Tribo-mechanical performance of brake composite material: A comprehensive review

1. Introduction

A composite is a mixture of ingredients with an interface bond that provides the required properties for any application. The composite material for brake friction is a multi-component material, which is highly heterogeneous and anisotropic and differs from other materials as the rule of mixtures is invalid. Formulating composite material is an art rather than a science [1–5]. Its sacrificial part is connected to a steel back plate through adhesives or rivets in a braking device. Due to frictional heat, it sacrificed itself to slow and stop the vehicle soon. Hard brake pads are forced to touch the turning part, where energy transfer from kinetic energy to heat/thermal energy occurs at the interface [6–8]. Friction happens when the disc and pad are forced together, turning kinetic energy into heat. The energy is then released into the atmosphere as heat [9].

Further, the friction content additives are considered most important because of environmental degradation, aquatic species life and human life, which are much more significant than the excellent composition of brake pads. The versatility of asbestos has dominated longer in the friction industry, but the dangerous presence of asbestos has been shown to cause human lung cancer. Asbestos prohibition began in 1970, and only a few countries introduced laws prohibiting asbestos and its different types [10,11]. Recently, under the prevention of copper consumers, copper has been banned by the US government [12]. Each item in a friction formulation has its function, which varies in terms of brake pads’ physical, mechanical and tribological properties, such as improving the fade resistance, regulating friction coefficient, reducing porosity, noise, and increases in pressure, etc. A series of tests must be conducted to ensure good brake friction to meet the minimum criteria for a given application [13–15]. It is essential to understand the properties of the material and its effect on frictional efficiency. Therefore, the basic wording of frictional material and its forms are discussed in this review study.

Moreover, the different materials and their effect on brake efficiency are discussed. These brakes are classified into several types based on the mode of operation and the direction of forces acting. Based on the method of operation, they are classified as, Hydraulic brakes, Air brakes and Magnetic brakes. Based on the direction of forces working, they are classified as Radial brakes and axial brakes. Hydraulic brakes use fluid power to transfer pressure from a controlled mechanism to a mechanical one. These brakes are very efficient and depend on fluid properties, material characteristics, etc. [16,17].

More technically, an air brake system has a compressed air brake, a frictional brake device used by vehicles to press a piston to avoid the car's brake pad. Air brakes are used in heavy vehicles with many braking systems trailers, such as lorries, buses and trailers [18,19]. Magnetic brakes are a recent development. It has a higher degree of protection, which makes it very well-known. These brakes are not in contact with discs but can efficiently reduce the vehicle's speed. Magnetic brakes consist of one row or two neodymium magnets [20].

As a metal fin moves through the magnet rows, eddy currents are created in the fin, creating an anti-motional magnetic force. The resulting braking force is directly proportional to the rate of movement of the end element through the frequency element. But the limitation of magnetic braking is that the eddy force cannot hold a train by itself in perfect condition [21]. The train will also be maintained with an additional set of fine brakes or ‘kickers’, plain rubber tubes which effectively touch and park the train. Magnetic brakes can be found in two configurations, where the brake elements are mounted on or off the rail, and the fins are installed on the train's bottom or at the top, which looks like fine frictional braking. The other configuration is that the fins are mounted, and the braking components are attached to the train's underside. In Radial brakes, the force that acts on the brake drum is directed radially in those brakes. The radial brake can be separated into external and internal frequencies. The axial brakes are just the axial orientation of force acting on brake drums such as Disc and Cone Brakes.

Further, the disc brake type was introduced, developed and utilized in England during the eighteenth century [23]. In Birmingham, UK Lanchester, Frederick William Lanchester invented the first auto disc brake type of calliper type in 1902. However, in those eras, copper had to be utilized in brake on the disc system due to a restricted variety of metals [24,25]. The bad quality of roads at that time, including dusty, robust tracks, made the worn copper quickly, which made the disc braking system inviolable. The innovative new disc brakes required 50 more years.

A disc brake is fastened to the centre of the wheel, as illustrated in Figure 1. A fixed casing is known as a calliper. A permanent item is attached to callipers such as an axle box or stub-axle, cast into a piston and divided into two parts [22,26]. In the interim part, a friction pad is kept in one place by holding pins, spring plates, etc., between each piston and the disc. These passages are generally associated with a bleeding route. In any cylinder, it is a ring between the piston and the cylinder. The brake disc typically consists of galvanized iron but frequently consists of polysaccharides, ceramic material and metal alloys [27–29]. The carbon reinforces the composites. The disc brake system has superior stopper efficiency to the drum braking system because the drive is more relaxed, and the braking pad has a more excellent impact surface. The disc brakes and discs are less likely to regenerate fast after immersion. There is one leading shoe servo effect [30,31] for several drum brake types. A disc brake has no automated action but is continually in line with the resistance of a brake servo, braking pedal or lever and makes the driver feel better to prevent impending lockup.

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Figure 1

Fixed calliper disc brake system [22].

