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Influence of weave of carbon fabric on low amplitude oscillating wear performance of Polyetherimide composites
Wear 262 (2007) 727–735
Influence of weave of carbon fabric on low amplitude oscillating wear performance of Polyetherimide composites Rekha Rattan, J. Bijwe ∗ , M. Fahim Industrial Tribology Machine Dynamics and Maintenance Engineering Centre (ITMMEC), Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Received 11 March 2006; received in revised form 24 July 2006; accepted 3 August 2006 Available online 20 September 2006
Abstract Three composites were fabricated based on Polyetherimide (PEI) matrix and carbon fabric (CF) (55 vol.%) of different weaves, viz. plain, twill and satin (4H) by impregnation technique. These composites were evaluated for various mechanical properties and tribological performance in low amplitude oscillating wear (LAOW)/fretting wear mode. It was observed that CF reinforcement led to a significant enhancement in all strength and modulus properties of PEI except elongation to break (e). Twill weave proved to be the most effective followed by satin and plain weave in almost all the properties. The LAOW mode evaluation under various loads revealed that the wear performance order was exactly opposite to the above trend. Overall, plain weave composite was the best performer followed by twill and satin. Various wear mechanisms, such as fiber-matrix debonding due to repetitive reciprocating shearing stresses, micro-cracking, micro-cutting and pulverization of fibers followed by removal of debris from the contact zone were operative during such wear situation. Amongst these processes, generation of fiber debris, their temporary retention in fabric weaves and subsequent removal from the crater played a key role in overall wear performance. The ability of plain weave to hold back the debris was maximum that resulted in lowest wear while satin weave had the minimum retention ability that led to the highest wear. The proposed wear mechanisms were supported by SEM studies. © 2006 Published by Elsevier B.V. Keywords: Low amplitude oscillating wear; Fretting wear; Carbon fabric reinforced composites; Fabric-weave tribology
1. Introduction Wear of components under oscillatory relative motion (either linear or torsional) of small amplitude of displacement is termed as low amplitude oscillating wear (LAOW) or fretting wear. Such oscillatory motion can be intentional or unintentional. Rolling bearings used for small oscillatory movement, bearing’s outer ring and housing, wire ropes, electrical switchgears, multilayer leaf springs, palliatives, spline couplings, flanges, seals, riveted and pinned joints, etc. may suffer from such type of wear damage [1–4]. Reinforced polymers and composites are frequently used to make such components [5–7]. In spite of this fact, not much is available on fretting wear behavior of these materials compared to vast literature available for metals and alloys [2]. Among reinforced polymers, carbon fiber reinforced polymers
∗
Corresponding author. Tel.: +91 11 26591280; fax: +91 11 26591280. E-mail addresses: [email protected], [email protected] (J. Bijwe).
0043-1648/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.wear.2006.08.005
(CFRP) have immense potential in numerous applications in which fretting wear situations may arise. CFRP ropes, for example, if used in the place of steel rope for the construction of light weight suspension bridge, low amplitude oscillatory wear would occur between the retaining parts and the CFRP ropes [7]. CFRP components are used in aeronautical and space applications in large scale [8] and are prone to similar damages in some situations, which can be fatal and lead to accidents. It was a technical snag that had forced the pilot for an emergency water landing to avoid an accident. Later, the inspection of aircraft parts revealed that fitting beams had undergone fretting wear and cracking that were responsible for the mishap [9]. Among fabric reinforced or bi-directional (BD) composites, as they are popularly called, CFRPs are being increasingly used in aircraft industry as structural material because of very high specific strength and modulus. However, hardly any literature is available on the fretting wear behavior of such BD composites [10,11]. In our earlier work, significant benefits endowed by inclusion of carbon fabric (69 vol.%) in PEI matrix were reported
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R. Rattan et al. / Wear 262 (2007) 727–735 Table 1b Properties of three weaves of carbon fabric measured in the laboratory Carbon fabric (g/cm3 )
Fig. 1. Schematic showing different weave patterns (i) plain (one weft over one warp), (ii) twill (two warp over two weft) and (iii) satin (4H) (one warp over three weft).
[10]. In a subsequent work, a series of CF reinforced BD composites containing various amounts of fabric was developed and evaluated to investigate optimum amount of CF in PEI for best combination of tribo-performance and strength [11]. Though weave of fabric is very important in controlling strength properties of the composites, no paper is reported on the influence of weave of CF in any matrix in LAOW mode. Interestingly, a little is reported in case of glass fabric reinforced PEI composites in abrasive wear mode [12], and in thermoset composites in adhesive wear mode [13]. Hence, it was proposed to investigate influence of three weaves of CF in PEI on strength properties and LAOW performance. The results are presented in subsequent sections. 2. Experimental 2.1. Fabrication of composites GE plastics USA supplied the PEI material (ULTEM 1000) as granules. The carbon fabric used as reinforcement was procured from Fiber Glast Ltd. USA. Carbon fabrics of three different weaves, viz. plain weave (P), twill weave (T) and satin weave-4 harness (S) as shown in Fig. 1 were selected. The properties of these fabrics were studied in the laboratory and are compiled in Table 1. The three composites were developed keeping the fabric concentration constant (55 vol.% or 65 wt.%) using impregnation technique (I). The plies (280 mm × 260 mm) were cut from the carbon fabric roll and the open strands from all the four sides were sealed with a PTFE coated glass fabric tape to avoid the fiber misalignment. These plies were immersed individually in separate containers filled with viscous solution of PEI (prepared with Dichloromethane) for 12 h. The containers were properly sealed to avoid evaporation of solvent, which was required for adequate wetting of fiber strands with the PEI solution. The plies
Density Area (g/cm2 ) Towa Tex Denier Crimp (%) Count Warp (in.) Weft (in.) Thickness (cm) Bending length (cm) Tensile strength (kg/cm2 ) Elongation (%) a
Plain
Twill
Satin (4H)
1.85 196 3K 20 185 0.64 28 12 12 0.34 7.2 3 1.25
1.85 198 3K 22 198 0.70 26 16 16 0.34 5.9 1.47 1.85
1.85 193 3K 19 171 0.30 31 14 14 0.36 5.0 1.2 1.52
Supplier’s data.
