Effect of Alumina Microparticle-Infused Polymer Matrix on Mechanical Performance of Carbon Fiber Reinforced Polymer (CFRP) Composite

Abstract

In recent times, fiber reinforced polymer composite materials have become more popular due to their remarkable features such as high specific strength, high stiffness and durability. Particularly, Carbon Fiber Reinforced Polymer (CFRP) composites are one of the most prominent materials used in the field of transportation and building engineering, replacing conventional materials due to their attractive properties as mentioned. In this work, a CFRP laminate is fabricated with carbon fiber mats and epoxy by a hand layup technique. Alumina (Al₂O₃) micro particles are used as a filler material, mixed with epoxy at different weight fractions of 0% to 4% during the fabrication of CFRP laminates. The important objective of the study is to investigate the influence of alumina micro particles on the mechanical performance of the laminates through characterization for various physical and mechanical properties. It is revealed from the results of study that the mass density of the laminates steadily increased with the quantity of alumina micro particles added and subsequently, the porosity of the laminates is reduced significantly. The SEM micrograph confirmed the constituents of the laminate and uniform distribution of Al₂O₃ micro particles with no significant agglomeration. The hardness of the CFRP laminates increased significantly for about 60% with an increase in weight % of Al₂O₃ from 0% to 4%, whereas the water gain % gradually drops from 0 to 2%, after which a substantial rise is observed for 3 to 4%. The improved interlocking due to the addition of filler material reduced the voids in the interfaces and thereby resist the absorption of water and in turn reduced the plasticity of the resin too. Tensile, flexural and inter-laminar shear strengths of the CFRP laminate were improved appreciably with the addition of alumina particles through extended grain boundary and enhanced interfacial bonding between the fibers, epoxy and alumina particles, except at 1 and 3 wt.% of Al₂O₃, which may be due to the pooling of alumina particles within the matrix. Inclusion of hard alumina particles resulted in a significant drop in impact strength due to appreciable reduction in softness of the core region of the laminates.

1. Introduction

Advanced materials known for their outstanding mechanical qualities, such as stiffness, high strength-to-weight ratio and exceptional resistance to corrosion and fatigue, are Carbon Fiber Reinforced Polymer (CFRP) composites. Because CFRPs are lightweight and resilient to extreme environments, they are widely employed in sports equipment, automotive, civil engineering and aerospace. The continuous advancement of CFRPs is revolutionizing engineering and design by providing high-performance, sustainable solutions that open up new avenues for innovation across several industries.

