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Applied Surface Science 257 (2010) 531–537

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

aser gas-assisted processing of carbon coated and TiC embedded Ti–6Al–4Vlloy surface

.S. Yilbasa,∗, S. Akhtara, B.J. Abdul Aleema, C. Karatasb

Mechanical Engineering Department, King Fahd University of Petroleum & Minerals, Saudi ArabiaFaculty of Engineering, Hacettepe University, Turkey

r t i c l e i n f o

rticle history:eceived 24 April 2010eceived in revised form 8 July 2010ccepted 10 July 2010vailable online 16 July 2010

a b s t r a c t

Laser gas-assisted treatment of Ti–6Al–4V alloy surface is carried out. The alloy surface is initially coatedby a carbon layer, in which the TiC particles are embedded prior to laser processing of the surface.The carbon coating with the presence of TiC particles on the workpiece surface is expected to resultin carbonitride compound in the surface vicinity after the laser treatment process. Optical and scanningelectron microscopes are used to examine the morphological and the metallurgical changes in the laser

eywords:aseritridingesidual stressi alloy

treated layer. The residual stress formed in the surface region after the laser treatment process is criticalfor the practical applications of the resulting surface. Therefore, the residual stress formed in the lasertreated region is predicted from the analytically equation. The X-ray diffraction technique is incorporatedto obtain the residual stress formed in the surface region. It is found that the residual stress predictedagrees with the X-ray diffraction data. The dense structures consisting of TiCxN1−x, TiNx, Ti2N, and TiCcompounds are formed in the surface region of the treated layer. This, in turn, significantly increases the

face.

microhardness at the sur

. Introduction

Ti–6Al–4V alloy is used widely in industry because of its highoughness to mass ratio. However, the poor surface properties ofhe alloy limit its applications in practice. One of the methodso improve the surface properties of the alloy is to form a hardayer at the surface. In this case, the laser surface treatment pro-ess offers advantages over the conventional methods. Some ofhese advantages include precision of operation, control process-ng, short process duration, and the low cost. Moreover, the carbonoating, blended with TiC powders, at the surface prior to the laserreatment enables to form carbonitride compounds in the treatedegion. The treatment process involves with the initial prepara-ion of the surface and the laser controlled melting of the preparedurface at nitrogen gas ambient. Since the process is complex inature, the detailed investigation into the surface treatment pro-ess becomes essential.

Considerable research studies were carried out to examine the

aser treatment of titanium alloys. The laser gas-assisted nitridingf Ti–6Al–4V alloy and thermal stress development in the laserreated surface were examined by Labudovic et al. [1]. They showedhat the high cooling rates in the laser melt zone produced high

∗ Corresponding author. Tel.: +966 860 5223; fax: +966 3 860 2949.E-mail address: bsyilbas@kfupm.edu.sa (B.S. Yilbas).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.07.028

© 2010 Elsevier B.V. All rights reserved.

values of the surface residual stress in the surface region. Themicrostructural assessment of the laser nitrided Ti–6Al–4V alloywas carried out by Xin et al. [2]. They identified two zones in thelaser treated region, including well defined dendrites of fcc TiNx inthe first zone, and needles of hcp �′-Ti in the second zone. Theyfurther indicated that the different microstructures developed bythe laser nitriding producing non-equilibrium phases were con-sidered to be due to the effects of solidification rate, cooling rateof solid, and diffusion of nitrogen. Moreover, when a low laserenergy input and high-pressure nitrogen environment were used,a high cooling rate in the solidification stage of the material anda high concentration of nitrogen on the liquid surface would beexpected. The pulsed laser deposition of (Ti,Al)CxN1−x coatings, andmechanical and the tribological properties of the resulting coatingswere examined by Lakner et al. [3]. They indicated that hardnessand elastic modulus were depended on the crystallinity and highcarbon content, which led to the formation of the pure carbon pre-cipitations. However, the low density of defects was observed onthe coating composition. Atomic force microscopy (AFM) graphsrevealed the presence of small clusters of about 10–15 nm heightand 100–200 nm diameter. The clusters were the tops of crys-talline micro-columns in the coating, which were embedded in an

amorphous matrix. The density of these clusters was decreasedon the surface of the coatings of high carbon content. The laserpulsed deposition of titanium nitride thin layer on metallic andnon-metallic substrates was investigated by Major et al. [4]. Theyindicated that deposition of metallic titanium in argon environ-