were taken out carefully to avoid the misalignment in weave and dried in oven for 2 h at 100 ◦ C in a stretched condition. These 20 prepregs were used to attain the desired thickness in the range of 3–3.5 mm and were stacked in the mould carefully to avoid misalignment. PTFE coated glass fabric was placed on the top and bottom of the stacked prepregs. During compression molding, the mould was heated to attain the temperature in the range of 385–390 ◦ C within 2 h. The prepregs were then compression molded at this temperature at an applied pressure of 7.35 MPa. During the total compression time of 20 min at high temperature four intermittent breathings (each of 2 s) were applied to expel any residual solvent. The composites were allowed to cool under ambient conditions and applied pressure 7.35 MPa. The test specimens were cut from the composites with the help of diamond cutter as per required standards for mechanical and tribological testing. 2.2. Characterization of the composites The plain, twill and satin weave reinforced composites were designated as IP55 , IT55 and IS55 , respectively. The three composites fabricated for the present study were characterized for physical properties and composition using Soxhlett apparatus and Dichloromethane as a solvent. The extraction temperature and time were 40 ◦ C and 36 h, respectively. Composites were also evaluated for various mechanical properties as per ASTM standards. The mechanical properties of these composites are shown in Table 2. (Properties of PEI were provided by the supplier). 2.3. Oscillating wear studies
Table 1a Characteristic properties of various weaves of fabric [16] Property
Plain
Twill
Satin
Good stability Good drape Low porosity Smoothness Balance Symmetrical Low crimp
Good Poor Acceptable Poor Good Excellent Poor
Acceptable Good Good Acceptable Good Acceptable Acceptable
Poor Excellent Excellent Excellent Poor Very poor Excellent
The studies were done on SRV Optimol Tester [10], in which a chromium steel ball of diameter 10 mm was oscillated against a polymer composite plate (10 mm × 10 mm × 3–4 mm). The direction of fabric and fibers with respect to oscillation is shown in Fig. 2. The operating parameters were as follows. Load: 100, 150, 200, 250 and 300 N Stroke length (full oscillation width)—1 mm;
R. Rattan et al. / Wear 262 (2007) 727–735 Table 2 Details of the composition and properties of fabricated composites Composites Density (g/cm3 ) ASTM D 792 Contents of fabric (%, v/w) Tensile strength (MPa) ASTM 638 Tensile modulus (GPa) ASTM 638 Elongation at break (%) ASTM 638 Toughness (MPa) ASTM 638 Flexural strength (MPa) ASTM 790 Flexural modulus (MPa) ASTM 790 Inter laminar shear strength (ILSS) (MPa) ASTM 2344 *
PEI*
The specific wear rate was calculated using the equation:
IP55
IT55
IS55
1.55
1.53
1.54
55/65 535
55/65 888
55/65 575
03
73
106
76
60
0.54
0.08
0.32
– 150
3.8 589
2.2 951
2.8 832
3.3
40
54
46
–
49
66
63
1.27 – 105
729
Properties of PEI as per supplier’s data.
K0 =
V 2AνtFN
where Ko is the specific wear rate in m3 /N m, V the wear volume (m3 ) and FN the applied normal load (N). 2Aνt (where A is the full oscillation width (m), ν the frequency (Hz) and t is the experimental duration in (s)) indicates total sliding distance. 3. Results and discussion As seen from Table 2, the strength performance of the composites was in the order; IT55 > IS55 > IP55 ≫ PEI for all properties except toughness and elongation, where the order was reverse. The friction and wear performance of the selected composites under various loads is shown in Fig. 3. Possible wear mechanisms are shown in Fig. 4 and SEM micrographs of worn pin surfaces are shown in Figs. 5–7. Fig. 3a shows the variation of friction coefficient (μ) with increasing loads for all the three composites. The μ decreased with increasing loads and was in the range of 0.3–0.4. For
Fig. 2. Schematic showing ball on plate (composite) configuration.
Test duration—2 h; Oscillating frequency—50 Hz, Temperature—25 ◦ C. The load was increased on the ball fretting against a composite plate after each experiment in the step of 50 N till limiting load1 reached.
1 In the case of LAOW all selected parameters including amplitude are set before the experiment starts. Once the oscillation starts, generally amplitude falls below the set value and it is reset with the control panel. However, when load is very large and shearing forces are high, amplitude cannot be maintained even after applying maximum range provided in the control unit. This shows that the limiting load has reached. Sometimes amplitude reduces because of trapping of large particles of metal or polymer and falls to a low value for a short time. It regains once the particle either is thrown off or gets “wear thinned”. However, if the value falls below 80% of original value for a long time even after applying full possible force through control unit, it is realized that the material has reached the “failure limit” and cannot withstand the selected load. The experiment is abandoned and lower load is selected for next experiment.
Fig. 3. (a) Coefficient of friction and (b) specific wear rate (Ko ) as a function of load.
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Fig. 4. (a) Cross-section view of unit cell of (a) plain, (b) twill and (c) satin weave fabric; L1, L2 and L3 denote distance between mid-point of contact and the point where the crimp starts. (b) Schematic of the events of most probable sequence during fretting wear of plain weave fabric reinforced composite. A—Situation prior to fretting: (a) top view of weave and direction of fretting, (b) cross-section of unit cell and (c) contact configuration. B—Just initiation of fretting: (a) wear debris generation due to fiber damage, (b) larger sized fiber/matrix debris in contact zone and (c) sub-surface damage. C—Fretting continued: (a) initiation of travel of debris towards pockets, (b) retention of debris in pockets and (c) thick layer of entrapped fibrous and polymeric debris leading to third body interphase. D—Fretting continued: (a) wear thinned fibrous debris accumulation in pockets, (b and c) shearing of debris. E—Fretting in advanced stage: (a) pocket saturated with fibrous debris offering more wear resistance by further wear thinning, (b and c) accumulation beneath the crimp leading to saturation; excess debris escape the contact zone leading to measurable wear.
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Fig. 5. Scanning electron micrographs (×15) of IP55 worn under (a) 100 N and (b) 250 N; and IT55 worn under (c) 100 N and (d) 250 N loads, respectively.
almost all loads plain weave composite showed the lowest μ followed by twill and satin composites. μ of PEI, however, was always high 0.41 and was independent of load as reported in our earlier paper [10]. Thus, inclusion of CF reduced μ by approximately 25%, which was due to the lubricating action of CF. A lower μ at higher loads was attributed to the lubricity provided by the fine CF powder formed due to crushing and grinding of fiber pieces that accumulated at the contact zone. The specific wear rate (Ko ) plotted as a function of load in Fig. 3b indicates that Ko increased with increase in load for all the composites, which is a general trend observed in the case of fiber-reinforced composites [3,10,11]. The limiting load (and hence range of utility of the composite in harsh operating conditions) of plain and satin weave composites was 300 N while that of twill weave was slightly lower (250 N). IP55 was the best performer from tribological point of view, but poorest from strength point of view. Composite IS55 showed second best wear performance under only moderate loads (100–150 N). At higher loads, however it showed highest wear rate. Its friction behavior was also poorest in all conditions. Twill weave composite IT55 on the other hand was second best in both wear as well as friction performance under higher loads. Ko was least dependent on load as compared to other composites. Overall order of friction and wear performance of composites was: IP55 > IT55 > IS55 . The strength and modulus of the composites, however, did not follow the same order. The wear performance of the composites was compared with that of Neat PEI (data taken from Ref. [10]) in the form of relative wear
resistance which is calculated as per following equation RWR =
Wear resistance of composite Wear resistance of PEI
Since limiting load for PEI was 200 N, the data for comparison was selected accordingly. As seen from Table 3, a significant increase in wear performance due to CF inclusion can be seen. Approximately 8.4 times increase in performance was showed by IP55 . Overall, the improvement was in the range 4–8.4 for various composites evaluated under different loads. It is a well known fact that wear behavior in fretting wear mode is very different than that in sliding wear mode. It is the reciprocating motion in the former, which makes the major difference in wear mechanisms in two modes. Wear processes in such dry frictional contacts are considered as a succession of individual events involving particle detachment from the contacting bodies i.e., debris formation, its trapping and shearing in the contact zone and subsequent escape from the same. Among
Table 3 Relative wear resistance of composites at various loads (RWR = Ko of composite/Ko of PEI) Material
PEI IP55 IT55 IS55
Relative wear resistance (RWR) 50 N
150 N
200 N
1 6.55 4.08 7.03
1 8.38 6.00 6.55
1 7.28 6.08 5.17
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Fig. 6. SEM (×500) of composites worn under 100 N (WR -IS55 ∼ = IP55 IT55 ); (a) long fibers, longitudinally wear thinned and embedded in the matrix, (b) backtransfer of matrix, (c) magnified view (×1500) of fiber damage with micro-cracking, (d) shows worn surface of IP55 and back-transfer of matrix in the form of patches and (e) shows the worn surface of the composite IT55 and debris embedded in the matrix.
