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The effect of laminate orientations on friction and wear mechanisms of glass reinforced polyester composite
WEAR ELSEVIER
Wear 195 (1996) 186191
The effect of laminate orientations on friction and wear mechanisms of glass reinforced polyester composite N.S.M. El-Tayeb a**, I.M. Mostafa b aMechanical b Production
Design Department,
Engineering
Faculty of Engineering
Department,
Faculty of Engineering
and Technology,
Helwan University, Mataria, Cairo, Egypt
and Technology,
Helwan University, Helwan. Cairo, Egypt
Received 18 April 1995; accepted 26 October 1995
Abstract Friction and wear properties of polyester composite reinforced with laminated glass fibres are experimentally examined in three different orientations, namely, cross-laminar CL, and inter-laminar (normal, NL and parallel, PL) The rubbing experiments of the composite specimens are carried out against abrasive paper (silicon carbide, P600) under various sliding speed and loading conditions. Experimental results show that PL orientation gives the highest value of friction coefficients followed by NL and CL. Microscopic investigations of the worn surfaces are conducted to identify the operating wear mechanism. They reveal that the weaving configuration in CL orientation inhibits an easy detachment of fibres during the wear process. Therefore, intermediate values of wear resistance between PL and NL orientation are obtained. Furthermore, entrapped soft components of matrix resin between laminates in CL orientation play an important role in reducing the friction coefficient. In NL orientation, the individual fibres within the laminates do not get mutual support as in PL orientation. As a result, fibres suffer bending at their ends, leading to an easy shear mechanism. Keywords:
Friction; Abrasive wear; Glass laminate; Orientation; Composite
1. Introduction Fibre reinforced composites as viable structural materials strength, stiffness and density
have progressively emerged due to their high specific properties. The use of such
materials in primary and secondary structural components renders their mechanical properties a primary importance in design considerations. These materials are used under certain working conditions which also dictate a complete knowledge of their tribological properties. Such conditions are mainly met in aerospace and automobile industries in which composites are increasingly used. Recently, there has been a considerable amount of research directed towards acquiring a comprehensive understanding of the tribological behaviour of these materials, [ l-81. Other researchers are interested in studying the effect of fibre orientation on these properties, [7-131. The results showed that such an effect is still controversial. Some recent work on friction and wear characteristics of plain weave fabric-reinforced phenolic resin composites, made with E-glass, carbon, and kevlar-49, is presented in [ 11. Experimental results with a glass-phenolic composite
* Corresponding author. 0043-16448/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDlOO43.1648(95)06849-X
showed poor wear resistance in comparison with the other two composites tested. This was attributed to the changes in roughness of the counterface surface due to the abrasive effect of entrapped loose glass particles. In this work, it was mentioned that in a woven configuration, the simultaneous existence of parallel and anti parallel oriented carbon fibres has a synergistic effect on the enhancement of wear resistance of the composite. The wear rate for a woven graphite fibrePEEK composite was an order of magnitude lower than that of the unidirectional composite. The effect of sliding conditions on the friction and wear properties of a unidirectional E-glass fibre reinforced epoxy composite was investigated in [ 21. In this investigation, the friction coefficient and wear rate were improved by an average factor ranging between 1.3 and 3.3 in friction and 2 in wear rate when debris was prevented from entering the sliding contact. Furthermore, introducing water into the sliding contact as a lubricant showed a reduction of about 65% in friction and 45% in the wear rate. An improvement in the tribological properties of unidirectional E-glass fibre composite was gained by adding mica particles to the composite [ 51. An earlier investigation on the friction and wear properties of graphite fibre composites [ 61 showed that the tribological properties are greatly influenced by the fibre orientations.
N.S.M. El-Tayeb, I.M. Mostafa/ Wear 195 (1996) 186-191
This was attributed mainly to the anisotropic properties caused by the micro structure of oriented graphite crystals in the carbon fibres and the macro structure of fibre orientation in the matrix. The effects of fibres orientation angle on the friction coefficient and wear of graphite, kevlar-49, and glass fibre composites were experimentally investigated in Ref. [7]. The attained results showed that the lowest values of wear rate and friction coefficient were obtained for fibres oriented normal to the sliding surface and the highest values were obtained when sliding axes were transverse to the fibre axes. Sliding along the longitudinal direction of the fibres yielded intermediate wear rates and friction coefficients. In [ 81, on graphite-fibre-reinforced glass another investigation matrix composites sliding against metal, it was shown that the lowest values of friction coefficient and wear rate were observed when the fibres were oriented parallel to the direction of sliding. The effect of fibre orientation on wear of composites has been claimed to be dependent on the fibre volume fraction of the composites [5,8,14]. For composites with low volume fractions (less than 20%) the longitudinal direction has a higher wear rate while for composites with volume fractions (greater than 40%) there is no effect of fibre orientation. On the basis of the aforementioned work, it seems that much of the research carried out on composite materials has focused on either randomly or unidirectionaly oriented fibre composites. Woven fabric reinforced composites, on the other hand, are gaining popularity in many industrial applications because of their balanced properties in the fabric plane as well as their ease of handling during fabrication. However, very little attention has been given to studying the frictional and wear properties of such composites. The effect of laminate orientations with respect to the sliding direction is a major micro structural parameter which should be considered when assessing the tribological properties. Therefore, the objective of the present work is to investigate experimentally the influence of glass laminate orientations on the sliding friction and wear behaviour of this composite.