532 B.S. Yilbas et al. / Applied Surface Science 257 (2010) 531–537

Table 1L

mm)

mTtlBpTsaculTTpcpstwldamaspoiwhtacfdsndwp[ifirirtban

iTTcXmX

XRD technique: The measurement relies on the stresses in finegrained polycrystalline structure. The position of the diffractionpeak undergoes shifting as the specimen is tilted by an angle .The magnitude of the shift is related to the magnitude of the resid-

aser gas-assisted nitriding conditions.

Feed rate (mm/min) Power (W) Frequency (Hz) Nozzle gap (

800 80 1500 1.5

ent led to the formation of a new tetragonal phase of the TiNx type.he examinations of crystalline size and lattice strain revealed thathe residual stress was compressive of approximately −1.5 GPa. Theaser surface alloying for hard coatings of steel was carried out byabu et al. [5]. They indicated that nitrogen shielding during therocess increased the surface hardness of the resulting coating.he effect of laser gas nitriding on the microstructure and corro-ion properties of Ti–6Al–4V alloy was carried out by Razavi etl. [6]. They showed that the microstructures consisted of a thinontinuous layer of titanium nitride followed by nearly perpendic-lar dendrites, and below this, a mixture of small dendrites and

arge needles were formed. Laser nitriding and powder alloying ofi–6Al–4V alloy surface were carried out by Baker and Selamat [7].hey showed that the preheating generated due to the overlap-ing was influenced by the laser power intensity and the nitrogenoncentration in laser processing. In addition, laser alloying withre-placed SiC powder produced the smoothest surface with theurface roughness (Ra) less than 2 �m. They further showed thathe main phases present at the surface and a depth of 300 �mere, first TiNx, where x is varied 0.5 and 0.75 depending on the

aser track position at the surface, being TiN0.75 at the surface, andecreasing with depth. Laser assisted surface nitriding of Ti–6Al–4Vlloy was carried out by Biswas et al. [8]. They showed that theicrohardness of the nitrided surface was improved considerably

s compared to as received workpiece surface hardness. In addition,urface nitriding increased the pit formation significantly as com-ared to as received alloy. The microstructure and wear propertiesf TiCN/Ti coatings on titanium alloy through a laser cladding werenvestigated by Yang et al. [9]. They showed that the claded zone

as composed of TiCN dendrites, TiO2, and Ti. In addition, micro-ardness of the laser-claded zone was increased about 3–6 times ofhe base substrate hardness. The surface carburizing of Ti–6Al–4Vlloy by a laser melting was studied by Saleh et al. [10]. They indi-ated that microhardness was directly associated with the volumeraction of the TiC compound and the volume fraction of the den-rites decreased with increasing laser power or increasing traversepeed. Yilbas et al. [11] examined the laser re-melting of plasmaitrided surface of Ti–6Al–4V alloy. They showed that white layereveloped at the workpiece surface during the plasma nitridingas mixed with the underlying material after the laser treatmentrocess without any defects such as cracks and voids. Yilbas et al.12] studies the laser gas-assisted nitriding of Ti–Al–6Al alloy. Theyndicated that uniform nitride layer with 50–80 �m depth could beormed in the surface region of the alloy and the microhardnessmproved because of the formation of TiN compound in the surfaceegion. Yilbas et al. [14] investigated the laser gas-assisted nitrid-ng of titanium alloys and the nitrogen diffusion into the surfaceegion. They demonstrated that the nitrogen diffusion is high inhe surface region and reduces significantly with increasing depthelow the surface. However, the studies were limited to the gas-ssisted nitriding process and the carbonitriding of the surface wasot incorporated in the previous studies.

