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Wear 268 (2010) 473–480

Contents lists available at ScienceDirect

Wear

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

ry sliding wear behavior of Fe3Al alloys prepared by mechanical alloying andlasma activated sintering

ian Wang ∗, Jiandong Xing ∗, Li Cao, Wei Su, Yimin Gaotate Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, 28 Xianning West Road,i’an, ShaanXi Province 710049, PR China

r t i c l e i n f o

rticle history:eceived 28 August 2008eceived in revised form 27 August 2009ccepted 14 September 2009

a b s t r a c t

Room dry sliding wear behavior of ultrafine grained Fe3Al alloys prepared by mechanical alloying withsubsequent plasma activated sintering was investigated on a pin-on-disk wear tester using Fe3Al alloyspins and AISI 304 stainless steel discs. The results showed that hardness and wear resistance of Fe3Al alloys

vailable online 24 September 2009

eywords:e3Al alloystainless steel

were enhanced owing to fine microstructure and precipitation of hard Fe3AlC0.5.Volume wear rate of Fe3Alalloys increased as applied load increased, to a peak at 30 N, and then decreased. Moreover, it decreasedwith the increase of sliding speed. Worn surface of Fe3Al alloys and wear debris were analyzed by scanningelectron microscopy equipped with energy dispersive spectroscopy. It was found that stainless steeltransferred to the worn surface of Fe3Al alloys during sliding. Applied load and sliding speed had differenteffect on incrassation and peeling-off of transfer layers, thus causing different wear rate tendency of Fe Al

ardness

liding wear alloys.

. Introduction

Fe3Al alloys, one of the most important intermetallic alloys, haveood oxidation resistance, excellent sulfidation resistance, rela-ively low cost and density, so they have attracted considerablettention as potential candidates for high temperature structuralaterials [1–4]. Moreover, high elastic modulus and high work

ardening ability of Fe3Al alloys make them as promising wearesistant materials [5–8].

However, poor hardness and fracture toughness at room tem-erature prevent Fe3Al alloys from industrial applications asribological components. Hardness and wear resistance of as-caste3Al alloys were found to be comparable with AISI 1060 car-on steel and AISI 304 stainless steel (SS304), but much lowerhan high Cr white cast iron and ceramics [9]. By addition of largemount of hard phase particles such as TiC, TiB2, Fe3AlC0.5 ando on [9–11], hardness of Fe3Al alloys was significantly improved,owever, the improvement of hardness was generally accompa-ied with a reduction of toughness, which is harmful to their wearesistance.

Hardness and toughness of materials can be improved simulta-eously by grain refinement to nanometer, and it will be helpfulo improve their wear resistance. However, conventional fabri-ation processes, such as casting and powder metallurgy, are

∗ Corresponding authors. Tel.: +86 29 82665636; fax: +86 29 82665479.E-mail addresses: jwangxjtu@163.com (J. Wang), jdxing@mail.xjtu.edu.cn

J. Xing).

043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2009.09.006

3

© 2009 Elsevier B.V. All rights reserved.

difficult to obtain nano-materials. In recent years, some effec-tive methods, such as mechanical alloying (MA), hot isostaticpressing (HIP), plasma activated sintering (PAS), etc., have beenconducted to prepare nanocrystalline Fe3Al alloys with rela-tively high hardness and ductility [12–15]. Thus, in the presentwork, uiltrafine grained Fe3Al alloys were prepared by mechani-cal alloying with subsequent plasma activated sintering (MA-PAS),and then the hardness and wear behavior of Fe3Al alloys underdry sliding against SS304 at ambient conditions were investi-gated.

