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Composites: Part B 43 (2012) 1743 1748 Contents lists available at SciVerse ScienceDirect Composites: Part B journal homepage: www.elsevier.

1 Composites: Part B 43 (2012) Contents lists available at SciVerse ScienceDirect Composites: Part B journal homepage: Enhancement of mechanical properties of aluminium/epoxy composites with silane functionalization of aluminium powder H.J. Kim a, D.H. Jung a, I.H. Jung b, J.I. Cifuentes b, K.Y. Rhee b,, D. Hui b,c a Ocean Development System Laboratory, Korea Research Institute of Ships and Ocean Engineering, Taejon, South Korea b Department of Mechanical Engineering, Kyunghee University, Yongin, Republic of Korea c Department of Mechanical Engineering, University of New Orleans, LA 70148, United States article info abstract Article history: Received 8 August 2011 Accepted 28 December 2011 Available online 6 January 2012 Keywords: A. Polymer-matrix composites B. Fracture toughness B. Wear D. Mechanical testing In this study, the effect of surface modification of aluminum powders on the tensile, fracture, and tribological behaviors of aluminum/epoxy composites was investigated. Aluminum powders were surfacemodified with 3-aminopropyltriethoxysilane. Aluminium/epoxy composites were fabricated by cast molding method using 10 wt.% and silane-treated aluminium powders. Tensile, mode I fracture, and tribological tests were performed on both composites. The results showed that the tensile modulus and strength of silane-treated aluminum/epoxy composites were 9% and 12% greater, respectively than those of The results also showed that the fracture toughness and wear resistance of silane-treated aluminum/epoxy composites were 32% and 56% greater than that of Scanning electron microscope (SEM) examination showed the improvement of tensile and fracture properties of silane-treated aluminum/epoxy composites was attributed to the improved dispersion and bonding of aluminum particles in the epoxy, due to the silanization of aluminium powders. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Aluminum/polymer composites are attractive materials for a wide range of industrial applications due to the combination of properties such as low density, corrosion resistance, thermal stability, and ease of fabrication. Accordingly, many studies have been made to investigate the mechanical, thermal, and electrical properties of aluminum powder reinforced polymer composites [1 11]. One of the main issues in the fabrication of aluminum/polymer composites is to achieve the good dispersion and interfacial strength between aluminium powders and the polymer. To address this issue, some studies have been performed for the surface treatment of aluminum powders. Jallo et al. [12] reported the effect of surface modification on aluminium powder on its flowability, combustion and reactivity. Silanization provides lower surface energy values and such materials are expected to flow better after surface treatment. Chen et al. [13] investigated modification of aluminium powders applying methyltrichlorosilane. The basic procedure involved mixing of aluminium powder with silane solution in hexane. Crouse et al. [14] demonstrated functionalization of oxide-passivated aluminium nanoparticles using 3-methacryloxypropyltrimethosilane (MPS) to provide chemical compatibility within various solvent and Corresponding author. Tel.: ; fax: address: rheeky@khu.ac.kr (K.Y. Rhee). polymeric systems. However, not much work has been made to investigate the surface modification of aluminum powder to improve fracture and tribological properties of aluminum reinforced epoxy composites. In this study, the effect of silane treatment of aluminum powders on the tensile, fracture, and tribological behaviors of aluminum/ epoxy composites was investigated. Aluminium/epoxy composites were fabricated by cast molding method using 10 wt.% unmodified and silanized aluminium powders. FTIR analysis was made on the and silanized aluminium powders to determine the chemical change on the surface of aluminum powders due to the silane treatment. Tensile, mode I fracture, tribological tests were performed on both composites. After fracture and tribological tests, scanning electron microscopy (SEM) examination was made to determine fracture and wear mechanisms of aluminum/epoxy composites, depending on the silane treatment of aluminum powders. 2. Experimental 2.1. Materials The reinforcing material used in this study was aluminium powders of 45 lm size (Strem Chemicals, USA). The epoxy used was diglycidil ether or Bisphenol A (YD-115, Kukdo Chemical, Korea), and the curing agent was polyamidoamine (G-A0533, Kukdo /$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi: /j.compositesb