In the light of the previous studies [11–14], the present studys carried out to examine the laser gas-assisted carbonitriding ofi–6Al–4V alloy surface initially coated with carbon film, in which

iC particles are imbedded. The microstructural and morphologicalhanges in the laser treated layer are examined using SEM, EDS, andRD. The residual stress developed in the irradiated layer is deter-ined from the analytical expression [15] and measured using theRD technique.

Nozzle diameter (mm) Focus setting (mm) N2 pressure (kPa)

1.5 127 600

2. Experimental

The CO2 laser (LC-ALPHAIII) delivering nominal output power of2 kW at pulse mode with different frequencies is used to irradiatethe workpiece surface. The nominal focal length of the focusing lensis 127 mm. Nitrogen assisting gas emerging from the conical nozzleand co-axially with the laser beam is used. The nitriding conditionsare given in Table 1.

The workpiece accommodated is Ti–6Al–4V alloy at 3 mm inthicknesses (30 mm × 40 mm × 3 mm: width × length × thickness).The water soluble phenolic resin (thermoset viny-phenolic resin)was applied to the workpiece surfaces and kept at 8 bar argon pres-sure and 175 ◦C for 2 h in the furnace [16]. Fig. 1 shows the SEMmicrograph of carbon coated surface. After ensuring the conver-sion of the phenolic matrix into carbon layer through monitoringthe density, TiC particles were inserted mechanically into the car-bon layer formed at the workpiece surfaces. The averaged size ofthe TiC particles was in the order of 6 �m and the volume fractionof TiC particles inserted into the carbon film was about 20%.

Material characterization of the laser nitrided surfaces was car-ried out using SEM, EDS, and XRD. Jeol 6460 electron microscopy isused for SEM examinations and Bruker D8 Advanced having MoK�radiation was used for XRD analysis. A typical setting of XRD was40 kV and 30 mA. The parabolically shaped Göbel Mirror was usedin Bruker D8 Advanced, which provided the highly parallel X-raybeams. The parallel beams from it removed the traditional lim-itations of the Bragg-Brentano geometry for powder diffractionand thin film investigations. Microphotonics digital microhardnesstester (MP-100TC) was used to obtain microhardness at the surfaceof the nitride layer. The standard test method for Vickers indenta-tion hardness of advanced ceramics (ASTM C1327-99) was adopted.The measurements were repeated three times at each location.

2.1. Residual stress measurement

Fig. 1. SEM micrograph of the surface of carbon coating.

B.S. Yilbas et al. / Applied Surface Science 257 (2010) 531–537 533

Fi

us

�

wanls(wdi±nTtdwu1ss(

2

ptmt

ig. 2. The d spacing measurement as a function of sin2 for Ti and TiN. The errors in the order of 2%.

al stress. The relationship between the peak shift and the residualtress (�) is given [17,18]:

= E

(1 + �)sin2

(dn − do)do

(1)

here E is Young’s modulus, � is Poisson’s ratio, is the tilt angle,nd dn are the d spacing measured at each tilt angle. If there areo shear strains present in the specimen, the d spacing changes

inearly with sin2 . The d spacing measurement as a function ofin2 is shown in Fig. 2a. The calculations were performed for TiN111) planes (2� = 36.6◦) and the elastic modules of laser nitridedorkpiece as 450 GPa [19]. It can be observed that no shear straineveloped in the specimen surface region and the residual stress

s compressive in the order of −1.63 GPa. The error estimated is35 MPa. Moreover, in the analytical formulation, the effect ofitrogen diffusion on the resulting residual stress field was omitted.his is mainly because of: (i) the penetration depth of MoK� radia-ion source, which is in the order of 20 �m, and (ii) TiN compoundistribution within this depth is not uniform. In this case, the stressas determined from the �-Ti (1 0 1) plane of Ti–6Al–4V alloy whilesing the elastic module and Poisson’s ratio of Ti–6Al–4V alloy as13 GPa and 0.342 [20], respectively. Fig. 2b shows d spacing mea-urement as a function of sin2 for Ti–6Al–4V alloy. The residualtress determined is compressive and it is in the order of −320 MPa±6 MPa).

