Dry-sliding wear studies of Fe3Al-ordered intermetallic alloy

Materials Science and Engineering A 386 (2004) 408–414

Dry-sliding wear studies of Fe3Al-ordered intermetallic alloy

Garima Sharmaa,∗, P.K. Limayeb, R.V. Ramanujanc, M. Sundararamana, N. Prabhud

a Material Science Division, BARC, Mumbai 400085, Indiab Refuelling Technology Division, BARC, Mumbai 400085, India

c School of Materials Engineering, Nanyang Technological University, Singapored Indian Institute of Technology, Mumbai, India

Received 6 March 2004

Abstract

Room temperature dry-sliding wear behavior of iron aluminides (Fe–28 at.% Al–3 at.% Cr) has been investigated using a ball on plate weartester. The aluminides were heat treated to produce an ordered DO3 structure. It was found that the wear rate of the aluminides increased withan increase in applied normal load and sliding speed. The wear rates of the aluminides were also found to decrease with an increase in slidingdistance. SEM observation of the worn surface showed that microploughing, microcutting and surface delamination were the dominant slidingw©

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eywords: Iron aluminide; Sliding wear; Wear; Intermetallics; Fe3Al; Ordered structures

. Introduction

Intermetallics are potentially excellent-elevated temper-ture materials for structural applications because of theirxcellent properties at high temperature. Material propertiesetermining resistance to wear and erosion are complex, mak-

ng it difficult to predict the service behavior of a particularaterial. Generally, high hardness, rapid work hardening, andood oxidation and corrosion resistance can all contribute toear and erosion resistance[1]. Alloys based on aluminidesre of much interest because these compounds form an imper-ious and protective oxide layer in aggressive oxidizing, sul-dizing and carbonizing environments. Further, these com-ounds show high hardness and work-hardening rate whichean that they can perform in severe wear and erosion condi-

ions. To date, only a few studies had been reported regardinghe wear characteristics of intermetallics like iron aluminidesnd nickel aluminides alloy[2–11].

Iron aluminides based on DO3 or B2 ordered structurere now receiving extensive attention as materials with good

potential for industrial applications as replacement for htemperature oxidation-resisting or corrosion-resisting sless steel[12–15]. These are ordered intermetallic alloy toffer lower material cost than many stainless steel. Compto stainless steels, these materials have a lower densihence they offer a better strength-to-weight ratio. The laductility at room temperature and a decrease in strength a873 K had retarded their development as a structural matHowever, the properties of iron aluminides can be improby adding specific elements like Cr, Mo, Si, Ti etc. Amothese, Cr is a major alloying element, which contributesmensely to the improvement of room temperature ductiliiron aluminides. It also leads to an improvement in theirrosion and oxidation resistance[15–17]. Efforts are continuing at present to develop iron aluminides for high-temperastructural applications. Much effort has been paid to sture control with the aim of improving and optimizing tmechanical properties of Fe3Al-based alloys. Maupin et a[6,7] had shown that the Fe3Al alloy having DO3 structurepossesses marginally lower wear rate as compared wiB2 structure. The wear resistance of Fe3Al alloy was found

∗ Corresponding author. Tel.: +91 22 5505 151; fax: +91 22 1559 0457.E-mail address:[email protected] (G. Sharma).

to vary with ductility and yield strength. It was found that theaddition of aluminum to iron has the effect of improving the

921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2004.07.053

G. Sharma et al. / Materials Science and Engineering A 386 (2004) 408–414 409

wear resistance compared with that of pure iron whether thealloy is ordered or not. The reported work-on-wear deals withonly abrasive and erosive wear as a function of varying alu-minum content[5,6]. The abrasive wear resistance of Fe3Alalloy was found to be comparable with the wear resistance ofAISI 1060 carbon steel and SS 304[6,7]. The abrasive wearof Fe3Al compares favorably with that of Hadfield steel, ahigh toughness material used in the mining applications. Inour earlier studies, it has been found that the addition of Crresulted in improving the wear resistance of iron aluminides[18]. The present study focuses the sliding wear resistance ofiron aluminides based on DO3 structure at different load andsliding speed. The nominal composition of alloy used for thewear study was Fe–28Al–3Cr (at.%).

