Wear 257 (2004) 1154–1166

Tribological behavior of Stellite 21 modified with yttrium

Iulian Radua,∗, D.Y. Lia, R. Llewellynb

a Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6b Institute for Fuel Cell Innovation, National Research Council, Vancouver, British Columbia, Canada V6T 1W5

Received 11 March 2004; received in revised form 14 June 2004; accepted 2 July 2004Available online 8 September 2004

Abstract

Cobalt-base alloys have found a wide variety of tribological applications particularly at elevated temperatures or in corrosive environments inmany industries, such as aerospace, automotive, power and gas turbines. One of the standard Co-base alloys, Stellite 21, is used predominantlyto resist the synergistic effects of corrosion and mechanical attack, especially at elevated temperatures and continuing efforts have been madeto improve its properties. One approach is to add reactive elements, such as yttrium, in order to beneficially affect its oxidation behavior.

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Research was conducted to investigate the effects of Y additions on Stellite 21 on its microstructure, mechanical behavior,emperature wear performance. These studies employed various experimental tools, such as micro-mechanical probe, XRD, SEMigh-temperature tribometer. The effects of Y addition on the properties of the oxide film formed on Stellite 21 were also investigarazing XRD and nano-mechanical probing techniques.It has been demonstrated that Y additions benefited the wear behavior of Stellite 21, especially at elevated temperatures. The

eveloped at 600◦C also showed markedly enhanced mechanical properties when Y was alloyed to the alloy.2004 Elsevier B.V. All rights reserved.

eywords:Yttrium; Stellite 21; High-temperature wear; Oxide; Nano-indentation; Micro-scratch

. Introduction

Cobalt-base alloys, most commonly known as Stellite®

lloys,1 have been used extensively as bearing materials forpplications that require high resistance to wear, particularlyt elevated temperatures and/or in corrosive environments

1–6].Stellite 21 is used predominantly in casting and hardfacing

orms to resist the synergistic corrosion and mechanical at-ack especially at high temperatures, e.g., wear of valve seatsn nuclear power plants and in automobile engines[7–11].

The tribological behavior of Stellite alloys benefits fromhe low stacking fault energy of cobalt, the formation ofard carbides, high corrosion resistance, and a solid solutiontrengthened matrix[1,4].

∗ Corresponding author. Tel.: +1 780 4925157.E-mail address:radu@ualberta.ca (I. Radu).

1 Stellite is a registered trade mark of Deloro Stellite Company Inc.

Pure cobalt has two allotropes:� phase (hcp) stablelow temperatures, and� phase (fcc) stable at higher tempetures. The� → � transformation in pure cobalt takes plac430◦C (As) by means of a diffusionless martensitic mecnism[4]; it is reversible with� → � transformation occurrinat 390◦C (Ms) on cooling. Alloying of pure cobalt changthe stability of the allotropes and thus modifies the statransformation temperaturesAs andMs. For instance,As isincreased to 970◦C for a Co–27Cr–5Mo–0.05C alloy[12].The transformation on cooling tends to be sluggish in allocobalt due to limited chemical driving force. As a result,alloy can be a mixture of two phases:� and metastable�, atroom temperature[4,13]. Volume fractions of the two cobaallotropes influence the mechanical properties of the[14]. For example, ductility is increased with an increasthe amount of� phase[4,15].

The� → � transformation can be:

(1) induced by plastic deformation i.e., strain-induced trformation (SIT)[13,16],

043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved.

oi:10.1016/j.wear.2004.07.013

I. Radu et al. / Wear 257 (2004) 1154–1166 1155

(2) promoted by quenching from the temperature range ofstable� phase[17],

(3) enhanced if the alloy is aged at temperatures near 800◦Cprior to cooling[12,18].

The transformation under these conditions is affected bythe grain size of the parent phase[13,16], cooling rate[17],temperature, and the duration of solution heat treatment priorto aging[12]. SIT in Stellite 21 alloy is responsible for itsexcellent galling resistance[19] and high ability to accom-modate stress. Furthermore, the residual� phase has a lowstacking fault energy (SFE)[13], which increases the work-hardening capability and the resistance to galling wear[20] byhindering dislocation movement. For lower SFE (15 mJ/m2)the SIT mechanism prevails, while twinning is the predomi-nant deformation mode at 20 mJ/m2 [13].

Chromium improves the resistance to corrosion andoxidation and is the main carbide former (e.g., Cr23C6). Theamount of carbon present affects the formation of carbidesand thus influences hardness, ductility and wear resistance ofthe alloy. Molybdenum is utilized as solid–solution strength-ener to provide additional strength to the matrix by formingon intermetallic compound Co3Mo. Molybdenum benefitsthe corrosion resistance of the alloy[4]. Molybdenum andchromium decrease the SFE[13] and can stabilize the�p SFEap gram[ .T d tot er of� ium,h

ite 21a on-c alt-b aloyf neu-t[ onsi-b loyr icalp -s madet nu-c

t al-l e, tof ion.A andt ti og-i

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main beneficial effects of yttrium additions are: (1) reducingthe growth rate of oxide scale and (2) improving the scale ad-herence to substrate[41–46]. Several possible mechanismswere proposed for the improvement in the scale adherenceto substrate, such as oxide keying or pegging, enhanced ox-ide plasticity, impurity gathering, and reduction in interfacialstress[44,45,47].

