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Effect of SiC particles on the mechanical properties of glass-phenolic composites
Materials S(ience and Engineering, A 136 ( 1991 ) 179-182
179
Effect of SiC particles on the mechanical properties of glass-phenolic
composites D. Sarath Chandra, G. S. Avadhani and Kishore Department of Metallurgy, Indian Institute of Science, Bangalore 560012 (India) (Received April 30, 1990; in revised form September 6, 1990)
Abstract Glass-phenolic composites~ filled with SiC particles in the range 1-4 vol.% of resin, were prepared and flexure, interlaminar shear strength and impact tests were done to evaluate the mechanical properties. In all the tests, a minimum in values was recorded, following which a rising trend was observed. Scanning microscopy examination was also done to analyse the fracture features of failed samples.
1. Introduction Polymer composites are considered as futuristic materials as they play an important role in aviation and aerospace industries and are light and easy to fabricate. However, these materials exhibit insufficient toughness, which was overcome to some extent by additions of rubber [1], glass [2] and hard particles [3]. The yield stress of such a class of materials showed a rise if there is a good bonding between the filler and the resin [ 3 ] . Poor bonding between the two has been reported to result in a drastic fall in strength values for increasing filler content [4]. Epoxies and polyesters were among the widely studied resins, for modification purposes, owing to their superior compatibility with many of the reinforcements. Phenolics did not attract the same attention from investigators as their compatibility with reinforcements is inferior and also because of extensive void formation [5] in the resin matrix during curing due to the release of water vapour. Phenolics, however, which have inferior mechanical properties to epoxies and polyesters, find very good applications in high temperature areas as ablatives. On the contrary, the employability of epoxies and polyesters in thermal fields is highly restricted because of the release of harmful gas at high temperatures, causing asphyxiation and choking, and because they are highly inflammable [5]. The fillers tried with phenolics were additions 0921-5093/91/$3,50
involving sawdust, paper and asbestos [6] which gave slight improvement in strength values. It has been reported that filled resins attract more water vapour than unfilled resins owing to stress concentration at the interface [7] and high specific surface area of filler. So it would be very interesting first to study the filled phenolics further reinforced with glass, secondly to determine the role played, if any, by the fillers in interacting with the water vapour and finally to investigate the void formation and content of the matrix and attendant changes in mechanical properties [8]. With these objectives the glass-phenolic systern has been chosen with a rough surface containing SiC particles as fillers; the strength values were evaluated as a function of filler content and further the fracture features studied using fractographic techniques.
2. Experimental techniques E-glass of 7 × 10 -3 in thickness, plain weave and having a density of 2.75 g cm 3 formed the reinforcement material. The resin employed was the hot-curing grade R-88084 phenolic resin (supplied by Bakelite Hylam, Hyderabad). No hardener was used as the resin was self-setting type. Irregularly shaped SiC particles with a rough surface and a size range 5-25 ~m constituted the filler material. Laminates of thickness 3.2 mm were prepared by hand from the cloth pieces, 18 © Elsevier Sequoia/Printed in The Netherlands
180
in all, measuring 225 mm x 225 mm. After application of the resin these were clamped in iron plates and cured at 160 °C for 2 h followed by cooling in the oven. The amount of resin and glass cloth had a ratio of 1:1 by weight. SiC particles were introduced into the resin in the range 1-4 vol.% of resin. Care was taken to avoid agglomeration of particles,
2.1. Sample preparation Flexure samples had an l/d ratio of 16 in one case and 20 in the other (where l is the span length and d the thickness of the specimen). The former had dimensions of 60 mm x 25 mm x 3.2 mm, while the latter measured 80 mm x 25 mm x 3.2 mm. The cross-head velocity adopted was 0.025 mm s- ~. The loading set-up had drums with a diameter of 1 in as per ASTM standard specifications [9]. Unnotched impact samples had dimensions of 55 mm x 12.6 mm x 3.2 ram. The testing was done on an Avery drop-weight testing machine and the strike velocity was 2.44 m s-t The interlaminar shear strength (ILSS) test was conducted by the three-point bend test method with an l/d ratio of 5. Samples had dimensions of 15 m m x l 0 m m x 3 . 2 mm. The cross-head velocity was 0.021 mm s- ~ and the machine used was the servohydraulic Instron-8032 type.
