Microstructure, microhardness and corrosion resistance of Stellite-6coatings reinforced with WC particles using laser cladding

Dariusz Bartkowski a,n, Andrzej Młynarczak a, Adam Piasecki a, Bartłomiej Dudziak b,Marek Gościański b, Aneta Bartkowska a

a Poznan University of Technology, pl. M.Sklodowskiej-Curie 5, 60-965 Poznan, Polandb Industrial Institute of Agricultural Engineering, Starolecka 5, 60-963 Poznan, Poland

a r t i c l e i n f o

Article history:Received 6 August 2014Received in revised form28 November 2014Accepted 3 December 2014

Keywords:Laser claddingTungsten carbideMetal matrix composite coating

a b s t r a c t

The paper presents the method of preparation and study results of metal matrix composite coatings(MMC coating) in the system of Stellite-6 and tungsten carbides. Changes in microstructure, corrosionresistance, microhardness, phase and chemical composition as well as surface conditions wereinvestigated. Stellite-6/WC MMC coatings were prepared by laser cladding technology using a 1 kWcontinuous wave Yb:YAG disk laser with a powder feeding system. Two different powder mixturescontaining 30% and 60% of WC and three different values of laser beam power were used. It was foundthat increasing WC values caused an increase in microhardness on the cross-section of the producedcoating in comparison to the substrate. Depending on the laser beam power, the coatings produced with30% WC achieved microhardness in the range from about 350 HV0.05 (700 W) to about 680 HV0.05(550 W). Twice as large amount of WC particles in the powder mixture resulted in increase ofmicrohardness from about 700 HV0.05 (700 W) to about 1500 HV0.05 (550 W). In the coating M7C3,M6C and M23C6 carbides were identified by an X-ray diffraction method. Special attention was given tobondings between carbide particles and metal matrix, which had a characteristic microstructure. Areduction of corrosion resistance with increasing WC content in coating was also discovered.

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1. Introduction

Modern methods and techniques of surface engineering enablethe production of different types of composite and complex coatings.The most frequently reported in the literature are thermal spraying,plasma and laser remelting of galvanic coatings, diffusion layers orprecoat in the form of pastes [1–6]. Using laser technology it ispossible to modify properties of the surface of various materials suchas light metals and various species of steels and cast iron [7,8]. Lasercladding method [9–12] is utmost importance for various industrialbranches wherein a suitably designed nozzle simultaneously emitsthe laser beam and adds powder mixtures to it. It is thus possible toeasily produce a coating with new unique properties. In industry, 5-axis CNC laser processing centers which allow production at coatingonly in zones where they are required are being used. An importantproblem often discussed in studies is wear resistance. This problemwas described for example in the paper [13]. Less frequent meets theinformation focus on wear in soil environment. The production of

wear-resistant coatings on agricultural tools may be a good examplefor using laser cladding technology. During exploitation such toolswear away only in some zones, so modifying their entire surface iseconomically unjustified. Increased durability only in the zonesexposed to wear seems to be a good solution. Laser cladding maybe a successful alternative to diffusion methods and other methodswhich are more energy- and time-consuming.

There are many publications where the authors describe metalmatrix composite coatings (MMC). Due to the presence of hardparticles of carbides such as WC, VC, etc. located in metallicmatrix, MMC coatings have a much better wear resistance. Micro-structure of this type of coatings is similar to microstructure ofsintered materials. So far, majority of studies were focused onproduction of MMC coatings on Fe-based alloys or Ni-based alloys[14,15]. However, there is far fewer papers concerning productionof MMC coatings with Stellite-6 as a matrix [16,17], compared tothe amount of papers describing Stellite-6 coatings without hardparticles [18–23]. This alloy is often used for production of coat-ings on machine components which operate in conditions requir-ing high strength as well as good wear and corrosion resistance.Increased temperature does not change these properties. Highdurability of Stellite-6 is associated with its chemical composition

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Optics & Laser Technology

http://dx.doi.org/10.1016/j.optlastec.2014.12.0050030-3992/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author.E-mail address: dariusz.w.bartkowski@doctorate.put.poznan.pl (D. Bartkowski).