Recent studies[26] have demonstrated that an yttriumaddition can make Stellite 6 considerably more resistant tohigh-temperature wear due to the enhancement of mechanicalproperties of its surface oxide scale and its adherence to thesubstrate. Also, it was demonstrated that yttrium additionslead to a markedly improved resistance of stainless steel andaluminide coatings to corrosion and corrosive wear[48–50].

In this work, research was conducted to investigate theeffects of yttrium on the wear behavior of Stellite 21 espe-cially at high temperatures. Changes in microstructure, wearperformance, mechanical properties of both the alloy andits oxide scale were investigated using various techniques,including optical microscopy, X-ray diffraction, SEM(scanning electron microscopy), EDS (energy dispersivemicroscopy), micro- and nano-mechanical probes, micro-scratch tests, and high-temperature pin-on-disc wear tests.

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hase, while iron, nickel and manganese increase thend can stabilize the� phase[4]. Carbon also stabilizes the�hase. However, according to a Co–C binary phase dia

3], carbon should stabilize the� phase rather than� phasehe opposite role that carbon plays is possibly attribute

he fact that carbon in cobalt-base alloys act as a gett-stabilizing elements, such as molybdenum and chromence could appear as a�-phase stabilizer.

Recently, attempts have been made to replace Stelllloy for nuclear applications because of radioactivity cerns[21]. The source of radiation was found to be cobased alloy valve wear debris, which may reach the Zirc

uel rod in the reactor core. This debris is activated byron flux leading to the formation of radioactive Co60 isotope21]. It was estimated that the valve wear debris is resple for 10% of total radioactivity. However, attempts of aleplacement failed to duplicate all the required tribologroperties of Stellite 21 alloy[22,23], especially its galling reistance. Consequently, efforts have been continuouslyo improve Stellite 21 and other Stellite alloys not only forlear application but also for other tribological uses[24–29].

The surfaces of high-temperature oxidation-resistanoys generally react with oxygen at elevated temperaturorm protective oxide layers, which prevent further oxidatn oxide layer may also reduce wear by reducing friction

he synergism of wear and oxidation.[30–34]. It appears thamproving the oxidation behavior could benefit the tribolcal properties of high-temperature alloys.

It has been demonstrated that alloying reactive elemith high affinity for oxygen such as yttrium can improvexidation resistance of high-temperature alloys[35–41]. The

. Experimental procedure

.1. Alloy preparation and heat treatment

Stellite 21 alloy provided by Deloro Stellite Inc. we-melted with added yttrium powder (40-mesh) inrc-melting furnace (MRF Inc. SA338-V&G). The meltind solidification conditions are given inTable 1. In order

o increase the degree of homogenization, all samplesurned over and re-melted consecutively for five times.ominal composition of the supplied Stellite 21 alloyiven in Table 2. Several groups of samples were m

able 1xperimental conditions for re-melting of Stellite 21

arameters Melting conditio

acuum pressure before filling with Ar gas 27–28 psiatio of vacuum/back-fill with Ar gas 3 timesrotection gas Argonas flow during melting and cooling 2 l/minressure during melting and cooling 0 psilectrode type Thoriated tungstlectrode diameter 4.8 mmrc current 160/240Ao. of turning-over and re-melting 5 times

able 2ominal composition of Stellite 21

lement Co Cr Mo Ni Fe Si Mn Cercentage composition(wt.%)

Bal 27 5.5 2 3 1 1 0.2

1156 I. Radu et al. / Wear 257 (2004) 1154–1166

containing 0, 0.5, 1, 2, and 5 wt.% yttrium, respectively. Thecast samples had the following dimensions: 10 mm× 10 mm× 30 mm. The samples were homogenization-annealed at815◦C for 3 h followed by air-cooling. After homogenizationtreatment, samples were cut using an abrasive disc cuttermachine (Buehler, NY, USA).

2.2. Microstructure examination

Standard techniques were used to prepare specimens foroptical metallography. Final polishing was performed us-ing a 0.05�m alumina slurry. A solution of HCl and 30%H2O2 (volume fraction 6:1) was used to etch the samples.Microstructures of the samples were observed using an op-tical microscope (Olympus PME3-ADL). The samples forX-ray examination had a dimension of 10 mm× 10 mm×

Ff

1 mm. A Rikagu X-ray diffractometer using Cu–K� radiation(λ = 1.54056A) was used to determine the crystal structuresof phases developed in the samples. The samples were furtherexamined using SEM and EDS.

2.3. Evaluation of the mechanical properties of alloys

Macro-hardness was measured using a conventionalRockwell hardness tester under a load of 1471 N. TheHRC value reported is an average of five measurements.Micro-hardness values of different phases developed in thesamples were determined using a micro-mechanical probe(Fisher Technology Ltd., Winsor, CT, USA) under differentmaximum loads (50, 100, 200, 400, 600, 800, and 1000 mN,respectively). Each reported value is an average of 10measurements. Theη value, a measure of elastic behavior,

ig. 1. Optical microstructures of Stellite 21 alloy modified with: (a) yttrium-fror different domains resulted from changes in composition and formation of

ee; (b) 0.5% Y; (c) 1% Y; (d) 2% Y; (e) 5% Y (the changes in the grey degreesa new phase, Co2YSi2, which influenced the etching effect).

