MECHANICAL BEHAVIOR OF METAL PARTICLES REINFORCED POLYMER MATRIX COMPOSITES

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1 MECHANICAL BEHAVIOR OF METAL PARTICLES REINFORCED POLYMER MATRIX COMPOSITES 1 ANSHUL VERMA, 2 V.K. SRIVASTAVA 1 M.Tech Iit (Bhu) Varanasi, 2 Phd Iit (Bhu) Varanasi 1 anshul.verma.iit@gmail.com, 2 vijayks210@gmail.com Abstract- Polymers have long been used as a substitute to various conventional materials. There mechanical properties have also been reconstructed several times, for different purposes, using reinforcing agents. In the present study, the purpose was to investigate the effect of metallic fillers on the mechanical properties of a commercial epoxide resin (PL-411). Copper and aluminium were thus reinforced as filler particles, having size less than 100µm. Tensile strength, compressive strength and vicker s hardness were evaluated and compared. Fillers were varied in their weight percentage (i.e., 1%, 5%, 8%, and 10%) and their behavior in matrix was investigated under scanning electron microscope. Experimental data from tensile tests were compared with Bigg s equations. With the addition of ductile fillers, tensile strength gradually degraded. Both hardness and compressive strength increased with an increase in filler content Index Terms- Aluminium, Compressive strength, Copper, Scanning electron microscopy, Tensile strength, Vicker s hardness. I. INTRODUCTION Composites are classified on the basis of morphology of reinforcement (fiber, particulate reinforced and laminate composites) or on the matrix material (metal, ceramic, polymer matrix composite). In a two phase material, matrix is a polymer and the reinforcement is in the form of dispersed particles which acts as the second phase. A wide range of microstructures can be obtained with combinations of matrix material and reinforcement [1]. The relation between the properties and the structure of two phase materials has been studied in the past decades by many authors [2]. In commercial production, low cost particulate fillers are added to plastics primary reason being economic and improvements in molding characteristics. It is not only the material properties of the two components or the volume fraction of the filler which governs the deformation behavior, but shape, size, orientation and the state of adhesion between the filler and the matrix also governs it [3]. Polymer matrix composites (PMC) can exhibit unique properties with the combination of particles as fillers like thermal conductivities [4], dielectric constants and ductility [5], scratch resistant and modulus [6], toughness and wear properties [7], impact performance [8], compressive strength [9] and many more. Such polymers have also found their practical importance in several novel end applications [10]. In polymer matrix composites, thermoset polymers have been of much greater importance [11]. Epoxy (thermoset) is a widely used polymer in numerous applications ranging from coatings, corrosion protectants, flooring, electrical and electronic instruments, sports equipment s and many more. However its behavior is brittle when compared to metals and possess low thermal conductivity and limited mechanical properties. Reinforcement of fillers with lower aspect ratio can improve some of its mechanical properties [12]. Hussian et al [13] reported that ceramic fillers as reinforcement in epoxy composites results in improved physical, rheological and thermal properties. Youngs modulus and flexural modulus can also be improved. Goyanes et al [14] reported that for aluminium-epoxy composites, density increases with increasing filler content which also follows the rule of mixture. A correlation between the Youngs modulus and its density for filled epoxy composites was also justified by Tilbrook et al [15]. Space between the particles was assumed to be filled with matrix and no air bubbles or voids were present. Under these conditions, the matrix volume is at a minimum for a given system and acts as individual pockets or segments to support a tensile or compressive load [16]. II. TENSILE STRENGTH For a glass bead filled polypropylene system as suggested by Ramsteiner and Theysohn [17] that at higher temperature (about 70ºC) the tensile strength is independent of filler concentration, i.e. the model is valid at lower temperature where the matrix is brittle and the beads are not stress bearing. Fig. 1 Tensile strength of Al and Cu with varying weight 104

