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Synthesis and Mechanical Characterisation of Aluminium-Copper-Alumina Nano Composites Powder Embedded in Glass/Epoxy Laminates

P K Dash*, Prof. B. S. Murty, R B Karthik Aamanchi

Department of Aeronautical Engineering, IARE, Hyderabad, Telangana

Abstract: This paper presents the synthesis and mechanical properties study of Aluminum-Copper nanocomposite powders with variation in volume percentages of alumina. The powders were synthesized using mechanical alloying (high energy ball milling technique). Samples of size 2010 mm were produced from nanocomposite powders by spark plasma sintering technique and conventional sintering method. The microstructural verifications were carried out using X-ray diffraction. Transition electron microscopy were used to determine the phases formed and size of the particles. Thermal analysis and hardness of these samples were measured by conducting DSC and Vickers’s Hardness Test. Also, the powders of ACANC were embedded into Glass/Epoxy laminates for further identification of NC powders effects on mechanical properties like tensile and compressive strength. The samples prepared using conventional sintering technique had gone through two different types of annealing before sintering and shown enhanced hardness, yield strength and increment in density. The nanocomposite embedded laminates have shown improved tensile, compression and hardness values in compare to virgin specimens.

Keywords: Al-Cu-Alumina-Nano-Composite (ACANC), plasma sintering technique, X-ray diffraction, TEM, DSC, GFRC, mechanical properties

Received May 06, 2015; Revised June 05, 2015; Accepted July 15, 2015

Citation: P K Dash, Prof. B. S. Murty, R B Karthik Aamanchi, Synthesis and Mechanical Characterisation of Aluminium-Copper-Alumina Nano Composites Powder Embedded in Glass/Epoxy Laminates. American Journal of Nanomaterials, 3 (1). 28-39. January 2015. doi:10.12691/ajn-3-1-4

1. Introduction

Nanocomposite, a high performance material exhibit unusual property combination and unique design possibilities [1,2]. With an estimated annual growth rate of about 25% and fastest demand to be in engineering plastics and elastomers, their potential is so striking that they are useful in several areas ranging from packaging to biomedical applications. Present day, a spurt of research activity is witnessed in the area of synthesis, fabrication and utilization studies of different size and shape of metal nano-particles [1-50]. With polymer NCs, properties related to local chemistry, degree of thermoset cure, polymer chain mobility, polymer chain conformation, degree of polymer chain ordering or crystallinity can all vary significantly and continuously from the interface with the reinforcement into the bulk of the matrix [17-31]. Also, it can have an observable effect on the macroscale properties of the composite [32-50]. For example, adding carbon nanotubes improves the electrical and thermal conductivity. Other kinds of nano-particulates may result in enhanced optical properties, dielectric properties, heat resistance or mechanical properties such as stiffness, strength and resistance to wear and damage [10-50].

The metallic NC plays a significant role in modern day engineering due to its excellent conductivity and low cost nano-particles [2]. These nanostructural materials can be synthesized in controlled processes by the methods like, highly energetic reactive milling, precipitation from solution (sol-gel), hydrothermal synthesis, electrochemical synthesis, and internal oxidation etc [1-16]. One of them is Aluminum Copper alumina nano particles which will gain increasing importance as is expected to be an essential component in the future nano-devices due to its excellent thermal properties as well as good biocompatibility and its surface enhanced Raman scattering (SERS) activity [3]. Also, the addition of metallic copper nano-crystals, dispersed homogeneously in silica layers have attracted great attention recently for the development of nonlinear optical devices [4].

