTi,CoTi2,NiTi)are beneficial to the improvement of the prop-erties of the cladding coatings[12e14].In order to further improve the properties,different ceramic particles have been added in the Co-based self-fluxing alloys,such as WC[15],SiC[16] and TiC[17].Besides,TiN is also regarded as an outstanding reinforced phase in laser cladding layer owing to its high hardness and excellent chemical stability[18].However,laser cladding with Co-based alloy and TiN on titanium alloys is rarely reported.Furthermore,the ef-fects of certain rare earth oxides on the microstructure and prop-erties of the cladding coatings have been studied,such as CeO2[19], Y2O3[20]and La2O3[21].Though the beneficial effects of the rare earth oxides are notable,the mechanisms need more extensive study.*Corresponding authors.Jing Shi Road#17923,Ji'nan250061,Shandong,PR China.E-mail addresses:wengfeisdu@(F.Weng),yhj2001@(H.Yu), czchen@(C.Chen),jianli21s@(J.Liu),zhaoljsdu@ (L.Zhao).Contents lists available at ScienceDirectJournal of Alloys and Compounds journal homepage:http://www.e lse /locate/jalcom/10.1016/j.jallcom.2015.07.2950925-8388/©2015Elsevier B.V.All rights reserved.Journal of Alloys and Compounds650(2015)178e184In this paper,TiN reinforced Co-based composite coatings were fabricated on Ti e 6Al e 4V titanium alloy by laser cladding with Co-based self-fluxing alloy and TiN powders,and then the effect of Y 2O 3on the microstructure,microhardness and wear properties of the cladding coatings was studied comparatively.2.Materials and methodsThe substrate material used in this study was Ti e 6Al e 4V alloy.Specimens with dimensions of 30mm Â10mm Â10mm were prepared and the 30mm Â10mm planes were abraded with SiC grit paper prior to preparing the preplaced coating.The cladding materials were comprised of Co42(!99.5%purity,50e 100m m),TiN (!99.5%purity,20e 60m m)and Y 2O 3(!99.9%purity,0.05e 5m m).The chemical composition of the Co42Co-based self-fluxing alloy powder is shown in Table 1.Water glass solution (Na 2O ∙nSiO 2:H 2O ¼1:3,vol.%)was used to mix the preplaced powders into mash,and then the mixed powders were preplaced on the substrate with a thickness of 0.8mm.A 1.5kW continuous wave CO 2laser was employed to carry out laser cladding.The process parameters and the cladding materials used in this paper are shown in Table 2.The molten pool was protected by the shielding gas (Ar)during the laser cladding pro-cess.The schematic diagram of the laser cladding process is shown in Fig.1a.The cladding coatings fabricated in this study showed good macro morphology,as shown in Fig.1b.The cross section samples were cut perpendicular to the scanning direction,prepared by standard mechanical polishing procedure and etched in H 2O-8vol%HF-25vol%HNO 3water solution.Phase constituents of the specimens were analyzed by D/max 2500PC X-ray diffractometer (XRD).Microstructures of the cladding coatings were analyzed by S-3400N scanning electron microscope (SEM)equipped with an attached energy dispersive spectrometer (EDS).Microhardness distributions of the coatings were measured by DHV-1000Vickers microhardness tester with a test load of 200g and a load-dwell time of 10s.MM200wear tester was used to test the wear resistance with a load of 50N.The rotational speed of the grinding wheel (YG6cemented carbide)was 400r/min and the linear velocity was 0.84m/s.The wear mass loss was measured every 10min by the electronic balance with an accuracy of 0.1mg.3.Results and discussion3.1.Microstructure of the laser cladding coating3.1.1.Phase constituent analysisAs shown in Fig.2,the composite coatings were mainly comprised of g -Co/Ni,CoTi,CoTi 2,NiTi,TiC,Cr 7C 3,TiN,TiB,Ti 5Si 3and TiC 0.3N 0.7phases.The phase constituent was similar to one of our previous study [22].While,TiB 2was not detected in this paper,probably owing to the limited amount of B in the claddingmaterials.Diffraction peaks of Y 2O 3didn't appear,probably due to