.2. Analytical expression of the residual stress

The analytical expression for the residual stress was developedreviously is used [15]. The previous expression is based on thehermal expansion mismatched between the coating and the base

aterial. Although the analytical expression is developed for thehick coating (coating thickness of about 300–600 �m), it can also

Fig. 3. Optical and SEM micrographs of the laser treated workpiece surface.

be applied for the melted surface with thicknesses within the rangeof 50–100 �m when the thermal expansion mismatched betweenthe laser melted layer and the base material is present. More-over, the formation of cabonitride species in the surface vicinitycontributes to the elastic modules of the laser treated layer. Thiscontribution is omitted due to the depth of the formation of car-bonitride species, which is mainly limited in the surface vicinity.Therefore, the residual stress is:

� = [Ec(Tf − TR)(˛c − ˛s]

1 + 2(Ectc/Ests

) (2)

where Ec and Es are the elastic modules of the coating and the basematerial, tc and ts are the coating and base material thicknesses,Tf is the maximum temperature during laser heating and TR is theroom temperature after the cooling period is over, ˛c and ˛s arethe thermal expansion coefficient of the coating and the base mate-rial, respectively. Table 2 gives the data used for the residual stresscalculation using Eq. (2).

3. Results and discussion

The alloy surface was initially coated with a carbon film usinga water soluble phenolic resin and TiC powders were imbedded inthe coating prior to the laser treatment process.

Figs. 3 and 4 show optical and SEM micrographs of the lasertreated surface. The presence of the laser scanning trackers isevident from the SEM micrograph. The overlapping ratio of theirradiated spot is in the order of 60%, which appears as continu-ous melting of the surface in the micrograph. In addition, no cracksand surface defects, such as voids and large cavities are observed.

It should be noted that the large cavities are developed becauseof the evaporation of the surface during the excessive heating athigh laser power intensity. The crack free surface reveals that ther-mal stress developed in the surface region does not form cracksin this region. In addition, the level of the micro-stress formed,

534 B.S. Yilbas et al. / Applied Surface Science 257 (2010) 531–537

Table 2The data used in Eq. (2) [12].

˛c (1/K) ˛s (1/K) TR (K) Tf (K)

3 9.40 × 10−6 11 × 10−6 300 2300

dcsbsTcfgrocitsstasafpordhstf

lrirtrtlmt

Table 3EDS spectrums (wt%) and their locations at the surface. The error related to quan-tification of elemental analysis is in the order of 0.5%.

Spectrum N Al Ti V

Spectrum 1 5 2 Balance 3

Ec (GPa) Es (GPa) tc (m) ts (m)

606 115 55 × 10−6 3.0 × 10−

ue to the grain refinement at high cooling rates, is not signifi-antly high to cause the crack initiation at the micro-level in theurface region. Moreover, during the multi-scanning of the surfacey the laser beam acts as post-heat treatment process for initiallycanned tracks while lowering the stress levels in the treated layer.he similar observation is reported in the previous study [2]. Thelose examination of the surface at high magnification, as observedrom SEM micrograph, reveals that fine structures with nano-sizedrains are formed in the surface vicinity as compared to that cor-esponding to some depth below the treated surface (Fig. 5). Thisccurs because of the followings: (i) formation of TiN and Ti(C,N)ompounds causes volume shrinkage in the surface region result-ng in dense structures [3] and (ii) high cooling rates after the laserreatment process causes grain refinement in the surface region. Ithould be noted the formation of fine structures consist of nano-ized grains are measured in the surface region of the laser nitrideditanium [21]. It should be noted that temperature at the surfacettains considerably high values, since TiC appears to be almost dis-olved totally in the surface region and TiC particles is not clearlypparent at the treated surface. The averaged roughness of the sur-ace (Ra) is in the order of 4.5 �m, which is less than the TiC averagearticle size. This also indicates the melting and/or partial meltingf TiC particles. In addition, the attainment of relatively high surfaceoughness value is associated with the high surface temperatureuring the treatment process. In this case, the melt flow under theigh assisting gas pressure contributes to the enhancement of theurface roughness. The microscale surface texture irregularities dueo the melt flow at high assisting gas pressure are also observedrom SEM micrograph (Fig. 4).

