surface treatment of titanium by laser irradiation to improve resistance to dry-sliding friction

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Ž . Wear 236 1999 39–46 www.elsevier.comrlocaterwear Surface treatment of titanium by laser irradiation to improve resistance to dry-sliding friction B. Courant ) , J.J. Hantzpergue, S. Benayoun Laboratoire de Physico-Chimie des Surfaces, ENSAM, 2 BouleÕard du Ronceray, BP 3525, Angers Cedex 49035, France Received 1 February 1999; received in revised form 20 May 1999; accepted 13 June 1999 Abstract This experimental study was performed with intent to improve the surface properties of titanium and to minimize the friction on ceramic. Its main object was to strengthen the resistance against wear resulting from dry-sliding contact. Titanium-covered or not, graphite powder was irradiated and superficially melted by means of a pulsed Nd–YAG laser beam. Different irradiations were carried out, changing the distance between irradiated surface and laser beam focal plane. The variations in the dry-sliding friction coefficient were recorded using a pin-on-disc tribometer. An optimum irradiation of graphite-covered titanium generated hard granular titanium carbide and lubricating graphite inclusions which drastically reduced dry friction and wear of the Laser-Melted Zone. In this way, a self-lubricating composite coating has been successfully synthesized on titanium from the laser melting process. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Antiwear treatment; Dry-sliding friction; Laser melting; Self-lubricating coating; Titanium 1. Introduction Titanium and its alloys are materials used in aeronautics or astronautics because of their light density and toughness comparable to the best steels. However, their behaviour during dry friction with sliding or rolling contact is rarely satisfactory. Surface melting by laser irradiation with material adding is the process that allows the most possibilities among the surface treatments which are able to improve the tribologi- cal behaviour of metallic materials. The addition elements wx wx wx are varied, e.g., other metal 1 , nitrogen 2 , bore 3 or wx carbon 4 . Acting on the irradiation parameters, it is possible to range strongly the concentration of addition Ž . element in the transformed zone Laser-Melted Zone and also its thickness from about 10 mm up to some millime- ters. The power laser beam allows to irradiate and to modify locally the surface of the parts, like other energy beams. Moreover, its use is easier because the surface does not need to be in vacuum during irradiation, contrary to the use of ion or electron beams. ) Corresponding author. LAMM-CRTT, Boulevard de l’Universite, BP ´ 406, Saint-Nazaire Cedex 44602, France. Tel.: q33-2-40-14-26-27; fax: q33-2-40-17-26-18; E-mail: [email protected] Great progress has been recently noticed concerning the synthesis of coatings reinforced by particles of ceramic Ž . wx such as titanium carbide TiC 5 , and the development of wx low friction and antiwear coatings 6 , with a view to improve the resistance of the coated materials to dry friction. wx A previous study 7 concerned the formation of surface alloys made of titanium a-Ti and carbide, TiC . The 1yx pulsed laser irradiation of titanium surfaces covered with a graphite powder deposit 20 mm thick allowed the Vickers microhardness in the Laser-Melted Zone to be multiplied by four. Graphite as a solid lubricant has been well-known for a wx wx long time 8 . The mechanisms of friction and wear 9 are clarified for this self-lubricating solid, and also its lubrica- w x tion mechanism 10 . Such a material can be mixed with sintered metals to achieve self-lubricating characteristics w x 11 . The purpose of this study is to improve the surface properties of titanium and to minimize especially the fric- tion on ceramic and the wear by dry-sliding contact. Irradiation of titanium by pulsed Nd–YAG laser beam with deposition of graphite powder gives two possibilities. The first one squares with the surface melting which induces partial carbidation and hardening of the metal, as 0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0043-1648 99 00254-9

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Page 1: Surface treatment of titanium by laser irradiation to improve resistance to dry-sliding friction

Ž .Wear 236 1999 39–46www.elsevier.comrlocaterwear

Surface treatment of titanium by laser irradiation to improve resistanceto dry-sliding friction

B. Courant ), J.J. Hantzpergue, S. BenayounLaboratoire de Physico-Chimie des Surfaces, ENSAM, 2 BouleÕard du Ronceray, BP 3525, Angers Cedex 49035, France

Received 1 February 1999; received in revised form 20 May 1999; accepted 13 June 1999