the low content.During the laser cladding process,complicated chemical reactions took place in the molten pool,forming various phases mentioned above.The formation of CoTi,CoTi 2and NiTi was attributed to the reaction between Co,Ni in Co42and Ti diluted from the Ti e 6Al e 4V substrate.According to the Ti e Co binary alloy phase diagram (Fig.3),there are various intermetallics between Ti and Co.However,only CoTi and CoTi 2were detected in this study,probably owing to the limited amount of Ti diluted into the molten pool.It could be deduced that the constituent of Ti and Co fell into the zone (1),as annotated in Fig.3.Therefore,CoTi and CoTi 2formed in the molten pool and solidi fied rapidly in pace with the laser scanning.Similar results were reported by Xue and Wang [23]and the in situ formed CoTi/CoTi 2dual-phase structure was proved bene ficial to the wear resistance of the coatings due to the comprehensive effects of the ductile CoTi and hard CoTi 2.Besides,the NiTi phase has also been proved effective in enhancing the mechanical properties of the materials [24].In Ref.[24],the NiTi 2/NiTi intermetallic alloy fabricated by laser melting exhibited notable wear resistance owing to the combination of high hardness of NiTi 2and excellent ductility and toughness of NiTi.However,NiTi 2was not detected in our study probably due to the insuf ficientTable 1The chemical composition of the Co-based self-fluxing alloy powder (wt.%).C Cr Fe W Ni Si B Co 1.0e 1.218e 243.04e 614e 162.5e3.21.2e 1.6Bal.Table 2The process parameters and the preplaced powder compositions.Number Powders composition (wt.%)Laser power (W)Scanning speed (mm s À1)Spot diameter (mm)Specimen 1Co42e 10TiN950e 10005.04.0Specimen 2Co42e 10TiN e 1.0Y 2O3Fig.1.Schematic diagram of the laser cladding process (a)and the macro photos of cladding coatings (b).F.Weng et al./Journal of Alloys and Compounds 650(2015)178e 184179Ti.It is worthy to note that the phase constituents in the cladding coating are related to both the thermodynamic and kinetic factors under non-equilibrium conditions of laser scanning [25].Apart from the intermatallics,TiC,TiB and Ti 5Si 3were also in situ synthesized owing to the high af finity between C,B,Si and Ti [26,27].Besides,the diffraction peak of TiN and TiC 0.3N 0.7appeared in the XRD results.TiC 0.3N 0.7derived from the combination be-tween TiC and TiN [28].Previous studies [29]revealed that partial replacement of nitrogen atoms by carbon atoms could cause distortion of the TiN lattice and result in the formation of harder TiC 0.3N 0.7.The multiple effects of TiN and TiC 0.3N 0.7on the micro-hardness and wear resistance of the cladding coatings have also been reported by Yang et al.[28].In summary,the composite coatings in this paper were comprised of the ductile phases of CoTi and NiTi,the reinforced phases of TiN,TiC 0.3N 0.7and in situ synthesized CoTi 2,TiC,Ti 5Si 3,TiB and Cr 7C 3.All the phases mentioned above distributed in the g -Co/Ni matrix and formed a concrete-like composite structure,which was bene ficial to the wear performance of the coatings.Theaddition of Y 2O 3had little in fluence on the phase types in the coatings.In this paper,we put the emphasis on the in fluence of Y 2O 3on the microstructure and property of the coatings,which will be discussed later.3.1.2.SEM analysisFig.4shows the cross section morphologies of the specimens 1and 2.Results showed that the microstructure became denser and more compact with the addition of Y 2O 3.It was worthy to note that the top right corner of Fig.4b was not cracks but the microstruc-tures caused by uneven etch during the preparation of the cross section samples.As shown in Fig.4a and b,the coatings showed sound metallurgical bonding free of pores and cracks with the substrate.By comparison,the bonding line was clearer in specimen 1without Y 2O 3(Fig.4a).Besides,the substrate near the bonding line was characterized by acicular martensites (a 0-Ti),which was also recognized as heat affected zone (HAZ)owing to the rapid heating and cooling rate in pace with laser scanning.Besides,the precipitates near the bonding line penetrated into the substrate like wedges (as annotated by arrows in Fig.4b),which was more bene ficial to the combination between the coating and the substrate.Fig.4c and d demonstrate the typical morphologies of the middle layer in the cladding coatings.During the laser cladding process,TiN absorbed a lot of energy and dissolved into Ti and N atoms.Subsequently,TiN formed again by the combination be-tween Ti and N.The newly formed TiN tended to grow into den-dritic morphology,similar with the report by Lin et al.[30].In our study,the dendritic TiN crystals were also picked out in the clad-ding coatings (as annotated by arrows in Fig.4c).However,the dendrites were notably re fined with the addition of Y 2O 3,as shown in Fig.4d.What's more,the microstructure of the coatings were relatively even (Fig.4c and d),probably owing to the drastic Mar-angoni convective current in the molten pool [31].In order to investigate the action mechanism of Y 2O 3,the microstructure of the coating was observed at higher magni fication and analyzed by reference to the EDS results (Fig.5).It could be deduced that the crescent-shape phase (Point 1)was Y 2O 3and the undeveloped dendrite (Point 2)was TiN.Partial N atoms were probably replaced by C atoms as mentioned earlier,which could be con firmed by the small quantity of C in point 2.Besides,Ti,C,Cr,Ni and Co were found rich in point 3,a thin layer around the crescent-shape phase (Y 2O 3).Thus,point 3mainly consisted of TiC.Cr 7C 3,Ni e Ti and Co e Ti intermetallics were probably in situ formed in the same location.Under the non-equilibrium condition,the in situ formed compounds mentioned above could nucleate on the surface of Y 2O 3,forming the core e shell structure.In this case,the Y 2O 3particles were fixed in the matrix of the coating and then the growth of other phases near the core e shell structure was sup-pressed.As shown in Fig.5,the growth of the TiN (Point 2)was suppressed by the Y 2O 3centered core e shell structure (Point 1and 3).In the study of Li et al.[20],the addition of 2.0wt.%Y 2O 3re fined the microstructure of the cladding coatings,improved the micro-structural uniformity,and increased the microhardness,fracture toughness and wear resistance.However,the direct evidence of the action mechanism of Y 2O 3was not presented.In our study,the unmelted Y 2O 3was picked out,as shown in Fig.5.The energy density of the CO 2laser beam used in this study followed typical Gaussian distribution model,resulting in uneven temperature distribution in the molten pool.Hence,partial Y 2O 3remained in the cladding coatings and acted as the nuclei for heterogeneous nucleation.Besides,Y 2O 3also had a dragging effect on the move-ment of grain boundaries,suppressing the growth of TiN (Fig.5).Similarly,the re fining effect of Y 2O 3on the microstructure oftheFig.2.X-ray diffraction diagram of (a)specimen 1,(b)specimen2.Fig.3.Ti e Co binary alloy phase diagram [23].F.Weng et al./Journal of Alloys and Compounds 650(2015)178e 184180Co-based cladding coatings on Ni-based alloy was reported [32].Results showed that nano-Y 2O 3particles acted as the nucleation sites in the molten pool and promoted the formation of equiaxed grains.Fig.6shows the diagram illustration for the action mechanism of Y 2O 3in the re finement of TiN.Firstly,partial Y 2O 3decomposed into Y and O atoms and the added TiN dissolved into the molten pool (Fig.6a).The decomposed O atoms reacted with B and Si intheFig.4.SEM micrographs of the composite coatings in specimen 1(a,c)and specimen 2(b,d)(a,b)the bonding zone,(c,d)the middlelayer.Fig.5.SEM image and the corresponding EDS results of certain phases in specimen 2.F.Weng et al./Journal of Alloys and Compounds 650(2015)178e 184181Co42self-fluxing