Fig. 5 shows SEM micrographs of the cross-sectional view of theaser treated surface. The fine structures are evident in the surfaceegion of the laser treated layer. However, fine dendrites are formedn the vicinity of the surface indicating nitrogen-rich species in thisegion [6]. In addition, feathery-like structures are also observed inhis region, which is also associated with the presence of nitrogen-ich nitride compounds. Moreover, the volume shrinkage due to

he formation of dense structure in the surface region results inocally scattered few microsize voids, which is observed in SEM

icrograph. However, partially melted TiC particles embedded inhe surface region is also observed. This appears to be few in number

Fig. 4. SEM micrographs of the laser treated surface texture irregularities.

Spectrum 2 4 2 Balance 3Spectrum 3 9 4 Balance 2Spectrum 4 10 3 Balance 3

and mainly located in the vicinity of the surface. A typical partiallymelted TiC particle is shown in Fig. 6. Moreover, a dense layer isformed around the partially melted particle. This indicates the for-mation of nitride species in the molten part of the particle as wellas its surrounding. Although the thermal expansion coefficient ofnitride species and TiC are different, no micro-crack is observedaround the partially melted particles. The elongated dendrites areobserved at some distance from the treated surface towards thesolid bulk. The presence of the elongated dendrites reveals the for-mation of titanium nitride compounds in this region. The possibilityof forming nitrocarbide compounds in this region is less likely, sincethey are associated with small dendrites or fine structures. As thedepth below the treated surface increases further, the short needle-like structures are observed. This shows the presence of ˛′-Ti/TiNx

in this region [2]. However, as the distance increases further, smallneedle-like structures replaces with large needle-like structures,which is associated with the heat-affected zone. The size of heat-affected zone is narrow because of the low thermal diffusivity ofthe alloy which suppresses the heat conduction in the heat-affectedzone.

Fig. 7 shows XRD diffractograms of as received and laser treatedsurfaces. It is evident that TiC, TiNx, Ti2N, TiCxN1−x peaks is visible inthe diffractogram. It should be noted that TiN, Ti(C,N), TiC are iden-tified in accordance with ICDD 38-1420 card, ICDD 42-1488 card,and ICDD 32-1383 card, respectively. The presence of TiC peaksreveals that partially dissolved TiC particles are present in the sur-face region. In addition, the formation of new TiC compounds atelevated temperature is also possible due to the presence of car-bon film at the workpiece surface, which is formed prior to thelaser treatment process. The presence of TiNx is associated withthe fast solidification rate, which leads to a non-equilibrium statedue to the time shortage for the interaction between Ti and N toform stoichiometric TiN in the surface region. Moreover, TiN2 peakis associated with the formation of metastable TiNx compound. Themetastable phase is formed because of high cooling rates resultingin non-equilibrium process [2]. Due to the gradual heating resultedfrom the laser scanning of the neighboring tracks, the dissolu-

tion of metastable titanium nitride and the diffusion of nitrogenfrom the supersaturated titanium would give rise to formation ofTi2N compound in the surface region. This takes place according toaTiNx → bTi2N+cTi. Moreover, the presence of non-uniform nitro-

Table 4Microhardness test results at the surface.

Microhardness (HV)

As received surface 305Present study 1650 ± 40Laser nitrided [8,21] 1260 ± 30Plasma nitrided [11] 1050Plasma nitrided [22] 925

B.S. Yilbas et al. / Applied Surface Science 257 (2010) 531–537 535

laser treated layer cross-section.

gicp

pi[

TTl

Fig. 5. SEM micrographs of

en concentration at the surface is evident from EDS results, whichs given in Table 3. This is attributed to formation of different nitrideompounds at the surface as well as non-equilibrium process takinglace during non-uniform cooling of the surface.

