Dry Sliding Wear Characteristics of Glass Epoxy Silicon Carbide

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Dry sliding wear characteristics of glass–epoxy composite filled

with silicon carbide and graphite particles

S. Basavarajappa n, S. Ellangovan

Department of Studies in Mechanical Engineering, University B.D.T. College of Engineering, Davangere 577004, India

a r t i c l e i n f o

Article history:

Received 19 November 2011

Received in revised form30 July 2012

Accepted 2 August 2012Available online 10 August 2012

Keywords:

Sliding wear

Polymer matrix composites

Wear testing

Surface topography

a b s t r a c t

The dry sliding wear characteristics of a glass–epoxy (G–E) composite, filled with both silicon carbide

(SiCp) and graphite (Gr), were studied using a pin-on-disc test apparatus. The specific wear rate was

determined as a function of sliding velocity, applied load and sliding distance. The laminates werefabricated by the hand lay-up technique. The volume percentage of filler materials in the composite

was varied, silicon carbide was varied from 5 to 10% whereas graphite was kept constant at 5%. The

excellent wear resistance was obtained with glass–epoxy containing fillers. The transfer film formed on

the counter surface was confirmed to be effective in improving the wear characteristics of filled G–E

composites. The influence of applied load is more on specific wear rate compared to the other two wear

parameters. The worn surfaces of composites were examined with scanning electron microscopy (SEM)

to investigate the probable wear mechanisms. It was found that in the early stage of wear, the fillers

contribution is significant. The process of transfer film, debris formation and fiber breakage accounts for

the wear at much later stages.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Over the past decades, polymer matrix composites are made

and most widely used for structural applications in the aerospace,

automotive, and chemical industries, and in providing alterna-

tives to traditional metallic materials [1]. The features that make

composites so promising as industrial and engineering materials

are their high specific strength, high specific stiffness and oppor-

tunities to tailor material properties through the control of fiber

and matrix compositions. Composites are developed for superior

mechanical strength and this objective often conflicts with the

simultaneous achievement of superior wear resistance [2]. As a

result of this, these materials are found to be used in mechanical

components such as gears, cams, wheels, impellers, brakes,

clutches, conveyors, transmission belts, bushes and bearings. Inmost of these services the components are subjected to tribolo-

gical loading conditions, where the likelihood of wear failure

becomes greater. Of the large number of matrices available

commercially, only a small portion is in significant use for these

kinds of applications.

The use of fillers in the matrix, gives rise to many combina-

tions that provide increasing load withstanding capability,

reduced coefficient of friction, improved wear resistance and

improved thermal properties. In addition to this, fillers in poly-meric composite reduce the cost due to the less consumption of

matrix material. Fibers are the principal constituents in a fiber

reinforced composite materials. They occupy the largest volume

fraction and share the major portion of the load acting on a

composite [3]. In case of dry sliding it is effective in reducing the

wear rate, this reduction in wear is due to the load carrying

capacity of the fibers, their higher creep resistance and thermal

conductivity. But the higher load makes it more sensitive to fiber

breaking, pulverizing of the fibers and transfer [4]. Generally, the

wear behavior of polymer matrix composites is different from

that of conventional metallic materials. The material removal

from the polymer matrix composites in contact with a counter

surface is characterized by several mechanisms. The primary one

is adhesive wear, wherein fine particles of polymer gets removedfrom the surface, and also fiber–matrix debonding and fiber

breaking. On the other hand, the presence of either the fused

polymer or the grooves at the interface is interpreted to indicate

that the materials are wearing out by abrasion instead of

adhesion [5].

The question of why fibers and fillers usually improve the

wear resistance of a polymer matrix has been the subject of

intense study in recent years [6–9]. Zhang et al. [10] studied

dry sliding friction and wear behavior of PEEK and PEEK/SiC-

composite coatings and concluded that the influences of SiC fillers

in the composite effectively reduce the plough and the adhesion

between the two relative sliding parts. Chauhan et al. [11]

Contents lists available at SciVerse ScienceDirect

journal homepage: www.elsevier.com/locate/wear

Wear

0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.wear.2012.08.001

n Corresponding author. Tel./fax: þ 91 8192 2224567.

E-mail address: [email protected] (S. Basavarajappa).

Wear 296 (2012) 491–496

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reported a study on the effect of SiCp filled glass fiber–vinyl ester

composites on dry sliding wear and water lubricated conditions.

Pure vinyl ester material has higher specific wear rate because of

the lower mechanical properties and reinforcement of glass fiber

and SiC filler improves the wear resistance both under dry and

water lubricated conditions. Basavarajappa et al. [12] studied the

effect of graphite fillers in glass–epoxy composites under dry

sliding conditions. They reported that addition of graphite in

glass–epoxy composite leads to lower wear volume loss. This isdue to a thin coherent and uniform film that was transferred on

the disc and the interphase also contained lubricant particles,

thereby reducing the severity of the wear. Hyung Cho et al. [13]

investigated tribological properties of solid lubricants (graphite,

Sb2S3, MoS2) for automotive brake friction materials wherein it is

reported that the friction stability, fade resistance, anti-fade, and

wear of gray iron disks and friction materials were affected by the

relative amounts of solid lubricants in the friction materials.