alloy,forming low density oxidation products floating on the surface of the molten pool (this process was not annotated in Fig.6).Whereafter,TiC was in situ synthesized together with the re-formation process of TiN (Fig.6b).In order to simplify the diagram illustration,the in situ formation processes of other compounds (e.g.Co e Ti and Ni e Ti intermetallics)were not concluded here.In consequence,the in situ formed TiC grew on the residual Y 2O 3,forming the core e shell structure (Fig.6c).The newly formed TiN nucleated and grew up owing to compositional fluc-tuation and energy fluctuation in the molten pool.Corresponding to above analysis,the growth of TiN was restricted by the Y 2O 3centered core e shell structure.Besides,the Y atoms would distribute on the interface of the solid e liquid interface and further restrain the growth of TiN.Previous studies [33,34]have also indicated that Y 2O 3can decompose into Y atoms and O atoms in the molten pool.As a surface-active element,Y has an effect of reducing the surface tension and the interfacial energy,which is bene ficial to the wet-ting effect between the reinforced phases and the melt as well as the in situ homogeneous nucleation at the interface.Furthermore,Y tends to accumulate at the interface of the crystalline phases and the grain boundaries,in fluencing the microstructural morphology and suppressing the growth of the crystals,respectively.Moreover,the decomposition of Y 2O 3consumed lots of energy and the longevity of the molten pool was reduced.Hence,the TiN in specimen 2had no enough time to grow up into developed den-drites.In the study of Sun et al.[35],the glass forming abilities of Al e Ni e Ce,Al e Ni e La and Al e Ni e Y were investigated and the Al e Ni e Y system showed a higher glass forming ability.Besides,the Al e Ni e Y amorphous phases have been reported in the cladding coatings [36].Though Al,Ni,and Y all existed in the molten pool,Al e Ni e Y amorphous phase was not spotted out in this paper,which might be attributed to the limited amount of Al and Y.Developing cladding coatings based on the Al e Ni e Y materials system might be a new trend in future studies.In summary,the effect of Y 2O 3on laser cladding process is probably a comprehensive function of the initial Y 2O 3and the decomposed Y atoms in the molten pool.However,the effect of Y element in laser cladding stays at the theoretical analysis.More intensive interface studies should be done to con firm the distri-bution of Y atoms at the interfaces.3.2.Microhardness and wear resistanceThe microhardness distributions of the two specimens are shown in Fig.7a.Results indicated that the microhardness of the laser cladding coatings were as high as 3e 4times the Ti e 6Al e 4V substrate.The enhancement of the microhardness was mainly attributed to the comprehensive effects of solution strengthening (g -Co/Ni)and dispersion strengthening (TiN,TiC,Cr 7C 3,TiB,Ti 5Si 3,TiC 0.3N 0.7,Ni e Ti and Co e Ti intermetallics),in accordance with the XRD results.By comparison,the average microhardness of the cladding layer on specimen 2with Y 2O 3addition (~1197.9HV 0.2)was higher than that of specimen 1(~1078.6HV 0.2).Thus,the addition of Y 2O 3enhanced the microhardness of the coating.As mentioned above,Y 2O 3could re fine the microstructure of the specimens signi ficantly.And then the microhardness increased owing to the comprehensive effects of re finement strengthening and dispersion hardening.Fig.7b shows the dry sliding wear test results of the cladding coatings and the Ti e 6Al e 4V substrate.The wear resistance of the cladding coatings were better than the bare Ti e 6Al e 4V substrate and specimen 2(with Y 2O 3addition)exhibited the lowest wear mass loss,as shown in Fig.7b.The average wear mass loss rates of specimen 1,specimen 2and the substrate were3.150Â10À4g min À1, 2.525Â10À4g min À1and2.995Â10À3g min À1,respectively.Thus,the wear resistance of the laser