The most crucial part of a mechanical brake is the disc. Hence, discs’ mechanical, chemical and physical properties should be efficient to ensure effective braking. The materials used for brake mechanism should possess different properties in that some increase compared to other materials used. These properties include sustainability and being harmless to the natural environment, high friction coefficient, ability to withstand high temperatures, resistance to wear and corrosion, flexibility and conformability to any surface, and the thermal stability of material at elevated temperatures along with stabilized friction coefficient. Unfortunately, high temperatures are produced due to friction in the contact zones. The pad's structure might be destroyed, and its parts might separate. For this reason, lubricants are employed, which typically enhance the pad's thermal conductivity. Improved energy removal from the contact region and cooling of the friction components are achieved by lubricants [32,33].

Other parameters include less fade and high recovery, good compressibility, rotor friendly, high mechanical strength to satisfy the application requirement and Less Disc Thickness Variation (DTV). Thus, the material must possess these properties to be used as an efficient brake material. The other characteristics of composite material used for braking applications depend on the properties of reinforcement materials, Binders, Friction modifiers and fillers. The reinforcement material in a composite provides shape and structure for the composite, i.e. fibres. The fibre scale has been described as 5 microns, and at least 1:3 would be an aspect ratio by the national institute for occupational safety and health.

In a brake patch, the reinforcing material serves as a power booster. It is primarily fibres that give a rare combination of structural integrity in the braking pad, friction and thermal stability [34]. Fibres that are synthetically produced chemicals may be natural organic or inorganic. The collection of fibres and the optimal inclusion is considered most important since frictional material will be used too often unless the interface between fibre and matrix materials is appropriately interconnected. Compared to asbestos fibres, the advent of metal fibres proved best and offered ample load-bearing plateaus and wear improvement [35]. The reinforcement agent mainly uses carbon fibres, basalt, rock wool, aramid, lapinus and asbestos (cellulose) [36,37].

Further, the binders are those materials that help hold the fibre and matrix together. Usually, a powder type among the matrix content or phenol-formaldehyde is used to formulate a brake pad. In the face of temperature and stress, this binder retains the multi-components/ingredients in a polymeric brake pad. The formaldehyde reacted with phenol and formed a phenol bridge [38]. Compression moulding created friction-resistant sisal fibre/nano-silica phenol-formaldehyde resin composites. Different surface modifiers were applied to the sisal fibre (SF) to improve the SF and polymer matrix adhesion. Scanner electron microscopy (SEM) was used to examine the worn surfaces of composite materials. The outcome demonstrates that the heat fading of the friction materials can be reduced by the nano-silica phenol-formaldehyde resin matrix [39]. The choice of binder depends on the method.

Moreover, adding binders in a mixture is impossible, so that undue inclusion will lead to high fade and brake pad wear. The binder controls and maintains the operation by controlling the mechanical characteristics of the friction surface to provide the necessary frictional properties. The low binder concentration is contributed to matrix fibre disconnection, thereby increasing the wear rate and affecting the matrix materials [40]. Phenolic resins can be converted to modify forms by physical or chemical procedures to produce stable output at a high-temperature range. Different modifications were made in phenolic resins, including silicone-modified, condensed polynuclear aromatics (COPNA) resin, epoxy-modified resin, ester cyanide resin, rubber resin changed, modified resin, acrylic resin-altered resin (CSNL) [41].

The friction modifiers include the two groups known as Abrasives and Lubricants, where the abrasives have their application to reduce the friction and stabilizing them towards the high range of temperatures so that the friction coefficient increases. The single abrasives known as solid lubricants are not included, but as an alternative, the soft and hard abrasive mixture is used to maintain steady friction at high temperatures [42]. Vital lubricants have no way of cooling the system and thus contribute to a rise in working temperature. Therefore, lubricating solid compounds expose the tension that has formed and reduces it as it rises sharply. When its struggles with the abrasive, stick-slip phenomenon and the noise is closely related to it.

The abrasive range is generally focused primarily on the toughness, acuteness, ductility, form and scale of Mohs [43]. High Mohs abrasive hardness suggested that the equivalent can be abraded further. The abrasives are used for quartz, silica, alumina, carbide, mullite, feldspar and dolomite [44]. A correct tribo-film must be formed on the friction–pair interface when working with grains and could reproduce tribo-film, stabilizing the friction coefficient. Usually, vital lubricant includes many metal sulphides including, di-sulphide of molybdenum, sulphide, antimony, iron sulphide, sulphide of copper, etc., as well as numerous graphite forms, such as natural graphite, flake, synthetic and polarized graphite [45–47].

Brake friction filler means a substance that will fill up the field, or the material space in a brake friction fluid occupies a significant portion of space. They are added to reduce processing costs and product costs. The chosen filler material shall only assist the other materials without affecting their consistency. However, it must not be very polite or lubricant. The position of the filler is less important than the role of different materials such as reinforcing, wrapping, etc. Organic and inorganic fillers are the primary classification of filler materials, among which the organic fillers are commonly used materials and could work under certain low temperatures. The brake pad is incorporated with organic fillers such as calcium carbonate, mica, vermiculite, carbon black, petroleum coke and molybdenum trioxide [48,49].

Furthermore, suppose the organic or inorganic filler is applied to achieve some purpose. In that case, it is called a practical filler, and if it is added to reduce costs, it is called an inert filler. The inorganic filler is chosen as an abrasive by considering of form, scale and friability of particulate matter [50,51].