Optics & Laser Technology 68 (2015) 191–201

(Table 1) and its microstructure. The chromium content contri-butes both to increasing corrosion resistance and wear resistance,which are a result of formation of two types of carbides M7C3 andM23C6. Alloying additives like tungsten and molybdenum causeformation of MC and M6C carbides and intermetallic phases.Carbides are embedded in the solid solution matrix, which ischaracterized by dendritic microstructure with high chromiumcontent. Co-based alloys may be a good alternative to Ni-basedalloys primarily due to a relatively lower price. Available publica-tions mainly concentrated on production of MMC coatings using aCO2 [22] and Nd:YAG laser [23]. Laser production of coatings withmatrix of Stellite-6 reinforced mainly by tungsten carbides mayfind a number of applications in mechanical and automotiveindustry and oil and gas mining.

The paper presents results of studies on Stellite-6/WC compo-site coatings on low carbon steel using Yb:YAG laser disk. Micro-structure, chemical and phase composition were investigated.Additionally, microhardness as well as resistance to wear andelectrochemical corrosion was measured. Bonding between tung-sten carbide particles and metal matrix was also examined.

2. Experimental detail

2.1. Materials

Low-carbon steel specimens with dimensions of 20�20�7 mm3

were investigated. In this study, mixtures consisting of commerciallyavailable Co-based alloy (Stellite-6) powder and tungsten carbidespowder (WC) in two different proportions were used. Chemicalcomposition of the steel used is shown in Table 1, while chemicalcomposition of Stellite-6 powder is shown in Table 2. The Stellite-6powder particles were spherical with size in the range of 25–53 mm,whereas WC powder particles were irregular with size of 50–100 mm. Particle shapes and sizes of powder mixtures are shownin Fig. 1. Before the laser cladding process specimens were polished,cleaned with alcohol and degreased by acetone.

2.2. MMC coating preparation and parameter selection for lasercladding

For the preparation of composite powder mixtures the ball millwas used. Powders were dry mixed together for 2 h and immedi-ately prior to the laser cladding process were additionally dried for3 h at 120 1C. Stellite-6/WC metal matrix composite coatings wereprepared by laser cladding technology using a 1 kW continuouswave Yb:YAG disk laser with a powder feeding system. Wavelengthof laser beam was equal to 1030 nm. The process was carried outusing a TRUMPF Laser Cell 3008 5-axis CNC device with TruDisk1000 laser. This device enables 2D and 3D laser processing of small-size and medium-size machine parts. It allows precise directing oflaser beam and powder feed control during the manufacturingprocess. The laser beam mode (TEM00) of circular shape wasapplied. The Gaussian laser beam is not ideal to perform lasercladding (high dilution, deep substrate damage); in particular todaywhen different high power laser sources (diode lasers or notGaussian fiber lasers) or beam shaping techniques are studied forapplications which requires large spot and uniform distribution(such as laser cladding). The use of a disk laser allows generatinglarge laser spot diameter and its uniform distribution. The laser

beam was directed on the steel substrate and a melt pool wascreated. The prepared powder mixture was blown simultaneouslyinto the laser beam as well as into the melt pool. Helium was usedas carrier gas for the powder and argonwas used as shielding gas toprotect the molten pool from oxidation during the laser claddingprocess. The device manufacturer recommends use of helium to thelaser cladding process. However, in the industry due to the reduc-tion of costs, helium was partially replaced by argon. In order torepresent industrial environments these studies were carried out inthe same mixture of gases. Flow rates of both gases were the same(8 l/min). The nozzle is designed so that three powder streamsconverge at the same point on laser beam. The distance betweenthe tip of nozzle and surface of specimen (substrate) was 12 mm.The angle between the nozzle and the specimen was 901. Para-meters used in the experiment were theoretical laser beam dia-meter (d) of 1.64 mm with 40% overlapping. Other parameters ofthe laser cladding process associated with power laser beam, itspower density (Pd) and its scanning speed are shown in Table 3. Thepowder absorbed laser energy and partially melted before reachingthe substrate. A part of the laser heat energy was absorbed by thesubstrate to cause its melting. The value of laser beam fluence (F)was calculated using the following formula:

F ¼ PEtπr2

ð1Þ

where P—laser beam power (W); r—radius of the laser beam (mm);Et—exposure time of laser beam on material(s).

Exposure time of laser beam on material (Et) was measuredaccording to equation:

Et ¼df r

sð Þ ð2Þ

where d—laser beam diameter (mm); f r—scanning speed (mm/s).The exposure time of laser beam depends not only on the laser

beam diameter and scanning speed, but also how the selected pointon the surface of the material is removed from the scan trajectories.However, the two first factors have the most influence on laser beamfluence. The formula (2) is the simplification adopted by the authorsof paper.