2 1744 H.J. Kim et al. / Composites: Part B 43 (2012) Chemicals, Korea). 3-Aminopropyiltriethoxysilane with a purity of 99% (Aldrich, USA) was used as a silane functionalization agent. (12 mm in diameter). At least, three specimens of each case were tested to ensure reliability of tests results Silanization of aluminium powders 60 g of unmodified aluminium powders were dispersed in 250 ml of ethanol via mechanical stirring for 30 min. Then the reaction was conducted with 3-aminopropyltrietoxysilane and the solution was stirred at 60 C for 6 h. After completion of reaction, the aluminum powders were purified by repeated washing with distilled water followed by acetone. The resulting silanized aluminium powders were separated by filtration and dried under vacuum oven at 80 C for 14 h Fabrication of aluminum/epoxy composites Untreated aluminum powders (U-aluminum) and silane-treated aluminum powders (S-aluminum) were mixed with the ethanol at 10 wt.%. Ultrasonication was performed for 5 min for better dispersion. After sonication, the U-aluminum and S-aluminum were mixed with the epoxy resin and then stirred for approximately 12 h at 80 C to completely evaporate the ethanol. After the ethanol was removed, hardener was added (at an epoxy to hardener ratio of 2:1) and then poured into a Teflon mold with the cast molding method. The mixture was de-gassed in a vacuum oven for 30 min and cured at 100 C for 15 h. Then, then it was post cured for 2 h at 125 C. The fabricated composites plates were machined as tensile, fracture, and wear specimens. Thickness and gage length of the tensile specimens were 5 mm and 80 mm, respectively. Single edge notched specimens (SENT) of 4 mm thick 90 mm long 18 mm wide with a 6 mm initial crack were prepared for fracture tests. Disk type specimens of 30 mm and height of 8 mm were prepared for wear tests Characterization FTIR spectral analysis was made on the and silanetreated aluminum powders, and SEM examination was made to study fracture surface morphology. Tensile and fracture tests were performed using a universal testing machine at a cross-head speed of 0.5 mm/min and 0.2 mm/min, respectively. Fracture tests were performed at room temperature according to the ASTM E procedure [15]. The tribological test was performed by applying sliding speed of 0.12 m/s, sliding distance of m, load of 29.5 N. The counter-wear material used was zirconia ball 3. Results and discussion FTIR spectral analysis was made to determine the chemical change on the surface of aluminum powder due to the silane treatment. Fig. 1 shows the FTIR spectra of and silane-treated aluminium powders. For aluminium powder, the peaks at 3440 cm 1 (Fig. 1a) and 1048 cm 1 (Fig. 1b) were caused by hydroxyl groups (Al OH) on the surface of aluminium powder due to the presence of atmospheric moisture on the surface of aluminum powder. For the silane-treated aluminum powder, the characteristic absorption peak at 1070 cm 1 (Fig. 1b) is attributed to the formation of networked system of (Si O AL)n groups following extensive cross-linking through silanization of Al OH group. This confirms the reaction of 3-aminopropyltriethoxysilane organic coating film on aluminium powder surface. Fig. 2 shows the typical tensile stress versus strain curves of and silane-treated As shown in the figure, the tensile stress increased linearly with strain at an early stage and nonlinear behavior occurred for both composites. The figure also shows that the tensile strength and elastic modulus of aluminum/epoxy composites were affected by the silane treatment of aluminum powders. Table 1 shows the comparison of tensile strength between and silane-treated The tensile strength of was determined by measuring the maximum stress in the stress strain curve. As shown in the figure, the tensile strength of silane-treated aluminum/epoxy composites is 12% higher than that of Specifically, the average tensile strengths of and silane-treated aluminum/epoxy composites were 45.1 MPa and 50.5 MPa, respectively. The table also shows the comparison of elastic modulus between and silane-treated The elastic modulus was determined by measuring the slope in the beginning linear region of the stress strain curve. As shown in the figure, the elastic modulus of silane-treated aluminum/ epoxy composites is 9% greater than that of aluminum/epoxy composites. Specifically, the average tensile moduli of and silane-treated aluminum/epoxy composites were 2.1 GPa and 2.3 GPa, respectively. Fig. 3 shows typical load displacement curves of both composites. As shown in the figure, both composites exhibited brittle fracture behavior. That is, the load increased almost linearly with displacement up to fracture. The figure also shows that fracture Fig. 1. FTIR spectra of and silane-treated aluminium powders.