Abstract

This experimental study was performed with intent to improve the surface properties of titanium and to minimize the friction onceramic. Its main object was to strengthen the resistance against wear resulting from dry-sliding contact. Titanium-covered or not,graphite powder was irradiated and superficially melted by means of a pulsed Nd–YAG laser beam. Different irradiations were carriedout, changing the distance between irradiated surface and laser beam focal plane. The variations in the dry-sliding friction coefficientwere recorded using a pin-on-disc tribometer. An optimum irradiation of graphite-covered titanium generated hard granular titaniumcarbide and lubricating graphite inclusions which drastically reduced dry friction and wear of the Laser-Melted Zone. In this way, aself-lubricating composite coating has been successfully synthesized on titanium from the laser melting process. q 1999 Elsevier ScienceS.A. All rights reserved.

Keywords: Antiwear treatment; Dry-sliding friction; Laser melting; Self-lubricating coating; Titanium

1. Introduction

Titanium and its alloys are materials used in aeronauticsor astronautics because of their light density and toughnesscomparable to the best steels. However, their behaviourduring dry friction with sliding or rolling contact is rarelysatisfactory.

Surface melting by laser irradiation with material addingis the process that allows the most possibilities among thesurface treatments which are able to improve the tribologi-cal behaviour of metallic materials. The addition elements

w x w x w xare varied, e.g., other metal 1 , nitrogen 2 , bore 3 orw xcarbon 4 . Acting on the irradiation parameters, it is

possible to range strongly the concentration of additionŽ .element in the transformed zone Laser-Melted Zone and

also its thickness from about 10 mm up to some millime-ters. The power laser beam allows to irradiate and tomodify locally the surface of the parts, like other energybeams. Moreover, its use is easier because the surface doesnot need to be in vacuum during irradiation, contrary to theuse of ion or electron beams.

) Corresponding author. LAMM-CRTT, Boulevard de l’Universite, BP´406, Saint-Nazaire Cedex 44602, France. Tel.: q33-2-40-14-26-27; fax:q33-2-40-17-26-18; E-mail: [email protected]

Great progress has been recently noticed concerning thesynthesis of coatings reinforced by particles of ceramic

Ž . w xsuch as titanium carbide TiC 5 , and the development ofw xlow friction and antiwear coatings 6 , with a view to

improve the resistance of the coated materials to dryfriction.

w xA previous study 7 concerned the formation of surfacealloys made of titanium a-Ti and carbide, TiC . The1yx

pulsed laser irradiation of titanium surfaces covered with agraphite powder deposit 20 mm thick allowed the Vickersmicrohardness in the Laser-Melted Zone to be multipliedby four.

Graphite as a solid lubricant has been well-known for aw x w xlong time 8 . The mechanisms of friction and wear 9 are

clarified for this self-lubricating solid, and also its lubrica-w xtion mechanism 10 . Such a material can be mixed with

sintered metals to achieve self-lubricating characteristicsw x11 .

The purpose of this study is to improve the surfaceproperties of titanium and to minimize especially the fric-tion on ceramic and the wear by dry-sliding contact.Irradiation of titanium by pulsed Nd–YAG laser beamwith deposition of graphite powder gives two possibilities.The first one squares with the surface melting whichinduces partial carbidation and hardening of the metal, as

0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0043-1648 99 00254-9

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( )B. Courant et al.rWear 236 1999 39–4640

Fig. 1. Formation of coatings made of titanium and carbon by laserirradiation of titanium covered with graphite powder.

the result of the formation of surface alloy. The second onesquares with the incorporation of a strong graphite amountunder inclusion state which induces the surface synthesisof a real self-lubricating composite material.

2. Experiments

The surface of titanium was irradiated by means of aNd–YAG laser beam which was pulsed by relaxed mode.Wavelength of laser radiation was 1.064 mm, whereasmaximum transmitted power was 300 W. The samples

Ž 2 .were square 20=20 mm titanium foils 3.2 mm thick,with grade of 99.67%. Each titanium sample was polishedbefore irradiation in order to exhibit always the samesurface state. Addition material was graphite powder of

Ž .micrometric grain size 1 to 2 mm . This powder wasmixed with water and ethanol to obtain regular deposit ontitanium surface. The graphite deposit was then dried inhot air. Thickness of graphite deposit ranged always be-tween 18 and 22 mm. This one was deduced from theweighings before and after graphite deposition, knowingthe area of deposit and the specific mass of graphite.