Alaa Al Mushaikeh et al. [1] explored in their study about the manufacturing methods of Carbon Fiber Reinforced Thermoplastic (CFRTP) recovery techniques for carbon fibers and their versatile and potential applications across industries. CFRTPs combine the lightweight and high-strength properties of carbon fibers with the versatility and recyclability of thermoplastics, making them an attractive choice for advanced engineering solutions. However, challenges such as cost, production efficiency and recycling complexity were recommended for addressing their maximum potential. The influence of fiber orientation on the mechanical properties of Carbon Fiber Reinforced Polymer (CFRP) composites was studied by Subhedar et al. [2], specifically examining how different laminate configurations affect their strength, stiffness and flexibility. To evaluate these effects, a variety of composite laminates with different fiber angles and layer arrangements were tested, offering valuable insights into their mechanical behavior under various loading conditions. Four primary fiber orientations were explored in the study such as 0°, 45°, 90° and cross-ply configurations that combined multiple fiber angles within a single laminate. The objective of the study was to understand how the alignment of fibers influences the performance of the composite, which was critical for optimizing CFRP materials in engineering applications. Mayank and Prabhakaran [3] in their study investigated the effect of fiber sizing on the mechanical properties of Carbon Fiber Reinforced Composites, highlighting its critical role in improving the fiber-matrix interface. Fiber sizing refers to the application of surface treatments or coatings to carbon fibers, designed to enhance their bonding with the surrounding polymer matrix. This interface was very crucial in determining the overall mechanical performance of the composite, including its strength, stiffness and durability. With tailor-made surface treatments, manufacturers can create materials with enhanced strength and reliability, making them suitable for advanced applications in aerospace, automotive and structural engineering. In a detailed review work carried out by Meltem and Hasan [4], it was summarized that the CFRP was a composite material renowned for its exceptional mechanical properties, including high stiffness, strength and fatigue resistance. In CFRP, the fibers serve as the primary load-bearing component, while the matrix, typically made from resin, holds the fibers together, protecting them and transferring loads between them. This feature allowed the CFRP to outperform traditional materials like steel and concrete in many applications, particularly where high strength-to-weight ratios and durability are required. Abdullah et al. [5] examined the CFRP composites, focusing on their mechanical and thermal advantages in comparison to traditional materials. CFRP is known for its high strength, lightweight properties and excellent resistance to fatigue and thermal expansion, making it ideal for demanding applications in industries like aerospace and automotive. In these sectors, CFRP has been used for components that require precision, durability and performance under extreme conditions. However, despite its benefits, CFRP poses significant machinability challenges, which are critical for ensuring the quality and precision of the final components, particularly in product based production sectors. A review on the role of fillers in the mechanical performance of fiber reinforced polymer composites was carried out by Senthil et al. [6], who found very important implications that were used as a motivational factor for the present study. Addition of hard fillers in the polymer matrix influences significantly the behavior of the matrix phase in the composite due to their chemical nature, size, structure and shape etc., which in turn enhances the bonding between the matrix and reinforcement through increased surface area and interfacial boundary. The mechanical behavior of these hard fillers improves the load carrying capacity of the composite laminate subjected to various types of loading.

Overall, CFRP has a much higher strength-to-weight ratio than materials like steel or aluminum, making it ideal for applications where weight reduction is critical without sacrificing strength. Unlike metals, CFRP does not corrode when exposed to moisture or chemicals, making it suitable for harsh environmental conditions [7,8]. CFRP laminates are used in many structural applications for a larger extent and hence the inter-laminar shear and flexural characteristics become more important to deal with. In this aspect, investigating fillers like alumina micro particles mixed with epoxy targeting for enhanced mechanical behavior will really help to bridge the research gap and the outcomes of the investigation will be beneficial for academicians and researchers working in this field of research. An attempt is made in the present study to investigate the mechanical performance of CFRP produced by a hand layup technique and the highlight of the study is to analyze the effect of infusing alumina micro particles in the epoxy on the mechanical behavior of the CFRP laminates being influenced by the structure and surface area of alumina micro particulates. The optimized quantity of ceramic particulate that need to be infused in epoxy was also analyzed in this investigation. This study advances our knowledge of CFRP composites and highlights its increasing significance in providing high-performance, sustainable solutions for a variety of engineering domains.

2. Experimentation

2.1. Materials

In this study, a polymer matrix composite, Carbon Fiber Reinforced Polymer (CFRP) composite laminate, was fabricated using the materials and epoxy resin (chemically called polyepoxides—0.473 L (0.236 L resin and 0.236 L hardener), supplied by Vision International Ltd., Muscat, Oman), which was a thermoset polymer with resin and hardener mixing ratio of 1:1. The reinforcement was dry carbon fiber mats (3.048 m long and 3.048 m wide), supplied by Vision International Ltd., Oman, without any pretreatment, each with an average thickness 0.25 mm. Aiming to enhance the mechanical performance of the composite laminates, the filler alumina micro particles (supplied by Loba Chemie, Mumbai, India) of an average particle size of 75 microns were used along with resin in various weight fractions so that the total resin content in the composite is maintained at 40%. The composition of resin and fiber content in the composite was 40:60 by weight percentage. The various compositions used in the study for the fabrication of composite laminates are shown in

Table 1. Compositions considered during fabrication.

Composition	Resin (wt.%)	Fiber (wt.%)	Filler (wt.%)
1	40	60	0
2	39	60	1
3	38	60	2
4	37	60	3
5	36	60	4

. The physical and mechanical properties of the materials used in the CFRP composite are listed in

Table 2. Parameters and their levels considered during fabrication.

