不锈钢表面激光熔覆Ni-WC复合涂层

Wear resistance of diode laser-clad Ni/WC composite coatings at different temperaturesZhikun Weng a ,Aihua Wang a ,⁎,Xuhao Wu b ,Yuying Wang a ,Zhixiang Yang aaState Key Laboratory of Material Processing and Die and Mould Technology,School of Materials Science and Engineering,Huazhong University of Science and Technology,Wuhan,430074,PR China bRuian Demonstration Center for Laser Application,Ruian 325207,PR Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 21February 2016Revised 27June 2016Accepted in revised form 29June 2016Available online 1July 2016Ni/WC composite coatings with different weight percentage (0–60%)of WC particle were produced on a stainless steel by diode laser-cladding technology with the aim to improve wear resistance of the stainless steel in the present study.The effects of laser power,WC particle content and rare earth element (La)on the quality of the coatings were investigated.The in fluences of WC content on microstructure and hardness were investigated.The friction and wear behavior of the laser-clad coatings at room temperature and elevated temperatures of 600°C and 700°C were evaluated using a ring-on-block tribometer.Results revealed that the laser-clad compos-ite coatings with WC content ranging from 20wt.%to 60wt.%were free of cracks and pores by controlling laser power level and adding 0.4wt.%La.An increase in WC content increases wear resistance signi ficantly at three test temperatures except for the Ni-20%WC coating.The phase structure of the oxidation films formed during the wear test process played important role on the wear behavior of the laser-clad coatings.©2016Published by Elsevier B.V.Keywords:Laser claddingNi/WC composite coatings Wear resistance High temperature Oxidation film1.IntroductionNickel-based alloys are widely used in the severe environment suf-fering from high temperature,wear,corrosion,impact and fatigue for its high wear resistance and corrosion resistance.However,single nickel-based alloys are hard to meet the requirements of the work piece under severe posite coatings consisting of metallic matrix and reinforcement possess comprehensive properties,which have been widely employed to improve the lifetime of components such as roller,wearing plates,piston rods and turbines.The wear resis-tance of the Ni-based alloy coatings can be increased by the addition of hard ceramic particles such as WC,TiC and VC [1,2].Tungsten carbide (WC)has been extensively used as a reinforcement in Ni-based alloys for its good wettability with Ni,high hardness (2500~2700HV)and high wear ser cladding has been an effective approach to build a metal matrix composite coating.However,good quality of metal-ceramic composite coating without any defects such as pores and cracks is hard to achieve for the different properties between metal matrix and reinforcement,and the heat damage of WC particle during laser cladding [3].Other authors have studied that the pores formed in the coatings are mainly resulted from dissolution of WC par-ticles [4]and gas trapping due to large fluid viscosity induced by WC particles in the melt pool [5].Pores in the coating are increased withthe increase in laser power and high laser power can reduce the WC content in the coating [6].Furthermore,many methods have been put forward to eliminate the cracks in the composite coating during laser cladding,such as preheating the substrate [7],functionally graded coat-ing [8],adding the rare earth or nano-particles [9]and the laser induc-tion hybrid rapid cladding [3].Some alloy elements such as Cr have an obvious effect to cracking sensitivity [10].Recently,various investigators have put to developing different methods to improve the quality of the Ni/WC composite coating.Farahmand et al.reported laser cladding of Ni-60%WC composites using a diode laser with induction heating and the addition of nano-WC and La 2O 3,the composite coatings without any defects possess good mechanical properties in this study [9].Tobar et al.investigated laser cladding of NiCrBSi –WC composite coatings on stainless steel using a 2.2kW industrial CO 2laser and results showed that dense and pore free layers could be obtained as far as the WC content was main-tained below 50wt.