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DOI: 10.1177/1350650112469166published online 2 January 2013

2013 227: 423 originallyoceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering TribologyTakuya Nakase, Shinji Kato, Takashi Kobayashi and Shinya Sasaki

during sliding against different steelswt% silicon alloy surface treated with dispersed hard particles12ibological properties of aluminium

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Special Issue Article

Tribological properties ofaluminium12 wt% silicon alloysurface treated with dispersedhard particles during slidingagainst different steels

Takuya Nakase1,2, Shinji Kato1, Takashi Kobayashi1 and

Shinya Sasaki2

Abstract

ADC12 is a eutectic AlSi die-cast alloy (Al12Si) widely used instead of cast iron for industrial parts in order to reducetheir weight. Here, to improve the wear resistance, SiC and diamond particles were dispersed into the surface of theADC12 alloy. The tribological properties of the hard-particle-dispersed ADC12 samples under lubrication were eval-uated using a vane-on-plate tribometer and compared to those of hypereutectic AlSi alloy (Al15Si). Three differentkinds of vane materials, carbon steel, die steel and high-speed steel, were used as counter materials. The diamond-dispersed ADC12 alloy showed the most significant wear reduction effect when sliding against die steel and high-speedsteel, and the lowest coefficient of friction was obtained when high-speed steel was used as the counter material. Thedurability tests using a conventional pump and the modified materials confirmed the significant improvement in the wearresistance of hard-particle-dispersed ADC12.

KeywordsEutectic AlSi alloy, hard particle dispersion, wear, friction

Date received: 31 July 2012; accepted: 25 October 2012

Introduction

Al alloys are widely used to reduce the weight of vari-

ous industrial products, especially automotive compo-

nents. While Al itself has inferior wear resistance

because of its ductility, an increase in the Si content

in an Al alloy can mitigate this effect. Therefore, muchresearch has been conducted on increasing the wear

resistance of these alloys, mainly on engine cylinder

liners using hypereutectic AlSi alloys with more than

12.6 wt% Si; their typical Si contents range from 15 to

20 wt%.15 On the other hand, the high production

costs of hypereutectic AlSi alloys restrict their use

to premium parts. These costs mainly arise from the

difficulty of controlling the size of the primary Si par-

ticles and the poor machinability of the alloys result-

ing from the high Si content. In contrast, the eutectic

AlSi alloy ADC12, which contains 9.612 wt% Si,

has good mechanical properties, good castability

and relatively lower manufacturing costs. Hence,ADC12 has been widely used as an excellent gen-

eral-purpose aluminium die-cast alloy.

In addition to the engine components, aluminium

alloys are used for several other automotive compo-

nents including, for instance, hydraulic pumps for

auxiliary equipments of engines or transmissions.

The hydraulic vane pumps used in such applications

are composed of a housing, a cover and a side-platemade of aluminium alloy as well as several vanes, a

cam and a rotor which are made of steel or sintered

steel. In the vane pump, the vanes and rotor are sand-

wiched between the side-plate and the cover inside the

cam ring. In order to improve the efficiency of the

1Materials Engineering Section, Basic Technology R&D Center, KYB

Corporation, Sagamihara-shi, Japan2Department of Mechanical Engineering, Tokyo University of Science,

Tokyo, Japan

Corresponding author:

Takuya Nakase, Materials Engineering Section, Basic Technology R&DCenter, KYB Corporation, 1-12-1 Asamizodai, Minami-ku, Sagamihara-

shi, Kanagawa-ken 252-0328, Japan.

Email: [email protected]

Proc IMechE Part J:

J Engineering Tribology

227(5) 423432

! IMechE 2013

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DOI: 10.1177/1350650112469166

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of the hard-particle-dispersed ADC12 samples were

investigated by SEMEDX, as shown in Figure 2.

The SEM images reveal hard particles of a few mm in

diameter dispersed over the surface. In addition, the

quantitative EDX showed approximately 3 wt% of C

on the surface of the diamond-dispersed ADC12 (a

representative measured area is shown in Figure 2(b)).Untreated samples were also prepared for compari-

son. The untreated ADC12 and Al15Si alloy samples

were ground with abrasive paper and polished with

1 mm diamond paste, after which they were etched

with 5% aqueous sodium hydroxide solution at

room temperature for about 40 s. The surface was

examined using laser microscopy, which showed that

after etching the Si particles are exposed outside of the

Al matrix and have thus become load-carrying aspe-

rities. The height of the exposed Si was approximately

200 nm. Figure 2(e) and (f) shows SEM images of the

surfaces of untreated samples.

