Available online at www.sciencedirect.com

Wear 264 (2008) 157165

Improving the wear properties of StellonH.irmin

ber 22007

Abstract

This resea by dend, a serie nitrobearing gas d laymicrohardne ng a rExperimenta haseboth carbon recipStellite 21 a atmeNaCl solutio 2007 Else

Keywords: Stellite 21; Plasma surface alloying; Wear; Carbon and nitrogen

1. Introduction

Owingare widelyrials in a rvalves, valv[1,2]. Becaalloys are a(i.e. corrospetrochemially, thesestrengthenibides and t

Howevesome demacause lossical or mecin severalvalve seats

CorresponE-mail ad

radioactive Co60, which results in high risks and costs during

0043-1648/$doi:10.1016/jto their high wear resistance, cobalt-based alloysused as wear-resistant bulk and hardfacing mate-

ange of industrial sectors for such applications ase seats, bearings, bushings, sleeves, dies and punchesuse of their good corrosion resistance, cobalt-basedlso used for combating wear in corrosive mediums

ionwear), and find applications in the chemical &cal, medical and food processing industries. Gener-good properties arise mainly from the solid solutionng of chromium and carbon, the formation of car-he Cr-induced good corrosion resistance [13].r, wear and corrosionwear are still concerns fornding applications. Wear and corrosionwear can

of material from solid surface as a result of mechan-hanicalchemical action, and wear debris will resultproblems. For example, cobalt in wear debris fromused in nuclear power plants can be converted into

ding author. Tel.: +44 121 414 5197; fax: +44 121 414 7373.dress: h.dong.20@bham.ac.uk (H. Dong).

operation [4,5]; bearings made of Stellite alloys are commonlyused to support guiding rolls in hot dip continuous galvaniz-ing lines. Because the severe working environment of elevatedtemperature (450470 C) and corrosive medium (molten zinc)makes it difficult to adequately lubricate the bearings, the bear-ings often experience severe wear in a relatively short periodof time [6]. Recently, Malayoglu and Neville [7] have inves-tigated the erosioncorrosion behaviour of Stellite 6 and 706alloys under liquidsolid erosion conditions in 3.5% NaCl liquidmedium.

Plasma surface alloying (PSA) technology has been success-fully applied to austenitic stainless steels to improve their wearresistance through the formation of an interstitial (carbon ornitrogen) supersaturated hard surface layer, named as S-phase,which has good wear, corrosion and corrosionwear resistance[813]. Recently, a novel plasma surface alloying process hasbeen developed to achieve combined improvements in corrosionand wear properties of cobalt-based alloys [14]. This paperreports the mechanical and tribological properties of plasmasurface alloyed Stellite 21 alloy simultaneously with carbonand nitrogen. Based on the experimental results, the mecha-nisms involved in the surface hardening and wear reduction

see front matter 2007 Elsevier B.V. All rights reserved..wear.2006.12.012surface alloying with carbJ. Chen, X.Y. Li, T. Bell,

Department of Metallurgy and Materials, the University of BReceived 15 August 2006; received in revised form 18 Decem

Available online 21 March

rch was directed at improving the wear resistance of Stellite 21 alloys of plasma surface alloying (PSA) treatments with both carbon andmixture at temperatures ranging from 400 to 550 C. The surface alloyess tester. Detailed tribological characterization was carried out by usil results have demonstrated that C and N supersaturated metastable pand nitrogen but at a high temperature of 550 C, chromium nitrides plloy can be significantly increased by 23 times following the PSA tren can be reduced by 99% and 96%, respectively.vier B.V. All rights reserved.ite 21 alloy by plasmaand nitrogen

Dong gham, Birmingham B15 2TT, UK006; accepted 19 December 2006

eveloping novel surface engineering technologies. To thisgen were conducted using a DC plasma unit with CNers were analyzed by XRD, GDS, SEM/EDX, TEM and aeciprocating tribometer in air and in 3.5% NaCl solution., i.e. S-phase, can be formed by the PSA treatments withitated in the surface treated layer. The surface hardness ofnts. The wear factor of Stellite 21 alloy in air and in 3.5%

158 J. Chen et al. / Wear 264 (2008) 157165

Fig

following pdiscussed.