%[11].Zhou et al.achieved the NiCrBSi +35wt.%WC composite coatings free of cracks and pores produced by laser induction hybrid rapid cladding [12].There are plenty of scienti fic investigations containing information about wear resistance of laser-clad Ni/WC composite coatings.Howev-er,most of the investigations were concentrated on the friction and wear behavior of the coatings at room temperature.Huang et al.inves-tigated the abrasive wear performance of laser-clad WC/Ni layers pro-duced on H13tool steel substrates with a pulsed Nd:YAG laser,the abrasive wear performance of the layers was 5–10times higher than that of the substrate [13].Fretting and wear behaviors of Ni/nano-WCSurface &Coatings Technology 304(2016)283–292⁎Corresponding author at:School of Materials Science and Engineering,Huazhong University of Science and Technology,Wuhan,430074,PR China.E-mail address:ahwang@ (A.Wang)./10.1016/j.surfcoat.2016.06.0810257-8972/©2016Published by ElsevierB.V.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 tcomposite coatings in dry and wet conditions were studied by Benea, results showed that the Ni/nano-WC coatings have higher nanohardness and wear resistance in dry and wet conditions as com-pared with pure Ni coatings[14].Guo et al.investigated the wear resis-tance of NiCrBSi and NiCrBSi/WC–Ni composite coatings at500°C,they found that the NiCrBSi/WC–Ni composite coating showed better high temperature wear resistance than NiCrBSi coating and the composite coating only experienced mild abrasive and fatigue wear when sliding against the ceramic counterpart[15].Xu et al.have found that the wear resistance of Ni/WC composite coating is strongly dependent on the content of WC and microstructure of the coating and a defined amount of WC in the coating can possess a best wear resistance[16]. The excellent wear resistance of the WC/Ni composite coating is the combined result of the touch Ni-based matrix,the distribution of the WC particles and the good bonding between the WC particles and the matrix[17].So far,few publications are currently available about the wear behavior of the laser-clad Ni/WC composite coatings with differ-ent content of WC,in particular,at different test temperatures.Therefore,the aim of this article was to investigate the influences of laser power,WC content and the addition of element La on the macroquality,microstructure and hardness of the laser-clad composite coatings.Furthermore,the effect of temperature on the wear behavior of the composite coatings was investigated.2.Materials and experimental procedureIn this experiment,an austenitic stainless steel AISI304with the di-mension of200mm×150mm×12mm was used as the substrate.The clad material used in this study was a mixture of a Ni-based self-fluxing powder(see Table1)and a crushed WC powder.The size of the powder particles was in the range of80μm–100μm for the Ni-based alloy and 45μm–100μm for the WC powder.Four different mixed powders with the WC contents of0,20,40and 60wt.%were performed in the experiments.The substrate surfaces were polished with sand paper and cleaned with acetone before the experiments.Laser cladding was carried out using a3kW high-power diode laser (DILAS SD3000/S)with980±10nm wavelength.An off-axial auto-feeding powder equipment was used as the powder feeder and the lat-eral nozzle was kept at an angle of45°to the horizontal.Both the laser and the nozzle werefixed to a6-axis KUKA robot system.Finally, argon gas was used as shielding and powder carrying ser process-ing parameters used in the experiments included the following:laser power(P)was in the range of1.3–1.9kW at a scanning speed(V s)of 11mm/s,the laser spot diameter was kept constant ing these parameters power densities and interaction time between66.2 and96.8W/mm2and0.45s,respectively,were achieved.The powder delivery velocity(V p)was set at25–26.5g/min.A50%overlap ratio was used for multi-track laser cladding.Prior to cladding,the substrate was preheated to300°C to reduce thermal