The most extensively used methods for the production of Al-Cu-alumina composite materials (ACACMs) are based on casting, [8] mechanical alloying [9,10] and other techniques. [11,12] However, conventional casting techniques are not practical because of the poor uniformity of dispersed particles. But, ACACMs are increasingly produced by powder metallurgy (P/M) method where the composite powder used as the raw material for P/M processing and prepared through high energy ball milling, [9,13] internal oxidation, [14,15] and chemical process. [16,17] These traditional techniques for the synthesis of such composite powders still have several limitations, such as manufacturing costs, grain size, and the homogeneity of the final product. Studies on the fabrication of nanostructured CACMs have recently drawn scientific interest [18,19] because the nanostructured materials are expected to possess several unique physical and mechanical properties.

A number of studies [16-31] on the preparation and charcterisation of ACACMs had been undertaken. However, studies on mechanical characterization embedded with Glass polymer epoxy composites have been less reported, especially operations under tensile and compressive load.

In this paper, the manufacturing method of ACACMs by using ball milling process and characterization of this material through TGA, X-ray diffraction technique, TEM, hardness determination and mechanical properties like tensile strength and stress-strain properties estimation by embedding to glass fibre epoxy composite have been carried out and presented.

2. Experiment

2.1. Selection of Materials

The powders of Aluminium, Copper and Alumina of 30-40 μm with 99.97 % pure are taken to produce the al-cu-alumina nanocomposites. The densities of Aluminium, Copper and Alumina are 2.70 g/cc, 8.90 g/cc and 3.96 g/cc respectively. The composition of the NC powders that were prepared was given in the Table 1.

Glass fiber of 210 GSM bidirectional woven cloth and Araldite epoxy resin LY 556 were taken to prepare a 9 layered composite laminate. HY 951 hardener was used. Fiber to binder ratio was taken as 1:1.5 and resin to hardener was taken as 10:1.

Table 1. Composition of Powder and SPS parameters

2.2. Preparation of Nanocomposite through High Energy Ball Milling Process

Powder metallurgy is one of the highly established methods to synthesize metals, alloys and composites, and sintering is one of the important steps in powder metallurgy method. Three reinforcements were targeted: (a) silicon carbide (a microwave susceptor), (b) alumina (a microwave transparent material) and (c) copper (a conducting material). The powders of Aluminium, Copper and Alumina were taken and wet ball milled at 300 rpm for 20 hours to obtain nano composite powders taking ball to powder ratio as 10:1. Tungsten balls of 10 mm in diameter and tungsten vial was used for the ball milling technique (Figure 1). The powder was kept in the vial and tungsten balls were added to it. The vial was rotated at a high speed and caused the balls to gain momentum and collided with each other at high speed. The powder gets entrapped between two balls, imparted a large force which leads to the grinding of powder breaking them into smaller sizes. These ball mills were rotated around a horizontal axis and internal cascading effect reduced the powder size to as small as 5 nm. After every half an hour of milling, half an hour time was given for cooling. Toulene was added as a reducing agent and also to prevent adherence of powder to the walls of the vials. The powders were collected at regular intervals to perform X-ray diffraction to ensure the mixing of the powders and also to check the phases formed along with crystallographic structures as shown in Figure 7a & b. In addition to XRD, the transition electron microscopy is also used (Figure 6a & b) for verification of nanocomposite configuration and distribution. The Table 2 is showing the details of powders collected at different milling times. The nanocomposite powders as shown in Figure 3(i) and Figure 3(ii), here with referred as Sample-A and Sample-B, were kept in the desiccators to prevent the oxidation of the sample.

Figure 1. Fritsch P-5 high energy ball milling apparatus

Table 2. Composition of Composite Laminate with different percentage of NC powder reinforcement

Figure 2. Compression Molding Machine

2.3. Density Calculations

Using the rule of mixture, the theoretical density of these NCs are calculated and presented in Table 4. Initially the densities of the corresponding materials should be taken and their volume percentages in the total sample have to be calculated. The below formula is used to calculate the individual volume percentages that are to be taken for milling.