Shyam [14] found that wear depends upon the cohesion of the

transfer film, adhesion of the transfer film to the counterface and

the protection of rubbing polymer surface from metal asperities

by transfer film.

Hashmi et al. [15] demonstrated graphite modified cotton fiber

reinforced polyester composites under sliding wear conditions.

They stated that significant reduction in the contact-surface-

temperature was observed on addition of graphite in cotton–

polyester composites. Bahadur and Polineni [16] investigated the

glass fabric-reinforced polyamide composites filled with CuO and

PTFE, and reported that 11.3 vol% glass fabric–25 vol% CuO–

10 vol% PTFE composite showed the lowest steady state wear

rate. It was 60–75% lower than the wear rate that could be

obtained by using glass fabric or CuO reinforcement alone or in

combination. Suresha et al. [17] described the role of SiC and Gr

on friction and slide wear characteristics in glass–epoxy compo-

sites by adding them separately. They stated that the influence of

these inorganic fillers has a significant role in reducing friction

and exhibited better wear resistance properties under dry sliding

conditions.

In view of above an attempt is made in this present investiga-

tion to combine the benefits of two inorganic fillers SiCp and Gr

into the G–E composite, to enhance the wear resistance. The dry

sliding wear behavior of G–E composites was characterized by

observing the scanning electron microscopy (SEM) images. This

approach was adopted to elucidate the mechanism of wear in the

composites.

2. Experimental details

2.1. Specimen details

The matrix material used was a medium viscosity epoxy resin

(LAPOX L-12) and a room temperature curing polyamine hardener(K-6). This matrix was chosen since it provides good adhesive

properties owing to the cross-linking chain between the resin

polymer and the hardener. Hence, the shrinkage after curing is

usually lower. The reinforcement material employed was bidirec-

tional perpendicular yarns of 7-mil E-glass fiber. SiCp (15 mm) and

Gr (15 mm) powders were selected as the filler materials on the

basis of their demonstrated ability to withstand high tempera-

tures, and to form transfer film during sliding and low thermal

expansion. The composites were prepared in the form of blocks

(250 mm 250 mm 3 mm) by the hand lay-up technique. The

fillers SiCp and Gr are mixed with known amount of epoxy resin.

The detail composition of the composite is given in Table 1. The

laminate was cured at room temperature for a period of about

24 h. The cured laminates are cut using a diamond tipped cutter

to yield wear test specimen of size 6 mm 6 mm 3 mm.

Dry sliding wear behavior tests were performed on 6 mm 6 mm

face.

2.2. Test details

A pin-on-disc wear test apparatus was used for the dry sliding

wear experiments (as per ASTM G-99 standard). The disc used

was an alloy steel with 165 mm diameter and 8 mm thick,

hardness of 62 HRc and with a surface roughness of 1.2 mm. The

test was conducted on a track of 130 mm diameter for a specified

test duration, applied load and sliding velocity. The surface of thespecimen was perpendicular to the contact surface. Prior to

testing, the specimen pin was rubbed over a 600-grade SiC paper

to ensure proper contact between the specimen surface and the

disc counter surface during sliding. The surfaces of both the

specimen and the disc were cleaned with a soft paper soaked in

acetone before the test. The initial and final weights of the

specimen were measured by using an electronic digital balance

with an accuracy of 0.0001 g. The difference between the initial

and final weights is the measure of weight loss. The weight loss

was then converted into wear volume using the measured density

data. The specific wear rate (W s) parameter provides a more

comprehensive measure of the wear loss characteristics of the

materials. The specific wear rate was calculated from

W s ¼ DV =Ld ðmm3=NmÞ ð1Þ

where DV is the volume loss in mm3, L is the applied load in

Newton and d is the sliding distance in meters. SEM observations

were carried out; the features of interest regions were recorded.

The specimen being non-conducting, it was sputter coated with a

layer of gold before SEM examination.

3. Results and discussion

When SiCp and graphite fillers are embedded in the G–E

composite, the wear trend is as shown in Figs. 1–3. The specific

wear rate of G–E composite has been found to be affected by the

sliding speed. This is true in both filled and unfilled G–E

composites. In both the cases a general trend has been foundfor the effect of sliding speed as show in Fig. 1.

The effect of sliding speed on wear of polymer matrix compo-

sites has been investigated quite extensively. The specific wear

rate increased as the sliding speed increased. Plowing by the wear

debris and the asperities on the counter surface is the major

activity on the surface. At high speed the interface temperature

increases because of the poor conductivity of the polymer

composite. The high temperature can give rise to a molten layer

at the interface and it can affect the fiber–matrix bonding on the

subsurface. It can also promote degradation wear and crack

propagation on the subsurface. So, the specific wear rate increases

exponentially at higher speeds [18]. The higher thermal conduc-

tivity of the fillers is one of the reasons why filled G–E composites

have superior wear resistance to that of unfilled G–E composite.