Table 1Operative parameters of pulsed irradiationThe values have been selected from numerous experiments achieved

w xduring a preliminary study 12 .y1 y1Ž . Ž . Ž . Ž . Ž .E J t ms f s V mm s DY mm

5.0"0.3 12.0"0.1 25.0"0.1 7.34 500

Table 2Distances between focal plane and irradiated sample surface correspond-ing to the different irradiation conditions

Ž .Irradiation condition Distance of focal plane mm

1 y0.32 4.73 7.74 9.75 14.7

Fig. 2. Schemas of the area irradiated during one pulse and the superposi-tion of two areas successively irradiated.

The titanium samples, with or without graphite deposit,were mounted on an X–Y numerically driven table andthen were irradiated by scanning of the pulsed laser beam.Their surface therefore melted with or without carbidationof titanium. The titanium surfaces without graphite depositwere blackened in smoke to induce a strong absorption ofthe laser radiation as for titanium covered with graphite.

Ž 3 y1.An argon flow 10 dm min prevented the oxidation ofŽ .titanium and graphite during their irradiation Fig. 1 . The

first micrometer depth of the samples was then analysedwith the electron microprobe by energy dispersion spec-

Ž .trometry EDS . Under these circumstances, the surfaceoxygen content always was below the detection threshold,

w xi.e., below the atom percentage by about 3% 12 .The testing of the dry-sliding friction was performed

with a pin-on-disc tribometer. The plane titanium samplewas mounted on the rotary holder after irradiation and

Žrubbed on a spherical ceramic pin 6 mm diameter ruby.ball with the load force, L. The friction force, F, was

measured by an inductive sensor which recorded the strainof the pin holder arm. The friction coefficient, FrL, wasrecorded vs. the rotation number under the conditions ofdry-sliding in moist air at 258C with relative moisturecontent of about 60%. The parameters for these frictiontests were chosen as follows:

Ž .– load force, L 5 N ;Ž .– friction track radius 4 mm ; and

Ž y1 .– rotation speed 120 turns min .The relief induced on titanium sample by laser meltingprevented the use of a profilometer to measure the wear ofthe disc after pin-on-disc tests. Titanium sample and ruby

Fig. 3. Relative variations of the pertinent parameters D and R vs.radius, r, of area irradiated during one pulse.

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( )B. Courant et al.rWear 236 1999 39–46 41

Fig. 4. Mass percentage of incorporated carbon vs. distance between focalplane and irradiated sample surface. This percentage is related to theirradiated carbon mass.

ball were therefore weighed before and after friction test todeduce the mass loss resulting from wear.

3. Irradiation conditions

The irradiation conditions were dependent on the opera-tive parameters below:

– pulse energy, E;– pulse duration, t ;– frequency of pulse generating, f ;– scanning speed along the axis X of the laser beam, V;and– translation along the axis Y between two scannings ofthe beam, DY.

Fig. 6. Evolution of the friction coefficient of the titanium sampleŽ .blackened in smoke and then irradiated under condition 5 .

The same values of operative parameters were used forŽ .all the irradiations Table 1 . Only distance between focal

Ž .plane and sample surface was modified Table 2 . Thisdistance was deduced from the distance between samplesurface and condensation lens of the beam, knowing thefocal length, i.e., 62.4 mm. So, only the radius of the laserbeam in the interaction plane, r, was modified for thedifferent irradiations. Two pertinent parameters directlyregulated the heat transfer induced in the metal volumefrom the irradiation and therefore regulated the thickness

w xof the resulting Melted Zone 13 . This concerned thesurface density of energy deposited by each pulse, D, andthe superposition ratio of two areas successively irradiated,R. These two parameters are dependent on the four opera-tive parameters E, t , f and V, and also on the radius of

Ž . Ž .Fig. 5. Evolutions of the friction coefficient, after irradiation under conditions 1 to 4 , of titanium samples either blackened in smoke, or covered withgraphite powder layer 20 mm thick.

