COMPOSITESSCIENCE AND

Composites Science and Technology 64 (2004) 2031–2038

TECHNOLOGY

www.elsevier.com/locate/compscitech

Comparison of short carbon fibre surface treatments onepoxy composites

II. Enhancement of the wear resistance

Hui Zhang, Zhong Zhang *

Institute for Composite Materials, University of Kaiserslautern, Erwin Schr€odinger Str. 58, 67663 Kaiserslautern, Germany

Received 7 August 2003; accepted 22 February 2004

Available online 9 April 2004

Abstract

In this study, pitch-based short carbon fibres were treated by both an air oxidation and a cryogenic treatment, thereafter these

fibres were incorporated into epoxy matrix for wear investigations. It was found that under an appropriate air oxidative condition,

the wear resistance of carbon fibre reinforced epoxy was significantly improved at low contact pressure, whereas that was reduced at

high pressures which may due to the damage of fibre mechanical properties after oxidation. On the contrary, carbon fibres pre-

formed better at high sliding pressure after cryogenic treatment. Correlated wear mechanisms were discussed.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Wear resistance; Short carbon fibre; Fibre treatment; Air oxidation; Cryogenic treatment; Epoxy matrix

1. Introduction

Possessing of excellent mechanical and tribological

properties, carbon fibre reinforced plastics (CFRP) are

increasingly being used for many different applications.

More and more industrial components, e.g., bushings,

cams, gears, rollers, wheels, brakes, conveyors, andsliding shoes, etc., are manufactured from short/con-

tinuous carbon fibres reinforced composites, where

friction and wear are key parameters to be taken into

consideration. In the past years, many researchers have

focused on the tribological behaviours of CFRP in

various directions. Lancaster co-workers [1–5] found

that the wear performance of CFRP under lubricated

conditions was influenced by the types of carbon fibreand matrix materials, the counterface and its surface

roughness, the experimental temperature, and the for-

mation of transferred film as well. Voss and Friedrich

* Corresponding author. Tel.: +49-631-201-7213; fax: +49-631-201-

7196.

E-mail address: zhong.zhang@ivw.uni-kl.de (Z. Zhang).

0266-3538/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2004.02.010

[6] reported that, compared to glass fibres, short carbon

fibres show better potential to an enhanced wear re-

sistance for PEEK matrix at different pv-factors

(product of contact pressure and sliding velocity). The

effect on friction and wear of carbon fibre orientation

and surface temperatures of unidirectional fibre com-

posites were also investigated in detail by Tripathy andFurey [7]. Lu and Friedrich [8,9] studied systematically

the influence of carbon fibre volume fraction of their

composites on friction and wear, and declared that an

optimum range for short carbon fibre in PEEK matrix

is 15–25 vol% according to the improved specific wear

rate.

In the theoretical analysis, Voss and Friedrich [6] also

developed a semi-empirical formula for sliding wear ofshort fibre composites as following

WsC ¼ 1

ð1� VFÞW �1sM þ 0:5ð1þ V c

F ÞVFW �1sFs

þ 0:5ð1� V cF ÞVFWsFci; ð1Þ

where WsC is the total wear rate of composites, WsM is the

wear rate of the unfilled matrix, WsFs is the wear rate of

Table 1

Details of specimens and treatment conditions

Treatment

approach

Sample no. CF volume

content

Treatment

details

– CF_0 15% As-received

Oxidation (in air) CF_450 15% 450 �C/1 h

CF_500 15% 500 �C/1 h

CF_550_1 15% 550 �C/1 h

CF_550_2 15% 550 �C/2 h

CF_600 15% 600 �C/1 h

Cryogenic treatment CF_1 15% 1 min

(in liquid nitrogen) CF_5 15% 5 min

CF_10 15% 10 min

CF_20 15% 20 min

Block Specimen

Normal Force Dead Weight

Ring: Counterpart Steel 100Cr6 (DIN) Polished

Fig. 1. A schematic diagram of a block-on-ring test apparatus.

