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Influence of impingement angle on solid particle erosion of carbon fabric reinforced polyetherimide composite
Wear 262 (2007) 568–574
Influence of impingement angle on solid particle erosion of carbon fabric reinforced polyetherimide composite R. Rattan, Jayashree Bijwe ∗ Industrial Tribology Machine Dynamics and Maintenance Engineering Centre (ITMMEC), Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Received 9 February 2006; received in revised form 3 July 2006; accepted 18 July 2006 Available online 28 August 2006
Abstract Polyetherimide (PEI) composite reinforced with plain weave carbon fabric (CF) (40% by volume) was developed and characterized for physical and mechanical properties. The erosive wear behaviour of PEI and its composite was evaluated using silica sand particles at a constant impact velocity but varying angles of impingement. It was confirmed that though all the mechanical properties of PEI improved substantially by CF reinforcement, the erosion resistance (WR ) deteriorated by a factor of almost four–six times at all angles of impingement. Both materials showed minimum wear at normal incidence (90◦ impingement). In spite of the fact that PEI is not a very ductile polymer (elongation to break-60%), it showed maximum wear at 15◦ which is a characteristic of ductile and semi-ductile mode of failure. The composite (elongation to break-1%) also showed highest wear at 30◦ (impingement at 15◦ was not studied). These phenomena were explained using scanning electron micrographs of the eroded surfaces. © 2006 Elsevier B.V. All rights reserved. Keywords: Erosive wear of polymer composites; Polyetherimide composites; Carbon fabric reinforced polyetherimide composite; Mechanical properties of composites
1. Introduction Recently, carbon fabric (CF) reinforced composites based on high performance thermoplastics, such as polyether-etherketone (PEEK) are gaining an edge over the CF-thermoset based composites (mainly epoxy based) due to better mechanical properties, ease of processing and unlimited shelf life and are replacing them in various aircraft parts. CF reinforced epoxy composites proved a better choice for a number of applications in aircraft construction compared to lightweight alloys of aluminium, titanium and magnesium because of the substantially higher specific strength of composite form [1]. Carbon fiber/fabric reinforcement is a good choice since for military aircraft both high modulus and high strength are desirable while for satellite applications, such as reflector dishes, antennas and their supporting structures high modulus fibers provide stability as well as stiffness. Given the choice between fiber and fabric reinforcement, the latter is preferred mainly because of its
∗
Corresponding author. Tel.: +91 11 26591280; fax: +91 11 26591280. E-mail address: [email protected] (J. Bijwe).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.07.001
bi-directional (BD) strength, ease of handling the fabric and processing of BD composites. Moreover, the possibility of tailoring the properties through a wide selection of styles of fabric is an added advantage. Another unique advantage of fabric is its ability to drape or conform to curved surfaces without wrinkling. This is very important for tailoring the composite to curved surfaces and complex shapes. In many aircraft applications, these materials encounter wear and damage processes due to erosion by solid particles. Hence, a study on the erosive wear behaviour of such composites is important. PEI is a high performance specialty thermoplastic polymer with most of its properties either comparable or superior to PEEK (e.g. easy processability, lower cost, etc.). The friction and wear performance of PEI and its carbon fabric reinforced composites with various amount of CF and varying weaves have been investigated in detail in the authors’ laboratory. However, efforts were previously focused on the study of the influence of the material rather than the operating parameters, such as angle of impingement of erodent, particle velocity, etc. [2,3]. Angle of impingement is the most important and widely studied parameter in the case of materials in the literature [4–14]. Among the
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available literature on erosive wear behaviour of polymer composites [6–14], a little is reported on the erosive wear behaviour of BD polymer composites [2,3,6,12–14]. Hence, it was thought worthwhile to investigate the influence of angle of impingement on erosion of CF reinforced PEI composite along with neat PEI. The results of these investigations are discussed in the present paper. 2. Experimental 2.1. Fabrication of composites GE plastic USA supplied the PEI material (ULTEM 1000) in a granular form. The carbon fabric (CF) of plain weave (P) used as reinforcement was procured from Fibre Glast Ltd., USA. Properties of CF were studied in our laboratory and are shown in Table 1. It was used in a concentration of 40 vol.% (50 wt.%) to develop the composite. The composite was prepared by an impregnation technique (I) followed by compression molding and was designated as IP40 . The 20 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 fiber misalignment. Dichloromethane (CH2 Cl2 ) was used as a solvent to prepare the solution of PEI (containing 31 wt.%). These plies were immersed individually in the separate containers filled with the viscous solution of PEI for 12 h. The containers were sealed to avoid evaporation of solvent, which was required for adequate wetting of fiber strands with the PEI solution. The plies were taken out carefully to avoid misalignment of the weave and then dried in an oven for 2 h at 100 ◦ C in a stretched condition. These prepregs were then stacked in the mould to attain the desired thickness in the range of 3–4 mm. PTFE coated glass fabric was placed on the top and bottom of the stacked prepregs. The mould was then heated to attain a temperature in the range 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, two intermittent breathings (each of 2 s) were given to expel the possible residual solvent. The composite was allowed to cool under natural conditions
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Table 2 Details of the composition and mechanical properties of fabricated composite based on carbon fabric and PEI Composites
PEI
IP40
Density (g/cm3 ) ASTM D 792 Percentage of fabric (vol./wt.%) 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 (GPa) ASTM 790 Interlaminar shear strength (ILSS) (MPa) ASTM 2344
1.27 – 105 3.0 60 – 150 3.3 –
1.49 40/50 330 54 0.87 3.0 505 29 35
Data on PEI-provided by the supplier.