2. Experimental procedure

2.1. Preparation of Fe3Al alloys

Fe3Al alloys were prepared by MA-PAS in the following ways:iron powder (200 mesh, 99 wt.% purity) and aluminum powder (200mesh, 99 wt.% purity) were mixed in a composition of Fe–28 at.%Al.59.5 g of the mixed powders, 595 g of 10 mm sized stainless steelballs and 0.5 ml methanol as process control agent were packedin a stainless steel vessel. The MA was performed in a planetaryball mill at 250 rpm for 50 h under an argon atmosphere. 30 g ofmilled powders were packed in a graphite mold under a continuouspressure of 30 MPa using a PAS apparatus (Ed-PASIII, Elenix Ltd.,

Japan) whose inner atmosphere in chamber was a vacuum of 1.5 Pa,and then consolidated at 1273 K for 180 s.

Density of the compacts was measured by Archimedes method.Bulk hardness was determined on each specimen by a universalVickers indenter at a load of 50 g for 30 s.

474 J. Wang et al. / Wear 268 (2010) 473–480

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Fe3Al alloys. It can be seen that the wear rate of Fe3Al alloys is lowerthan that of SS304, and changes little for every 250 m. Same resultswere also found in other test conditions.

Fig. 1. Schematic illustration of pin-on-disc dry sliding wear tester.

.2. Wear tests

Wear tests were carried out using a pin-on-disk wear tester,s schematically shown in Fig. 1. The pin specimen was fixed, andhe disk specimen rotated at different speeds. The pin specimens,ith a size of 5 mm × 5 mm × 10 mm, were made of Fe3Al alloysrepared by MA-PAS; the disk specimens, with a size of 44 mm iniameter and 6 mm in thickness, were made of SS304. The surfaceoughness of pin specimens and disc specimens were Ra = 0.3 and.15 �m, respectively.

Friction and wear tests were conducted under dry condition atoom temperature. The sliding speed was varied from 0.065 m/so 0.39 m/s, and the applied load was varied from 10 N to 50 N.ach new pair had a 500 m sliding distance under certain speednd load, and each test was repeated three times. Before and afteresting, the specimens were ultrasonically cleaned in acetone bathor 15 min. The friction force was measured with a load cell and wasonverted into the friction coefficient. The wear mass loss (�m) waseasured by an electronic balance with an accuracy of 0.1 mg. The

olume wear rate (�V) was calculated according to the followingxpression:

V = �m

�L(1)

here � was the density of the test materials, L was the slidingistance, and �m was the average of wear mass loss of three spec-

mens.The worn surface of pins and wear debris collected during

ear tests were examined by scanning electron microscopy (SEM)quipped with energy dispersive spectroscopy (EDS) to analyze theear mechanism of Fe3Al alloys.

. Results and discussion

.1. Microstructure and mechanical properties of Fe3Al alloys

The microstructure of Fe3Al alloys prepared by MA-PAS is shownn Fig. 2. No porosities are observed even at high magnification.he density of Fe3Al alloys measured by Archimedes method is.65 ± 0.01 g/cm3. It can also be seen that the grain size of the com-act is about 1 �m and that a few precipitations disperse in theatrix. Result of XRD analysis, as shown in Fig. 3, indicates that theain phase of the compacts is Fe3Al alloys, and that the precip-

tations in the matrix are Fe3AlC0.5 which formed from methanol

sed during MA. Owing to ultrafine microstructure and precipitatesf hard Fe3AlC0.5 (595HV) [16], hardness of Fe3Al alloys preparedy MA-PAS (524 ± 7 HV) is much higher than that of as-cast Fe3Allloys (235 HV) [5] and SS304 (230 ± 5 HV).

Fig. 2. SEM (a) and TEM (b) images of Fe3Al alloys prepared by MA-PAS.

3.2. Effect of sliding distance on friction coefficient and wear

Fig. 4 shows a typical variation of friction coefficient with slidingtime. It can be seen that the friction coefficient is relatively high atthe very beginning, and after about 100 s, the friction coefficientdecreases and periodically fluctuates in the range of 0.45–0.55.Fig. 5 shows the volume wear rate comparison between SS304 and

Fig. 3. XRD patterns of Fe3Al alloys prepared by MA-PAS.