187
CSM was cut to the required size which was 300 X 300 mm. A wooden mould with the preassigned dimension was prepared for producing the sample. The lay-up surface of the mould was cleaned thoroughly and coated with paraffin wax as a release agent. A consecutive placement of the CSM followed by resin brushing was carried on until the required thickness is obtained. A similar coating process was done on the upper surface of the laminate. The trapped air within the matrix was frequently removed using a hand-roller. The sample was removed from the mould after 1 h to allow a complete gel process of the matrix. 2.2. Testing procedure Experiments were performed on three principal glass laminate orientations of the GLRP composite with respect to the sliding direction, i.e. normal-laminate (NL),parallel-laminate (PL) , and cross-laminate (CL) as shown in Fig. 1. NL and PL are called inter-laminate (IL). Dry sliding wear was carried out at an ambient temperature using a pin-on-ring apparatus [ 21. Fig. 2 shows schematically the main features of the test apparatus. It includes a stationary pin holder directly loaded by dead weights. The composite specimen, in the form of a cylindrical pin (8 mm diameter and 20 mm length), was clamped in the pin holder to be rubbed against abrasive paper which was adhered to a rotating steel shaft of 60 mm diameter. This arrangement has the advantage that friction force can be continuously monitored during the tests.
(A)
(6)
(Cl
Fig. 1. Position of glass-laminate orientations with respect to the sliding direction: (A) normal-laminate (NL); (B) parallel-laminate (PL); (C) cross-laminate (CL) sliding direction.
2. Experimental
work
2.1. Preparation of test specimens The material tested in this study was a glass laminate reinforced polyester (GLRP) composite. The resin used was NEOXEL 886TA16, which is an aerospace-grade fire-resistant polyester. It is normally supplied pre-accelerated with cobalt naphthalate (0.5% by weight). The catalyst used in this particular resin was 1% by weight of methyl ethyl ketone (MEK) peroxide. A chopped-strand-mat (CSM) of 450 g m -’ was used here as a reinforcement material (35.5% by weight). The stacking sequence of the fibre in the fabric layers was 0” fibre/90” fibre. The test sample from which wear specimens were cut had a nominal thickness of 20 mm. The sample was laid-up of CSM and polyester resin. The
Fig. 2. Schematic diagram of the test set-up.
N.S.M. El-Tayeb, 144. Mostafa / Wear 195 (I 996) 186-191
188
The specimens were rubbed against the abrasive paper (P600) designated as SIA- 1727 SIAWAT-FC. The test apparatus was adjusted to run at different sliding speeds from 0.05 upto 1.2ms-’ and the normal load was varied from 4 N up to 22 N. The weight loss for each specimen was determined after 10 min of continuous running at the preassigned speed and load. After the running-in period, when the ring had rubbed its circular profile into the specimen, the weight loss was determined using three specimens, each tested in the same conditions. Each specimen was slid against fresh track on the abrasive paper, so the initial surface conditions were the same. Before and after each test, the mass losses of the specimens were measured using a Mettler balance (sensitivity 0.01 mg).
f : a: :.
2.40
QQPPD
w
0
1.60
e!w
Pardiel-Laminate Normal-Laminate Cross-Lnnllnate 136 m
at 12N
SlidingVelocity. (m/w)
Fig. 5. Wear resistance vs sliding velocity for different laminate orientations.
3. Results and discussion 3.1. Effect of laminate orientation
on wear behaviour - ’
A comparison of the effects of different orientations on friction coefficient and wear results, Figs. 3-6, is made here for the steady-state condition, which in terms of the wear life of the systems is considered as the dominant period. In the present work, the wear results, Fig. 3 and Fig. 5, are presented in terms of wear resistance which is the inverse of specific wear rate [ 1,4]. The specific wear rate is calculated according to: 8.80
0;
8.00
5
7.20
t_ 6.40 5
5.60
"0 4.60 ; 4.00 d 3.20 s ;; 2.40 '; 1.60 g
\,,i,,7=
,/,,,,,,I
0.60
0 0.00 3
0
10 15 Normal Load (N)
5
Fig. 3. Wear resistance
20
25
vs normal load for different laminate orientations.
1.00 ,
0.60
Fig. 4. Coefficient orientations.
