Brake pads and linings are part of the disc brakes used in motor cars [1]. They are used in braking systems to control the vehicle’s speed by converting the vehicle’s kinetic energy to thermal energy through friction and releasing the heat generated into the environment. Drum brake and disc brake linings are the two types of brake linings used in automotive braking systems. The drum brake is housed inside a drum, and when the brakes are applied, the brake lining is pulled outward and pressed against the drum, whereas disc brakes work comparably but are unprotected [1,2,3,4,5]. The asbestos matrix is a component of the disc brakes, which are made up of a variety of materials [6]. Asbestos, which is a major constituent in brake lining production, is not readily available and is expensive. Asbestos is linked with the disease asbestosis, which is life-threatening [3]. Because of its carcinogenic and destructive characteristics, asbestos is being avoided in both developed and developing countries [1, 3, 4, 7, 8].
With the ongoing research on alternative materials for friction lining, the use of copper in friction lining is also reduced, which results in a meaningful change in the composition of friction materials in 2020 (a reduction of 0.5%). Due to the wear debris from friction materials that are washed into rivers when braking and contain copper, rules have been introduced in California and Washington to reduce this. The copper in this detritus is harmful to molluscs and prevents salmon from spawning. Therefore, copper-free brake friction materials have advanced, and there are also a few research articles and patents related to it [4].
The braking system was designed to assist in the gradual slowing down or complete stoppage of a moving vehicle [5, 9,10,11]. When the vehicle brake is applied, it is driven outwards and against the drum. Friction occurs between the revolving disc and the pad when the brake is applied. The brake pads must absorb heat fast, withstand high temperatures and not wear. When the brake is heated up by making contact with a drum or rotor, it begins to transfer a small quantity of friction material to the disc or lining [12]. This is why a brake disc is a dull grey [1, 3, 10, 13, 14]. Friction linings contain over ten ingredients mixed properly to achieve the expected combination of performance properties [3, 7, 15]. Aside from managing friction and wear resistance, friction linings are divided into four categories: binder, fibres, friction modifiers and fillers. In friction materials, a binder is a critical component that tightly binds the ingredients so they can perform the desired function [16]. Friction modifiers are used to adjust the required range of friction, whereas amalgamated fibres are frequently incorporated for strength. Functional fillers are used to improve particular characteristics of composites, such as fade resistance, and space/non-reactive fillers are used to save money [17, 18].
Due to in situ deterioration that begins at room temperature, these resins are heat- and humidity-sensitive, with a short shelf life. The features that make brake pads suitable for usage are assessed. Properties to be studied include abrasion resistance, hardness, friction coefficient, compressive strength, specific gravity, water and oil soaks, tensile strength, thermal conductivity, disc temperature and stopping time [3, 19].
The compositional design of friction materials is well-known to be difficult with multiple criteria optimization that involves not only the problems of handling different categories of ingredients but also reaching an expected level of performance [20]. The use of coconut shell (CS) and palm kernel shell (PKS) has been developed for asbestos-free brake pad materials [3, 6, 14]. Agro-industrial wastes are being studied as a source of raw materials in the industry all over the world. This method of disposal will not only be cost-effective, but it may also result in foreign exchange gains and environmental protection. After the phase-out of asbestos, which had received global recognition as a carcinogen, new materials and elements are now utilized in vehicle brake friction material, even though the asbestos prohibition in some countries was only implemented in 1989.
According to the previous study by Adeyemi et al. [21], palm kernel shells (PKS) can be used to fabricate brake pads and friction lining material since they showed good potential in the evaluation. PKS is both affordable and readily available. It has an impact on polymer composite manufacturing adhesion and dispersion.
Coconut shell brake pads were developed by Egeonu et al. [22]. Ground coconut shells (filler), epoxy resin (binder–matrix), iron chips (reinforcement), methyl ethyl ketone peroxide (catalyst), cobalt naphthenate (accelerator), iron and silica (abrasives) and brass were added in the composition (friction modifier). The pulverized filler was sieved using a sieve with a 710-m aperture.
Breaking strength, hardness, compressive strength and impact all decreased as the proportion of ground coconut powder increased, showing that a high percentage of ground coconut powder causes brittleness [23].
Elakhame et al. [22] used periwinkle shells to produce brake pads. Periwinkle shell asbestos-free brake pad shape and characteristics were investigated. Crushed, powdered and sieved sun-dried periwinkle shells have sieve sizes of + 710, + 500, + 355, + 250 and + 125 m. The recipe called for periwinkle shell powder, phenolic resin (phenol-formaldehyde), motor oil (SEA 20/50), and water.
Idris et al. [6] studied and manufactured brake pads utilizing a binder made from peels of banana instead of asbestos. In a 5-wt. per cent interval, the resin was changed from 5 to 30% wt. Physical, mechanical, wear and morphological aspects of the brake pad have all been investigated. The results revealed that increasing the weight per cent of resin added increased compressive strength, hardness and specific gravity of the samples. The resin concentration grew as the oil soak, water soak, wear rate and % burned reduced. All the qualities were improved in the samples containing 25% uncarbonized banana peels (UNCBp) and 30% carbonized banana peels (CBp).