under the same pressure. It was then cut using a diamond cutter as specified in the standards for mechanical and tribological testing (see Table 2). 2.2. Characterization of the composites The composite was characterized for various properties as per standards (Table 2). The composition was determined using a Soxhlett apparatus and CH2 Cl2 as solvent. The extraction temperature and time were 40 ◦ C and 36 h, respectively. 3. Erosive wear studies The solid particle erosion experiments were carried out as per ASTM G76 on the erosion test rig shown schematically in Fig. 1. The test rig consisted of an air compressor, an air drying unit, a conveyor belt-type particle feeder and an air particle mixing and accelerating chamber. The dried and compressed air was then mixed with the silica sand (106–120 m size) which was fed constantly by a conveyor belt feeder into the mixing chamber. Samples of composite (30 mm × 40 mm × (3–4) mm) were held at selected angles (30◦ , 45◦ , 60◦ and 90◦ ) with respect to the flow of the impinging sand particles and eroded. Similarly, PEI was studied at angles of impingement 15◦ , 30◦ , 45◦ , 60◦ , 75◦
Table 1 Properties of plain weave of carbon fabric evaluated in the laboratory Carbon fabric
Plain weave
Density (kg/m3 ) Area (kg/m2 ) Towa Tex Denier Crimp (%) Count Warp (m) Weft (m) Thickness (m) Bending length (m) Tensile strength (MPa) Elongation (%)
1850 1960 3K 20 185 0.64 28 468.75 468.75 0.0034 0.072 0.3 1.25
a
Supplier’s data.
Fig. 1. Schematic of air jet erosion test rig.
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and 90◦ . The silica sand particles were accelerated by passing through a converging tungsten carbide nozzle of 3 mm diameter to bombard the target. The distance between the target material and the nozzle was approximately 10 mm. The operating parameters were as follows. Dose of erodent Flux rate Impinging velocity of particles Duration of erosion Impact angles
80 g 8 g/min 26.88 m/s 10 min 15◦ , 30◦ , 45◦ , 60◦ , 75◦ and 90◦
Two tests were performed on each sample and an average value calculated. Wear was measured by the weight loss method. Samples were cleaned with petroleum ether (40–60 ◦ C) before and after weighing. Eroded samples were cleaned with a brush to remove fine sand particles attached to the surface and then wiped with a cotton plug dipped in petroleum ether. The wear rate was expressed in terms of Wc (g)/Ws (g); where Wc was the loss in weight of the composite and Ws was the total weight of erodent used (80 g). Wc was determined by weighing the sample before and after the experiment on a weighing balance having an accuracy of 0.1 mg. An incubation period or weight gain was not observed for the neat PEI or its composite. 4. Results and discussion As seen in Table 2, CF reinforcement has improved significantly all the mechanical properties of PEI except elongation to break. The tensile strength (TS) and tensile modulus TM of PEI increased by 3 and 18 times, respectively, while the flexural strength (FS) and flexural modulus (FM) increased by approximately 3.5 and 7 times, respectively. The elongation to break however reduced drastically from 60 to <1%. The erosive wear rates of neat PEI and its composite as a function of angle of impingement are shown in Fig. 2. Figs. 3–7 are micrographs of the eroded surfaces of PEI and its composite.
Fig. 2. Erosion rate (10−5 g/g) vs. angle of impingement of: (a) neat polyetherimide (PEI) and (b) IP40 .
