Fang LUO*，Anrw COCKBURN，Martin SPARKES，**，Roo LUPOI，Zhi-jun CHEN，W illiam O'NEILL，Jian-hua YAO，Rong LIU

aCollege of Zhijiang，Zhejiang University of Technology，Hangzhou 310024，Zhejiang，China

bInstitute for Manufacturing，Departmentof Engineering，University of Cambridge，CB3 OFS，UK

cDepartmentofMechanical&Manufacturing Engineering，Trinity College Dublin，Dublin 2，Ireland

dCollege of Mechanical Engineering，Zhejiang University of Technology，Hangzhou 310012，Zhejiang，China

eDepartmentofMechanical&Aerospace Engineering，Carleton University，1125 Colonel By Drive，Ottawa，Ontario K1S 5B6，Canada

Received 26 February 2014；revised 29 August 2014；accepted 19 September 2014 Available online 26November 2014

Performance characterization of Ni60-WC coating on steel processed w ith supersonic laser deposition

Fang LUOa，*，Andrew COCKBURNb，Martin SPARKESb，**，Rocco LUPOIc，Zhi-jun CHENd，W illiam O'NEILLb，Jian-hua YAOd，Rong LIUe

aCollege of Zhijiang，Zhejiang University of Technology，Hangzhou 310024，Zhejiang，China

bInstitute for Manufacturing，Departmentof Engineering，University of Cambridge，CB3 OFS，UK

cDepartmentofMechanical&Manufacturing Engineering，Trinity College Dublin，Dublin 2，Ireland

dCollege of Mechanical Engineering，Zhejiang University of Technology，Hangzhou 310012，Zhejiang，China

eDepartmentofMechanical&Aerospace Engineering，Carleton University，1125 Colonel By Drive，Ottawa，Ontario K1S 5B6，Canada

Received 26 February 2014；revised 29 August 2014；accepted 19 September 2014 Available online 26November 2014

Ni60-WC particles are used to improve thewear resistance of hard-facing steel due to their high hardness.An emerging technology that combines laser with cold spraying to deposit the hard-facing coatings is known as supersonic laser deposition.In this study，Ni60-WC is deposited on low-carbon steel using SLD.Themicrostructure and performance of the coatings are investigated through SEM，opticalm icroscopy，EDS，XRD，microhardnessand pin-on-discwear tests.The experimental resultsof the coating processed with the optimalparametersare compared to those of the coating deposited using laser cladding.

Ni60-WC；Supersonic laser deposition；Laser cladding；M icrostructure；Wear

1.Introduction

Ni60 alloy，in combination w ith hard tungsten carbide（WC），isw idely used to improve thewear resistanceof steelas surface coating.Ow ing to an excellentwear resistance，Ni60-WC has been applied in many fields of industry，such as m ining，transport，engineering，manufacturing industries，etc. However，Ni60-WC coatings have been found often to have cracksand pores in them.In recent years，the researchershave attempted to apply various deposition techniques on this type of coating to achieve the crack/flaw-free coatings.The traditional techniques，such as thermal spraying，plasma spraying， arcwelding，and laser cladding（LC），have often been used to produceWC-reinforced Ni60 composite coatings.Wang etal.［1］studied the abrasive resistance of Ni-based coatings w ith WC hard phase prepared by plasma spraying w ith laser posttreatm ent，and tested the abrasive wear resistances of three Ni-WC com posite coatings w ith different contents of WC particles.The results show that the best coating contains the composition of Ni60+60%WC（w t%）but ithas the pores in it.Although the cold spraying techniqueoffershigh deposition efficiency and low oxidation and retains the initial composition and/or phases of coating materials，it cannot be used to deposit the hard particles.In contrast，the arc welding and laser cladding techniques can be used to deposit the hard particles and offer a good metallurgical bonding because a molten pool is formed on the surface of the substrate，but the coating and substrate can be severely deformed due to excessive energy input.In these processes，WC particles are largely dissolved in the melt pool during deposition，whichreduces the hardness of the final coating.Additionally，the thermal stresses induced during the course of welding or cladding can lead to crack formation in the coating，and the low cladding efficiency increases the processing cost of cladding over large areas［2，3］.Zhou etal.［4-6］observed the cracks in the Ni60 coatings deposited by laser cladding but achieved fully dense and crack-free Ni-based WC composite coatings and Ni60 coatings prepared using laser induction hybrid rapid cladding w ith an elliptical spot.However，this technique restricts the geometry of substrate to simp le shape，such as flat，shaft，etc.It is not suited for preparing the comp lex products.The key reason why Ni60-WC coatings cannot be app lied in m any areas is that the carbide particles result in the formation of cracksand pores.Therefore，the contentand size of WC grain have an important effect on the characteristics of the coatings［2，7］.Matthew et al.［8，9］successfully combined laser cladding w ith cold spraying together to prepare high-density coatings consisting of Tiand Ti alloy，and used cold N2as a carrier gas to reduce the cost of cold spraying.The supersonic laser deposition（SLD）method offers many advantages by combining laser w ith cold spraying，in particular，the use of a laser to control the deposition temperature，which allows hard materials，such as Stellite 6 and carbide［10，11］，to be deposited whilemaintains the key advantagesoffered by solid-state cold spraying and replaceshigh costhelium w ith nitrogen.Moreover，the high-speed impactof particleson the substrate produces severe plastic deformation，resulting in a good bonding of the coating and the substrate. Meanwhile，SLD avoids the melting of the coating material and therefore retains the finemicrostructure of the coating and preventsWC from degrading.