2. Experimental

The ball-on-plate tests were performed on a micro-frictionmachine (TE 70 Plint) offering friction evaluation and weartesting facilities. Rolled sheet of iron aluminide of composi-tion Fe–28Al–3Cr were cut into plates of 22 mm× 40 mmcross-section. The plates were heat treated at 540◦C for170 h followed by furnace cooling to achieve DO3 ordering atroom temperature. Room temperature XRD was performedt sw rfacer aira mmd rbideb cturet stc wasm (9, 15a tant.

F Op

The tests were performed at 3 and 5 N-load conditions. Foreach data point, three tests were done and results were within±1%. Wear track profiles were formed on the plate due to ballsliding. The wear profiles were ellipsoidal in shape whosemajor, minor axes and depth were measured by stereomi-croscope. Wear rate was calculated by dividing the volumeof the wear groove by the sliding distance. Micro-hardnesstests were carried out in the region near and far away fromthe wear groove to test the effect of strain hardening on thesample. The micro-mechanisms responsible for sliding wearwere studied in detail by SEM.

3. Results and discussion

Wear test on the iron aluminide alloy was performed withdifferent sliding frequencies and normal loads.Fig. 2showedthe variation of coefficient of friction with sliding time. Fric-tion coefficient increased rapidly to a value of 0.60 withinfirst 25 s of the start of the experiment.Fig. 2(a) showed fourstages; A: an initial sharp increase, B: gradual increase, C:second sharp increase, and finally D: a plateau, with nearlyconstant value. Stage D is maintained after prolonged sliding,friction coefficient, though it fluctuates slightly, remains rel-atively constant in the range of 0.56–0.57 (Fig. 2(b)). Fig. 3s of

Fig. 2. Variation of coefficient of friction with sliding time. (a) Initial 120 sof the start of the experiment and (b) prolonged sliding.

o confirm DO3 ordering in the sample (Fig. 1). These plateere metallographically polished to have an average su

oughness (Ra) of 0.25�m. Wear tests were carried out int room temperature using a tungsten carbide ball of 6iameter as a mating material. The abrasive tungsten caall has a hardness of approximately 22 GPa and a fra

oughness of 7.0 MPa m1/2. In order to retain uniform-teonditions, a new ball was used for each test. The ballade to slide on the plate sample with three frequenciesnd 21 Hz) keeping the sliding amplitude of 1 mm cons

ig. 1. Room temperature XRD of Fe–28Al–3Cr showing mainly D3

hase.

hows the variation of coefficient of friction as function

410 G. Sharma et al. / Materials Science and Engineering A 386 (2004) 408–414

Fig. 3. Variation of coefficient of friction with sliding distance at 3 and 5 N load (a) sliding speed of 0.018 m/s and (b) sliding speed of 0.042 m/s.

sliding distance at various loads and sliding speeds. Thesteady-state values of the coefficient of friction showed littledependence on the applied load, though the value at a lowerload exhibited a slightly higher one. The coefficient of frictionwas found to increase with the increase in sliding speed.

Wear rates versus sliding distance for different sliding fre-quencies are shown inFigs. 4 and 5. These plots showed thatthe wear rate decreased as the sliding distance increased.It was found that with the increase in sliding frequencythere was a corresponding increase in wear rate. Compar-ing Figs. 4 and 5, showed that as the normal load increasedfrom 3 to 5 N, there was an increase in wear rate. Wear rateof iron aluminides was found to decrease with an increasein sliding distance (Figs. 4 and 5). The micro-hardness mea-surements performed on the tested sample showed that slid-ing of the ball resulted in an increase in the hardness. Theregion surrounding the wear groove showed a hardness valueof 395 Kg/mm2 as compared to the bulk sample with hard-ness value of 374 Kg/mm2. This increase in hardness was due

to high rate of work hardening of this alloy. Deformation ofFe3Al alloys is propagated by the movement of a group offour a/4〈1 1 1〉 dislocations[19]. Two types of APBs are gen-erated between leading and trailing pairs of dislocations andalso in between these pairs. McKamey et al.[20] observedan increase in separation between all these dislocations withchromium addition, which is indicative of reduction in APBenergy. A consequence of this decrease in APB energy is theuncoupling of leading and trailing pairs of dislocations in thecase of chromium-containing alloys. This could result in theease of cross slip of dislocations resulting in higher ductilityand work-hardening rate. Similar observations regarding therole of chromium addition on mechanical properties of Fe3Alalloys containing 25% Al[21] and 28% Al[20,21]have beenreported. In fact, an increase in aluminum content from 25 to28 at.% resulted in decrease in hardness of Fe3Al alloys [5].A simultaneous decrease in yield strength and an increasein work-hardening rate with increasing aluminum contenthave also been observed. This effect has been attributed to