3 H.J. Kim et al. / Composites: Part B 43 (2012) tensile stress (MPa) displacement of silane-treated of aluminum/epoxy composites is greater than that of aluminum/epoxy composites, in which the fracture displacement are defined as the displacement at which fractured occurred. It can be also seen in the figure that similar to tensile strength results, the fracture load of the silanetreated of aluminum/epoxy composites is greater than that of The effect of silane treatment on the fracture toughness of aluminum/epoxy composites was investigated by comparing the fracture toughness of silanetreated of aluminum/epoxy composites with that of aluminum/epoxy composites. Generally, fracture toughness becomes equal to the critical stress intensity factor when the material exhibits a linear elastic fracture behavior. In this study, the fracture toughness, K IC was determined as follows [15]: K IC ¼ Y Pcr pffiffiffi a BW tensile strain (mm/mm) Fig. 2. Typical tensile stress versus strain curves of and silane-treated Table 1 Comparison of tensile strength and elastic modulus between and silanetreated Tensile strength (MPa) Elastic modulus (GPa) Untreated 45.1 ± ± 0.2 Silane treated 50.5 ± ± 0.2 fracture load (N) displacement (mm) Fig. 3. Typical load displacement curves of and silane-treated aluminum/epoxy composites. where P cr is the fracture load, B is the thickness of the specimen, W is the width of the specimen, a is the crack length, and Y is the shape ð1þ fracture toughness, K IC [MPa m 1/2 ] Fig. 4. Comparison of fracture toughness between and silane-treated function, which is a function of the shape of specimen. For the present study, the shape function was determined by: Y ¼ 1:99 0:41 a W þ 18:7 a ðwþ 2 38:48ða=WÞ3 þ 53:85 a ðwþ 4 Fig. 4 shows the comparison of fracture toughness between and silane-treated As shown in the figure, the fracture toughness of aluminum/epoxy composites was improved by the silane treatment of aluminum powder. Specifically, the average fracture toughness of silane-treated aluminum/epoxy composites was 32% greater than that of FESEM analysis was performed on the fracture surfaces of the and silane-treated aluminum/epoxy composites to investigate the fracture mechanism of aluminum/epoxy composites due to the silane treatment. Fig. 5 shows the fracture surfaces of and silane-treated aluminium/epoxy composites. For the aluminium/epoxy composites, as shown in Fig. 5a, debonding occurs at many interfaces between aluminum powders and epoxy matrix. It also shows that crack propagated from the debonded interface due to the weaker interfacial adhesion between the aluminium and polymer. For the silane-treated aluminium/epoxy composites, as shown in Fig. 5b, better dispersion of aluminum powders in the epoxy matrix was observed. It can be seen in the figure that several microcracks but less interfacial debonding occurred, which indicates the improved interfacial adhesion. Thus, more energy is needed to break this interlink and start the fracture. The tribological property of composite materials is characterized by the friction coefficient and wear rate. The effect of silane treatment on the tribological behavior of aluminum/epoxy composites was investigated by determining the change in friction coefficient as a function of wear distance. Fig. 6 shows the change in friction coefficient as a function of the wear time for and silane-treated As shown in the figure, vibrational sliding friction behavior occurred in the aluminum/epoxy composites while stable tribological behavior occurred for silane-treated aluminum/epoxy composites, which indicates that silane-treated aluminum/epoxy composite is more compact and uniform. It can be seen in the figure that the average friction coefficient of silane-treated aluminum/epoxy composites is far lower than that of aluminum/epoxy composites. Specifically, the friction coefficient of and silane-treated aluminum/epoxy composites are in the ranges of and , respectively. The worn surfaces of the and silane-treated aluminum/epoxy composites were examined using surface profiler to determine the wear loss. Fig. 7 shows the comparison of worn ð2þ

$1746 H.J. Kim et al. / Composites: Part B 43 (2012) 1743 1748 (a) (b) Fig. 5. SEM images of fracture surfaces of (a) and (b) silane-treated aluminium/epoxy composites.$ 7a and b, the wear track of aluminum/epoxy composites was wider and deeper compared to that of silane-treated Fig.