As seen from Fig. 2b, CF reinforcement reduced the wear performance of PEI considerably (at α = 90◦ , the reduction was four times while at 30◦ , it was almost six times). However, since components are made in the composite form due to the requirements of high specific strength and modulus, the use of the composite is inevitable in numerous applications. Hence, efforts are needed to minimize the wear, probably by developing composites with hybrid fibers (CF, Aramid, etc.). It was also observed that the angle of impingement at which the wear rate was lowest (αmin ) for both materials was 90◦ while αmax for PEI and its composite were observed to be 15◦ and 30◦ , respectively, out of the selected angles of impingement. (The composite could not be studied at 15◦ because a sample of the required size was unavailable. Since the shape of both the curves match very well, there is a high probability that αmax for the composite is in the same range as that of neat PEI.) In general, for ductile materials, αmax is in the range of 15–30◦ , and αmin 90◦ , while for brittle materials the behaviour is the opposite [4]. However, in the present study, PEI and its composite have shown behaviour similar to ductile materials, though they are not ductile as can be seen from their properties listed in Table 2. In the literature, there are no fixed trends correlating ductility or brittleness of materials with αmax or αmin . It is found that some polymers erode in a ductile manner; some show evidence of both ductile and brittle characteristics [7,9,11]. Thermoplastics generally exhibit a more ductile response than the thermosets [12]. Pool et al. [13] reported that the maximum erosion rate occurred at normal incidence for the UD and woven graphite reinforced epoxy composites implying a brittle type erosion behaviour. However, woven aramid reinforced epoxy composites exhibited a maximum erosion rate between 34◦ and 45◦ indicating semi-ductile behaviour. Chopped fiber-PPS material showed a maximum erosion rate at 25◦ which indicates ductile behaviour even though the material was not ductile. Thus, though the use of terms, such as failure by ‘ductile’ and ‘brittle’ mechanisms is frequent and useful in understanding erosion of materials, it is not strictly true in all cases. A ductile material can show peak erosion at 90◦ if the failure mechanism is different as seen, for example, in the case of erosion of metals by hard spherical particles, which remove metal by low-cycle plastic fatigue process rather than by cutting [4,15]. In the case of the impact of hard angular particles on brittle materials, plastic indentation takes place along with generation of long cracks extending from the plastic zone. The most important are lateral cracks, which curve back towards the eroding surface. Wherever such multiple cracks converge and intersect, a loose particle is removed as debris. In a brittle homogeneous material, cracks are not stopped until they reach the surface and hence lead to material removal. Material removal is more severe because impact causes crack propagation rather than plastic indentation. Impact at 90◦ leads to the greatest depth of plastic zone and hence a larger volume is bounded by lateral cracks and thus removed leading to maximum wear. On the other hand, ductile materials, especially metals, undergo erosive wear by the formation and detachment of heavily strained surfaces where erodent contacts repeatedly. The material removal process is enhanced at low angles, where cut-
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Fig. 3. Scanning electron micrographs showing erosion features of neat polyetherimide at different angles of impingement: (a) 90◦ , (b) 75◦ , (c) 60◦ , (d) 45◦ , (e) 30◦ and (f) 15◦ .
ting and micro-machining mechanisms are most effective. When impingement takes place, the normal component of the impacting force causes indentation on the surface while the tangential component removes material by lateral displacement after repeated impacts. Impact at 90◦ may deform a large amount of material. It will, however, simply be displaced backwards and
forwards in the plane of surface by repeated impacts. Such a damage process does not lead to effective removal of material [15]. In the case of PEI and its composite, αmax or αmin are at (15–30◦ ) and 90◦ , respectively. Evidence of material removal by indentation was seen at 90◦ and micro-machining at lower angles in SEM studies as discussed in the subsequent section.
Fig. 4. (a) SEM showing erosion features of IP40 at normal incidence (90◦ ) and (b) magnified view.
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Fig. 5. (a) SEM showing erosion features of IP40 at 60◦ and (b) magnified view.
Fig. 6. (a) SEM showing erosion features of IP40 at 45◦ and (b) magnified view.
In the case of fiber reinforced thermoplastic composites (nonductile materials similar to IP40 ), the material removal process may be different. αmax for this composite was not at 90◦ but at 30◦ similar to unfilled PEI, which is a comparatively ductile material. In contrast to bulk homogeneous brittle material, during normal impact (90◦ ), cracks if generated cannot propagate so easily because of the fibers present. During energy dissipation, it causes fracture of a fiber. Crack propagation in the forward and backward directions towards the eroding surface is very difficult since cracks have to cross the ductile matrix between the fibers. The matrix thus restricts the crack growth. Thus, wear of such composites was mainly due to easy fracture of brittle fibers and subsequent removal of fiber debris. Wear of composites was hence was higher than that of neat PEI. The overall wear
of the composites, therefore, was not because of the generation, propagation (to and fro) and intersection of cracks as in case of unfilled brittle material, but because of fiber fracture. At oblique angles (<90◦ ), both the processes, viz. fiber fracture and fiber cutting followed by successive removal of polymer and fiber debris are more dominating and occur easily leading to enhanced wear. Thus, the higher wear of IP40 as compared to PEI was due to excessive fracture of carbon fibers and effective removal of the debris from the surface. It was also clear that with decrease in the angle of impingement, the wear of PEI increased because of the increasing component of micro-cutting and micro-machining processes rather than lateral cracking. This was strongly supported by SEM analysis as discussed in the following section.
Fig. 7. (a) SEM showing erosion features of IP40 at 30◦ and (b) magnified view.