posites exceeded that of unfilled composites. For l/d -= 20 also, the flexure strength decreases, but less drastically. A decrease is seen until around 2% after which it started to show a rising trend. For both I/d ratios employed, the role of SiC seems to be quite critical around 3%-4%. Like the above test results, a fall in values in the ILSS test data was also recorded until about 1%, after which an increase was observed. For large additions the values tend to saturate (Fig. 2). When the particle-filled samples were subjected to an impact test (Izod), the impact energy as in the previous mechanical data shows first a decrease and then an increase. A good saturation trend is clearly discernible from an inspection of Fig. 3. Scanning electron microscopy (SEM) examination was done to analyse the fractured samples. Many voids which tended to be present in preferred areas in the matrix of unfilled samples could be seen (Fig. 4). In the samples containing 1% SiC particles, voids occurred much less fiequently (Fig. 5). They were conspicuous by their
18
3. Results Figure 1 shows the variation in flexure strength with additions of SiC. As emphasized earlier, flexure tests were done for lid ratios of 16 and 20. For lid = 16 there is a fall in flexure strength until about 2%, following which the values increase. It is clear that, for the larger amount of fillers only, the flexure strength of filled corn-
~
14©
,
~ 2 4 O~os,c ~o,u~e Fig. 2. ILSSvalues vs. percentageof SiC additions.
32C [
~~_~" ~2~~4o:~s,o--ol ~ :16: /~ / 0
I 2,0 °/o SiC volume
4,0C
_'~~3.~'6~E~ ,~ c~z °
I 4.0
Fig. 1. Variation in flexure strength in composites with SiC additions.
i 0
I 2.0
f 4.Q
v. s,c volume Fig. 3. Variation in impact energy (Izod) with SiC additions.
181
Fig. 4. Void formation in unfilled composite. l~ig. 6. SEM fractograph of a 4% SiC-filled composite.
Fig. 5. Occurrence of voids in 1% SiC-filled composite.
near absence in samples containing 4% SiC fillers, as seen in Fig. 6. Larger particles of SiC which tended to settle at the resin-fibre interface is evident in Fig. 7. 4. Discussion From the data in Figs. 4-6, it is clear that as the percentage of particles increases, there is a drop in void content. Generally phenolics release water vapour during curing, which leads to void formation. In the presence of fillers, one or more of the following situations may prevail. Thus there is a likelihood that this vapour will be adsorbed by the particles or will settle at the matrix-filler interface, owing to stress concentration [7] and high specific surface area, or will reach the resin-fibre interface through the resin-filler
Fig. 7. Regions of decohesion seen in the resin fibre area which arose because SiC particles settled at such an interface.
interface if the filler is nearer to a fibre, or that the voids themselves will be filled up by the particles, causing a decrease in the amount of void content. This decrease in void content might help to improve the mechanical properties [8], particularly the flexure strength and ILSS at higher percentages of fillers. The decrease in mechanical properties with initial additives of filler material can be traced to the inability of the particles to adsorb the released water vapour fully because of the smaller content of fillers. The above data make one infer that, for a gain in mechanical properties of filled composites over unfilled composites, a critical amount of filler has to be exceeded, above which the positive role of filler can be dominant. At this juncture the
182
work of Kalnin and Turner [10] on the effect of lithium aluminium silicate fillers on the mechanical properties of mixture of bisphenol A and glycidyl methacrylate (BISGMA) and triethylene glycol dimethacrylate may be mentioned; these workers found that a minimum in yield stress, hardness and modulus occurred at around 3%-5% filler additions. The fact that, in the present investigation, a minimum in properties was obtained at around 1%-2% can be attributed to the additional presence of glass reinforcement, However, the commonality of the behavioural trend of the composite in the above-mentioned work with the present investigation is heartening to note. Further the work on a pure epoxy matrix containing SiC particles and having no reinforcement of glass, yielded a 25% rise in modulus values for about 10% filler additions [11], whereas in the present case in a matrix of phenolic resin filled with SiC and further reinforced with glass the flexure values recorded a similar trend for more than 3% of additions. When the fillers tend to preferentially settle at the resin-fibre interface, as seen in Fig. 7, they add one more dimension to the complex situation prevailing in the region between the resin and the fibre. The present results reveal that the changes in the values of impact energies are not very dependent on the filler additions unlike the flexure and ILSS test values. The impact test, which is high strain rate test, depends more on parameters such as the resin-fibre systems, the orientations of plies, and the presence of notches. On the contrary, flexure and ILSS test values come under the domain of slow strain rate tests where shear deformations are important and hence depend on the fillers added since they do alter the resin-fibre interface and interply bonding considerably even if present in small amounts [12]. 5. Concluding remarks The present study deals with the effect of SiC particle fillers on the mechanical properties and
fractographic features of glass-phenolic composites. Initially the inclusion of fillers yielded lower values for the ILSS, impact and flexure tests. However, after a minimum and for higher contents of the fillers the properties showed a rising trend. This increase has been attributed to the decrease in void formation tendency. Moisture adsorption on the surface of SiC particles has been suggested as a possible reason for the decrease in the void content. It is noticed that larger SiC particles show a tendency to settle at the glass-phenolic interface. Further studies such as the effect of the particle size, distribution and quantity, the modulus of the filler used and the cure cycle on the mechanical properties and fractographic features need to be done to assess the overall utility of such filler materials in a phenolic matrix.