3. Parameters influencing the friction material formulations

Since asbestos fibre is carcinogenic and may pose health hazards, its usage is discouraged. A novel brake pad made with phenolic resin (phenol-formaldehyde) as a binder and banana peel waste to replace asbestos were under investigation. The resin content ranged from 5 to 30 wt.%, with a 5 wt.% gap. The brake pad's morphology, physical, mechanical and wear characteristics were investigated. The findings demonstrate that while oil soak, water soak, wear rate and percentage burned dropped as the weight percentage of resin grew, compressive strength, hardness and specific gravity of the generated samples were shown to rise. Overall, samples with 25 wt.% of uncarbonized banana peels (BUNCp) and 30 wt.% of carbonized banana peels (BCp) had better attributes. The findings of this study suggest that banana peel particles can successfully replace asbestos in the production of brake pads [52].

In 1939, the semi-metallic formulation was initially employed as an asbestos replacement. Semi-metal formulation denotes the formulation of resin-bonded friction comprised of 30–35 wt.%, or more than 50 wt.% of other ferrous fibres [53]. Over 50% of the iron powder is in the conventional half-metallic composition. The structure comprises steel wool fibres in a novel semi-metallic friction substance with iron powder. This type of friction is worth it for streetcars. The friction coefficient increases as steel fibres increase but decays during the descent of a hill and working at a high velocity, particularly at high temperatures and on flat roads. It also corrodes in humid environments, contributing to audible noise during braking [54]. Semi-metallic friction material comprises fibre and metal powder combinations. Ceramic, metallic, carbon and aramid are the primary fibres in use. Metallic powders are generally made of iron and copper. They have good tribological characteristics and thermal solid resistance.

Low-frequency noise, easy rusting and significant brake disc degradation are the major inconveniences of semi-metallic matrix formulation. The semi-metallic friction material formulation is widely utilized in the motor and automobile industry. In the investigation, 10 semi-metallic pad formulations were developed through the Powder Metallurgic Route, consisting of eight (8) to 14 (14) components. The pathway comprises dry mixing, backing plate preparation, pre-forming, hot compression, post-baking and finishing. The procedures are described as: SM1, SM2, SM3, SM4, SM5, SM6, SM7, SM8, SM10 and SM2 samples were included in the following prototypes.

The produced brake pads are not having homogeneous material. It was found that each element's particle size is not uniform, and the distribution of the element in the matrix is not homogeneously distributed. Test samples were created by adjusting the amount of phenolic resin in the banana peel particles from 5 to 30 wt.% with a 5 wt.% interval. The particles from the carbonized and uncarbonized banana peels were used to create two sets of samples. The mixture was thoroughly dry-mixed in a mixer to establish homogeneity, and then it was transferred to a mould held in a hot platen press for two minutes at 150°C and 9.81 × 10⁷ N/m² pressure [52]. Fine abrasive particles are typically rounded in this formulation because the abrasive shape influences the wear rate. The low-steel formulae are used in front axles, where heavy loads operate during braking. The abrasives in this formulation are usually smaller than in the low-metal formulation. In addition, some compounds are combined with lubricants and rubber, as opposed to semi-metallic formulas [55,56]. Non-metallic friction matrix materials include fibres like Kevlar, carbon, etc., in organic form as reinforced fibres in combination with additional elements like rubber powder-modified resin, frictional additives and filler materials. Hot pressing strengthens these materials. Many non-metallic friction matrix materials are available, which are very resistant and robust to carbon–carbon friction material with exceptional thermal stability. C–C composite friction material is now primarily utilized in aero and racing aircraft.

Further, the forms for the ceramic matrix are incorporated in a wide variety of functions in many ceramic materials. It was found that the semi-metal formulation does not fade rapidly. This style of formulation is no longer based on steel fibre. It has a stable friction coefficient at widespread temperatures [57]. The construction of the material is very complicated since many materials are used in noise reduction. The original equipment manufacturer (OEM) and suppliers of post-market brake pads should be misguided in finding an authentic ceramic brake friction material. The brake pads include high wt.% but are bound with resin [58] in ceramic components. To ensure the brake pad's exceptional thermal strength [59–61], its temperature of more than 1000°C should be resisted. Ceramics comprise ceramic enhancing agents, such as Ca–Mg Silicate Fibre, Rockwool Fibre, Basalt Fibre, etc. [62]. This mixture comprises soft and hard abrasives, which may affect friction at low and high temperatures. Figure 2 shows the Ceramic Matrix Composite material fabricating brake discs and friction pads.

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Figure 2

Tribology applications of Hybrid-CMC (Ceramic Matrix Composites): (a) bike brake discs; (b) Unidirectional reinforced composite friction pad; (c) 2D reinforced composite friction pad [60].