After finding out the volume percentages, again rule of mixture is used to determine the density of Al-Cu by using formula ρAl-Cu = (ρAl x Vol % of Al) + (ρCu x Vol % of Cu), where ρAl-Cu is density of Aluminum-copper, ρAl is density of Aluminum and ρCu is density of Copper. All are in g/cc. Using rule of mixture, the Density of Sample-A will be:

2.4. Weight Calculations

The weight percentage is estimated using following formula, i.e.,

And presented in Table 3.

Figure 3. NC powders synthesized through ball milling technique (i) Al-4Cu-10 % Alumina (Sample-A) and (ii) Al-4Cu-20 % Alumina (Sample-B)

2.5. Sintering of nc Powders

The pellets with 20 mm diameter and 6 mm thick, were prepared from the powder of Sample-A and Sample-B by using spark plasma sintering (SPS) technique and microwave sintering (Figure 4). The main characteristic of SPS is to pulse DC current through the graphite die, as well as the powder compact. The heat is generated internally. This facilitates a very high heating or cooling rate (up to 1000 K/min) and caused the sintering process faster (within a few minutes). The general speed of the process ensures it has the potential of densifying powders with nanosize or nanostructure while avoiding coarsening which accompanies standard densification routes. The conditions of sintering are presented in the Table 3. The powder was annealed for 2 hours at 200°C to remove residual strains present. Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat is added to increase the rate of diffusion by providing the energy needed to break the bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in metals and (to a lesser extent) in ceramics. This alteration in dislocations allows metals to deform more easily, so increases their ductility. The amount of process-initiating Gibbs free energy in a deformed metal is also reduced by the annealing process. In practice and industry, this reduction of Gibbs free energy is termed "stress relief". The relief of internal stresses is a thermodynamically spontaneous process; however, at room temperatures, it is a very slow process. The high temperatures at which the annealing process occurs serve to accelerate this process. All composition in form of nanocomposite are shown in Figure 5.

Figure 4. Compaction Machine used to prepare conventional sintering NC

Figure 5. Composites prepared with various percentage of NC powder [ a) C1 b) C2 c) C3 d) C4 e) C5]

Figure 6. The palate with sample name [(b) P_A (c) P_B (d) P_CS_A (e) P_CS_B]

The samples were exposed inside the oven because the oven was large enough to place the work piece in a position to receive maximum exposure to the circulating heated air to complete the annealing process. Some worked pieces are left in the oven to have a controlled cooling process and other materials and alloys were removed from the oven for quench hardening in air, water, oil, and salt. These nanocomposite powders were used for preparation of the pellets through compaction technique (Figure 3b) at 8 tons load for 30 seconds of holding time. These pellets were referred as P_CS_A and P_CS_B as shown in Figure 6b to 6e and were sintered conventionally at 500°C for one hour.

2.6. Preparation of nc Embedded Glass/Epoxy Laminates

NC powders of sample-A and sample-B were mixed into resin using mechanical stirrer with 40 RPM speed for two hours with 2.5 and 5 weight % of resin weight respectively. Hardener was added 2 minutes before the application of the resin to the fiber. The compression molding technique was used where preheated molding material were placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. The material to be molded is positioned in the mold cavity and the heated platens are closed by a hydraulic ram. Bulk molding compound (BMC) or sheet molding compound (SMC), are conformed to the mold form by the applied pressure and heated until the curing reaction occurs. SMC feed material usually is cut to conform to the surface area of the mold. The mold is then cooled and the part removed. Materials are heated above their melting points, formed and cooled. The more evenly the feed material is distributed over the mold surface, the less flow orientation occurs during the compression stage. The properties of the composite were given in the Table 4 and prepared products were shown in Figure 4.

The following precautions were taken during the fabrication of laminates,

•  Mylon sheet was placed in between spacer plates for easy removal of the laminate after curing.

•  Special precaution is taken for homogeneous distribution of NC

•  Precaution has been followed during the curing process so that thermosetting resin should not be overstressed or form any micro cracks.