these, the escape of wear debris is minimal if the slip amplitude is very small. In the case of engineering polymers, the effect of contact zone kinematics on the wear is significant when the third body formation leads to abrasion or the transfer of polymer films to the counterface as well. The wear process is also affected by the formation of ‘loose’ or ‘tight’ compacts from the debris and its subsequent rheological behavior [3]. Thus, the wear behavior of materials is quite different from that in other wear modes and cannot be predicted a priori. There are a number of cases reported where inclusion of solid lubricants or fibers or both did not render the expected benefits. The performance on the other hand deteriorated [14,15]. The wear resistance (WR ) (inverse of wear rate, Ko ), was highest for plain weave composite followed by twill and satin. This was not related to any strength properties of these composites, rather it was mainly because of the difference in wear
mechanisms. The wear process involves various mechanisms such as fiber-matrix debonding followed by fiber breakage as a result of micro-cracking, micro-cutting and micro-pulverization due to reciprocating shearing stresses. The wear also depends on the amount of fiber debris produced and its escape from the contact zone. Amongst all weaves, plain weave is the most tight (least flexible) followed by twill and satin. This would result maximum fiber damage to the plain weave composite. However, the same weave is also responsible for the retention of fiber debris in the contact zone. Thus, fretting wear of fabricreinforced composites is a complex phenomenon that comprises multiple mechanisms which are operative successively and then simultaneously. Fig. 4a shows the unit cells of three weaves. It highlights the variation in distance between mid-point of contact and the point of starting of crimp. This distance has critical impact on wear mechanisms. Higher the distance,
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Fig. 7. SEM of IP55 worn under 250 N showing (a) weave of fabric (marked as 1), (b) magnified view (×1000) fiber debris embedded in the matrix, (c) retention of fiber debris in the weave pockets marked as 1 and fiber-matrix debonding marked as 2, (d) magnified view (×1000) for IT55 showing cavities (marked as 1), fiber piece escaped from cavity (marked as 2), fiber-matrix debonding (marked as 3) and (e) worn surface of (×500) IS55 showing long fibers and cavities.
lower is the probability of fiber debris reaching to crimp and getting entrapped. If entrapped, they will cause negative wear. The distance being largest in satin weave, the probability of entrapment was lowest. This was one of the reasons for highest wear of satin weave composite. The same distance was lowest for plain weave composite and hence offered highest possibility of retention of debris beneath the crimp leading to lowest wear. Fig. 4b is self-explanatory and depicts fretting mechanisms of plain weave composite. Various events that take place sequentially are collected in Table 4. The impact (positive or negative) on weight loss of a composite is also indicated in the same table. The mechanisms suggested in Table 4 and Fig. 4b are supported by SEM studies as discussed in the subsequent section. In the case of satin weave composite, similar mechanisms (Fig. 4a) take place except for the difference in the degree of
extent. The satin weave is the least compact (Table 1a) amongst three followed by twill and plain. Hence, the tendency of retention of debris beneath the crimp and pockets is minimum for satin and maximum for plain weave composite, a fact that supports the highest and lowest wear rates for the former and latter. Thus, the initial fiber breakage process though is maximum in plain weave composite, the further damage processes are minimal and retention processes are maximum. Satin weave being loose, on other hand, fiber breakage is minimal because of highest length of fiber, and hence maximum flexibility, between crossover points. Thus, plain weave composite showed highest WR , despite maximum fiber pulverization, mainly because of its highest tendency of retention of wear debris in the pockets and beneath the crimp point. The maximum tightness of the plain weave and maximum crimp points (Table 1a) were responsible for retention of debris leading to lowest wear of this composite.
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Table 4 Possible wear mechanisms and extent of their contribution towards weight loss/wear rate
1 2
3 4 5 6 7 8 9
Parameters and events during fretting of a ball on a composite plate (WR , wear resistance)
Effect on wear rate
Wear thinning of fibers parallel to the direction of slip; higher the better for WR . Longer the fibers more the wear thinning Breakage of fibers due to micro-cracking and micro-cutting. Lesser, the better for WR Generation of wear debris. Lesser, the better for WR Length of the fibers between crimp points and associated flexibility. Higher, the better for WR ; LS > LT > LP Wear debris escape from the contact zone. Lesser the probability, the better for WR Retention of debris trapped in the pockets of the weave. Higher, the better for WR Formation of intermediate layer due to shearing of debris. Thicker the better for WR Embedment of debris retained in the zone followed by wear thinning; Higher, the better for WR Tightness of weave responsible for efficient entrapment of debris; tighter, the better for WR Quick entrapment of debris beneath the length of crimp (crossover points); Higher, the better for WR
Extent of Effect Plain
Twill
Satin
−
Minimum
Moderate
Maximum
+
Maximum
Moderate
Minimum
+ +
Maximum
Moderate
Minimum
+
Minimum
Moderate
Maximum
−
Maximum
Moderate
Minimum
−
Maximum
Moderate
Minimum
−
Maximum
Moderate
Minimum
−
Maximum
Moderate
Minimum
−
Maximum
Moderate
Minimum
(+) Sign indicates contribution to weight loss; (−) sign indicates protection from wearing. For plain weave composites events 1–3 are unfavorable for increasing WR while factors 4–9 are favorable. Resultant of these led to highest WR . Exactly reverse is true for satin weave composite. However, extent of contribution of these mechanisms depends on operating parameters, especially applied load.