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4.1. SEM analysis of eroded surfaces Scanning electron micrographs of the surfaces of both neat PEI and its composite eroded at different angles of impingement are shown in Figs. 3–7. Micrographs (×1000-magnification, unless specified), are arranged in decreasing order of angle of impingement and increasing order of wear rates. As seen in Fig. 3a for α = 90◦ , the surface of PEI showed unusual features. PEI is not as ductile as metals. Hence, the surface shows mixed damage processes. As discussed for ductile materials, repeated impacts lead to plastic indentation processes and heavily strained regions on the surface. In the case of brittle materials, on other hand, the propagation of curved cracks towards the surface and their intersection to form a wear particle separated from the surface leads to wear. As seen in Fig. 3a, both processes appear to be operative. Initially the surface was strained and displaced backwards and forwards in the plane by repeated impacts. Regions were formed due to simultaneous generation of cracks characteristic of brittle materials. The extent of plastic indentation, however, decreased as the angle of impingement decreased as seen in other micrographs (Fig. 3b–f). When the angle of impingement was changed to 75◦ , the features seen were quite different (Fig. 3b). The normal component of the impact force was still effective in producing plastic indentation creating patches similar to Fig. 3a (marked as 1). The tangential component, on other hand was now operative in cutting action. Most parts of the micrograph show evidence of material flow in the direction of impingement. As seen in Fig. 3c (60◦ ), d (45◦ ) and e (30◦ ), the dominance of plastic indentation reduced with angle, though micro-cracking persisted. Fig. 3f (15◦ ) shows unique features. The entire surface shows the dominance of the micro-machining process, a characteristic failure feature for ductile materials at very low angle. This mechanism was responsible for the highest material removal. In the case of worn surface studies of the composite, however, the most visible dominant features were fracture and cutting of fibers. Failure of the matrix was not clearly observed. Fig. 4a (×500) and b (×1000) for the composite eroded at 90◦ show mainly fiber fracture. Comparatively more fibers appeared damaged but not extensively removed due to normal impact of erodent. Fig. 5a and b show the surface of the composite eroded at 60◦ . Fig. 5a shows the weave of fabric and crossover points. As seen in Fig. 5b, features, such as an increased extent of fiber–matrix de-bonding, fiber breakage and removal of fiber debris can be seen clearly. The tangential component of the impact force (cos θ), increases with decreasing angle of impingement and results not only in material removal (both fiber and polymer debris), but also in an increase in fiber/matrix debonding leading to enhanced wear. Fig. 6a (×500) and b (×1000) are for a surface eroded at 45◦ . The process of fiber damage and pulverization has increased with decrease in angle of impingement leading to higher wear of this composite. Fig. 7a (×500) and b (×1000) are for the surface eroded at 30◦ . The process of fiber damage and pulverization appears to be the most dominant mechanism. The tangential component of the impact force is more effective in micro-cracking followed by micro-cutting of
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fibers in many places leading to severe pulverization of fibers. Fiber debris is clearly seen on the surface (circled on the micrograph) before removal by successive impingement of erodent particles. 5. Conclusions Carbon fabric (40% by volume) reinforced PEI composite fabricated using the impregnation technique exhibited improved mechanical properties compared to neat PEI except for elongation to break. The reinforcement, however, adversely affected the erosion wear resistance of PEI which can be attributed to severe damage to the fibers caused by excessive fiber–matrix de-bonding, fiber fracture, micro-cutting and pulverization by successive impacts of high speed impinging particles. PEI and its composite though neither are ductile materials, exhibited erosion peaks at low angles of impingement (15◦ and 30◦ , respectively), which is generally seen in the case of ductile materials. SEM of eroded surfaces showed microstructural features that were characteristic of both brittle as well as ductile modes of failure. Acknowledgements The authors are grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi India for funding the work reported in this paper. The authors are very thankful to Prof. I.M. Hutchings for extensive discussion on the phenomenon of the correlation between peak angle of impact and failure mechanisms. References [1] C. Soutis, Carbon fiber reinforced plastics in aircraft construction, Mater. Sci Eng. A 412 (2005) 171–176. [2] J. Bijwe, R. Rattan, M. Fahim, Erosive wear of carbon fabric reinforced polyetherimide composites: role of amount of fabric and processing technique, J. Mater. Sci., in press. [3] Rekha Rattan, J. Bijwe, Carbon fabric reinforced polyetherimide composites: influence of weave of fabric and processing parameters on performance properties and erosive wear, Mater. Sci. Eng. 420 (part A) (2006) 342–350. [4] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, E. Arnold (Division of Hodder and Soughton), London, 1992. [5] G.W. Stachowiak, A.W. Batchelor, Engineering Tribology, Tribology Series, vol. 24, Elsevier, Amsterdam, The Netherlands, 1993. [6] M. Roy, B. Vishwanathan, G. Sundararajan, The solid particle erosion of polymer matrix composite, Wear 171 (1994) 149–161. [7] A. Hager, K. Friedrich, Y.A. Dzenis, S.A. Paipetis, Study of erosion wear of advanced polymer composites, in: K. Street, B.C. Whistler (Eds.), Proceeding of ICCM 10, Woodhead Publishing, Cambridge, 1995, pp. 155–162. [8] H.A. Aglan, T.A. Chenock Jr., Erosion damage features of polyimide thermoset composites, SAMPE Q. 24 (2) (1993) 41–47. [9] K.R. Karasek, K.C. Goretta, D.A. Helberg, J.L. Routbort, Erosion in bismaleimide polymers and bismaleimide polymer composites, J. Mater. Sci. Lett. 11 (1992) 1143–1144. [10] P.J. Mathias, W. Wu, K.C. Goretta, J.L. Routbort, D.P. Groppi, K.R. Karasek, Solid particle erosion of a graphite-fiber-reinforced bismaleimide polymer composite, Wear 135 (1989) 161–169. [11] N.-M. Barkoula, J. Karger-Kocsis, Processes and influencing parameters of the solid particle erosion of polymers and their composites, J. Mater. Sci. 37 (2002) 3807–3820.
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[12] P. Lee-Sullivan, G. Lu, Erosion of impact-notched holes in GFRP composites, Wear 176 (1994) 81–88. [13] K.V. Pool, C.K.H. Dharan, I. Finnie, Erosive wear of composite materials, Wear 7 (1986) 1–12.
