IJEMS 18(4) 268-282

Indian Journal of Engineering & Materials Sciences

Vol. 18, August 2011, pp. 268-282

Influence of load and temperature on the dry sliding wear behavior of

aluminium-Ni3Al composites

Mehtap Demirela & Mehtap Muratoglub*

aVocational High School, Adiyaman University, 02040 Adiyaman, Turkey

bDepartment of Metallurgy and Material Engineering, Engineering Faculty, Firat University, 23119 Elazig, Turkey

Received 31 May 2010; accepted 25 March 2011

The suitability of Ni3Al intermetallics as reinforcements for Al-base materials for tribological application has been

investigated. For this purpose, an Al/Ni3Al (5 wt%, 10 wt% and 15 wt%) composite is prepared by powder metallurgy

techniques and tested on a pin-on-ring apparatus. The effects of the applied load (83-150 N) and temperature (25-150°C) at a

constant sliding velocity of 0.4 m/s on the wear behavior of Al-Ni3Al composites and wear mechanisms during dry sliding

are investigated. The worn surfaces are examined by scanning electron microscopy (SEM) and energy dispersive

spectrometry (EDS). It is found that the wear resistance of Al-Ni3Al composites decreased with increasing load and with an

increasing fraction of reinforcement Ni3Al particles. With an increasing fraction of Ni3Al particles, the wear resistance of

the composites increased at higher test temperatures, but not at lower test temperatures, and generally with increasing test

temperatures, the weight loss of composite materials increased slightly. It is also observed that a significant amount of

Fe-rich oxide particles become incorporated into the Al matrix during wear, forming a tribolayer.

Keywords: Metal matrix composites, Intermetallics, Tribology, Electron microscopy

Aluminium alloy matrix composites (AMCs)

reinforced with ceramics have been considered

suitable for a wide variety of applications

(i.e. automobile industry and marine structures). The

tribological characteristics of AMCs reinforced with

ceramics have been extensively investigated to

determine the effect of different combinations of

matrix alloy and reinforcements on wear behavior1-4

.

The resultant matrix/reinforcement bonding in these

AMCs is often undesirable as it may induce reduced

ductility and fracture toughness. Thus, the

introduction of new reinforcements is being

investigated5,6

.

A large number of Al-base composites reinforced

with Ni-base intermetallics are being developed for

high performance materials in aerospace applications;

NiAl has been considered as a potential reinforcement

capable of increasing wear resistance for die-cast Al

alloys5. More recently, it was found that Ni3Al

particles improve the resistance of Al matrices7.

Strafellini et al.8 proposed that friction materials are

receiving particular attention because of the possibility

of using these materials for disc brakes in automotive

applications. AMC discs offer promising advantages,

such as lower density and higher thermal conductivity.

Varin et al.9 studied AMCs reinforced with

intermetallic ribbons and concluded that they might

be appropriate for high temperature applications,

particularly when the intermetallic is Ni3Al-based.

One of the most important advantages of employing

Ni3Al as a reinforcement can be inferred from the fact

that its thermal expansion coefficient at room

temperature, 13 × 10-6

K-1

, is much closer to that of Al

alloys10,11

, 18 to 24 × 10-6

K-1

, when compared to

those of ceramic reinforcements, for example,

3.3 × 10-6

K-1

for SiC12

. This small difference in the

thermal expansion coefficient will lower residual

stresses that appear at reinforcement matrix interfaces

while exposing the composite to thermal cycles7.

Al/Ni3Al composites reduce ductility because the

interface between the reinforcement and the matrix is

altered by diffusion reaction products12. Martin

et al.13

analyzed the wear behavior of AMCs in the

case of external heating at temperatures ranging up to

200°C. It was found that wear increased as

temperature increased due to thermal softening of the

composite, and became severe at a critical

temperature8,13

. However, Wang et al.3 found that the

enhanced high temperature behavior of the composite ________________________

*Corresponding author (E-mail: [email protected])

DEMIREL & MURATOGLU: DRY SLIDING WEAR BEHAVIOR OF ALUMINIUM-Ni3Al COMPOSITES

269

will promote better deformation resistance at high

wear loads and speeds than unreinforced Al.