Moreover, the sintered substance is another term for a metal friction material, where metal particles bind under heat and pressure that defuse the metal particles. The sintered friction content consists of two kinds, i.e. copper-based sintered metal and iron-filled metal. However, the high energy densities are primarily due to using synthetic metals in friction materials [63]. The sintered metals have been coated on cotton to make a metallic substance of a fibrous structure. It was the first actual sintered brake pad. It is usually made of many metal powders such as copper, iron, tin, lead, etc. and could be used with iron for dry and wet friction [64–66]. Subsequently, copper was added to the matrix of friction content as a base metal with a percentage of iron powder [67–69]. The iron-based metal was used in the finished product, approximately 61 wt.% The matrix included mineral abrasives, ceramic particles and zinc [70]. The idea of carbonization is used in the development of carbon on a material's surface. The carbonated fibres are pulverized and used in the matrix or carbonated output as friction content and moulded in a desired size. It is used in high-speed trains, main war tanks, aircraft and automobiles because it provides intense thermal stability up to 1500°C. Its benefits include 2.5-fold higher output than steel, higher temperature, 40% lighter weight and double life cycle [71–73]. The organic friction material is divided into (a) organic asbestos and (b) organic non-asbestos. The semi-metal brake pads often come with an organic material not made from asbestos, of which the wear rate depends on the binder decomposition [74]. The production of asbestos in the 1950s was difficult to import. Instead of asbestos, they used metal fibres and other fibre products, including glass fibres [75]. This type of formula requires 50% metallic materials, phenolic resin, graphite, frictional dust, etc. It quickly became apparent that a single fibre was insufficient for asbestos. As a result, a mixture of fibres was used to replace asbestos. The aramid fibre was mixed in 2:3 with stiff ceramic fibre, and para-aramid fibre is integrated into the 1.67:1 ratio with carbon fibre [76–78].

The semi-metallic formulation was the first updated formulation to substitute the asbestos-based material. The steel wool fibres have been strengthened and shown to be superior in transferring thermal energy effectiveness, reducing binder loss, and improving friction and wear resistance [79–81]. In addition, by adding stainless wool fibres in a semi-metallic formulation, the thermal conductivity is improved, the fades efficiency is decreased, and performance improvements and μ recovery [82]. Still, with the addition of stainless-steel wool fibre, other steel fibres of stainless steel and oxide laminated steel fibres increased the density, hardness and wear resistance in friction materials [83–85]. Further, it was reported that the steel fibre also has a compacted secondary platform, which enhances the wear resistance of the brake pad. Since 1913, copper has been used in friction products. Heat transfer was one of the challenges faced by manufacturing companies over the decades of asbestos-based brake pads. It was found that the copper fibre acted efficiently when it was positioned generally on the surface of friction and spread in the matrix for heat transfer [86–88]. The copper fibre then became an unavoidable component in all the friction formulations as practical fillers or additives because of their ability to convey heat, minimize the wear rate and increase the negative fading of the composite at a high-temperature level [89–91]. The surface temperature of copper can be regulated, and the hot spot formed due to the reduction of overheating [92].

Further, three essential functions of thermal conductivity, primary panelling and smooth sliding at high temperatures are found [93–95]. Copper in a brake friction material has evenly spread the temperature during curing, eliminating composite cracking during moulding. This chemical assault may influence the degradation of the phenolic binder during slipping, which forms copper oxide with oxygen [96]. The Brass fibre is a member of the copper family, and although it performs similarly to copper in heat dissipation, it has shown low wear output [97].

Furthermore, the zinc fibre increases regeneration and stabilizes the coefficient of friction. Basalt fibre is a black, rigid mineral fibre made of igneous rock and many oxides [98,99]. It is created to substitute asbestos through this fibre. They have shown similar characteristics of asbestos, such as heat resistance, durability and strong binding nature. Basalt fibres enhanced friction and fad. It recovers when it is combined with aramid fibres [100]. The different natural fibres, matrix material, method of preparation, tribological properties and application have been given in Table 1 [101–111].

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Table 1

Different natural fibres, matrix material, method of preparation, tribological properties and application[101]

Fibre used	Matrix	Treatment	Method	Properties	Application
Woven banana and kenaf fibre	Unsaturated polyester	10% NaOH and 10% sodium lauryl sulfate	Hand lay-up technique	Abrasion loss is 0.8 g when 2 kg load was applied for 8 h	Clutch plate
Sisal fibre	Phenolic resin	NaOH–ammonia base liquor, H2SiO3, then steep in Na2B4O7–HCHO–NaHSiO3	Compression moulding	Optimum wear and friction were observed at a 3:4 resin/sisal fibre ratio	Brake composites
Fibres from (a) Tapsi, (b) Abutilon indicum and (c) Prosopisjuliflora	Epoxy	NaOH	Hand lay-up technique	The friction coefficient is highest for the composites
Jute/Hemp/Flax mats	Epoxy		Hand lay-up	The friction coefficient is highest, while the specific wear rate is lowest for the composites
Caryota urens fibre	Phenolic resin		Compression moulding	The incorporation of 5 wt.% Caryota urens fibre in brake formulation increases the wear properties	Brake pads
Areca sheath fibres	Phenolic resin		Compression moulding	Incorporation of 5 wt.% of areca sheath fibre in brake formulation increases the wear properties	Brake pads
Eulaliopsis binata fibre	Epoxy		Hand lay-up technique	The composite containing 20 wt.% of the fibre gives the best wear properties
Phoenix sp. fibre	Epoxy		Compression moulding technique	The composite containing 20 mm fibre length and 40 wt.% fibre content gives the best abrasion resistance
Phoenix sp. fibre	Epoxy	Sodium hydroxide	Compression moulding	Fibre treated with 15% NaOH showed the best properties
Short wood fibres	Acrylic resin		Co-rotating twin screw	Incorporation of 50 wt.% fibres showed the lowest specific wear rate, the impregnation of sunflower oil reduced the friction and wear of the composites considerably

Further, it was noticed that the stainless-steel swarf is the emerging option to eliminate the copper powder. The use of stainless-steel fibre improved the thermal density and diffusiveness. The higher content of steel swarf in friction composite resulted in an excellent fading and recovery ratio and an overall friction coefficient superior to copper friction material [112].