In the present research，since SLD hasmany advantages，such as high deposition rate，low dilution，it could be used to deposithardmaterialon the steel，etc.This paper dem onstrates the influenceof laser poweron the characteristicsof Ni60-WC composite coatings deposited using SLD and compares the results of those deposited using LC.

Fig.1.Ni60 particles and size distribution.

Fig.2.WC particles and size distribution.

2.Experimentalmethods

2.1.Powder preparation

The coating m aterial was a m ixed pow der consisting of Ni60 and 30%WC w ith different particle sizes.The contentproportion of Ni60 to WC was optim ized from many experimentsof laser cladding，spray remelting and AC-HVAF spray［1，15］.The size of particle in the feed stock powder was analyzed by laser dispersion.The diameter of powder particle was selected based on Refs.［12-17］，and the product of the Ni60 spray powder is commercially available.The diameters ofmost particles in Ni-based self-fluxing alloy are in the range from 30μm to 50μm，asshown in Fig.1，and the diametersof most particles in WC powder are approximately 10μm，as shown in Fig.2.It can be observed that the clusters of WC particlesare stuck together and the individualWC particlesare hardly found.The chem ical composition of Ni60 and the weight percentage of each element are given in Table 1.The m ixed 70%Ni60 and 30%WC powder was deposited on carbon steel substrate using a SLD system［8，9，18］，and the process parameters for a single track are listed in Table 2.

Table 1 Chem ical composition of Ni60 pow der.

Table 2 Process parameters for a single track of SLD.

Fig.3.Characteristics of coating specimen 1.

Fig.4.Characteristics of coating specimen 3.

Initially，the single track was sprayed over a range of operating conditions to identify the laser power ranges required for deposition.To quickly identify the optimal laser power，the operating pressure was kept at 30 bar to give a gas velocity of approxim ately 1000 m s-1and a traverse rate of 10mm s-1for SLD.Low-carbon steel platesw ith dimensions of 160×55×2 mm were used as substrates.

2.2.Microstructure analysis

The obtained SLD coating structureswere examined under an opticalmicroscope，in combination w ith A4i image analysis software.The cross-sections of the coating specimens were polished and then etched w ith the chemical solution which consists of 5 g FeCl3，2 m l HCl and 99 m l ethanol.

A scanning electron m icroscope was used to analyze the surface topography and microstructure of coating specimen. The chem ical compositions of the coatings were evaluated using an energy dispersive spectroscopy（EDS），and the phases in the m icrostructures were identified using an X-ray diffractometer and the X'Pert Pro analysis software.