The prepared composite laminated were post cured for 7 days in open atmosphere as suggested by the suppliers. The hardness test have been carried out for identifying any strength loss during the fabrication.

3. Characterization of nc and nc Embedded Composite Laminates

Microstructural characterizations of NC were carried out by using X-ray diffraction technique and transition electron microscopy (TEM). Hardness, tensile and compressive properties of NC and NC embedded laminates were carried out using Vicker’s hardness testing machine and computerized Universal testing machine. Followings are the method followed for characterizations.

3.1. X-ray Diffraction Technique

X-ray scattering techniques, a non-destructive analytical techniques which revealed the information about the crystallographic structure, chemical composition, and physical properties of materials. X-ray diffraction with Cu-K as cathode from 20° to 90° of Bragg angle with step size of 0.02°/sec was used. The diffraction data of the nanocomposite powder are presented as a diffractogram in which the diffracted intensity I is shown as function either of the scattering angle 2θ or as a function of the scattering vector q. (Figure 7a and Figure 7b).

Figure 7(a). XRD pattern of Sample-A, (b): XRD pattern of Sample-B

3.2. Transition Electron Microscopy

TEM at different magnification was carried out. The sample powder was first ultrasonicated in acetone solution for 15 minutes to ensure dispersion. Then this powder solution was taken on a copper grid and is placed inside the machine. Bright field and dark field images were taken. Thicker regions of the sample, or regions with a higher atomic number will appear dark, whilst regions with no sample in the beam path will appear bright – hence the term "bright field".

Table 3(a). Relative Densities of Sintered Samples

Table 3(b). Hardness and yield strength values of composed pellets

3.3. Vicker’s Hardness

The samples were placed under the indentation point and a load of 10 kg with 20 seconds hold-time was applied. The observations are shown in Table 3b.

3.4. Mechanical Properties of Composites

The square plate with dimensions 300x300 mm containing NC and without NC are prepared using compression molding machine (Figure 2) for identification of nanocomposite effect on mechanical properties of glass fiber composite. Accordingly the tensile and compression testing on the composite laminates were carried out as per ASTM D3039 and D659. A computerized UTM with maximum load of 400 KN was used. The stress-strain graphs obtained during both tensile and compression testing are shown in Figure 11a to Figure 11m. Hardness of the composite laminates was determined by Barcol hardness tester using ASTM D2583 standards. UTM tested results are shown in Table 4 and hardness values are shown in Table 3b.

Table 4. Mechanical Properties of NC embedded Composite Laminate

4. Results and Discussion

The sample-A, sample-B and NC embedded composite laminates are analyzed in following lines.

4.1. X-ray Diffraction

Figure 7(a) is showing the XRD pattern of Sample-A at different milling times. The peaks are only showing Aluminium and Copper in 20 minutes of milling but as milling time increases the copper got alloyed with aluminium [1,2,5,6,9,14,16,19]. The copper precipitated into aluminium matrix. The peaks are showing Al (111), Al (022), Al (002), Al (113), Al (222) and Copper (111), (002) and (022). No Alumina peak is visible as it might have lost its crystalline state and became amorphous or the alumina crystallite size is very small so that the peaks are not visible in XRD. There might be a high chance of Al2Cu phase formation but is not visible in XRD. This might be because the percentage of copper is very less, so the amount of second phase formation is not indexed as it might have gone in background. From the peak positions the microstructure can also be identified which is face centered cubic structure (FCC).This is same in case of sample-B and shown in Figure 7b for reference.

Figure 8(a). TEM micrographs of Sample-A (a) bright field image (b) dark field

Figure 8(b). TEM micrographs of Sample-B (c) bright field image (d) dark field image

4.2. TEM Analysis

Figure 8(a) is the bright and dark field images of sample-A. From the figures it is clearly evident that after milling, the powders are in nano scale of order 30-50 nm. In the dark field image, the bright spots correspond to the Alumina. The bright field image shown has an agglomerated particle of size 500 nm. But also the small sized NC can be found which are of order 30-50 nm. In the dark field image, the higher atomic number elements tend to appear as bright spots. Similarly Figure 8(b) presents the microscopic details of Sample-B.

