Overview of the Important Factors Influencing ...

The braking system is a vital component for automotive safety. The durability and quality of brake pads significantly influence the effectiveness of this mechanism. Brake pads are integral parts of a vehicle that hold the wheel rotation to enable braking. Recently, asbestos, a material harmful to human health, has been extensively used in manufacturing brake pads. Researchers are actively seeking natural alternatives to replace asbestos due to its health risks. Notable materials under investigation include coconut fiber, wood powder, bamboo fiber, and shell powder. This review aims to analyze the key parameters that affect brake pad performance, including the filler composition and fiber types used in polymer composites. Additionally, previous studies related to the fabrication and testing of brake pads are discussed. The insights from this review may provide researchers and academicians with valuable information for future studies.

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Biomass from agricultural activities, including plant and animal waste products, has emerged as a trend in brake pad manufacturing due to its commercial viability and environmental friendliness. Various agricultural wastes like palm trees, bamboo, corn stalks, sugarcane bagasse, banana, cashew nutshells, coir (coconut shell), rice straw, and pineapple are extensively utilized. These wastes contain a rich concentration of natural fibers that can serve as reinforcement materials in polymer composites. Agricultural waste boasts high strength, ecological benefits, low cost, abundance, and sustainability, marking its potential in composite applications. Due to significant health concerns regarding carcinogenic properties and the drive for improved brake pad performance, various researchers have explored a range of materials and processing practices to replace asbestos with agricultural-based alternatives in brake pad production.

Friction materials in the market are classified into three categories: sintered metals, non-asbestos organic (NAO), and semi-metal (SM). These materials typically comprise iron powder, steel fibers, graphite, rubber, organic fibers, and ceramics combined with various abrasives, lubricants, and fillers. The friction material is generally a composite formed by bonding several materials with a phenolic resin (thermosetting). This review will investigate innovative materials aimed at replacing asbestos and inorganic resins to develop high-performance, safe brake pads.

Asbestos is recognized by the International Agency for Research on Cancer (IARC) and the World Health Organization (WHO) as carcinogenic, with potential to cause lung cancer due to the dust produced. The ban of asbestos in the automotive industry necessitates the development of alternative friction materials that can maintain mechanical integrity and performance.

Heat conduction transfers heat energy generated during braking to interacting components. Excessive thermal load can lead to disc thickness variation, surface cracking, and severe wear at contact areas. Elevated temperatures can also result in overheating brake fluid, seals, and other components, risking malfunction of the braking system. Brake pads absorb energy through friction force, which necessitates the development of materials that achieve lower wear rates and stable friction coefficients across diverse operating conditions such as temperature, pressure, speed, and environmental sustainability.

Asbestos, which possesses excellent properties like strength, durability, resilience against corrosion, heat, and fire resistance, has been widely utilized in various goods across several industries, including brake pads. Brake pads are one part of the larger braking system, alongside the master cylinder, wheel cylinder, and hydraulic control system. Given their significant role and potential environmental impacts, brake pads have garnered substantial research interest. The materials used in brake pad production can generally be categorized into binders, friction modifiers, fillers, and reinforcements. It is important to note that asbestos fibers are typically included in the polymeric matrix of brake pads, in combination with various other materials.

Natural fibers possess several advantages over synthetic fibers, attributed to their low density, availability, recyclability, biodegradability, renewability, and relatively high strength and stiffness. Natural fiber-reinforced composites are gaining popularity as they are sustainable, require less energy in production, and are biodegradable. Rising environmental concerns and dependency on petroleum are leading to an increased demand for such sustainable natural resources. Consequently, natural fibers are emerging as preferred reinforcement materials in several sectors including automotive, furniture, packaging, and construction.

2. Factors Impacting Brake Pad Performance

2.1. Composition of Eco-Friendly Materials

Brake pads consist of multiple layers. The underlayer functions as an adhesive, securing the friction material to the other layers. This underlayer's essential role is to mitigate vibrations caused by friction materials contacting the disc. The backplate provides necessary rigidity to the brake pads, allowing them to move smoothly along caliper guides. To reduce unnecessary noise during braking, some manufacturers implement specific interference shims. The friction material in direct contact with the disc during braking is the key layer of brake pads, composed of various ingredients, each tailored to specific applications.

The friction material in brake pads comprises binders, reinforcements, fillers, and abrasives. Binders are essential as they hold all components together. This material needs to have a stable high friction coefficient, endure high temperatures and rapid temperature changes, and maintain light weight. Reinforcements, which are fibrous materials added to the binder, significantly enhance its mechanical properties. The durability of brake pads is substantially influenced by the types of reinforcing materials utilized. While asbestos is a suitable reinforcing fiber, the associated health hazards warrant the search for alternative materials. Fillers are utilized to occupy gaps between components of brake pads, while abrasives serve to adjust the coefficient of friction. Hard materials such as steel, refractory oxides, and quartz are employed to improve the friction coefficient between the disc and brake pads, ultimately extending the service life of the pads.

Coconut shell waste is renewable, low-cost, recyclable, and biodegradable. Its physical properties and high compressive strength make it a viable candidate for non-asbestos friction materials, particularly in lightweight vehicles like motorcycles. For example, a study by Daut et al. showed the impact of coconut shell powder in brake friction material, where a combination of aluminum oxide, maleic anhydride, epoxy resin, and various percentages of coconut shell powder was optimized. Results indicated that the optimum formulation yielded impressive hardness, density, and porosity for friction materials, suggesting coconut shell powder as an excellent alternative to asbestos in brake pad manufacturing.