Further, glass fibre is also used to replace asbestos fibre as fibreglass because of its excellent heat and impact resistance [114]. It is one of the most frequent reinforcing products in organic friction materials. It is utilized as a cut beam for its outstanding thermal stability and strength. However, it may be combined with other textiles; therefore, it is fragile [115]. Glass fibre is combined with aramid steel tissue and alters the scent of a phenolic resin composite emitted during braking [116]. The result is abrasive wear waste at the interface that can damage the braking wall [117–119] besides causing excessive wear of brake pads. Glass fibre is very elastic and has a high phenolic resin melting point [120]. It has shown less heat conducting than 0.04 W mK⁻¹ asbestos [121,122].

Furthermore, carbon fibres are used for dimensional friction stock stability to reduce wear. The optimal outcomes are improved specific strength, rigidity and wear strength [123,124]. The carbon fibre in the composite has been incorporated by mixing fibres due to its cost and resilience [125].

The aramid, carbon and ceramic fibre compounds also increased the composite's wear resistance. The fade output [126–128] was also improved through the randomly spread fabric. However, carbon fibre has a detrimental effect because of its moisture absorption capacity, i.e. reduced friction level. Even the alignment of graphite crystallites could significantly impact the carbon layer participating in the third body generation [129]. Figure 3 shows the microstructure of the C/C-SiC composite with the presence of SiC. The composites’ carbon fibre dispersion and different PyC, SiC and Si concentrations are displayed (each phase is denoted in the image). A PyC ring ringed the carbon fibre during CVI (Chemical vapour infiltration) processing. The thickness of PyC around the interfibre is less than that of PyC around the interbundle because the space in the interfibre is much smaller than in the interbundle. Four samples, designated CS-2, CS-3, CS-4 and CS-5, were created with varied Chemical Vapour Infiltration Times of 2, 3, 4 and 5 h.

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Figure 3

Optical micrographs of polished surfaces of (a) CS-2, (b) CS-3, (c) CS-4 and (d) CS-5. F, carbon fibre; P, PyC [113].

With longer CVI durations, the PyC layer that surrounds the interfibre becomes thicker. It is clear from Figure 3(a) that the PyC layer thickness in the CS-5 sample is much higher than in the CS-2 sample. While the RMI processing did not degrade carbon fibre, the interaction between PyC and Si resulted in the formation of a SiC layer. The outstanding mechanical qualities of carbon fibre, particularly its tensile strength and modulus, may be maintained. For the SiC layers that were created, the interface between the SiC and C phases appears to be continuous, while the contact between the Si and SiC phases is rough. Different Si and C response mechanisms during RMI processing led to other interfaces. In the C/C-SiC composite (Figure 3(a–d)), Si is still present since it did not completely react with PyC. Due to the various porosities and structures of the C/C composites, the C/C-SiC composite contains different amounts of free Si. The CS-2 sample has the most silicon of the four types of composites, as seen in Figure 3(a) [113].

Filler materials have been shown to have a significant impact on the properties of the brake friction material. Among the most often-used materials, Barite is a low-cost filler, typically utilized to lower the final component's costs. The chemical reaction of the barium sulphate involves carbon from phenol resin, resulting in the formation of barium sulphide and the release of carbon dioxide. The appearance of sulphide is possible due to the extraction of oxygen from sulphate, which is converted to sulphide [130]. The effect of filler on the mechanical properties of non-asbestos brake pads has been given in Table 2 [131–135]

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Table 2

Effect of filler on mechanical properties of non-asbestos brake pads.

No.	Type of reinforcement	Fabrication method	Weight fraction (wt.%)	Hardness (Shore D)	Density (g/cm3)	Wear rate	Coefficient of friction
1	Coconut shell powder	Hot compression (80°C, 100 kN/cm2, 5 min)	2 wt.	21	2.05
4 wt.	70	2
6 t.	69.7	1.89
8 wt.	68	1.7
10 wt.	58	1.6
2	Grounded coconut shell	Hand lay-up	50 wt.	30 (HRF)	2.55	2.56 (10−6 g/min)
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	0.2193 (g/min)	0.374
38 wt. + 13 wt.	3.41 (kgf/mm2)	2.6	0.2733 (g/min)	0.383
15 wt. + 36 wt.	3 (kgf/mm2)	2.78	0.2007 (g/min)	0.362
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
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

The alkaline nature of the composites has offered by the pH modifiers such as calcium hydroxide (CaOH) and calcium oxide. Using calcium hydroxide during the curing process increases the deteriorating impact, and other compositions of calcium hydroxide are added as well. By adding 1.9 W% of the CaOH and 2%–10% of the CaOH, we increase the strength of the braking line by reacting with phenolic resin, and the hardness of the finishing product is improved.