3.1. SEM studies Scanning electron micrographs of worn composite surfaces in the order of increasing wear rate are shown in Figs. 5–7. Fig. 5 shows the general appearance of craters and influence of load on their size at lower magnification (×15). Micrographs 5a and b show the craters for composite IP55 worn under 100 and 250 N loads, respectively, while 5c and d are for composite IT55 worn under 100 and 250 N loads, respectively. The size of the craters has increased with load because of higher wear. Micrographs 6 and 7 show surfaces worn under lower (100 N) and higher (250 N) loads, respectively. At 100 N, wear performance of composites was in the order IP55 ≈ IS55 IT55 . Micrographs 6a–c show worn surface of IS55 . As seen in micrograph 6a, the weave being satin, long fibers, longitudinally wear thinned and well embedded in the matrix appear on the surface (event 1 described in Table 4). A little back-transfer of resin (marked 1) is also appearing on the surface. Micrograph 6b shows another location where various stages of fiber damage (marked 1) and back-transfer of the matrix (marked as 2) can be clearly seen. Few fibers are completely broken leaving behind cavities (marked as 3). Enhanced fiber-matrix debonding responsible for higher wear can also be seen (marked as 4). Micrograph 6c shows finer features of fiber damage which initiates with micro-cracking, followed by micro-cutting, pulverization in the form of powder and subsequent removal from the surface leaving behind cavities as seen in the micrograph. Micrograph 6d shows worn surface of IP55 . Surface, overall is very smooth supporting lowest wear rate (similar to IS55 ) of this composite. Most of the area was covered with back-transferred
matrix in the form of patches (marked as 1). The pulverized debris of fibers is efficiently embedded in these patches (this was one of the very critical factors i.e., retention of debrisresponsible for highest WR ). The dark boundaries of patches give the false notion of being cracks. Carbon fibers longitudinally wear thinned can also be seen. The length of such fibers, however, is shorter than that observed in IS55 . These broken fibers and pulverized debris are very well embedded in the matrix and no evidence of fiber-matrix debonding could be seen. Micrograph 6e shows the worn surface of the composite IT55 , that exhibited the highest wear rate at 100 N. The wear damage to the fibers is severe as compared to mild wear observed in earlier micrograph (6d). Another difference was in the nature of back-transferred layer of matrix (marked as 1). The extent of debris embedded in the matrix and hence retention of wear debris was less, which could be the cause of highest wear rate of the composite. Micrographs of composites worn under higher load (250 N) are shown in Fig. 7. At this load order of WR was IP55 > IT55 > IS55 . The features in micrographs 7a–c support the highest WR of IP55 . Though the fiber damage is severe (events 2, 3 in Table 4) hardly any cavities are seen (events 6, 7 in Table 4). This indicated the high capability of this composite for wear debris retention, which was suggested as main reason of highest WR of the composite. The micrograph also shows the weave of fabric (marked as 1) (event 5 in Table 4). Micrograph 7b shows magnified view (×1000) of the surface confirming well-embedded fiber debris in the matrix in various orientations that provided temporary wear protection. Micrograph 7c shows another location of the same composite surface (×1000) with
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evidence of retention of fiber debris in the weave pockets (marked as 1) (event 5 in Table 4). A location marked as 2 shows some evidence of fiber-matrix debonding because of higher frictional heat as a consequence of higher load. Micrograph 7d shows the magnified view (×1000) of worn surface of IT55 . The fiber damage though is less, the extent of removal of debris leaving behind cavities (marked as 1) is very high, which was a major cause of higher wear. The fiber piece (marked as 2) that was uplifted from the cavity is seen lying on the surface ready to be thrown out of the contact zone. Some evidence of fiber-matrix debonding because of higher frictional heat as a consequence of higher load can also be seen (marked as 3). Micrograph 7e (×500) shows the worn surface of IS55 , which has shown highest wear. The long fibers fractured at various places because of high load can be seen. The surface shows maximum damage as seen in the left portion of the micrograph. A number of cavities formed due to fiber debris removal and mostly responsible for highest wear of this composite can also be seen. 4. Conclusions Based on the fretting wear studies carried out on the selected composites of Polyetherimide reinforced with carbon fabric of three different weave patterns, it was observed that plain weave reinforced composite (IP55 ) provided maximum resistance to fretting wear and lowest coefficient of friction followed by twill (IT55 ) and satin weave composite (IS55 ) in severe operating conditions. Inclusion of CF improved WR of PEI significantly (4–8.4 times depending on load and weave of fabric). μ on other hand was benefited marginally (∼25%). With increasing loads, wear rate increased and μ decreased gradually. The difference in WR of the three composites was attributed to the difference in the capability of retention of pulverized fiber debris in the pockets of the weave and below the crimp points as supported by SEM. Plain weave was the most effective in retaining debris and provided highest wear protection while satin was the least effective in this aspect leading to poorest performance. Proposed wear mechanisms were supported by SEM.
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Acknowledgement Authors are grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for funding the work reported in this paper. References [1] K.H.Z. Gahr, Microstructure and Wear of Materials, Tribology Series 10, Elsevier, Amsterdam, 1987. [2] G.W. Stachowiak, A.W. Batchelor, Engineering Tribology, Tribology Series, 24, Elsevier, Amsterdam, The Netherlands, 1993. [3] K. Schulte, K. Friedrich, O. Jacobs, Fretting and fretting fatigue of advanced composite laminates, in: K. Friedrich (Ed.), Advances in Composite Tribology, Composite Materials Series 8, Elsevier, Amsterdam, The Netherlands, 1993, Chapter 8. [4] K. Friedrich (Ed.), Advances in Composite Tribology, Composite Materials Series 8, Elsevier, Amsterdam, The Netherlands, 1993. [5] R.C. Bill, Selected fretting wear resistant coatings for Ti-6 percent Al-4 percent V alloy, Wear 106 (1985) 283–301. [6] O. Jacobs, K. Friedrich, K. Schulte, Fretting fatigue of continuous carbon fiber reinforced plastics, Wear 145 (1991) 167–188. [7] N. Ohmae, K. Kobayashi, T. Tsukizoe, Characteristics of fretting of carbon fiber reinforced plastics, Wear 29 (1974) 345–353. [8] C. Soutis, Carbon fiber reinforced plastics in aircraft construction, Mater. Sci. Eng. A 412 (2005) 171–176. [9] www.ntsb.goc, Official website of National Transportation Safety Board (NTSB) (Jan 1998) Aviation Accidents, site accessed on July 2006. [10] J. Bijwe, J. Indumathi, B.K. Satapathy, A.K. Ghosh, Influence of carbon fabric on fretting wear performance of PEI composites, ASME J. Tribol. 124 (2002) 834–839. [11] J. Bijwe, Rekha Rattan, Influence of amount of carbon fabric on the low amplitude oscillating wear performance of polyetherimide composites, Tribo. Lett., in press. [12] J. Bijwe, J. Indumathi, A.K. Ghosh, Influence of weave of glass fabric on oscillating wear performance of polyetherimide composite, Wear 253 (2002) 803–812. [13] B. Vishwanath, A.P. Verma, C.V.S. Rao, Effect of fabric geometry on friction and wear of glass fiber reinforced composites, Wear 145 (1991) 315–327. [14] T.C. Chivers, S.C. Gordelier, Fretting fatigue palliatives: some comparative experiments, Wear 96 (1984) 153–175. [15] P. Rehbein, J. Wallaschek, Friction and wear behavior of polymer/steel and alumina/alumina under high frequency fretting conditions, Wear 216 (1998) 97–105. [16] J. Schaff, Properties and Performance in ASM Handbook of Composites, vol. 21, 2002.