2032 H. Zhang, Z. Zhang / Composites Science and Technology 64 (2004) 2031–2038

the fibre, WsFci accounts for fibre cracking and sub-

sequent processes of fibre–matrix separation at the in-

terface, VF is the fibre volume fraction, and c determines

how fast the transition from one process to another

occurs once VF is altered. In this equation, the totalvolume wear rate, WsC, is composed by two items. The

first item reflects the contribution of fibre and matrix to

the wear rate, whereas the second item reflects the in-

fluence of fibre–matrix interface due to the fibre crack-

ing, the subsequent fibre–matrix debonding and

separation at the interface. According to Eq. (1), it is

clear that the enhancement of the fibre–matrix interfa-

cial bonding is expected to be beneficial to an improvedwear resistance of composites, which will be concen-

trated on the present paper.

It is well known that treatment and modification of

carbon fibre surface are, generally speaking, of help to

enhance the interfacial bonding between fibre and ma-

trix. In the first part of this paper [10], two different

kinds of surface treatments, i.e. air oxidation and

cryogenic treatment, were concentrated on short carbonfibres (CF), which effectively improved the mechanical

properties of CF/epoxy composites due to the en-

hancement on fibre–matrix interfacial bonding. In the

present study, the wear performance of untreated and

treated CF/EP composites under various nominal con-

tact pressures were compared using a block-on-ring test

apparatus. The worn surfaces were observed by SEM,

and wear mechanisms were discussed.

2. Experimental

2.1. Materials

The matrix used was a bisphenol-A type resin

(DER331, Dow) hardened by an amine curing agent(HY2954, Dow). Pitch-based short carbon fibres with-

out any treatments were supplied by Kureha Co.

(M-2007S). Details of fibre surface treatments by hot air

or liquid nitrogen, as well as the preparation procedures

of epoxy composites, were given in the first part of this

paper [10] (represented in Table 1 as well).

2.2. Wear test

Unlubricated sliding wear tests were carried out on a

block-on-ring apparatus designed and constructed at the

IVW (Fig. 1). A hardened and polished carbon steel ring

(German Standard 100Cr6) with a diameter of 60 mm

and initial surface roughness of Ra ¼ 0:1 lm served as

counterpart. The sliding velocity was kept constant at

1 m/s, while the contact pressure was adjusted in a rangebetween 0.5 and 5 MPa. The test duration was set to

20 h. All measurements were performed at room tem-

perature. The specimen�s weight loss after experiments,

Dm, was measured by an analytical balance with a pre-

cision of 0.01 mg. Finally, the specific wear rate Ws andthe time-related depth wear rate Wt were calculated ac-

cording to the following equations

Ws ¼DmqFNL

ðmm3=N mÞ; ð2Þ

Wt ¼ k� � pv ðm=sÞ; ð3Þwhere q is the density of the specimen, FN is the normal

load applied on the specimen during sliding, L is the

total sliding distance, p is the nominal contact pressure

on the specimen and v is the sliding velocity of the ring.

k� is often referred in literatures as the ‘‘wear factor’’,which is equal to Ws numerically. k� is a factor depended

on the properties of the two materials in contact and the

dominant wear mechanisms. In the case of moderate pv

product, k� should be constant, however k� value may

significantly increase at higher pv-factor, in which severe

wear occurs correlated to different wear mechanisms

[11]. At least six specimens of each composition were

measured, and an average value was reported with anerror scatter of the maximum absolute error.

2.3. SEM worn surface observation

After wear test, the worn surfaces were investigated

by both a scanning electron microscopy (SEM) and a

laser profilometry. The roughness of worn surface was

H. Zhang, Z. Zhang / Composites Science and Technology 64 (2004) 2031–2038 2033

calculated by software supplied by UBM Messtechnik

Company.

3. Results and discussion

3.1. Neat matrix and untreated-CF/epoxy at a pv-factor

of 1 MPa m/s

Measured by a block-on-ring apparatus after a test

duration of 20 h at a pv-factor of 1 MPa m/s, the specific

Fig. 3. Worn surfaces of untreated CF/EP composites (p ¼ 1 MPa, v ¼ 1 m/s)

fibre thinning; (b) fibre broken; and (c) fibre peeling off.