2. Experim

2.1. Mater

The matbased alloy(UK). Theis 29.8Cr,microstrucsolid soluti

Plate sp5 mm thick1200 grit. Pwere clean

PSA trecarried outment condiconducted.

2.2. Mater

Phase idX-ray Diff0.154 nm).size and tim(XTEM) sa

Fig. 2. Schematic diagram of reciprocating tribometer.

trea5 mm

sur

m inth sim.

mm

k sidasse

lishece o

en stECOr chal m

nvesfacehardnf 25 gata,eas

ribol

ecipresistr coar te

Table 1PSA treatmen

Temperature (Time (h)Total layer thThickness of. 1. Typical microstructure of as-cast Stellite 21 alloy.

lasma surface alloying with carbon and nitrogen are

ental

ials and PSA treatments

erial used in this research was cast Stellite 21 cobalt-which was supplied by Deloro Stellite Company

main chemical composition (at.%) of the material1.4C, 2.4Ni, 3.3Mo and balance Co. The typical

ture of the material consisted of dendrites of Co-richon matrix and darker interdendritic carbides (Fig. 1).ecimens 20 mm 20 mm 5 mm were cut fromsheets and then wet ground with SiC paper down torior to plasma surface alloying treatments, samples

ed in acetone and dried with hot air.atments with methane, nitrogen and hydrogen werein a 40 kw Klockner DC plasma unit. Different treat-tions, as summarized in Table 1, were designed and

430 Cand 1.treated1000tre. Boto 200and 1unstucforcedIon Pointerfawas th

A Lused fosectionwere i

SurMicroload oness dthree m

2.3. T

A rwear r

ones foing weials characterization

entification was carried out with an Xpert Philipsractometer using a Cu K radiation (wavelengthThe scanning was from 20 to 100 at a scan stepe of 0.02 and 3 s, respectively. Cross-section TEMmple was prepared using the following steps. The

WC/Co basurface undboth in aiwas 10 mmsliding disby integratby a stylus

t conditions and thickness of the total and SN layers

Temperature effectC) 400 460 500

15 15 15ickness (m) 6.26 9.78 13.01SN layer (m) 2.42 2.63 4.62ted sample was sectioned into 2 slabs (3 mm in widthin thickness). The slabs were then glued with the

faces facing each other. The assembly was cut tothickness with the glued treated layers in the cen-

des of the assembly were grinded and polished downThe thin assembly was then glue to a 3 mm (o.d.)

(i.d.) brass reinforcing disc using G-1 epoxy. Thee was further polished to 60m. Finally, the rein-mbly was ion beam thinned by a Gatan 691 Precisionr System (PIPSTM) to form the thin area around thef the two glued treated surface. The XTEM sampleudied by Philips CM20.

GDS-750 QDP glow discharge spectrometer wasemical composition depth-profile analysis. The cross-icrostructures of plasma surface engineered samples

tigated using SEM (JOEL7000) with EDX.hardness was measured using a Leitz Miniload 2ess Tester with a Vickers indenter under an appliedf (0.25 N). In view of the scatter of the microhard-

the hardness value reported was a mean of at leasturements made under the same conditions.

ogical tests

ocating tribometer was used to evaluate the slidingance of the treated samples as well as the untreatedmparison. The schematic diagram of the reciprocat-ster is shown in Fig. 2. The counterpart, a 12.5 mmll, moved backwards and forwards on the sampleser the applied load, 10 N. The tests were conducted

r and in 3.5% NaCl solution. The sliding stroke, the frequency was 32 rpm (0.53 HZ) and the total

tance was 160 m. The wear volume was calculateding the area across the wear scar profile measured

profilometer, and then multiplying the length of the

Time effect

550 460 460 46015 10 15 2027.93 9.50 9.78 11.17

7.98 2.55 2.63 3.80

J. Chen et al. / Wear 264 (2008) 157165 159

Fig. 3. Effectdepth profiles

tracks. Furthe wear vexaminatio

3. Results

3.1. Surfac

The depsurface layare given isurface wawith the dbon increait decrease(SN) layersThe thicknTable 1.