stress and then to avoid cracking of the laser-clad coatings.After laser cladding,a liquid dye penetrant testing was used to detect the crack presence in the clad area.The transverse cross sections of each laser-clad coating were prepared for the microstructure analysis.The cross-sectional samples were polished and etched by75%HCL and 25%HNO3to reveal the microstructure of the coating.Microstructure characterization was analyzed using an optical microscopy(XJL-03) and environmental scanning electron microscope(ESEM Quanta200 with EDX microanalysis system equipped with light elements).The phase structures of the laser-clad coatings were analyzed by X-ray dif-fraction using Cu-Kαradiation at40kV and30mA(XRD-7000S).Microhardness of the coatings was measured using a Vickers-1000 tester,with12s dwelling time and under load of200g.The average hardness of the coatings was measured using a Rockwell hardness tes-ter.Friction and wear behavior of the laser-clad coatings were evaluated using an MM-U10G ring-on-block wear tester at room temperature, 600°C and700°C.Fig.1shows the scheme of MM-U10G wear tester. The sliding was performed at laser-clad coating block(with dimension of30mm×30mm×12mm)sliding against the Ni-60%WC laser-clad composite coating ring.The sliding tests were carried out under ap-plied loads of200N,rotated at50rpm and180min application time in air.The friction coefficient was continuously recorded by the computer connected to the tester.Prior to sliding test,the specimens were all ground and polished by a grinder and then washed in acetone.The wear mass loss of the samples was determined by an electronic analyt-ical balance with an accuracy of0.1mg.The worn surfaces were ana-lyzed using SEM and color3D violet laser scanning microscope(VK-×1000).Oxide phases formed after high temperature wear test were conducted by Raman spectroscopy using a LabRAM HR800Raman spec-trometer with a laser wavelength of532nm.The spectra were mea-sured in the range100–1600cm−1with a laser power of50mW.3.Results3.1.Effect of power density and WC content on the quality of the laser-clad coatingsThe cross-sectional micrographs of the laser-clad Ni-60%WC coat-ings produced at laser power density ranging from66.2W/mm2to 96.8W/mm2are shown in Fig.2.With the increase of laser power den-sity to a higher value,the porosity of the composite coatings increases clearly.The pores in the clad coatings tend to concentrate at the over-lapped zone where less WC particles are distributed,and the amount of WC particles decreases significantly when the power density in-creases from66.2W/mm2to96.8W/mm2.Sound laser-clad coatings without any cracks and obvious pores could be produced at power den-sity66.2W/mm2and preheating temperature300°C,as shown in Fig.2(a)and Fig.3.Fig.4shows the cross-sectional micrographs of the laser-clad coat-ings with different WC content(0,20,40and60wt.%).It can be seen that no cracks are observed through the laser-clad coatings,the crushed WC particles are distributed homogenously with the appropriate laser power and scanning velocity.Most of the WC particles remain in theirTable1Composition(in wt.%)of the Ni-based alloy powder.Elements C Cr Fe Ni Mo Si Co Ni0.3 4.0 6.6bal.0.1 3.20.1Fig.1.Schematic illustration of the wear test. 284Z.Weng et al./Surface&Coatings Technology304(2016)283–292initial shape.The pores on the overlapped tracks could not be thorough-ly eliminated by optimizing the processing parameters alone when the WC particle content is as high as 60wt.%.Therefore,0.4wt.%La is intro-duced into the Ni-60wt.%WC coating material with an aim to eliminate pores,the addition of La to the coating was performed in a powder shak-er using agate balls to avoid powder agglomeration for a duration of 2h before laser cladding.Fig.5shows the cross-section of the laser-clad Ni-60wt.%WC-0.4wt.