Fig. 2. Worn surface of (a) neat epoxy, and (b) untrea

wear rates of neat epoxy and untreated short-CF/EP

composites were 6.92� 10�6 and 1.48� 10�6 mm3/N m,

respectively. It is clear that the addition of 15 vol% of

untreated short-CF could significantly improve the wear

resistance of epoxy, which was also reported for otherpolymers [1,2,6,8,12].

In order to understand the corresponding wear

mechanisms, worn surfaces of epoxy and its composite

were observed by SEM (as show in Fig. 2). Regarding

to neat epoxy, it is clear that the fatigue wear is the

dominative wear mechanism which was caused by the

, which demonstrate the three sequential stages during sliding wear: (a)

ted CF/EP composites (p ¼ 1 MPa, v ¼ 1 m/s).

2034 H. Zhang, Z. Zhang / Composites Science and Technology 64 (2004) 2031–2038

repeated very high local stresses in the course of sliding.

Many fatigue wear particles can be recognized on the

worn surface of neat epoxy in Fig. 2(a). In this case,

wear is mainly determined by the crack initiation, crack

growth and facture, which are relatively poor of neatepoxy. Regarding to untreated CF/EP, the worn surface

given in Fig. 2(b) is smoother than that of the neat ep-

oxy. Even the major wear mechanism is still fatigue

1.160.47 0.61

6.92

1.030.751.48

0

2

4

6

8

10

Neat

CF_0

CF_450

CF_500

550_

1

550_

2

CF_600

Ws

[10-6

mm

3 /Nm

]

Fig. 4. Influence of air treatment conditions on the enhancement of

wear resistance of short carbon fibre reinforced epoxy composites

(p ¼ 1 MPa, v ¼ 1 m/s).

Fig. 5. Worn surfaces of air-treated CF/EP composites at different treatmen

wear, short carbon fibre thinning, broken, and peeling

off become dominant [12–14]. As shown in Fig. 3(a)–(c)

of the three sequential stages of fibre wear mentioned

above, it can be found that carbon fibres were exposed

on the sliding surface, supported mainly the sliding loadapplied, and protected matrix material to avoid being

ploughed and torn during wear process [12].

3.2. Oxidative-treated CF/EP at a pv-factor of 1 MPa m/s

Fig. 4 illustrates the effect of air treatment conditions

on the specific wear rate of CF/EP composites. Experi-

ments were performed at room temperature under acontact pressure of 1 MPa and a sliding velocity of 1 m/s.

All oxidative-treated samples more or less exhibited an

improved wear resistance, although the tendency was not

very clear. This also implied that the wear mechanisms

might be controlled by several factors. When the treat-

ment temperature was between 450 and 500 �C, the

variation of Ws was indistinct. Under a treatment con-

dition of 550 �C for 1 h (CF_550_1), a lowest Ws wasachieved, i.e., 0.47� 10�6 mm3/N m, which is less than

one-third of that of untreated ones. When continually

t conditions: (a,b) 550 �C/1 h, (c) 600 �C/1 h (p ¼ 1 MPa, v ¼ 1 m/s).

Table 2

Wear factors and roughness of worn surface of untreated and treated

CF/EP composites

No. k� (10�6 mm3/N m) Fitted scope

of pressure

(MPa)

Roughness of

worn surface at

1 MPa m/s (lm)

CF_0 3.16 0.5–5 0.80

CF_550_1 1.31 0.5–2 0.71

CF_20 2.25 0.5–5 0.87

30 CF_0

H. Zhang, Z. Zhang / Composites Science and Technology 64 (2004) 2031–2038 2035

increasing the treatment temperature or time, Ws, how-

ever, increased again.