Fig. 4 s15 h PSA trbe seen tha400 C con(460 C) andouble-lays of the treatment time on the (a) nitrogen and (c) carbon depth profiles, and effects.

ther, the wear factor was calculated through dividingolume by the load and sliding distance. Post-wearn of wear tracks was conducted using SEM and EDX.

e characteristics

th distribution of carbon and nitrogen in the treateders was probed using a GDS machine and the resultsn Fig. 3. These diagrams showed that the treateds rich in nitrogen, which then decreased graduallyepth. When the depth reached a few microns, car-sed rapidly to a peak value (about 47 at.%) befored slowly. The depth of the N-rich (SN) and C-rich

increased with the treatment time or temperature.ess of the SN and total diffusion layer is also given in

hows typical cross-sectional microstructures of theeated Stellite 21 alloy at 400, 460 and 550 C. It cant whilst the sample treated at a low-temperature oftained a single surface layer, medium-temperatured high-temperature (550 C) treated samples were

ered. Detailed EDX analyses further revealed that the

single surfaupper layerrich (SN) athe 550 Cand Cr andlayer. It waand nitridePSA treatm

The XRing treatedpeaks for tand HCP diffractionangles were-Co or antion Databas those geized austenS-phase waStellite 21 sS(1 1 1) andthe substratreatment teHowever, wof the treatment temperature on the (b) nitrogen and (d) carbon

ce layer formed at 400 C is carbon-rich (SC). Theformed at medium temperature 460 C is nitrogen-

nd the lower layer is carbon-rich (SC). However, fortreated sample, the top layer was rich in O, N, Clacking in Co. the followed layer was the diffuseds probably that precipitation of chromium carbidess might have occurred during the high-temperatureent of Stellite 21 alloy.

D patterns of the untreated and plasma surface alloy-samples are shown in Fig. 5. It can be seen that

he untreated material can be assigned to fcc -Co-Co phases. For the 400 C/15 h sample, two mainpeaks at low angles and some minor peaks at highdetected. These peaks could not be matched to-Co,

y other phases given in the existing Powder Diffrac-ase. However, they exhibited similar characteristicsnerated by the S-phase in plasma nitrided or carbur-itic stainless steel. It was therefore assumed that thes probably produced on the plasma alloying treatedamples. These two main S-phase peaks, indicated asS(2 0 0), corresponded to the (1 1 1) and (2 0 0) of

te but appeared at lower angles. With the increase ofmperature, these peaks shifted more to lower angles.hen the treatment temperature reached 500 C or

160 J. Chen et al. / Wear 264 (2008) 157165

above, theand the S-p

The formby the XTEselected areS-phase fostainless st

Fig. 5. XRD cand untreatedFig. 4. Cross-sectional microstructures of the 15 h PSA treated samples

peaks corresponding to CrN precipitates appearedhase peaks shifted back.ation of the S-phase in Stellite was further confirmedM analysis of the 430 C treated layer (Fig. 6). Thea diffraction pattern shows the similar pattern of the

rmed in low-temperature plasma nitrided austeniticeels. Detailed analysis of the diffraction pattern indi-

hart as a function of treatment temperature for the 15 h PSA treatedmaterials.

cated thatphase. It caa higher cocarbon absof slip linesFor high-te

Fig. 6. Surfac: (a) 400 C, (b) 460 C and (c) 550 C.

the treated layer has a distorted fcc structure, i.e. Sn be seen from Fig. 6 that the treated layer showedntrast than the substrate because the large amount oforbed by the surface layer. In addition, large amountand dislocations were observed in the S-phase layer.mperature (500 C) plasma treated samples, car-

e hardness as a function of PSA treatment temperature and time.

J. Chen et al. / Wear 264 (2008) 157165 161

Fig. 7. Microstructure and corresponding SAD pattern of the surface layer of430 C treated Stellite 21.

bides precipitated from the S-phase, which is in line with theabove XRD results (Fig. 5).

Fig. 7 shows the surface hardness as a function of PSA treat-ment temperature and time. It can be seen that all treated samplesexhibited significantly improved surface hardness as comparedwith the untreated ones. The surface hardness for 15 h PSAtreated sampled increased with the increase of treatment tem-perature up to 500 C and then decreased. No significant effect

Fig. 8. Wear factor of the PSA treated and untreated samples tested in air(unlubricated) and in 3.5% NaCl solution.

of treatment time on the surface hardness of 460 C PSA treatedwas observed.