Stellite-6/WC MMC coatings were produced in 18 passes. Aftereach working movement (each track) laser head returned to initialposition and shifted by 1 mm. CAD model of a specimen withMMC coating is shown in Fig. 2, whereas schematic diagram of thelaser cladding process is shown in Fig. 3.

2.3. Macroscopic observations and microstructural examination

To analyze the surface condition after the laser cladding process astereoscopic microscope was used. Widths of tracks, porosity andcracks presence as well as heat tints of MMC coatings were observed.Additionally, coatings were examined by dye penetrant inspection. Itis a non-destructive method used for determining consistency of weldbut it can also be used to determine the coating quality. In this

Table 1Chemical composition of S355 steel [wt%].

Material Co Fe Si Ni Mn Mo C Cr W P S Al CuAISI S355 – Base 0.35 o0.03 1.5 – 0.2 o0.03 – o0.04 o0.04 o0.2 o0.3

Table 2Chemical composition of Stellite-6 powder [wt%].

Material Co Fe Si Ni Mn Mo C Cr WStellite-6 Base o2.0 1.2 28.5 4.6

D. Bartkowski et al. / Optics & Laser Technology 68 (2015) 191–201192

method cracks ranging from 30 to 50 μm appear as red marks on awhite background. In this study cleaner, dye penetrant and whitedeveloper (Skincric fromWeldline) were used. The process consists of:surface cleaning, red dye penetrant application, excess red dyepenetrant removing, white developer application, and finally inspec-tion of the coating.

After laser cladding processes, specimens were cut perpendi-cular to surface and were polished using abrasive paper of dec-reasing granularity and finished using Al2O3 with a grain size of0.05 μm. A two-step etching procedure was used. A 2% nital foretching specimens was used to visualize substrate microstructure.A 3:1 solution of HCl and HNO3 was used for etching specimens to

Fig. 1. Particle shapes and sizes of powder mixtures: (a) Stellite-6, (b) tungsten carbides, (c) Stellite-6 and 30% WC mixture (wt%), and (d) Stellite-6 and 60% WC mixture (wt%).

Table 3Laser cladding parameters.

Specimenno.

Powder mixture composition Laser beam parameters

Stellite-6(%)

WC(%)

Number of turns offeeder

Laser beampower (W)

Feed rate(mm/min)

Laser beam fluence(J/mm2)

Laser beam power density(W/mm2)

Exposure time of laser beam onmaterial (s)

a 70 30 0.8 400 340 51.2 189 0.27b 70 30 0.8 550 340 70.4 260 0.27c 70 30 0.8 700 340 89.6 331 0.27d 40 60 0.8 400 340 51.2 189 0.27e 40 60 0.8 550 340 70.4 260 0.27f 40 60 0.8 700 340 89.6 331 0.27

D. Bartkowski et al. / Optics & Laser Technology 68 (2015) 191–201 193

reveal coating microstructure. Specimens microstructures wereobserved using a Neophot 32 optical microscope and a TescanVega 5135 scanning electron microscope.

Extraction replicas were made from the produced MMC coat-ings. The prepared specimens were observed with the scanningelectron microscope. Specimens in the form of thin films were alsoprepared and observed by a PHILIPS EM 300 transmission electronmicroscope (TEM).

A quantitative analysis of the amount of carbides in the coatingwas carried out. For this purpose, selected representative areas inthe microstructures characterized by the same dimensions wereevaluated. Photographs of the coating microstructures were con-verted to black and white images. The ratio of carbides (black area)to matrix (white area) was determined by a software for proces-sing graphics.

2.4. Microhardness measurement

Microhardness along the depth of the polished and etchedspecimen in its cross-section was measured with a BuehlerMicromet II microhardness tester. An indentation load of 50 gand loading time of 15 s were used in this study. Microhardness ofcarbides particles and the metal matrix in their environment wasalso investigated using an indentation load of 10 g.

2.5. Chemical and phase composition

Chemical composition of the Stellite-6/WC MMC coatingsproduced by the laser cladding method was investigated usingthe scanning electron microscope (SEM) equipped with a PTGPrism Si(Li) energy dispersive X-ray spectrometer (EDS). Pointanalysis of the specimen cross-sections was performed. Phaseidentification was carried out with a PANalytical EMPYREAN

X-ray diffractometer using the Cu Kα radiation and operating at45 kV and 40 mA and 25 1C.