Figure 9. DSC Traces of (a) Sample-A (b) Sample-B

4.3. DSC-TGA of NC Powders

DSC-TGA analysis of nanocomposite Al-Cu-Al2O3 powder obtained by the thermochemical procedure are shown in Figure 9a and Figure 9b and one endothermic peaks was obtained at approximately 150°C, which are related to evaporation and dehydration of the residual moisture. Exothermic peak at 324°C is accompanied by mass increment of 5,88%, which represents the beginning of the oxidation process of the present fine copper powder. Intensive mass increase at the TG curve is recorded at the temperature approximately 550oC, after which the TG curve is stabilized, showing insignificant mass increase of some percents only [1].

4.4. Hardness and Yield Strength of nc:

The hardness and yield strength of the pellets are presented in Table 3b and simultaneously plotted in Figure 10. For the pellets P_A and P_B which are sintered using spark plasma sintering and microwave sintering have hardness of 142.2 and 178.29 VHN. The pellets which are prepared from annealed powders through conventional sintering have hardness of 227 and 266 VHN. The difference of the hardness is related to sintering and solidification of the materials. The NC pellets P_A and P_B hardness increased for P_B as the harder phase which is alumina percentage is increased by 10 %. But the relative density is little less because there might be strain present in the powder which was generated during the ball milling process.

Figure 10. Hardness of samples observed

The NC pellets P_CS_A and P_CS_B have hardness of 227 and 266 VHN. NC powders before doing conventional sintering were subjected to annealing at 200°C in tubular furnace for one hour in ambient atmosphere. Due to annealing the strain present in the powders was reduced. Then the powders were compacted to produce the increased hardness. Also, this is the reason for the enhancement of density. Again, the sample with more alumina has more hardness and yield strength and it is due to the compactness as explained in above paragraph.

The Table 4 is showing that there is an increase in the strength values of the samples. In two cases, the powder with more alumina has higher strength and hardness. This is because of the increment of content of Alumina which has the hard phase of the composite. The reason for this increase can be examined from two perspectives. One might be due to greater interfacial area between the hard and soft phases. Secondly, the defects in the coarse-grained particles are more than the fine-grained ones which results in its easy fracture under tension²¹. And also conventional sintering pellets have shown higher hardness values. This is mainly attributed due to the removal of strain from the powder by annealing process. The comparative graph of these samples was shown in Figure 2.

4.5. Mechanical Properties of Composite Laminates

The composite laminates were tested for tensile and compression properties using ASTM D3039 and ASTM D695. The hardness of these laminates was tested as per ASTM D2583. The Figure 1 and Figure 2 are the stress-strain curves of the laminates for tensile and compression. And also the comparative graphs in case of hardness were shown in Figure 3 and Figure 4. The Table 3 gives the ultimate strengths both in tensile and compression and elastic moduli of the composites. The table shows that the ultimate strengths are increasing from 1 to 2 and 3 and also from 1 to 4 & 5. The ultimate tensile strength is increasing from 180 N/mm2 to 235 and 250 in case of C1, C2 and C3 respectively. Similarly the percentage of elongation is also increasing from C1 to C3. The young’s modulus is increasing from C1 to C2 but decreased in C3. This is attributed to the material behavior translation to be more ductile. In case of compression also the ultimate strength is increased from 230 to 245 and 262 N/mm2 respectively.