Carbon fibre surface treatment is usually accompa-

nied with the change of the fibre�s mechanical properties

(cf. [10]), which may consequently affect the wear rate ofcomposites. Therefore, the wear rate of composites is

considered to be governed by two essential causes, i.e.,

fibre–matrix interfacial bonding and fibre mechanical

properties. In the case of oxidative treatment, the first

item in Eq. (1) can be considered as constant since a

constant VF was applied. For the second item in Eq. (1),

due to the enhanced fibre–matrix interfacial adhesion,

few fibres debonding occurred and the broken fibres weremaintained in the matrix. As a result, the value of the

second item of Eq. (1) was reduced, accordingly reduced

the wear rate of composites. On the worn surface

of oxidative-treated specimen (CF_550_1) given in

Fig. 5(a), less groove and fibre peeling-off can be de-

tected, comparing to that of untreated one in Fig. 2(b).

The high magnification micrograph of Fig. 5(b) illus-

trates that the oxidative treatment improved the fibre–

1.160.47 0.61

6.92

1.030.751.48

0

2

4

6

8

10

Neat

CF_0

CF_450

CF_500

CF_550

_1

CF_550

_2

CF_600

Ws

[10-6

mm

3 /Nm

]

Fig. 6. Influence of cryogenic treatment conditions on the enhancement

of wear resistance of short carbon fibre reinforced epoxy composites

(p ¼ 1 MPa, v ¼ 1 m/s).

Fig. 7. Worn surfaces of cryo-treated composites (a,b) (p ¼ 1 MPa, v ¼ 1 m

matrix interface significantly when compared to the un-

treated ones in Fig. 3(a)–(c) at the same sliding condition.

However, over oxidative etching was also unfavour-

able to improve the wear resistance of composites. Since

the fibres were probably damaged by an excessiveoxidation, fibre broken may easily occur, which will

/s). The treatment condition: immersed in liquid nitrogen for 20 min.

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00

5

10

15

20

25

Wt [

10-9 m

/s]

pv-Factors [MPa m/s]

CF_550_1 CF_20

Fig. 8. Influence of pv-factor on the depth wear rate, Wt, of untreated

and treated CF/EP composites. The treatment condition: for air-

treatment: 550 �C/1 h; for cryo-treatment: 20 min.

2036 H. Zhang, Z. Zhang / Composites Science and Technology 64 (2004) 2031–2038

contribute negatively to the wear resistance of compos-

ites. Under a treatment condition of 550 �C/1 h, the wornsurface (Fig. 5(a)) is relatively smooth with very small

debris. However, when the treatment temperature in-

creased to 600 �C for the same treatment time, largenumber and size debris can be observed, even with a piece

of broken fibre (marked by white arrow in Fig. 5(c)). It

can, therefore, be concluded that an appropriate oxida-

tive treatment could improve the wear resistance of CF/

EP composites due to the enhanced interfacial bonding,

however, over treatment should be avoided.

Fig. 9. Worn surfaces of (a,b) untreated, (c,d) air-treated (550 �C/1 h), and (e

contact pressure of sliding (p ¼ 3 MPa, v ¼ 1 m/s).

3.3. Cryo-treated CF/EP at a pv-factor of 1 MPa m/s

Fig. 6 illustrates the effect of cryogenic treatment on

the Ws of CF/EP composites under a pv-factor of 1 MPa

m/s. Although it was believed that the fibre–matrix in-terfacial bonding increased after treatment in our pre-

vious work [10], the Ws of all treated samples increased

lightly compared to that of untreated one, which indi-

cates that cryogenic treatment shows somehow negative

effect to an enhanced wear resistance, at least at the

present pv-factor condition.

,f) cryo-treated (20 min in liquid nitrogen) CF/EP composites at higher

H. Zhang, Z. Zhang / Composites Science and Technology 64 (2004) 2031–2038 2037

Fig. 7 demonstrates the worn surface of cryo-treated

CF/EP composite, in which short-CF were treated in

nitrogen for 20 min (CF_20). As shown in Fig. 7(a), the

worn surface of cryo-treated sample was much rougher

in comparing with untreated (Fig. 2(b)) and oxidative-treated composites (Fig. 5(a)). This observation was also

confirmed by the roughness of worn surface of these

three samples measured by laser profilometer (as given

in Table 2). A high magnification micrograph as shown

in Fig. 7(b) shows that cryo-treated carbon fibres were

not easy to be thinned, probably due to the fibre

strengthening effect [15]. Therefore, a poor wear resis-

tance of its composites was observed. Further investi-gations are still needed to fully understand the

mechanism behind.