Fig. 4. Variation of wear rate with sliding distance for various sliding fre-quencies, at a normal load of 3 N.

an increase in aluminum content or ternary chromium ad-dition on the anti-phase boundary energy in DO3 structureFe3Al alloys. It has been well established that the change inmicro-structural morphology due to wear can be attributed totwo concurrent processes: plastic deformation and frictionalheating. This frictional heat, depending on the material prop-erties, can raise the contact temperature. The plastic deforma-tion along with frictional heating can lead to micro-structuralevolution, e.g. formation of small grained from initial large-grained microstructure. In the case of iron aluminides, if thecontact temperature reaches to sufficiently high temperatureit may result in re-crystallisation[7]. The bulk temperaturewas measured with a thermocouple adjusted very close to thesample. The bulk temperature was found to be 28◦C. Con-sequently, the flash temperature produced by the contact ofthe mating material with sample may be the critical factor inthe re-crystallisation process. In order to determine the in-stantaneous temperature at the point of contact (Tc), the bulk

F fre-q

temperature (Tb) and the flash temperature (Tf ) must be de-termined. The instantaneous contact temperature is simplythe sum of the bulk and flash temperatures.

Tc = Tb + Tf

For estimating the flash temperature, the equation devel-oped by Archard[22] as described below was used.

Tf = µV (πFNPy)

8k

1/2

whereµ is the coefficient of friction,V is the sliding speed,FN is the normal force acting on the sample,Py is the yieldpressure of Fe3Al (approximately equal to hardness) andkis the thermal conductivity. Using 7 W m−1 ◦C−1 value fork, 392 kg/mm2 for hardness, 0.57 forµ, the flash tempera-ture was calculated to be approximately 365◦C. The instan-taneous contact temperature was calculated to be 393◦C.Maupin et al.[7] have reported that a temperature rise of410◦C resulted in dynamic re-crystallisation. The differencein the temperature rise in the two studies may be due todifference of composition and test conditions. The DO3 toB2 transformation temperature as reported in the literature is540◦C. The presence of Cr has no significant effect on thistransformation temperature[23]. A temperature rise of thismagnitude along with plastic strains was enough to cause re-c alct wears sup-p how-i bulkh

alsoa d inms e aso th thei raturer h toc earr creasei se inw ad isis inede orkh xida-ts rfacea ains,w ruc-t duet ashtd ighv the

ig. 5. Variation of wear rate with sliding distance for various slidinguencies, at a normal load of 5 N.

rystallisation in the case of Fe3Al alloys. Since the thermonductivity of iron aluminides is lower than pure iron[7],he re-crystallised zone should be localized to the nearurface region. The presence of this localized zone isorted by micro-hardness results in the present study s

ng high hardness in the wear zone as compared to theardness.

At such high-temperature oxidation of the surface ispossibility and in fact oxide layers have been observeany worn surfaces of these alloys. Kim and Kim[5] have

hown the formation of mixed oxide layer on the surfacne of the reason for the increased wear resistance wi

ncrease in sliding distance. The instantaneous tempeise in the present study was found to be sufficiently higause a thin-oxide layer thereby resulting in improved wesistance. The present observations regarding the den wear rate with increase in sliding distance and increaear rate with increase in sliding frequency as well as lo

n conformity with earlier studies on binary Fe3Al alloys neartochiometric compositions. This was due to the combffect of large-friction heat generated as well as the wardening, subsequent dynamic re-crystallisation and o

ion of the surface during sliding. Maupin et al.[6,7] had alsohown that no ordering was observed on the deformed sund large grains were replaced by fine nanocrystalline grhich was relatively free of dislocations. Such microst

ures could develop only if the temperature of the surfaceo friction is very high. They have determined that the flemperature near asperities due to friction heat in Fe3Al alloysue to their poor-thermal conductivity could reach very halue, sufficient to cause dynamic re-crystallisation of


Fig. 6. SEM micrograph showing (a) microploughing grooves after a sliding distance of 54 m, (b) microploughing and microcutting after a sliding distance of216 m and (c) wider and deeper ploughing grooves with small regions of microcutting with sliding of 540 m, tested at 3 N load.

deformed surface. They attribute the large increase in Knoophardness with sliding distance to the fine-sized grains devel-oped on the surface. In addition, disordering of the surfacelayer could result in higher plastic flow and work harden-ings resulting in increased wear resistance. From the abovediscussion, one could conclude that the wear rate should de-crease with the sliding distance. Higher sliding speed induceshigher deformation rate of the wearing surface, which seemsto result in increase in wearing rate.