S. No.	Property	Epoxy	Carbon Fiber	Alumina
1	Mass density (g/cc)	1.2–1.25	1.8	3.9
2	Viscosity @ 25 °C (cps)	550	-	-
3	Tensile strength (MPa)	11.14	5880	262
4	Elastic modulus (MPa)	3100	228,000	370,000
5	Size	-	8 µm	75 µm
6	Color	Transparent	Black	Grey

, which also compares some basic properties with conventional materials like steel used for many structural applications.

2.2. Fabrication

The laminates were fabricated using a hand layup technique at room temperature and the step-by-step procedure is shown in Figure 1. The carbon fiber mats were stacked together by applying the resin uniformly on the entire surface of the mat and continued until the thickness of laminate reached around 2.5 mm, which was about 10 layers of carbon mats approximately. The resin was distributed to the entire surface of the fiber mat and the uniformity was ensured using a roller. After laying, the stacked laminates were loaded with a heavy weight and allowed to cure for 24 h. After curing, the composite laminates were removed from the fabrication set up, trimmed on all the edges and cut for the required dimensions for various mechanical tests as per the ASTM standards. The edges of all the specimens were filed to confirm the accuracy of the dimensions required for the tests.

2.3. Testing

A microstructural analysis as per ASTM E2809 [9] was carried out using a Scanning Electron Microscope, SEM (Model: Axia ChemiSEM, make: Thermo Fisher Scientific Limited, The Netherlands), to verify the uniform distribution of alumina particles in the resin and also confirm the composition of constituents in the CFRP composite. The agglomeration of filler particles, if any, within the laminate was also verified by this micrographical study. The sample considered for this analysis is CFRP with 4% Al₂O₃ due to the higher concentration of the filler particles. The chemical composition of the constituents and their concentration in the laminate is observed using Energy Dispersive X-ray (EDX) analysis.

Porosity is the ratio of void volume to total volume in a material and it happens in the production process of the material in different ways. It reduces the strength and stiffness of composite laminates. The porosity of the fabricated CFRP laminates was estimated using theoretical and experimental density using Equations (1)–(3). The following Equations (1)–(3) are used based on rule of mixtures according to ASTM D792 [10]. The experimental density is measured from the mass and volume of the specimen as the specimens have regular size and shape.

Theoretical density,

$${\rho }_{th}=v_f{\rho }_f+v_m{\rho }_m$$

(1)

where v_f $={\displaystyle \frac{v_f}{vtotal}}$ and $v_m={\displaystyle \frac{v_m}{vtotal}}$.

Experimental density,

$${\rho }_{ex}={\displaystyle \frac{mass}{Total volume}}$$

(2)

and

$$\%\mathit{porosity}={\displaystyle \frac{{\rho }_{th}-{\rho }_{ex}}{{\rho }_{th}}}$$

(3)

$$\mathrm{Mass}\mathrm{density}\mathrm{of}\mathrm{resin}\mathrm{and}\mathrm{filler}\mathrm{mixture},{\rho }_m{v}_r{\rho }_r+v_{fi}{\rho }_{fi}$$

where $v_r={\displaystyle \frac{v_r}{v_m}}$, $v_{fi}={\displaystyle \frac{v_{fi}}{v_m}}$.

v_f—Volume of carbon fiber mats (cm³)

v_m—Volume of resin and filler (cm³)

v_total—Total volume of laminate (cm³)

$\rho$_f—Mass density of carbon fiber (g/cc)

$\rho$_m—Mass density of resin and filler mixture (g/cc)

v_r—Volume of resin (cm³)

v_fi—Volume of filler (cm³)

$\rho$_r—Mass density of resin (g/cc)

$\rho$_fi—Mass density of filler (g/cc)