In addition, steel fibres and CaOH are added to enable the brake lining or pad's ability to endure high dynamic loads during braking. The pad material utilized for the investigation is shown in Figure 4 [136–138]. Mica and vermiculite are used in the early days as a lubricant, providing a great property to shear well on the basal plane. They are phyllosilicate minerals and have a hardness of 2–3 Mohr [139]. And because of its flat net shape, the structure has also been employed as an additive to minimize squeaky sounds for over a century [140–142]. Weathered mica made of laminar silicate sheets has low thermal conductivity and high abrasion resistance [143,144]. The wear resistance to composite has been enhanced by 25–50 wt.% of vermiculite as a substitute for asbestos and by 2 wt.% vermiculite [145,146]. Sponge iron is another filler material, a reduced iron powder mainly used in the friction substance as a metal filler. It is also observed that noise reduction may be accomplished using the sponge iron course grade in semi-metal brake pads [147].

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Figure 4

Cross-sections of brake pads used in the tests: (a) first group and (b) third group [136].

There are various materials used as friction modifiers in brake friction materials. Silica is widely used in friction materials. This is due to their excellent friction coefficient. Thus, it is significant to consider materials with optimum friction coefficient in the friction materials. Also, Tripoli and rottenstone, an impure form of silica, are used as brake materials. Also, Alumina is an abrasive that enhances the friction output by 15% and provides wear resistance [148,149]. The friction coefficient during the high-temperature fade cycle has been increased due to the addition of a 2% concentration of alumina. Also, mullite is a naturally obtained alumina-silicate abrasive used in the sintered friction material. If a compressive force acts on them, the mullite would split and provide a new abrasive surface and allow a constant friction coefficient over an extensive range of temperatures, especially in the case of railway brakes [150,151]. Silicon carbide is widely used in friction industries. A slight increase in its concentration yields an excellent friction coefficient, improved friction stability and an increased wear rate [152–154]. Figure 5 illustrates the compression strain at room temperature and 400°C for friction composites. The abrasive powder coating in a sodium silicate solution enhanced green or pre-burnish friction [155]. Zinc oxide is a soft elastomeric substance that ensures friction coefficient stability. It also does not cause extra wear and may affect the physical features of the ultimate brake pad [156]. The metal sulphides are generally employed to minimize friction as a solid lubricant since they may be lubricated when metals react well and eventually form a thick trio layer. On the surface, metal sulphides with copper create metal alloys [157]. Another widespread usage in the friction industry is Molybdenum di-sulphide since the molybdenum slides across the mattress component. In addition, the molybdenum di-third Sulphur's corporal layer is stable at 350–400°C but transformed to molybdenum oxide by the sulphur loss at higher temperatures [158,159]. Low-metal and semi-metallic formulations, also called friction-enhancing solid lubricants, improve friction by 5% [160,161]. The copper–iron sulphide reacts with the mating part in the heat generated due to friction during braking. As a result, it gets decomposed, and sulphur gas is liberated. As a result of this reaction, a layer of iron pyrites is formed on the surface, affecting the brake friction material's performance [162]. But the load-bearing capacity of the brake friction materials is increased by the addition of copper sulphide and zinc sulphide [163]. The fade and recovery properties are enhanced by the presence of cupric sulphide and cuprous sulphide [164]. Cupric oxide provides a higher friction coefficient than cupric sulphide but lacks to provide friction stability. Among the group of metal sulphides, Tungsten sulphide is an effective solid lubricant used in brake friction materials. They reduce the wear rate and enhance the strain recovery behaviour by forming tribo-layer effectively [165].

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Figure 5

Compression strain of the friction composites: (a) at room temperature and (b) at the temperature of 400°C [153].

Tribology is the study of a better understanding of wear, friction and lubrication. There are various terminologies and parameters to be considered while analysing a material. The term tribology has been given importance because most materials require wear analysis as they tend to fail in significant cases. With a better understanding of the tribology of materials, the material's durability and resistance to wear can be optimized, and the material can be provided with the property requirements that would satisfy the application. The third body layer resembles a continuous layer at a friction-based pair interface for all the brake pad materials, where energy transformation occurs. According to the viewpoint on the third physical layer, the friction pair interface of wear debris is trapped into a continuous sheet rather than particle isolation from the surface during the braking phase [166]. This layer is influenced by frictional heat, and a rotor film has known as transmission film, i.e. distributed and on the surface, it continues to degrade. The third body layer improves the wearing strength of the brake pad by supplying a lubricant barrier on the surface [167] and minimizes the counterpart loss. It dissipates the velocity gradient generated from kinetic to thermal intensity during an energy transfer. The vehicle's speed is balanced and scattered in this layer as the speed gradient [168]. There are 20 mechanisms to accommodate the two bodies; the interlayer between a third body and a second body, called screens, or between two so-called third bodies [169,170] can be used.

