Indian Journal of Engineering & Materials Sciences Vol. 9, August 2002, pp. 299-306

Sliding wear behaviour of woven glass fibre reinforced polyester composites

Navin Chand & Somit Neogi Regional Research Laboratory, Hoshangabad Road, Bhopal 462 026, India

Received 19 Jun e 2001; accepted 25 February 2002

The dry sliding wear behaviour of woven glass fibre reinforced polyester composites has been studied by usi ng pin-on­disc machine. The friction and wear experiments have been conducted on three different orientation of glass fibre wi th respect to sliding di rection. The coefficient of friction and wear of the composites at various applied load and sliding speeds have been determined. The lowest coefficient of friction and wear values observed for the fibres oriented in 0°_90° direction and highest are for normal-longi tudinal (N-L) orientation. The applied load further increase the friction and wear of the composi tes for all orientations. The friction and wear behav iour have been dominated by a number of mechanisms. The wear of the fibres has been dominated by the fibre fracture. The fibres have been fractured because of bending of fibre due to dragging by the steel disc in the sliding direction . The microscopic observations of the worn surfaces revealed and supported the involved mechani sm.

Fibre reinforced polymer composites are widely used for many structural applications owing to their high specific modulus and strength. The friction and wear behaviour of fibre reinforced polymer composites have been the subject of intensive study for many years because of its practical importance. The literature reveals that the friction and wear of fibrous polymeric composites are governed by the type of fibre, their orientation, the matrix resin and the type of fillers used 1

•10 apart from various experimental

parameters. However, most of the studies on the friction and wear behaviour of polymeric composites have confined to unidirectional or randomly oriented composites. Lancasterll reported that in carbon fibre reinforced polyester composites, the minimum wear was observed longitudinal when fibres were oriented normal to the sliding direction. According to Tsukizoe and Ohmea12

, relative wear rates of longitudinal and transverse fibres depend on the type of carbon fibres .

Researchers are now shifting their interest towards the use of woven fabric composites because of their balanced properties in the fabric plane as well as the ease of fabrication 13

. Most of the researchers have chosen either fabric geometry or type of resin system for their, studies I 4

•19

• Vishwanath et al. 14 have used various forms of glass fibre fabric geometries (such as woven roving, plain weave and satin weave) as reinforcement in modified phenolic resin. They found that plain weave glass fabric showed lowest wear rate. In another report 15, they used different resin systems for the slid ing wear study. EI-Tayeb and Mostafal6

studied the effect of orientation of laminates on the friction and wear of glass fabric reinforced polyester composite and concluded that the laminates parall el to sliding direction had low coefficient of friction and wear as compared to the normal and perpendicular directions . El-Tayeb and Gadelrab l7 conducted their friction and wear tests in the normal direction of fibre orientation for different sliding surface conditions. They concluded that friction and wear improved when the counter-face was free from contamination or was wet.

The understanding of both the mechanical properties and tribological behaviour of glass fibre reinforced polymer composites is necessary for its applications. Therefore, in the present investigation, an experimental study on the friction and wear behaviour of glass woven polymer composites sliding under dry condition against stainless steel counter­face has been carried out. The matrix sys tem used is, unsaturated isophthalic polyester resin . Three different orientations of GFRP laminates, have been used to understand the friction and wear anisotropy with respect to the sliding direction under various conditions of sliding such as, sliding velocity and normal load.

Materials and Methods

Laminate Preparation The commercially available E-glass fibres in plain

weave woven fabric of 360 GSM manufactured by the Fibre Glass Pilkinton (FGP) India Ltd. was used fo r

300 INDIAN 1. ENG. MATER. SCI. . AUGUST 2002

laminate preparation in the present study. 2% Cobalt nap than ate as accelerator was mixed thoroughly in the commercially available unsaturated isophthalic polyester resi n and then 2% methyl ethyl ketone peroxide (MEKP) as hardener was mixed in the resin . The glass fabric was kept on the plate having releasing agent and gelcot on it. The resin system was then applied on the glass fabric by using brush. The rubber roller was then used to consolidate the fabric c loth and to remove any trapped air. Then the laminates were kept in the press at 120 psi for 2 h to squeeze out excess resin. The laminates were cured at room temperature under the pressed condition for 2 h. The cured laminates were post-cured in an air circulating oven at 120°C for 2 h. The details of the processing technique were discussed elsewhere20

. The bas ic constituents and their compositions along with some properties are g iven in Table 1.