%La coating at power density 66.2W/mm 2,interac-tion time 0.45s,overlapping ratio 50%and substrate temperature 300°C.Signi ficantly,no pores or cracks are found in the laser-clad coat-ing,which means that the addition of La eliminated pores effectively.3.2.Microstructure characteristics of the laser-clad coatingsFigs.6(a)–(d)show the microstructure of the laser-clad coatings with different WC content.The microstructure of the laser-clad pure Ni-based alloy coating is characterized by fine long primary dendrite and short second dendrite,as shown in Fig.6(a).The addition of WC particles changes the dendrite morphology of the Ni-based matrix.The microstructure of the laser-clad Ni –WC coatings changes gradually from dendrite to equiaxed grain with the increase of WC content from 20wt.%to 60wt.%,and a small amount of WC fragmentation was ob-served in the Ni-based matrix for the Ni-60%WC composite coating.The degree of dissolution of WC particles is so low that few precipitatedcarbides can be seen around the WC particle.The average grain size of the Ni-matrix decreases with the addition of WC particle from 20wt.%to 60wt.%.Fig.2.Cross-sectional micrographs of the laser-clad Ni-60%WC coatings at different power density (V p =26.5g/min).(a)66.2W/mm 2,(b)76.4W/mm 2,(c)86.6kW/mm 2,(d)96.8W/mm 2.Fig.3.Liquid penetrants test morphology for cracks inspection (Ni-60%WCcoating).Fig.4.Cross-section microstructure of the laser-clad coatings with different content of WC (wt.%),(a)0%,(b)20%,(c)40%and (d)60%.Fig.5.Cross-section of the laser-clad Ni-60wt.%%coating.285Z.Weng et al./Surface &Coatings Technology 304(2016)283–292Fig.6.Microstructure characterization of the laser-clad coatings.(a)Ni-0%WC,(b)Ni-20%WC,(c)Ni-40%WC,(d)Ni-60%WC.Fig.7.SEM images of periphery regions of WC particles in (a,b,c,e)Ni-60%WC and (d)Ni-60%WC with 0.4%La.286Z.Weng et al./Surface &Coatings Technology 304(2016)283–292Fig.7presents a SEM image of periphery regions of WC particles in the Ni-60%WC coatings with and without La element.Fine matrix/WC interface was achieved without any defects.Angular WC particles are well embedded in the Ni-based matrix(see Fig.7a).Partially melting on the surface of WC particles usually occurred and then dissolved into the Ni-alloy matrix under the laser beam heating,which resulted in blocky carbides precipitated near the WC particle.The addition of La was not beneficial to refine the microstructure of the matrix,as shown in Fig.7(b)and(d).Based on the morphological feature of the carbides and EDX analysis(see Table2),the white carbide particles (marked A in Fig.7b)are the original cast WC.The W-rich blocky car-bides(marked D in Fig.7b)are distributed in the Ni-based alloy matrix around the original cast WC.The small blocky carbides near the WC par-ticle(marked E in Fig.7d)contained a high concentration of Ni,W and Si.EDX analysis results of matrix dendrite–interdendrite for Ni,Cr,W,Fe and Si by line scanning is shown in Fig.7(e).It can be noted that the amounts of Si,Cr and W are higher in the primary dendrite as compared to that in the interdendrite eutectic structure.And,also the interdendrite eutectic structure contains more amounts of Ni and Fe than in the primary dendrite.Sound metallurgical interface between the coating and the substrate was generated,as shown in Fig.7(c).The XRD patterns of the laser-clad Ni/WC composite coatings withdifferent WC contents are shown in Fig.8.It can be seen that the laser-clad Ni coating consists mainly ofγ-Ni,Cr7C3and Fe3C,while Ni/ WC composite coatings contain new phases such as WC,W2C,Ni4W and Cr23C6.Particularly,the existence of Ni4W suggests the formation of intermetallic phase in the matrix of the laser-clad Ni/WC coating, which indicates the possible reaction between WC or W2C and Ni matrix.3.3.Hardness measurementFig.9(a)shows the microhardness profile of the Ni-based alloy ma-trix along the depth direction of the laser-clad coatings.The undissolved WC particle area in the coating was avoided during microhardness test. It is can be seen that the microhardness of the Ni matrix of the Ni/WC composite coatings is obviously higher than that of the pure Ni coating. The microhardness of the Ni matrix increases slightly with increasing the WC content in the coating,which may be attributed to the partly dissolution of WC particles into the matrix.A microhardness compari-son between the Ni-60%WC coatings with