Failure and tribological behaviour of the AA5083 and AA6063

composites reinforced by SiC particles under ballistic impact

M.B. Karamis*, A. Tasdemirci, F. Nair

Department of Mechanical Engineering, Erciyes University, 38039 Kayseri, Turkey

Received 7 January 2002; revised 1 January 2003; accepted 13 January 2003

Abstract

The wear behaviour of two different MMCs, namely AA5083 and AA6063 reinforced by 45, 30 and 15% SiCp, respectively, are

investigated under condition of high-velocity impact. The tests are carry out by firing 7.62 armour piercing rounds into these composite

materials. The wear and failure mechanisms are evaluated by examining the projectile tips and the hole surfaces produced by high-velocity

impact using SEM and optical microscopy. The hardness differences of the two regions on the hole surfaces, i.e. the plastically compressed

regions surrounding the projectile hole is higher than unaffected matrix. The wear mechanisms on the friction surfaces of the matrix are

predominantly abrasion and melt wear. It is observed that the projectile nose is plastically deformed when it impacts the armour. The

projectile surface is also scratched by the SiC particles on the hole surface produced by the impact. The wear mechanism on the projectile

surface is predominantly abrasive.

q 2003 Elsevier Science Ltd. All rights reserved.

Keywords: A. Metal-matrix composites (MMCs); A. Particle-reinforcement; B. Wear; B. Impact behaviour

1. Introduction

Discontinuously reinforced metal matrix composites are

typically a two-component system consisting of a dispersed

ceramic phase in a metallic matrix. Metal-matrix compo-

sites (MMCs) such as Al, Ti and Ni alloys reinforced with

Al2 O3 and SiC particulates or whiskers have the potential to

provide desirable mechanical properties including high

specific stiffness, high plastic flow strength, good thermal

expansion, thermal stability, creep resistance, and good

oxidation and corrosion resistance. Particulate and whisker

reinforced MMCs have further advantage that traditional

manufacturing process such as forging, rolling and extru-

sion can be used to generate the finished product. This suite

of properties makes particle reinforced MMCs attractive to a

wide range of applications in automobile, aerospace, and

defence industries [1–5].

In recent years, ceramic-particle and whisker-reinforced

MMCs have emerged as an affordable material candidate

for commercial and military applications. MMC heat

management packaging in electronics and automotive

brake and structural components are in widespread use

today. On the military side, MMCs are used in jetfighters

as structural and high-temperature engine components. The

Army is conducting applied research into further appli-

cation of MMCs in vehicle trackshoes and helicopter

landing gears. The recognition of MMCs as a maturing

materials technology stimulated exploratory research into

the armour potential of this material [6]. Largely driven by

these applications, extensive theoretical and experimental

studies have been made in recent years to uncover the

mechanics and mechanisms underlying the behaviour of

particle reinforced MMCs, e.g. Ref. [1]. On the negative

side, composites are usually more brittle than their

corresponding unreinforced alloys, exhibiting a low

elongation to tensile, failure and low fracture toughness

[5].

Most metals harden with increasing strain rate; so do

MMCs. Under dynamic loading conditions, such as during

penetration of MMCs armours, collision of cars and the

impact of foreign objects on aerospace structures, the

components made of particulate reinforced MMCs often

sustain high strain rate deformation. Improved penetration

resistance was observed with high-strength ceramic

1359-835X/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S1359-835X(03)00024-1

Composites: Part A 34 (2003) 217–226

www.elsevier.com/locate/compositesa

* Corresponding author. Tel.: þ90-352-4375755; fax: þ90-352-

4375784.

E-mail address: karamisb@erciyes.edu.tr (M.B. Karamis).

particulate reinforced MMCs. The hypothesis that MMCs

exhibit excellent wok hardening under dynamic loading

supports the observed ballistic performance [1,4,6].

Projectiles are in competition with armours. On the one

hand the armour materials have been improved against

primitive projectiles in recent years. On the other hand the

projectiles are aimed to pierce the armour. Therefore it is

desirable that the armour should be able to withstand the

projectile in itself. This is a measure on the performance of

the armour whether it is strong or not.

Therefore, improving the metal matrix composites and

understanding their strength against projectiles are import-

ant areas for investigation.