Strength tests

The laminated GFRP composite samples were tested according to ASTM standards Nos . ASTM 0-790 for flexural strength, ASTM 0-638 for tensile strength and ASTM 0-256 for impact strength. The strength (tensile and flexural) test on the composite laminates were conducted in an Instron Universal Testing Machine. The impact strength test was conducted on an Impact Testing Machine. The values obtained from the above tests are shown in Table 1.

Sliding wear tests

Dry sliding wear characteristics of GFRP composites were determined by using a pin-on-disc friction and wear testing machine (type TR-20 LE,

Oucom India). The GFRP composite pins of dimension 8 mm diameter and 25 mm length held perpendicularly to the steel (EN-32) di sc, hardened to 65 HRC. Fig. 1 illustrates the shape of the composi te pin and the orientation of glass fibre in three di ffere nt sliding directions, e.g. , longitudinal-transversal or L-T (0°_90°), 45°-45°, and normal-Ion,gi tudinal (N- L) orientation of the glass fibres wi th respect to the sliding direction. The dimension of the steel di sc was 215 mm in diameter and 8 mm in thickness. The steel disc was rotated in the clockwise direction at different speeds. Load on the sample was applied by using dead load through loading lever and cable. The load selected was in the multiples of 0.5 kg. (5 N) and maximum load was restricted according to the

fa 25 f---'-l

l~ L- T 45-45 N'L

Fig. I-Schematic di agram of composite pin and the orientation of glass fibres with respect to sliding direction (all spec ifi cati ons are in mm)

Table I - Constituent, composition and some properties of composites used in the present study

Sample Resin Glass Fibre Density Tensile

Flexural Impact

fibre type wt% g/cm3 strength

strength MPa strength

name system MPa kJ/m2

GP- l Isophthalic

WR 67.2 2. 1 284.2 20 1.2 189.0 polyester

GP-2 Isophthalic

WR 50 1.86 254.8 159.0 174.0 polyester

Table 2-The details of the experimental parameters and their corresponding values used for the calculation of wear rate and specific wear rate/resistance of sliding wear tested composi tes

Applied load: 5, 10, 15, 20,25,30,35 N Time and corresponding sliding distance at different speeds are given below

Time RPM =300 RPM =600 RPM =900 (min) Velocity = 1.66 m /s Velocity = 2.5 m Is Velocity = 3.33 m /s

Sliding distance (m) Sliding distance (m) Sliding di stance (m)

3.0 300 450 600 6.0 600 900 1200

9.0 900 1350 1800

CHAND & NEOGI : WOVEN GLASS FIBRE REINFORCED POLYESTER COMPOSITES 30 1

response of the composite. The frictional force was measured by the means of a load cell (strain-gauge force transducer) and it was displayed digitally on the control panel in the terms of frictional force. The coefficient of friction was computed by dividing the frictional force by normal applied load . The wear volume of the composites was computed from the weight loss measurements taken after each test using an electronic balance of I x 10-4 g accuracy. Before weighing, the samples were kept in desiccator to avoid moisture absorption after the test.

The wear tests of composite pin were preformed on purely dry sliding condition against smooth steel counterface. Three tests runs were performed for each set of test and the average values of wear and friction were reported. The roughness of the counter-face was kept constant at the start of each test. A new set of pin samples was used for each set of tests. The sliding speed and the corresponding time along with the applied load was varied over a range during the tests (Table 2).

Scanning electron microscopy A scanning electron microscope (SEM) JEOL 35

was used to observe the worn surfaces of the composites. The samples were coated with a thin layer of gold using ion-sputtering technique prior to surface observation.