Al-based MMCs reinforced with intermetallics

were first proposed by Yamadi and Unakoshi14

. Later,

Ruutopold et al.15

suggested that Ni3Al would be an

optimum intermetallic reinforcement, but observed

extensive reaction between the intermetallic and the

matrix when composites were produced via the

casting route. In the experimental results, it was

determined that increasing drill hardness and feed

rates decreased the surface roughness of the drilled

surface for all heat-treated conditions14,15

. Izciler

et al.16

used the rwat to characterize the low-stress

abrasive wear behavior of 2124 Al alloy composite.

SiC and Al2O3 abrasive particles were used as the

abrasive medium. It was found that the wear rate of

the composites was increased by increasing the load

wear rate of the composites; those abraded by SiC

abrasive particles showed higher values than those of

the composites abraded by Al2O3 abrasive particles16

.

In the present work, for a meaningful

understanding of the role of the Ni3Al intermetallic

particles in the wear behavior of composites, the use

of Ni3Al as a wear resistance reinforcement for Al-

based composites was investigated. The main

objective of this work is to compere the effects of the

load and the temperature together with the weight

percentage of the reinforcement on dry silding wear

behaviour of the that composites.

Experimental Procedure Materials

Commercial Al powder supplied by ALPACO

(Aluminium Powder Company) of average size 70 µm

was employed as the matrix, and Ni3Al particles

supplied by Alfa (USA) with a mean particle size of

<149 µm were used as the reinforcements. The SEM

micrograph of the Ni3Al particles is shown in Fig. 1.

The particle-reinforced composites contained 5, 10

and 15 wt% Ni3Al were fabricated by powder

metallurgy (PM) techniques. The powders were mixed

for 30 min in a stainless steel cup. After mixing, the

powder mixtures were uniaxially cold pressed at

350 MPa and sintered at 500°C for 45 min in an Ar

which has 99.9% purity atmosphere. Vacuum degree is

1×10-3 Pa. The dimensions of the cylindrical specimens

were a diameter of 12 mm and a height of 7.5 mm. Friction and wear tests

Fig. 1Scanning electron micrograph of the Ni3Al particles

INDIAN J. ENG. MATER. SCI., AUGUST 2011

270

Sliding wear and friction tests were performed on a

pin-on-ring apparatus with the composite specimen

serving as the pin under dry conditions. A schematic

diagram of the experimental arrangement is shown in

Fig. 2. The dry sliding wear tests were carried out at a

constant sliding velocity of 0.4 m/s within an applied

normal load range of 83-150 N and a normal

temperature range of 25-150°C. Steel rings with a

diameter of 35 mm were used as the counterface. The

counterparts in the experiments were fabricated from

GCr15 steel. Prior to the tests, the contact surfaces of

the composite specimens were polished using 600-,

800-, 1000- and 1200- grit SiC emery paper in

running water. Specimens and rings, ultrasonically

cleaned and washed in acetone, were weighed to the

nearest 0.1 mg using an electronic analytical balance

before and after each wear test. The results were taken

as the average from three tests. The coefficients of

friction were obtained periodically by measuring the

tangential force on the specimen using a strain gauge

bridge. The microstructures of the specimens were

examined by scanning electron microscopy (SEM)

and energy dispersive microanalysis (EDS).