2.6. Electrochemical corrosion resistance tests

Prior to starting the corrosion tests, the specimens were polishedin order to smoothen the surface. This was necessary due toconstruction of the corrosion tester. Potentiodynamic corrosion testsin 5% NaCl solution were carried out using an Atlas Sollich – ATLAS0531 EU&IA potentiostat. The specimens with the area of 50 mm2

were studied at constant temperature of 22 1C and the rate ofpotential change was equal to 0.5 mV/s. The specimens werepolarized in the direction of the anode in the potentials range from�1.5 to 1.5 V. Corrosion current and corrosion potential weremeasured. A commercial saturated calomel electrode (SCE) was usedas the reference electrode and the auxiliary electrode was made ofplatinum. The results were recorded and processed using AtlasLaband AtlasCorr software.

3. Results of studies

3.1. Macroscopic tests and description of laser parameters

Surface topography was characterized by surface waviness andsurface inequality typical for the coatings produced with high-energymethods. However, the laser cladding method decreased surfaceinequality with higher powers of laser beam. This is due to melting ofpowder particles. When laser beam power was low, powder particleson surface were not completely melted, but only were glued. Such asurface is shown in Fig. 4. The SEM image shows the Stellite-6 andtungsten carbide particles which were not melted. It is probable thatelimination of these particles will result in satisfactory surfacecondition. Although if these coatings had to be applied for preciseproducts, it would be necessary to carry out additional machining. Itcan be concluded that increase of laser beam power from 400W to700W resulted in reduction of the surface roughness.

Dye penetrant inspections and macroscopic studies revealed nocracks and no gas porosity. The coatings did not delaminate, whichindicates their good bonding with the steel substrate. Macroscopicobservations of surfaces and cross-section of the specimens indicatethat during the deposition process the matrix powder was comple-tely melted, while WC particles were partially remelted. Coatingssimilar to sintered carbide were finally obtained. With increasingpower of laser beam the steel substrate more heat is removed. Thedark blue heat tints on steel specimens proved that their tempera-ture had not reached a value greater than approximately 300 1C.Similar coatings were produced by means of two powder mixtures(containing 30% WC and 60% WC).

The manufacturer of the laser system states that one revolutionper minute of feeder is equal to 4 g of powder with mass density ofabout 8 g/cm3. The density of the applied Stellite-6 was 8.44 g/cm3;it may therefore be assumed that one feeder rotation caused theapplication 4 g of Stellite-6 powder. Due to the fact that the usedpowder mixtures contained an addition of WC particles of higherdensity (15.7 g/cm3) the amount of the applied powder can bedetermined by the following formula:

PQ ¼ Rf A%MpρMp� �

100þ %RpρRp� �

100

� �

where PQ—the amount of the feed powder (g/min); Rf—the numberof feeder turns; A—the adopted coefficient A¼ 4=8¼ 0:5; %Mp—thepercentage of the matrix powder; %Rp—the percentage of thereinforcing phase powder; ρMp—the matrix density (g/cm3); ρRp—

the reinforcing phase density (g/cm3). The developed formula can be

Fig. 2. CAD model of specimen with MMC coating.

Fig. 3. Schematic diagram of the laser cladding process.

D. Bartkowski et al. / Optics & Laser Technology 68 (2015) 191–201194

used to each type of applied powder, provided that its physicochem-ical properties are known.

The interaction time of the laser beam with the material ispresented in Table 3. Both the substrate and the powder should betreated as materials which carry away the heat. It is possible toestimate the powder amount which is added to the weld pool andto the laser beam when it impacts on the material. It is calculatedusing the following formula:

PQE ¼ PQEt60

where PQE—the powder quantity during action of the laser beamon the material (g), PQ—the powder feed rate (g/min), and Et—theexposure time of the laser beam on the material(s).

