Mechanical Strength and Corrosion Rate of Aluminium Composites ( Al / SiO 2 ) : Nanoparticle Silica ( NPS ) as Reinforcement

The fabrication and characteristics of amorphous silica reinforced Al matrix composites are studied in this paper. The major starting materials were commercial Al powder and extracted nanoparticle SiO2 (NPS) powder from Indonesian silica sands. Two different active solutions, namely N-butanol and tetramethylammonium hydroxide (TMAH), were introduced during synthesis. Characterisations in terms of physical, mechanical, microstructural and corrosion rate examinations were also employed. Introducing the SiO2 nanoparticles into the Al matrix has decreased the density and increased the porosity of the composites. The addition of N-butanol into Al/SiO2 (Al/SiO2(B)) led to broader and lower X-ray diffraction profiles than the addition of TMAH (Al/SiO2(T)). From the microstructural analysis, we found that the SiO2 particles enter and agglomerate into the opening gap of the Al sheets. Furthermore, yield strength, ultimate compression strength and modulus of elasticity tended to reduce the addition of SiO2. The corrosion rate of Al/SiO2(T) was lower than that of Al/SiO2(B) composites.


INtRODUCtION
Aluminium is one of the most popular non-ferrous metal matrix composites (MMCs) due to its excellent characteristics in terms of low density, good corrosion, high acid resistance, and high thermal and electrical conductivities. 1,2Aluminium-based MMCs with Al 2 O 3 and SiC reinforcements have enhanced some important material properties like tensile strength, elastic modulus, hardness, wear resistance, thermal conductivity and corrosion resistance. 3,48][19] Their corrosion rate increases along with the increasing temperature, medium concentration and profile of pitting corrosion in the surface.
This study deals with the simple approach of producing Al/SiO 2 composites prepared by two different active solutions, i.e., N-butanol and tetra-methyl-ammoniumhydroxide (TMAH), as well as their microstructural and mechanical characteristics.Furthermore, the corrosion rate of the composites will also be studied.To the best of our knowledge, a comprehensive examination of the properties of amorphous nano-silica-reinforced aluminium employing various active solutions as media mixing matrix and filler has, so far, has rarely been reported.

EXPERIMENtAL
The raw materials were Al powders (Merck) and extracted amorphous SiO 2 powders with an average crystallite size of around 35 nm that were prepared from natural silica sands. 20The elemental contents of those raw materials are detailed in Table   Phase compositions within the composites were evaluated using Cu-Kα radiation X-ray diffraction (XRD) data.Elemental mapping and microstructures were analysed by scanning electron microscopy and energy dispersive spectrometry (SEM-EDX) module.Density and porosity were measured using the Archimedes principle, while the mechanical properties were investigated through a compression test.The corrosion rate was studied by potentiodynamic polarisation using stainless steel as the calculating electrode, Al/SiO 2 composite as working electrode, and Ag/AgCl as a reference electrode.ANOVA analysis was employed to evaluate the Tafel curve using Butler-Volmer mathematical expression, and polarisation resistance was calculated using the Stern-Geary equation. 15[18] 3.