A water absorption test following ASTM D570 [11] is used to find the quantity of water absorbed under certain conditions. The factors that influence the water absorption include the type of material being tested, environmental temperature and test duration. For the water absorption test, the specimens are weighed in a dry condition and then immersed in distilled water and 0.9% NaCl solution (prepared by dissolving 9 g of NaCl, supplied by Vision International Ltd., Oman, in 1 L of distilled water) for 24 h at room temperature. Later the specimens are removed, patted dry with a lint-free cloth and weighed using a weighing machine (supplied by Lachoi, Instruments, Dubai, UAE, of a capacity 2 kg and accuracy 0.02 g). This analysis was carried out to determine the moisture absorption capacity of the CFRP laminate and the role of alumina filler in affecting the moisture absorbing rate as well. The weight gain % due to water absorption was calculated using Equation (4).

$$\%\mathrm{weight}\mathrm{gain}={\displaystyle \frac{\mathrm{Wet}\mathrm{weight}-\mathrm{Dry}\mathrm{weight}}{\mathrm{Dry}\mathrm{weight}}}\times 100\%$$

(4)

The hardness of the CFRP laminates is measured using Brinell hardness tester (Model: WP 300 of 20 kN capacity, make: Gunt Hamburg, Germany) as per ASTM E10 [12] with an approximate load of 10 kN using a hardened steel ball indenter of 10 mm diameter. The loading speed is maintained at 1 kN/s. The hardness is measured at three different locations on the same specimen and the average hardness is estimated.

The composite laminate specimens for various tests are shown in Figure 2. The laminates were tested for their tensile behavior using a Universal Testing Machine of a 50 kN capacity (make: Lloyd EZ-50, UK), shown in Figure 3a. The tensile test specimens with the dimensions 250 × 25 × 2.5 mm were prepared according to the standard ASTM D3039 [13]. The tensile test was carried out with a gripping length of 25 mm on both sides and a strain rate of 5 mm/min and the data were recorded using the Data Acquisition (DAC) system(Software: Nexygen 4.1) attached to the machine. The stress-strain behavior of the composite laminates is explained in detail in the next section. The composite laminates were also subjected to test their flexural, shear and impact behaviors and the corresponding specimens were prepared as per the standards ASTM D790, ASTM D2344 and ASTM D256 [14,15,16], respectively. The experimental set up is shown in Figure 3. The flexural test and inter laminar shear test was carried in a 20 kN Universal Testing Machine (Make: Gunt Hamburg—WP 300, Germany), shown in Figure 3b. The span of the flexural test specimen was 100 mm and the transverse load was applied at a uniform speed of 5 N/s. The load and corresponding lateral deflections were recorded using the DAC system attached to the machine. The loading conditions remain same for the inter-laminar shear test and the testing characteristic was a short beam test with a 40 mm span between the supports. The flexural strength and inter-laminar shear strength of the laminates were determined using Equations (5) and (6). The impact strength of the composite laminate was determined by conducting an Izod impact test to study the influence of alumina micro particles enhancing the core toughness and damping capacity of the laminate. The impact strength of the laminate was calculated using Equation (7).

$$\mathrm{Flexural}\mathrm{strength},\mathsf{\sigma }={\displaystyle \frac{3FL}{2b{d}^2}}$$

(5)

$$\mathrm{Inter}-\mathrm{laminar}\mathrm{shear}\mathrm{strength}(\mathrm{ILSS}),\tau ={\displaystyle \frac{0.75\mathrm{F}}{\mathrm{b}\times \mathrm{d}}}$$

(6)

$$\mathrm{Impact}\mathrm{strength},\mathrm{I}={\displaystyle \frac{\mathrm{Impact}\mathrm{Energy}\mathrm{in}\mathrm{Joules}}{\mathrm{w}\times \mathrm{t}}}$$

(7)

where

F—maximum compressive force (N)

L—span length (mm)

b—width of the laminate (mm)

d—thickness of the laminate (mm)

w—width of the laminate at notch (mm)

t—thickness of the laminate at notch (mm)