References 1 A. J. Kinloch and R. J. Young, Fracture Behaviour of Polymers, Applied Science Publishers, London, 1983, Chapter 11. 2 J. Spandoukis and R. J. Young, J. Mater. Sci., 19 (1984) 473. 3 A. C. Moloney, H. H. Kausch and H. R. Stieger, J. Mater. Sci.,18(1983)208. 4 J. Spandoukis and R. J. Young, J. Mater. Sci., 19 (1984)
487. 5 s. M. Tavakoli, R. A. Palfrey and M. G. Phillips, Composites, 20(2)(1989)159. 6 A. A. K. Whitehouse, E. G. K. Pritchett and G. Barnett, Phenolic Resins, Plastic Institute, London, 1967. 7 V.F. Janasand R. L. McCullough, Compos. Sci. Technol., 29(1987) 293. 8 L. J. Broutman and R. H. Krock, Modern Composite Materials, Addison-Wesley, Reading, MA, 1967, Chapter 2.5. 9 ASTM Stand. D 790 M, 1984, in Annual Book of ASTM Standards, ASTM, Philadelphia, PA, 1987. 10 M. Kalnin and D. T. Turner, J. Mater. Sci. Lett., 4 (1985) 1479. 11 A. C. Moloney, H. H. Kausch, T. Kaiser and H. R. Beer, J. Mater. Sci., 22 (1987) 381. 12 D. Hull, An Introduction to Composite Materials, Cam-
bridge Solid State Science Series, Cambridge University
Press, Cambridge, 1981, Chapter 3.
179
Effect of SiC particles on the mechanical properties of glass-phenolic
composites D. Sarath Chandra, G. S. Avadhani and Kishore Department of Metallurgy, Indian Institute of Science, Bangalore 560012 (India) (Received April 30, 1990; in revised form September 6, 1990)
Abstract Glass-phenolic composites~ filled with SiC particles in the range 1-4 vol.% of resin, were prepared and flexure, interlaminar shear strength and impact tests were done to evaluate the mechanical properties. In all the tests, a minimum in values was recorded, following which a rising trend was observed. Scanning microscopy examination was also done to analyse the fracture features of failed samples.
1. Introduction Polymer composites are considered as futuristic materials as they play an important role in aviation and aerospace industries and are light and easy to fabricate. However, these materials exhibit insufficient toughness, which was overcome to some extent by additions of rubber [1], glass [2] and hard particles [3]. The yield stress of such a class of materials showed a rise if there is a good bonding between the filler and the resin [ 3 ] . Poor bonding between the two has been reported to result in a drastic fall in strength values for increasing filler content [4]. Epoxies and polyesters were among the widely studied resins, for modification purposes, owing to their superior compatibility with many of the reinforcements. Phenolics did not attract the same attention from investigators as their compatibility with reinforcements is inferior and also because of extensive void formation [5] in the resin matrix during curing due to the release of water vapour. Phenolics, however, which have inferior mechanical properties to epoxies and polyesters, find very good applications in high temperature areas as ablatives. On the contrary, the employability of epoxies and polyesters in thermal fields is highly restricted because of the release of harmful gas at high temperatures, causing asphyxiation and choking, and because they are highly inflammable [5]. The fillers tried with phenolics were additions 0921-5093/91/$3,50
involving sawdust, paper and asbestos [6] which gave slight improvement in strength values. It has been reported that filled resins attract more water vapour than unfilled resins owing to stress concentration at the interface [7] and high specific surface area of filler. So it would be very interesting first to study the filled phenolics further reinforced with glass, secondly to determine the role played, if any, by the fillers in interacting with the water vapour and finally to investigate the void formation and content of the matrix and attendant changes in mechanical properties [8]. With these objectives the glass-phenolic systern has been chosen with a rough surface containing SiC particles as fillers; the strength values were evaluated as a function of filler content and further the fracture features studied using fractographic techniques.