3.Results and discussion

3.1.Single track for SLD

3.1.1.Coating structure

The coating specimens 1，3 and 5，asnumbered in Table 2，were selected for analysis.The structures of these specimens are shown in Figs.3-5（a），respectively.A large amount ofsmall，brightand white particles，which sizesof the range from 1μm to 3μm are observed from Fig.3（a）；the small pits and poresaredistributed in thematrix.Theelementdistributionsare illustrated in Fig.3（b），show ing peaks and valleys，where the former of the curve（white particle）indicates the peaks：high tungsten and carbon contents，the later（rectangle）indicate the valleys：low tungsten content，and high chrom ium and iron contents.Fig.3（c）shows the scanning graph of elements of coating specimen 1.The brightandwhiteobjectshould beWC，the thin and white dendritic crystals in thematrix are likely to be chromium carbide.These results confirm that the brightand whiteobjectisWC particles，which aredistributed in thematrix w ith FeNi and chromium carbides.Fig.4（a）shows that the largerbrightobjectsaredistributed in thematrixw ith largerpits and pores.Fig.4（b，c）show that the larger polygonal and bright object are WC particles，the small white crystals probably are Cr7C3and Cr23C6，and thematrix may bew ith FeNi.

Fig.5.Characteristics of coating specimen 5.

Fig.6.XRD spectra of Ni60-WC pow der and Ni60-WC coatings.

Fig.7.M icrohardnesses of cross-sections of 5 coating specimens.

Fig.8.Topographies of coating cross-sections.

From Fig.4（a）it can be seen that，for coating specim en 3 which was prepared w ith higher laser power，the sizes of the white particles and the smallwhite crystals increasew ith the increase in laser power；the sizes of some WC particles are over 2μm，and they exhibit partially dendritic crystals，as indicated by arrow at top left corner.W ith the increase in laser power，the largeWC particles（light）of coating specimen 5 in Fig.5 are distributed in thedendritic crystalsof Cr7C3，Cr23C6and FeNimatrix.

It can be seen from Figs.3-5（a）that the crystals of chrom ium carbides grew from thin dendritic crystals to coarse dendritic crystals w ith the increase in laser power；so did the WC particles.Hard phases，such as Cr7C3，Cr23C6and WC particle，are the key factors of improving the hardness and anti-wearing properties of the coatings.

X-ray diffraction（XRD）analysis was conducted on the coating specimens to confirm the phases formed during deposition.The XRD spectra are shown in Fig.6.The phases of the coatingsdeposited by SLD areCr7C3，Cr23C6，F(xiàn)eNi，and WC.This isquite different from the normal laser cladding ofWC-Ni，where WC in the coating hard ly dissolves and is diffused into laser cladding and form compound.

3.1.2.Microhardness of cross-sections

Since the change in the amount of WC results in a large variation in hardness data，the microhardness of coating is m easured along horizontal direction and vertical direction from the interface to the coating surface at constant intervals of 0.4 mm using a digital micro-hardometer.The average hardnessesof five coatingsare shown in Fig.7.It can be seen from Fig.7 that them icrohardnessof coating 1，HV0.3742，is the highest，and themicrohardnessof coating 5，HV0.3687，is the lowest.These results show that the differencesamong the m icrohardnesses are small.Itmay infer that the hardness is dependent on the structure of carbide.Consider this again from Figs.3-7，there is also a correlation among laser power，coating structure and coating hardness.

Fig.9.Porosity analysis of the coatings.

Table 3 Process parameters for single tracks and multi-tracks of LC.

3.1.3.Cross-section topography

Fig.8 shows the topographies of the coating specimens in Table 2.Coating specimen 1 in Fig.8（a）shows the highest density among the coatings.At the top of coating layer appear many bright and fine WC particles，which is consistentw ith the hardness test result.

Many pores and smallWC are observed in coating specimen 2 in Fig.8（b）.These carbides particles are uniform ly distributed in the coating，but their amount is very little.

The pits or pores in the coating are increased w ith the increase in laser power.This is because the coating material starts to melt partially in this case.The sizes of Ni60 and carbide particleswere found to be increased w ith the increase in laser power.Oxygen in air can possibly react with some chem ical elements of the coating，such as carbon，to produce gases，resulting in the formation of pores.Fig.8（c-e）show the pits in the coatings.However，most pitsmay notbe pores，as shown in Fig.9；they are the holes leftwhenWC particles are spalled off during polishing.

Fig.9 presents the analysis of red areas in percentage of pits in the coatings.The dark areasmay represent the pores formed in the SLD process，and the holes due to spallation of WC particles during polishing.It can be seen from Fig.9 that thepercentage of dark areas in coating specimen 1（2 kW laser power）is the least.