and without La element indi-cates that the addition of La has no contribution to the microhardness of the coating.Fig.9(b)shows the average Rockwell hardness of the mate-rial,It is can be seen that the average hardness of the laser-clad coatings are significantly improved when WC particles are added in the coatings.3.4.Friction and wear behavior of the laser-clad coatingsThe weight loss of the laser-clad coatings at various temperatures is illustrated in Fig.10.At the given test temperature of both room tem-perature and600°C,the weight loss of the coatings decreases with an increase in WC content,indicating that wear resistance is improved sig-nificantly with increasing the WC content in the laser-clad coatings.The laser-clad Ni-60wt.%WC coating exhibits the best wear resistance at room temperature,600°C and700°C among all the laser-clad coatings.The variation of friction coefficients of the laser-clad coatings with sliding time at different temperature is shown in Fig.11.It can be seen that the Ni-60%WC composite coating possess the lowest friction coefficient as compared with other coatings at room temper-ature.The friction coefficients of the coatings initially increase and then decrease with increasing the content of WC at600°C,and for the same WC content,the friction coefficients of all the coatings de-crease to lower value at700°C in comparison with the friction coeffi-cients at600°C.3.5.Worn surface morphologiesIn order to understand the wear mechanism of the laser-clad coat-ings,the worn surfaces of the coatings are examined by SEM and3D vi-olet laser scanning microscope.Fig.12shows the worn surfaces of the coatings at room temperature,and Fig.15illustrates3D surface map-ping of the corresponding wear scars.Deep grooves and large pits are observed on the worn track of the samples containing0to20wt.% WC,as shown in Figs.12(a)–(b).The grooves become much shallower and the pits become smaller for the test samples containing40wt.%to 60wt.%WC,as shown in Fig.15(a).The worn surface of the Ni-0%WC coating shows signs of adhesion wear as well as severe plastic defor-mation,as shown in Fig.12(a)and Fig.15(a).Contrary to the Ni-0% WC coating,no obvious sign of adhesion wear is visible and only shallow grooves on the worn surfaces of the laser-clad coatings con-taining20%to60wt.%WC,indicating that these composite coatings experience abrasive wear.The WC particles in the surface region of Ni-20%WC coating have also become dislodged during running-in. For the samples with40%and60wt.%WC,no obvious sign of plastic deformation is visible and the WC particles possess good combining with the matrix.After the wear testing at high temperature between600°C and 700°C,oxidation happened during wear processing.At600°C,the worn surfaces of all the samples show signs of adhesion wear and severe spalling of oxidefilm,as shown in Fig.13and Fig.15(b),indicating that these samples experience adhesion wear and oxidation wear.The WC particles in the surface region of Ni-20%WC and Ni-40%WC coatings have become dislodged and shallow grooves are visible.As the temper-ature increases to700°C,fine oxidefilm forms and no obvious peeled off as large pieces on the surface of the samples(see Fig.14).However, large pits are observed on the worn track of Ni-20%WC coating,which possess the size approximate to the WC particle(see Fig.14b and Fig.15c),indicating that WC particles pull out from the matrix.Table2Element concentration of periphery regions of WC particles.ElementC(wt.%)W(wt.%)Cr(wt.%)Fe(wt.%)Ni(wt.%)Si(wt.%) PointA 5.394.70000B 6.289.20.60.4 3.60C 2.719.5 2.1 4.868.2 2.8D 4.876.8 3.5 1.014.00E7.810.3 2.7 4.466.47.8Fig.8.XRD patterns of the laser-clad coatings with different contents of WC.