The surface morphologies of the calcinated rice husk at 600°C were studied using FESEM setup by FEI Netherlands. The micrographs displayed in the Fig. 2 display that the sample consists of consists of the gathering of hexagonal block-like silicates with various size particles with shape stand almost same. As it is seen in higher magnifications it is clear that the shape is like a floret of cauliflower. The surface is very fluffy and it has loose accumulation of particles. It is reported in the literature that the dual size structures are vital in super hydrophobicity generation in material [16, 17].
Energy dispersive spectroscopy or EDX analysis of the calcinated rice husk confirms the maximum silica content.
3.1 Mechanical Properties evaluation
In composites the enhancement of the mechanical properties such as tensile, flexural, compressive etc. depend on several factors such as type of filler, matrix material, size or shape of the reinforcement, dispersion of filler in matrix, fiber orientation, filler concentration and adhesion between the matrix and filler etc.
Tensile Properties
Tensile properties play an important role to analyze the strength of any engineering material. The tensile tests were conducted of the fabricated samples according to ASTM standard at room temperature, using a universal testing machine (INSTRON).
The tensile test results of RH and Silica particle-based polymer composites with varying wt.% (2 wt.%, 4 wt.% and 6 wt.%) are shown in Fig. 3.
It is seen that with the addition of RH filler with polymer, the strength is substantially increasing at 2 wt.% compared to pure epoxy composite. But a sudden change was also observed that the higher filler loading ie 4 wt.% and 6 wt.% of RH had a negative impact on the tensile strength of the composite. Which occurs due to the weak interfacial bonding between the filler and the polymer, because the filler is hydrophilic whereas polymer is hydrophobic in nature. The compatibility of polymer and natural filler is not up to the mark.
On the other hand, 4 wt.% silica particle filler composites were found to be effective to improve the tensile properties of the composite. The filler-matrix interaction was found to be better in polymer composite when compared to pure matrix and rice husk filler composite. Because of calcination process at 600oC, amorphous silica is formed, which is hydrophobic in nature. Epoxy is also hydrophobic in nature, so there is a good interfacial bonding occurs between the amorphous silica and pure polymer. Which leads to increase the strength of the composite. Further increasing of filler content, the strength of the composite decreased. It may be due to silica has a lower bulk density and larger average particle size which increases the filler content and start forming agglomerations.
Figure 4 illustrates the load-displacement curves of the tensile strength for neat epoxy, various filler loading (2,4,6wt%) of rice husk and silica. The highest tensile failure load is observed 4wt% silica filler composites followed by 2wt% silica and 6wt% silica. When the filler percentage increases from 2wt % to 4wt% of silica the resistance to failure also increases but at higher filer loading the load resistance is reduced. Where as in rice husk the maximum load is observed in 4wt% followed by 2wt% rice husk the least load is observed for 6wt% rice husk filler loading. The results also indicate that the behavior of failure is not pure brittle where as for pure epoxy the failure is brittle which has less elongation.
Figure 5 illustrates the micrographs of the fracture surface samples of silica and rice husk for 4wt% filler loading. It is clearly observed that the composite with rice husk filler has many cracks and filler pull out due to tensile loading is noticed this may be due to agglomerations and week interface between the filler and the matrix. But in silica filler no cracks are observed and the filler and matrix interface are good.
Flexural properties
Flexural test was carried out according to ASTM D790 standard method. Figure 6 shows flexural strength of the RH and silica particle-based polymer composite. It is observed from the figure that due to incorporation of RH fiber or silica particle with polymer the flexural strength increasing significantly as compared to polymer. Among all the filler concentration, 6 wt.% silica particle-based polymer composite shows better strength. Normally silica has good resistance to deformation due to presence of hydrogen bond in composites. It is also observed that the flexural strength increases as the filer loading increases up to 6wt% where as in rice husk as the filler loading increase from 4 to 6wt% the strength decreases this is due to poor interfacial bonding between the fillers at higher filler loading.