Influence of weave of carbon fabric on low amplitude oscillating wear performance of Polyetherimide composites Rekha Rattan, J. Bijwe ∗ , M. Fahim Industrial Tribology Machine Dynamics and Maintenance Engineering Centre (ITMMEC), Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Received 11 March 2006; received in revised form 24 July 2006; accepted 3 August 2006 Available online 20 September 2006
Abstract Three composites were fabricated based on Polyetherimide (PEI) matrix and carbon fabric (CF) (55 vol.%) of different weaves, viz. plain, twill and satin (4H) by impregnation technique. These composites were evaluated for various mechanical properties and tribological performance in low amplitude oscillating wear (LAOW)/fretting wear mode. It was observed that CF reinforcement led to a significant enhancement in all strength and modulus properties of PEI except elongation to break (e). Twill weave proved to be the most effective followed by satin and plain weave in almost all the properties. The LAOW mode evaluation under various loads revealed that the wear performance order was exactly opposite to the above trend. Overall, plain weave composite was the best performer followed by twill and satin. Various wear mechanisms, such as fiber-matrix debonding due to repetitive reciprocating shearing stresses, micro-cracking, micro-cutting and pulverization of fibers followed by removal of debris from the contact zone were operative during such wear situation. Amongst these processes, generation of fiber debris, their temporary retention in fabric weaves and subsequent removal from the crater played a key role in overall wear performance. The ability of plain weave to hold back the debris was maximum that resulted in lowest wear while satin weave had the minimum retention ability that led to the highest wear. The proposed wear mechanisms were supported by SEM studies. © 2006 Published by Elsevier B.V. Keywords: Low amplitude oscillating wear; Fretting wear; Carbon fabric reinforced composites; Fabric-weave tribology
1. Introduction Wear of components under oscillatory relative motion (either linear or torsional) of small amplitude of displacement is termed as low amplitude oscillating wear (LAOW) or fretting wear. Such oscillatory motion can be intentional or unintentional. Rolling bearings used for small oscillatory movement, bearing’s outer ring and housing, wire ropes, electrical switchgears, multilayer leaf springs, palliatives, spline couplings, flanges, seals, riveted and pinned joints, etc. may suffer from such type of wear damage [1–4]. Reinforced polymers and composites are frequently used to make such components [5–7]. In spite of this fact, not much is available on fretting wear behavior of these materials compared to vast literature available for metals and alloys [2]. Among reinforced polymers, carbon fiber reinforced polymers
∗
Corresponding author. Tel.: +91 11 26591280; fax: +91 11 26591280. E-mail addresses: [email protected], [email protected] (J. Bijwe).
0043-1648/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.wear.2006.08.005
(CFRP) have immense potential in numerous applications in which fretting wear situations may arise. CFRP ropes, for example, if used in the place of steel rope for the construction of light weight suspension bridge, low amplitude oscillatory wear would occur between the retaining parts and the CFRP ropes [7]. CFRP components are used in aeronautical and space applications in large scale [8] and are prone to similar damages in some situations, which can be fatal and lead to accidents. It was a technical snag that had forced the pilot for an emergency water landing to avoid an accident. Later, the inspection of aircraft parts revealed that fitting beams had undergone fretting wear and cracking that were responsible for the mishap [9]. Among fabric reinforced or bi-directional (BD) composites, as they are popularly called, CFRPs are being increasingly used in aircraft industry as structural material because of very high specific strength and modulus. However, hardly any literature is available on the fretting wear behavior of such BD composites [10,11]. In our earlier work, significant benefits endowed by inclusion of carbon fabric (69 vol.%) in PEI matrix were reported
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R. Rattan et al. / Wear 262 (2007) 727–735 Table 1b Properties of three weaves of carbon fabric measured in the laboratory Carbon fabric (g/cm3 )
Fig. 1. Schematic showing different weave patterns (i) plain (one weft over one warp), (ii) twill (two warp over two weft) and (iii) satin (4H) (one warp over three weft).
[10]. In a subsequent work, a series of CF reinforced BD composites containing various amounts of fabric was developed and evaluated to investigate optimum amount of CF in PEI for best combination of tribo-performance and strength [11]. Though weave of fabric is very important in controlling strength properties of the composites, no paper is reported on the influence of weave of CF in any matrix in LAOW mode. Interestingly, a little is reported in case of glass fabric reinforced PEI composites in abrasive wear mode [12], and in thermoset composites in adhesive wear mode [13]. Hence, it was proposed to investigate influence of three weaves of CF in PEI on strength properties and LAOW performance. The results are presented in subsequent sections. 2. Experimental 2.1. Fabrication of composites GE plastics USA supplied the PEI material (ULTEM 1000) as granules. The carbon fabric used as reinforcement was procured from Fiber Glast Ltd. USA. Carbon fabrics of three different weaves, viz. plain weave (P), twill weave (T) and satin weave-4 harness (S) as shown in Fig. 1 were selected. The properties of these fabrics were studied in the laboratory and are compiled in Table 1. The three composites were developed keeping the fabric concentration constant (55 vol.% or 65 wt.%) using impregnation technique (I). The plies (280 mm × 260 mm) were cut from the carbon fabric roll and the open strands from all the four sides were sealed with a PTFE coated glass fabric tape to avoid the fiber misalignment. These plies were immersed individually in separate containers filled with viscous solution of PEI (prepared with Dichloromethane) for 12 h. The containers were properly sealed to avoid evaporation of solvent, which was required for adequate wetting of fiber strands with the PEI solution. The plies
Density Area (g/cm2 ) Towa Tex Denier Crimp (%) Count Warp (in.) Weft (in.) Thickness (cm) Bending length (cm) Tensile strength (kg/cm2 ) Elongation (%) a
Plain
Twill
Satin (4H)
1.85 196 3K 20 185 0.64 28 12 12 0.34 7.2 3 1.25
1.85 198 3K 22 198 0.70 26 16 16 0.34 5.9 1.47 1.85
1.85 193 3K 19 171 0.30 31 14 14 0.36 5.0 1.2 1.52
Supplier’s data.