3.2. Wear behaviour

Reciprocating sliding wear tests were conducted both in air(dry wear) and in 3.5% NaCl solution (corrosionwear) and theFig. 9. SEM micrographs showing: (a) and (b) wear track and (c) wear debris.

162 J. Chen et al. / Wear 264 (2008) 157165

results are summarized in Fig. 8. It can be seen that all the PSAtreated samples exhibited considerably reduced wear comparedto the untreated one.

3.2.1. Dry wearAs shown in Fig. 8, the improvement in wear resistance of

Stellite 21 by PSA treatments was dependent on the treatmentconditions. The wear factor of the 15 h PSA treated samples firstdecreased with increasing the temperature and then reached thebottom for the 500 C sample before it finally increased with thetreatment temperature. The 500 C sample exhibited the lowestwear factor, 4.5 1016 m3 N1 m1, which was only about 4%of that of the untreated material.

Following the dry wear tests, the wear track and wear debriswere examined using SEM and EDX, and the results for theuntreated samples are shown in Fig. 9. It can be seen thatthe untreated surface (Fig. 9a and b) was worn mainly by theabrasion action of the hard slider and the formation/shearingof adhesion junctions at the real contact surfaces as evidencedby many parallel abrasion marks in conjunction with adhesioncraters. Both large block-like and very fine powder-like weardebris was observed (Fig. 9c).

On the other hand, the treated samples displayed totally dif-ferent wear track and wear debris morphologies. For example,Fig. 10 shows typical wear morphologies, wear debris and EDXresults of tested PSA samples. The wear track, as shown inFig. 10a, was very shallow and superficial. Most area of the weartrack was covered by dark debris and EDX analysis revealedthat it contained a very high level of oxygen (Fig. 10b). Fig. 10cshows that the wear debris was composed of very fine compactedparticles, and it was likely that oxidation wear dominated thewear process.

3.2.2. CorrosionwearCorrosion wear test results are also shown in Fig. 8 for

comparison. It was evident that all the PSA treated samplespossessed a better corrosionwear resistance as comparedwith the untreated material. However, except for the 460 Ctreated sample, all other samples exhibited a higher wear factorwhen tested in 3.5% NaCl solution than in air, indicating thatcorrosion may have played an important role. In addition, itwas also found that although similar treatment temperaturedependence of wear was found in both dry and corrosionweartests, the 460 sample showed the lowest wear factor in

Fig. 10. (a d in ai) Wear track, (b) EDX chart and (c) wear debris for PSA treated samples teste r. (a) Middle of the wear track. (b) One end of the wear track.

J. Chen et al. / Wear 264 (2008) 157165 163

Fig. 11. Wear track morphologies of untreated material tested in 3.5% NaCl solution.

corrosionwear tests while in dry wear tests the 500 C sampleexhibited the lowest wear factor.

Fig. 11 displays the wear tracks of the untreated Stellite 21samples afwear trackof the solutallel abrasiwear mustaddition, m3.5% NaCland the tribwear track

The we460 C/15wear trackin the untrmarks wereas comparein the untrwear trackimplied thacorrosion

4. Discussion

4.1. Formation of S-phase and hardening

as bg wi

rdnesplas0.02

reasemainexpahaseatur

ere ae inObvt cothaP sto to

s retter corrosionwear tests in 3.5% NaCl solution. Theseemed bright probably caused by the scouring effection and the wear track was also covered by many par-on grooves (Fig. 11a), which indicated that abrasivehave played an important role in the wear process. Inany small pits caused by the corrosion effect of thesolution can be found in the wear track (Fig. 11a),o-corrosion products piled up at the end area of the(Fig. 11b).ar track morphologies of the corrosion wear testedsamples are shown in Fig. 12. It can be seen that thewas much smaller and shallower than that producedeated samples (Fig. 11). Although parallel abrasionalso observed in the wear track, they were much finerd with those observed in the wear track producedeated samples (Fig. 11). Only at the edges of thewere very few pits observed. These morphologiest very mild abrasive wear might have dominated thewear process of the PSA treated samples.

