Ishaya Musa Dagwa
| Ogochukwu Eugenia Okeke | Musa Tanko Zarmai | Ibrahim Momoh-Bello Omiogbemi | Adekunle Akanni Adeleke | Peter Pelumi Ikubanni | Emmanuel Omotosho*
© 2025 The authors. This article is published by IIETA and is licensed under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
OPEN ACCESS
This study examines the development and application of composite materials based on dry and carbonized bamboo culm particles for friction material applications, particularly as a potential replacement for asbestos-based materials. Due to the environmental and health risks associated with asbestos, sustainable, high-performance alternatives are essential. Carbonized bamboo particles offer excellent thermal stability, while bamboo culm enhances strength. The development involved selecting bamboo culm composites, carbonizing, drying, and integrating them with other materials to achieve the desired friction properties. The composite materials were developed in a laboratory setting, and mechanical and thermal experiments were used to characterize the materials' properties. A systematic experimental design approach, including the Taguchi method, was employed to optimize the formulation and processing parameters. Test results show that the developed composite material has high mechanical strength, with an average tensile strength of 10.31 ± 0.21 MPa, a modulus of elasticity of 80.07 ± 1.60 MPa, an impact strength of 0.6912 J/mm, and a Vickers hardness of 107 HV. Thermal stability was further confirmed during testing, with a maximum temperature of 950℃ at a heating rate of 10℃/min. The developed composite material also performed well in friction material tests, with a friction coefficient of 0.378 and a wear rate of 0.15 mm³/Nm. Thermogravimetric analysis showed that optimized carbonized bamboo brake pads had lower temperature degradation than commercial ones. Results indicated that dry and carbonized bamboo culm composite materials are effective for friction applications, performing comparably to commercial brake pads.
carbonization, bamboo culm, composite, mechanical properties, TGA, SEM
Automobile brakes are essential for controlling vehicle speed by converting kinetic energy into heat, which is primarily dissipated into the atmosphere [1-3]. However, the use of asbestos fibers in brake pad manufacturing has raised significant health and safety concerns due to their carcinogenic properties. Although asbestos was favored in brake pads for its sound absorption, moderate tensile strength, and heat resistance, it has been recognized as a carcinogen since the 1970s linked to severe health issues such as malignant lung cancer, mesothelioma, and asbestosis (an interstitial lung disease caused by inhalation of asbestos fibers). Although the U.S. Environmental Protection Agency (EPA) banned asbestos in brake pads in March 2024, risks remain for those exposed to older brake pads containing asbestos, highlighting the need for continued awareness and research into safer alternatives [2, 4-6]. To address these concerns, researchers have explored alternative materials such as metals, ceramics, carbon, and organic composites for brake pad production [7]. Natural fibers, particularly agricultural waste, have been increasingly utilized as sustainable replacements for asbestos fibers [8]. Ibhadode and Dagwa [7] developed asbestos-free friction lining materials from palm kernel shells, which performed satisfactorily compared to commercial asbestos-based brake pads. Similarly, Ruzaidi et al. [9] investigated palm slag-based brake pads, demonstrating their effectiveness as a friction material. Other studies have explored combinations of coconut fiber and palm kernel shell [10, 11], kenaf fibers [12], and bagasse [13] as potential replacements for asbestos.
Bamboo culm, a lignocellulose biomass, presents a promising alternative due to its rapid growth rate, high mechanical strength, and organized fiber distribution. Its primary chemical components—cellulose, hemicellulose, and lignin—contribute to its structural integrity [14-16]. Azeez and Orege [14] found that the chemical composition of bamboo fibers varies with age, while Vivas [15] linked bamboo’s mechanical properties to factors such as species, density, and moisture content. Yunhai et al. [16] demonstrated that carbonized bamboo fibers enhance friction material performance by reducing noise and retaining wear debris.
Brake pads are composed of several key materials that work together to ensure effective braking performance [17, 18]. These include binding components, or binders, which hold the other materials together; abrasive components that create friction against the brake rotor; and performance components such as temperature-specific lubricants that enhance braking efficiency. Additionally, filler materials are added to improve specific properties, while structural materials help maintain the pad's shape during operation. The precise formulation of these components is crucial for optimizing braking characteristics [17-22]. Figure 1 illustrates the transformation process from fresh bamboo culm to carbonized bamboo culm. While various bio-based materials have been explored in brake pad production, the potential of bamboo ash (BA) remains under-researched, presenting an opportunity for further investigation into its applications in friction materials.
As previously discussed, asbestos-based friction materials have long been used in brake pad production but were later identified as carcinogenic, prompting widespread concern and subsequent research into safer alternatives [23, 24]. This study investigates the potential of locally sourced bamboo culm as a sustainable and eco-friendly base material for brake pad production, providing an alternative to carcinogenic asbestos fibers. This research adopts an innovative approach by combining carbonized bamboo culm with additives such as epoxy resin, cast iron filings, graphite, and aluminum oxide to create a composite brake pad. The objectives of this study are: (i) to process and optimize bamboo culm for friction material applications, (ii) to fabricate a composite brake pad using carbonized bamboo, (iii) to evaluate its mechanical and tribological properties, including wear rate, hardness, and friction coefficient, and (iv) to compare its performance against commercial brake pads. By focusing on bamboo culm as a renewable resource, this study contributes to sustainable material development while addressing health and environmental concerns associated with asbestos use.