Higher laserenergy can result in stronger adhesion between theWC particles and Ni60 matrix，but it can also reduce the WC content.When the laser powerwas increased from 2 kW to 4 kW，the amount of WC particles decreased，as show in Fig.9.However，the laser power was not sufficiently high to m elt the alloy comp letely；the deposited particles were not comp letely melted.Some areas show the good adhesion between the WC particles and Ni-based matrix，w ith the spallation ofWC particles pull-outbeing avoided during polishing. However，notonly the adhesion between the Ni-based coating and WC isaffected by the input energy，but it isalso affectedby the coefficients of thermal expansion of Ni60 and WC.If their coefficients of thermal expansion are the same，the grow th ratesof these particlesare thesame，the amountof pits or pores would be reduced due to good matching w ith each other，the adhesion between the Ni-based coating and WC would be better at the same laser power.

The above analysis indicates that coating specimen 1，deposited under a N2atmosphere at a pressure of 30 bar，a temperature of 450°C and a deposition power of 2 kW，is the best among the coating specimens under study.Single track coatings prepared w ith these parameters were further investigated in dilution rate.

Fig.10.M ulti-track LC coatings.

Fig.11.Cross-section of SLD coating.

Table 4 Optimal process parameters formulti-tracks.

3.2.Comparison of SLD coating with LC coating

Ni60-WC powderwasdeposited on low-carbon steelby LC w ith laser power of 1.8-2.0 kW and scanning velocity of 3-10 mm s-1，as detailed in Table 3.Only one coating（No.2in Fig.10（a））deposited at laser power of 2 kW and scanning velocity of 10 mm s-1was crack-free.

Multi-tracksw ith a deposition w idth of 4mm and overlap of 2 mm were made to investigate the performances of the coatingsunder the deposition conditionsw ith laser power of 2 kW and traverse rate of 10 mm s-1.The cracks were also found in these coatings.The cross-sections of LC coatings are shown in Fig.10（b），whichwere compared w ith the SLD coating having a deposition w idth of 4 mm and three layers under the conditions of optimal process（N2atmosphere，pressure of 30 bar，temperature of 450°C，deposition power of 2 kW and traverse velocity of 10 mm s-1）.The optimal process parameters of multi-track coating are listed in Table 4.

Cracksarehardly found in the firstand second layersof the multi-track SLD coating，but they are observed in the third layer，as shown in Fig.11.The cracking may be due to the phase transition of the material at different temperatures，resulting in the change of volume or stress.

Fig.12.Element line scan of SLD and LC coatings.

Fig.13.Structures of Ni60-WC coatings.

Table 5 Element compositions of LC coating.

3.2.1.Analysis of element dilution

During the LC process，a high pressure gas supp ly was split and delivered to a nozzle directly via a high pressurepow der feeder where the m etal pow der particles are entrained.The temperature in a deposition zone wasmonitored by a high-speed IR pyrometer（Kleiber）.The consistent deposition site conditions weremaintained by using the pyrometer to control the laser power via a PID loop built in to the control software.

Fig.14.Distribution curves ofmicrohardness and friction coefficients for the Ni60-WC coatings.

The substrate was held stationary while the nozzle，laser head and pyrometer can bemoved using a CNC X-Y stage，allow ing the samples to be up to 1600mm in diameter.

Fig.12（a，b）show the line scan of Ni，Cr and W from the interface to the coating（or substrate）along the cross-sections of SLD and LC coatings，the testing distance is 2.7 mm and 2.4mm，respectively.Since thesubstratematerialof SLD does not contain Ni，Cr and W，these results exhibita low dilution rate from the coating to the substrate for SLD.

Asillustrated in Fig.12（c），the contentsofNi，CrandW in the coatings are higher，and the contents of Ni，Cr and W in the substratesare lower.ThecontentsofNi，W and Crin SLD coating are higher than those in LC coating.Thisshow s that theW orCr reactedw ithC-formingcarbidesin theLCcoating，andNireacted w ith Fe-forming FeNi，which resulted in thedecrease of theelements in the LC coating，that is，higherelementdilution.

Fig.15.Structures of coating cross-sections.

3.2.2.Microstructural analysis

Asshown in Fig.13（a），the smallpolygonaland brightWC particlesare embedded in Ni60matrix.The pits in blackmay be the holes due to the spallation of WC particles during polishing or the pores formed due to the impact among the particles.