(a)Ni-0%WC, (b)Ni-20%WC,(c)Ni-40%WC and(d)Ni-60%WC.287Z.Weng et al./Surface&Coatings Technology304(2016)283–2924.Discussion4.1.Microstructure characteristics of the laser-clad Ni –WC coatings During the laser-cladding process,the excessive heating input re-sulted in the partial dissolution of the WC particles and generating car-bon.The carbon reacted with the atmospheric oxygen and generated gas (CO and CO 2),which had no enough time to flee from the molten pool after rapid solidi fication.As a result,pores formed in the laser-clad Ni/WC coatings [4].Furthermore,the amount of pores in the laser-clad coatings increased with an increase in laser power density,which would produce a higher degree of WC heat damage and generat-ed more gas.The pores tend to concentrate at the overlapped zone where less WC particles were observed,this can be explained by the laser-induced overheating and thus decomposition to the WC particles at the overlapped zone.The pores in the Ni-60%WC composite coating cannot be thoroughly eliminated as the dissolution of WC at the over-lapped zone and gas trapping due to large fluid viscosity induced by a large amount of WC particles in the melt pool and the encapsulated bubbles [18].The Ni-60%WC composite coating free of pores when La is added can be explained as follows:(1)the rare earth La is a surface active element and it may easily react with oxygen,silicon,nitrogen and other harmfulelements such as sulfur in the melt pool [19,20],some stable com-pounds such as La 2O 3,LaCrO 4and LaNi 8C 2can be formed during laser element can decrease the activity of carbon [21].Therefore,La reduces the amount of oxygen available for reaction with carbon to form CO 2/CO gas porosity during laser cladding.(2)La atom tends to be mobile in the molten pool as this element is a very active element,La atom has a relatively lower surface tension than Ni,which resulted in improving the wettability and flowability of Ni with respect to WC particles [9].As a result,the addition of La can effectively reduce the pores in the composite coating under a deoxidizing and re fining action.As compared the microstructure and microhardness of the Ni-60%WC coating with and without La element,it is seen that the addition of La has no contribution to the microstructure and microhardness of the matrix,which indicated that the addition of La would not effectively dissolve into the matrix of the coating.During cladding process,most of the La combined with oxygen and other harmful elements in the melt pool,which resulted in the formation of high melting point compounds.The compounds have lower density than that of the Ni-based alloy and floated on the liquid phase before solidi fication,then,slag may occur on the surface of the laser-clad coating.Results suggest that the increase of WC content causes the dendrite size of the Ni binder decreased.This can be attributed to the change in the nucleation.Because the melting point of WC is higher (about 2700°C)than that of the Ni-based alloy binder (about 1100°C),when the coatings cooled from the liquid state,the WC particles experienced just a partial melting status.The undissolved WC particle acted as a small heat sink that would increase the nucleation rate and promote grain nucleation [22].4.2.Analysis of wear behaviorThe results of wear test reveal that the content of WC particles had signi ficant in fluence on the wear resistance and wear mechanisms of WC reinforced composite coatings at different temperatures.From Fig.10,it can be seen that the wear resistance of the laser-clad coatings are improved by the high content of WC particles in the coatings.In room temperature,three wear mechanisms are identi fied in this study,namely,adhesion wear,abrasive wear and fatigue wear.For the coating without WC,multi-plastic deformation and adhesive wear dominate the wear process of the coating,as obvious plastic deforma-tion and adhesive desquamation appeared on the worn surface of Ni coating (Fig.12a).By increasing the percentage of WC,abrasive wear takes more relevance and adhesive wear decreased,which can be ex-plained that WC particles can reduce adhesive wear [18].During the wear process,the hard WC particles can protect the soft metalmatrixFig.9.Hardness measurement of the laser-clad coatings.(a)Microhardness pro files of the Ni-based alloy matrix of laser-clad coatings,(b)average hardness of the laser-cladcoatings.Fig.10.Weight loss of the laser-clad coatings at different temperature.288Z.Weng et al./Surface &Coatings Technology 304(2016)283–292Fig.11.Variation of friction coefficients of the laser-clad coatings at different temperature with sliding time.