SEM micrographs of the worn surfaces are shown inFigs. 6 and 7. The micrograph of iron aluminides tested at3 N at different sliding distance is shown inFig. 6. Fig. 6(a)showed mainly narrow-ploughing grooves and microcuttingon sliding surface after a sliding distance of 54 m. With fur-ther increase in sliding distance to 216 m, the microploughingand microcutting were found to take place simultaneously(Fig. 6(b)). Fig. 6(c) shows mainly deep-ploughing grooveswith small pockets of microcutting after a sliding of 540 m.These results showed that mainly microploughing takes placeduring the initial stages. With continued sliding the groovesbecomes deeper and microcutting was also found to takeplace. Both the mechanisms were taking place simultane-

ously, however, microploughing was found to be the domi-nant mechanism for all wear tests at 3 N load condition. Ironaluminides when tested at 5 N load showed wider and deeperploughing grooves on the surface (Fig. 7(a)). However, withthe increase in sliding distance to 540 m, traces of the detach-ment of deformed surface layer were also found to take place(Fig. 7(b)) along with microploughing and microcutting. Asthe load increased, the surface suffers extensive damage anddeformed surface platelets got detached from the surface withsubsequent sliding of the tungsten carbide ball. This featurewas observed only at higher load condition. In brittle or hardmaterial, very few slip systems are available and during wear,strains accumulate at the surface resulting in the formation ofsubsurface deformation zone. Cracks initiate in this zone andthe delamination of the zone results in higher wear rate. Thiscould explain the observation that with the increase in loadthere is an increase in the wear rate. Iron aluminides did notshow any sharply delineated cleavage faces or the cracks ofbrittle nature on the wear surface. These observations showedthat the sliding wear of iron aluminides occurred by plasticdeformation not by fracture. During microploughing, mate-rial may be ploughed aside repeatedly by passing particles


Fig. 7. SEM micrograph showing (a) wider and deeper ploughing groovesand (b) detachment of deformed surface layer, tested at 5 N load after asliding of 540 m.

and may break off. Similar observations were reported byMaupin et al.[6,7] where they found microploughing, micro-cutting and micro-fracture to be the dominant abrasive wearmechanism of iron aluminides. Johnson et al.[3] reportedthat microploughing and microcutting were the two domi-nant material removal processes during abrasion wear of ironaluminides. These mechanisms were responsible for the nar-row, deep grooves on the worn surfaces of the abraded sam-ples. However, Kim and Kim[5] showed that under low-loadcondition, microploughing was the predominant wear mech-anism for iron aluminides, whereas under high-load condi-tions, formation of platelets and their subsequent detachmentfrom the wearing surface was observed as a dominant-wearmechanism. These observations were found to be in goodagreement with the results obtained in this study. SEM ex-amination in the present case indicated that microploughingand microcutting were the dominant wear mechanisms. Thisis indicative of the fact that considerable plastic deformationmight have occurred on the surface. Based on these obser-

vations, it can be postulated that the wear mechanism of theiron aluminides was plastic deformation dependent.

4. Conclusion

The sliding wear behavior of iron aluminide had beenevaluated in this study using a ball on plate tester, micro-hardness tester and SEM observations. The followingconclusions were reached:

1. The sliding wear rate of iron aluminide was quantitativelymeasured and was found to decrease with an increase insliding distance.

2. An increase in the sliding frequency and load resulted inan increase in the wear rate.

3. SEM examinations of the abraded surfaces indicated thatmicrocutting and microploughing were the wear mech-anisms. With the increase in load, detachment of thedeformed surface layer along with microploughing andmicrocutting were the dominant sliding wear mecha-nisms. Wear mechanism of the iron aluminides was plasticdeformation dependent.

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cknowledgements

The authors would like to thank Mr. Jadhav, Refuelechnology Division for the help extended in performingxperiments. The authors would also like to thank Dr. P.Kead of the Materials Science Division, Shri R.G. Agarwead of the Refuelling Technology Division and Shri Noni, section head of the Fluid Power and Tribiology Sec

or their encouragement.

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Dry-sliding wear studies of Fe3Al-ordered intermetallic alloy