cladding coatings was enhanced by 9.5and 11.9times compared with the Ti e 6Al e 4V substrate,evaluated by the relative wear resistance.The test results of wear resistance agreed well with the microhardness.In general,higher hardness results in better wear resistance,as reported by the relevant studies on the laser cladding [37,38].In another word,the cladding coatings showed a well matching of strength and toughness,otherwise the abnor-mality relationship between the hardness and the wear resistance would be obtained [39].Moreover,sound metallurgical bonding was obtained between the cladding coatings and the substrate (Fig.4a and b),propitious to their application under harsh wear conditions.The worn surface morphologies were observed by SEM,as shown in Fig.8.It could be concluded that the bare Ti e 6Al e 4V substrate and the cladding coatings showed different wear mech-anisms owing to the different characteristics of worn morphol-ogies.The worn surface of the Ti e 6Al e 4V substrate showed deep grooves and slight plastic deformation,owing to the low micro-hardness (~350HV 0.2).During the wear process,the hard asperities on the surface of the grinding wheel could easily penetrate into the sliding surface of the Ti e 6Al e 4V substrate,forming micro-cutting and resulting in deep grooves (Fig.8a).Besides,spalling feature appeared on the worn surface of the substrate due to the alter-nating stress from the grinding wheel [40].By comparison,the micro-cutting process was notably suppressed owing to the higher microhardness of the coatings.Hence,the worn surfaces of the coatings on specimen 1and specimen 2were smoother (Fig.8bandFig.6.The diagram illustration for the action mechanism of Y 2O 3:(a)dissolution of Y 2O 3and TiN,(b)re-formation of TiN and in situ formation of TiC,(c)growth of TiN,TiC and the distribution of Y atoms.F.Weng et al./Journal of Alloys and Compounds 650(2015)178e 184182c).In addition,slight spalling also appeared on the worn surface of specimen 1without Y 2O 3addition (Fig.8b).During the dry sliding wear test,micro-cracks easily formed on the interfaces between the reinforced phases and the matrix.The micro-crack would propa-gate and lead to spalling under the effect of alternating stress from the grinding wheel.Fig.8c presented the worn surface morphology of the cladding coating with Y 2O 3addition.It could be recognized that the reinforced phases were tightly embedded in the matrix and distributed relatively evenly,as annotated in the dashed circle of Fig.8c.During the wear process,the embedded reinforced phases could withstand the external load from the grinding wheel effectively.What's more,the microstructure was re fined notably and the microhardness was enhanced further under the effect of Y 2O 3.Subsequently,specimen 2with Y 2O 3addition exhibited bet-ter wear resistance.However,the amount of Y 2O 3should be controlled to a certain extent,otherwise cracks would form.Pre-vious study [39]demonstrated that 3wt.%Y 2O 3led to the pro-duction of micro-cracks in the cladding coating,detrimental to the wear resistance.4.ConclusionsIn this study,composite coatings were fabricated on Ti e 6Al e 4V substrate by laser cladding with Co42self-fluxing alloy,TiN and Y 2O 3mixed powders.Microstructures and phase constituents of the cladding coatings with and without Y 2O 3addition were investigated comparatively.Results showed that the coatings were mainly comprised of g -Co/Ni,TiN,CoTi,CoTi 2,NiTi,TiC,Cr 7C 3,TiB,Ti 5Si 3and TiC 0.3N 0.7phases.The composite phase constituents were bene ficial to the wear resistance of the coatings.The coatings showed metallurgical bonding with the substrate without anypores and pared with the Ti e 6Al e 4V substrate,the microhardness and wear resistance of the coatings was enhanced by 3e 4times and 9.5e 11.9times,respectively.With 1.0wt.%Y 2O 3addition,the microstructure of the coating 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