8. Effect of brake wear particle size

In 2000, Garg et al. examined the brake dust produced by seven typical brake pads, showing that airborne PM brake wear is around 35%, with an output of 3.3–8.8 mg/km [222]. Furthermore, an average of 18% of the in-air particulate matter is determined to be carbonaceous. In 2002, Westerlund and Johansson analysed brake wear emissions using plasma emission spectrometry for material identification, such as copper, zinc, chromium, nickel and plumage for brake linings [223]. But the examination of rotor materials cannot be considered. They computed the annual consumption of vehicle brake lining based upon assumptions of a fixed driving distance lining replacement, weight lining and the effective lifespan of lining, i.e. 70% of their life before replacement.

Furthermore, their calculation showed the emissions of brake wear for passenger cars to be 17 mg/kg/vehicle. Abu-Allaban et al. conducted trials to examine the exhaust and non-exhaust PMs released by cars [224]. The test techniques include modelling the mass balance receptor and scanning electron microscopy in dry and wet areas. In 2003, Sanders et al. found the mean brake debris diameter being about 6 μm and the most significant emission of LM brake pads and the loss of airborne particulate matter in more than 50% of the total PM [225]. In the end, Fe, Cu, and Ba in PMs have been observed. Later in 2004, it was revealed that exhaust and non-fuel account for around 35%–50% of PM 10.4 and 40%–60% of PM 2.5 from atmospheric air data obtained from EU nations [226]. Also, the PM10 emission is higher than PM 2.5, which leads to the fact that there is no standardized method of analysing the emissions. Many of the tests were carried out in labs using dynamometers, while others published their findings based on collecting samples from roadways and studying them in laboratories. Also, the lack of adequate legislation makes Brake Linen Composites diverse. Separating brake wear from tyre wear and resuspension is difficult. In the event of the laboratory investigation, interface temperature generated frequency and severity of the deployment of braking systems.

9. Effect of composition of brake wear particles

In 2004, Chan and Stachowiak reported that binders, fillers, reinforcing fibres, lubricants and abrasives are included in the main components through the investigation of comparing a variety of reinforcing fibres such as glass, metallic, aramid, potassium titanate, sepiolite and ceramic fibres for brake components [227]. Among these, ceramic fibres were found to be an appropriate reinforcing material for brake material. This is due to its high melting temperature and high stiffness-weight ratio. Later, brake wear tests were performed by Mosleh et al. by making use of pin-on-disk configuration [228]. The materials used for the disc are grey cast iron type 40, and the pin consists of the material for the braking pad. The investigation is done with various contacts between 0.125 and 1.25 MPa with a speed of 0.275 m/s and 5 m/s. The cast iron wear debris was found to comprise Fe, C and O₂. It was stated that the scrap was more significant than 1 μm.

The wear particle size rises with increasing load and is included in Cu, Mg, Si, S, Al and Sb. It is also reported. The wear of the brake pad is also demonstrated to change the total Cu emissions by around 80% in the environment. A 3D chemistry transport was developed to investigate the formation of vehicles and sinks of oxidants, particulate matter and heavy metals [229]. The reports given by Barlow et al. show the inconstancy and insufficient data on non-exhaust emission data. It is proposed that the emission rates for vehicles less than 3.5 tons and higher than 7.5 tons gross vehicle weight were around 10–20 and 50–80 mg/km. And it is observed that about half of the particles enter the atmosphere, and about 80% appear to be PM10 [230]. The brake dynamometer abrasion test was conducted by Iijima et al., where the disk is made of cast iron, and the study estimated that around 74% of at 400°C to 92% at 200°C of the dust could be produced as PM2.5. It is noted that a large amount of Fe dust is obtained from cast iron [231]. This shows that the emission is higher at lower temperatures. Thorpe and Harrison checked brake wear, and the results show that the particles of the brake can be determined from the Cu and Sb contents. However, it would become hard to establish the frequency of emission of brake dust already on the road surface [232]. They reported that identifying the source of non-exhaust particles is restricted as brake dust particles mixed with tyre dust. The existence of brake wear particles and other elements has been further investigated [233] by Fe, Cu, Ba and Sb. However, the composition of the brake materials remains unknown.

Kukutschova et al. reported that the wear particles, sizing between 0.01 and 20 μm, are airborne by studying low metallic brake pads using a dynamometer [235]. An increase in the nano-sized matter was released as the temperature approached 300°C. However, according to Pant and Harrison, there is insufficient data on the characterization and quantification of non-exhaust sources. Also, the data availability is less for the effect of the emission of particulate matter on human health [236]. It is observed that the non-exhaust emissions lie between PM2.5 and PM10. Later, another study of brake wear was reported using the development of NAO friction material by introducing a particular composition of Cu particles [234]. Figure 6 shows the SEM images obtained during the study of wear in the brake disc. It is also observed that the thermomechanical properties, such as density, thermal conductivity, hardness, etc., showed a considerable increase in their readings. The brake wear rates depend on the vehicle's speed, where the wear is high for forced fast deceleration [237]. The study by Hussain et al. to estimate brake wear reported that the size of the particles influences the applied load [238]. But the brake speeds affect the size and shape of the particles at a significantly lower percentage.