I. Radu et al. / Wear 257 (2004) 1154–1166 1157

and indentation depth were also determined. The ratio (η) ofthe elastic work (We) to the total work (Wtot) is a measure ofthe contribution of elasticity to the deformation[53].

Deformed and un-deformed specimens were examined us-ing the X-ray diffraction technique in order to check if the� → � transformation was induced by stress. Un-deformedspecimens were polished using a 0.05�m alumina slurry thenetched. The deformation was achieved by hammering, whichwas performed in the same manner to all samples. Statisti-cally, the hammering effect should be the same to all samples.Macro-hardness (HRC) of the deformed specimens was alsomeasured in order to determine the degree of strain hardeningfor all samples.

2.4. Evaluation of oxide scale’s mechanical properties

Stellite 21 samples with and without yttrium were heatedin a furnace and oxidized in air at 600◦C for 20 min.The mechanical properties of oxide scales on yttrium-free and yttrium-containing samples were investigated atroom temperature after being oxidized at 600◦C using anano-mechanical probe (Hysitron, Minneapolis, USA) anda micro-scratch tester (Center for Tribology Inc., Moun-tain View, CA, USA) respectively. The sample’s dimensionswere 15 mm× 8 mm × 6 mm. The surface of the speci-m eo hree-s r-f 80,1 erer calesw d wasa tationd ate-r idalt 0 to1 /s.T andt con-t cale.T entst f thes nce tosd nceX andt

2

turep tel,S mm× itsd dings nder

a normal load of 2 N. Surfaces of specimens were polishedusing a 6-�m diamond paste.

The volume loss was determined by determining the pro-file of wear track. The reflected light microscopy and SEMwere used to observe morphologies of worn surfaces. EDSwas used to determine the local chemical composition of theworn surfaces. Mechanical properties of glaze formed duringwear at elevated temperatures were assessed at room temper-ature using a micro-mechanical probe (Fisher TechnologyLtd., Winsor, CT, USA) under a maximum load of 5 mN.

3. Results

3.1. Microstructure and phase analysis

Fig. 1 illustrates microstructures of Stellite 21 sampleswith and without yttrium additions. All the samples showeda typical hypoeutectic dendritic microstructure. The unmod-ified Stellite 21 sample had a finer form than the yttrium-modified ones.

Fig. 2. X-ray diffraction patterns of Y-free, 1%, and 5% yttrium-containingsamples, respectively:�-M23C6; �-Co3Mo; �–� Co;γ–� Co;�-NiCrCoMo;�-Co7Mo6; �-Co2YSi2.

ens was polished with a 0.05�m alumina slurry beforxidation treatment. The nano-mechanical probe was a tided diamond pyramid (tip angle = 90◦). The test was peormed, respectively under four maximum loads (40,00, and 150�m) and corresponding load-depth curves wecorded. Hardness and elastic behavior of the oxide sere determined from the curves and each value reporten average of 10 measurements. The smaller the indenepth under a given maximum load, the harder is the mial. The micro-scratch test was performed using a pyramungsten carbide tip under a linear increasing load from0 N. The tip moved at a horizontal velocity of 0.05 mmhe electrical contact resistance (ECR) between the tip

he sample was monitored in situ. A drop of the electricalact resistance (ECR) corresponds to failure of an oxide she critical load corresponding to the ECR drop repres

he critical force required to scratch the surface oxide ofample and is therefore a measure of the oxide adhereubstrate. Structures of the oxide scales formed at 600◦C onifferent samples were investigated using the low incide-ray diffraction technique. The angle between X-ray

he sample surface was 2◦.

.5. Wear test

Sliding wear tests were performed on a high-temperain-on-disc tribometer (CSEM Instruments, Neuchawitzerland). The disc was the sample under study (158 mm× 5 mm) and the pin was an alumina ball with

iameter equal to 6 mm. All tests were performed at a slipeed of 1 cm/s along a circle path of 0.8 mm in radius u

1158 I. Radu et al. / Wear 257 (2004) 1154–1166

Fig. 2 presents X-ray diffraction patterns of Y-free,1 and 5% yttrium-containing samples, respectively. Thesamples for X-ray analysis were ground then polished withthe 6-�m diamond paste. X-ray patterns of 0.5 and 2%yttrium-containing Stellite 21 samples were similar to thatof the sample containing 1% yttrium. Identified phaseswere: fcc M23C6 (where M was Cr, Co, Mo, or Fe, e.g.,Cr17Co4Mo2C [3,4]), intermetallic ordered compound:

Co3Mo (hcp), � phase: Co7Mo6 (rhombohedral), NiCr-CoMo (fcc), �-Co (fcc), and�-Co (hcp). The X-ray resultwas consistent with those reported in literature[4]. Additionof a small amount of yttrium did not result in the formationof any new phase. However, a new phase (Co2YSi2)was formed at higher yttrium contents, e.g., at 5%. Allsamples contained both� and � cobalt allotropes at roomtemperature.

Fig. 3. EDS compositional dot maps of Stellite 21 alloy

modified with: (a) yttrium-free; (b) 0.5% Y; (c) 5% Y.

I. Radu et al. / Wear 257 (2004) 1154–1166 1159

Fig. 4. Macro-hardness (HRC) values of yttrium-free and yttrium-containing Stellite 21 samples.