Figure 1. Material transformations (a) fresh bamboo culm, (b) dry cut bamboo culm, (c) bamboo fibres, and (d) carbonized bamboo culm
2.1 Materials
The materials utilized in this study include bamboo culm, epoxy resin (Epochem 205: Bisphenol-A-glycidyl ether), hardener (Epochem 105: Triethylenetetramine), graphite, aluminum oxide, and cast-iron filings, all sourced from a chemical shop in Zaria, Nigeria. The bamboo culm was harvested from a swampy riverside area in Sun-City, Abuja, where bamboo trees grow abundantly in the wild.
2.2 Experimental procedure
The sample of brake pads produced in this research work was carefully done according to Lobovikov et al. [25], following the standard procedure as described by Azeez and Orege [14] and Deng [26], evaluated with the standard test method adopted by Vivas [15]. The following methods were adopted during this research work: processing of the material composites and property determination of the prepared samples.
2.2.1 Processing of the bamboo culm
Three dozen bamboo culms, each length 100 mm, were collected and sun-dried for three weeks to remove the moisture contents from 48% to 15%, after which it was cut into smaller sizes using a hand saw. The bamboo culm was ground into a powdery form using a grinding machine (Model: Black & Decker CDJ15) and then sieved with various sieve sizes of 75, 150, and 212 microns to get the fine powder that was used for the experimental analysis. The ground-based material was divided into two parts; one part was carbonized, and the other part was left uncarbonized. The first part was carbonized using a muffle furnace at different temperatures (400, 500, 700, and 800℃) and time. The effect of carbonization temperature (400, 500, 700, and 800℃) on the mechanical properties of the bamboo-based composite shows a progressive improvement up to an optimal point. As temperature rises, notable enhancements in hardness, density, and wear resistance occur due to increased carbon content and structural transformation [27-29]. Figure 2 depicts the flow chart of the carbonization process from inception to completion.
2.2.2 Formulations of the hybrid
Table 1 shows various compositions of carbonized and un-carbonized base materials that were used in determining the effects of different particle sizes of the ground base material. The experiment was conducted by measuring and weighing all the materials used in the right proportion using the standard rule of mixture which agrees with what researchers have reported in previous literature [8, 30, 31], 30 ml of epoxy resin and 15 ml of hardener was poured into a container and thoroughly stirred for about 2 minutes. Thereafter, the other additives (Graphite, Aluminum oxide, and Cast-iron fillings) were poured consecutively into the earlier mixture of epoxy resin with a hardener and stirred for another 3 minutes to obtain a homogenous mixture. The mixture was further poured into a fabricated mold of 5 mm, which had been covered with paper foil, which was then put into a compression molding machine at a temperature of 130℃ and pressure of 2.5 MPa and allowed to cure for 5 minutes. Thereafter, it was allowed to cool for 2 minutes and then removed from the mold and weighed. This process was repeated for all the carbonized and un-carbonized samples. The samples were cut to different standard dimensions for various analyses. The 2.5 MPa pressure was used to avoid air traps on the composite material, as well as for compaction purposes. Based on some previous works by researchers [7, 13, 32, 33], trial formulations were carried out using 75, 150, and 212 µm particle sizes of bamboo culm base material to ascertain the optimal particle size.
The molding parameters (temperature, pressure, time, and heat treatment) were optimized using the Taguchi L9(3)4 orthogonal array method of experimental design. The method comprises 4 factors (molding parameters) with 3 levels each, as indicated in Table 2. The orthogonal array for these parameters can be seen in Table 3.