Results and Discussion

Microstructure and hardness

The SEM micrographs in Fig. 3 illustrates the

typical microstructure of Al- 10 wt% Ni3Al

composite specimen prepared by powder metallurgy

techniques. The composite showed Ni3Al particles

uniformly distributed throughout the Al matrix and

signs of low porosity. Bulk hardness measurements

were made using a Brinell hardness tester with a

contact pressure of 15 gr. The results of the hardness

test on the composites are given in Table 1. The

hardness values of the composites are much higher

than that of the unreinforced Al matrix and clearly

increase with Ni3Al weight percentage. And also

XRD pattern of the compacted Al composite samples

(Al- 10 wt% Ni3Al) after sintering are given in

Fig. 2 Schematic diagram of pin-on-ring apparatus

Fig. 3 Scanning electron micrograph of the Al-10 wt% Ni3Al composite specimen


271

Fig. 4. This shows that aluminum oxides observed

on composite samples after sintering althoug

sintering was made in controlled atmosphere. Relationships between weight loss and load

The relationship between weight loss and load for

various reinforcement (Ni3Al) amount within the

sliding distance of 1500 m are given in Fig. 5. Weight

losses of these materials increase linearly with

increasing load. The wear behavior could lead to the

conclusion that Ni3Al particles improve the wear

resistance of pure Al in the load range investigated, as

the weight loss of the composite specimens were lower

than those of the Al specimens. For the composites

investigated, the wear resistance of the composite

decreases with increasing Ni3Al weight percentage.

Table 1 Hardness of the unreinforced Al matrix and

Al-Ni3Al composite specimens at ambient temperature

Specimen Average hardness HB

Al 36

Al-5 wt.%Ni3Al 55

Al-10wt.%Ni3Al 57

Al-15wt.%Ni3Al 60

Fig. 4 XRD pattern of the compacted Al-10 wt% Ni3Al composite samples after sintering

Fig. 5 The variation of weight loss as a function of load for Al/Ni3Al composites and unreinforced Al at ambient temperatue


272

(a)

(b)

Fig. 6 Scanning electron micrographs of worn surface of the Al-15 wt% Ni3Al composite specimen for (a) 83 N and (b) 150 N, at

ambient temperatue (P: zones with creaters and G: zones with grooves, arrow indicates sliding direction)


273

The presence of Ni3Al particles is also useful in

preventing the aluminium matrix from early fracture

because the particles can maintain the specimen’s

structural integrity. However, increasing the weight

percentage of Ni3Al particulate at increasing loads

tends to increase the weight loss of the composites.

Particulate resulted in a reduction in the extent of

plastic deformation of the matrix, and increasing the

load tends to cause extensive plastic deformation of

the matrix and crack nucleation at the particle-matrix

interface, which can cause particle decohesion17

. Due

to the occurrence of work-hardening of the plastic

deformation in the subsurface materials, cracks

nucleated around the reinforcement particulate. Under

repeated loading and deformation, the cracks

propagated in the matrix. Eventually, the propagating

cracks joined together, causing particles to pull out17

.

When the load is increased to reach the fracture

strength of the particle, the particles began to fracture

and lose their ability to support the load. For particle-

reinforced composites, the particles near the contact

surface may more readily induce the nucleation of

cracks due to the interface debonding between the

particles and the matrix in comparison to unreinforced

Al. In the sliding wear process, these cracks may

propagate and connect to form subsurface cracks; the

subsurface damage process is increased by the

presence of particles18

.

Figures 6(a) and (b) show SEM images of wear

tracks of Al- 15 wt% Ni3Al composites at different

loads (83 N and 150 N). Wear tracks on Al/Ni3Al

composite material surfaces consisted of zones with

elongated craters in the sliding direction together with

smooth zones with longitudinal grooves. At 83 N,

narrow areas with long grooves and large areas with

small craters were present. Increasing the load to

150 N resulted in an increase in large areas of long

grooves together with lengthened craters; the number

of craters increased with increasing magnitude of the

applied load on the worn surface. The worn surface

was characterized by wide grooves progressed with

high plastic deformation, and there was also

fragmentation with material displaced to the sides of

the wear grooves. Figure 6b also showed the worn

surface of the composites indicated high weight losses

between composite specimens. Related to this

observation, taken surface roughnees measurement

was given in Table 2. Al- 15 wt% Ni3Al composite

specimen worn at 150 N showed the highest surface

roughnees value between composite specimens. The

wear surface of a Al-5 wt% Ni3Al specimen shown in

Fig. 7a was smoother than that of the Al-15 wt%

Ni3Al composite. In addition, worn surfaces of Al-5

wt% Ni3Al showed a decrease in craters while the

number of large grooves increased. Figure 7b shows

SEM images of the worn unreinforced Al surface at

150 N. The appearance of the worn composite surface

was different from that of the unreinforced matrix at

150 N. The wear surface of unreinforced Al exhibited

large areas with long grooves and craters with

adhesion in the sliding direction. So, it showed the

highest weight loss at 150 N for the all test specimens.