These quantities are correlated to laser beam scan rate andhave a direct impact on the final microstructure. The calculationresults of PQ and PQE for each sample are shown in Table 4

3.2. Microstructure analysis

In this paper Stellite-6 with addition 30% and 60% WC wereinvestigated. The change in the share of tungsten carbide in matrixdependent on increasing the laser beam power is shown in Fig. 5.In case of application the laser beam power of 400 W and 550 W,the twice increase WC content in powder mixture caused atwofold increase in WC in MMC coating. With increasing the laserbeam power to 700 W, the increase in amount of WC was about10%. It should be noted that only primary carbide particles werecounted. The overall content of tungsten carbide is much higherdue to nucleation of secondary carbides during the laser claddingprocess. However, it can be concluded that increase in laser beam

power reduces the share of primary tungsten carbide particles incoating. Some of them reduced their size by partial melting, theother were completely dissolved in the matrix. Quantitative mea-surements of carbides were performed on the same area sizes ofspecimens.

Microstructures of the composite coatings produced by the lasercladding method are shown in Figs. 6–11. The microstructure of thecoating produced with the powder mixture of Stellite-6 containing30% WC at velocity of 4.2 g/min is shown in Fig. 6. Along with theincrease in laser beam power reduction in the carbide share in thematrix was observed.

In each case the coatings were well bonded to the substrate. Whenthe laser beam power of 400W (Fig. 6a) was used the boundarybetween the coating and the substrate was a relatively straight line.Gradual increase in the laser beam power (Fig. 6b) led to formationthe parabolic shape bond line between the coating and the substrate.This shape is characteristic of laser processing and it was most clearlyvisible in the coatings produced by using the beam power equal to700W (Fig. 6c). No porosity or crack was observed on the entiresurface of the cross-section. The thickness of modified surface layerincreases with increasing laser beam power. At higher values of thisparameter remelting of primary carbides was observed. Carbides arerelatively uniformly distributed throughout the entire coating.

Microstructure of the coating produced by Stellite-6/30% WCpowder mixture with the velocity of powder feeding of 4.2 g/minis shown in Fig. 7. It was characterized by a higher proportion ofprimary WC carbides. The increase in WC carbides amount led tothe occurrence of a few cracks in coatings produced with laserbeam power equal to 400 W (Fig. 7a). The layers were pores.Increasing the carbide content significantly affected the matrixmicrostructure. A greater amount of secondary WC carbides wasobserved as compared to a mixture of 30% WC.

The melting degree of carbides was different. This increase isassociated with an increase in the amount of carbides in matrix. Itincreases thermal conductivity in the total powder material. Thethermal conductivity of Stellite-6 is equal to 14.82W/(m K) whereasfor tungsten carbide it is equal to 84.02W/(m K). Nearly six timeshigher thermal conductivity of WC results in the increased thermalconductivity of the total powder mixture. It resulted in delivery ofmore heat to the substrate and thus its better mixing with additionalmaterial. As result, a significant increase in thickness of MMC coatingin comparison to coatings produced using a mixture with a lowercontent of WC was observed. All the discussed coatings were chara-cterized by the composite microstructure which was composed ofprimary carbides WC in the Co-based alloy matrix. Laser parameterswere chosen so as not to cause melting of the tungsten carbideparticles (which have a melting point of 2870 1C), but to melt matrixof coating and steel substrate. The Stellite-6 matrix used was in fact

Fig. 4. Coating surface made by Stellite-6/30% WC powder mixture (400 W,340 mm/min) (SEM image).

Table 4Parameters related to powder amount fed to laser beam.

Parameters Specimen name

a–c d–f

PQ ðg= minÞ 4.2 5.1PQE ðgÞ 0.019 0.022

Fig. 5. Amount of primary carbides WC in Stellite-6/WC composite coating.

D. Bartkowski et al. / Optics & Laser Technology 68 (2015) 191–201 195

melted in the temperature range at 1260–1357 1C. Application ofrelatively low power density resulted in partial melting of the surfaceof tungsten carbide particles and consequently these are well metal-lurgically joined with the matrix.

A characteristic feature of all composite coatings was dendriticmicrosegregation appearing with a diversity of cooling rates oncross-section of coatings. The following phenomena had a significantimpact on the shape differences of dendrites: addition of WC primarycarbides (which contribute to enrichment of matrix in carbon andtungsten by diffusion), as well as modification of the steel substratewith the modified surface layer near the melting line, namelyconnection area between the coating and the substrate. The largestnon-uniformity in microstructure occurred in coatings produced athigher laser beam power. It resulted from mixing the substrate withan additive material and then rapid cooling, which removed theopportunity to homogenize the coating.