Figure 2. Flow chart of the carbonization process
Table 1. Different particle size formulations
|
|
Materials |
Carbonized Particle Sizes |
Un-Carbonized Particle Sizes |
||||
|
|
|
75 µm |
150 µm |
212 µm |
75 µm |
150 µm |
212 µm |
|
|
|
A1 |
A2 |
A3 |
A1 |
A2 |
A3 |
|
1 |
Epoxy Resin |
30 |
30 |
30 |
30 |
30 |
30 |
|
2 |
Hardener |
15 |
15 |
15 |
15 |
15 |
15 |
|
3 |
Base Material |
40 |
40 |
40 |
40 |
40 |
40 |
|
4 |
Graphite |
5 |
5 |
5 |
5 |
5 |
5 |
|
5 |
Aluminium Oxide |
5 |
5 |
5 |
5 |
5 |
5 |
|
6 |
Cast Iron Fillings |
5 |
5 |
5 |
5 |
5 |
5 |
|
|
TOTAL |
100 |
100 |
100 |
100 |
100 |
100 |
|
|
|
B1 |
B2 |
B3 |
B1 |
B2 |
B3 |
|
1 |
Epoxy Resin |
30 |
30 |
30 |
30 |
30 |
30 |
|
2 |
Hardener |
15 |
15 |
15 |
15 |
15 |
15 |
|
3 |
Base Material |
35 |
35 |
35 |
35 |
35 |
35 |
|
4 |
Graphite |
5 |
5 |
5 |
5 |
5 |
5 |
|
5 |
Aluminium Oxide |
10 |
10 |
10 |
10 |
10 |
10 |
|
6 |
Cast Iron Fillings |
5 |
5 |
5 |
5 |
5 |
5 |
|
|
TOTAL |
100 |
100 |
100 |
100 |
100 |
100 |
|
|
|
C1 |
C2 |
C3 |
C1 |
C2 |
C3 |
|
1 |
Epoxy Resin |
30 |
30 |
30 |
30 |
30 |
30 |
|
2 |
Hardener |
15 |
15 |
15 |
15 |
15 |
15 |
|
3 |
Base Material |
30 |
30 |
30 |
30 |
30 |
30 |
|
4 |
Graphite |
10 |
10 |
10 |
10 |
10 |
10 |
|
5 |
Aluminium Oxide |
5 |
5 |
5 |
5 |
5 |
5 |
|
6 |
Cast Iron Fillings |
10 |
10 |
10 |
10 |
10 |
10 |
|
|
TOTAL |
100 |
100 |
100 |
100 |
100 |
100 |
Table 2. Molding parameters and levels
|
S/N |
Factors |
Levels |
||
|
1 |
2 |
3 |
||
|
1 |
Molding Temperature (℃) |
125 |
135 |
145 |
|
2 |
Molding Pressure (MPa) |
2.0 |
2.5 |
3.0 |
|
3 |
Molding Time (Min) |
4.0 |
6.0 |
8.0 |
|
4 |
Heat Treatment (h) |
1.0 |
3.0 |
5.0 |
2.3 Research characterization and optimization of brake pad samples
To determine the elemental compositions of carbonized and uncarbonized composites, samples were prepared and analyzed using Energy Dispersive X-ray Fluorescence (EDXRF) spectroscopy. This technique provides qualitative and quantitative data on elemental constituents, including major oxides like SiO2. The EDXRF spectra are acquired and analyzed to identify peaks that indicate specific elements and their concentrations. Findings are documented in tables and figures to clearly present the elemental compositions of the composites.
To obtain the empirical values of the important analytical brake pad properties, standard tests were performed on each set of samples.
Table 3. Orthogonal array for the molding parameters
|
Experimental Run |
Moulding Parameters |
|||
|
Molding Temperature (℃) |
Molding Pressure (MPa) |
Molding Time (min) |
Heat Treatment (Annealing) (h) |
|
|
T1 |
125 |
2.0 |
4.0 |
1.0 |
|
T2 |
125 |
2.5 |
6.0 |
3.0 |
|
T3 |
125 |
3.0 |
8.0 |
5.0 |
|
T4 |
135 |
2.0 |
6.0 |
5.0 |
|
T5 |
135 |
2.5 |
8.0 |
1.0 |
|
T6 |
135 |
3.0 |
4.0 |
3.0 |
|
T7 |
145 |
2.0 |
8.0 |
3.0 |
|
T8 |
145 |
2.5 |
4.0 |
5.0 |
|
T9 |
145 |
3.0 |
6.0 |
1.0 |
2.3.1 Density determination
According to Naik et al. [34], the mass of the samples was analyzed to an accuracy of ±0.2 g to determine the composites' density using the ASTM D792-20 standards [35]. Each sample was weighed on a computerized scale to obtain the various masses (m), which were then used to calculate the densities of each sample. The average was calculated after this experiment was run in triplicate per sample. Eq. (1) was used to evaluate the density of the composites.
Density (ρ)=[M(g)V( cm3)] (1)
where, M = mass of sample (g) and V = volume of sample (cm3).
2.3.2 Water absorption test
A testing procedure used to verify the moisture content of the produced hybridized composite as a proportion of its dry weight is called the water absorption analysis. This test was carried out in accordance with ASTM D570, which Edokpia et al. [33] and Nishino et al. [36] adopted, using a rectangular sample of dimension (50 × 50 × 5 mm) acquired from the developed composite samples. To remove any potential moisture, the samples were dried in an oven at 50℃ for 24 hours [37]. For 24 hours, the samples were completely submerged in distilled solution at 25℃ (77 °F). After that, they were taken out, dried with filter paper, and weighed once more with a digital scale (Model: BL20001). In this instance, the percentage water absorption (WA) formula from Eq. (2) was used to evaluate the water absorption utilizing the material's weight gain approach.
WA=[W2−W1 W1]×100 (2)
where, W1 is the dry weight and W2 is the wet weight of samples.