$ 0
I
I,
I
,I
T
I
5 Ndrkl
of friction
(I
I,
15 Load (N)
vs normal
load
‘A
20
25
for different
laminate
0.60 {
uuuNormal-laminate *M Cross-laminate
0.56 0.00
0.20
Fig. 6. Coefficient orientations.
w,=” -x 1
0.40 0.60 0.60 Sliding Velocity(m/w)
of friction
vs sliding
velocity
1.00
1. 0
for different
laminate
IO9
AT F,VFP W, is the specific wear rate in mm3 N-’ m-l, AmlATis the mass loss during test time in kg s, V, is the sliding velocity in m SK’, p is the density of the specimen in kg rnt~~, and F, is the normal load in N. It can be seen that the wear resistance of the composite, Fig. 3 is clearly dependent on the laminates orientation with respect to the sliding direction. With varying normal load, PL orientation gives the highest wear resistance followed by CL and NL, respectively. This distinction is clear at the higher load levels, but less so at low loads because of scatter. This scatter may be explained in terms of the worn surface state of the GLRP composite. During the sliding process, most polymers wear by debris transfer to the counterface. With the incorporation of the reinforcing fibre, the transfer process includes fragments of these fibres. The surface traction exerted during ploughing by hard asperities or hard particles induces plastic shear deformation, which accumulates with repeated loading. This leads to matrix cracking and nucleation of delamination on the composite surface. Material is thus removed from the composite surface in the form of small flakes which are further rubbed in between the sliding surfaces [ 21. A typical feature is an accumulation of wear debris in the form of a patchwork layer in some regions of the worn surface, Fig. 7. This compacted wear debris plays an important part as a third body in the wear process [ 1,151. If a large amount of debris layer is deposited on the worn surface, this
N.S.M. El-Tayeb, I.M. Mostafa / Wear 195 (1996) 186-191
Fig. 7. Optical graph of worn surface showing patchwork
layer.
NL PL
Fig. 8. Weaving configuration show sliding direction).
of glass fibres within the laminate
(arrows
in turn results in a decrease in the mass loss of the bulk material. In addition, the change in the thickness and size of the debris layer can cause broad variations in the wear rate, especially at low loads. In the case of the PL orientation, Fig. la (which shows the highest wear resistance) the location of the individual lami-
189
nate with respect to the sliding surface of the counterpart controls wear processes. The counterpart meets every single glass fibre (which is the hardest and stiffest component in the GLRP composite) in a plane either normal or tangential to its axis, Fig. 8. The fibres stacking within the laminate in the PL orientation allow a mutual reinforcement amongst the neighbouring fibres. Such stacking reinforcement takes place along the entire rubbing surface. It also reduces bending of the fibre axis under the effect of the applied load and rotating motion. As a result, the main wear process takes place at the projection of the fibre cross section and to a lesser extent at the longitudinal section. This important part, which the PL orientation allows the individual fibres to play, reduces the contribution of the matrix (as the softer component in the GLRP composite) in the wear process. Therefore, the entire architecture of the composite components in the PL orientation gives the highest wear resistance. Fig. 9(A) shows a micrograph of a worn surface for a specimen with PL orientation subjected to a relatively high normal load ( 17 N), in which the two main mechanisms responsible for the wear of PL orientation are elucidated. An individual fibre worn along its longitudinal axis is shown as L, while fibre worn at the projection of its cross section is indicated by X. The same “architectural” argument of the fibres can be extended to discuss the wear mechanism in the other orientations. In the case of CL, the wear resistance is less than that in the case of PL orientation, Fig. 3. Here, the surface of the counterpart meets the fibres within the laminates at their circumferential surface in parallel and anti parallel directions as sketched in Fig. 8. Although the fibres’ cross-section projection is absent here as a major wear resistive part, yet the
Fig. 9. Microscopic graphs of the worn surfaces: (A) PL-orientation (17 N, 0.3 m s-l, orientation (12 N, 0.3 m s-‘, 500 s); (D) NL-orientation (22 N, 0.5 m s-‘, 500 s).
500 s); (B) CL-orientation
(17 N, 0.3 m s-‘,
500 s); (C) CL-
190
NAM.