Bondings between WC particles and Stellite-6 matrix alloy wereexamined. It was been found that it is at a diffusive type. Under theinfluence of heat supplied by laser beam, the surface layer of WCprimary carbides was remelted. Afterwards, the tungsten and carbon(contained inWC) changed the chemical composition of matrix. Duringsolidification, which in the described method occurs in a very shorttime, the matrix crystallized, while in places where there was excess oftungsten and carbon, spherical precipitates were formed. They wereprobably complex carbides. These precipitates were resided on theboundary between the primary WC carbides and Stellite-6 matrix.

An example of an analyzed tungsten carbide particle, whichwas present in a MMC coating produced with a powder mixturecontaining 30% WC, is shown in Fig. 8a. Around carbide particles, acharacteristic brighter zone was observed with the microstructuredifferent from the rest of matrix. This zone is enlarged in Fig. 8b,

whereas Fig. 8c shows a modified part of the zone around thecarbide. It was a eutectic which was characterized by nanometricsize. On the carbide-matrix boundary, the spherical precipitatesranking in size from 250 nm to 1.5 um were observed (Fig. 8d).

The presence of spherical precipitates was confirmed also onextraction replicas. The surface SEM image of the extracted tungstencarbide is shown in Fig. 9. These spherical precipitates were discussedabove. When performing a replica, the authors proved that thebonding between WC and Stellite-6 was very good. Despite a verylong and intense etching in mixture of concentrated hydrochloric acidand nitric acid in a volume ratio of 3:1 only a few carbides wereseparated from the matrix. Usually these were carbides with mostspherical-shape, whereas the more irregularly shaped carbidesremained in the metallic matrix. This may be valuable informationduring the design of coating for specific industrial applications.

Observations using the transmission electron microscope (TEM)revealed a large share of fine grained eutectic between dendrites ofthe solid solution (Fig. 10). The eutectic consisted of carbides ranking insize from approximately 50 nm to 150 nm and solid solution. Obser-vations were carried out on thin films in the areas around the primarycarbides WC, namely in the zone with the altered microstructure.

Fig. 11a presents an example of a tungsten carbide particle presentin MMC coatings produced using a powder mixture containing 60%WC. Around this particle a characteristic zone of an altered micro-structure compared to the rest of matrix was also observed. The highcontent of WC particles resulted in enrichment of the matrix intungsten and increase in the content of secondary carbides in themetal matrix as seen in the SEM image, which was made usingbackscattered electron (BSE). In Fig. 11b the transition from columnalcrystals to dendrites characterized by a butterfly shape can be seen.Fig. 11c shows the details of the area closely adhering to the primary

Fig. 6. Microstructure of MMC coating produced on low-carbon steel using Stellite-6/30% WC powder mixture. Specimens designation and applied laser cladding parametersaccording to Table 3.

Fig. 7. Microstructure of MMC coating produced on low-carbon steel using Stellite-6/60% WC powder mixture. Specimens designation and applied laser cladding parametersaccording to Table 3.

D. Bartkowski et al. / Optics & Laser Technology 68 (2015) 191–201196

carbide, and which was formed by diffusion during the process. As canbe seen, an increase in tungsten carbide in the coating and hence theincreased tungsten content caused a total change in the nature of theprecipitates. They were not spherical-shaped but more elongated andjoined to each other. These precipitates were distributed around thecircumference of primary carbides and their size was about 1.5 mm. Itcan be concluded that these precipitates were the crystallization startof columnar crystals.

4. Results of microhardness tests

The results of microhardness measurements are shown in Figs. 12,13 and 15. These measurements were made along a straight line onlyin the Stellite matrix. The influence of carbide particles on the matrixhardness was determined. For MMC Stellite-6/30% WC coatings theirregular nature of the microhardness profiles was observed (Fig. 12).A significant difference in microhardness was found between the

Fig. 8. Bondings between WC particles and matrix in Stellite-6/30% WC coating.

Fig. 9. SEM image of surface of tungsten carbide extracted from Stellite-6 matrix.Fig. 10. Microstructure Stellite-6/30% WC (TEM).

D. Bartkowski et al. / Optics & Laser Technology 68 (2015) 191–201 197

zone close to primary carbide and the zone far away from thecarbide. Fig. 13 shows the change in microhardness of the producedcoating: from tungsten carbide (2426 HV0.01) through the transitionzone to the matrix, which was not enriched in tungsten.