Fig.13（b）show s the clear formation of dendritic crystals. Thewhite eutecticswere distributed along the crystal boundary，and the circular areas of the LC coating weremeasured and analyzed using an energy dispersive X-ray（EDX）detector.The resultsare presented in Table 5.It can be seen that the iron and nickel contents in area A are higher than those in area B，and the tungsten and carbon contents in area B are higher than those in area A.These results，together w ith X-ray diffraction（XRD）analysis results in Fig.6，were used to identify the eutectics form ed during the LC deposition.The phases of the coating deposited using LC are Cr7C3，Cr23C6，F(xiàn)eNi，and Fe7W6；the LC coating has no WC because Fe reactsw ithW-forming Fe7W6.Thismay be attributed to the fact that the inputenergy of LCwasmuch higher than thatof SLD，which led to the formation of Fe7W6.If the processparameters in Table 4 are used，the energy density of SLD is the same as the optim ized energy density of LC，but the laser melts the powder in LC，while the laser heats the deposition area inSLD.Therefore，for LC，themain phase in area A is FeNi，and themain phases in area B are Cr7C3，Cr23C6and Fe7W6.

A comparison of the phases among Ni60-WC powder，LC coating and SLD coating shows that B2O3is not present in SLD and LC coatings.The reason for thismay be that the area B is oxidized when itwas exposed in air during the powder preparation.B2O3would be vaporized atover 1500°C.

Fig.16.Worn surface of Ni60-WC coating prepared using SLD.

Table 6 Element compositionsof debris on SLD coating.

3.3.M icrohardness and wear resistance

The frictional and wear forces to which machinery components are subjected result in great loss of energy and material.Hardness is a key property that accounts for the antiwearing ability of materials.Thermal spraying，laser cladding and other surface techniques can be used to deposit the hard particles on the substrates to improve their hardness and wear propertiesagainst the frictional and wear forces thatact on the surfaces of components［15，19-24］.

Fig.14（a）provides the averagem icrohardnessesmeasured for 3 or 5 indentations at constant longitudinal interval of 0.3 mm from the interface to the coating surface（or substrate）using a digital micro-hardometer.The friction coefficients were measured using a pin-on-disc machine with a Si3N4ceramic ball having 5 mm in diameter.The rotational radius was 2.5 mm，the normal load was 200 g，the rotational speed was800 rmin-1，and the testduration timewas2 h.Asshown in Fig.14，the highesthardness value is HV0.3690 that is for the SLD coating.This is due to the fact that the amount of carbides in the coating ashard phases isgreater than that in the LC coating.The lowesthardness value is HV0.3394 that is for the LC coating.This is because of the higher energy density induced by decomposing amassof carbidesashard phases to form Fe7W6.Moreover，F(xiàn)ig.14（a）shows that themicrohardnessof the interface close to thesubstrate ishigher than thatof substrate，which is due to the fact that the input energy resulted in a phase change to lathmartensite.Fig.14（b）shows that the friction coefficient of LC coating isalmost tw ice that of SLD coating.The friction coefficients of LC and SLD coatings show the fluctuations between 0.10-0.40 and 0.13-0.23，respectively.Furthermore，in Fig.15（a），the martensite is present in thinner dentric crystal due to lower energy density of SLD in comparison w ith that of LD in Fig.15（b）.Chen et al.［25］proved that themicron scale WC particles can play a very effective role in resisting wearing.

The worn surface of Ni60-WC coating processed using SLD is shown in Fig.16.Fig.16（a）shows thewear track，and Fig.16（b）shows thewear debris.Thescarswere caused by the abrasion of Si3N4ball，and the scratches are likely due to the cracking of carbide.Fig.16（c）show s the debris at high m agnification；it can be deduced that som e relatively larger bulksw ith irregular shapes are possibly WC particles broken due towearing or coating withWC.The debriswasexaminedusing EDX，and the results are presented in Table 6，which shows that the debris consistsmainly ofWC and FeNi.