Fig. 3 presents EDS maps of molybdenum and yttriumin Stellite 21 samples containing 0, 0.5, and 5 wt.% yttrium,respectively. It was demonstrated that molybdenum wasconcentrated in the interdendritic regions. The 1 and 2%yttrium-containing samples had similar molybdenum andyttrium distributions. Yttrium was not homogeneouslydistributed but appeared to be concentrated in some smallregions in the interdendritic regions especially where themolybdenum was rich. Corresponding SEM back-scatteredelectron images are also presented inFig. 3.

3.2. Mechanical properties

Results of the macro-hardness measurement (HRC) aregiven in Fig. 4. It was demonstrated that yttrium did notmarkedly influence the hardness of the alloy.

Localized mechanical properties of various phases or do-mains were evaluated using a micro-mechanical probe. Thetechnique is based on Vickers indentation, which providesinformation on local hardness and elastic behavior[52]. Theelastic behavior was established using the ratio of elastic de-formation energy (We) to the total deformation energy (Wtot)[53]. The area enclosed by the loading curve and the maxi-mum depth of penetration (area ABC) represents (Wtot), andWe is represented by the area DBC (Fig. 5).

ta-t eas

curve f

shown inFig. 1. Yttrium did not significantly modify me-chanical properties of the alloy, although some decrease inhardness was observed in the dendritic region when the yt-trium content was increased to 5%.

Figs. 7 and 8, respectively present X-ray diffraction pat-terns of deformed and un-deformed yttrium-free and 5%yttrium-containing samples. The samples were polished andetched in order to remove additional strain caused by polish-ing. Relative fractions of metastable� phase (x), and� phase(1−x) were estimated from integrated intensities of (1 01 1)�,and (2 0 0)� peaks using the following equation[51]:

x

1 − x= 1.5

I fcc200

Ihcp1011

(1)

The quantitative analysis (Eq. (1)) indicated that the frac-tion of � phase in the stress-free specimens was 0.2 for theyttrium-free sample, and 0.3 for the 5% yttrium-containingsample, respectively. Alloying yttrium did not significantlyaffect the ratio of the cobalt allotropes. After deformation,the fraction of� phase increased to 0.45 in the yttrium-freesample, and to 0.75 in the 5% yttrium-containing sample, re-spectively. Clearly,� → � transformation was induced inStellite 21 by strain, and alloying with yttrium enhancedthe transformation. Such a transformation could improvet nicala

asedf ent;w 4 to4 hada

3

byn ofy pec-t ei d

Fig. 6 presents plots ofη ratio and depth versus indenion load for dendritic and slightly harder interdendritic ar

Fig. 5. A load–depth

rom micro-hardness test.

he performance of the alloy when subjected to mechattack.

The hardness of 5% yttrium-containing sample increrom 37.4 to 42 HRC caused by the deformation treatmhile that of the yttrium-free sample increased from 39.1 HRC. It appeared that the yttrium-containing samplehigher strain-hardening capability.

.3. Mechanical properties of the oxide scale

The effects of yttrium on oxide scale were evaluatedano-indentation testing.Fig. 9shows load–depth curvesttrium-free, 0.5 and 5% yttrium-containing samples, resively. Indentation depth andη values determined from thndentation test are presented inTable 3. The oxide forme

1160 I. Radu et al. / Wear 257 (2004) 1154–1166

Fig. 6. Curves ofη ratio and indentation depth versus indentation load: (a)dendritic regions; (b) interdendritic regions.

on the unmodified Stellite 21 sample was softest with thelargest indentation depth and the smallestη ratio, while 0.5%yttrium-containing sample showed the best mechanical prop-erties.

Fig. 10 shows results of micro-scratch test for yttrium-free, 0.5 and 5% yttrium-containing samples, respectively.

Fig. 7. X-ray diffraction patterns of unstrained and strained yttrium-free samples.P1–(1 01 1)�, (4 0 0)M23C6, (2 0 1)Co3Mo, (0 2 4)Co7Mo6;P2–(2 0 0)�, (6 0 0)M23C6, (2 0 0)NiCrMo.

Fig. 8. X-ray diffraction patterns of unstrained and strained 5%yttrium-containing samples.P1–(1 01 1)�, (4 0 0)M23C6, (2 0 1)Co3Mo,(0 2 4)Co7Mo6, (2 0 0)Co2YSi2; P2–(2 0 0)�, (6 0 0)M23C6, (2 0 0)NiCrMo,(2 0 2)Co2YSi2.

I. Radu et al. / Wear 257 (2004) 1154–1166 1161

Fig. 9. Nano-indentation curves of the oxide scales on the yttrium-free and yttrium-containing samples.

The yttrium-containing samples showed higher values of crit-ical load than the unmodified sample.

The micro-scratch results indicate that the oxide scale de-veloped on yttrium-containing samples was harder, more ad-herent to the substrate, and behaved more elastically.

Fig. 11presents low incidence X-ray diffraction patternsof oxide scales developed at 600◦C on yttrium-free, 0.5,and 5% yttrium-containing samples, respectively. The ox-ide scale developed on unmodified Stellite 21 consisted ofthe following phases: Cr2O3, MoO3, MO (where M was Co,Cr, or Ni), QCr2O4 (where Q was Co or Ni), Mo17O47,CoMoO3, CoMoO4. The oxide scale grown on yttrium-containing samples contained an additional new phase,Y2O3.