[14] E. Moos, J. Karger-Kocsis, Solid particle erosion of knitted glass fabricreinforced polyethylene terephthalate composites, Adv. Compos. Lett. 8 (1999) 59–63. [15] Personal communication with Prof. I.M. Hutchings.
Influence of impingement angle on solid particle erosion of carbon fabric reinforced polyetherimide composite R. Rattan, Jayashree Bijwe ∗ Industrial Tribology Machine Dynamics and Maintenance Engineering Centre (ITMMEC), Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Received 9 February 2006; received in revised form 3 July 2006; accepted 18 July 2006 Available online 28 August 2006
Abstract Polyetherimide (PEI) composite reinforced with plain weave carbon fabric (CF) (40% by volume) was developed and characterized for physical and mechanical properties. The erosive wear behaviour of PEI and its composite was evaluated using silica sand particles at a constant impact velocity but varying angles of impingement. It was confirmed that though all the mechanical properties of PEI improved substantially by CF reinforcement, the erosion resistance (WR ) deteriorated by a factor of almost four–six times at all angles of impingement. Both materials showed minimum wear at normal incidence (90◦ impingement). In spite of the fact that PEI is not a very ductile polymer (elongation to break-60%), it showed maximum wear at 15◦ which is a characteristic of ductile and semi-ductile mode of failure. The composite (elongation to break-1%) also showed highest wear at 30◦ (impingement at 15◦ was not studied). These phenomena were explained using scanning electron micrographs of the eroded surfaces. © 2006 Elsevier B.V. All rights reserved. Keywords: Erosive wear of polymer composites; Polyetherimide composites; Carbon fabric reinforced polyetherimide composite; Mechanical properties of composites
1. Introduction Recently, carbon fabric (CF) reinforced composites based on high performance thermoplastics, such as polyether-etherketone (PEEK) are gaining an edge over the CF-thermoset based composites (mainly epoxy based) due to better mechanical properties, ease of processing and unlimited shelf life and are replacing them in various aircraft parts. CF reinforced epoxy composites proved a better choice for a number of applications in aircraft construction compared to lightweight alloys of aluminium, titanium and magnesium because of the substantially higher specific strength of composite form [1]. Carbon fiber/fabric reinforcement is a good choice since for military aircraft both high modulus and high strength are desirable while for satellite applications, such as reflector dishes, antennas and their supporting structures high modulus fibers provide stability as well as stiffness. Given the choice between fiber and fabric reinforcement, the latter is preferred mainly because of its
∗
Corresponding author. Tel.: +91 11 26591280; fax: +91 11 26591280. E-mail address: [email protected] (J. Bijwe).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.07.001
bi-directional (BD) strength, ease of handling the fabric and processing of BD composites. Moreover, the possibility of tailoring the properties through a wide selection of styles of fabric is an added advantage. Another unique advantage of fabric is its ability to drape or conform to curved surfaces without wrinkling. This is very important for tailoring the composite to curved surfaces and complex shapes. In many aircraft applications, these materials encounter wear and damage processes due to erosion by solid particles. Hence, a study on the erosive wear behaviour of such composites is important. PEI is a high performance specialty thermoplastic polymer with most of its properties either comparable or superior to PEEK (e.g. easy processability, lower cost, etc.). The friction and wear performance of PEI and its carbon fabric reinforced composites with various amount of CF and varying weaves have been investigated in detail in the authors’ laboratory. However, efforts were previously focused on the study of the influence of the material rather than the operating parameters, such as angle of impingement of erodent, particle velocity, etc. [2,3]. Angle of impingement is the most important and widely studied parameter in the case of materials in the literature [4–14]. Among the
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available literature on erosive wear behaviour of polymer composites [6–14], a little is reported on the erosive wear behaviour of BD polymer composites [2,3,6,12–14]. Hence, it was thought worthwhile to investigate the influence of angle of impingement on erosion of CF reinforced PEI composite along with neat PEI. The results of these investigations are discussed in the present paper. 2. Experimental 2.1. Fabrication of composites GE plastic USA supplied the PEI material (ULTEM 1000) in a granular form. The carbon fabric (CF) of plain weave (P) used as reinforcement was procured from Fibre Glast Ltd., USA. Properties of CF were studied in our laboratory and are shown in Table 1. It was used in a concentration of 40 vol.% (50 wt.%) to develop the composite. The composite was prepared by an impregnation technique (I) followed by compression molding and was designated as IP40 . The 20 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 fiber misalignment. Dichloromethane (CH2 Cl2 ) was used as a solvent to prepare the solution of PEI (containing 31 wt.%). These plies were immersed individually in the separate containers filled with the viscous solution of PEI for 12 h. The containers were sealed to avoid evaporation of solvent, which was required for adequate wetting of fiber strands with the PEI solution. The plies were taken out carefully to avoid misalignment of the weave and then dried in an oven for 2 h at 100 ◦ C in a stretched condition. These prepregs were then stacked in the mould to attain the desired thickness in the range of 3–4 mm. PTFE coated glass fabric was placed on the top and bottom of the stacked prepregs. The mould was then heated to attain a temperature in the range 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, two intermittent breathings (each of 2 s) were given to expel the possible residual solvent. The composite was allowed to cool under natural conditions
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Table 2 Details of the composition and mechanical properties of fabricated composite based on carbon fabric and PEI Composites
PEI
IP40
Density (g/cm3 ) ASTM D 792 Percentage of fabric (vol./wt.%) 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 (GPa) ASTM 790 Interlaminar shear strength (ILSS) (MPa) ASTM 2344
1.27 – 105 3.0 60 – 150 3.3 –
1.49 40/50 330 54 0.87 3.0 505 29 35
Data on PEI-provided by the supplier.