J. Wang et al. / Wear 268 (2010) 473–480 475

Fig. 4. Variation of friction coefficient with sliding time.

atctW

Table 1Fiction coefficient, wear mass loss and volume wear rate of Fe3Al alloys at differentapplied load (sliding speed: 0.13 m/s, sliding distance: 500 m).

Applied load(N)

Friction coefficient Mass loss (g) Volume wear rate(×10−7 cm3/m)

10 0.8253 ± 0.1 0.0023 ± 0.0002 6.85 ± 0.2220 0.7090 ± 0.08 0.0033 ± 0.0004 9.82 ± 0.36

peak at 30 N, and then decreases.

Fig. 5. Comparison of volume wear rate of pin and disc for every 250 m.

There have been a number of reports concerning the frictionnd wear behavior of metal-to-metal couple. Ref. [17] indicated

hat when two solid surfaces were brought on to contact, adhesionould occur at the asperities between the two solid surfaces, andhis adhesive force at the interface acted as a resistance to motion.

ith the relative movement of the sliding surfaces, the adhesive

Fig. 6. Distribution of Cr and Al elements in the worn surface of Fe3Al alloys

30 0.6000 ± 0.18 0.0038 ± 0.0002 11.31 ± 0.2640 0.6547 ± 0.10 0.0030 ± 0.0001 8.93 ± 0.2050 0.6100 ± 0.15 0.0029 ± 0.0002 8.78 ± 0.24

points or areas would be torn off. The fracture did not usually occurin the adhesive junction but rather in the hardness lower of the twomaterials and transfer took place. In the present study, the lowerhardness of SS304 resulted the bulge was pulled out and transferredto the worn surface of Fe3Al alloys. The transfer phenomenon wasconfirmed by area distribution of elements in the worn surface ofFe3Al alloys by EDS (see Fig. 6). It can be seen that Cr element,which only exists in SS304, presents on the Fe3Al alloys surface. Itindicates that SS304 transferred to the Fe3Al alloys surface duringsliding. The transferred layers can partly prevent Fe3Al alloys frombeing worn, so the wear rate of Fe3Al alloys is lower than that ofSS304.

Heilmann et al. [18] investigated the transfer phenomena inCu/steel friction couple, and found that the adhesion and trans-fer occurred at the very beginning of the experiment, even beforeloose wear debris could be detected. In the light of the report,higher friction coefficient at the very beginning was mainly due tothe adhesion-transfer effect and the friction coefficient decreasedafter transfer layers formed. During the following stage, due torepeatedly shearing and compressive stress, the transfer layerswere broken away from the worn surface and wear debris formed.There was a competition between adhesion-transfer and peeling-off of transfer layers, and then the friction coefficient fluctuatedperiodically.

3.3. Effect of applied load on wear of Fe3Al alloys

Table 1 shows the fiction coefficient, wear mass loss and volumewear rate of Fe3Al alloys at different applied load. It can be seen thatwear rate of Fe3Al alloys increases as applied load increases, to a

Generally, shear stress and compressive stress increases withthe increase of applied load, and high shear stress and compressivestress are the main reasons for the peeling-off of transfer layers.The peeling-off of transfer layers was confirmed by SEM exami-

(applied load: 10 N, sliding speed: 0.39 m/s, sliding distance: 500 m).

476 J. Wang et al. / Wear 268 (2010) 473–480

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On the other hand, the increase of applied load will also rise theflash temperature of the friction interface, which will increase thetransfer layers thickness and decrease the wear rate of Fe3Al alloys.Fig. 9 gives the variation of flash temperature with the increase of

Fig. 7. SEM morphologies of worn surface of Fe3Al alloys, sliding speed

ation of the worn surface of Fe3Al alloys (Fig. 7). The arrows inig. 7 point to the peeling-off pits. Moreover, local microfracturef Fe3Al alloys may occur due to repeated shearing and com-ressive stresses, and then the fracture fragments are pulled outnd then are taken away with the peeled stainless steel trans-er layers, and this will rise the wear rate of Fe3Al alloys. The

racture fragments of Fe3Al alloys were confirmed by analyz-ng the composition of the wear debris by EDS (Fig. 8). Fig. 8ndicates that the wear debris is a mixture of Fe3Al alloys andS304.