Table 3Mechanical properties of the oxide scale obtained from nano-indentationtest (error range depth = 0.4 nm,η = ±2%)

Material Maximumload (�N)

Maximum depth ofindentation (nm)

η (%)

Yttrium-freeStellite 21

40 4.8 44.880 6.7 46.5

100 8.6 47.5150 11.6 43.7

Stellite 21–5% Y 40 2.8 66.1

Y

S

S

Fig. 10. Micro-scratch test with in situ monitoring changes in electric contactresistance (a) yttrium-free; (b) 0.5% Y; (c) 5% Y.

80 4.9 61.5100 6.1 63150 8.2 63.6

ttrium-free Stellite21–1% Y

40 3.9 59.580 6.6 58

100 7.5 58.1150 10.2 57.4

tellite 21–2% Y 40 3.2 6480 5.7 63

100 6.7 61.3150 9.6 58

tellite 21–5% Y 40 3.2 6180 6.3 56.4

100 8.3 60.8150 11 56.5

1162 I. Radu et al. / Wear 257 (2004) 1154–1166

Fig. 11. Low angle X-ray diffraction patterns of the oxide scales developed on: (I) yttrium-free; (II) 0.5% Y; (III) 5% yttrium-containing samples, respectively,where (a) Cr2O3; (b) CoO; (c) CrO; (d) CoCr2O4 and NiCr2O4; (f) Mo17O47; (g) CoMoO3; (h) CoMoO4; (j) NiO; (k) MoO3; (y) Y2O3; (s) substrate (CPSstands for counts per second).

3.4. Wear behavior

Fig. 12shows the wear volume losses versus temperatureunder a normal load of 2 N for 4000 laps. All samples reachedwear maxima around 200◦C followed by a decrease and thenan increase in the wear loss as the temperature was continu-ously raised. The wear loss of the yttrium-free Stellite 21 sam-ple was the greatest at all temperatures. The difference in thewear volume between yttrium-free and yttrium-containingsamples became more obvious at higher temperatures.

Fig. 13 illustrates optical micrographs of wear tracks onthe samples after sliding wear tests at 600◦C for 4000 laps.The yttrium-free Stellite 21 sample showed larger (broaderand deeper) wear track compared with the yttrium-containingsamples.

After sliding at elevated temperatures, glaze layers witha shiny and smooth appearance were observed on the wornsurfaces. Such glaze layers formed not necessarily at hightemperature; it was observed that the glaze formed even ona surface worn at 150◦C. The EDS analysis indicated that

I. Radu et al. / Wear 257 (2004) 1154–1166 1163

Fig. 12. Wear loss with respect to temperature.

Fig. 13. Micrographs of wear tracks at 600◦C–4000 laps: (a) yttrium-free; (b) 0.5% Y; (c) 1% Y; (d) 5% Y.

the glaze layers formed on the worn surfaces were heavilyoxidized. The composition of the glaze layers was similar tothat of the alloy but contained oxygen. Distribution of theglaze was not uniform over the wear track; some portionsof the wear track did not retain any glaze layer (Fig. 14a).At 600◦C, the glaze layer was more compacted and coveredmore areas (Fig. 14b). It appeared that the wear debris par-ticles were agglomerated and compacted under the externalforce to form the glaze layer (Fig. 14c).

The effects of yttrium on the mechanical properties of theglaze layer were evaluated by performing micro-indentationtests on glaze layers, respectively on yttrium-free and yttrium-

containing samples. It was demonstrated that yttrium did notinfluence either the mechanical properties or the coverage ofthe glaze layer.

4. Discussion

4.1. Microstructure

Exposure at 815◦C for 3 h (homogenization treatment)promoted the diffusion of molybdenum from the dendriticregions to the interdendritic regions and probably favoredthe formation of intermetallic ordered phase Co3Mo within

1164 I. Radu et al. / Wear 257 (2004) 1154–1166

Fig. 14. Morphology of wear track on sample after sliding for 2000 laps at: (a) 150◦C–0.5% Y; (b) 600◦C–2% Y; (c) 600◦C–2% Y, a closer view.

these zones[5,29]. Yttrium additions promoted microstruc-tural coarsening of this alloy. It appeared that yttrium wassurrounded by rich molybdenum phases. It is known thatmolybdenum solutes diffuse preferentially to stacking faults[54], and it is postulated that they might also provide morespace to accommodate yttrium atoms which have a larger ra-dius (0.181 nm), compared to other elements in the alloy (e.g.Co, Ni, Fe, Cr, and Mo). This might be a reason why yttriumwas more concentrated in the molybdenum-rich regions.

The XRD results demonstrate that both� and� cobalt al-lotropes were present in the structure. For 0.5, 1, and 2 wt.%yttrium-containing samples, no extra X-ray diffraction peakswere detected, probably because the amount of yttrium wasnot sufficient for the formation of new phases or more proba-bly because any phases formed were too small to be detected.

At room temperature, the main cobalt allotrope presentwas the metastable� phase rather than the� phase, whichwas expected to exist at room temperature. This happenedbecause� phase was hard to generate under normal coolingconditions, such as air-cooling[55]. The metastable� phasecould fully transform to� phase if the� phase is aged at815◦C prior to cooling.