The mechanical properties of MMCs have an important

role on their performance against the projectile action. The

tensile properties of an aluminium composite reinforced by

Al2O3 particles were investigated and it was observed that

the total elongation was dependent on the strain rate [4]. The

penetration behaviour of the high-velocity projectiles has

been investigated both theoretically and experimentally, and

it has been found that the strength of MMC is an important

factor for penetration depth [7–9]. In connection with this,

the dynamic properties of MMCs have also been investi-

gated from strain-rate behaviour point of view [10,11]

Although the strength of MMCs and their failure

mechanisms against ballistic attack have been widely

investigated, the wear mechanisms of the MMC during

the impact of projectiles having higher speed are not fully

understood. On the other hand, wear mechanism of the

projectile surface at high sliding speeds has also great

importance in the development of armour materials. For this

reason, the determination of the wear mechanisms on the

MMC hole and projectile surfaces will be able to help better

understand the armour material performance used in

defence applications.

Particulate reinforced aluminium alloy composites are

currently being considered for a number of engineering

applications requiring improved strength, modulus and

wear resistance compared to their unreinforced matrix

counterparts. In terms of the tribological behaviour of Al

alloy MMCs their resistance to both dry and lubricated

sliding wear, abrasive wear and erosive wear has been

investigated [12].

Since the friction and wear behaviour are important in

failure mechanisms of MMCs, some research on the

properties of hole surfaces generated by high velocity

projectiles has been realized and even though novel

experimental techniques have been established for simu-

lation of friction characteristics of high-velocity projectiles

[13,14]. It can be easily estimated that the friction

characteristics between the high-velocity projectile and the

hole surface affect both the penetration depth and the

direction of projectile in MMC. Therefore, the friction and

wear behaviour of the hole surface should be examined.

The friction and wear behaviour of steel projectiles

at very high penetration velocities were investigated

experimentally and it was found that dominant wear

mechanisms are surface melting followed by subsequent

removal of a portion of the molten surface layer [15]. On

the other hand, the wear tests conducted with AISI 1020,

304 steels and pure titanium (75A) materials at relatively

lower speed (i.e. 0.5–10 m/s) show that friction coeffi-

cient of all material decreases with sliding speed due to

oxide formation [16].

The friction and wear characteristics of particle

reinforced MMCs were examined not only under laboratory

conditions with relatively lower speed to understand friction

characteristics and wear mechanisms [17–20] but also

through the pin-on-disc configuration under high speed [21,

22].

Although a lot of experimental work has been one to

understand wear and friction mechanisms at high velocity

[4,21–24], the experimental studies about projectile and

MMCs friction under real conditions have not been

adequate yet. There is a relationship between the ballistic

efficiency and volume fraction of reinforcement composites.

It was observed that the ballistic efficiency increased

steadily as the volume fraction for a silicon carbide

particle-reinforced system increased (DWA) [23].

Although abrasion, oxidation and adhesion as wear

mechanisms were observed, increasing particle size in the

matrix was more effective than the volume fraction on the

transition of wear mode. While the wear rate of Al–SiC

decreases with increasing fraction of SiC, the corresponding

effect on coefficient of friction is only marginal [12]. On the

other hand, it was observed that a transition from mild wear

to severe wear occurs when the applied normal load reaches

a certain critical value [19].

A plastically deformed layer surrounding the hole

generated by projectile occurs and is composed of a number

of distinct layers like the mechanically mixed layer [12].

When the compressing effect of projectile is considered, it

can be seen that there is also a strong correlation between

the characteristics of the mechanically mixed layer and

Table 1

The chemical compositions (%) of the matrix materials

Materials Si Fe Cu Mn Mg Cr Zn Ti

5083 0.4–0.7 0.4 0.1 0.4–1 4.0–4.9 0.05–0.25 0.25 0.15

6063 0.2–0.6 0.35 0.1 0.1 0.45–0.9 0.1 0.1 0.1

M.B. Karamis et al. / Composites: Part A 34 (2003) 217–226218

the wear behaviour of Al-MMCs [20]. High-speed impact

tests were conducted on MMC and the performance of the

material was observed. When the hole surface was

examined, a thin aluminium rich layer was detected,

resulting from possible local melting, on the surface

which was observed [25]. These studies, however, are not

enough to fully understand the tribological properties of

MMC during the friction with projectile impact.