Mechanical Properties
Figure 1 shows the characteristic density and porosity of the prepared composites.
In general, for samples with 5% to 20% SiO 2 content, the density decreased and therefore, the porosity increased.This was in line with the previous report. 14The lowest density (the highest porosity) was shown by the 20% SiO 2 sample, while the highest density (the lowest porosity) was shown by 5% SiO 2 sample, which proved to be the best among the samples.Figure 1 illustrates that there is no strong influence in employing N-butanol and TMAH solutions as mixing media during the sample preparation.Nevertheless, the porosity should be considered for evaluating the modulus of elasticity. 8,22Since we introduced SiO 2 as discontinuous reinforcement, the modulus of elasticity with porosity involvement can be best computed using Sprigg's equation, where E pore and E o are respectively the theoretical moduli of elasticity with and without porosity considerations, b is a constant for the filler ratio factor having value of around 3.95, and P is porosity.As it is well known, the E o can be calculated using the Halpin-Tsai expression. 21,22In Figure 2(b), the E pore characteristic of the composites is seen as showing the same trends but having larger values as compared to the E c characteristic.These trends accord well with another report introducing nano-sized SiO 2 into the Al matrix, which would enhance the modulus of elasticity up to about 14 Gpa. 7Meanwhile, for micron-sized SiO 2 , reinforcement may give only around 0.240 Gpa. 11Our present study exhibited that the maximum modulus of elasticity attained was 61.28 GPa and 62.43 GPa respectively for Al/SiO 2 (B) and Al/SiO 2 (T) samples with 5% SiO 2 .This finding signifies that the addition of 5% SiO 2 leads to the most superior Al/SiO 2 composite in terms of its modulus of elasticity. 21,23The characteristics of yield strength (YS) and ultimate compression strength (UCS) of Al/SiO 2 are given in Figure 3, and they agreed with other investigations. 7,11,14Both UCS and YS of Al/SiO 2 (T) were slightly greater than those of Al/SiO 2 (B), implying that the use of TMAH may enable more homogenous distribution of SiO 2 within the composites.

Microstructures
Figures 4 and 5 show the representative XRD spectra for Al/SiO 2 (B) and Al/ SiO 2 (T) composites after their phase identification.From Figure 4, we can see that only Al (reference code 00-004-0787) and γ-Al 2 O 3 (00-002-1420) are present in all composites.Figure 2 also clarifies the noncrystalline SiO 2 content through the absence of any SiO 2 peak, implying that the whole treatment in all the processes did not transform the structure of silica from amorphous to crystalline state.The occurrence of γ-Al 2 O 3 , especially in Figure 5, indicates that γ-Al 2 O 3 is likely to form due to sintering Al/SiO 2 mixtures.[26][27][28] Adding N-butanol and TMAH as the active solutions along with Al/SiO 2 synthesis strongly affects the crystalline states and sizes of the composites.More amorphous state and smaller crystallite size, which are qualitatively indicated by the broader and lower XRD peaks, are obtained by Al/SiO 2 (T) rather than Al/SiO 2 (B) (see Figures 4 and 5).This evidence indicates that the addition of TMAH may reduce the dominance of Al in the composites.Also, more γ-Al 2 O 3 formed in Al/SiO 2 (T).The more detailed micrographs of non-homogenous distribution of amorphous SiO 2 within Al matrix are presented in Figure 9 (#25-B sample).Even supposing that SiO 2 has a large specific surface area due to its nano-sized structure, its agglomeration remains observed, shown in Figure 9(c).The distribution of the reinforcement, in this case SiO 2 , is affected by its size. 1,7,21Also, the large specific surface area of SiO 2 plays an important role in the interfacial bonding of the composite. 8In Figure 9, the presence of interfacial bonding between Al and SiO 2 with sphere-plane characteristic is also illustrated.This type of bonding contributes to the enhanced mechanical properties of the composites.The elemental mapping in the selected grain and the grain boundaries of Al/SiO

Corrosion Rate
The results from the Tafel curve analysis and its parameters for Al/SiO 2 composites with volume fraction 0% SiO 2 or bulk-pure Al are represented in Figure 11 and Table 2.The corrosion parameters obtained from the Tafel curve analysis for Al/ SiO 2 composites are shown in Table 3.The corrosion rate and the current density were calculated using the Stern-Geary equation: 27 and where C is a constant that labels the corrosion rate units divided by the specimen density and area, and W eq is equivalent to the weight of the material.the synthesis leads to lower corrosion rates than those produced by N-butanol.

CONCLUSION
Al/SiO 2 composites were successfully produced using a simple metallurgical approach and employing commercial Al powders and extracted amorphous SiO 2 powders from Indonesian natural silica sands.From the phase and microstructural evaluations, it was shown that besides crystalline Al and amorphous SiO 2 , γ-Al 2 O 3 were formed within the composites as had been predicted as the physical consequences of the presence of active solutions and sintering.
The addition of SiO 2 reduced the density or increased the porosity of the composites.Reduction of other physical properties in terms of yield strength and compression strength, ultimate compression strength, and modulus of elasticity were also observed due to the addition of SiO 2 .The composite with 5% SiO 2 exhibited the highest values of yield strength, modulus of elasticity and ultimate compression strength.Furthermore, corrosion rates (V Corr ) in the case of Al/SiO 2 (T) were lower than that of Al/SiO 2 (B) composites.