Counter materials. The counter materials used were

three different steels, carbon steel (CS), die steel

(DS), and high-speed steel (HSS), and their chemical

compositions are presented in Table 2. Figure 3 shows

the microstructure of the steels; the sizes of the car-

bide particles in the martensite matrices of DS and

HSS are different.

Hardness. The AlSi alloys and steels consist of severalcrystallized phases with different hardnesses from that

of the matrix. It is important to know the hardness of

each component in order to understand the wear

mechanisms occurring when these materials are

mated. In this way, the microscopic behaviour in the

real contact area of the asperities related to the hard

phases may be understood. A nanoindentation tester

was used to measure the hardness of each material at

100 randomly distributed points. For the AlSi alloys,

the resulting values were then assigned to either the

matrix or the Si particles using the SEM image, as

presented in Table 3, which also includes the hardnessvalues of SiC and diamond. The hardness and

Youngs modulus of diamond were taken from the

literature Richter et al.12 The steels were characterized

Figure 2. SEMEDX images of the surfaces of (a) SiC-dispersed ADC12; red arrows indicate SiC particles, (b, c) diamond-dispersed

ADC12; yellow arrows indicate diamond particles, (d) EDX image of (c), (e) ADC12 etched and (f) Al15Si etched.

SEMEDX: scanning electron microscopy/energy dispersive X-ray spectroscopy.

Nakase et al. 425



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in a similar way, but in the case of CS only five points

were measured because of the absence of hard crystal-

lized phases. As the results, the hardness of diamond

is predominantly higher than the other materials. This

suggests that a few micron sized diamond particle has

a chance to have good running-in behaviour against

the steels even when the fraction of the particle in

aluminium surface is very low.

Tribo-test

Vane-on-plate sliding test. A vane-on-plate tribometer

was used to evaluate the tribological properties of

the aluminium alloys against steels lubricated with

automatic transmission fluid (Figure 4). Three steel

vanes (1.4 8 15mm3) were inserted radially into

a holder. The holder was then loaded at the centre

with a ball so that the assembly could rotate on the

AlSi alloy plate. This arrangement allowed the load

to be applied equally onto each vane. The contact

geometry is plane on plane using the 1.4 8 mm2

flat side of the vane. Furthermore, the holder has a

mechanism that applies the normal force on the centre

top of vane by an orthogonal small cylinder so that the

vane contacts longitudinal exactly to the plate. Even if

there is a small misalignment, the contact area becomes

completely flat after a running-in stage. Before testing,

all the samples were cleaned ultrasonically in acetone

for 5 min. All tests were carried out mainly in a mixed

lubrication regime (at the initial stage) at a normal load

of 250 N (contact pressure is approximately 7 MPa)with a sliding speed of 1.9 m/s at a radius of 12 mm, a

lubricant temperature of 50 C and a test duration of

3 h. The lubricant used was an automatic transmission

fluid of which the viscosities are 41 mm2/s at 40 C and

7.5 mm2/s at 100 C. After the test, two profiles of the

wear scar were recorded using a laser profilometer and

then averaged to determine the amount of wear on the

AlSi alloy plate. Figure 4 shows an example of a wear

scar profile. The wear volume of the plate was calcu-

lated using equation (1)

V Z r2

r1

2xh x dx 1

where x is the lateral distance from the rotation

centre, h(x) the wear depth, r1 the inner radius of

Table 2. Chemical compositions of steels (wt%).

C Si Mn P S Cr Mo W V Fe

CS 0.420.48 0.150.35 0.60.9


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the wear scar, r2the outer radius of the wear scar and

Vthe wear volume of the plate. The frictional torque

was measured using a torque meter during the testand converted into the coefficient of friction.

Vane pump test. An evaluation using actual conven-

tional pumps confirmed the durability of the

ADC12 alloys treated with the hard particles. The

endurance test was performed under the conditions

presented in Table 4. The pumps used were modified

to apply a higher discharge pressure than their ori-

ginal one, in order to evaluate the durability of the

pump under more severe conditions. The pressure was

pulsed between normal pressure and a maximum dis-

charge pressure of 9.8 MPa with a holding time of 2 s

at low and high pressures. The wear depths of thecovers were measured after 1 and 222 h. The cover

materials used were untreated ADC12, SiC-dispersed

ADC12 and diamond-dispersed ADC12. The surface

of the untreated ADC12 was prepared by grinding.