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( )B. Courant et al.rWear 236 1999 39–4642

area irradiated during each pulse, r. According to Fig. 2,Ž .the parameters D and R are defined by expressions 1

Ž .and 2 below:

EDs , 1Ž .22t Vrqp r

12 rq ty Vž /bc f

Rs s . 2Ž .ac 2 rqt V

w xA preliminary study 13 showed from weighings thatthe percentage of carbon incorporated into titanium bymetal surface melting was strongly dependent on the su-perposition ratio, R. For different irradiation conditionswith the ratio Rs0.9, the incorporated mass percentageranged from 53.3% to 72.5%. With the ratio Rs0.5, thisone ranged only from 17.1% to 36.9%. A strong superposi-

tion ratio reduces the losses of graphite powder by ejectaand therefore increases the proportion of carbon incorpo-rated into titanium.

As shown in Fig. 3, the radius growth of the areairradiated during one pulse produces a decrease of energydensity, D, very superior to the increase of superpositionratio, R. Area irradiated during one pulse is greater ifdistance between sample surface and focal plane of beamis longer. The small increase of superposition ratio, R, haslittle influence on the quantity of incorporated carbon. Incompensation, the strong decrease of energy density, D,reduces strongly the thickness of the Laser-Melted Zonewhich is more enriched with carbon. It is therefore possi-ble to obtain a Laser-Melted Zone made of metallic tita-

Ž . Ž .nium a-Ti and titanium carbide TiC which encloses1yx

numerous graphite inclusions. These inclusions resultedfrom the graphite quantity which have not been dissolved

Ž .Fig. 7. Evolution of the friction coefficient of the titanium sample covered with graphite powder layer 20 mm thick and then irradiated under condition 5 .

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( )B. Courant et al.rWear 236 1999 39–46 43

Fig. 8. Feature of the friction track after 475 000 rotations on titanium; the Laser-Melted Zone is rich in graphite inclusions.

in the melting bath. This presence of graphite in theLaser-Melted Zone allows us to expect an improved tribo-logical behaviour.

4. Results and discussion

A titanium foil covered with graphite deposit 20 mmthick and a titanium foil blackened in smoke were irradi-ated for each distance given in Table 2. The objective wasto appraise the actual influence of carbon incorporated intotitanium of the Laser-Melted Zone.

The mass percentage of incorporated carbon is plottedvs. the distance between focal plane and irradiated surfaceŽ .Fig. 4 . This percentage is weakly dependent on this

distance. On the other hand, the energy density, D, is adecreasing function of this same distance. Therefore, themore the focal plane is distant, the more the Laser-MeltedZone is thin and the more the global concentration ofcarbon incorporated to titanium is increased.

The variations in friction coefficients are comparedbetween titanium samples with or without pre-deposit of

Ž . Ž .graphite Fig. 5 which were irradiated in conditions 1 toŽ . Ž .4 Table 2 . No improvement of tribological behaviour of

Ž .titanium sliding on ceramic ruby was noticed after theseirradiations which induced melting of the metal surface.

Friction tests by sliding on non-modified titanium sur-w xface have been discussed in a preliminary study 14 . Three

successive transitional periods of friction and wear wererecorded before a virtually steady state was reached.

Fig. 9. Feature of the ruby ball after friction on titanium surface rich in graphite inclusions during 475 000 rotations. The feeding suggests a weak titaniumŽ .transfer on ceramic ruby by adhesion.

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( )B. Courant et al.rWear 236 1999 39–4644

Ø The first was characterized by a fast increase offriction coefficient and corresponded to the accommoda-tion of the sliding contact between both bodies, withtitanium adhesion on the ruby ball and formation of atransfer layer. Titanium first suffered from adhesive wear.

Ø The second period began after stripping and fragmen-tation of the transfer layer to give fine titanium flakes asthird body. Accumulation of these particles on frictiontrack and morphology of these particles induced a weakdecrease of friction. The adhesive wear was softened dur-ing this period.

Ø The third friction period was characterized by astrong increase of friction whereas formation of scratcheson the track disclosed that titanium was subject to abrasivewear. The grinding of the third body gave finer particleswhich became oxidized, hard and abrasive.

Ø The abrasive wear was continuing and the coefficientof friction between titanium and ceramic stabilized about0.6 during the fourth and last period.