ACKNOWLEDGEMENtS
One of the authors (Munasir) is grateful to the Universitas Negeri Surabaya, Institut Teknologi Sepuluh Nopember, and the Ministry of Research, Technology and Higher Education of the Republic of Indonesia (grant number: 170/UN.38.9/HK/LT/2015).Acknowledgement is extended to Bundesanstalt für Materialforschung und-prüfung (BAM), Germany, which has provided author Munasir with the opportunity to study corrosion and conduct research on geothermal and surface treatment of corrosion in geothermal environments.

Figure 1 :Figure 2 :
Figure 1: Plots of (a) density, and (b) porosity of Al-SiO 2 composites.Measured modulus of elasticity (E c ) of the Al/SiO 2 composite is given in Figure2(a).The sample with 5% SiO 2 content demonstrated the highest E c , and the sample with 5% SiO 2 showed the lowest E c .The decreasing trend of E c may be due to individual E c of both Al and nano-SiO 2 , respectively having values of about 69 GPa and 72 Gpa.6,7As we may observe in Figure2, the modulus of elasticity does not merely decrease; it increases when the SiO 2 content exceeds 20%.This behaviour can be explained by the rule of mixture principle in which the upper bound and lower bound for the two components, Al and SiO 2 are relatively closer.21

Figure 3 :
Figure 3: Plots of (a) yield strength, and (b) ultimate compression strength of Al/SiO 2 .

Figures 6 ,
Figures 6, 7 and 8 show respectively the micrographic images as well as the elemental mapping for pure Al, 5% SiO 2 in Al/SiO 2 (B), and in Al/SiO 2 (T).In the figures, it is seen that Al appears like sheets, with SiO 2 particles entering and agglomerating into the opening gaps in the sheets.Grains of Si, Al and O elements are present.The sheets of Al appear very clearly in Figure6.Also, larger and more homogenous grains of Al are found in Figure7(a) as compared to Figure8(a).In Figure8(b), the elemental mapping viewpoint, more of γ-Al 2 O 3 can be detected.Different elemental contents of the samples of Al/SiO 2 (B) and Al/SiO 2 (T) with 5% SiO 2 are shown in Figures7(c) and 8(c).These results seem to support the XRD data as previously discussed.

Figure 12 :
Figure 12: Corrosion rate (V Corr ) of Al/SiO 2 .T refers to TMAH solution and B refers to the N-butanol solution.
. Al/SiO 2 composites were synthesised with varying content of SiO 2 (in vol.%): 0, 5, 10, 15, 20, 25 and 30 by simply mixing Al and SiO 2 in two different active solutions, namely N-butanol and TMAH, for 2 h.The mixtures were then dried at 120°C for 12 h.The dried powders were uniaxially pressed in a metal die by applying a pressure of 200 N m -2 to produce cylindrical samples with a thickness of 10 mm and a diameter of 13 mm.Heat treatment was given to the samples by pre-sintering at 200°C for 0.75 h, followed by vacuum sintering at 500°C for 2 h.The sintered samples were labeled as Al/SiO 2 (B) and Al/SiO 2 (T), standing respectively for N-butanol and TMAH treatments.

Table 1 :
Elemental contents of Al and nano-SiO 2 .

Table 2 :
Data of Tafel analysis with NOVA software for Al/SiO 2 sample (#0, example).
based environments.But, the presence of 5% of SiO 2 in the Al/SiO 2 (T) has a corrosion resistance stronger (V Corr ≈ 0.03461 mm y -1 ) than in the Al/SiO 2 (B) (V Corr ≈ 0.03968 mm y -1 ).It is because of the more amount of γ-Al 2 O 3 phase formed at Al/SiO 2 (T) than Al/SiO 2 (B) (EDX or elemental mapping shown in Figures