Figures 8(a) and (b) show EDS results of the worn

surface of Al-15 wt% Ni3Al tested at 83 N and 150 N.

For both loads, the EDS spectra indicated that the

composite materials contained Al, Fe, Ni and O. The

higher intensity of Fe indicates the transfer of

counterface materials to the surface of a Ni3Al/Al

composite, and the concentration of Ni shows that

worn Ni3Al fragments spread onto the surface. An Al-,

Fe- and O- was present in both materials, but in much

greater quantities at the highest load. The

concentration of Ni decreased together with

increasing load; this result showed that deformed

Ni3Al could not abrade the worn surface and Ni3Al

particles have fewer chemical interactions with the

counterface19. The reason for the superior wear

resistance of the composite at 150 N was believed to

be the interaction with the counterface. The true

contact areas of the composites were smaller than that

of the unreinforced matrix, and the chemical

interactions with the counterfaces of the composites

were less than that of the unreinforced matrix.

Wear products coming either from the counterface

(Fe-rich oxides) or the worn surface itself penetrate

the soft Al matrix. This leads to the formation of a

mechanically mixed tribolayer (MML) basically made

up of oxide particles and Al27,19

. The wear surface

shows the predominant mode at the lower

load (large areas with craters) and changes to a

Table 2 Surface roughness parameters of the test specimens

for the load

Surface Roughness (Ry)

Load Al

matrix

Al-5 wt%

Ni3Al

Al-10 wt%

Ni3Al

Al-15 wt%

Ni3Al

83 2.62 2.29 2.46 2.6

100 2.65 2.3 2.64 2.73

150 2.85 2.35 2.74 2.8


274

(a)

(b)

Fig. 7 Scanning electron micrographs of worn surfaces of the (a) Al-5 wt% Ni3Al composite specimen tested at 150 N and

(b) unreinforced Al specimen tested at 150 N, for ambient temperature (arrow indicates sliding direction)


275

(a)

(b)

Fig. 8 EDS spectra of large scanned areas of the worn surface of Al-15 wt% Ni3Al composite specimen tested at (a) 83 N and

(b) 150 N


276

predominantly abrasive mode at the higher loads (large

smooth areas with long grooves). In addition, the

transfer of Fe from the counterface applied pressure to

the matrix; severe plastic deformation was observed in

the matrix around these Fe particles. The total depth of

severe deformation in other areas of the matrix was not

large20

. This situation results in weight loss of all

specimens that increased with increasig load.

The results of the coefficient of friction of

unreinforced Al and the composites with different

applied loads at a constant temperature of 25°C are

given in Table 3. The coefficient of friction of the

composites is lower than that of the unreinforced Al

matrix and clearly increases with increasing Ni3Al

weight percentage especially at high loads (100 N and

150 N). The coefficient of friction for all materials

increased with increasing load. This result is in good

agrement with the results reported in the

literatures19,21

. The variations in the reported results

and the present results can be taken nearly constant

for the friction coefficient after 100 N. In fact, the

friction coefficient is in general determined by two

contributions; the first due to the adhesive interaction

between the contacting asperities and the second

related to the ploughing contribution due to

abrasion21

. At high loads, the contribution of adhesion

increases in importance because its greater hardness

reduces the contribution of abrasion, and the friction

cofficient increases with load because of the increase

in the real area of contact.