3. Results and Discussions

3.1. Micrographs

The composition of the constituents, matrix and reinforcement in the CFRP laminate and also the uniformity in the distribution of alumina particles in the laminate were analyzed by the SEM micrograph. Figure 4a shows the SEM micrograph of CFRP laminate with 4% Al₂O₃. The reason for selecting this laminate was the higher weight % of Al₂O₃. The SEM micrograph revealed the fact that the alumina micro particles were uniformly distributed in the matrix pool of the laminate with no significant agglomeration. The carbon fibers and the resin particles were clearly observed in the micrograph. It was also observed from the micrograph that the carbon fibers in the laminate have a uniform diameter around 5 µm and their mat pattern arrangement. The cross-linkages of carbon fiber in the mats were clear in the micrograph. The resin and alumina particles are well embedded with the carbon fiber, which was clear from the micrograph.

The corresponding EDX spectrum of CFRP laminate with 4% Al₂O₃ obtained along with the SEM micrograph is shown in Figure 4b. The elemental weight percentage at various locations on the SEM micrograph from the EDX spectrum is listed in

Table 3. Elemental weight %.

Element	Weight %
	Point 1	Point 2	Point 3	Point 4
Cu	0.0	0.0	0.0	0.1
C	33.6	22.4	20.0	30.0
O	34.8	50.9	58.0	52.3
Si	0.5	0.3	0.4	0.2
S	13.2	12.0	9.6	7.9
Ca	17.9	14.4	12.0	9.5

, which confirmed that there were no impurities or other chemicals present in the constituents of laminate except some traces of copper and platinum. This may be due to the impurities added from the metallic roller used during the fabrication of the CFRP laminate.

3.2. Mass Density and Porosity

It was observed from Figure 5a the density of the laminates was gradually increased with increase in the weight % of Al₂O₃. The denser and harder alumina particles increased the density of the CFRP laminate appreciably. The percentage of voids decreased with increase in the percentage of alumina particles in the resin. The highest void content of around 32% was observed for CFRP specimens with 0% Al₂O₃, whereas the lowest for CFRP specimens with 4% Al₂O₃. By adding a filler material like alumina, the interlocking between the matrix and fiber in the composite was increased significantly, which in turn reduced the porosity. The alumina micro particles orient and align themselves in the voids between the fiber and the matrix, which results in the steady increase of density. The crystalline structure of the alumina particles enhances the positioning within the interlocking areas of the laminate. It was observed that a drastic drop in porosity resulted from increasing the alumina from 0 to 1%, whereas on the other side, when the alumina content was increased further to maximum of 4%, porosity dropped very marginally. This may be attributed to the fact that the irregular crystalline structure of alumina particles resists themselves in aligning among them and leaving voids between them.

3.3. Water Absorption

It was observed from the results of water absorption, shown in Figure 5b, that the weight gain percentage due to water absorption was highest at about 27.84% for CFRP laminate with 0% Al₂O₃. In general, carbon fibers do not absorb water and the resin absorbs the water and increases its plasticity. In CFRP laminate with 0% Al₂O₃, the water absorption is higher due to the appreciable reactivity of the resin and the absence of the filler material. The water absorption took place by a diffusion process in the interfaces between the carbon fiber and the resin. The water gain % gradually dropped with an increase in the weight % of Al₂O₃ micro particles. The improved interlocking between the fibers due to the addition of filler material reduced the voids in the interfaces and increased the path for the water diffusion and in turn reduced the plasticity of the resin too. These results complement the results of hardness being increased significantly by adding the alumina fillers. In general, water absorption in the polymer composite laminates occurs due to the relaxation of segments of the polymer chain in the composite laminate and in turn the increase the diffusion rate of water molecules. This relaxation process of the polymer chain segments might be disturbed significantly with 0.9% NaCl solution due to the presence of salt ions. The trend of water gain % as seen in Figure 5b complements the results of hardness of CFRP laminates. A non-linear trend exists after 2% alumina, which may be due to possible agglomeration at a higher weight fraction of alumina. The irregular structure of alumina particles at higher weight fractions resulted in voids across the pool of particles, which in turn increased the path for water absorption. These voids gradually reduced at further, higher weight fractions of alumina particles.