Table 4 gives the microhardness results obtained from theresent study and the previous studies in relation to laser nitrid-

ng [8] and [22] as well as plasma nitriding of Ti–6Al–4V surfaces11] and [23]. It should be noted that the microhardness across

able 5he residual stress predicted and measured at different depths inside the nitrideayer.

Predictions (GPa) XRD technique (GPa)

Present study −1.63 −1.55 ± 0.30(Ti,Al)N coating [23] −3.11Laser Nitrided Ti–6Al–4V [11] −0.265

Fig. 6. SEM micrograph of partially melted TiC particles in the surface vicinity.

536 B.S. Yilbas et al. / Applied Surface Science 257 (2010) 531–537

ed Ti–

ttsbwrftsutfhsctfttlf

4

rTgmriuratT

Fig. 7. XRD diffractogram for as receiv

he treated layer could not be measured with accuracy, since thereated layer thickness extends 50 �m depth below the surface. Theurface microhardness increases significantly as compared to thease material hardness. The increase in the hardness is associatedith the formation of compact and fine structures in the surface

egion of the workpiece. In addition, the microhardness resultedrom the carbonitrided surface is higher than those correspondingo the laser nitrided surface [8] and [22], and the plasma nitridedurfaces [11] and [23]. Table 5 gives the residual stress predictedsing the analytical formula developed previously [15] and fromhe XRD technique. In addition, the residual stresses determinedrom the XRD technique for laser assisted nitriding [12] and (Ti,Al)Nard coating [24] are also included for comparison. The residualtress predicted agrees with the measured data. However, the dis-repancy between both results is associated with the assumption inhe formulation, which considers the uniform structure at the sur-ace. This may not be very true. It should be noted that the phaseransformation during forming Ti2N changes the stress states inhe surface region [2]. Consequently gradual heating during theaser scanning of the neighboring tracks reduces the residual stressormed in the surface region.

. Conclusions

Laser gas-assisted processing of Ti–6Al–4V alloy surface is car-ied out. The workpiece is initially coated with the carbon layer andiC particles were impeded in the carbon coating prior to the laseras-assisted processing. Optical and SEM micrographs are used fororphological and microstructural analysis while XRD is incorpo-

ated to obtain the residual stresses in the laser treated layer. Its found that the laser treated surface is free from surface irreg-

larities such as cavities and cracks. Due to the high overlappingatio of the irradiated spot at the surface, the laser scanning tracksppear as continuous melting sites. The melt flow, because of highemperature heating at the surface, and the partial dissolution ofiC particles increase the average surface roughness of the treated

6Al–4V alloy and laser treated surface.

layer. The carbon coating and TiC embedded in the coating facilitatethe formation of TiCxN1−x, which contributed to surface hardnessand the formation of the compact and dense layer in the surfaceregion of the treated layer. This results in a few scattered micro-sized voids in the surface vicinity of the treated layer. The smalldendrites and feathery-like structures are formed in the surfaceregion, which reveal the presence of nitride species in this region.As the depth increases below the surface needle-like structuresare formed. The size of the heat-affected zone is narrow becauseof the low thermal diffusivity of the titanium alloy. TiCxN1−x, TiN,TiNx, TiC, and Ti2N compounds are formed in the surface region,which contribute to the enhancement of the surface hardness of thetreated layer. The microhardness increases significantly after thelaser treatment process; however, non-uniform formation of thenitride reach compounds in the surfaces vicinity alters the micro-hardness at the treated surface. The residual stress predicted fromthe early formulation agrees with the data obtained from the XRDtechnique.

Acknowledgements

The authors acknowledge the support of King Fahd Universityof Petroleum and Minerals, Dhahran, Saudi Arabia, for the currentwork.

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