2. Experimental techniques E-glass of 7 × 10 -3 in thickness, plain weave and having a density of 2.75 g cm 3 formed the reinforcement material. The resin employed was the hot-curing grade R-88084 phenolic resin (supplied by Bakelite Hylam, Hyderabad). No hardener was used as the resin was self-setting type. Irregularly shaped SiC particles with a rough surface and a size range 5-25 ~m constituted the filler material. Laminates of thickness 3.2 mm were prepared by hand from the cloth pieces, 18 © Elsevier Sequoia/Printed in The Netherlands
180
in all, measuring 225 mm x 225 mm. After application of the resin these were clamped in iron plates and cured at 160 °C for 2 h followed by cooling in the oven. The amount of resin and glass cloth had a ratio of 1:1 by weight. SiC particles were introduced into the resin in the range 1-4 vol.% of resin. Care was taken to avoid agglomeration of particles,
2.1. Sample preparation Flexure samples had an l/d ratio of 16 in one case and 20 in the other (where l is the span length and d the thickness of the specimen). The former had dimensions of 60 mm x 25 mm x 3.2 mm, while the latter measured 80 mm x 25 mm x 3.2 mm. The cross-head velocity adopted was 0.025 mm s- ~. The loading set-up had drums with a diameter of 1 in as per ASTM standard specifications [9]. Unnotched impact samples had dimensions of 55 mm x 12.6 mm x 3.2 ram. The testing was done on an Avery drop-weight testing machine and the strike velocity was 2.44 m s-t The interlaminar shear strength (ILSS) test was conducted by the three-point bend test method with an l/d ratio of 5. Samples had dimensions of 15 m m x l 0 m m x 3 . 2 mm. The cross-head velocity was 0.021 mm s- ~ and the machine used was the servohydraulic Instron-8032 type.
posites exceeded that of unfilled composites. For l/d -= 20 also, the flexure strength decreases, but less drastically. A decrease is seen until around 2% after which it started to show a rising trend. For both I/d ratios employed, the role of SiC seems to be quite critical around 3%-4%. Like the above test results, a fall in values in the ILSS test data was also recorded until about 1%, after which an increase was observed. For large additions the values tend to saturate (Fig. 2). When the particle-filled samples were subjected to an impact test (Izod), the impact energy as in the previous mechanical data shows first a decrease and then an increase. A good saturation trend is clearly discernible from an inspection of Fig. 3. Scanning electron microscopy (SEM) examination was done to analyse the fractured samples. Many voids which tended to be present in preferred areas in the matrix of unfilled samples could be seen (Fig. 4). In the samples containing 1% SiC particles, voids occurred much less fiequently (Fig. 5). They were conspicuous by their
18
3. Results Figure 1 shows the variation in flexure strength with additions of SiC. As emphasized earlier, flexure tests were done for lid ratios of 16 and 20. For lid = 16 there is a fall in flexure strength until about 2%, following which the values increase. It is clear that, for the larger amount of fillers only, the flexure strength of filled corn-
~
14©
,
~ 2 4 O~os,c ~o,u~e Fig. 2. ILSSvalues vs. percentageof SiC additions.
32C [
~~_~" ~2~~4o:~s,o--ol ~ :16: /~ / 0
I 2,0 °/o SiC volume
4,0C
_'~~3.~'6~E~ ,~ c~z °
I 4.0
Fig. 1. Variation in flexure strength in composites with SiC additions.
i 0
I 2.0
f 4.Q
v. s,c volume Fig. 3. Variation in impact energy (Izod) with SiC additions.
181
Fig. 4. Void formation in unfilled composite. l~ig. 6. SEM fractograph of a 4% SiC-filled composite.