The phases formed in the Stellite 21 alloy are consistentwith previous observations reported in literature[1,3,28]. Themechanical behavior of the alloy was improved by the� → �tt o,

Co and Fe can substitute, e.g., Cr17Co4Mo2C). The M23C6carbides were formed in the interstices of dendritic arms andat grain boundaries as well. It appears that yttrium did notaffect the existence/extent of these phases but produced acoarser microstructure.

4.2. Mechanical properties

The estimated amount of� phase in the Stellite 21 sampleswas around 0.3 and considerably less than the saturation levelof 0.9 [16]. This may make the strain-induced� → � trans-formation important when subjected to mechanical attack.As demonstrated, the fraction of� phase reached 0.75 in thedeformed 5% yttrium-containing sample and 0.45 in the de-formed yttrium-free sample. It appears that yttrium additionspromoted� → � transformation. Although, the differencein hardness between the deformed yttrium-free and yttrium-containing samples was small, the “strain-hardening” capa-bility and the capability of accommodating stress with lessdamage due to the strain-induced phase transformation wereincreased by the yttrium addition (seeSection 3.2). The� →� transformation promoted by alloying yttrium could be re-sponsible for such improvement, which could be the reasonwhy the yttrium-containing samples displayed higher room-temperature wear resistance than the yttrium-free containings for-m

ransformation as well as by the formation of the Co3Mo in-ermetallic and by M23C6 carbides (M was usually Cr but M

amples, although the latter was slightly harder than theer (seeFig. 4).

I. Radu et al. / Wear 257 (2004) 1154–1166 1165

4.3. Wear at elevated temperatures

The wear peaks at 200◦C (seeFig. 12) were probablycaused by the increase in the stacking fault energy of thematrix with temperature[56], thus lowering the “strainhardening” capability. However, the wear loss decreased inthe temperature range from 200 to 450◦C. This could beattributed to the formation of oxide scale and a compactedglaze layer.

The yttrium effect on properties of the oxide scale was notobvious at lower temperatures[41] because of the lower rateof oxidation. Above 200◦C, the wear rates decreased due tothe development of the oxide scale and the compacted glazelayer. When the temperature exceeded about 450◦C, the wearrate increased again probably due to increased softening ofthe matrix, which could not effectively support the protectiveoxide scale and the glaze layer.

The yttrium addition improved the wear resistance of Stel-lite 21 at elevated temperatures, through its positive effect onthe oxide’s mechanical properties and the oxide adherence tosubstrate. In addition, at higher temperatures, the diffusionof yttrium to grain boundaries could inhibit the outward dif-fusion flux of chromium ions and favored inward diffusionof oxygen ions[35,41,42]. The inward diffusion of oxygenions may benefit the oxide’s mechanical properties (due totp due tot le in-t ssedi k-n ls

ce ofS r tot tecty nota

5

1

2 in

3 cted

4 -. Ither

5 icalenceium-

6. The presence of yttrium particularly at 0.5% levelmarkedly enhanced the high-temperature wear perfor-mance of Stellite 21, especially at temperatures above500◦C. Such improvement may largely benefit from thesuperior mechanical properties of the oxide scale contain-ing Y2O3 phase. The formation of glaze layer also bene-fited the high-temperature wear resistance to some degree.However, the yttrium addition did not show any noticeableinfluence on the mechanical behavior of the glaze layer.

References

[1] P. Crook, Cobalt, Cobalt alloys, in: ASM Metal Handbook, vol. 2,Materials Park, Ohio, 1993, pp. 446–456.

[2] E. Lemarie, M. Le Calvar, Evidence of tribocorrosion wear in pres-surized water reactors, Wear 249 (2001) 338–344.

[3] ASM International, Nickel, Cobalt and Their Alloys, Materials Park,Ohio, 2000.

[4] T.C. Sims, N.S. Stoloff, W.C. Hagel (Eds.), Superalloys II, Wiley,New York, 1987.

[5] K.C. Antony, Wear-resistant cobalt base alloys, J. Metals 35 (1983)52–60.

[6] W.J. Huppmann, M.A. Clegg, The tribological behavior of poly-crystalline cobalt as related to crystallographic texture and structure,ASLE Trans. 16 (1972) 107–114.

ruc-de-

989)

of a

nical997)

[ sur-999)

[ yb-e Stel-

[ mar-.05C

[ sfor-ater.

[ roometall.

[ Mo–

[ o

[ y:

[ rma-

[ wear279.

[ the981)

he growth of a fine-grained oxide scale containing the Y2O3hase) and the oxide adherence to substrate (probably

he inhibition of vacancy coalescence at the metal–scaerface, and formation of oxide pegs at interface as discun references[57,58]). Yttrium could also reduce the thicess of oxide scale[41-45], thus minimizing the interfaciatress and further benefiting interfacial bonding.

The glaze layer may also enhance the wear resistantellite 21. The composition of the glaze layer was simila

hat of the alloy but it contained oxygen. EDS did not dettrium in the glaze layer. This implies that yttrium mayffect the mechanical behavior of the glaze layer.

. Conclusions

. Stellite 21 consisted of M23C6, Co3Mo, Co7Mo6, NiCr-CoMo,�, and� cobalt phases.

. A small addition of yttrium did not form new phasesthe alloy but coarsened its microstructure.