Results and Discussion

Analysis of wear data Fig. 2 compares the coefficient of friction of GP-l

and GP-2 composites with sliding time at applied load of 20 Nand 3.3 mls sliding speed. The coefficient of friction values of GP-I composite were on the higher

15,-----------------------------------,

GP· 1 GP·2

0 0· 90" . 0 · 90" 12

- 0 45-45" . 45-45" ,. -

~: .6.N · l c

.~ 09

~ : ; ~ ; : II : "0 ~ C ~ II

8 06 ~ ; ~ ~ : 0 -0

t ~ r ~ ! ~ 8 (J

03

10

Sliding Time, min

Fig. 2- Variation of coefficient of friction with sliding time for GP-\ and GP-2 composite at applied load of 20 N and sliding speed of 3.33 mls

side as compared to GP-2 composite. The N-L (GP-l composite) showed highest coefficient of frict ion , which gradually decreased and 0°_90° (GP-2 composite) showed lowest coefficient of friction . The 45°-45° orientation (GP-I and GP-2 composite) showed similar values and trend of coefficient of friction. Further, the N-L orientati on (GP-" I composite) showed highest scattering in the coefficient of friction, it might be because fibres were oriented in normal direction and their tips were in contact with the steel disc . During the sliding the tips' blunted and new surface came in the contact, wh ich tends to increases the coefficient of friction. On the other hand, the coefficient of friction reduced with the reduction in the scattering, when the fibres oriented in 0°_90°. In this case, fibre oriented either parallel or at 90° to sliding with lengthwise fibres in contact with the steel disc.

Fig. 3 shows the variation in coefficient of friction with applied load for GP-I and GP-2 composite at .. constant sliding speed of 3.3 mls. The coefficient of friction increased with applied load in all composites . GP-I composite having N-L orientation showed highest coefficient of friction in all loads and GP-2 composite (0°_90° orientation) showed the lowest. GP-I composite showed higher coefficient of friction as compared to their counterpart in GP-2 composite. This may be attributed to the fact that GP-I composite contains 67.2 wt. % of glass fibres. The nature of glass fibres was highly brittle and hard as compared to' unsaturated isophthalic polyester resin . These qualities restricts the deformation of glass fib res, rather it ultimately fractures leading to the formation of new surfaces. The lack of deformation and formation of new surface creates hindrance to the

1.5 ,---------------------------------------,

1.2

0.3

GP - 1 GP - 2

00-90' . 0_90'

0 45-45' .45-45'

~ N - L ~;/

~ 10 15 20 25 30

Applied Load, N

40

Fig. 3-Variation of coefficient of friction with applied load for GP-l and GP-2 composite at sliding speed of 3.33 mls and aLe'· ' min sliding time

302 INDIAN J. ENG. MATER. SCI., AUGUST 2002

motion of steel disc as reflected from the increased coefficient of friction .

Fig. 4 shows the wear volume of GP-l composite as a function of sliding distance at applied loads of 10 and 30 N. In general. the wear volume increased with sliding distance for all the glass fibres orientations. The N-L orientation showed highest wear volume followed by 45°-45° orientation, 0°-90° orientation showed the lowest, at applied load of 30 N. At an applied load of 10 N, the N-L orientation, showed highest value of wear volume, however 45°-45° and 0°_90° orientation showed almost similar values and trend in the increase in wear volume. Further, the N-L orientation shows a sudden increase in wear volume after 1200 m of slidtlflg distance. It may be attributed to the fact that glass fibre laminates are oriented perpendicularly to the sliding (steel disc) direction. By this time, the polymer matrix removed and the naked fibres were in contact with the steel counter-

GP-1

0-90' o10N . 30 N 45·45' o 10N . 30 N

"e N-L 0 10N _ 30 N e .,; § 4 "0 • > ~ ~ -3: •

Sliding Distance. m

Fig. 4-Variaton of wear volume with sliding distance for GP-l composite at applied loads of 10 and 30 N and sliding speed of 3.33 m/s

12,---------------------------------.---.