Fig. 5. Occurrence of voids in 1% SiC-filled composite.
near absence in samples containing 4% SiC fillers, as seen in Fig. 6. Larger particles of SiC which tended to settle at the resin-fibre interface is evident in Fig. 7. 4. Discussion From the data in Figs. 4-6, it is clear that as the percentage of particles increases, there is a drop in void content. Generally phenolics release water vapour during curing, which leads to void formation. In the presence of fillers, one or more of the following situations may prevail. Thus there is a likelihood that this vapour will be adsorbed by the particles or will settle at the matrix-filler interface, owing to stress concentration [7] and high specific surface area, or will reach the resin-fibre interface through the resin-filler
Fig. 7. Regions of decohesion seen in the resin fibre area which arose because SiC particles settled at such an interface.
interface if the filler is nearer to a fibre, or that the voids themselves will be filled up by the particles, causing a decrease in the amount of void content. This decrease in void content might help to improve the mechanical properties [8], particularly the flexure strength and ILSS at higher percentages of fillers. The decrease in mechanical properties with initial additives of filler material can be traced to the inability of the particles to adsorb the released water vapour fully because of the smaller content of fillers. The above data make one infer that, for a gain in mechanical properties of filled composites over unfilled composites, a critical amount of filler has to be exceeded, above which the positive role of filler can be dominant. At this juncture the
182
work of Kalnin and Turner [10] on the effect of lithium aluminium silicate fillers on the mechanical properties of mixture of bisphenol A and glycidyl methacrylate (BISGMA) and triethylene glycol dimethacrylate may be mentioned; these workers found that a minimum in yield stress, hardness and modulus occurred at around 3%-5% filler additions. The fact that, in the present investigation, a minimum in properties was obtained at around 1%-2% can be attributed to the additional presence of glass reinforcement, However, the commonality of the behavioural trend of the composite in the above-mentioned work with the present investigation is heartening to note. Further the work on a pure epoxy matrix containing SiC particles and having no reinforcement of glass, yielded a 25% rise in modulus values for about 10% filler additions [11], whereas in the present case in a matrix of phenolic resin filled with SiC and further reinforced with glass the flexure values recorded a similar trend for more than 3% of additions. When the fillers tend to preferentially settle at the resin-fibre interface, as seen in Fig. 7, they add one more dimension to the complex situation prevailing in the region between the resin and the fibre. The present results reveal that the changes in the values of impact energies are not very dependent on the filler additions unlike the flexure and ILSS test values. The impact test, which is high strain rate test, depends more on parameters such as the resin-fibre systems, the orientations of plies, and the presence of notches. On the contrary, flexure and ILSS test values come under the domain of slow strain rate tests where shear deformations are important and hence depend on the fillers added since they do alter the resin-fibre interface and interply bonding considerably even if present in small amounts [12]. 5. Concluding remarks The present study deals with the effect of SiC particle fillers on the mechanical properties and
fractographic features of glass-phenolic composites. Initially the inclusion of fillers yielded lower values for the ILSS, impact and flexure tests. However, after a minimum and for higher contents of the fillers the properties showed a rising trend. This increase has been attributed to the decrease in void formation tendency. Moisture adsorption on the surface of SiC particles has been suggested as a possible reason for the decrease in the void content. It is noticed that larger SiC particles show a tendency to settle at the glass-phenolic interface. Further studies such as the effect of the particle size, distribution and quantity, the modulus of the filler used and the cure cycle on the mechanical properties and fractographic features need to be done to assess the overall utility of such filler materials in a phenolic matrix.
References 1 A. J. Kinloch and R. J. Young, Fracture Behaviour of Polymers, Applied Science Publishers, London, 1983, Chapter 11. 2 J. Spandoukis and R. J. Young, J. Mater. Sci., 19 (1984) 473. 3 A. C. Moloney, H. H. Kausch and H. R. Stieger, J. Mater. Sci.,18(1983)208. 4 J. Spandoukis and R. J. Young, J. Mater. Sci., 19 (1984)
487. 5 s. M. Tavakoli, R. A. Palfrey and M. G. Phillips, Composites, 20(2)(1989)159. 6 A. A. K. Whitehouse, E. G. K. Pritchett and G. Barnett, Phenolic Resins, Plastic Institute, London, 1967. 7 V.F. Janasand R. L. McCullough, Compos. Sci. Technol., 29(1987) 293. 8 L. J. Broutman and R. H. Krock, Modern Composite Materials, Addison-Wesley, Reading, MA, 1967, Chapter 2.5. 9 ASTM Stand. D 790 M, 1984, in Annual Book of ASTM Standards, ASTM, Philadelphia, PA, 1987. 10 M. Kalnin and D. T. Turner, J. Mater. Sci. Lett., 4 (1985) 1479. 11 A. C. Moloney, H. H. Kausch, T. Kaiser and H. R. Beer, J. Mater. Sci., 22 (1987) 381. 12 D. Hull, An Introduction to Composite Materials, Cam-
bridge Solid State Science Series, Cambridge University
Press, Cambridge, 1981, Chapter 3.