Surface analysis

In order to understand the wear mechanisms, the sur-

faces of the samples were inspected by optical and

electron microscopy as well as EDX before and

after the tests.

Results and discussion

Friction and wear properties

Table 5 lists the wear volume of the plate and the

vanes as well as the corresponding coefficient of fric-tion evaluated using the tribo-test. Figure 5 shows the

friction behaviour as a function of sliding time. The

Table 3. Results of nanoindentation test.

Materials Measure points Hardness (GPa)

Vickers hardness

(HV100g)

Youngs

modulus (GPa)

ADC12 Al matrix 2.1 87.5

Si particle 11.2 90.7

Al15Si Al matrix 1.9 95.2

Si particle 11.4 102.8

CS Ferrite and perlite 4.6 276 271.5

DS Matrix 7.2 686 293.6

Carbide 17.9 293.4

HSS Matrix 8.4 831 242.2

Carbide 22.0 317.8

SiC 22.6 305.0

Diamond 95117a 9211137a

CS: carbon steel; DS: die steel; HSS: high-speed steel.aFrom Richter et al.12

Figure 4. Scheme for vane-on-plate tribo-test and profile of a wear scar on an Al plate.

Nakase et al. 427



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friction and wear properties are dependent on the

presence of hard particles and on the hardness of

the counter material.

The untreated ADC12 experienced 0.51 mm3 of

wear loss against CS. However, the wear to one of

the vanes was too small to measure. When DS was

used for the vane, the untreated ADC12 showedsevere wear. A sharp increase in friction is interpreted

as a typical sign of seizure behaviour, and the test was

stopped if such behaviour was detected, even in the

initial stage (Figure 5(a)). SiC-dispersed ADC12 and

diamond-dispersed ADC12 showed significant

improvements in wear resistance against all of the

steels. It is interesting to note that the least hard

steel, CS, caused more wear on both the hard-parti-

cle-dispersed ADC12 plate and the Al15Si than did

the harder DS. This is because, first, the CS vanes had

been roughened by the dispersed hard particles and,

second, the rough surface of the vanes abraded the Alsurface. When the hardest HSS was used, the wear

loss of the vane was negligible, and a good, smooth

run-in surface was observed. However, the wear loss

of the SiC-dispersed ADC plate was 0.11 mm3, while

the wear loss of the diamond-dispersed ADC12 plate

was almost zero. On the other hand, the Al15Si alloy

also showed almost zero wear, but small signs of wear

on top of the Si particles on the surface were observed

using optical microscopy. However, on the surface of

its counter material HSS, only a slight, less polished

run-in was observed. Thus, it might be concluded that

with regard to wear resistance, the diamond-dispersedADC12 is similar to Al15Si alloy.

The tribo-test also showed that the two hard-par-

ticle-dispersed ADC12 alloys and the Al15Si alloy

exhibited different frictional behaviour. For the SiC-

dispersed ADC12 alloy, the coefficient of friction

against CS and DS was about 0.03, while the one

against HSS was somewhat higher, at 0.08 in the ini-

tial stage and 0.05 at the end of the test (Figure 5(c)).

Throughout the entire test, the lowest coefficient of

friction of 0.002 was measured for the combination

of the diamond-dispersed ADC12 and HSS (Figure

5(d)). This might be due to the ultrahigh hardness ofthe diamond, which could provide superior running-

in behaviour, as is discussed in more detail below. For

the Al15Si alloy, the coefficient of friction at the end

of the tests against the three steels was within the

range from 0.02 to 0.05 (Figure 5(b)).

Durability

The pumps equipped with covers made of hard-parti-

cle-dispersed ADC12 alloy were tested using the endur-

ance tester. The wear depths measured after the test of

the ADC12 covers are shown in Figure 6. The ground

ADC12 cover showed severe wear after 1 h of runningtime. However, a significant improvement in the wear

resistance for the hard-particle-dispersed ADC12 alloy

has been confirmed by the test result. The depth of

wear on the SiC-dispersed ADC12 cover was about

Table 5. Results of vane-on-plate test.

Al plate Hard particle Vane

Wear loss of

Al plate (mm3)

Wear loss of

vane (mm3)

Coefficient of friction

(averaged after 1 h) Notes

ADC12 (Polished and etched) CS 0.62 0.061

DS >75.62 0.079 Seizure

SiC CS 0.07 0.11 0.031

DS 0.03 0.02 0.025

HSS 0.11 0.058

Diamond CS 0.08 0.21 0.017

DS 0.01 0.023

HSS


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2 mm, while that of the diamond-dispersed ADC12

cover was about 1 mm. These results suggest that

these materials treated with hard particles may be

used in pumps to increase the lifetime while providing

equally good efficiency and increased wear resistance.