Sathyamoorthy et al. [11] in their review paper focused on providing in-depth details regarding the frequently used chemicals, manufacturing processes and properties of brake friction compounds.
Sathyamoorthy et al. [24] in another study, examined the impact of several abrasives (red mud, steel slag and fly ash) on the tribological properties of non-asbestos brake friction materials. By altering the ratios of crucial elements including red mud, steel slag, and fly ash while keeping the ratios of other basic ingredients, three different brake friction composites were created. As a result, friction composites made with various abrasives were created, and their mechanical, chemical and physical characteristics were assessed in accordance with industry requirements. The tribological characteristics were determined experimentally in accordance with IS2742 part 4 using the Chase friction test equipment. According to the experimental findings, fly ash particles in friction composites demonstrated consistent fade and recovery behaviours with a lower wear rate. In contrast, the recovery behaviour of steel slag-based friction composites was better.
In another study, iron sulphide is used as a solid lubricant with red mud as an abrasive to create brake friction composites utilizing traditional production methods in the shape of ordinary brake pads. By changing important ingredients like red mud and iron sulphide while maintaining the same levels of the other parental ingredients, three distinct brake friction composites were created. According to industry norms, experiments on the generated composites’ physical, chemical, mechanical and thermal properties were conducted. Using a Chase friction test rig, the tribological performance was experimentally examined in accordance with IS2742 part 4. The experimental effort led to the conclusion that red mud and iron sulphide particles combined produced friction composites with stable fade and recovery behaviours. Conversely, the friction composites based on iron sulphide showed reduced wear. The surface properties of the friction composites evaluated by Chase were revealed by scanning electron microscopy (SEM) [25].
Akıncıoğlu et al. [26] reported in their study that 3.5% hazelnut shell dust was used as a natural additive material to create an eco-friendly brake composite sample. The Chase-type test machine and a newly created device were used to conduct friction testing on the manufactured pad sample and a commercial pad. The evaluation of the two distinct test device outcomes is given using a separate strategy. The braking performance of the sample with hazelnut shell dust was found to be in conformity with international standards after the trial results were compared using the Taguchi method. The findings from their study led to the nominal friction coefficient value being determined to be 0.505. The shearing forces of the eco-friendly brake composite pad and commercial pad samples were measured to be 607.3 and 850.5 N, respectively.
Another study investigated how brake pads made with walnut shell powder as a natural additive material affected braking performance. Two distinct kinds of samples of brake pads were made with 3.5 and 7% (2A and 2B) of walnut shell dust in the mix. As a benchmark, a commercial Clio brake tip was employed. The developed brake pads underwent microstructure analysis, testing for thermal conductivity, friction wear, density and hardness. Wear friction tests were conducted using a Chase-type equipment, and results were obtained in accordance with SAE-J661 (Brake Lining Quality Test Procedure) guidelines. The performances of the natural additive brake pads were assessed by comparing the experimental data to those of the commercial brake pads. Walnuts have been added [27].
Edokpia et al. [28] created and assessed eggshell (EG)-based eco-friendly (biodegradable) brake cushions. Gum Arabic (GA) was utilized as a folio. Both added substances were explored as a conceivable substitution for asbestos and formaldehyde gum which are carcinogenic in nature and non-biodegradable. The brake lining formulation was delivered by changing the GA from 3 to 18 wt %. Tests carried out on tests included wear rate, thickness swelling in water and SAE oil, warm resistance, particular gravity, compressive quality, hardness values and microstructure. Comes about appeared that definitions containing 15 to 18 wt % of GA created great holding. The test with 18 wt % of GA in ES particles gave the most excellent properties. Maize husk (MH)-based composite brake pads were developed by Adeyemi et al. [7]. The filler particulate size in their research was 300 microns, and the binder was epoxy resin. The study showed that decreasing the filler content increases the developed brake pad’s hardness, wear rate, tensile strength, compressive strength and thermal conductivity, while increasing the filler content weight per cent enhances density, coefficient of friction, water absorption and oil absorption. The findings suggested that MH particles could be used to replace asbestos in the manufacturing of brake pads for automobiles. The new composite brake pad, unlike asbestos-based brake pads, is environmentally friendly and has no recognized health risks [15]. In their study, the physical, mechanical and tribological properties of the newly developed automotive brake pad were investigated. The results for the agro waste-based brake pads were superior to those of commercial brake pads and other existing brake pads.
Research has shown that agro wastes are potential materials for the development of friction linings [3, 7, 19, 22]. This work would improve on the existing PKS-based friction linings by mixing them with pulverized coconut shells, brass filings, iron filings, calcium carbonate and burnt vehicle tyres. PKS is economical and is available in abundance. It has the characteristics of influencing the adhesion and dispersion of polymer composite fabrication. The physical and chemical characteristics of palm kernel shells were investigated [3, 21].