2.3.3 Oil absorption test
The oil absorption test was conducted by prior weighing each composite sample on a digital weighing balance. Afterward, the samples were immersed separately in SAE60 automotive engine oil at time intervals of 24 hours. The composite samples were removed from the oil, cleaned using a dry cloth, and re-weighed. The difference in weight between the initial and final weight was calculated and recorded, as well as used to determine the absorption rate as posited by Edokpia et al. [33]. The weight measurements were taken periodically at time intervals of 24 hours for the whole sample. The percentage of oil absorption of the composites was calculated using Eq. (3) as adopted by Smales [38];
OA=OA1−[OA0OA0×100] (3)
where, OA (%) is the percentage of oil absorption, OA0 is the initial weight of the composite before immersion, and OA1 is the final weight of the composite after immersion. The oil absorption result for the carbonized composite materials was recorded and tabulated which was used to ascertain the diffusion mechanism. Figure 3(a) and (b) shows the absorption test samples soaked in distilled water and SAE60 automotive oil, respectively.
Figure 3. Absorption test samples soaked in (a) distilled water and (b) SAE60 automotive oil
2.3.4 Hardness test
To determine the hardness values of the composites, the Vickers hardness testing machine (Model MV1-PC) was used in accordance with the ASTM D-2240 standard as reported by some researchers [30, 31]. Three sections of surfaces of each sample of the composite with dimensions of 30 mm, 30 mm, and 5 mm were subjected to Vickers hardness tests, and an average was recorded.
2.3.5 Tensile test
The Hounsfield Monsanto Tensometer (Model: 386083-W9), operating at a cross speed of 10 mm/min, was used to conduct the tensile test in accordance with the ASTM D-638-03 standards adopted by some researchers [30, 31, 39]. A tensile force and tensile properties, such as tensile strength, were applied to the samples cut to standard for experimentation with the dimensions as shown in Figure 4. This test was done on the produced and commercially available samples.
Figure 4. The sample set up for tensile analysis (all measurements in mm)
2.3.6 Impact test
The impact test was conducted using the Izod impact testing machine (Model 4123-07-15269C) with a maximum capacity of 15 kg-m (150 J). The test sample of dimension (80 × 20 × 5) mm having a notch angle of 40° with a 2 mm notch depth and a notch radius of 0.25 mm was mounted on the impact testing machine while the pendulum was released to calibrate the sample which was horizontally gripped in a vice with the required force applied from the swinging pendulum to break the bar. The experiment was conducted three times for the samples with various readings obtained and recorded accordingly [39]. This test was conducted on both the produced and commercially available samples.
2.3.7 Compression test
The test was carried out to determine the behavior of the material as it experiences a compressive load. The rectangular-shaped sample used was gripped to the universal testing machine (Model: Cat. Nr. 261) with 100 kN capacity, and a load was applied with strain recorded at intervals. The samples for brake pad development were subjected to compressive force continuously until failure occurred, and the load at which failure occurred was recorded [13]. This test was carried out on both the samples produced internally and those obtained from commercial sources.
2.3.8 Thermogravimetry test
Thermogravimetric analysis (TGA) is an indispensable method that is mostly used in materials science and engineering to analyze the thermal stability of natural and other kinds of fibers. TGA can assess the quantity and intensity of weight variations as a function of temperature or time in a wide range of controlled environments. Thermal stability may typically be predicted using TGA results within a temperature range of up to 1000℃ [30, 40]. Utilizing Perkin Elmer's thermal analysis (Model: TGA 4000), TGA was used to determine the thermal stability of the composite in a nitrogen environment with a heating rate of 10℃/min. The samples were heated at a temperature range of 30℃ to 950℃.
2.3.9 Wear test
A ball-on-disc tribometer testing machine (Model: 20-8054 Graz-Austria) was used for the wear test analysis. The sample used for the experiment was held against the counter face of the rotating disc with a wear track radius of 5 mm, and the test was conducted under a load of 5 N. Each sample of wear resistance had a 110 mm diameter and a 5 mm thickness. Before the test, each sample was weighed and recorded using a BL20001 4-digit electronic weighing scale. With the help of a nut in the center, the sample was secured to a machine's 100 mm-diameter rotating disc. The sample affixed to the rotating disc was permitted to come into direct touch with two emery wheels located on the machine's top arm. According to Ademoh and Olabisi [41], the machine was powered by a 220 V, and the disc revolved at a steady speed of 85 r.p.m. for a pre-set time of 1000 seconds. The two emery wheels in close proximity to the specimen rotated in tandem with the disc as it spun, wearing down the sample as a result. The equipment automatically shuts off after the predetermined period. The item was meticulously cleaned, and the final weight was measured and noted. Eq. (4) [41] was used to calculate the wear rate from the specimen's weight loss using the difference between the final and initial weights.
Wear Rate =W0−W1S=ΔWS=ΔW2πND×t (4)
where, W0 represents the starting weight, W1 represents the end weight, ∆W represents the weight loss, S represents the sliding distance (mm), D represents the disc diameter (mm), N represents the radial speed (rpm), and t represents the exposure time (s) for the sample to wear.