. The hardness of Stellite 21 was not significantly affeby the yttrium addition.

. Strain-induced� → � transformation (SIT) played an important role in the wear behavior of Stellite 21 alloyappears that yttrium promoted the SIT, leading to higwear resistance at room temperature.

. Alloying yttrium markedly enhanced the mechanproperties of the oxide scale as well as its adherto the substrate. The oxide scale developed on yttrcontaining samples contained a new phase, Y2O3.

[7] S. Atamert, H.K.D.H. Bhadeshia, Comparison of the microstture and abrasive wear properties of Stellite hardfacing alloysposited by arc welding and laser cladding, Metals Technol. 20 (11037–1054.

[8] S.C. Agarwal, H. Ocken, The microstructure and galling wearlaser-melted cobalt-base hardfacing alloy, Wear (1990) 140.

[9] V. Otterloo, J. Hosson, Microstructural features and mechaproperties of a cobalt-based laser coating, Acta Mater. 45 (11225–1236.

10] J.N. Aoh, Y.R. Jeng, E.L. Chu, L.T. Wu, On the behavior offace clad layers under high temperature, Wear 225–229 (11114–1122.

11] J.C. Shin, J.M. Doh, J.K. Yoon, D.Y. Lee, J.S. Kim, Effect of moldenum on the microstructure and wear resistance of cobalt-baslite alloys, Surf. Coat. Technol. 166 (2003) 117–126.

12] J.S. Garcia, M.A. Medrano, A.S. Rodriguez, Formation of hcptensite during the isothermal aging of an fcc Co–27Cr–5Mo–0implant alloy, Metall. Mater. Trans. A 30 (1999) 1177–1184.

13] L. Remy, A. Pineau, Twinning and strain induced fcc to hcp tranmation on the mechanical properties of Co–Ni–Cr–Mo alloys, MSci. Eng. 26 (1976) 123–132.

14] G. Bouquet, B. Dubois, Influence of the fcc phase retained attemperature on the mechanical properties of cobalt, Scripta M12 (1978) 1079–1081.

15] A. Halstead, R.D. Rawlings, Structure and hardness of Co–Cr–Si wear resistant alloy, Metal Sci. 18 (1984) 491–500.

16] P. Huang, H.F. Lopez, Strain induced�-martensite in a Co–Cr–Malloy: grain size effects, Mater. Lett. 39 (1999) 244–248.

17] P. Huang, H.F. Lopez, Athermal�-martensite in a Co–Cr–Mo allograin size effects, Mater. Lett. 39 (1999) 249–253.

18] P. Huang, H.F. Lopez, Role of aging on the martensitic transfotion in a cast cobalt alloy, Scripta Mater. 45 (2001) 427–433.

19] P. Crook, C.C. Li, The elevated temperature metal to metalbehavior of selected hardfacing alloys, Wear Mater. (1983) 272–

20] K.J. Bhansali, A.E. Miller, Role of stacking fault energy ongalling and wear behavior of a cobalt base alloy, Wear Mater. (1179–182.

1166 I. Radu et al. / Wear 257 (2004) 1154–1166

[21] H. Ocken, Reducing the cobalt inventory in light water reactors,Nucl. Technol. 68 (1984) 18–28.

[22] A.J. Persson, S. Jacobson, S. Hogmark, Effect of temperature onfriction and galling of laser processed Norem 02 and Stellite 21,Wear 255 (2003) 498–503.

[23] J. Vikstrom, Galling resistance of hardfacing alloys replacing Stellite,Wear 143 (1994) 143–146.

[24] S.H. Kang, Y.K. Shinoda, Y.K.H.S. Jeong, Thermal fatigue charac-terisitcs of PTA hard-faced steel, Surf. Eng. 17 (2001) 498–504.

[25] V. Kuzuku, M. Ceylan, I. Aksoy, An investigation of Stellite 6 alloycontaining 5 wt.% Si, J. Mater. Process. Technol. 79 (1998) 47–51.

[26] L. Wang, D.Y. Li, Effects of yttrium on microstructure, mechanicalproperties and high-temperature wear behavior of cast Stellite 6 alloy,Wear 255 (2003) 535–544.

[27] V. Kuzuku, M. Ceylan, I. Aksoy, Phase investigation of a cobalt basealloy containing Cr, Ni, W and C, J. Mater. Process. Technol. 74(1998) 137–141.

[28] V. Kuzuku, M. Ceylan, I. Aksoy, Microstructure and Phase Analysesof Stellite 6 Plus 6 wt.% Mo Alloy, J. Mater. Process. Technol. 69(1997) 257–263.

[29] A. Osma, E.S. Kayali, M.L. Ovecoglu, The effect of elevated temper-ature and silicon addition on a cobalt-based wear resistant superlloy,J. Mater. Sci. 31 (1996) 4603–4608.

[30] F.H. Stott, High-temperature sliding wear of metals, Tribol. Int. 35(2002) 489–495.

[31] F. Archard, W. Hirst, The wear of metals under unlubricated condi-tions, Proceedings of the Royal Society of London, A236, 1990, pp.89–97.

[32] F.H. Stott, J. Glascott, G.C. Wood, The sliding of wear commer-cial Fe–12%Cr alloys at high temperatures, Wear 101 (1985) 311–

[ s in

[ Int.