El-Tayeb, I.M. Mostafa/Wear195(1996)
weaving configuration of the fibres within the laminate gives a relatively stiff fabric at the entire contact area. Such a weaving configuration partially inhibits an easy detachment of the fibre during the wear process. Fig. 9(B), (C) shows a micrograph of individual fibres partially worn along their circumferential surfaces (C) as well as a slight bending (B) , in the direction of wear, taking place in other fibres. In the case of the NL orientation, neither the high stiffness of the fibre stacking arrangement nor that of the weaving configuration is present. The main active component in the wear process here is still the projection of the cross section of the individual fibre. The absence of any stiffness effect is due to the direction in which the counterpart rotates against the laminate. In the present orientation, NL, the fibres do not get a similar mutual support as in the PL orientation. The fibres are backed by the matrix, which is the soft component. Therefore, they suffer a little bending at their ends followed by an easy shear effect. Consequently, lower wear resistance is provided, Fig. 3. The flattened ends of the fibres (F), Fig. 9(D), indicate the dominating wear mechanism. 3.2. Effect of laminate orientation
on friction coeficient
Friction coefficients reaches steady-state values at the start of the experiments, i.e. no “running-in” was observed. Typical results of friction coefficient are shown in Fig. 4 and Fig. 6. The figures show the variation of friction coefficient (average value at each test condition) with normal load and sliding velocity for the three laminate orientations of the GLRP composite. In general, friction coefficient increases with an increase in both normal load and sliding velocity and the heights value is obtained for the PL orientation followed by NL and CL. There are small differences in the friction values between Fig. 4, Fig. 6 which may possibly be considered within the experimental scatter. In the present composite, the influence of laminate orientation on the friction coefficient is not attributed to the orientation of the crystals, as in the case of graphite fibres [ 61, since the micro structure of the glass is essentially amorphous. The case here is mainly governed by the wear debris generated from rubbing between the mating surfaces. In the PL orientation the relative motion of the counterpart with respect to the laminates does not permit accumulation of wear debris on the cross section of the fibres. The relative motion drives the glass debris away from the laminated domain. Most of this component is swept away along the matrix domain, which suffers a higher level of wear rate and operates as a tunnel exit. The whole process allows exposure of fresh fibre to the counterpart leading to a higher friction coefficient. The same mechanism is also active in the case of the NL orientation. However, here the relative motion of the counterpart does not allow continuous drain of the relatively soft component observed in the PL orientation. It rather leads to a successive accumulation of such component on the front of the laminate domain. In addition, the same component is trapped between consecutive laminates. The pile up of this
186-191
component adjacent to the laminate domain leads to an overall soft rubbing process. As a consequence, a lower friction coefficient is obtained. In the CL orientation, the extent to which the soft component is entrapped between fibres is much higher. This, in turn, leads to lower friction coefficients than in the other two orientations. 3.3. ESfect of sliding velocity on friction and wear An increase in the sliding velocity generates an additional mechanism to those already acting during the wear process. With such an increase, a temperature rise in the sliding interface occurs. The increase in temperature causes thermal penetration as a result of softening of the matrix resin. Consequently, the rate of the increase in friction coefficients with sliding speed decreases as seen in Fig. 6. Regarding the effect of sliding velocity on the wear process, the interference of the data points in Fig. 5 is explored. Apparently, the softening of the matrix resin, due to heat generation, causes weakening of the bonds at the fibre-matrix interface. The reinforcing fibre thus may become loose in the matrix and break due to frictional thrust [I]. This in turn, causes a higher rate of looseness and breakage for the NL orientation followed by CL and then PL, Fig. 5. The difference in such resistance is not that high due to the presence of the aforementioned active mechanisms of the wear process.
4. Conclusions Based on friction and wear results of the glass laminate reinforced polyester (GLRP) composite examined, the following conclusions can be drawn: Friction and wear properties of a GLRP composite are greatly influenced by the laminate orientations with respect to the sliding direction. At all normal loads and sliding velocities studied, the friction coefficients obtained for the PL-orientation are higher than those of NL and CL orientations. This is due to the continuous exposure of fresh fibres to the counterpart in the case of the PL orientation. The higher wear resistance observed here is attributed to the insignificant contribution of the matrix resin (as a softer component). NL-orientation shows intermediate values of friction coefficient. This is due to a successive accumulation of the soft component leading to a soft rubbing process. The fibres suffer a little bending at their ends followed by an easy shear effect. Consequently, lower wear resistance is provided. CL-orientation exhibits the lowest values of friction coefficient. Herein, the soft components are entrapped between fibres within the laminates leading to lower friction coefficients. Furthermore, weaving configuration inhibits an easy detachment of the fibre during the wear process, providing an intermediate wear resistance.
N.S.M. El-Tayeb, I.M. Mosrafa/
4. The mechanisms responsible for wear resistance and friction coefficient in the different orientations considered are enhanced by increasing the normal load and sliding velocity.
References [l] B. Vishwanath, A.P. Verma and C.V.S. Kameswara Rao, Effect of reinforcement on friction and wear of fabric reinforced polymer composites, Wear, 167 (1993) 93. (21 N.S.M. EL-Tayeb and R. Gadelrab, Friction and wear properties of E-glass fiber reinforced epoxy composite under different sliding contact conditions, Wear, 192 (1996) 112-117. [3] V.K. Srivastava, J.P. Pathak and K. Tahzibi, Wear and friction characteristics of mica-filled fibre-reinforced epoxy resin composites, Wear, 152 (1992) 343. [4] Z. Lu and K. Friedrich, W. Pannhorst and J. Heinz, Wear and friction of a unidirectional carbon fiber-glass matrix composite against various counterparts, Wear, 162-164 (1993) 1103. [ 51 J.K. Lancaster, The effect of carbon-fibm reinforcement on the friction and wear of polymers, J. Appl. Phys., 1 (1968) 549.