GP-2 10

0.90' o 10N . 30 N

45 ·45' o lON . 30 "

•

•

==6

400 800 1200 1600 2000

Sliding Distance, m

Fig. S-Variation of wear volume with sliding distance for GP-2 composite at applied loads of 10 and 30 N and sliding speed of 3.33 m/s

face. The perpendicularly held fibres were dragged by the rotating disc in the direction of sliding and applied load too supports the process of bending the glass' fibres. However, the brittleness of fibre restricted the bending, leading to the initiation of fibre fracture. The fractured fibre comes out as worn out particle increasing the wear volume. On the same time, the formation of new surface at the contact due to fracture increases the coefficient of friction of N-L orientation of fibres . In case of 45°-45° orientation of glass fibres , the glass laminates were placed fla t on the steel disc with length-wise fibres in contact with the disc. Once the matrix removed from the surface, the separated fibres were dragged by the steel disc. Because the :fibres were horizontal to the sliding direction, which require multiple fracture process to facilitate the wear. Hence, the wear loss reduced in 45° -45° orientation of GP-l composites . Similar, results of increase in wear volume were revealed for

12 t

GP-1 GP-2 ) 10 00-90' . 0 -90'

0 45 - 45' .45 - 45' "e 8

o N - L e /

.,; E

6 :> "0 > ~ • " 4 3:

400 BOO 1200 1600 21)(

Sliding Distance.

Fig. 6--Variation of wear volume with sliding di stance fo r GP-l and GP-2 composite at applied load of 20 N and sliding speed of 3.33 rnIs

12,---------------------------- --------,

J/ GP -1 GP - 2

10 00-90' •. 0 -90'

045 - 45 ' .. 45 - 45 '

"~8 o N _ L

iii § 6 "0 > :. ~ 4

10

~ 15 20 25 30 35 40

Appied Load. N

Fig. 7-Variation of wear volume with applied load for GP-l and GP-2 composite after sliding distance of 1800 m

.. .

•

"lit '

..

..., ....

CHAND & NEOGI : WOVEN GLASS FIBRE REINFORCED POLYESTER COMPOSITES 303

GP-2 composites, when tested at applied loads of 10 and 30 N as shown in Fig. 5.

Fig. 6 shows the variation in wear volume with sliding distance for GP-I and GP-2 composites at an applied load of 30 N. GP-2 composite showed higher wear volume as compared its counterpart in GP-l composite. This trend of wear volume was just opposite to the trend observed for the coefficient of friction of the same composites. Further, the wear volume increased suddenly after 1200 m of sliding distance. This increase was quite high in case of GP-2 composite having 45°-45° orientation as compared to 0° -90° fibre orientation. In case of GP-l composite, N-L orientation showed highest wear volume. It showed similar values and trend of increase in wear volume as observed for GP-2 composite (45°-45° fibre orientation). In GP-l composite, the 45°_45° fibre orientations showed higher wear volume as compared to 0°-90° fibre orientation. However, the similar trend of increase in wear volume was observed for both the cases.

Fig. 7 shows the variation of wear volume with applied load after sliding distance of 1800 m for GP-l and GP-2 composites. The interesting feature observed was that GP-2 composite showed the maximum limit of applied load (30 N), whereas GP-l composite showed higher applied load limit up to 35 N. The similar trend of increase in wear volume for both the orientation of GP-2 composite were observed. In the same manner, the increasing trend in wear volume of GP-I composite showed similar behaviour. However, the trend of increase in wear volume of GP-l is quite different from GP-2

composite. The sudden increase in wear volume of GP-2 composite may attribute to the fact that it contains 50 wI. % of glass fibres. Once the polymer matrix removed, the separated fibres were exposed to the steel disc. These glass fibres (oriented in 90° or 45°) were dragged by the steel disc in the direction of motion, but the brittleness of glass fibre prevents them from bending. At this movement, the process of initiation and propagation of crack takes place from the point of weakness. This process initiated the micro-fracturing in the glass fibre leading to fractured· and worn out particles of glass fibres.