2 But however corresponding to a tough matrix state, some stress can be transferred from the matrix into the beads. The simplest model proposed by Bigg [18] in which the tensile strength of particulate filled composites can be described as (1) Where σ m is the tensile strength of matrix, σ c is the tensile strength of composite and V f is the volume fraction. Value of f depends upon the adhesion quality between the matrix and filler such that f = 1.21 means poor adhesion and f =1.1 means better adhesion. For estimating the strength of elastically deforming material which are susceptible to brittle fracture, diametrical compression test is used. A mathematical expression was developed by Hertz [19] to determine the stress states of the elastic disks under diametrical compression under point loading conditions. (2) (3) (4) III. COMPRESSIVE STRENGTH The limit performance of a material to the stress is determined by strength test such as tensile test, diametrical compression test and the simple compression test. Diametrical compression test is sometimes referred to as diametrical tensile test, Brazilian disc test or compact crushing test. It is a mechanical testing technique used from concrete, ceramics, metal composites and many more as per ASTM designation D3967. Where σ x is stress in horizontal direction when specimen loaded in vertical direction, σ y is stress in vertical direction, σ xy is shear stress, P is applied load, R is radius of disk, t is thickness and β 1 2 = (R - y) 2 + x 2, β 2 2 = (R + y) 2 + x 2. When the bonding in particles is enough to prevent the crushing of disk, it is predicted by the Hertz solution that the maximum principle stress occurs at the center of the disk. This stress is assumed to be responsible for the failure of the disk, the tensile and compressive strength thus can be obtained by putting x = y = 0 in (2), (3), and (4) (at the center of the disk) [20]. (5) (6) (7) IV. MATERIALS AND EXPERIMENTATION Fig. 2 Compressive strength of Al and Cu with varying weight 2.1 Metal fillers For this study aluminium (Al) and copper (Cu) powder were selected as the metal fillers. Aluminium powder was obtained from CDH Laboratories New Delhi and Copper powder was obtained from Otto Kemi, Mumbai. Particle size for both the powders was in micrometers but less than 100µm. 2.2 Epoxy resin Epoxy resin contains a three membered oxide ring called as epoxide group. It can be regarded as a compound containing on an average more than one epoxide group per molecule and is polymerized with these epoxide group, using a cross-linking agent to form a tough three-dimensional network. Epoxy PL-411 and hardener PH-861 were used as the matrix material. Fig. 3 Vickers hardness for Al and Cu with varying weight Metal fillers were added into the epoxy resin in various weight percentage, namely 1%, 5%, 8%, and 10%. The mixture was stirred for almost 10 minutes to ensure proper mixing. Hardener was then added to the 105

3 International Journal of Mechanical And Production Engineering, ISSN: , epoxy in the ratio of 1:10 by weight and again mixed thoroughly till the mixture started to cure. Compression test was conducted on compression testing machine (Loading rate of 2 mm per minute). Compression load was obtained from the machine and compression strength was then calculated using (5) as shown in the fig. 2. Hardness was also calculated using Vicker s Hardness Testing Machine as shown in the fig. 3. Finally scanning electron microscopy (SEM) was done on the tensile test samples to study the behavior of the crack formation. Weight fraction of the fillers in the matrix was converted into volume fraction by taking densities as 1 g/cm3 for epoxy resin, 8.96 g/cm3 for Cu particles and 2.7 g/cm3 for Al particles. V. RESULTS AND DISCUSSIONS Fig. 4 Tensile strength of Al at different weight percentage in comparison to Bigg s equation. Fig.1 shows that the tensile strength decreases continuously with increase in the weight percentage of the fillers. Al with a weight percentage of 1% experienced maximum tensile strength of all and then decreased thereafter. For 10% of Cu and Al, the maximum tensile strength was less than that of pure epoxy (85 MPa). At lower weight percentage, when the particles were added to the matrix, they were uniformly distributed and this caused crack deviation as shown in fig. 6 obtained by SEM. Due to pinning action, the stress lines deviated from their actual path and caused the tensile strength to increase. But with further increase in the weight percentage, agglomeration of the filler particles took place and this agglomeration thereafter decreased the strength. Fig. 7 shows the patches of Al formed at 8% weight fraction and fig. 8 shows the agglomeration of Cu particles which diminished the effect of pinning action at 5% weight fraction. Similar work was done by Chang [21] in which a relatively lower tensile strength was achieved with agglomerated powder that easily fractured in a brittle manner. The tensile strength was supposed to follow (1), but the experimental results has shown a Fig. 5. Tensile strength of Al at different weight percentage in comparison to Bigg s equation. The mixture was then poured into different molds to prepare the desired samples. For tensile test, dog-bone shaped samples based on the ASTM standard D638 and for compression test brazalian (circular) disk samples were prepared as per ASTM standard D3967. Tensile test was conducted on Instron Tensile Testing Machine (loading rate of 2 mm per minute). Load and deflection of the tensile tests were obtained from the machine itself and from the load deflection curve, tensile strength at different weight percentages was obtained as shown in the fig. 1. Fig. 7. SEM inage of sample having 8% weight fraction of Al. Fig. 6. SEM inage of sample having 1% weight fraction of Al. 106