2.3.10 Coefficient of friction test
The coefficient of friction test was conducted using an inclined plane apparatus (Model: SKU PH0334). Each sample was set up as shown in Figure 5 on an inclined plane with a specified inclination and a 90° wedge. To increase the angle, the wedge location was changed until the specimen was about to slip down the plane. The resultant coefficient of static friction (µ) was calculated using Eq. (5) [41].
μ=tanθ= Opposite Adjacent (5)
Figure 5. Illustration of sample for the coefficient of static friction examination
2.3.11 X-ray diffraction (XRD) test
Phase characterization of the carbonized produced sample was performed using X-ray diffraction (XRD) on a Rigaku MiniFlex 600-C diffractometer, equipped with Cu-Kα radiation (wavelength 1.5406 Å), operating at 40 kV and 15 mA. To eliminate background interference, the analysis was conducted using a single-crystal quartz holder. The XRD scan was executed with a scan speed of 2°/min, covering a 2θ range of 2°–70°, with 0.02° increments in a locked coupled configuration adopting Sá Ribeiro et al. [42] and Chin et al. [43] methods.
2.3.12 Scanning electron microscope (SEM) test
SEM (Model: PW-100-012) microstructural analysis of the composite samples was performed to investigate the type of interfacial bonding that exists in the composite. To ensure excellent conductivity of the samples mounted on the stubs with a silver plate of the SEM machine, the surface of the samples for analysis was initially prepared by a vacuum-evaporation mechanism of a thin platinum film. Thereafter, the micrographs were taken seamlessly [44].
The elemental compositions of carbonized and uncarbonized composites are given in Table 4. The major oxide in the material, SiO2, has a hexagonal crystal structure and makes up roughly 12.91 wt.% of it, as shown in Table 4. However, Table 4's validation of the trigonal hematite crystal structure shows that it has a high degree of crystallinity. Due to its advantageous characteristics, including hardness, silicon oxide plays a crucial role in the manufacture of brake pads. According to Table 4, the hexagonal structure of SiO2 is approximately 1.939 wt.%, with a low crystallinity trigonal hematite crystal structure [45]. Al₂O₃, present at 2.266 wt.% in the carbonized composite sample and 0.416 wt.% in the uncarbonized composite sample, is renowned for its exceptional hardness and thermal resistance. This oxide plays a crucial role in significantly enhancing the wear resistance of the composite material [45, 46]. Potassium oxide (K₂O), widely used as a flux in ceramic applications, effectively reduces the melting point of silica, thereby enhancing the workability of the composite material. Iron (III) oxide (Fe₂O₃) and magnesium oxide (MgO), as basic oxides, make significant contributions to the mechanical properties and thermal stability of the carbonized composite. Phosphorus pentoxide (P₂O₅), recognized for its strong dehydrating properties, enhances the durability of the composite by promoting cross-linking during the curing process. Additionally, calcium oxide (CaO), or quicklime, is vital for improving both the mechanical strength and overall durability of the composite material [45-48]. The analysis indicates that the carbonized base material exhibits superior performance compared to the uncarbonized base material, as shown in Table 4.
Table 4. Compositions of carbonized and uncarbonized base material
|
Oxide Composition of Carbonized Base Material |
|||||||||||||
|
Element/Oxides |
Fe2O3 |
SiO2 |
Al2O3 |
MgO |
P2O5 |
SO3 |
TiO2 |
MnO |
CaO |
K2O |
CaO2 |
ZnO |
CuO |
|
Weight (wt.%) |
3.634 |
12.91 |
2.266 |
1.340 |
1.330 |
0.341 |
0.141 |
0.213 |
2.145 |
7.083 |
1.110 |
0.061 |
0.022 |
|
Oxide Composition of Uncarbonized Base Material |
|||||||||||||
|
Element/Oxides |
Fe2O3 |
SiO2 |
Al2O3 |
MgO |
P2O5 |
SO3 |
TiO2 |
MnO |
CaO |
K2O |
CaO2 |
ZnO |
CuO |
|
Weight (wt.%) |
1.014 |
1.939 |
0.416 |
0.040 |
0.990 |
0.218 |
0.028 |
0.101 |
0.531 |
1.650 |
0.039 |
0.033 |
0.013 |
Figure 6. Effects of particle sizes on density, water absorption, and oil absorption of (a) carbonized base material and (b) uncarbonized base material
3.1 Effects of particle sizes on the physical properties of the Composites
Figure 6(a) and 6(b) presents the results of tests conducted to evaluate the effects of different particle sizes of carbonized and uncarbonized base materials. The analysis includes density, water absorption, and oil absorption tests, which are critical for assessing the suitability of these materials for various applications [49]. In the figures, A1, B1, and C1 represent a particle size of 75 µm, while A2, B2, and C2 correspond to 150 µm, and A3, B3, and C3 indicate 212 µm. These classifications are further supported by data in Table 1. Upon comparing the outcomes illustrated in Figures 6(a) and 6(b), it was noted that sample B1 (75 µm) of the carbonized base material exhibited the lowest density at 1.1 g/cm³. Additionally, it demonstrated a water absorption rate of 0.768 and an oil absorption rate of 0.645, both of which were lower than those of the other samples tested. These findings highlight the superior performance characteristics of the 75 µm particle size. The study indicates that smaller particle sizes, particularly 75 µm, improve the mechanical properties of composites, including lower density and water and oil absorption. This implies that fine particles enhance interfacial bonding, promoting better material integrity and load transfer in the composite matrix. Consequently, this necessitated the selection of 75 µm as the optimal particle size for further investigation, guiding the development of composite materials with enhanced properties for practical applications [50, 51].