Table 1

Details of composites prepared.

Specimen

code

Matrix

volume (%)

Reinforcement

volume (%)

Fillers volume (%)

SiCp Gr

A 50 50 – –

B 40 50 5 5

C 35 50 10 5

S. Basavarajappa, S. Ellangovan / Wear 296 (2012) 491–496 492





a ploughing action on the surface of the composite. Thus the wear

debris was produced during the sliding process and further

decreased the wear resistance of the composite owing to an abrasive

wear effect.

Contrary to the above, the worn surface of filled G–E compo-

sites has nearly invisible peeling-off at the same sliding condi-

tions. This indicates that the fillers incorporated in the G–E

composite effectively act to enhance the bonding strength amongthe fibers and the matrix. Figs. 5–10 are micrographs of the worn

surfaces of the filled G–E composites in the order of increasing

load, showing that the fillers are protruded from the matrix. The

protrusion of fillers indicates that the fillers take up some portion

of the load during sliding and prevent severe adhesion between

the matrix and the counter surface (Fig. 5). Here the fibers are less

distinctly seen due to smearing by resinous material in which

glass fibers are arranged.

In the absence of severe adhesion the surface fracture is

significantly reduced. The matrix surface is covered with small

shallow and irregular patches of the thin dark film (marked ‘C’ in

Fig. 6) which are different from the markings on the unfilled G–E

composite. Here a few abrasion grooves (marked ‘D’ in Fig. 6) and

a number of ripple markings are exhibited. From Fig. 8, there are

Fig. 4. Worn surface of unfilled G–E composite at 60 N applied load.

Fig. 5. G–E composite filled with 5% SiCp–5% Gr at 20 N applied load.

C

D


E






some extent cracks, which are parallel and perpendicular to the

sliding direction (marked arrow). Further, these cracks become

more susceptible to propagation on to the surface, giving rise to

loosening of the fillers. Furthermore, the worn surface gives rise

to laminate type of debris (marked ‘F’ in Fig. 9) when applied loadis increased to 60 N. Hence it can be concluded that an increase in

the volume percentage of filler materials leads to less bonding

between the filler and matrix material. To produce a good

bonding and better wear resistance, an optimum level of filler is

required.

In the case of higher loads most of the matrix material has already

been removed and loosening of the fillers results in exposure of the

fibrous region to the sliding contact. In the regime under adhesive

forces, often transmitted through a film, the fiber ends undergo

severe thinning along their length (marked ‘E’ in Fig. 7); a similar

feature was observed by Hui Zhang et al. [22]. The thinning process

fractures the skin of the fibers and separates it from the surface,

whereas the rest remain embedded, still contributing to the wear

resistance of the composite for a certain time.

By comparing Fig. 10 with Fig. 7, it is possible to highlight the

effect of more SiCp particles in G–E composite. Here the epoxy

matrix and glass fibers are damaged more severely by crushing

and cutting action of abrasive particles (marked ‘G’). The worn

surface shows evidence of poor adhesion of matrix to the fibers as

several clean fibers appear on the worn surface. In fact, this

topography looks more likely as an abrasive wear case rather than

the adhesive wear. As a result the wear rate of the composites

increases slightly with increasing applied load but never reachesvery high levels.

The synergistic effect of fillers hinders the wear of G–E compo-

sites surface layer. Thus, a smoother worn surface and hence lower

wear rate was observed under dry lubrication as compared with

unfilled G–E composite. The better wear resistance exhibited by the

filled G–E composites depends on factors such as increasing

bonding strength, less voids and formation of transfer film by filler

materials.

5. Conclusions

The study of the wear behavior of filled and unfilled G–E

composites at various sliding velocity, applied load and slidingdistance reveals the following.

An increase in sliding velocity increased the specific wear rate.

Applied load has much more predominant effect, whereas sliding

distance has less effect. Inclusion of fillers in G–E composites leads

to better wear resistance; however higher the percentage of the

SiCp filler along with graphite higher the wear due to deteriorated

abrasive wear performance of the parent material and it is also seen

to depend on the amount and nature of the transfer film formed on

the steel counter surface. The wear mechanisms involved are well

indicated with SEM micrographs, which reveal multiple micro-

cracking, debris formation, fiber thinning, fiber breakage, fiber pull

outs, peeling of the matrix and fiber–matrix debonding.

References

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[4] J. Quintelier, P. De Baets, P. Samyn, D. Van Hemelrijck, SEM features of glass–polyester composite system subjected to dry sliding wear 261 (2006)703–714Wear 261 (2006) 703–714.

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Dry Sliding Wear Characteristics of Glass Epoxy Silicon Carbide