For coatings produced using the laser beam power of 400 W,the maximum microhardness value reached 570 HV0.05. This laserbeam power did not result in intensive partial melting of carbides;therefore matrix microhardness fluctuated around the valueattributed to the Stellite-6 alloy. After increasing power to 550 Wa substantial increase in microhardness to a value equal to 680HV0.05 was observed. This is related to partial melting of primarycarbides and enrichment of the matrix in the tungsten and carbonoriginating from those carbides. This parameter seems to be thebest among applied in this study because high microhardness wasobtained with relatively small reduction of carbides particles inthe matrix (small partial melting of WC particles).

The least preferred seems to be the use of a high power laserbeam. The power laser beam equal to 700W in theory should get amatrix with the highest microhardness, because tungsten and carbonenter into matrix. However, this does not happen because the laserbeam heat affects both carbides particles, the matrix and also veryintensively affects the steel substrate to the point of melting it. Thesubstrate and the coating materials mixed with each other andthereby significantly reduced the microhardness of the producedcoating. Hence the results of the microhardness profile for the laserbeam power of 700W were so low. The presence of iron wassupported by study results of chemical composition in the form ofEDSmapping (Fig. 14). These results refer to a specimenmade using alaser beam power of 700W. A significant share of iron in the coatingwas found.

For MMC Stellite-6/60% WC coating irregular nature of themicrohardness profiles was also observed (Fig. 16). Microhardnessvalues were significantly higher compared to the coating producedusing the powder mixture with 30% WC content.

Fig. 11. Interface between WC particles and matrix in Stellite-6/60% WC coating.

Fig. 12. Microhardness of MMC Stellite-6/30% WC coatings.

Fig. 13. Change in microhardness of produced coating: from WC through atransition zone to matrix.

D. Bartkowski et al. / Optics & Laser Technology 68 (2015) 191–201198

Coatings produced with the laser beam power of 400W achievedmaximum microhardness of 1150 HV0.05. Twice as large amount ofWC particles in the powder mixture finally resulted in enrichment ofthe matrix in carbon and tungsten, and thereby a significant increasein microhardness of coating. The Vickers hardness increases with theincrease of WC weight fraction also in other studies such as [24].With a laser beam power equal to 550W this increase was evenmore noticeable, and the maximummicrohardness of the matrix wasabout 1500 HV0.05. The best results with these values of laser beampower were obtained which may be explained in the same manner

as in the previous cases. Using a laser power equal to 700W themicrohardness decreased significantly due to the mixing of thesubstrate with the coating.

From the test results of microhardness and microstructure itcan be concluded that the use of higher laser power is not justified,as it reduces the amount of hard WC particles in the coatings,which in turn reduce the hardness of the matrix, which is alsodisadvantageous in terms of application.

5. SEM/EDS X-ray microanalysis

The study of chemical composition of the produced Stellite-6/30%WC coatings demonstrated an influence of the addition of tungstencarbide on the increased amount of tungsten in the matrix. Theresults of EDS microanalysis are presented in Fig. 16. In the analysis ofthe chemical composition it was concluded that the sphericalparticipates (Fig. 8) were probably complex carbides (Co, W, Cr,Fe)7C3. With increasing distance of the measurement points fromcarbide particles, a gradual decrease in the tungsten content wasobserved. Tungsten is gradually dissolved in cobalt which is the maincomponent of the matrix. Also, a higher carbon content and a lowercobalt content were detected in the eutectic than in dendrites.

The chemical composition study of the coatings produced usinga powder mixture Stellite-6/60% WC is shown in Fig. 17. It can beconcluded that the participates observed around the primarycarbides were most likely complex carbides (Co, W, Cr, Fe)23C6 or(Co, W, Cr, Fe)6C types.

6. Phase analysis

Results from XRD patterns of specimens obtained sequentiallywith 30% WC and 60% WC are shown in Figs. 18 and 19. XRD studieswere carried out for the coatings produced using the followingparameters: the laser beam power of 700W and the scanning Speerof 340 mm/min. In both these cases the phases were the same,whereas their intensity had changed, and consequently their occur-rence frequency changed too. In the coatings produced using themixture containing 30% WC, WC and W2C phases were found. M7C3phase was very intense and evident, while M23C6 and M6C phaseswere slightly visible and very difficult to identify. The results wereconsistent with the EDS X-ray microanalysis. Enrichment of thepowder mixture in with 60% WC finally produced a compositecoating in which a significant share of M7C3 phases was found.