Figure 7 shows SEM images of the wear scar on each

cover. Figure 7(a) indicates that only the Al matrix wasworn out, and Si particles had accumulated on the sur-

face. It seems that the much harder counter asperities

such as the carbides abraded the surface containing the

softer Si particles. However, for the hard-particle-dis-

persed ADC12 covers, only mild wear was observed

(Figure 7(b) and (c)).

Effect of hard particle dispersion

In order to understand the tribological mechanisms,

the worn surfaces were investigated using SEMEDX

analysis. Figure 8 shows an SEMEDX image of thewear scar of the diamond-dispersed ADC12 alloy that

was sliding against HSS, which was the combination

that showed the lowest coefficient of friction. The SEM

image clearly shows two diamond particles in the alloy

matrix. Around the diamond particles, an accumula-

tion of iron was detected by EDX. This is evidence that

iron is transferred, and thus the surface of the counter

body may be smoothed by the diamond particles.

Figure 9 shows higher magnification SEM images of

different area on the same sample of Figure 8. The

diamond asperities were also smoothed by the counter

body. And it may have low abrasive property because

of the flat contact asperities as well as the low amount

of iron transfer around the diamond particles.

Normally, diamond coatings like the shock wave dia-

mond coating11 show much high friction because of therelatively sharp and high fraction of diamond on the

surface, because it is much more difficult to abrade

such many ultra hard asperities. In contrast, the dia-

mond-dispersed ADC12 sliding against HSS showed

Figure 5. Friction behaviours of (a) untreated ADC12, (b) Al15Si, (c) SiC-dispersed ADC12 and (d) diamond-dispersed ADC12.

Figure 6. Wear depth of AlSi alloys after the pump test.

Nakase et al. 429



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Figure 7. Wear scars of ADC12 covers after the pump test: (a) untreated ADC12; (b) SiC-dispersed ADC12; and (c) diamond-

dispersed ADC12.

Figure 8. Wear scar of diamond-dispersed ADC12 after sliding against HSS.

HSS: high-speed steel.

430 Proc IMechE Part J: J Engineering Tribology 227(5)



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ultra low friction and negligibly small wear volume.

This low wear and the good run-in surface indicate

that the material combination maintained excellent

and stable lubricating ability. Similar effects wereseen for the SiC-dispersed ADC12 alloy. However,

the diamond particles are more efficient in causing

this running-in effect because of their higher hardness.

Furthermore, since the Si particles are not hard enough

to abrade the carbides, Al15Si may exhibit higher fric-

tion than the diamond-dispersed ADC12 when DS or

HSS is used as the counter material. In addition, the

difference between the tribological behaviour of DS

and HSS might be explained by the different sizes of

their carbides. The carbides in DS were around 30 mm

in diameter while the size of the diamonds was under

10 mm, which made it difficult to uniformly polish the

carbide so that the surface became roughened. ForHSS, because the sizes of the carbides and diamonds

are similar, the abrasiveness was much lower, leading

to a smoother surface and, consequently, improved

running-in behaviour.

Conclusions

1. Severe wear was observed on the ADC12 alloy

when the hardness of the dispersed particles was

lower than the hardness of the carbides in the

steel.

2. Dispersing hard particles into the surface ofADC12 significantly improved its wear resistance.

In particular, the wear resistance of the diamond-

dispersed ADC12 with 3 wt% C on the surface

was found to be almost the same as that of the

hypereutectic AlSi alloy. At this concentration of

dispersed particles, the wear loss of the vane was

negligible.

3. The lowest coefficient of friction of 0.002 was

observed for the combination of the diamond-dis-

persed ADC12 sliding against HSS. This can be

explained by the smoothing of the vane surface

and the formation of an iron transfer layer

around the diamond during the running-in period.4. The durability tests using the conventional pump

and the modified materials confirmed a significant

improvement in the wear resistance of both of the

hard-particle-dispersed ADC12 samples.

FundingThis research received no specific grant from any

funding agency in the public, commercial, or not-for-profit

sectors.

Acknowledgements

The authors thank Dr Wa sche (Federal Institute for

Materials Research and Testing, BAM) for useful sugges-

tions and discussions during the writing of this article.

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432 Proc IMechE Part J: J Engineering Tribology 227(5)


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