[ s in980)

[ ctive

[ orros.

[ sion,

[ rmalM6.

[ high-oys,

[ tionive

Elements in Oxidation at High Temperatures, Prentice Hall, Ohio,USA, 1989, pp. 111–127.

[42] K. Przybylski, G.J. Yurek, The reactive elements effect on high-temperature oxidation, Mater. Sci. Forum 43 (1989) 1–6.

[43] C.M. Cottel, G.J. Yurek, R.J. Hussey, D.F. Mitchell, M.J. Graham,The Influence of grain boundary segregation of Y in Cr2O3 on theoxidation of Cr metals, Oxid. Met. 34 (1990) 173–200.

[44] M.J. Bennett, A.T. Tuson, Improved high-temperature oxidation be-havior of alloys by ion implantation, Mater. Sci. Eng. 116 (1989)79–87.

[45] P.Y. Hou, J. Stringer, The Effect of reactive element additions on theselective oxidation, growth and adhesion of chromia scales, Mater.Sci. Eng. 202 (1995) 1–10.

[46] F.J. Perez, M.J. Cristobal, M.P. Hierro, F. Pedraza, G. Arnau, P.Merino, Effect of yttrium and erbium ion implantation on the oxida-tion behavior of the AISI 304 austenitic steel, Surf. Coat. Technol.126 (2000) 116–122.

[47] F.H. Stott, J. Glascott, G.C. Wood, The influence of alloying el-ements on the development and maintenance of protective scales,Oxid. Met. 44 (1995) 113–141.

[48] T. Zhang, D.Y. Li, Effects of yttrium on sliding wear of 304 stainlesssteel in dilute sulphuric acid and air, Mater. Sci. Technol. 15 (1999)1441–1446.

[49] X. Wang, D.Y. Li, Beneficial effects of yttrium on the mechanicalfailure and chemical stability of the passive film of 304 stainlesssteel, Mater. Sci. Eng. A 315 (2001) 158–165.

[50] X. Wang, D.Y. Li, Effects of yttrium on mechanical properties andchemical stability of passive film of alumide coating on 1045 steel,Surf. Coat. Technol. 160 (2002) 20–28.

[51] M. Sage, C. Guillaud, Methode d’analyse quantitative des varietes950)

[ ear

[ a-99)

[ of a.

[ ther-y,

[ ertiesnol.

[ Theidual

[ t, K.theum,

324.33] F.H. Stott, J. Glascott, G.C. Wood, The Effectiveness of oxide

reducing sliding wear of alloys, Oxid. met. 24 (1985) 99–113.34] F.H. Stott, The role of oxidation in the wear of alloys, Tribol.

31 (1998) 61–71.35] D.P. Whittle, J. Stringer, Improvement in properties: additive

oxidation resistance, Philos. Trans. R. Soc. London Ser. A 295 (1309–329.

36] B. Pieraggi, R.A. Rapp, Interfacial scaling reactions and the reaelement effect, Mater. High Temp. 24 (1994) 229–234.

37] J. Jedlinski, General aspects of the reactive element effect, CSci. 35 (1993) 863–869.

38] J. Stringer, The reactive element effect in high-temperature corroMater. Sci. Eng. A 120 (1989) 129–137.

39] S. Chevalier, G. Bonnet, Effects of a reactive element on isotheand cyclic oxidation of chromia forming alloys: SEM/EDX, TEand SIMS investigations, Mater. High Temp. 17 (2000) 247–25

40] L.V. Ramanathan, Role of the rare earth elements ontemperature oxidation behavior of Fe–Cr, Ni–cr and Ni–Cr–Al allCorros. Sci. 35 (1993) 871–878.

41] M.J. Bennet, D.P. Moon, Effect of active elements on the oxidabehavior of Cr2O3 formers, in: E. Lang (Ed.), The Role of React

allotropiques du cobalt par les rayons X, Rev. Metall. 37 (1139–145.

52] H. Berns, Micro-structural properties of wear resistant alloys, W181–183 (1995) 271–279.

53] R. Liu, D.Y. Li, Y.S. Xie, R. Liewellyn, H.M. Hawthorne, Indenttion behavior of pseudoelastic TiNi Alloy, Scripta Mater. 41 (19691–699.

54] R.N. Taylor, R.B. Waterhouse, A study of the aging behaviorcobalt based implant alloy, J. Mater. Sci. 18 (1983) 3265–3280

55] P. Huang, H.F. Lopez, Effects of grain size on development of amal and strain induced�-martensite in a Co–Cr–Mo implant alloMater. Sci. Technol. 15 (1999) 157–164.

56] F.H. Stoot, C.W. Stevenson, G.C. Wood, Friction and wear propof Stellite 31 at temperatures from 293 to 1073 K, Metals Tech4 (1977) 66–74.

57] S. Chevalier, C. Valot, G. Bonnet, J.C. Colson, J.P. Larpin,reactive element effect on thermally grown chromia scale resstress, Mater. Sci. Eng. A 343 (2003) 257–264.

58] M.F. Stroosnijder, J.D. Sunderkotter, M.J. Cristobal, H. JenetIsenbugel, M.A. Baker, Influence of yttrium ion implantation onoxidation behavior of powder metallurgically produced chromiSurf. Coat. Technol. 83 (1996) 205–211.