3.2 Hardness characterization for carbonized and uncarbonized base materials
Figure 7 presents the results of Vickers hardness characterization for carbonized and uncarbonized base material samples. It was seen from the results that the carbonized base material of sample No. 3 gave the highest hardness property (91.7 ± 2.75 HV) compared to the other carbonized and un-carbonized base material samples at the preliminary test. This performance was due to the increase in the bonding materials in the composition of sample No. 3 (epoxy resin and carbonized grounded bamboo culm) as well as the close packing that strain-hardened the composite brake pad and raised its hardness properties compared to other compositions. Due to the bond interaction of epoxy, which keeps the components together, the hardness value increased as reinforcement levels increased. The result is in consonant with the works of some researchers on brake pad application [15, 16]. At this stage, the sample No. 3 composition of carbonized samples was further used to carry out the next stage of experimentation using Taguchi experimental run (L9) method and the resultant hardness values (Samples R1-R9) and the control sample R10 are illustrated in Figure 7(b) while the uncarbonized base material was not considered for further specimen production. It can be observed also that from the outcome of the hardness test analysis (Figure 7(b)) from the Taguchi experimental run (L9) samples that sample R9 gave the highest hardness values (109.3 ± 2.73 HV) as compared to other samples R1, R2, R3, R4, R5, R6, R7, R8 and R10 with hardness values of 69.5 ± 1.34 HV, 81.7 ± 2.04 HV, 79.2 ± 1.91 HV, 90.8 ± 2.27 HV, 95.7 ± 2. 39 HV, 101.7 ± 2.54 HV, 104.7 ± 2.62 HV, 102.7 ± 2.57 HV, and 67.3 ± 1.68 HV, respectively. This showed that the control sample R10 gave the least hardness properties. It can be evident that sample R9 possesses toughened characteristics with the capacity to withstand indentation as viewed by Dagwa et al. [30] and Mohamed et al. [52]. The results obtained generally compared favourably with those of conventional brake pad (101 HV) as posited by Ademoh and Olabisis [41].
Figure 7. Hardness characterisation for carbonized and uncarbonized samples
Table 5. Tensile test results
|
S/N |
Samples |
Average Breaking Load (N) |
Average Extension (mm) |
Tensile Strength (MPa) |
Strain |
Modulus of Rupture (MPa) |
|
1 |
Produced |
396.00 ± 7.92 |
6.44 |
10.31 ± 0.21 |
0.1288 |
80.07 ± 1.60 |
|
2 |
Commercial |
485.00 ± 9.70 |
5.10 |
12.63 ± 0.25 |
0.1020 |
123.83 ± 2.48 |
3.3 Tensile analysis
Table 5 presents the tensile test results for the developed brake pad sample alongside a commercially obtained brake pad. The tensile strength of the developed sample was measured at 10.31 ± 0.21 MPa, with a modulus of rupture of 80.07 ± 1.60 MPa. In comparison, the commercial brake pad exhibited a higher tensile strength of 12.63 ± 0.25 MPa and a modulus of rupture of 123.83 ± 2.48 MPa. Despite this, the tensile strength of the developed brake pad falls well within the range reported for conventional brake pads, such as 7.00 MPa [41], and aligns closely with the performance of other developed brake pads [53, 54]. These findings suggest that the developed brake pad exhibits competitive and acceptable mechanical properties, making it a viable alternative to conventional and commercial brake pads.
3.4 Impact analysis
Figure 8 illustrates the impact energy results for both the developed and commercial brake pad samples. The developed brake pad exhibited an impact energy of 0.691 ± 0.013 J/mm, closely matching the commercial sample's 0.716 ± 0.014 J/mm, which was used as a control. Notably, the impact energy of the produced sample falls within the recommended range for brake pad applications, such as 0.082 J/mm, as highlighted by Abutu et al. [53]. The commendable impact energy of the developed sample can be attributed to the inclusion of cast iron filings as reinforcement, which significantly enhances its energy absorption capability. This favorable comparison demonstrates the efficacy of the produced brake pad in maintaining mechanical integrity under impact, highlighting its potential as a reliable and competitive alternative for braking system applications [55, 56].