Surface melting of titanium covered with graphite de-posit induces partial carbidation of the metal. The particles

Fig. 10. Scanning electron microscopy micrographies of the cross-section of the titanium sample covered with graphite powder and irradiated in conditionŽ . Ž . Ž .5 . a NAZsNon-Affected Zone of titanium with equiaxis microstructure; HAZsHeat-Affected Zone of titanium ;50 mm thick with acicular

Ž . Ž .microstructure; LMZsLaser-Melted Zone ;20 mm thick as composite coating made of a-Ti, TiC and C graphite ; FSCs free surface of coating:1y xŽ . Ž . Ž . Ž .1 a-Ti™hexagonal metallic titanium as matrix; 2 TiC ™granular titanium carbide grain size ;1 mm ; 3 C™carbon under the state of graphite1y x

Ž . Ž . Ž .inclusions of size 2–20 mm. b Local view of the Laser-Melted Zone LMZ showing the granular structure of the phase TiC grain size 0.5–1.5 mm .1y x

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( )B. Courant et al.rWear 236 1999 39–46 45

of the synthesized hard phase TiC are abrasive. The1yx

titanium transfer layer on ruby ball is subject to abrasivewear and this wear process prevents fragmentation of thelayer with titanium flake formation. Weak decrease offriction coefficient is therefore prevented, except for the

Ž .irradiation condition 1 where abrasive phase TiC is in1yx

the minority compared with ductile phase of metallictitanium.

A different behaviour is recorded for titanium irradiatedŽ .under condition 5 . The friction coefficient slowly in-

Žcreases during 6000 rotations up to a value about 0.6 Fig..6 . Wear of friction track and third body then become

suddenly visible whereas friction coefficient decreases upto about 0.5 because of the formation of third body. Thefriction coefficient then slowly reaches its previous valueof 0.6. The same periods of friction and wear follow one

Ž . Ž .another as for irradiation conditions 1 to 4 , but they aremuch longer. A thinner titanium layer is melted with

Ž .condition 5 because the laser beam is very defocused onsample surface. Titanium is blackened in smoke beforeirradiation only to allow stronger absorption of the laserradiation. In this case, the weak amount of depositedamorphous carbon is less diluted in titanium of Laser-

Ž . Ž .Melted Zone than under conditions 1 to 4 . The presenceof this carbon probably reduces titanium transfer on ce-ramic during the first period and therefore slows downfriction increase.

The strength of sliding friction on ceramic is minimizedand resistance to wear of titanium is strongly improvedŽ .Fig. 7 after treatment by deposition of graphite layer 20

Ž .mm thick and then irradiation under condition 5 . Friction<coefficient does not exceed 0.35 after 475 000 rotations,

whereas no mass loss of the titanium sample and the rubyball is recorded. Friction track is shallow and lack of

Ž .scratches suggests the lack of abrasive wear Fig. 8 . Thirdbody as a very fine grey powder is generated in infinitesi-mal amount. Adhesion of titanium on ceramic takes placebecause a fading of contact surface on ruby ball is appar-

Ž .ent Fig. 9 . The very thin Laser-Melted Zone about 20mm thick on this titanium sample contains titanium car-bide, TiC , but also numerous big inclusions of graphite1yxŽ .Fig. 10 . This graphite acts as solid lubricant duringsliding. Hard phase TiC and graphite reduce much the1yx

titanium transfer by adhesion on the ceramic and therefore,slow down much the wear of the metal.

5. Conclusion

It is possible to modify superposition ratio, R, of thepulses and also energy density, D, deposited on irradiatedsurface by each pulse, by changing distance betweengraphite-covered titanium surface and focal plane of thepulsed laser beam. Energy density strongly acts on totalconcentration of incorporated carbon in Laser-Melted Zone.

Tribological behaviour of titanium is slightly modified ifthis surface zone is only made of a cermet resulting fromthe mixture of titanium carbide as hard phase with metallictitanium as ductile phase. But this behaviour is muchimproved if the Laser-Melted Zone is more enriched withcarbon, owing to optimisation of the irradiation conditions.Such a surface zone is also made of a third phase in theform of numerous graphite inclusions acting as solid lubri-cant. Dry-sliding friction between ceramic and titaniumtreated in this manner is minimized whereas adhesive wearof the metal is reduced much, and its abrasive wear isprevented, by the self-lubricating composite coating syn-thesized from the laser melting process.

Acknowledgements

The authors are grateful to Dr. L. Fouilland-PailleŽ .ENSAM, Chalons-en Champagne, France and Dr. S.ˆ

Ž .Ettaqi ENSAM, Meknes, Maroc for their helpful discus-`Žsions. They also wish to thank Dr. A. Perronet LAMM,

.Saint-Nazaire, France for her contribution to Fig. 10.

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