(a)room temperature,(b)600°C and(c)700°C.Fig.12.SEM images of worn surfaces at room temperature.(a)Ni-0%WC,(b)Ni-20%WC,(c)Ni-40%WC,(d)Ni-60%WC.289Z.Weng et al./Surface&Coatings Technology304(2016)283–292from being cut by the counterpart to restrain abrasive wear,and the par-ticles are hard to pulled out from the matrix as the good bonding be-tween the WC particles and the matrix binder.Therefore,the addition of WC particle effectively improves the wear resistance of the composite coatings as the comprehensive results of “protect function ”of the WC particles and “support function ”of the Ni matrix [23].When the temperature is increased to 600°C ~700°C,the wear mechanism of the coatings changes,namely,adhesive wearandFig.13.SEM images of worn surfaces at 600°C.(a)Ni-0%WC,(b)Ni-20%WC,(c)Ni-40%WC,(d)Ni-60%WC.Fig.14.SEM images of worn surfaces at 700°C.(a)Ni-0%WC,(b)Ni-20%WC,(c)Ni-40%WC,(d)Ni-60%WC.290Z.Weng et al./Surface &Coatings Technology 304(2016)283–292oxidation wear are the main way at high temperature.The mean wear mass of the coatings increases from room temperature to600°C,but it turns to decrease at700°C(except Ni-20%WC coating).In order to un-derstand this phenomenon,the composition of the oxides on the worn tracks generated during the high temperature wear experiments were analyzed by Raman spectroscopy.The Raman spectra of the laser-clad coatings after wear tested at600°C and700°C in air are shown in Fig.16.These analyses showed that oxides of the worn surface on the pure Ni-based alloy coating mainly consists of NiO,NiCr2O4and NiFe2O4oxides while the Ni/WC composite coating mainly consists of mixtures of WO3oxide and NiWO4tungstates.At600°C,the laser-clad coatings became softening and plastic deformation improved.Fur-thermore,the thinner oxidation layer with poor mechanical properties could be removed during the friction process.Thus,the laser-clad coat-ings exhibit a lower wear resistance at600°C.After the test temperature increased to700°C,the oxidation rate of the coatings increased signifi-cantly allowing rapid and easy formation of a continuous oxide layer on the worn surfaces(see Fig.14).These results are in good agreement to Raman analysis(see Fig.16),which showed essentially strong signals of the oxides at700°C.The oxide acts as a protective tribofilm decreas-ing the wear loss of the coatings.Similar results were achieved and ex-plained by Fernandes and Polcar[24].Additionally,more NiWO4and WO3oxides were produced at the worn surface on the Ni/WC compos-ite coatings.The WO3with the structure of magnéli phase can act as a solid lubricant to reduce the friction coefficient[25]and the numerous NiWO4particles can increase the hardness and touchness of the worn coating surface[26].As a result,the friction coefficient of the coatings decreased,which indicates that oxidation inhibits wear.However,for the Ni-20%WC coating with a low ceramic content,a lot of WC particles pulled out from the matrix due to the large deformation of the matrix during friction process at700°C.Then the hard WC debris as the three bodies entrap into the contact surfaces and aggravate the abrasive wear.In such a case,the wear resistance of Ni-20%WC coating presents the worst among the coatings at600°C.5.Conclusions(1)Sound laser-clad Ni/WC composite coatings free of pores andcracks were successfully produced by diode laser.The power density and WC content have significant effects on the quality of the coatings.With the increase of power density and WC con-tent to higher value,porosity increases obviously in the coatings.Furthermore,the addition of La could effectively reduce porosity in the compositecoatings.Fig.15.3D topography of the worn surfaces of the laser cladding coatings with different wt.%WC after various temperature tests.(a)room temperature,(b)600°C and(c)700°C.Fig.16.Raman spectra of the worn surface on the laser-clad coatings at high temperature.(a)Ni-0%WC and(b)Ni-60%WC.291Z.Weng et al./Surface&Coatings Technology304(2016)283–292。