were taken out carefully to avoid the misalignment in weave and dried in oven for 2 h at 100 ◦ C in a stretched condition. These 20 prepregs were used to attain the desired thickness in the range of 3–3.5 mm and were stacked in the mould carefully to avoid misalignment. PTFE coated glass fabric was placed on the top and bottom of the stacked prepregs. During compression molding, the mould was heated to attain the temperature in the range of 385–390 ◦ C within 2 h. The prepregs were then compression molded at this temperature at an applied pressure of 7.35 MPa. During the total compression time of 20 min at high temperature four intermittent breathings (each of 2 s) were applied to expel any residual solvent. The composites were allowed to cool under ambient conditions and applied pressure 7.35 MPa. The test specimens were cut from the composites with the help of diamond cutter as per required standards for mechanical and tribological testing. 2.2. Characterization of the composites The plain, twill and satin weave reinforced composites were designated as IP55 , IT55 and IS55 , respectively. The three composites fabricated for the present study were characterized for physical properties and composition using Soxhlett apparatus and Dichloromethane as a solvent. The extraction temperature and time were 40 ◦ C and 36 h, respectively. Composites were also evaluated for various mechanical properties as per ASTM standards. The mechanical properties of these composites are shown in Table 2. (Properties of PEI were provided by the supplier). 2.3. Oscillating wear studies
Table 1a Characteristic properties of various weaves of fabric [16] Property
Plain
Twill
Satin
Good stability Good drape Low porosity Smoothness Balance Symmetrical Low crimp
Good Poor Acceptable Poor Good Excellent Poor
Acceptable Good Good Acceptable Good Acceptable Acceptable
Poor Excellent Excellent Excellent Poor Very poor Excellent
The studies were done on SRV Optimol Tester [10], in which a chromium steel ball of diameter 10 mm was oscillated against a polymer composite plate (10 mm × 10 mm × 3–4 mm). The direction of fabric and fibers with respect to oscillation is shown in Fig. 2. The operating parameters were as follows. Load: 100, 150, 200, 250 and 300 N Stroke length (full oscillation width)—1 mm;
R. Rattan et al. / Wear 262 (2007) 727–735 Table 2 Details of the composition and properties of fabricated composites Composites Density (g/cm3 ) ASTM D 792 Contents of fabric (%, v/w) Tensile strength (MPa) ASTM 638 Tensile modulus (GPa) ASTM 638 Elongation at break (%) ASTM 638 Toughness (MPa) ASTM 638 Flexural strength (MPa) ASTM 790 Flexural modulus (MPa) ASTM 790 Inter laminar shear strength (ILSS) (MPa) ASTM 2344 *
PEI*
The specific wear rate was calculated using the equation:
IP55
IT55
IS55
1.55
1.53
1.54
55/65 535
55/65 888
55/65 575
03
73
106
76
60
0.54
0.08
0.32
– 150
3.8 589
2.2 951
2.8 832
3.3
40
54
46
–
49
66
63
1.27 – 105
729
Properties of PEI as per supplier’s data.
K0 =
V 2AνtFN
where Ko is the specific wear rate in m3 /N m, V the wear volume (m3 ) and FN the applied normal load (N). 2Aνt (where A is the full oscillation width (m), ν the frequency (Hz) and t is the experimental duration in (s)) indicates total sliding distance. 3. Results and discussion As seen from Table 2, the strength performance of the composites was in the order; IT55 > IS55 > IP55 ≫ PEI for all properties except toughness and elongation, where the order was reverse. The friction and wear performance of the selected composites under various loads is shown in Fig. 3. Possible wear mechanisms are shown in Fig. 4 and SEM micrographs of worn pin surfaces are shown in Figs. 5–7. Fig. 3a shows the variation of friction coefficient (μ) with increasing loads for all the three composites. The μ decreased with increasing loads and was in the range of 0.3–0.4. For
Fig. 2. Schematic showing ball on plate (composite) configuration.
Test duration—2 h; Oscillating frequency—50 Hz, Temperature—25 ◦ C. The load was increased on the ball fretting against a composite plate after each experiment in the step of 50 N till limiting load1 reached.
1 In the case of LAOW all selected parameters including amplitude are set before the experiment starts. Once the oscillation starts, generally amplitude falls below the set value and it is reset with the control panel. However, when load is very large and shearing forces are high, amplitude cannot be maintained even after applying maximum range provided in the control unit. This shows that the limiting load has reached. Sometimes amplitude reduces because of trapping of large particles of metal or polymer and falls to a low value for a short time. It regains once the particle either is thrown off or gets “wear thinned”. However, if the value falls below 80% of original value for a long time even after applying full possible force through control unit, it is realized that the material has reached the “failure limit” and cannot withstand the selected load. The experiment is abandoned and lower load is selected for next experiment.
Fig. 3. (a) Coefficient of friction and (b) specific wear rate (Ko ) as a function of load.
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Fig. 4. (a) Cross-section view of unit cell of (a) plain, (b) twill and (c) satin weave fabric; L1, L2 and L3 denote distance between mid-point of contact and the point where the crimp starts. (b) Schematic of the events of most probable sequence during fretting wear of plain weave fabric reinforced composite. A—Situation prior to fretting: (a) top view of weave and direction of fretting, (b) cross-section of unit cell and (c) contact configuration. B—Just initiation of fretting: (a) wear debris generation due to fiber damage, (b) larger sized fiber/matrix debris in contact zone and (c) sub-surface damage. C—Fretting continued: (a) initiation of travel of debris towards pockets, (b) retention of debris in pockets and (c) thick layer of entrapped fibrous and polymeric debris leading to third body interphase. D—Fretting continued: (a) wear thinned fibrous debris accumulation in pockets, (b and c) shearing of debris. E—Fretting in advanced stage: (a) pocket saturated with fibrous debris offering more wear resistance by further wear thinning, (b and c) accumulation beneath the crimp leading to saturation; excess debris escape the contact zone leading to measurable wear.
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Fig. 5. Scanning electron micrographs (×15) of IP55 worn under (a) 100 N and (b) 250 N; and IT55 worn under (c) 100 N and (d) 250 N loads, respectively.
almost all loads plain weave composite showed the lowest μ followed by twill and satin composites. μ of PEI, however, was always high 0.41 and was independent of load as reported in our earlier paper [10]. Thus, inclusion of CF reduced μ by approximately 25%, which was due to the lubricating action of CF. A lower μ at higher loads was attributed to the lubricity provided by the fine CF powder formed due to crushing and grinding of fiber pieces that accumulated at the contact zone. The specific wear rate (Ko ) plotted as a function of load in Fig. 3b indicates that Ko increased with increase in load for all the composites, which is a general trend observed in the case of fiber-reinforced composites [3,10,11]. The limiting load (and hence range of utility of the composite in harsh operating conditions) of plain and satin weave composites was 300 N while that of twill weave was slightly lower (250 N). IP55 was the best performer from tribological point of view, but poorest from strength point of view. Composite IS55 showed second best wear performance under only moderate loads (100–150 N). At higher loads, however it showed highest wear rate. Its friction behavior was also poorest in all conditions. Twill weave composite IT55 on the other hand was second best in both wear as well as friction performance under higher loads. Ko was least dependent on load as compared to other composites. Overall order of friction and wear performance of composites was: IP55 > IT55 > IS55 . The strength and modulus of the composites, however, did not follow the same order. The wear performance of the composites was compared with that of Neat PEI (data taken from Ref. [10]) in the form of relative wear
resistance which is calculated as per following equation RWR =
Wear resistance of composite Wear resistance of PEI
Since limiting load for PEI was 200 N, the data for comparison was selected accordingly. As seen from Table 3, a significant increase in wear performance due to CF inclusion can be seen. Approximately 8.4 times increase in performance was showed by IP55 . Overall, the improvement was in the range 4–8.4 for various composites evaluated under different loads. It is a well known fact that wear behavior in fretting wear mode is very different than that in sliding wear mode. It is the reciprocating motion in the former, which makes the major difference in wear mechanisms in two modes. Wear processes in such dry frictional contacts are considered as a succession of individual events involving particle detachment from the contacting bodies i.e., debris formation, its trapping and shearing in the contact zone and subsequent escape from the same. Among
Table 3 Relative wear resistance of composites at various loads (RWR = Ko of composite/Ko of PEI) Material
PEI IP55 IT55 IS55
Relative wear resistance (RWR) 50 N
150 N
200 N
1 6.55 4.08 7.03
1 8.38 6.00 6.55
1 7.28 6.08 5.17
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Fig. 6. SEM (×500) of composites worn under 100 N (WR -IS55 ∼ = IP55 IT55 ); (a) long fibers, longitudinally wear thinned and embedded in the matrix, (b) backtransfer of matrix, (c) magnified view (×1500) of fiber damage with micro-cracking, (d) shows worn surface of IP55 and back-transfer of matrix in the form of patches and (e) shows the worn surface of the composite IT55 and debris embedded in the matrix.