3.4. Hardness

The Brinell hardness of the CFRP specimens is presented in Figure 6. A steady rise in hardness was observed in the CFRP specimens with increase in weight % of Al₂O₃. The lowest hardness of 24.71 kgf/mm² was observed for CFRP with 0% Al₂O₃. This was due to the soft matrix, with resin being the major content in the laminate. As the alumina content is increased in the laminates, the harder alumina particles contribute to the improvement of hardness of CFRP specimens. With a higher content of harder alumina, the interlocking capacity of the laminate is increased significantly and also increased the hardness due to increased surface area of the alumina. The highest hardness of 39.72 kgf/mm² was observed for CFRP with 4% Al₂O₃, which was around 60% higher than for 0% laminate. It was observed from the figure that a marginal rise exists between 3% and 4%.

3.5. Tensile Strength

The observations obtained during the tensile test of CFRP laminates is presented in Figure 7a as stress-strain diagrams. It was observed from the figure that the maximum tensile strength was obtained for CFRP laminate with 2% Al₂O₃ micro particles and the least strength was observed for CFRP laminate with 1% Al₂O₃. The stress-strain behavior of the CFRP laminates confirmed that the laminates were brittle in nature, which may be due to epoxy in the laminate, which was a thermoset polymer and the presence of alumina micro particles in the laminate. The inclusion of hard ceramic particulates with the epoxy had increased the strength of CFRP laminates significantly and on the other side reduced the plasticity or strain rate too. From Figure 7b, it can be noticed that a maximum tensile strength of 400 MPa was obtained for CFRP laminates with 2% and 4% Al₂O₃ with corresponding strain of 0.02 mm/mm. Initially, with the addition of Al₂O₃ micro particles, the flow behavior of the epoxy was restricted around the fibers and to a certain extent affected the tensile strength also. With subsequent increase in the content of Al₂O₃ micro particles, the tensile strength increases reasonably due to extended grain boundary and enhanced interfacial bonding between the fibers, epoxy and alumina particles [17,18,19,20]. The non-linear behavior of tensile strength was similar to that of the observation made in water gain %. Although there was a positive trend in tensile strength with respect to the weight fraction of alumina, particularly at 1% and 3%, there was a significant drop observed, which may be due to the disturbance in the alignment such as pooling of alumina particles within the matrix. It may be also due to certain uncertainty in the mechanical mixing of alumina fillers in the matrix. The non-uniform speed of mixing may also be one of the reasons for the non-linear behavior in the tensile strength with respect to the weight fraction of alumina. This was not due to experimental error because the observation plotted was the average of three trials with similar configured specimens and at the same conditions. The water absorption in the composite laminate led to the degradation of the material’s structure and interfacial strength between the matrix and fiber of the composite, which in turn weakens the bonding strength. However, there was a contradiction initially for 0 to 1% of Al₂O₃, after which the water absorption behavior correlated with the tensile strength behavior of the laminate.

The tensile stiffness computed from the tensile test of CFRP laminates is shown in Figure 7c. It was observed that the addition of Al₂O₃ micro particles to epoxy had reduced the resistance against deformation through breaking the bond between the resin and fiber initially but at the same time increased the content of Al₂O₃ micro particles and the tensile stiffness of the laminates increased significantly. This may be due to the fact that the irregular crystalline structure of Al₂O₃ micro particles increases the interfacial surface area with extended grain boundaries to bear the tensile load. It was observed from the failure behavior of the CFRP laminates under tension that the carbon fibers in the laminate across the entire cross-section were subjected to rupture along the direction of loading. The fiber rupture due to appreciable rise in brittleness had resulted in the failure of the composite laminate [21].