Figure 8. Impact analysis of produced and commercial samples
3.5 Compression analysis
Table 6 presents the results of the compression test conducted on three samples each of the developed and commercial brake pads, with their averages and standard deviations recorded. The analysis reveals that the developed brake pads reinforced with bamboo culm demonstrated superior performance, achieving an average compression strength of 13.747 ± 0.412 MPa. This performance is notably higher than that of the commercially available brake pads, which recorded a similar average compression strength. Furthermore, the results align well with the findings of previous studies, such as those by study [57], which investigated brake pads developed using coconut shell reinforcement materials, reporting minimum compression strengths ranging from 3.817 to 5.451 MPa. The enhanced compression strength of the developed brake pads indicates their robustness and suitability for reliable braking system applications.
Table 6. Compression strength results for produced and commercial samples
|
Sample Type |
Average Thickness (mm) |
Average Width (mm) |
Average Compression Strength (kN) |
|
Produced sample |
8.467 ± 0.254 |
19.067 ± 0.572 |
13.747 ± 0.412 |
|
Commercial sample |
9.933 ± 0.297 |
15.933 ± 0.478 |
6.450 ± 0.194 |
3.6 Thermogravimetry analysis
Figure 9(a) and (b) show the curve of TGA (changes in weight % to the corresponding temperature) of the produced composite sample (bamboo culm, cast iron fillings, alumina, and epoxy) and its first derivatives (DTG) and the commercial composite sample (control sample) respectively that show the temperatures of optimum decomposition. As evident from Figure 9(a) and (b) respectively, the produced composite material was stable up to about 319℃ and started degrading from a temperature of 320℃ until it became rapid at 580℃ while the commercial sample was stable up to about 291℃ and started degrading from a temperature of 292℃ until it became rapid at 550℃ which is closely related to the average degradation temperature of fiber-related brake pad materials [30, 58]. It is evident that the produced composite materials experienced lower temperature degradation as compared to their counterpart commercial sample of brake pad used as control. These findings are corroborated by the works of Dagwa et al. [30], Yao et al. [58], and Kozłowski and Przybylak [59] on the degradation characteristics of composite materials for different engineering applications.
Figure 9. TGA/DTG graph of (a) the produced composite sample brake pad and (b) the commercial sample brake pad
3.7 Effect of sample weight, coefficient of friction, and wear rate characterizations
Figures 10(a) and (b) present the wear rate characterizations of the produced sample of the brake pad. It is evident that the values of the wear rate decrease with the increased coefficient of friction of the samples (Figure 10(b)), while sample weight maintains relatively uniform values (Figure 10(a)). This can be attributed to the high hardness properties and compressive strength of the developed composite samples of brake pads, which slightly differ from the commercial brake pad sample [13]. Conversely, the obtained coefficient of friction values of the developed samples of brake pads and commercial samples (Figure 10(a) and (b)) fall within the standard range of 0.3-0.4 [41, 60]. Therefore, the coefficient of friction of the developed sample brake pad compares favourably with that of a conventional brake pad obtained from a standard [41]. In contrast, the friction coefficients of the composite samples 1, 4, 5, 6, 7, 8, and 9 ranged by 20.64, 21.22, 18.90, 15.41, 18.02, 22.09, and 20.06%, respectively, above the mean values of those of commercial brake pads (0.344). With variances of 0.02 µ, 0.06 µ, and 0.14 µ, respectively, the friction coefficient of sample 6 composition (0.173 µ) was higher than that of related works by Aigbodion et al. [13] that used bagasse filler, Yawas et al. [32] that used periwinkle filler, and Edokpia et al. [33] that used eggshell filler for composite brake pads. It also competed favorably with the works of Ibhadode and Dagwa [7], which used conventional filler with a variation of 0.09 µ. However, it is corroborated by the value (0.44) obtained by Dagwa and Ibhadode [60]. The wear rate of the composite brake pad composition 6 differed from that of the commercial brake pad by 0.010, making composite 6 a superior brake pad material to the conventional brake pad. In comparison with previous works, composition 6 was better than the brake pad made with conventional material [7], bagasse filler [13], periwinkle filler [32] and eggshell filler [33].
Figure 10. Wear rate properties of the developed sample brake pad material (a) sample weight and (b) comparison of coefficient of friction vs wear rate
3.8 XRD analysis
Figure 11 presents the XRD analysis of the carbonized bamboo culm-based composite sample. The XRD analysis of the carbonized bamboo sample revealed major peaks at 26°, 27°, 36°, 44°, 57°, and 69°, indicating a mix of graphitic and amorphous carbon phases [61, 62]. The dominant peak at 26° corresponds to the (002) plane of graphitic carbon, signifying partially ordered structures, while the peak at 27° suggests amorphous carbon contributions. The 36° and 44° peaks may indicate graphene nano-crystallites and the (100) plane of graphitic carbon, respectively, highlighting in-plane carbon ordering. The 57° and 69° peaks suggest higher-order reflections of turbostratic carbon, indicating stacked but disordered graphene layers. The high background noise confirms a significant amorphous content, which influences the material's mechanical strength, thermal stability, and wear resistance [27, 63]. These characteristics are critical for brake pad applications, as the presence of graphitic carbon enhances frictional performance, reduces wear, and improves heat dissipation, making carbonized bamboo a potential eco-friendly material for high-performance friction components [27, 42, 43, 61-63].