Fig.16（d）showstheweartrackathighmagnification，whereit canbeseen thatamassofWC isembedded in theNialloymatrix. Thematrix w ith the scars lefton theworn surfacewasabraded. High-hard WC on the coating during wearing can protect the coating from wearing.As a result，the wear resistance is improved，and the coating base can support the hard particles，avoidingWC fractureand abscission，retaining thehigh hardness ofWC and losing lessmass.During the later stagesofwearing，the supporting base is continually abraded，and the supporting effectof thebaseisgradually reduced.Asa result，partof theWC phase begins to crack and falloff.Theworn surface transforms into a new layer，and theprocess iscontinually repeated.

Theworn surface of the Ni60-WC coating prepared using LC is shown in Fig.17.Asseen in Fig.17（a），thewear track is quite different from that in Fig.16（a）.Fig.17（b）shows the wear debris.Fig.17（c）shows the obvious，deep and broad wear scars.The debris isgrind to form a laminate on theworn surface.Fig.17（d）shows thata part of the debrismoves away due to the brittle debris pulling outduring wearing，and reveals the topography of the wearing surface.Table 7 shows the element compositions of the debris for LC coating.It can be seen from Table 7 that the contentsof C andW are lower than those in SLD coating，and the contentof Fe ishigher than that of SLD coating.It can be seen from the dilution analysis that the elements of LC coating diffused into the substrate or the elements of the substrate diffused into the coating.It can be seen from Fig.6 and Fig.13（b）that the phases of Ni60-WC coating deposited by laser cladding exclude WC phase，and the tungsten and iron form the com pound Fe7W6.The lost weights，friction coefficients and average hardnesses of SLD and LC are shown in Table 8，respectively，which show that the lost weight and the friction coefficient of SLD coating are lower than thoseof LC coating，and themicrohardnessof SLD ishigher than thatof LC.These data prove thatSLD results in improved tribological properties of Ni60-WC coating in comparison to that produced using LC.

Fig.17.Worn surfaces of Ni60-WC deposited using LC.

Table 7 Element compositions of debris on LC coating.

Element Cr C W Fe Ni W t.%3.27 16.14 8.51 64.6 7.48

Table 8 Properties of Ni60-WC coatings.

4.Conclusions

1）The characteristics of the Ni60-WC com posite coating deposited on low-carbon steel using SLD w ith the variousprocess parameters were studied.The optimal deposition conditions are N2atmosphere，pressure of 30 bar，temperature of 450°C，traverse rate of 10 mm s-1and laser power of 2 kW with respect to themicrohardnessandwear properties of the coating.

2）The coating deposited using SLD contains Cr7C3，Cr23C6，F(xiàn)eNi，andWC particleswhile the coating deposited using LC contains Cr7C3，Cr23C6，F(xiàn)eNi，and Fe7W6.Both of the coatings do not have B2O3.The interface between the coating and the substrate bonds strongly.WC particles are embedded in the alloy m atrix in the SLD coating，while the carbide is distributed along the grain boundaries for the LC coating.A diluted layer is hard ly found in the coating deposited using SLD.

3）ThemicrohardnessandwearpropertiesofNi60-WCcoatings depositedusingSLDand LCwereexamined.Comparedwith theLCcoating，thehardnessof theSLD coatingishigher，and its friction coefficientand theweight lossare lower.

Acknow ledgm ents

This work was sponsored by the Centre for Industrial Photonics，Institute for M anufacture，Departm ent of Engineering，University of Cambridge；the Natural Science Foundation of China（51271170）；China International Science and Technology Cooperation Project（2011DFR50540）；Major Scientific and Technological Special Key Industrial Project of Zhejiang Province（2012C11001）.

We would like to express our gratitude to the Key Laboratory of Special Purpose Equipment and Advanced Processing Technology（Zhejiang University of Technology），the M inistry of Education，China，and sincerely thank M r.Yuanhang LU for providing assistance in sample preparation.

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.Tel.：+86 13867418595.

**Corresponding author.Tel.：+44（0）1223 332786.

E-mail addresses：luofang@zjut.edu.cn（F.LUO），m rs46@cam.ac.uk（M. SPARKES）.

Peer review under responsibility of China Ordnance Society.

http：//dx.doi.org/10.1016/j.dt.2014.09.003

2214-9147/Copyright?2014，China Ordnance Society.Production and hosting by Elsevier B.V.All rights reserved.

Copyright?2014，China Ordnance Society.Production and hosting by Elsevier B.V.A ll rights reserved.