The main purpose of the present work is to evaluate, from

tribological point of view, the properties of the hole and tip

surfaces after high speed projectile impact.

2. Experimental details

5083 and 6063 aluminium alloys are extensively used in

defence applications due to their favourable ballistic

properties, moderate strength, high corrosion resistance

and super plastic potential, so for this investigation these

materials were chosen as matrix reinforced with SiC

Fig. 2. (a) The micrograph showing that some particles have been pulled out

off their locations and been buried to hole surface (material

AA5083 þ 30%). (b) The micrograph showing that some of SiCp have

been shaked only and consequently gaps at their surroundings are

established by breaking down (material AA5083 þ 30%).

Fig. 1. (a) The projectile surface scratched by SiCp. (b) The hole surface

scratched by SiCp (material AA6063 þ 15%).

Fig. 3. The swelling and failure of projectile nose shot at 710 m/s speed.

M.B. Karamis et al. / Composites: Part A 34 (2003) 217–226 219

Fig. 4. Embedded particles cause the projectile to stop in the matrix (material AA6063 þ 15% SiCp).

Fig. 5. ED-X analyses at two different point of projectile surface after test (a) original surface; (b) after impact test.

M.B. Karamis et al. / Composites: Part A 34 (2003) 217–226220

particles. The chemical compositions of the materials are

given in Table 1.

Manufacturing of composites was conducted by squeeze

casting. SiC particles were incorporated into the matrix

material in 15, 30 and 45% volume fractions.

They were solidified under a pressure of 180 MPa in a

steel mold with a 650–700 8C temperature range. The SiC

particles were 250–500 mm in size. The manufactured

MMC specimens were disc shaped with a diameter of

140 mm and a thickness of 20 mm. Al-MMCs were used as

manufactured. The flashes on the samples generated during

the casting were removed by machining.

In this study, terminal ballistic tests with 7.62 mm

armour piercing projectile were performed on these

composites. The velocity of 7.62 mm projectiles was

710 m/s. Failure mechanisms and deformation behaviour

of composites and projectile were observed under impact

loading conditions. While some different failure mechan-

isms such as petalling, radial cracking, spalling, dishing, etc.

were determined on the composites, on the projectile tips,

swelling and starching, etc. were examined. In this study

friction and wear phenomena of the friction pair are

especially investigated. Also, the effective failure mechan-

isms are investigated. In fact, the main purpose of this study

is to shed light on high-speed tribological properties of

MMCs. In the existing literature the studies related to the

subject are very rare. The macroexaminations of the damage

zones were also conducted photographically. For micro-

structural analyses, the samples were cut with a diamond

saw in appropriate dimensions. SEM and ED-X studies were

also performed. To observe the fracture zones resulting

from the ballistic impact event, light and SE microscopy

studies were also carried out.

3. Results and discussion

It is well known that an important part of the energy input

to any mechanical system’s motion is consumed by friction.

Therefore, the frictional force for dynamic systems should

be as low as possible. In contrast, for some dynamic

systems, frictional forces are particularly required so that

the frictional parts can produce and sustain the performance

desired of themselves such as rolling, and perforating of

armour plates. The factors affecting penetration depth most

when using a high velocity projectile and armour are plastic

deformation of both projectile and armour plate, and

frictional characteristics of both materials. The frictional

characteristics of the armour material strongly depend on

the particle volume fraction and type. The strong friction

between the particles and the penetrator generally lowers

the penetration depth. It can be seen in Fig. 1(a) that the SiC

particles scratch the projectile surface by ploughing

mechanism. By this friction mechanism, the kinetic energy

of the projectile is dissipated and thus the penetration depth

is reduced. The main purpose of MMC usage in armour

applications is to prevent perforation. The prevention is also

provided by scratching the hole surface (Fig. 1b). When the

projectile enters the target, it meets with some particles and

transfer them to another zone on the surface by ploughing. If

the particles are brittle such as SiC, they are sometimes

shattered into smaller pieces and buried into the frictional

surfaces and this increases the frictional forces between

frictional pairs.