Wear 195 (1996) 186191
191
[6] Hyun H. Shim, Oh K. Kown and Jae R. Youn, Effects of fibre orientation and humidity on friction and wear properties of graphite fibre composites, Wear, 157 (1992) 141. ,[7] N.H. Sung and N. Suh, Effect of fibre orientation on friction and wear of fibre-reinforced polymeric composites, Wear, 53 (1979) 129. [ 81 E. Minford and K. Prewo, Friction and wear of graphite fiber reinforced glass matrix composites, Wear, 102 (1985) 253. [9] S.A.R. Naga, Effect of glass fiber content on the frictional behaviour of glass fiber reinforced polyamide-66,8rh Int. Colloquium Tribology 2000, Technische Ackademie Esslingen, 14-16 January, 1992. [ 101 S.A.R. Naga, Wear behaviour of glass fiber reinforced polyamide-66, Proc. AME, 5th Conj, 5-7 May, 1992, p. 157. [ 111 T. Tsukizoc and N. Ohmac, Unidiictionaly oriented carbon hbre reinforced plastics, Tribal. Inr., 8 (4) (1975) 171. [ 121 B.S. Tripathy and M.J. Furey, Tribologicalbehaviuor of unidirectional Wear, 162-164 graphite-epoxy and carbon-PEEK composites, (1993) 385. [ 131 N. Saka, N.K. Szeto and Turgay, Friction and wear of fiber-reinforced metal-matrix composites, Wear, 157 (1992) 339. [ 141 H.W. Chang, Wear characteristics of composites: effect of fibre orientation, Wear, 8.5 (1983) 81. [ 151 M. Godet, The third-body approach; a mechanical view of wear, Wear, 100 (1984) 437.
Wear 195 (1996) 186191
The effect of laminate orientations on friction and wear mechanisms of glass reinforced polyester composite N.S.M. El-Tayeb a**, I.M. Mostafa b aMechanical b Production
Design Department,
Engineering
Faculty of Engineering
Department,
Faculty of Engineering
and Technology,
Helwan University, Mataria, Cairo, Egypt
and Technology,
Helwan University, Helwan. Cairo, Egypt
Received 18 April 1995; accepted 26 October 1995
Abstract Friction and wear properties of polyester composite reinforced with laminated glass fibres are experimentally examined in three different orientations, namely, cross-laminar CL, and inter-laminar (normal, NL and parallel, PL) The rubbing experiments of the composite specimens are carried out against abrasive paper (silicon carbide, P600) under various sliding speed and loading conditions. Experimental results show that PL orientation gives the highest value of friction coefficients followed by NL and CL. Microscopic investigations of the worn surfaces are conducted to identify the operating wear mechanism. They reveal that the weaving configuration in CL orientation inhibits an easy detachment of fibres during the wear process. Therefore, intermediate values of wear resistance between PL and NL orientation are obtained. Furthermore, entrapped soft components of matrix resin between laminates in CL orientation play an important role in reducing the friction coefficient. In NL orientation, the individual fibres within the laminates do not get mutual support as in PL orientation. As a result, fibres suffer bending at their ends, leading to an easy shear mechanism. Keywords:
Friction; Abrasive wear; Glass laminate; Orientation; Composite
1. Introduction Fibre reinforced composites as viable structural materials strength, stiffness and density
have progressively emerged due to their high specific properties. The use of such
materials in primary and secondary structural components renders their mechanical properties a primary importance in design considerations. These materials are used under certain working conditions which also dictate a complete knowledge of their tribological properties. Such conditions are mainly met in aerospace and automobile industries in which composites are increasingly used. Recently, there has been a considerable amount of research directed towards acquiring a comprehensive understanding of the tribological behaviour of these materials, [ l-81. Other researchers are interested in studying the effect of fibre orientation on these properties, [7-131. The results showed that such an effect is still controversial. Some recent work on friction and wear characteristics of plain weave fabric-reinforced phenolic resin composites, made with E-glass, carbon, and kevlar-49, is presented in [ 11. Experimental results with a glass-phenolic composite
* Corresponding author. 0043-16448/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDlOO43.1648(95)06849-X
showed poor wear resistance in comparison with the other two composites tested. This was attributed to the changes in roughness of the counterface surface due to the abrasive effect of entrapped loose glass particles. In this work, it was mentioned that in a woven configuration, the simultaneous existence of parallel and anti parallel oriented carbon fibres has a synergistic effect on the enhancement of wear resistance of the composite. The wear rate for a woven graphite fibrePEEK composite was an order of magnitude lower than that of the unidirectional composite. The effect of sliding conditions on the friction and wear properties of a unidirectional E-glass fibre reinforced epoxy composite was investigated in [ 21. In this investigation, the friction coefficient and wear rate were improved by an average factor ranging between 1.3 and 3.3 in friction and 2 in wear rate when debris was prevented from entering the sliding contact. Furthermore, introducing water into the sliding contact as a lubricant showed a reduction of about 65% in friction and 45% in the wear rate. An improvement in the tribological properties of unidirectional E-glass fibre composite was gained by adding mica particles to the composite [ 51. An earlier investigation on the friction and wear properties of graphite fibre composites [ 61 showed that the tribological properties are greatly influenced by the fibre orientations.