7a and b, the wear track of aluminum/epoxy composites was wider and deeper compared to that of silane-treated Fig.

Comparison of friction coefficient change as a function of the wear time for and silane-treated ð3þ -220 0 2.5 5 7.5 10 x (mm) -220 0 2.5 5 7.5 10 x (mm) Fig. 8.

4 1746 H.J. Kim et al. / Composites: Part B 43 (2012) (a) (b) Fig. 5. SEM images of fracture surfaces of (a) and (b) silane-treated aluminium/epoxy composites. friction coefficient 100 (a) (b) 100 (µm) (µm) y y surfaces between and silane-treated aluminium/epoxy composites. As shown in Fig. 7a and b, the wear track of aluminum/epoxy composites was wider and deeper compared to that of silane-treated Fig. 8 shows the depth profile of the and silane-treated aluminum/ epoxy composites, respectively. The mean depth profile was obtained according to several points taken from the cross-section in the worn area. Fig. 9 shows the comparison of specific wear rate from sliding distance for both composites. The specific wear rate, K was determined by the following equation [16]: K ¼ V N l time (sec) Fig. 6. Comparison of friction coefficient change as a function of the wear time for and silane-treated ð3þ x (mm) x (mm) Fig. 8. Comparison of depth profile between (a) and (b) silane-treated wear rate [mm 3 /Nm] V ¼ 2prA ð4þ where K is the specific wear rate, N is the applied load, and l is the sliding distance, V is the wear volume loss, A is the cross-sectional area of wear track obtained from the depth profile using a surface profilometer, and r is the radius of the wear track. As shown in the figure, the wear rate of aluminum/epoxy composites decreased by the silane treatment of aluminum powders. Specifically, the Fig. 9. Comparison of wear rate between and silane-treated aluminum/ epoxy composites. wear rate of silane-treated aluminum/epoxy composites was 56% less than that of This agrees with previous results for MMT/epoxy composites [17]. Fig. 7. Comparison of worn surfaces between (a) and (b) silane-treated

H.J. Kim et al. / Composites: Part B 43 (2012) 1743 1748 1747 (a) (b) Fig. 10. SEM images of worn surfaces of (a) and (b) silane-treated aluminium/epoxy composites.

composites Fig. 10 shows the comparison of worn surfaces between and silane-treated aluminium/ epoxy composites. SEM image of the aluminum/epoxy composites (Fig.