under the same pressure. It was then cut using a diamond cutter as specified in the standards for mechanical and tribological testing (see Table 2). 2.2. Characterization of the composites The composite was characterized for various properties as per standards (Table 2). The composition was determined using a Soxhlett apparatus and CH2 Cl2 as solvent. The extraction temperature and time were 40 ◦ C and 36 h, respectively. 3. Erosive wear studies The solid particle erosion experiments were carried out as per ASTM G76 on the erosion test rig shown schematically in Fig. 1. The test rig consisted of an air compressor, an air drying unit, a conveyor belt-type particle feeder and an air particle mixing and accelerating chamber. The dried and compressed air was then mixed with the silica sand (106–120 m size) which was fed constantly by a conveyor belt feeder into the mixing chamber. Samples of composite (30 mm × 40 mm × (3–4) mm) were held at selected angles (30◦ , 45◦ , 60◦ and 90◦ ) with respect to the flow of the impinging sand particles and eroded. Similarly, PEI was studied at angles of impingement 15◦ , 30◦ , 45◦ , 60◦ , 75◦
Table 1 Properties of plain weave of carbon fabric evaluated in the laboratory Carbon fabric
Plain weave
Density (kg/m3 ) Area (kg/m2 ) Towa Tex Denier Crimp (%) Count Warp (m) Weft (m) Thickness (m) Bending length (m) Tensile strength (MPa) Elongation (%)
1850 1960 3K 20 185 0.64 28 468.75 468.75 0.0034 0.072 0.3 1.25
a
Supplier’s data.
Fig. 1. Schematic of air jet erosion test rig.
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and 90◦ . The silica sand particles were accelerated by passing through a converging tungsten carbide nozzle of 3 mm diameter to bombard the target. The distance between the target material and the nozzle was approximately 10 mm. The operating parameters were as follows. Dose of erodent Flux rate Impinging velocity of particles Duration of erosion Impact angles
80 g 8 g/min 26.88 m/s 10 min 15◦ , 30◦ , 45◦ , 60◦ , 75◦ and 90◦
Two tests were performed on each sample and an average value calculated. Wear was measured by the weight loss method. Samples were cleaned with petroleum ether (40–60 ◦ C) before and after weighing. Eroded samples were cleaned with a brush to remove fine sand particles attached to the surface and then wiped with a cotton plug dipped in petroleum ether. The wear rate was expressed in terms of Wc (g)/Ws (g); where Wc was the loss in weight of the composite and Ws was the total weight of erodent used (80 g). Wc was determined by weighing the sample before and after the experiment on a weighing balance having an accuracy of 0.1 mg. An incubation period or weight gain was not observed for the neat PEI or its composite. 4. Results and discussion As seen in Table 2, CF reinforcement has improved significantly all the mechanical properties of PEI except elongation to break. The tensile strength (TS) and tensile modulus TM of PEI increased by 3 and 18 times, respectively, while the flexural strength (FS) and flexural modulus (FM) increased by approximately 3.5 and 7 times, respectively. The elongation to break however reduced drastically from 60 to <1%. The erosive wear rates of neat PEI and its composite as a function of angle of impingement are shown in Fig. 2. Figs. 3–7 are micrographs of the eroded surfaces of PEI and its composite.
Fig. 2. Erosion rate (10−5 g/g) vs. angle of impingement of: (a) neat polyetherimide (PEI) and (b) IP40 .