The friction cofficient as a function of applied load and sliding speed with orientation of glass fibres ·is given in Table 3.

Worn surface analysis The SEM photomicrographs of worn surface of

composites were taken after sliding distance of 1800 m (Figs 8-10). Fig. 8 shows the photomicrograph for orientation of glass fibre at 0°_90° direction with the sliding counter-face. Fig. 8a shows the consequences of sliding wear at maximum applied load of 35 N. The photomicrograph shows the fibre oriented in directi.on of sliding counter-face. Photomicrograph reveals the matrix removal resulting in clear fibres due to poor interfacial bonding or may be due to the fibre-matrix de-bonding process that had taken place during sliding wear. Once separated from the matrix, glass fibre undergoes the process of wear dominated by the fracture2

) . The photomicrograph shows evidences of fractured fibres, fibre thinning, fibre wear and above all fractured fibres forming debris. In Fig. 8b, the fibres are oriented at 90° to the sliding and tested at

Table 3 - The fri ction coefficient as a function of applied load and sliding speed with orientation of glass fibres

Sample & Orientation Speed (m/s)

GP-I • 0°_90° 1.66 0°_90° 2.5 0°_90° 3.33

GP- I @ 45°-45° 1.66 45°_45° 2.5 45°-45° 3.33

GP-I e N-L 1.66 :: ~ .=~; N-L 2.5 . ~:..-:. ... , N-L 3.33

GP-2 0°_90° 1.66 0°_90° 2.5 0°_90° 3.33

GP-2 45°-45° 1.66 45°-45° 2.5 45°-45° 3.33

Applied load, N

10 15 20

0.52 0.56 0.57 0.55 0.58 0.60 0.56 0.59 0.62 0.57 0.65 0.68 0.63 0.66 0.70 0.66 0.69 0.74 0.66 0.75 0.78 0.67 0.74 0.82 0.69 0.76 0.82 0.40 0.39 0.39 0.38 0.41 0.44 0.43 0.44 0.50 0.50 0.57 0.58 0.51 0.57 0.62 0.54 0.58 0.63

25 30

0.62 0.66 0.70 0.76 0.74 0.82 0.75 0.80 0.78 0.83 0.80 0.87 0.83 0.89 0.88 0.91 0.91 0.98 0.58 0.62 0.62 0.64 0.65 0.66 0.63 0.64 0.68 0.69 0.72 0.75

35

0.71 1.0

1.09 0.95 1.1

1.14

0.64 0.69 0.74

304 INDIAN J. ENG. MATER. SCI., AUGUST 2002

.. ""'

Fi g. 8-SEM photomicrograph of worn surface of GP-I composite 0°_90° orientatiDns of glass fibres: (a) at 20 N applied load and fibres oriented 0" to sliding direction, (b) at 30 N applied load and fibres oriented <lit 90° to sliding direction and (c) fibres fracture at 30 N when fibre oriented at 90° to sliding direction

applied load of 25 1\1. The photomicrograph clearly shows the removal of polymer matrix from the surface and separated fibres. The separated fibres then removed from the surface by a number of process as mentioned earlier. Fig. 8c shows the magnified view

Fig. 9-SEM pbotomicrograph of worn surface of GP-I composite 45°-45° orientations of glass fibres :(a) at 20 N applied load and fibres oriented 0° to sliding direction, (b) at 30 N applied load and fibres oriented at 90° to sliding direction and (c)inclusion of foreign particle / wear debries at the operating surface

of the fractured fibre oriented at 90° to sliding counter-face. Further concentrating on the alignment of the fibre it is found that the separated and fractured fibre bend in the direction of sliding counter-face as compared to the intact fibre. The unique feature of

CHAND & NEOGI : WOVEN GLASS FIBRE REINFORCED POLYESTER COMPOSITES 305

Fig. 10000SEM photomjcrograph of worn surface of GP-2 composite at fibre cross-overs and an applied load of 20 N: (a) glass fibres oriented at 00 _900 to the sliding direction and (b) glass fibres oriented at 45 0 _45 0 to the sliding direction

fibre bending towards the sliding direction shows clear evidence of fibre dragging by the counter-face, leading to the fibre fracture and its removal. This micro-structural evidence shows good agreement with the mechanism discussed.