3.4. At higher pv-factor

In order to understand the wear behaviour of un-

treated and treated carbon fibre composites at higher

pv-factor, the depth wear rate, Wt, as a function of pv-factors were plotted in Fig. 8, where v was kept as

constant of 1 m/s and only p was varied from 0.5 to

5 MPa. The oxidative-treated condition is 550 �C/1 h

(CF_550_1), and cryo-treated one is 20 min in liquid

nitrogen (CF_20), as optimised from the previous study.

A high pv value leads to high depth wear rates of

materials generally. In the range of pv values applied,

the Wt of both untreated and cryo-treated specimensperformed almost linear dependences according to

Eq. (3), but the situation was different for the oxidative-

treated one. Fitted wear factors, k�, were summarized in

Table 1, determined by the slope of each case. For ox-

idative-treated one, only a rang of 0–2 MPa m/s of

pv-factor was applicable. It is clear that the oxidative-

treated one is very sensitive to the contact pressure,

p. At lower p condition, the oxidative-treated fibrepreformed the lowest wear factor in all three cases.

However, once p was higher than 2 MPa, the depth

wear rate increased sharply. This result confirms again

that the oxidative treatment may damage the fibre in

some degree, which leads to more fibre broken and

peeling-off at high pressure. The cryogenic treatment

performed a satisfied improvement of wear resistance at

high pressure, which may due to the less damage andeven an increased strength of fibres after this treatment.

To conclude, following causes may contribute to an

improved wear resistance of composites at high pres-

sure: (i) improved strength of both composites and fi-

bres [10]; (ii) a reduction of interface debonding and

fibre peeling-off due to the improved interface strength;

(iii) carbon fibres can still be thinned at higher pressure

applied, which is of help to reduce the frictional coeffi-cient during sliding.

The worn surfaces of these three cases measured at a

pv-factor of 3 MPa m/s are given in Fig. 9. To compare

the low magnification micrographs of Fig. 9(a), (c) and

(e), it can be seen that there are many grooves in the

oxidative-treated case of Fig. 9(c), which indicates that

fibre peeling-off can easily take place at higher pressure.

However, for the cryo-treated one in Fig. 9(e), the wornsurface is relatively smooth. The high magnification

micrographs given in Fig. 9(b), (d) and (f) show more

details about these effects, which confirm the wear

mechanisms discussed above.

4. Conclusions

Based on this work devoted to studying the effect of

surface treatment on wear resistance of short carbon

fibre reinforced epoxy composites, the following con-

clusions can be drawn:

1. Wear resistance of polymer composites can be en-

hanced by an improved fibre–matrix interface of car-

bon fibre surface treatments.2. Optimised oxidative treatment may significantly im-

prove the wear resistance at low pv-factor, e.g.

1 MPa m/s. However, negative effect occurred at

higher pv values, e.g. P 3 MPa m/s, which may due

to the fibre damage after treatment.

3. Cryogenic treatment performed very low wear rate at

high sliding pressure, even they were not as good as

untreated one at lower pressure. The improved fibrestrength and interface strength may contribute to this

effect.

Acknowledgements

Z. Zhang is grateful to the Alexander von Hum-boldt Foundation for his Sofja Kovalevskaja Award,

financed by the German Federal Ministry of Educa-

tion and Research (BMBF) within the German Gov-

ernment’s ‘‘ZIP’’ program for investment in the

future. The authors appreciate Prof. Dr.-Ing. Dr.

h.c.K. Friedrich, IVW, for his valuable discussions

during the course of this work and the preparation of

this paper.

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