It is obvious that some particles have been pulled out

of their locations and buried either to hole or to its

surface by projectile (Fig. 2a). On the other hand, some

of these particles, close to the surface, have been shaken

only and consequently gaps in their surroundings have

been formed due to particle displacement or the breaking

up of the particles (Fig. 2b). It is obvious that the

projectile is plastically deformed when it impacts against

the armour surface. However, it follows its direction

itself in the MMC matrix. It can be seen in Fig. 3 that a

projectile nose, shot at 710 m/s a speed of, has lost the

Fig. 6. Some traces of varying size generated from particle friction on the

frictional surface (material AA5083 þ 15% SiCp). (a) Three-body

abrasion; (b) two-body abrasion.

M.B. Karamis et al. / Composites: Part A 34 (2003) 217–226 221

sharpness and swell after impacting and stopped by in

the matrix. It can be seen clearly that while the nose was

plastically deformed, some coarse and finer SiC particles

were transferred to the projectile surface by adhering to

the projectile. On the other hand, some of the large

particles become free by being broken from the worn

surfaces. These events cause dissipation of the kinetic

energy of projectile and decrease in the penetration

depth. There is evidence that friction behaviour of the

projectile in the MMC is an important factor for its

ballistic performance. It can be claimed that the stronger

the friction, the smaller the penetration depth. The

projectile is slowed down by particle friction initially

but stopped by others embedding to both surfaces.

Examples for these friction mechanisms can be seen in

Fig. 4a–c. The figures show that embedded particles

cause the projectile to stop in the matrix. If the projectile

has a jacket, frictional drag occurred simultaneously at

both zones between matrix/jacked, and between jacked/

projectile core. The hard particles bury into the jacket

surface during the stopping of projectile, and its core

continues to travel by itself with more difficulty. Its path

may be changed when it departs from the jacket.

Therefore, the core does not progress more in the matrix.

When two different areas on the projectile tip surface are

examined chemically by SEM. It is observed that some

matrix material is transferred to projectile nose surface

and thus this transfer changes the chemical composition

of the surface. While the original nose composition

consists of Fe and C, some elements appear on the

surface after impact (Fig. 5a). For example, aluminium,

magnesium and lead appear as an important value such

as 15.94, 1.41 and 71.72%, respectively (Fig. 5b).

The high lead percent appeared on the tip surface is

generated from the lead jacked of the projectile after

impacting. Some lead is stuck to the projectile surfaces

because of the high friction between projectile core and

jacked. When the jacket is kept by matrix and core

Fig. 7. The crater, land and deep grooves caused by plastic yielding, and SiC particles. (a) Crater and land after impact; (b) deep grooves caused by frictioning

of SiC particles; (c) the wear tracks occurred by reinforcing particles on the noise surface; (d) scratching and swelling edge of the deep grooves with SiC

particles.

M.B. Karamis et al. / Composites: Part A 34 (2003) 217–226222

Fig. 8. The aluminum matrix melted by excess heating resulted from high velocity friction (a) melted material solidified over the hole (material

AA5083 þ 30% SiCp); (b) melted material solidified over the particle surfaces (material AA5083 þ 45% SiCp); (c) the surface melting generated from the

excessive heating resulting from high friction; (d) the areas yielded melted and spread over the surface.

Fig. 9. The rapid solidification cracks observed on the hole surface (material AA5083 þ 30% SiCp).

M.B. Karamis et al. / Composites: Part A 34 (2003) 217–226 223

propagate into the matrix without jacket. It can be explained

that the friction mechanism is not only adhesive but also

predominantly abrasive. The contribution of these modes

varies depending on the particle volume fraction and the

homogeneity of the particle distribution in MMC.

Although different wear mechanisms are observed on the

hole and tip surfaces, the wear mechanism is predominantly

abrasive. Fig. 6 shows some traces in varying size generated

from particle scratching have been observed on frictional

surface. The shallower grooves can be seen in Fig. 6. The

material surface has been abraded. This type of mechanism

is commonly observed on the worn surface after two or three

body abrasive friction. Some of the particles are set free and

a three-body abrasion occurs with the effect of projectile

(Fig. 6a). On the other hand, other particles, which cannot be

pulled out of, are shaken only by projectile and they cause

two-body abrasion (Fig. 6b).