Fig. 8. EDS spectrum of wear debris.

m/s; sliding distance: 500 m; applied load: (a) 10 N, (b) 30 N, (c) 50 N.

Fig. 9. Variation of flash temperature with the increase of applied load.

J. Wang et al. / Wear 268 (2010) 473–480 477

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Fi

ig. 10. Pear-shaped region of electronic beam and depth of penetration [20].

pplied load, which was calculated using the following equationeveloped by Archard [19]:

Tf = �vd(�FNpy)8k

(2)

here � was the friction coefficient which was measured duringhe sliding, �d was the sliding speed, FN was the applied load on thein, py was the yield pressure of the Fe3Al alloys (about equal to theardness) and � was the thermal conductivity of the Fe3Al alloys. Itan be seen that the flash temperature increases with the increasef applied load. Stainless steel layers on Fe3Al alloys surfaces areery thin. A precise determination of their thickness is very diffi-ult. In Ref. [20], EDS was adopted to do a rough determination ofhe relative thickness of transfer layers. When a high energy beam isnjected into a specimen surface, the beam will propagate normallynd laterally, forming a pear-shaped region as shown in Fig. 10. Theignals received by the detector of EDS are essentially coming fromhe entire region, so there should be a relation between film thick-ess ı and the relative amount of the elements. Fig. 11 shows theariation of contents of Cr and Ni of the transfer layers with thencrease of applied load. From Fig. 11, it can be qualitatively con-luded that the transfer layers becomes thicker with the increase

f applied load.

From the above analyses, it is not difficult to explain the effectf applied load on wear rate of Fe3Al alloys as follows: when thepplied load was lower than 30 N, the transfer layers were relatively

ig. 11. Variation of relatively contents of Cr and Ni in the transfer layers with thencrease of applied load.

Fig. 12. Variation of flash temperature with sliding speed.

thin, and wear of Fe3Al alloys was mainly owed to the peeling-offof transfer layers, the rate of which increased with the increaseof applied load, so the wear rate increased. However, when theapplied load was higher than 30 N, because of relatively high flashtemperature, the incrassation of transfer layers counteracted themoss loss caused by peeling-off of transfer layers and so the wearrate decreased.

3.4. Effect of sliding speed on wear of Fe3Al alloys

Table 2 shows the fiction coefficient, wear mass loss and volumewear rate of Fe3Al alloys at different sliding speed. It can be seenthat wear rate of Fe3Al alloys decreases rapidly with the increaseof sliding speed.

Different to the effect of applied load, the increase of slidingspeed has slight effect on the variation of shearing and compres-sive stresses, which are the main reason for peeling-off of transferlayers. Fig. 12 gives the variation of flash temperature with slidingspeed, which was also calculated according to Archard’s model [19].It can be seen that the flash temperature increases rapidly as slidingspeed increases. Comparing Figs. 9 and 12, it can be seen that theeffect of sliding speed on flash temperature is stronger than thatof applied load. SEM images of the worn surface of Fe3Al alloys areshowed in Fig. 13. The phenomenon of adhesion and peeling-offwas also obviously found. Fig. 14 shows the variation of contentsof Cr and Ni in the worn surface with the increase of sliding speed.It confirms that the transfer layers became thicker as sliding speedincreased. If the flash temperature is high enough, the transfer par-ticles should aggregate and grow up. For the specimens sliding athigher sliding speed (0.26 m/s and 0.39 m/s), large wear derbieswith flake shape were found (Fig. 15(c) and (d)). It clearly showsthat the transfer particles aggregated and grew into flakes becauseof high flash temperature. For comparison, the wear derbies formedat lower sliding speed are also presented in Fig. 15(a) and (b).