N.S.M. El-Tayeb, I.M. Mostafa/ Wear 195 (1996) 186-191
This was attributed mainly to the anisotropic properties caused by the micro structure of oriented graphite crystals in the carbon fibres and the macro structure of fibre orientation in the matrix. The effects of fibres orientation angle on the friction coefficient and wear of graphite, kevlar-49, and glass fibre composites were experimentally investigated in Ref. [7]. The attained results showed that the lowest values of wear rate and friction coefficient were obtained for fibres oriented normal to the sliding surface and the highest values were obtained when sliding axes were transverse to the fibre axes. Sliding along the longitudinal direction of the fibres yielded intermediate wear rates and friction coefficients. In [ 81, on graphite-fibre-reinforced glass another investigation matrix composites sliding against metal, it was shown that the lowest values of friction coefficient and wear rate were observed when the fibres were oriented parallel to the direction of sliding. The effect of fibre orientation on wear of composites has been claimed to be dependent on the fibre volume fraction of the composites [5,8,14]. For composites with low volume fractions (less than 20%) the longitudinal direction has a higher wear rate while for composites with volume fractions (greater than 40%) there is no effect of fibre orientation. On the basis of the aforementioned work, it seems that much of the research carried out on composite materials has focused on either randomly or unidirectionaly oriented fibre composites. Woven fabric reinforced composites, on the other hand, are gaining popularity in many industrial applications because of their balanced properties in the fabric plane as well as their ease of handling during fabrication. However, very little attention has been given to studying the frictional and wear properties of such composites. The effect of laminate orientations with respect to the sliding direction is a major micro structural parameter which should be considered when assessing the tribological properties. Therefore, the objective of the present work is to investigate experimentally the influence of glass laminate orientations on the sliding friction and wear behaviour of this composite.
187
CSM was cut to the required size which was 300 X 300 mm. A wooden mould with the preassigned dimension was prepared for producing the sample. The lay-up surface of the mould was cleaned thoroughly and coated with paraffin wax as a release agent. A consecutive placement of the CSM followed by resin brushing was carried on until the required thickness is obtained. A similar coating process was done on the upper surface of the laminate. The trapped air within the matrix was frequently removed using a hand-roller. The sample was removed from the mould after 1 h to allow a complete gel process of the matrix. 2.2. Testing procedure Experiments were performed on three principal glass laminate orientations of the GLRP composite with respect to the sliding direction, i.e. normal-laminate (NL),parallel-laminate (PL) , and cross-laminate (CL) as shown in Fig. 1. NL and PL are called inter-laminate (IL). Dry sliding wear was carried out at an ambient temperature using a pin-on-ring apparatus [ 21. Fig. 2 shows schematically the main features of the test apparatus. It includes a stationary pin holder directly loaded by dead weights. The composite specimen, in the form of a cylindrical pin (8 mm diameter and 20 mm length), was clamped in the pin holder to be rubbed against abrasive paper which was adhered to a rotating steel shaft of 60 mm diameter. This arrangement has the advantage that friction force can be continuously monitored during the tests.
(A)
(6)
(Cl
Fig. 1. Position of glass-laminate orientations with respect to the sliding direction: (A) normal-laminate (NL); (B) parallel-laminate (PL); (C) cross-laminate (CL) sliding direction.
2. Experimental
work
2.1. Preparation of test specimens The material tested in this study was a glass laminate reinforced polyester (GLRP) composite. The resin used was NEOXEL 886TA16, which is an aerospace-grade fire-resistant polyester. It is normally supplied pre-accelerated with cobalt naphthalate (0.5% by weight). The catalyst used in this particular resin was 1% by weight of methyl ethyl ketone (MEK) peroxide. A chopped-strand-mat (CSM) of 450 g m -’ was used here as a reinforcement material (35.5% by weight). The stacking sequence of the fibre in the fabric layers was 0” fibre/90” fibre. The test sample from which wear specimens were cut had a nominal thickness of 20 mm. The sample was laid-up of CSM and polyester resin. The
Fig. 2. Schematic diagram of the test set-up.
N.S.M. El-Tayeb, 144. Mostafa / Wear 195 (I 996) 186-191
188
The specimens were rubbed against the abrasive paper (P600) designated as SIA- 1727 SIAWAT-FC. The test apparatus was adjusted to run at different sliding speeds from 0.05 upto 1.2ms-’ and the normal load was varied from 4 N up to 22 N. The weight loss for each specimen was determined after 10 min of continuous running at the preassigned speed and load. After the running-in period, when the ring had rubbed its circular profile into the specimen, the weight loss was determined using three specimens, each tested in the same conditions. Each specimen was slid against fresh track on the abrasive paper, so the initial surface conditions were the same. Before and after each test, the mass losses of the specimens were measured using a Mettler balance (sensitivity 0.01 mg).
f : a: :.
2.40
QQPPD
w
0
1.60
e!w
Pardiel-Laminate Normal-Laminate Cross-Lnnllnate 136 m
at 12N
SlidingVelocity. (m/w)
Fig. 5. Wear resistance vs sliding velocity for different laminate orientations.