As seen from Fig. 2b, CF reinforcement reduced the wear performance of PEI considerably (at α = 90◦ , the reduction was four times while at 30◦ , it was almost six times). However, since components are made in the composite form due to the requirements of high specific strength and modulus, the use of the composite is inevitable in numerous applications. Hence, efforts are needed to minimize the wear, probably by developing composites with hybrid fibers (CF, Aramid, etc.). It was also observed that the angle of impingement at which the wear rate was lowest (αmin ) for both materials was 90◦ while αmax for PEI and its composite were observed to be 15◦ and 30◦ , respectively, out of the selected angles of impingement. (The composite could not be studied at 15◦ because a sample of the required size was unavailable. Since the shape of both the curves match very well, there is a high probability that αmax for the composite is in the same range as that of neat PEI.) In general, for ductile materials, αmax is in the range of 15–30◦ , and αmin 90◦ , while for brittle materials the behaviour is the opposite [4]. However, in the present study, PEI and its composite have shown behaviour similar to ductile materials, though they are not ductile as can be seen from their properties listed in Table 2. In the literature, there are no fixed trends correlating ductility or brittleness of materials with αmax or αmin . It is found that some polymers erode in a ductile manner; some show evidence of both ductile and brittle characteristics [7,9,11]. Thermoplastics generally exhibit a more ductile response than the thermosets [12]. Pool et al. [13] reported that the maximum erosion rate occurred at normal incidence for the UD and woven graphite reinforced epoxy composites implying a brittle type erosion behaviour. However, woven aramid reinforced epoxy composites exhibited a maximum erosion rate between 34◦ and 45◦ indicating semi-ductile behaviour. Chopped fiber-PPS material showed a maximum erosion rate at 25◦ which indicates ductile behaviour even though the material was not ductile. Thus, though the use of terms, such as failure by ‘ductile’ and ‘brittle’ mechanisms is frequent and useful in understanding erosion of materials, it is not strictly true in all cases. A ductile material can show peak erosion at 90◦ if the failure mechanism is different as seen, for example, in the case of erosion of metals by hard spherical particles, which remove metal by low-cycle plastic fatigue process rather than by cutting [4,15]. In the case of the impact of hard angular particles on brittle materials, plastic indentation takes place along with generation of long cracks extending from the plastic zone. The most important are lateral cracks, which curve back towards the eroding surface. Wherever such multiple cracks converge and intersect, a loose particle is removed as debris. In a brittle homogeneous material, cracks are not stopped until they reach the surface and hence lead to material removal. Material removal is more severe because impact causes crack propagation rather than plastic indentation. Impact at 90◦ leads to the greatest depth of plastic zone and hence a larger volume is bounded by lateral cracks and thus removed leading to maximum wear. On the other hand, ductile materials, especially metals, undergo erosive wear by the formation and detachment of heavily strained surfaces where erodent contacts repeatedly. The material removal process is enhanced at low angles, where cut-
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Fig. 3. Scanning electron micrographs showing erosion features of neat polyetherimide at different angles of impingement: (a) 90◦ , (b) 75◦ , (c) 60◦ , (d) 45◦ , (e) 30◦ and (f) 15◦ .
ting and micro-machining mechanisms are most effective. When impingement takes place, the normal component of the impacting force causes indentation on the surface while the tangential component removes material by lateral displacement after repeated impacts. Impact at 90◦ may deform a large amount of material. It will, however, simply be displaced backwards and
forwards in the plane of surface by repeated impacts. Such a damage process does not lead to effective removal of material [15]. In the case of PEI and its composite, αmax or αmin are at (15–30◦ ) and 90◦ , respectively. Evidence of material removal by indentation was seen at 90◦ and micro-machining at lower angles in SEM studies as discussed in the subsequent section.
Fig. 4. (a) SEM showing erosion features of IP40 at normal incidence (90◦ ) and (b) magnified view.
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Fig. 5. (a) SEM showing erosion features of IP40 at 60◦ and (b) magnified view.
Fig. 6. (a) SEM showing erosion features of IP40 at 45◦ and (b) magnified view.
In the case of fiber reinforced thermoplastic composites (nonductile materials similar to IP40 ), the material removal process may be different. αmax for this composite was not at 90◦ but at 30◦ similar to unfilled PEI, which is a comparatively ductile material. In contrast to bulk homogeneous brittle material, during normal impact (90◦ ), cracks if generated cannot propagate so easily because of the fibers present. During energy dissipation, it causes fracture of a fiber. Crack propagation in the forward and backward directions towards the eroding surface is very difficult since cracks have to cross the ductile matrix between the fibers. The matrix thus restricts the crack growth. Thus, wear of such composites was mainly due to easy fracture of brittle fibers and subsequent removal of fiber debris. Wear of composites was hence was higher than that of neat PEI. The overall wear
of the composites, therefore, was not because of the generation, propagation (to and fro) and intersection of cracks as in case of unfilled brittle material, but because of fiber fracture. At oblique angles (<90◦ ), both the processes, viz. fiber fracture and fiber cutting followed by successive removal of polymer and fiber debris are more dominating and occur easily leading to enhanced wear. Thus, the higher wear of IP40 as compared to PEI was due to excessive fracture of carbon fibers and effective removal of the debris from the surface. It was also clear that with decrease in the angle of impingement, the wear of PEI increased because of the increasing component of micro-cutting and micro-machining processes rather than lateral cracking. This was strongly supported by SEM analysis as discussed in the following section.
Fig. 7. (a) SEM showing erosion features of IP40 at 30◦ and (b) magnified view.