4 deviation ranging from 15% to 26% for Al filled epoxy composite and 2% to 10% for Cu filled epoxy composite (as shown in fig. 4 and fig. 5). This was because of the presence of voids in the samples as the formation of voids and air bubbles being practically unavoidable and these voids can act as a third phase of reinforcement. This degradation of tensile strength with increasing filler content showed that the fillers acted as a potential stress raisers. Fig. 2 shows the variation of compressive strength as a function of filler weight It can be noticed that the trend observed for compressive strength was quite different from those observed for tensile strength (fig. 1). Epoxy matrix being brittle in nature has good compressive properties. In the case of Cu, it could be assumed that the brittleness was retained by the matrix when weight percentage was 1%. Initial compressive strength was thus high for Cu particles with 1% by weight. But it decreased when the weight percentage was increased to 5%. The ductility of Cu particles caused the overall compressive strength to decrease. Because of the agglomeration of copper particles, matrix again gained its brittleness with increasing weight percentage of Cu to 10%. For Al particles, the initial compressive strength was low but increased with hardness increased with the increase in filler weight percentage but further increment ceased at 8% Cu by weight, beyond which it decreased. Lam et al [23] in their investigation found a similar trend in the hardness values in which the hardness increases up to a certain filler content. CONCLUSION Several important conclusions were inferred for Al and Cu particle reinforced epoxy matrix composites from the above discussion in regard to their mechanical properties. Tensile strength of the particulate composites degraded with the increase in filler content. Al reinforced epoxy showed better tensile properties, in comparison to Cu reinforced epoxy, with a maximum tensile strength of MPa (23% greater than neat epoxy) at a weight percentage of 1%. With increasing filler content, though Al filled epoxy gained compressive strength, Cu filled epoxy on the other hand showed the best results throughout with a maximum at 10% by weight. There has been a potential increase in the hardness of Cu filled epoxy composites with increasing filler content, with a maximum hardness at 8% weight fraction. Though the hardness then decreased for Cu filled epoxy composites, but showed better results than Al filled epoxy composites. REFERENCES Fig. 8. SEM inage of sample having 5% weight fraction of Cu. increase in its weight percentage because of the agglomeration of Al particles in the matrix. This increase in compressive strength was estimated as the favorable deformation process facilitated by the presence of fillers in the matrix. Thus under compressive loading, fillers aided the load bearing capability of the composite, rather than being a stress raiser as was in the case of tensile loading. Similar results were shown by Singha [22] in which the compressive strength was increased with the particle reinforcement. The vicker s hardness obtained from the experiment is shown in fig. 3. Hardness values for Al filled epoxy remained close to 21 Kgf/mm 2. For Cu, [1] M. F. Ashby, Criteria for selecting the components of composites, Acta Metallurgica et Materialia, vol. 41, no. 5, pp , May [2] J. A. Manson and L. H. Sperling, Polymer Blends and Composites, New York Plenum Press, Ch. 12, [3] S. Y. Fu, X. Q. Feng, B. Lauke and Y. W. Mai, Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate-polymer composites, Composites Part B: Engineering, vol. 39, no. 6, pp , Sep [4] A. S. Luyt, J. A. Molefi and H. Krump, Thermal, mechanical and electrical properties of copper powder filled low-density and linear-low density polyethylene composites, Polymer Degradation and Stability, vol. 91, no. 7, pp , Jul [5] Y. Xu, D. D. L. Chung and C. Mroz, Thermally conducting aluminum nitride polymer matrix composites, Composites Part A: Applied Science and Manufacturing, vol. 32, no. 12, pp , Dec [6] C. B. Ng, B. J. Ash and L. S. Schadler, A study of the mechanical and permeability properties of nano- and macron-tio2 filled epoxy composites, Advanced Composites Letters, vol.10, pp , [7] N. Singh, N. Ghildiyal, R. Khelawan, R. S. Singh, P. S. Venkataramani, G. N. Mathur and V. Shankar, Development of epoxy-metal particulate composite, In: Proceedings of the Second International Conference on Advances in Composites, ADCOMP, IISc Bangalore, vol. 96, pp , Dec [8] H. S. Kim and M. A. Khamis, Fracture and impact behaviors of hollow micro-sphere/epoxy resin composites, Composites Part A: Applied Science and Manufacturing, vol. 32, no. 9, pp , Sep

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