It halloyinthe haof the420HVan inccan beurated

S-ptemperthat thS-phasof Cr.since iknownare HCfcc -C-Co iFig. 12. Wear track morphologies of PSA samples testedeen shown in the present work that plasma surfaceth both carbon and nitrogen can significantly increases of Stellite 21 alloy (Fig. 7). The surface hardnessma surface alloyed material increased from about5 to as high as about 1150HV0.025, representingof 2.8 times. This significantly increased hardnessly attributed to the formation of interstitial supersat-nded fcc -Co, or S-phase in cobalt-based alloys.was first found by Bell and Zhang [9] in low-

e plasma nitrided austenitic stainless steel. It is knownre two essential requirements for the formation of thestainless steel: (a) fcc structure and (b) high contentiously, Stellite 21 can meet the second requirementntains about 30 wt.% Cr. In addition, although it ist under equilibrium conditions cobalt-based alloysructured (i.e. -Co), the phase transformation fromHCP -Co is so sluggish that a large amount of fcc

ained after cooling from high temperature to roomin 3.5% NaCl solution.

164 J. Chen et al. / Wear 264 (2008) 157165

temperature (Fig. 5). Therefore, it is possible to form S-phasein cobalt-based alloys during low-temperature plasma surfacealloying w

As showand carbon7 at.%, respnitrogen anof the S(1strated in Falloy and textremelylow-tempe

It can alpeaks haveis even hardples. It seemplayed a rosamples. Xcipitation oPSA treatmeffect mayinterstitialtion of fineIt is believening and pobserved hfore expectwould redution hardensupported(Fig. 7).

4.2. Wear

As hastreatment c21 alloy bo

It can betrend of weopposite trture. This imsurfaces, thtance. Thisthe dry weWC ball.

As showdry wear techaracterizthat abrasivresults in Fcess of bothwas also noby abrasivenism for thprobably bthe WC coufor the untr

sive wear for the PSA treated material was effectively reducedsince the surface hardness has been significantly improved. In

n, itdu

hardt toof ie tre

ry sixceps waulderace sopits

eracione. Thibite

whilestedar re

ar prsam

itatiosistaer hasam

orro

0 Cto it

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ed oing c

haseyinggh tehe su

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au

6)/De oft stud, foro tonce.ith nitrogen and/or carbon.n in Fig. 3, the maximum concentration of nitrogenin the PSA treated Stellite 21 can reach about 40 andectively. Clearly, the S-phase was supersaturated byd carbon, which is supported by the large shifting1 1) and S(2 0 0) peaks to lower angles as demon-ig. 5 because carbon and nitrogen diffused into thehus caused expansion of the fcc lattice. Therefore,strong solid solution hardening has been achieved inrature (460 C) PSA treated samples.so be found in Figs. 5 and 7 that although the S-phaseslightly back shifted for the 500 C treated sample, iter than the low-temperature (460 C) treated sam-s that other hardening mechanisms might have also

le in enhancing the hardness of the 500 C treatedRD (Fig. 5) study (Fig. 7) has revealed that pre-f fine coherent CrN has occurred during the 500 Cent. Therefore, although the solid solution hardeninghave been slightly reduced owing to the reduction inelement concentration in the S-phase, the precipita-coherent CrN would also greatly harden the S-phase.ed that this synergistic effect of solid solution hard-recipitation hardening might have contributed to the

igh hardness of the 500 C treated sample. It is there-ed that further increase in the treatment temperaturece hardness due to the rapid reduction in solid solu-ing and the coarsening of the precipitates. This is

by the hardness of the 550 C PSA treated material

reduction mechanism

been summarized in Fig. 8, plasma surface alloyingan effectively improve the wear properties of Stelliteth in air and in 3.5% NaCl solution.seen from Figs. 7 and 8 that when tested in air, the

ar factor with the treatment temperature followed theend of surface hardness with the treatment tempera-

plied that the higher the hardness of the PSA treatede lower the wear factor and the better the wear resis-may be related to the wear mechanisms operated inar of PSA treated samples sliding against the hard