On the other hand, in general, the hardness of metals decreasesat elevated temperature, and consequently the wear resistance willdecrease. However, hardness of Fe3Al alloys changes little until823 K [1]. The flash temperature during the test is lower than 823 K(Fig. 12), so the hardness of Fe3Al alloys changes little with theincrease of flash temperature.

From above analyses, it can be concluded that: increasing of slid-ing speed had stronger effect on incrassation of transfer layers, buthad slight effect on peeling-off of transfer layers and the hardnessof Fe3Al alloys, so the wear rate of Fe3Al alloys decreased with theincrease of sliding speed.

478 J. Wang et al. / Wear 268 (2010) 473–480

Table 2Fiction coefficient, wear mass loss and volume wear rate of Fe3Al alloys at different sliding speed (Applied load: 10 N, sliding distance: 500 m).

Sliding speed (m/s) Friction coefficient Mass loss (g) Volume wear rate (×10−7 cm3/m)

0.065 0.8335 ± 0.05 0.0057 ± 0.0006 16.96 ± 0.150.13 0.8253 ± 0.1 0.0023 ± 0.0002 6.85 ± 0.220.26 0.5125 ± 0.08 0.00045 ± 0.0005 1.40 ± 0.230.39 0.3715 ± 0.04 0.00025 ± 0.0005 0.74 ± 0.21

Fig. 13. SEM morphologies of the worn surfaces of Fe3Al alloys, applied load: 10 N; sliding distance: 500 m; sliding speed: (a) 0.065 m/s, (b) 0.13 m/s, (c) 0.26 m/s, (d) 0.39 m/s.

Fig. 14. Variation of relatively contents of Cr and Ni in the worn surface of Fe3Al alloys with sliding speed.

J. Wang et al. / Wear 268 (2010) 473–480 479

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Fig. 15. SEM morphologies of the wear debris, applied load: 10 N; sliding di

. Conclusion

From above test results and analyses, the following conclusionsan be summarized.

. Hardness of Fe3Al alloys prepared by MA-PAS was enhancedowing to fin microstructure and precipitation of hard Fe3AlC0.5,and consequently the wear resistance of Fe3Al alloys was higherthan that of SS304, which was comparable with as-cast Fe3Alalloys.

. During sliding wear process for Fe3Al alloys against SS304, stain-less steel was firstly transferred to Fe3Al alloys surface, and thenthe transfer layers was peeled off due to the repeated shear andcompressive stresses. When the transfer layers were peeled off,some Fe3Al alloys fragments were also pulled out and takenaway.

. Wear rate of Fe3Al alloys increased as applied load increased toa peak at 30 N, and then decreased. The effect of applied loadon wear rate of Fe3Al alloys was attributed to two concurrentprocessing: the incrassation of transfer layers and peeling-offof transfer layers. When the applied load was lower than 30 N,the transfer layers was relatively thin, and wear of Fe3Al alloyswas mainly owed to the peeling-off of transfer layers. When theapplied load was higher than 30 N, because of relatively highflash temperature, the incrassation of transfer layers counter-

acted the moss loss caused by peeling-off of transfer layers andso the wear rate decreased.

. Increasing of sliding speed had stronger effect on incrassationof transfer layers, but had slight effect on peeling-off of transferlayers, so the wear rate of Fe3Al alloys decreased with increasingsliding speed.

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: 500 m; sliding speed: (a) 0.065 m/s, (b) 0.13 m/s, (c) 0.26 m/s, (d) 0.39 m/s.

Acknowledgements

This research was supported by the National Nature ScienceFoundation of China (Grant No. 50871084). The authors wereindebted to Prof. Guanjun Qiao and Dr. Yajie Guo for their assistantduring the plasma activated sintering.

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