5 H.J. Kim et al. / Composites: Part B 43 (2012) (a) (b) Fig. 10. SEM images of worn surfaces of (a) and (b) silane-treated aluminium/epoxy composites. The worn surfaces of the and silane-treated aluminum/epoxy composites was analyzed using FESEM to investigate the effect of silane treatment of aluminum on the wear mechanism of aluminum/epoxy composites Fig. 10 shows the comparison of worn surfaces between and silane-treated aluminium/ epoxy composites. SEM image of the aluminum/epoxy composites (Fig. 10a) shows that typical wear mechanisms such as matrix cracking and exforlation of matrix occurred. The figure also shows that local plastic deformation process and welding between contacting asperities and zirconium counter material occurred, indicating the adhesive wear behavior occurred. For the silane-modified aluminum/epoxy composites (Fig. 10b), matrix cracking and exforlation also occurred. However, the matrix cracking was less compared to the case. Particularly, the figure shows the ploughing deformation and micro parallel scratching on the worn surface, which indicates that abrasive wear occurred. It can be also seen in the figure that the dispersion of aluminum of silane-treated composites was better than that of case. The abrasive wear behavior of silane-treated case occurs because the hardness of aluminum/epoxy composites was increased by the improved dispersion and interfacial characteristic of aluminum powders due to the silane treatment. 4. Conclusions This study demonstrated the enhanced mechanical properties of aluminium/epoxy composites by silane treatment of aluminium powders. The conclusions obtained from this study are as follows: First, the tensile modulus and strength of aluminum/epoxy composites are improved 9% and 12% by the silane treatment of aluminum powders. Second, the fracture toughness and wear resistance of aluminum/epoxy composites are also improved 32% and 56% with silane treatment of aluminum powders. Third, the wear mechanism shows abrasive and mild wear behavior in silane surface modified aluminum powder/epoxy composite and adhesive and severe wear in unmodified aluminum powder/ epoxy composite. Fourth, the improved dispersion and interfacial characteristics of aluminum powders in the epoxy is the reason of the improved mechanical properties and performance of aluminium/epoxy composite. Acknowledgements This work was financially supported by the National R&D Project of Development of Energy Utilization of Deep Ocean Water supported by the Korean Ministry of Land, Traffic and Maritime Affairs. References [1] Deflorian F, Rossi S, Fedrizzi L. Silane pre-treatments on copper and aluminium. Electrochim Acta 2006;51: [2] Lei X, Xiang J, Ma X, Wang C, Sun J. Surface modification of aluminum with tin oxide coating. J Power Sources 2007;166: [3] Stevens N, Tedeshi S, Powers K, El-Shall H, Moudgi B. Controlling unconfined yield strength in a humid environment through surface modification of powders. Powder Technol 2009;191: [4] Funatani K. Emerging technology in surface modification of light metals. Surf Coat Technol 2000; : [5] Esawi A, Morsi K. Dispersion of carbon nanotubes (CNTs) in aluminum powders. Composites: Part A 2007;38: [6] Hoseini M, Meratian M. Fabrication of in situ aluminum alumina composite with glass powder. J Alloys Compd 2009;471: [7] Chung S, Im YG, Kim HY, Park SJ, Jeong HD. Evaluation for micro scale structures fabricated using epoxy aluminum particle composite, its application. J Mater Process Technol 2005;160: [8] Nazari A, Didehvar N. Analytical modeling impact resistance of aluminum epoxy laminated composites. Composites: Part B 2011;42: [9] Costa-Mattos HS, Monteiro AH, Sampaio EM. Modelling the strength of bonded butt-joints. Composites: Part B 2010;41: [10] Saboktakin MR, Tabatabaie RM, Maharramov A, Ramazanov MA. Synthesis, rheological properties of poly(methyl methacrylate)/polymethacrylic acid nanocomposites as denture resins. Composites: Part B 2011;42: [11] Yang W, Song L, Hu Y, Lu HD, Yuen RKK. Enhancement of fire retardancy performance of glass fibre reinforced poly(ethylene terephthalate) composites with the incorporation of aluminum hypophosphite and melamine cyanurate. Composites: Part B 2011;42: [12] Jallo LJ, Schoenitz M, Dreizin EL, Dave RN, Johnson CE. The effect of surface modification of aluminum powder on its flowability, combustion and reactivity. Powder Technol 2010;204: [13] Chen Y, Jallo L, Quintanilla M, Dave R. Characterization of particle, bulk level cohesion reduction of surface modified fine aluminum powders. Colloids Surf A 2010;361:66 80.

6 1748 H.J. Kim et al. / Composites: Part B 43 (2012) [14] Crouse C, Pierce, Spowart. Influencing solvent miscibility and aqueous stability of aluminum nanoparticles through surface functionalization with acrylic monomers. ACS Appl Mater Interfaces 2010;2(9): [15] American society for testing and materials. E399-81, Standard test method for plane-strain fracture toughness of metallic materials. Annual Book of ASTM Standards, Part 10. Philadelphia: American Society for Testing and Materials; [16] Bayer RG. Mechanical wear fundamentals and testing. Marcel Dekker Inc.; [17] Ha SR, Ryu SH, Park SJ, Rhee KY. Effect of clay surface modification, concentration on the tensile performance of clay/epoxy nanocomposites. Mater Sci Eng A 2007;448:264 8.

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