7. Electrochemical corrosion

The important tests in the case of MMC coatings are testing ofcorrosion and erosion resistance. This paper applies to research onelectrochemical corrosion resistance. For the MMC coatings contain-ing 30% and 60% WC, the dependence curves of current density for agiven potential were determined. These are shown in Fig. 20. Corro-sion resistance of the MMC coatings was compared to the corrosionresistance of the Stellite-6 coating made in the same method usingthe same parameters. The electrochemical parameters were deter-mined based on the curves analysis and are presented in Table 5.

Analyzing the results listed in Table 5, it is clear that the smallestcorrosion current equal to 7:21� 10�8 mA/cm2 was characteristic forthe pure Stellite-6 coating, which points out to its high corrosionresistance. Whereas, the greatest corrosion current equal to 5:69�10�6 mA/cm2 was characteristic for the MMC Stellite-6/60%, whichproves its much lower resistance to corrosion. Comparing the corrosionresistance of both these coatings it was found that increase in thecontent of WC particles caused a decrease in their corrosion resistance.

Fig. 14. Mapping of chemical composition in Stellite-6/30% WC coating (700 W,340 mm/min).

Fig. 15. Microhardness of MMC Stellite-6/60% WC coatings.

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8. Conclusions

1. Metal matrix composite coatings of Stellite-6 with the additionof 30% and 60% tungsten carbide WC produced using the lasercladding method were characterized by enhanced lower elec-trochemical corrosion resistance as compared to layers pro-duced using pure-Stellite 6 powder. With increasing content of

carbide particles in the coating, the number of possible corro-sion cells increased. This causes destruction by electrochemicalcorrosion.

2. The addition of tungsten carbide causes changes in microstruc-ture of the matrix. Most of these changes are visible on thecarbide-matrix boundary which revealed the presence of pre-cipitates with spherical or irregular shape.

Fig. 16. EDS chemical composition produced Stellite-6/30% WC (550 W, 340 mm/min).

Fig. 17. EDS chemical composition produced Stellite-6/60% WC (550 W, 340 mm/min).

Fig. 18. XRD pattern of Stellite-6/30% WC coating. Fig. 19. XRD pattern of Stellite-6/60% WC coating.

D. Bartkowski et al. / Optics & Laser Technology 68 (2015) 191–201200

3. Increasing tungsten carbide content in powder mixture causedincrease in the microhardness of the modified surface layer.Depending on the laser beam power, Stellite-6/30% WC MMCcoatings achieved microhardness in the range from about 350HV0.05 (700 W) to about 680 HV0.05 (550 W). Stellite-6/60%WC MMC coatings achieved microhardness in the range fromabout 700 HV0.05 (700 W) to about 1500 HV0.05 (550 W).

4. Increase of a laser beam power causes reduction in content ofWC particles in the MMC coating.

5. In MMC coatings produced by laser cladding, the tungstencarbide particles were well bonded to the Stellite-6 matrix. Theuse of higher power laser beam causes melts in the surface ofcarbide particles and increased their bonding with the metalmatrix.

6. In produced MMC coatings Stellite-6/30% WC and Stellite-6/60% WC were detected in the following phases: WC, W2C,M7C3, M23C6 and M6C. The chemical composition of thesephases is most likely (Co, W, Cr, Fe)7C3, (Co, W, Cr, Fe)23C6

and (Co, W, Cr, Fe)6C.

Acknowledgments

The authors wish to thank Ph.D. M. Tuliński for his help in XRDanalysis.

Dariusz Bartkowski is a scholarship holder within the project“Scholarship support for Ph.D. students specializing in majorsstrategic for Wielkopolska's development”, Sub-measure 8.2.2Human Capital Operational Programme, co-financed by EuropeanUnion under the European Social Fund.

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Fig. 20. Corrosion resistance of MMC Stellite-6 coating reinforces by 30% and 60%WC and pure Stellite-6 coating.

Table 5Electrochemical parameters determined based on potentiodynamic curves.

Corrosion parameters Stellite-6 Stellite-6/30% WC Stellite-6/60% WC

Icorr (A/cm2) 7.21E�8 2.65E�07 5.69E�06Ecorr (V) �5.97E�01 �7.71E�01 �1.03E�00

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