3.6. Flexural Strength

The results of flexural behavior of CFRP laminates is shown in Figure 8a,b. Laminates with 0% Al₂O₃ micro particles were found to be weaker against flexural loads compared to that of other laminates. The carbon fibers in the laminate are inferior against transverse loads compared to that of tensile loads, which in turn led to weaker flexural strength. Increase in the weight fraction of Al₂O₃ micro particles in the epoxy resulted in an increase of flexural load capacity to a greater extent. Maximum flexural strength of about 1.5 MPa was observed for the CFRP laminate with 4% Al₂O₃. Adding ceramic particulates in the epoxy arrests the deformation against transverse load and withstands maximum load with least bending due to the improved interlocking capacity of the laminate. In contradiction to tensile behavior of the CFRP laminate, with respect to flexural loading, the flexural strength was significantly improved steadily with the increase of alumina micro particles. This may be due to the direction of loading, i.e., transverse direction, being resisted more by the carbon fibers and alumina micro particles [22,23,24].

3.7. Inter-Laminar Shear Strength

The inter-laminar shear strength of CFRP laminates is presented in Figure 9a. The trend of inter-laminar shear strength was similar to that of flexural strength. Compared to flexural strength, the inter-laminar shear strength of the CFRP laminates were found to be appreciably higher for all weight fractions of Al₂O₃ micro particles. Maximum shear strength of about 35 MPa was obtained for laminate with 4% Al₂O₃. This was about twenty times higher than that of the flexural strength for the same composition. The carbon fibers in the laminate with improved grain boundary surface resulted in an enhanced shear load capacity of the laminates. The results of water gain correlated with the inter-laminar shear behavior [25,26]. Improvement in the inter-laminar shear strength of the composite laminates had resulted in a significant drop in moisture absorption. Inclusion of alumina fillers played a vital role in this property enhancement.

3.8. Impact Strength

Inclusion of Al₂O₃ micro particles in the epoxy resulted in a negative effect with respect to impact strength, as shown in Figure 9b. A drastic drop in impact strength being observed with increase in weight fraction of Al₂O₃ micro particles from 0% until 4%. CFRP laminate with 0% Al₂O₃ had an impact strength around 0.2 J/mm², whereas for CFRP laminate with 4% Al₂O₃, it was reduced to 0.1 J/mm², almost half of its strength. The hard ceramic alumina particles had affected the toughness of the laminate abruptly with increase in hardness and in turn reduced the impact strength to a larger extent [27]. The core toughness of the composite laminate was suppressed by the addition of ceramic particles and affected the softness significantly [28].

4. Conclusions

The CFRP laminates were fabricated by a hand layup technique using carbon fiber mats and epoxy resin. Alumina micro particles were used as a filler, added with resin at five different weight fractions such as 0, 1, 2, 3 and 4%. The alumina micro particles had influenced the mechanical performance of the CFRP laminates significantly with the following conclusions.

➢

The alumina micro particles were uniformly distributed in the matrix pool of the laminate with no significant agglomeration. The carbon fibers and the resin particles were clearly observed in the micrograph. The denser and harder alumina increased the density of the CFRP laminate appreciably. The highest void content of around 32% was observed for CFRP specimen with 0% Al₂O₃, whereas the lowest was for the CFRP specimen with 4% Al₂O₃. The irregular structure with a larger surface area of alumina particles resulted in improved mass density and reduced porosity.

➢

The highest hardness of 39.72 kgf/mm² was observed for CFRP with 4% Al₂O₃, which was around 60% higher than for 0% laminate due to enhanced strain hardening that occurred at a higher weight fraction of ceramic filler. The water gain % gradually dropped with the increase in the weight % of Al₂O₃ micro particles. A significant difference in the percentage of water absorption was observed in the CFRP laminates in pure distilled water and 0.9% NaCl solution.

➢

The tensile strength, flexural strength and inter-laminar shear strength were significantly enhanced with the addition of alumina micro particles, except for a drop for tensile behavior at 1% and 3%, which may be due to agglomeration of the micro particles in the matrix pool of the laminate. The carbon fibers in the laminate with improved grain boundary surface by the addition of alumina micro particles, resulted in enhanced shear load capacity of the laminates. The hard ceramic alumina micro particles had affected the toughness of the laminate abruptly by reducing the ductility and softness of the material and in turn reduced the impact strength to a larger extent.