n in Figs. 9a and 10a, the wear tracks formed insts for both the untreated and treated samples wereed by parallel grooves with dark areas. This indicatede wear and oxidation wear (as evidenced by the EDXig. 10b) might have occurred during the dry wear pro-

that untreated and PSA treated samples. However, itted that while the untreated samples were dominatedwear by the hard WC ball, the predominant mecha-

e PSA treated materials was oxidative wear. This isecause the untreated material was much softer thannterface and therefore severe abrasive wear occurredeated material; on the other hand, the extent of abra-

additioformedon thesuppor

It iswith thare ve

First, esampleThis coical intwith thby thethe intcorros

surfacple exhwear)when tthe wethe we500 Cprecipsion rethe oth500 Cgood cthe 46owing

5. Co

Basfollow

1. S palloa hiin t

2. Thetimcar

3. Thesolpla

Ackno

TheS0846and onprojecStellitemust gassistais also known that the effectiveness of the oxide filmring a wear process in reducing wear also dependsness of the subsurface, which provides mechanical

the surface oxide film.nterest to note that although the trend of wear factoratment temperature in air and in 3.5% NaCl solutionmilar (Fig. 8), two noteworthy features were found.t for the 460 C sample, the wear factor for all others larger when tested in 3.5% NaCl solution than in air.be attributed to the fact that in addition to the mechan-tion with the WC ball (or wear), chemical interactionlution (or corrosion) has also occurred as evidencedobserved in the wear tracks (Fig. 11a). Moreover,

tion between mechanical and chemical actions (i.e.wear) would accelerate the material loss from theis can be used to explain the fact that the 500 C sam-d the best wear resistance when tested in air (i.e. dry

e the 460 C sample possessed the lowest wear factin 3.5% NaCl solution. This is because for dry wear

sistance is mainly dependent on the hardness sinceocess is dominated by abrasive wear. Therefore, theple was the best due to its highest hardness. However,n of CrN occurred in the 500 C sample so its corro-nce and thus corrosionwear would be reduced. Onnd, although the 460 C sample was not as hard as theple, no precipitation of CrN was observed and hencesion resistance should be maintained. As a result,sample showed the best corrosionwear resistance

s high hardness and good corrosion resistance.

sions

n the experimental results of the investigation, theonclusions can be drawn:

can be formed in Stellite 21 alloy by plasma surfacetreatments at low-temperatures (400 and 460 C). Atmperature of 550 C, chromium nitrides precipitatedrface treated layer.

face hardness of Stellite 21 can be increased by 23llowing the plasma surface alloying treatments with

and nitrogen.ing wear factor of Stellite 21 in air and in 3.5% NaClcan be reduced by 99% and 96%, respectively, by

surface alloying treatments.

gements

thors gratefully acknowledge the EPSRC (GR/TI (STI 3/05), UK for financial support to the project,us (J.C.) would like to express his appreciation for aentship. Thanks are due to Dr. H. Lovelock of Delorothe provision of materials. In addition, special thankstheir former colleague Dr. C.X. Li, for his technical

J. Chen et al. / Wear 264 (2008) 157165 165

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[9] Z.L. Zhang, T. Bell, Structure and corrosion resistance of plasma nitridedstainless steel, Surf. Eng. 1 (1985) 131136.

[10] Y. Sun, X.Y. Li, T. Bell, X-ray diffraction characterisation of low tempera-ture plasma nitrided austenitic stainless steels, J. Mater. Sci. 34 (19) (1999)47934802.

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[13] H. Dong, et al., Improving the erosion-corrosion resistance of AISI316austenitic stainless steel by low-temperature plasma surface alloying withN and C, Mater. Sci. Eng. A 431 (2006) 137145.

[14] T. Bell, H. Dong, C.X. Li, Plasma surface treatment of Co-Cr biomaterial,Patent Pending, UK.

Improving the wear properties of Stellite 21 alloy by plasma surface alloying with carbon and nitrogenIntroductionExperimentalMaterials and PSA treatmentsMaterials characterizationTribological tests

ResultsSurface characteristicsWear behaviourDry wearCorrosion-wear

DiscussionFormation of S-phase and hardeningWear reduction mechanism

ConclusionsAcknowledgementsReferences