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4.1. SEM analysis of eroded surfaces Scanning electron micrographs of the surfaces of both neat PEI and its composite eroded at different angles of impingement are shown in Figs. 3–7. Micrographs (×1000-magnification, unless specified), are arranged in decreasing order of angle of impingement and increasing order of wear rates. As seen in Fig. 3a for α = 90◦ , the surface of PEI showed unusual features. PEI is not as ductile as metals. Hence, the surface shows mixed damage processes. As discussed for ductile materials, repeated impacts lead to plastic indentation processes and heavily strained regions on the surface. In the case of brittle materials, on other hand, the propagation of curved cracks towards the surface and their intersection to form a wear particle separated from the surface leads to wear. As seen in Fig. 3a, both processes appear to be operative. Initially the surface was strained and displaced backwards and forwards in the plane by repeated impacts. Regions were formed due to simultaneous generation of cracks characteristic of brittle materials. The extent of plastic indentation, however, decreased as the angle of impingement decreased as seen in other micrographs (Fig. 3b–f). When the angle of impingement was changed to 75◦ , the features seen were quite different (Fig. 3b). The normal component of the impact force was still effective in producing plastic indentation creating patches similar to Fig. 3a (marked as 1). The tangential component, on other hand was now operative in cutting action. Most parts of the micrograph show evidence of material flow in the direction of impingement. As seen in Fig. 3c (60◦ ), d (45◦ ) and e (30◦ ), the dominance of plastic indentation reduced with angle, though micro-cracking persisted. Fig. 3f (15◦ ) shows unique features. The entire surface shows the dominance of the micro-machining process, a characteristic failure feature for ductile materials at very low angle. This mechanism was responsible for the highest material removal. In the case of worn surface studies of the composite, however, the most visible dominant features were fracture and cutting of fibers. Failure of the matrix was not clearly observed. Fig. 4a (×500) and b (×1000) for the composite eroded at 90◦ show mainly fiber fracture. Comparatively more fibers appeared damaged but not extensively removed due to normal impact of erodent. Fig. 5a and b show the surface of the composite eroded at 60◦ . Fig. 5a shows the weave of fabric and crossover points. As seen in Fig. 5b, features, such as an increased extent of fiber–matrix de-bonding, fiber breakage and removal of fiber debris can be seen clearly. The tangential component of the impact force (cos θ), increases with decreasing angle of impingement and results not only in material removal (both fiber and polymer debris), but also in an increase in fiber/matrix debonding leading to enhanced wear. Fig. 6a (×500) and b (×1000) are for a surface eroded at 45◦ . The process of fiber damage and pulverization has increased with decrease in angle of impingement leading to higher wear of this composite. Fig. 7a (×500) and b (×1000) are for the surface eroded at 30◦ . The process of fiber damage and pulverization appears to be the most dominant mechanism. The tangential component of the impact force is more effective in micro-cracking followed by micro-cutting of
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fibers in many places leading to severe pulverization of fibers. Fiber debris is clearly seen on the surface (circled on the micrograph) before removal by successive impingement of erodent particles. 5. Conclusions Carbon fabric (40% by volume) reinforced PEI composite fabricated using the impregnation technique exhibited improved mechanical properties compared to neat PEI except for elongation to break. The reinforcement, however, adversely affected the erosion wear resistance of PEI which can be attributed to severe damage to the fibers caused by excessive fiber–matrix de-bonding, fiber fracture, micro-cutting and pulverization by successive impacts of high speed impinging particles. PEI and its composite though neither are ductile materials, exhibited erosion peaks at low angles of impingement (15◦ and 30◦ , respectively), which is generally seen in the case of ductile materials. SEM of eroded surfaces showed microstructural features that were characteristic of both brittle as well as ductile modes of failure. Acknowledgements The authors are grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi India for funding the work reported in this paper. The authors are very thankful to Prof. I.M. Hutchings for extensive discussion on the phenomenon of the correlation between peak angle of impact and failure mechanisms. References [1] C. Soutis, Carbon fiber reinforced plastics in aircraft construction, Mater. Sci Eng. A 412 (2005) 171–176. [2] J. Bijwe, R. Rattan, M. Fahim, Erosive wear of carbon fabric reinforced polyetherimide composites: role of amount of fabric and processing technique, J. Mater. Sci., in press. [3] Rekha Rattan, J. Bijwe, Carbon fabric reinforced polyetherimide composites: influence of weave of fabric and processing parameters on performance properties and erosive wear, Mater. Sci. Eng. 420 (part A) (2006) 342–350. [4] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, E. Arnold (Division of Hodder and Soughton), London, 1992. [5] G.W. Stachowiak, A.W. Batchelor, Engineering Tribology, Tribology Series, vol. 24, Elsevier, Amsterdam, The Netherlands, 1993. [6] M. Roy, B. Vishwanathan, G. Sundararajan, The solid particle erosion of polymer matrix composite, Wear 171 (1994) 149–161. [7] A. Hager, K. Friedrich, Y.A. Dzenis, S.A. Paipetis, Study of erosion wear of advanced polymer composites, in: K. Street, B.C. Whistler (Eds.), Proceeding of ICCM 10, Woodhead Publishing, Cambridge, 1995, pp. 155–162. [8] H.A. Aglan, T.A. Chenock Jr., Erosion damage features of polyimide thermoset composites, SAMPE Q. 24 (2) (1993) 41–47. [9] K.R. Karasek, K.C. Goretta, D.A. Helberg, J.L. Routbort, Erosion in bismaleimide polymers and bismaleimide polymer composites, J. Mater. Sci. Lett. 11 (1992) 1143–1144. [10] P.J. Mathias, W. Wu, K.C. Goretta, J.L. Routbort, D.P. Groppi, K.R. Karasek, Solid particle erosion of a graphite-fiber-reinforced bismaleimide polymer composite, Wear 135 (1989) 161–169. [11] N.-M. Barkoula, J. Karger-Kocsis, Processes and influencing parameters of the solid particle erosion of polymers and their composites, J. Mater. Sci. 37 (2002) 3807–3820.
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