3. Results and discussion 3.1. Effect of laminate orientation
on wear behaviour - ’
A comparison of the effects of different orientations on friction coefficient and wear results, Figs. 3-6, is made here for the steady-state condition, which in terms of the wear life of the systems is considered as the dominant period. In the present work, the wear results, Fig. 3 and Fig. 5, are presented in terms of wear resistance which is the inverse of specific wear rate [ 1,4]. The specific wear rate is calculated according to: 8.80
0;
8.00
5
7.20
t_ 6.40 5
5.60
"0 4.60 ; 4.00 d 3.20 s ;; 2.40 '; 1.60 g
\,,i,,7=
,/,,,,,,I
0.60
0 0.00 3
0
10 15 Normal Load (N)
5
Fig. 3. Wear resistance
20
25
vs normal load for different laminate orientations.
1.00 ,
0.60
Fig. 4. Coefficient orientations.
$ 0
I
I,
I
,I
T
I
5 Ndrkl
of friction
(I
I,
15 Load (N)
vs normal
load
‘A
20
25
for different
laminate
0.60 {
uuuNormal-laminate *M Cross-laminate
0.56 0.00
0.20
Fig. 6. Coefficient orientations.
w,=” -x 1
0.40 0.60 0.60 Sliding Velocity(m/w)
of friction
vs sliding
velocity
1.00
1. 0
for different
laminate
IO9
AT F,VFP W, is the specific wear rate in mm3 N-’ m-l, AmlATis the mass loss during test time in kg s, V, is the sliding velocity in m SK’, p is the density of the specimen in kg rnt~~, and F, is the normal load in N. It can be seen that the wear resistance of the composite, Fig. 3 is clearly dependent on the laminates orientation with respect to the sliding direction. With varying normal load, PL orientation gives the highest wear resistance followed by CL and NL, respectively. This distinction is clear at the higher load levels, but less so at low loads because of scatter. This scatter may be explained in terms of the worn surface state of the GLRP composite. During the sliding process, most polymers wear by debris transfer to the counterface. With the incorporation of the reinforcing fibre, the transfer process includes fragments of these fibres. The surface traction exerted during ploughing by hard asperities or hard particles induces plastic shear deformation, which accumulates with repeated loading. This leads to matrix cracking and nucleation of delamination on the composite surface. Material is thus removed from the composite surface in the form of small flakes which are further rubbed in between the sliding surfaces [ 21. A typical feature is an accumulation of wear debris in the form of a patchwork layer in some regions of the worn surface, Fig. 7. This compacted wear debris plays an important part as a third body in the wear process [ 1,151. If a large amount of debris layer is deposited on the worn surface, this
N.S.M. El-Tayeb, I.M. Mostafa / Wear 195 (1996) 186-191
Fig. 7. Optical graph of worn surface showing patchwork
layer.
NL PL
Fig. 8. Weaving configuration show sliding direction).
of glass fibres within the laminate
(arrows
in turn results in a decrease in the mass loss of the bulk material. In addition, the change in the thickness and size of the debris layer can cause broad variations in the wear rate, especially at low loads. In the case of the PL orientation, Fig. la (which shows the highest wear resistance) the location of the individual lami-
189
nate with respect to the sliding surface of the counterpart controls wear processes. The counterpart meets every single glass fibre (which is the hardest and stiffest component in the GLRP composite) in a plane either normal or tangential to its axis, Fig. 8. The fibres stacking within the laminate in the PL orientation allow a mutual reinforcement amongst the neighbouring fibres. Such stacking reinforcement takes place along the entire rubbing surface. It also reduces bending of the fibre axis under the effect of the applied load and rotating motion. As a result, the main wear process takes place at the projection of the fibre cross section and to a lesser extent at the longitudinal section. This important part, which the PL orientation allows the individual fibres to play, reduces the contribution of the matrix (as the softer component in the GLRP composite) in the wear process. Therefore, the entire architecture of the composite components in the PL orientation gives the highest wear resistance. Fig. 9(A) shows a micrograph of a worn surface for a specimen with PL orientation subjected to a relatively high normal load ( 17 N), in which the two main mechanisms responsible for the wear of PL orientation are elucidated. An individual fibre worn along its longitudinal axis is shown as L, while fibre worn at the projection of its cross section is indicated by X. The same “architectural” argument of the fibres can be extended to discuss the wear mechanism in the other orientations. In the case of CL, the wear resistance is less than that in the case of PL orientation, Fig. 3. Here, the surface of the counterpart meets the fibres within the laminates at their circumferential surface in parallel and anti parallel directions as sketched in Fig. 8. Although the fibres’ cross-section projection is absent here as a major wear resistive part, yet the
Fig. 9. Microscopic graphs of the worn surfaces: (A) PL-orientation (17 N, 0.3 m s-l, orientation (12 N, 0.3 m s-‘, 500 s); (D) NL-orientation (22 N, 0.5 m s-‘, 500 s).