To explore the wear characteristics of eco-friendly friction linings for automotive drum brakes, Shinde et al. adopted experimental studies alongside finite-element analysis. They crafted drum brake liners using eco-friendly composites via an industrial hot compression molding process. Their findings demonstrated superior wear resistance for friction lining materials developed with coconut shell powder, confirming the efficacy of using such sustainable materials in brake applications.

In another study, Abutu et al. utilized grey relational analysis to fabricate brake pads employing varying ratios of epoxy resin (binder), coconut shell (reinforcement), aluminum oxide (abrasive), and graphite (friction modifier). The study concluded that optimized coconut shell-reinforced brake pads exhibited reduced noise and vibration during operation compared to commercially available samples. Curing time was determined to critically influence the mechanical properties of the resulting friction materials.

Research into the use of coconut shells, along with various binders, has yielded promising results, confirming their potential applicability for brake pad production through experimental methodologies.

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Coconut fiber, banana fiber, and rice husk powder have been utilized to produce bio-composite brake pads with varying fiber proportions. Utilizing a basic wet hand lay-up approach, the composites were created, with different combinations of each ingredient. Evaluating their wear characteristics revealed that increased natural fiber content resulted in enhanced hardness, showcasing that properly balanced compositions can produce effective non-asbestos brake pads.

Considering the insights gained from various studies, the demand for non-asbestos brake pads manufactured using agricultural waste demonstrates a significant impact on friction material performance, especially in terms of wear rate and coefficient of friction.

Table 2

No. Type of Reinforcement Fabrication Method Weight Fraction (wt.%) Hardness Density (g/cm3) Wear Rate Coefficient of Friction Ref. 1. Coconut shell powder Hot Compression (80 °C, 100 KN/cm2, 5 min) 2 wt.% 21
(Shore D) 2.05 [32] 4 wt.% 70
(Shore D) 2 6 wt.% 69.7
(Shore D) 1.89 8 wt.% 68
(Shore D) 1.7 10 wt.% 58
(Shore D) 1.6 2. Grounded coconut shell Hand lay-up 50 wt.% 30 (HRF) 2.55 2.56
(10−6 g/min) [35] 40 wt.% 39 (HRF) 2.54 2.1
(10−6 g/min) 30 wt.% 40 (HRF) 2.45 0.5
(10−6 g/min) 20 wt.% 58 (HRF) 2.22 0.25
(10−6 g/min) 10 wt.% 60 (HRF) 2.15 0.5
(10−6 g/min) 3. Palm kernel shell + coconut shell Compression
(16.75 KN/m2, 6 h) 25 wt.% + 25 wt. 3.3 (kgf/mm2) 2.55 0. (g/min) 0.374 [36] 38 wt.% + 13 wt. 3.41 (kgf/mm2) 2.6 0. (g/min) 0. wt.% + 36 wt. 3 (kgf/mm2) 2.78 0. (g/min) 0. 4. Candlenut shell powder + coconut shell powder Compression
(15 KN/m2, 4 h) 35 wt. + 25 wt. 87 (HR) 5.28 × 10−5 g/mm.s [37] 30 wt. + 20 wt. 89 (HR) 4.82 × 10−5 g/mm.s 25 wt. + 15 wt. 92 (HR) 3.67 × 10−5 g/mm.s 5. Wood powder + coconut fiber + cow bone Compression
(2 tons, 1 h) 0 wt. + 40 wt. + 10 wt. 23.9 (HV) 0.47 [38] 40 wt. + 0 wt. + 10 wt. 35.4 (HV) 0.56 [38] 0 wt. + 20 wt. + 10 wt. 32.1 (HV) 0.44 [38] 0 wt. + 25 wt. + 0 wt. 26.5 (HV) 0.40 [38] 6. Coconut shell powder + sugarcane powder Hand lay-up 21 wt. + 7 wt. 3.55 × 10−6 mg/m 0.48 [43] 14 wt. + 14 wt. 4.13 × 10−6 mg/m 0.48 [43] 0 wt. + 21 wt. 3.87 × 10−6 mg/m 0.48 [43] 7. Coconut fiber Hot Compression
(200 °C, kgf,
20 min) 29 wt. 37.14 HRB 0.323 mm3/N·mm 0.454 [44] 8. Bamboo fiber Hot Compression
(200 °C, kgf,
20 min) 20 wt. 44.10 HRB 0.242 mm3/N·mm 0.469 [44] 9. Palm kernel fiber Hand lay-up 10 wt. 2.11 HRC 0. mm3/N·m [45] 10 wt. 2.75 HRC 0. mm3/N·m [45] 30 wt. 2.84 HRC 0. mm3/N·m [45] 40 wt. 2.92 HRC 0. mm3/N·m [45] 50 wt. 2.98 HRC 0. mm3/N·m [45] 10. Palm kernel fiber + wheat fiber + nile rose fiber Hand lay-up 5 wt. + 2 wt. + 3 wt. 1.83 HRC 0. mm3/N·m [45] Additionally, the incorporation of kenaf fibers has been explored, highlighting their potential to enhance both thermal resistance and strength of brake pads.

In conclusion, investigating the performance of eco-friendly brake pads reveals that utilizing various natural materials significantly impacts the wear rate and friction coefficient. Each study indicates that these innovative approaches offer feasible alternatives to traditional materials, promoting both performance and sustainability in automotive applications.