these, the escape of wear debris is minimal if the slip amplitude is very small. In the case of engineering polymers, the effect of contact zone kinematics on the wear is significant when the third body formation leads to abrasion or the transfer of polymer films to the counterface as well. The wear process is also affected by the formation of ‘loose’ or ‘tight’ compacts from the debris and its subsequent rheological behavior [3]. Thus, the wear behavior of materials is quite different from that in other wear modes and cannot be predicted a priori. There are a number of cases reported where inclusion of solid lubricants or fibers or both did not render the expected benefits. The performance on the other hand deteriorated [14,15]. The wear resistance (WR ) (inverse of wear rate, Ko ), was highest for plain weave composite followed by twill and satin. This was not related to any strength properties of these composites, rather it was mainly because of the difference in wear
mechanisms. The wear process involves various mechanisms such as fiber-matrix debonding followed by fiber breakage as a result of micro-cracking, micro-cutting and micro-pulverization due to reciprocating shearing stresses. The wear also depends on the amount of fiber debris produced and its escape from the contact zone. Amongst all weaves, plain weave is the most tight (least flexible) followed by twill and satin. This would result maximum fiber damage to the plain weave composite. However, the same weave is also responsible for the retention of fiber debris in the contact zone. Thus, fretting wear of fabricreinforced composites is a complex phenomenon that comprises multiple mechanisms which are operative successively and then simultaneously. Fig. 4a shows the unit cells of three weaves. It highlights the variation in distance between mid-point of contact and the point of starting of crimp. This distance has critical impact on wear mechanisms. Higher the distance,
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Fig. 7. SEM of IP55 worn under 250 N showing (a) weave of fabric (marked as 1), (b) magnified view (×1000) fiber debris embedded in the matrix, (c) retention of fiber debris in the weave pockets marked as 1 and fiber-matrix debonding marked as 2, (d) magnified view (×1000) for IT55 showing cavities (marked as 1), fiber piece escaped from cavity (marked as 2), fiber-matrix debonding (marked as 3) and (e) worn surface of (×500) IS55 showing long fibers and cavities.
lower is the probability of fiber debris reaching to crimp and getting entrapped. If entrapped, they will cause negative wear. The distance being largest in satin weave, the probability of entrapment was lowest. This was one of the reasons for highest wear of satin weave composite. The same distance was lowest for plain weave composite and hence offered highest possibility of retention of debris beneath the crimp leading to lowest wear. Fig. 4b is self-explanatory and depicts fretting mechanisms of plain weave composite. Various events that take place sequentially are collected in Table 4. The impact (positive or negative) on weight loss of a composite is also indicated in the same table. The mechanisms suggested in Table 4 and Fig. 4b are supported by SEM studies as discussed in the subsequent section. In the case of satin weave composite, similar mechanisms (Fig. 4a) take place except for the difference in the degree of
extent. The satin weave is the least compact (Table 1a) amongst three followed by twill and plain. Hence, the tendency of retention of debris beneath the crimp and pockets is minimum for satin and maximum for plain weave composite, a fact that supports the highest and lowest wear rates for the former and latter. Thus, the initial fiber breakage process though is maximum in plain weave composite, the further damage processes are minimal and retention processes are maximum. Satin weave being loose, on other hand, fiber breakage is minimal because of highest length of fiber, and hence maximum flexibility, between crossover points. Thus, plain weave composite showed highest WR , despite maximum fiber pulverization, mainly because of its highest tendency of retention of wear debris in the pockets and beneath the crimp point. The maximum tightness of the plain weave and maximum crimp points (Table 1a) were responsible for retention of debris leading to lowest wear of this composite.
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Table 4 Possible wear mechanisms and extent of their contribution towards weight loss/wear rate
1 2
3 4 5 6 7 8 9
Parameters and events during fretting of a ball on a composite plate (WR , wear resistance)
Effect on wear rate
Wear thinning of fibers parallel to the direction of slip; higher the better for WR . Longer the fibers more the wear thinning Breakage of fibers due to micro-cracking and micro-cutting. Lesser, the better for WR Generation of wear debris. Lesser, the better for WR Length of the fibers between crimp points and associated flexibility. Higher, the better for WR ; LS > LT > LP Wear debris escape from the contact zone. Lesser the probability, the better for WR Retention of debris trapped in the pockets of the weave. Higher, the better for WR Formation of intermediate layer due to shearing of debris. Thicker the better for WR Embedment of debris retained in the zone followed by wear thinning; Higher, the better for WR Tightness of weave responsible for efficient entrapment of debris; tighter, the better for WR Quick entrapment of debris beneath the length of crimp (crossover points); Higher, the better for WR
Extent of Effect Plain
Twill
Satin
−
Minimum
Moderate
Maximum
+
Maximum
Moderate
Minimum
+ +
Maximum
Moderate
Minimum
+
Minimum
Moderate
Maximum
−
Maximum
Moderate
Minimum
−
Maximum
Moderate
Minimum
−
Maximum
Moderate
Minimum
−
Maximum
Moderate
Minimum
−
Maximum
Moderate
Minimum
(+) Sign indicates contribution to weight loss; (−) sign indicates protection from wearing. For plain weave composites events 1–3 are unfavorable for increasing WR while factors 4–9 are favorable. Resultant of these led to highest WR . Exactly reverse is true for satin weave composite. However, extent of contribution of these mechanisms depends on operating parameters, especially applied load.