Fig. 9 shows the SEM photomicrograph for the worn surface of GP-I composite, when the fibres oriented at 45°-45° to the sliding counter-face. Fig. 9a shows clear evidence of fibre fracture in the direction of sliding. The significant feature of the photomicrograph is the fibre fracture in the localised region. The propagation of the fractured region of fibre in the direction of sliding may be due to increased surface roughness of the steel disc resulted from continuous sliding at applied load of 25 N. Other micro-structural features are also observed, which have been discussed earlier. Fig. 9b shows the photomicrograph of worn surface at an applied load of 30 N. At high applied load the surface roughness of the steel disc increased and the dominating wear

mechanism is more like as an abrasive wear process controlled by the roughness of the disc. The micro­structural features also indicate towards the cutting action rather than the fracture in glass fibres , similar features were reported by Chand and Fahim l6 during the two-body abrasive wear of GFRP composites. Fig. 9c shows the presence of foreign particle or the debris . of the fractured glass fibres between the operating surface. The particle in the process has fractured the glass fibre. On close examination of worn surface in particular, it is found that the fracture is caused by particle. Further, the orientation of fractured glass fibre has been shifted or oriented in the direction of sliding, it is believed that due to the force exerted by the particle and sliding counter-face. .

Fig. 10 shows the SEM photomicrograph of worn surface of GP - 2 composites for fibres oriented in 0°_ 90° and 45°-45° directions respectively to the sliding direction at an applied load of 30 N. Fig. lOa shows, the consequence of sliding wear at the region of fibre crossover. The significant feature observed from the. photomicrograph is the removal of polymer matrix, separation of fibres and its ultimate removal22

.

Another feature is the propagation of crack through the fibre in sliding direction . Similar, features were found for the 45°-45° orientation of glass fibres .

Conclusions The coefficient of friction increases with applied

load for all orientations of fibres and composites. The friction was higher when the fibres were oriented in N-L direction to the sliding and was lower when oriented in 0°-90° direction. The higher weight fraction of fibre increases the coefficient of friction. The wear volume of the composite increases with sliding distance and applied load. The highest wear volume for the normally oriented fibre verifies the results obtained for the friction coefficient.

The sliding wear behaviour of the composites was dominated by brittle fractures in both matrix and fibres. The bending of glass fibres due to dragging by steel disc in the sliding direction leads to fibre fracture. This process of fibre fracture was mainly responsible for the formation of debris. In case of 45°_. 45° orientation, the bending of brittle fibres was responsible for the increase in the wear volume as compared to 0°_90° orientation. The wear process dominated by bending was severe for . the N-L orientation. Further, it was found that the 0° oriented fibre shows maximum wear resistance and the normally oriented fibre shows minimum resistance.

306 INDI AN J. ENG. MATER. SCI. , AUGUST 2002

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145 (1991) 3 15. 15 Vishwanath B, Verma A P, Rao C V S K & Gupta R K,

Composiles, 24 (1993) 347. 16 EI-Tayeb N S M & Mostafa 1M, Wear, I 95 ( 1996) 186. 17 EI-Tayeb N S M & Gadelrab R M, Wear, 192 ( 1996) 112. 18 Ramesh R, Kishore & Rao R M V G K, Wear, 89 (1983) 13 1. 19 Vishwanath 8 , Verma A P & Rao C V S K. Wear, ( 1989) 197. 20 RRUPWUB HEL, "/llIem al Report all FRP materials for

Traclioll MaIOI' Gear Case," India ( 1991 ). 21 Navin Chand, Naik Ajay & Neogi Somit, Wear, 242 (2000) .

22 Navin Chand & Somit Neogi, Tribal Lell, 4 ( 1998) 8 1.