For C4 and C5 also both the ultimate tensile and compression strengths were increasing when compared to C1. Even the young’s modulus has shown increment. The reasons for increment are discussed in the following paragraphs. The graph of composite-1 which is without NC powders has shown a typical character of a brittle material. For this composite the ultimate stress is around 180 N/mm2. At the starting linearity is observed which is not to be mistaken as plastic zone but it is because the machine is taking some time to adjust to the load acting on the material. Stress-strain curves of the other composites have shown some irregularities during the loading. This is mainly because of the presence of NC powders which are embedded into the resin. These NC powders are acting as the hindering agents which might be offering the resistance force for the tensile. NC powders in the composites binds to the reinforcement and thus increases the strength of the composite. Whenever the load is being applied the stress concentration will be increasing which is a universal truth, but due the presence of NC, the stress will be taken up by them and prevents the failure of the material. So at this point where the NC are taking up the load, the graph is showing decrement in the stress and then it moves to other fibers where the NC concentration is less and tries to fail the material at that point. This is the sole reason why it is most difficult to investigate where the composite fails whenever the NC powders are embedded in them. The other reason might be that the NC’s are preventing the crack growth. Whenever there is a hindrance to the propagation of crack in one direction, the cracks tries to change their direction of propagation which is very high in case of bidirectional fiber. This might also be the reason for the stress decrement in the stress-strain graph.

The tensile and compression test stress-strain graphs are shown in Figure 11a to 11(m). The composites with embedded NC’s have shown resistance to the load at the staring stages of loading. This is mainly because of the agglomeration of the NC in between the reinforcements. Whenever the load is acted upon any body, there will be increase in the internal resistance. When force is applied on the material especially compression, since there is decrease in the inter-fiber distances, the NCs particles comes near to each other and tries to combine to form a stable state. Whenever stability is obtained, the NCs exhibit normal behavior like any other metals. So at time of initial loading the load is taken up by the NC’s and later they might have formed agglomerations which decreased the resistance force. But the advantage is that the ultimate compression strength increased. This is clearly shown in the comparative graphs in Figure 12 and Figure 13.

Figure 11(a). Stress-Strain curve of GFRP without NC

Figure 11(b) Stress-Strain Curves of GFRP with 2.5% Wt. of Sample A

Figure 11(c) Stress –Strain Curve with 5% Wt. of Sample A

Figure 11(d) Stress – Strain Curve of GFRP with 2.5% Wt. NC powder in Sample B

Figure 11(e) Stress – Strain Curve of GFRP with 5% Wt. NC powder in Sample B

Figure 11(f) Stress-Strain Curves in Tension of C1. C2 and C3

Figure 11(g) Stress-Strain Curves in Tension of C1. C4 and C5

Figure 11(h) Stress-Strain Curves in Compression

Figure 11(i) Stress-Strain Curves in Compression

Figure 11(j) Stress-Strain Curves in Compression

Figure 11(k) Stress-Strain Curves in Compression

Figure 11(l) Stress-Strain Curves in Compression

Figure 11(m) Stress-Strain Curves in Compression

Figure 11(n) Stress-Strain Curves in Compression

Figure 12. Comparative graph showing ultimate strengths for C-1, C-2 and C-3

Figure 13. Comparative graph showing ultimate strengths for C-1, C-4 and C-5

5. Conclusions

Based on the studies carried out on synthesis and mechanical characterization of aluminum- copper-alumina nanocomposites powder embedded into glass/epoxy laminates, the following conclusion are drawn. The processed nanocomposite powders of Al-4Cu-Al2O3 with varying alumina percentages through high energy ball milling technique (Mechanical alloying) prepared to the pellets of two different techniques viz., spark plasma sintering and normal compaction unit have revealed significant variance in microstructural form. The phases determined by the X-ray diffraction technique had shown significance change in there nanocomposite compositional difference at different odder of preparation. The hardness of these two different samples was determined and comparative studies had shown that NC prepared with control cooling had higher hardness value. Tensile, compression and hardness of NC powder embedded glass/epoxy laminates prepared by compression molding technique, were determined and corresponding stress-strain curves are plotted. There was an increase in the ultimate strength both in tensile and compression and young’s modulus with increasing weight percentage of NC powders.

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