3.1. SEM studies Scanning electron micrographs of worn composite surfaces in the order of increasing wear rate are shown in Figs. 5–7. Fig. 5 shows the general appearance of craters and influence of load on their size at lower magnification (×15). Micrographs 5a and b show the craters for composite IP55 worn under 100 and 250 N loads, respectively, while 5c and d are for composite IT55 worn under 100 and 250 N loads, respectively. The size of the craters has increased with load because of higher wear. Micrographs 6 and 7 show surfaces worn under lower (100 N) and higher (250 N) loads, respectively. At 100 N, wear performance of composites was in the order IP55 ≈ IS55 IT55 . Micrographs 6a–c show worn surface of IS55 . As seen in micrograph 6a, the weave being satin, long fibers, longitudinally wear thinned and well embedded in the matrix appear on the surface (event 1 described in Table 4). A little back-transfer of resin (marked 1) is also appearing on the surface. Micrograph 6b shows another location where various stages of fiber damage (marked 1) and back-transfer of the matrix (marked as 2) can be clearly seen. Few fibers are completely broken leaving behind cavities (marked as 3). Enhanced fiber-matrix debonding responsible for higher wear can also be seen (marked as 4). Micrograph 6c shows finer features of fiber damage which initiates with micro-cracking, followed by micro-cutting, pulverization in the form of powder and subsequent removal from the surface leaving behind cavities as seen in the micrograph. Micrograph 6d shows worn surface of IP55 . Surface, overall is very smooth supporting lowest wear rate (similar to IS55 ) of this composite. Most of the area was covered with back-transferred
matrix in the form of patches (marked as 1). The pulverized debris of fibers is efficiently embedded in these patches (this was one of the very critical factors i.e., retention of debrisresponsible for highest WR ). The dark boundaries of patches give the false notion of being cracks. Carbon fibers longitudinally wear thinned can also be seen. The length of such fibers, however, is shorter than that observed in IS55 . These broken fibers and pulverized debris are very well embedded in the matrix and no evidence of fiber-matrix debonding could be seen. Micrograph 6e shows the worn surface of the composite IT55 , that exhibited the highest wear rate at 100 N. The wear damage to the fibers is severe as compared to mild wear observed in earlier micrograph (6d). Another difference was in the nature of back-transferred layer of matrix (marked as 1). The extent of debris embedded in the matrix and hence retention of wear debris was less, which could be the cause of highest wear rate of the composite. Micrographs of composites worn under higher load (250 N) are shown in Fig. 7. At this load order of WR was IP55 > IT55 > IS55 . The features in micrographs 7a–c support the highest WR of IP55 . Though the fiber damage is severe (events 2, 3 in Table 4) hardly any cavities are seen (events 6, 7 in Table 4). This indicated the high capability of this composite for wear debris retention, which was suggested as main reason of highest WR of the composite. The micrograph also shows the weave of fabric (marked as 1) (event 5 in Table 4). Micrograph 7b shows magnified view (×1000) of the surface confirming well-embedded fiber debris in the matrix in various orientations that provided temporary wear protection. Micrograph 7c shows another location of the same composite surface (×1000) with
R. Rattan et al. / Wear 262 (2007) 727–735
evidence of retention of fiber debris in the weave pockets (marked as 1) (event 5 in Table 4). A location marked as 2 shows some evidence of fiber-matrix debonding because of higher frictional heat as a consequence of higher load. Micrograph 7d shows the magnified view (×1000) of worn surface of IT55 . The fiber damage though is less, the extent of removal of debris leaving behind cavities (marked as 1) is very high, which was a major cause of higher wear. The fiber piece (marked as 2) that was uplifted from the cavity is seen lying on the surface ready to be thrown out of the contact zone. Some evidence of fiber-matrix debonding because of higher frictional heat as a consequence of higher load can also be seen (marked as 3). Micrograph 7e (×500) shows the worn surface of IS55 , which has shown highest wear. The long fibers fractured at various places because of high load can be seen. The surface shows maximum damage as seen in the left portion of the micrograph. A number of cavities formed due to fiber debris removal and mostly responsible for highest wear of this composite can also be seen. 4. Conclusions Based on the fretting wear studies carried out on the selected composites of Polyetherimide reinforced with carbon fabric of three different weave patterns, it was observed that plain weave reinforced composite (IP55 ) provided maximum resistance to fretting wear and lowest coefficient of friction followed by twill (IT55 ) and satin weave composite (IS55 ) in severe operating conditions. Inclusion of CF improved WR of PEI significantly (4–8.4 times depending on load and weave of fabric). μ on other hand was benefited marginally (∼25%). With increasing loads, wear rate increased and μ decreased gradually. The difference in WR of the three composites was attributed to the difference in the capability of retention of pulverized fiber debris in the pockets of the weave and below the crimp points as supported by SEM. Plain weave was the most effective in retaining debris and provided highest wear protection while satin was the least effective in this aspect leading to poorest performance. Proposed wear mechanisms were supported by SEM.
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Acknowledgement Authors are grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for funding the work reported in this paper. References [1] K.H.Z. Gahr, Microstructure and Wear of Materials, Tribology Series 10, Elsevier, Amsterdam, 1987. [2] G.W. Stachowiak, A.W. Batchelor, Engineering Tribology, Tribology Series, 24, Elsevier, Amsterdam, The Netherlands, 1993. [3] K. Schulte, K. Friedrich, O. Jacobs, Fretting and fretting fatigue of advanced composite laminates, in: K. Friedrich (Ed.), Advances in Composite Tribology, Composite Materials Series 8, Elsevier, Amsterdam, The Netherlands, 1993, Chapter 8. [4] K. Friedrich (Ed.), Advances in Composite Tribology, Composite Materials Series 8, Elsevier, Amsterdam, The Netherlands, 1993. [5] R.C. Bill, Selected fretting wear resistant coatings for Ti-6 percent Al-4 percent V alloy, Wear 106 (1985) 283–301. [6] O. Jacobs, K. Friedrich, K. Schulte, Fretting fatigue of continuous carbon fiber reinforced plastics, Wear 145 (1991) 167–188. [7] N. Ohmae, K. Kobayashi, T. Tsukizoe, Characteristics of fretting of carbon fiber reinforced plastics, Wear 29 (1974) 345–353. [8] C. Soutis, Carbon fiber reinforced plastics in aircraft construction, Mater. Sci. Eng. A 412 (2005) 171–176. [9] www.ntsb.goc, Official website of National Transportation Safety Board (NTSB) (Jan 1998) Aviation Accidents, site accessed on July 2006. [10] J. Bijwe, J. Indumathi, B.K. Satapathy, A.K. Ghosh, Influence of carbon fabric on fretting wear performance of PEI composites, ASME J. Tribol. 124 (2002) 834–839. [11] J. Bijwe, Rekha Rattan, Influence of amount of carbon fabric on the low amplitude oscillating wear performance of polyetherimide composites, Tribo. Lett., in press. [12] J. Bijwe, J. Indumathi, A.K. Ghosh, Influence of weave of glass fabric on oscillating wear performance of polyetherimide composite, Wear 253 (2002) 803–812. [13] B. Vishwanath, A.P. Verma, C.V.S. Rao, Effect of fabric geometry on friction and wear of glass fiber reinforced composites, Wear 145 (1991) 315–327. [14] T.C. Chivers, S.C. Gordelier, Fretting fatigue palliatives: some comparative experiments, Wear 96 (1984) 153–175. [15] P. Rehbein, J. Wallaschek, Friction and wear behavior of polymer/steel and alumina/alumina under high frequency fretting conditions, Wear 216 (1998) 97–105. [16] J. Schaff, Properties and Performance in ASM Handbook of Composites, vol. 21, 2002.