It can be easily supposed that a big crater can result from

impact leading to the plastic deformation of the nose

(Fig. 7a). Also many deep grooves can be seen on the

transition areas between the nose and the vertical surface of

the projectile (Fig. 7b). The projectile surface is scratched

by the SiC particles and the plastic deformation is also

occurred during the scratching while some wear debris is

generated by frictional effect. Fig. 7c also shows evidence

that there are many deep grooves caused by reinforcing

particles on the friction surface parallel to the propagation

direction. The strong friction leads to the deceleration of the

projectile in the armours.

Only 10% of the groove volume is wear debris, while

the other 90% is swelled by the occurrence of plastic yield

[26]. Some cracked SiC particles are also observed on

the grooves adhering loosely to the surface (Fig. 7d). It is

possible that the wear debris generated by the abrasion can

be melted over the surface. In addition to this, the

aluminium matrix is also melted by excess heating resulting

from high velocity friction and this melted material

solidified over the hole, particle and projectile surfaces

(Fig. 8a–c). It can be seen clearly that the yielded areas are

melted and spread over the surface and scratched again

(Fig. 8d).

In the melted region, the wear mechanism is changed into

melting wear mode. This mechanism can also be observed

in much slower velocities than 30 m/s in Ref. [21] involved

in the present work.

Fig. 10. The plastic shear localized at the subsurface (material AA5083 þ 30% SiCp). (a) The rapid compression as a result of the imposed strain concentration

at the projectile edge; (b) flowing matrix material around the SiC particles due to the softening because of excessive heating; (c) consolidated particles in the

highly deformed zone.

Fig. 11. The microhardness distribution of the deformed zone.

M.B. Karamis et al. / Composites: Part A 34 (2003) 217–226224

Some cracks are observed on the highly compressed

surface due to the rapid plastic deformation. The rapid

solidification also led to the occurrence of these cracks on

the surface (Fig. 9). Since the compression occurs suddenly

and it does not repeat, the cracks cannot propagate on the

surface. Fig. 10 illustrates the plastic shear localized at

the subsurface during the rapid compression as a result of

the imposed strain concentration at the projectile surface

(Fig. 10a). These locations can also be seen around the SiC

particles. During the subsequent plastic deformation, the

matrix near the surface is highly deformed and compacted.

Matrix material easily flows around the SiC particles owing

to the softening caused by the excessive heating (Fig. 10b).

At the same time, SiC particles are translated together by

this compression effect with the deformed matrix region.

These consolidated particles in the highly deformed zone

can be seen in Fig. 10c.

On the other hand, the severe and rapid plastic

deformation causes the hardening of the subsurface

deformed zone relative to the matrix. The microhardness

distribution of the zone can be seen in Fig. 11 as an example.

X and Y-directions marked on the photograph represent

hardness measurement and projectile entrance directions,

respectively. The nearest region to the hole surface has a

hardness of 33 HV0.04, while the average matrix hardness is

20 HV0.04 The slope of the hardness curve is gradually

reduced to hardness original matrix (i.e. 20 HV0.04)

4. Conclusions

The following conclusions could be drawn from the

present study:

1. The frictional behaviour of MMC armour is strongly

affected by the contact conditions between projectile and

the matrix of the composite. The more reinforcement

particles the projectile comes into contact, the greater is

the friction. This will reduce the penetration depth of the

projectile. If the particles are brittle, such as SiC, they are

sometimes broken up into smaller pieces and buried into

the frictional surfaces, thereby increasing the friction

between the projectile and the MMC armour. This further

reduces the penetration depth.

2. The projectile nose is plastically deformed when it

impacts the MMC armour. Because of swelling and the

presence of SiC particles in the matrix, the sliding of the

projectile through the armour hole surface is now more

difficult. This reduces the penetration depth.

3. The predominant wear mechanisms are abrasion and

melt wear. Three and two-body abrasion mechanisms are

also observed as a result of the frictional conditions

explained above. In addition to these mechanisms, the

aluminium matrix is also melted by excessive heating

generated from higher friction and then re-solidified over

the sliding (frictional) surfaces when cooled down.

4. The cracks caused by rapid plastic deformation and

solidification are also observed on hole surfaces. These

cracks do not propagate to the subsurface.

5. The matrix near the surface is highly deformed and

compacted, and a compacted zone around the projectile

is observed. The compacted zone has a different hardness

distribution from the matrix.

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