500 s); (B) CL-orientation
(17 N, 0.3 m s-‘,
500 s); (C) CL-
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NAM.
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weaving configuration of the fibres within the laminate gives a relatively stiff fabric at the entire contact area. Such a weaving configuration partially inhibits an easy detachment of the fibre during the wear process. Fig. 9(B), (C) shows a micrograph of individual fibres partially worn along their circumferential surfaces (C) as well as a slight bending (B) , in the direction of wear, taking place in other fibres. In the case of the NL orientation, neither the high stiffness of the fibre stacking arrangement nor that of the weaving configuration is present. The main active component in the wear process here is still the projection of the cross section of the individual fibre. The absence of any stiffness effect is due to the direction in which the counterpart rotates against the laminate. In the present orientation, NL, the fibres do not get a similar mutual support as in the PL orientation. The fibres are backed by the matrix, which is the soft component. Therefore, they suffer a little bending at their ends followed by an easy shear effect. Consequently, lower wear resistance is provided, Fig. 3. The flattened ends of the fibres (F), Fig. 9(D), indicate the dominating wear mechanism. 3.2. Effect of laminate orientation
on friction coeficient
Friction coefficients reaches steady-state values at the start of the experiments, i.e. no “running-in” was observed. Typical results of friction coefficient are shown in Fig. 4 and Fig. 6. The figures show the variation of friction coefficient (average value at each test condition) with normal load and sliding velocity for the three laminate orientations of the GLRP composite. In general, friction coefficient increases with an increase in both normal load and sliding velocity and the heights value is obtained for the PL orientation followed by NL and CL. There are small differences in the friction values between Fig. 4, Fig. 6 which may possibly be considered within the experimental scatter. In the present composite, the influence of laminate orientation on the friction coefficient is not attributed to the orientation of the crystals, as in the case of graphite fibres [ 61, since the micro structure of the glass is essentially amorphous. The case here is mainly governed by the wear debris generated from rubbing between the mating surfaces. In the PL orientation the relative motion of the counterpart with respect to the laminates does not permit accumulation of wear debris on the cross section of the fibres. The relative motion drives the glass debris away from the laminated domain. Most of this component is swept away along the matrix domain, which suffers a higher level of wear rate and operates as a tunnel exit. The whole process allows exposure of fresh fibre to the counterpart leading to a higher friction coefficient. The same mechanism is also active in the case of the NL orientation. However, here the relative motion of the counterpart does not allow continuous drain of the relatively soft component observed in the PL orientation. It rather leads to a successive accumulation of such component on the front of the laminate domain. In addition, the same component is trapped between consecutive laminates. The pile up of this
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component adjacent to the laminate domain leads to an overall soft rubbing process. As a consequence, a lower friction coefficient is obtained. In the CL orientation, the extent to which the soft component is entrapped between fibres is much higher. This, in turn, leads to lower friction coefficients than in the other two orientations. 3.3. ESfect of sliding velocity on friction and wear An increase in the sliding velocity generates an additional mechanism to those already acting during the wear process. With such an increase, a temperature rise in the sliding interface occurs. The increase in temperature causes thermal penetration as a result of softening of the matrix resin. Consequently, the rate of the increase in friction coefficients with sliding speed decreases as seen in Fig. 6. Regarding the effect of sliding velocity on the wear process, the interference of the data points in Fig. 5 is explored. Apparently, the softening of the matrix resin, due to heat generation, causes weakening of the bonds at the fibre-matrix interface. The reinforcing fibre thus may become loose in the matrix and break due to frictional thrust [I]. This in turn, causes a higher rate of looseness and breakage for the NL orientation followed by CL and then PL, Fig. 5. The difference in such resistance is not that high due to the presence of the aforementioned active mechanisms of the wear process.
4. Conclusions Based on friction and wear results of the glass laminate reinforced polyester (GLRP) composite examined, the following conclusions can be drawn: Friction and wear properties of a GLRP composite are greatly influenced by the laminate orientations with respect to the sliding direction. At all normal loads and sliding velocities studied, the friction coefficients obtained for the PL-orientation are higher than those of NL and CL orientations. This is due to the continuous exposure of fresh fibres to the counterpart in the case of the PL orientation. The higher wear resistance observed here is attributed to the insignificant contribution of the matrix resin (as a softer component). NL-orientation shows intermediate values of friction coefficient. This is due to a successive accumulation of the soft component leading to a soft rubbing process. The fibres suffer a little bending at their ends followed by an easy shear effect. Consequently, lower wear resistance is provided. CL-orientation exhibits the lowest values of friction coefficient. Herein, the soft components are entrapped between fibres within the laminates leading to lower friction coefficients. Furthermore, weaving configuration inhibits an easy detachment of the fibre during the wear process, providing an intermediate wear resistance.
N.S.M. El-Tayeb, I.M. Mosrafa/
4. The mechanisms responsible for wear resistance and friction coefficient in the different orientations considered are enhanced by increasing the normal load and sliding velocity.
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