In tensile strength 4wt% filler loaded composites shows better strength where as in flexural 6wt% shows better strength it is because high density filler when added in to low density polymers all the fillers are settled at lower level of the composites at the time of fabrication in higher filler loading some particles are settled in the lower layers some are distributed to the top layers and form like functional polymer composites which enhances the bending properties.
In natural fibers due to hydrophilic nature this type of distribution is not possible and a clear agglomeration of the fillers takes place which decreases the strength.
Ultimate compressive strength
The results of the compressive strength of RH and silica reinforced composites are presented in the Fig. 7. Compared to the neat polymer composite, the optimum compressive strength was found to be at 2 wt.% for both filler composite. Furthermore, addition of filler at 4 wt.% and 6 wt.% reduces the compressive strength, which is due to the weak interlingual bond between the filler and the matrix material, resulting in a weak load transfer from the reinforcement, leading to failure.
Maximum ultimate strength was found at 62 MPa in silica particle polymer composite followed by 57 MPa and 52MPa in RH and epoxy composite respectively. Further increased in silica particles with polymer, the strength is gradually decreasing. This may be due to the lower bulk density and larger average particle size of silica.
3.2 Effect of Erosion wear on fabricated composite
In the engineering field mostly, the behavior of the materials is grouped by two vital categories such as “Brittle and Ductile”. Even though this grouping is not conclusive [18]. Among the several polymer matrix composite, thermoplastic polymer matrix composite generally shows ductility in nature because of peak erosion rate occurs at 30o impact angle, since cutting mechanism is predominant in erosion. Even though thermosetting polymer matrix composites erode in a brittle manner with the peak erosion occurring at normal impingement angle ie. 90o. Still there is a controversy about this failure classification because erosive wear behavior sturdily depends on experimental conditions and the composition of the target material.
In the present work erosion tests is conducted for different compositions of RH filler such as 2 wt.%,4 wt.% and 6wt.% composite at different impingement angle (30o, 45o, 60o and 90o) and their respective erosion curves are plotted with keeping other parameters are constant (impact velocity = 72m/s, stand-off distance = 10mm and erodent size = 200 ± 50 µm).
Figure 8 shows the dependence of erosion rate of RH filler epoxy composite with different filler composition at various impingement angle. Among different wt.% of filler, 2 wt.% RH filler epoxy composite shows minimum wear rate. It can be observed that the peak erosion rate occurs at θ\(=\)30o impact angle for all the samples irrespective of filler content. This shows the ductile behavior of the RH polymer composite. As the literature articulates that the nature of the epoxy polymer is semi ductile (θ = 60o), but after incorporating of RH filler with epoxy, the behavior of the composite changes to ductile. It is further noted that with increased filler content with polymer the erosion rate of the composites significantly increasing. It may be due to in 2 wt.% filler the polymer content is more, so composite is taking some time to eroded but in higher content of filler the wear rate of the composite is more, it may be due to the rice husk is a soft fiber, so it is eroded easily.
The erosion wear rate of silica particle filler composites as a function of impingement angle (°) are shown in Fig. 9. It can be seen that reinforcing with silica particles reduces wear rate of the silica-epoxy composites quite significantly. As the filler content increases the wear rate of the Si-epoxy composite decreases. The 6 wt.% silica particles filled composites, shows minimum wear while for both 2 wt.% and 4 wt.% silica content composites shows maximum erosion wear rate. The peak erosion occurs is found to be at θ = 30o which is purely ductility in nature. It is also important to note that the sample with minimum filler content exhibits better erosion resistance. Rather than of silica particle rice husk also consists of some hardest element such as CaO, MgO, which helps to increase the wear resistance of the composite.
The Fig. 10 shows the comparison of RH filler with Si particle based polymer composite. It is clearly observed that 6 wt.% silica particle filler composite shows minimum wear rate as compared to other comosite.