Relationships between weight loss and temperature

The effect of applied temperature on the friction

and wear behavior of Al/Ni3Al composites was

studied by varying the temperature in the range from

25°C to 150°C at a constant load of 83 N. The

variation in the weight loss of the composites and in

the coefficient of friction with applied temperature are

plotted in Figs 9(a) and (b), respectively. The weight

(a)

(b)

Fig.9 The variation of (a) weight loss and (b) coefficients of friction of the Al-Ni3Al composite specimens tested at 83 N as a function

of the test temperatures


277

loss of unreinforced Al increased with increasing

temperature. For composites, the effect of the

reinforcement was strengthened with increasing

temperature, and the weight loss for composite

materials with increased amounts of Ni3Al particles

was lower with increasing temperature. In addition,

the weight loss of the composites was generally lower

than that of unreinforced Al. It is evident that the

coefficients of friction for the composites were lower

than those for the unreinforced Al matrix. Also with

an increasing weight percentage of Ni3Al particulate,

the coefficient of friction for the composite slightly

decreased. And also one of the most important

advantages of employing Ni3Al as a reinforcement

can be inferred from the fact that its thermal

expansion coefficient, 13 × 10-6 K-1

, is much closer to

that of Al alloys10,11, 18 to 24 × 10-6 K-1

. This small

difference in the thermal expansion coefficient will

lower residual stresses that appear at

reinforcement/matrix interfaces while exposing the

composite to thermal cycles. A lower degree of failure

originated at the particle/matrix interface can,

therefore, be expected. In addition, a high thermal

stability of Ni3Al in an aluminum matrix was

observed after long-term annealing at 300°C12

makes

this intermetallic an advantageous reinforcement for a

wear-resistant composites. The enhanced high

temperature behaviour of the composite will promote

better deformation resistance at high temperature than

unreinforced Al.

The SEM micrographs of the worn surface of the

specimens presented in Fig. 10. It shows that wear

mechanisms involving plastic deformation, cracking

and pulling out of particles occured at a wear

temperature of 150°C. Wear in the composite is

proceeded by the mechanism already observed at

ambient temperature for load tests. Wear tracks on all

specimen surfaces occurred from craters together

with grooves in the sliding direction. However, these

microstructures appear in different dimensions, sizes

and shapes under different wear conditions. Effects of

the weight percentage on the wear surface of the

composites were more pronounced. Figure 10a shows

the SEM micrograph of worn surface of the

unreinforced Al specimen at 150°C. This surface

showed predominantly adhesive mode at the high

temperature. Crack formation was produced by the

extremely high strains induced in the matrix by the

pressure of the steel counterface, which were

amplified as the matrix became softer because of the

thermal softening and recrytallization at elevated

temperature21

, and by the development of adhesive

forces between the two bodies in contact. These

adhesive forces were responsible for the chunks of

material glued to the steel sphere after high

temperature testing. Because of the high hardness and

low weight loss of the Ni3Al particles in the

composites, these particles very effectively resist

penetration and cutting into the surface. For

composites, the softer matrix around the particulate

was fractured under both load and temperature.

Contact surfaces arose between the steel and the

Ni3Al particle21

. Ni3Al particles were fractured and

became fragmented (Fig. 10(b)-(d)).

The effect of temperature and weight percentage

on the wear surface of the composites were more

pronounced. The SEM micrographs of the worn

surface of the Al-15 wt% Ni3Al composite tested at

50°C is shown in Fig. 11. For 50°C, wear was by

abrasion and adhesion and the friction coefficient was

quite high. The results showed that the Al-15 wt%

Ni3Al composite tested at 150°C indicated to have a

better worn surface than that of at 50°C. While the Al-

15 wt% Ni3Al composite showed large areas with

long craters, its grooves seemed to increase and there

were cracks on the wear surface. For 150°C

(see Fig. 10d), smooth areas with long grooves

increased, but small craters lengthened and their

number decreased. The EDS analysis (Fig.12)