Figure 11. XRD spectrum of the carbonized bamboo culm-based composite sample
3.9 Scanning electron microscope (SEM) analysis
Figures 12(a-c) present the SEM micrographs of the produced composite samples. Evident in the SEM micrographs are uniform dark regions, which indicate the presence of graphite that was used to reduce wear in the composition, while the white regions indicate the presence of aluminum oxide which serves as an abrasive in the formulation. Also, the reinforcing material is seen at a mixed region of dark and white with the addition of iron filings that aided the composite sample’s structural strength. The 30% resin aided the interfacial bonding of the produced sample. The SEM also shows a good combination of the element/oxide distribution in the formulation with a fair bonding of the resins and the bamboo culm particles. The XRF reveals the predominance of CaO (51.16%), SiO2 (18.31%), and Cl (10.12%), as well as the presence of other elements/oxides (Figures 12(a-c)). In the composites, there have been visible fiber pull-out and tiny pores. According to Dagwa et al. [30] and Chaturvedi et al. [64], the rough surface of the fiber of bamboo culm has stronger mechanical interactions with a matrix, leading to outstanding mechanical properties. Figure 12(d-f) presents the SEM micrographs of the commercial composite sample, which shows a more segmented, broken, and scattered orientation. The void spaces can be seen with some white dotted structures, which may indicate the presence of aluminum oxide. The XRF reveals the predominance of MgO (21.67%), CaO (19.27%), SiO2 (17.16%), and Fe2O3 (15.98%) as well as the presence of other elements/oxides (Figure 11(d-f)). The compositions of SiO2 (18.31%) and Al2O3 (3.14%) in the produced samples compare favorably with the compositions of SiO2 (17.16%) and Al2O2 (4.57%) in the commercial sample. These findings agree with the works of other researchers [30, 64-67] on the mechanical and microstructural characterizations and fiber interactions with the matrix in the sample compositions.
Figure 12. SEM micrographs of (a) produced composite sample (300x) with 30% resin, (b) produced composite sample (500x) with 30% resin, (c) produced composite sample (1000x) with 30% resin and XRF spectra results (d) commercial composite sample (300x) (e) commercial composite sample (500x) and (f) commercial composite sample (2000x), and XRF results
Figure 13(a-c) presents SEM micrographs of the worn surface of a carbonized bamboo culm-based composite material with 30% resin, captured at magnifications of 500×, 1000×, and 2000×, respectively, while Figure 13(d) displays the EDS spectrum results. The micrographs reveal a heterogeneous surface morphology exhibiting signs of abrasive wear, material pull-out, and surface fragmentation [62, 64]. The EDS analysis confirms a dominant carbon content (84.57 wt.%), which is critical for thermal stability and mechanical integrity in frictional applications, while the presence of Al, Ca, Si, and Fe suggests the formation of reinforcing compounds that enhance wear resistance [8, 55]. The observed wear mechanisms are predominantly abrasive and adhesive, as evidenced by micro-fractures and surface irregularities resulting from mechanical stresses during frictional contact. The high carbon composition contributes to self-lubricating properties, reducing the coefficient of friction and minimizing wear rates [62]. Additionally, the presence of silicate and oxide compounds associated with Si, Al, and Ca enhance the composite’s ability to withstand extreme temperatures and wear conditions [64]. The existence of plowing grooves and asperities suggests material removal due to hard particle interactions, while wear debris formation during frictional engagement accelerates surface roughening [27]. The resin content plays a crucial role in maintaining structural cohesion, preventing excessive fragmentation, and improving wear resistance. These wear characteristics align with prior studies on biomass-derived composite materials, further confirming the potential of carbonized bamboo culm composites for high-performance applications such as brake pads and other tribological components [8, 27, 52, 62, 64, 67, 68].
Figure 13. SEM micrographs of produced composite sample with 30% resin and magnifications of (a) 500x, (b) 1000x, (c) 2000x, and (d) EDS spectrum results
At the completion of this study, the following conclusions were reached:
The long-term durability and industrial feasibility of bamboo culm composites require attention. Challenges include moisture degradation, thermal instability, and bio-degradation. Industrial adoption faces raw material variability, machining difficulties, and cost concerns. Future research should optimize processing, enhance durability, and assess large-scale feasibility to ensure competitiveness with conventional brake pad materials in real-world applications.
The management of the University of Abuja, Gwagwalada, Abuja, Nigeria, is appreciated by the authors for providing the TETFund IBR Grant (Award No. TETFund IBR-UARG/002) for this study.
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