performed on wear tracks of Al-15 wt% Ni3Al

composite for 50°C and 150°C is indicated that an Al-

Fe which obviously transferred from the counterface,

-O- containing nanocrystalline phase was present in

both materials, but in much greater quantities in the

specimens tested for 150°C. A mechanically mixed

surface layer (MML)8,22

was present for both

materials at all temperature like as for load tests. All

surfaces contained an MML to some degree, which

would also be expected to have higher stiffness and

Table 3 The coefficient of friction obtained according to the

applied loads for the unreinforced Al matrix and Al-Ni3Al

composite specimens at ambient temperature

Coefficient of friction

Load Al Al-5 wt%

Ni3Al

Al-10 wt%

Ni3Al

Al-15 wt%

Ni3Al

83 0.16 0.15 0.15 0.15

100 0.25 0.24 0.25 0.25

150 0.26 0.24 0.26 0.26


278

(a)

(b)

Fig.10 (a,b) Scanning electron micrographs of worn surfaces of the (a) unreinforced Al specimen, (b) Al-5 wt% Ni3Al composite specimen


279

(c)

(d)

Fig.10 (c,d) Scanning electron micrographs of worn surfaces of the (c) Al-10 wt% Ni3Al composite specimen and (d) Al-15 wt%

Ni3Al composite specimen, tested at 150°C (arrow indicates sliding direction)


280

Fig.11 Scanning electron micrographs of worn surface of the Al-15 wt% Ni3Al composite specimen tested at 50°C

(C: zones with creaters and G: zones with grooves, arrow indicates sliding direction)

(a)

Fig.12 (a) EDS spectra of large scanned areas of worn surface of the Al-15 wt% Ni3Al composite specimen tested at 50°C


281

flow stress than the substrate and should therefore

have influenced the true contact area. As can be

observed, the Fe and O content increased together

with increasing temperature, however the

concentrations of Ni decreased because of the

decreasing broken amount of Ni3Al reinforcement.

Interestingly, 150°C showed less weight loss than

50°C. The result showed that this transfer layer may

acted as a solid lubricant, and the effect of this layer

was more pronounced together with increasing

temperature. The surface of specimens for high

temperature was protected by this layer and also

decreases the friction coefficient (see Fig. 10b).

Interestingly, the presence of an MML is largely

ignored in the classical theories of wear. But, It was

recognised that MMLs play a significant role in dry

sliding wear of material in this study.

Conclusions

The following conclusions can be drawn from this

study:

(i) The effect of the weight percentage of Ni3Al

particulate on the sliding wear resistance of the

composites varied with the load. Though the

wear resistances of the composites were higher

than that of the unreinforced matrix, the wear

resistance of the composites decreased with

increasing weight percentages of Ni3Al.

(ii) In wear experiments carried out at different

temperatures, weight loss of both the

unreinforced matrix and lower percentage

reinforced composites (5 wt% and 10 wt%)

increased with increasing temperature. But

also, the weight loss of higher percentage

reinforced composite (15 wt%) was decreased

at higher temperatures (100°C and 150°C).

(iii) Some amount of Fe (from the counterfaces)

was incorporated into the Al matrix. The Fe

content increasing with load and temperature.

Ni3Al particles limited the penetration of

oxide particles, which protected the matrix.

According to EDS analysis, a mechanically

mixed surface layer (MML) was present for

all materials at all loads and all temperatures,

the effect of which increased approximately

linearly with load and temperature. MML

contained appreciable quantities of Fe for

high temperature and high weight percentage

of the Ni3Al.

(iv) In this work, the coefficients of friction for

the composites were approximately

independent of the load and the temperature.

The coefficients of friction for the composites

were lower than that for the unreinforced

matrix under both load and temperature.

However, although the coefficients of friction

Fig.12 (b) EDS spectra of large scanned areas of worn surface of the Al-15 wt% Ni3Al composite specimen tested at 150°C


282

for the composites increased with increasing

weight percentages of Ni3Al particulate at

different loads, they decreased with

increasing weight percentages of Ni3Al

particulate at different temperatures.

Acknowledgements The authors would like to acknowledge the Fırat

University Research Fund (FUBAP-1209) for

financial support throughout this study.

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IJEMS 18(4) 268-282

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