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Figure 6

SEM images of the fracture surface in the brake disc [234].

The particles were between 300 and 700 μm, and the presence of elements such as C, O₂, Mg, Al, Si, S, Fe, Ba and Tb in the NAO debris. The brake wear emissions were between 16% and 55% in the study. It is reported that about half of the total brake wear was observed to be emitted as airborne particulate matter PM10 and confirmed the presence of Fe, Cu, Ba and Pb [239]. The brake dust particles were collected and investigated, and it is reported that the size ranges between 10 and 15 nm for the smallest particles and up to 100 μm for the PMs. And the presence of Na, Mg, Al, Fe, Cu, Zn, Ba and Bi was confirmed [240]. Hagino et al. reported airborne PM10 between 0.04–1.4 mg/km and 0.04–1.2 mg/km for PM2.5. They used a brake dynamometer for the brake wear analysis. The study proved the presence of Fe, Cu, Ba and Sb [241].

Two critical groups for field studies are (1) on-vehicle measurement and (2) airborne particulate matter sampling. When measuring, sampling and assessing ambient brake wear particles during field tests, it may be difficult to distinguish natural brake wear from other factors such as resuspended road mud, pneumatic wear, concrete and other related sources of pollutants. Thus, these simulations can be carried out only with the contribution of the measured brake pads and rotors in a stable environment. The literature has just a few field experiments on brake wear particles. To allow a comparison with dynamometer brake tests, Hagen et al. conducted wind tunnel, track tests and field tests with a full-size car and LM frequency block pads [261]. At speeds of 96 km/h, initial braking temperatures of 130°C and steady winds at 64 km/h, the wind tunnel was tested. These measurements are free of road dust because of the smooth stainless-steel roller's low wear and tyre wear, which provides the load in wheels. The production of wear particles from LM and NAO brake materials was supplied with a high-speed path and public roads. In all the samples, particle size distributions were measured. Both stops were made with variable deceleration from 60 km/h on the test circuit. Filters were mounted on the vehicle to trap the airborne particles produced and to conduct chemical analysis to compare the waste obtained during dynamometer testing with the wear composition. Dynamometer measurements were associated with field tests in the distribution of particulate matter. However, for the dynamometer tests, the mean weighted mass diameter drops to about 3 mm from about 6 mm for the test track results. In all experiments and all materials, the submicron fraction made a marginal contribution to the overall wear particle mass. This is determined by the chosen particle quantification parameter and the fact that the finest particles, especially those of nano size, are ‘weightless’. With 109 released particles per stop for LM and NAO materials, it was possible to distribute maximum numbers around 1 mm. The wear mass of PM10 was created by dynamometer testing for LM pads, w8 mg/stop/braking for NAO and w2 mg/stop/brake, whilst the airborne wear particle mass was w21 mg/stop/brake after the wind tunnel tests for LM pads. Several field experiments were done in addition to using a passenger vehicle with LM brake pads [262–264].

The tests were conducted on a test track that mirrored city driving, highways and motorways. Experiments were performed on rainy days to reduce the effect of resuspended traffic-generated particles. The number of particles per m3 measured was 1.7 107 and a maximum of 410 nm [265]. A significant variation in concentrations determined on the brake and the front of the car was noted. Many types of research focused on sampling and characterization of PM from roadside setups, but only a few studied the indirect detection of brake wear contaminants using an appropriate tracer [266].

12. Future challenges

For the automotive industry, the future of frictional brake materials depends very much on the pace at which effective ‘regenerative braking’ technology is created. As renewable braking efficiency improves to mature, low-cost and durable technology, the friction brake will likely be used alone as an emergency or parking device. Instead, accelerated construction of self-contained cars, where driving speed and stopping time is optimized, usually involves more frequent friction braking, which leads to higher demands on friction materials. Further developing their wear tolerance would be essential if environmental issues are discussed. The challenge, however, is complicated and involves designing new brake frozen materials and an ultimately improved brake architecture with more effective heat dissipation/administration, vibration and associated sound, while minimizing the braking system mass. A rise in demand for reduced travel times, increased vehicles’ top speed, and the associated enhanced safety measures can also be anticipated. Therefore, the above-mentioned ‘overlap’ between the prevalent use of friction brakes and the use of regenerative braking, which is entirely and fully efficient, would be characterized by the fact that a significantly increased demand is required for braking applications. Through the advent of modern and enhanced products, this is possible. When the cost of braking devices, including brake components, is optimized or reduced, the friction industry is under tremendous strain. New and better friction materials would be expected for automotive technology in the not-so-far future. However, for a given community of cheap materials, it seems unlikely to accomplish these demanding requirements, and this paradigm will likely shift shortly. In designing braking mechanisms for the not-so-far future, it is possible to anticipate the implementation of research and knowledge regarding innovative materials and new developments.

13. Conclusion

The brake composite materials are proving a higher significance in the automobile industry. The utmost safety measures taken in an automobile include brake material characterization. Thus, the study to prepare the brake composite materials is essential. From the study, the following conclusions may be drawn. Various material formulations, such as semi-metallic, ceramic, non-asbestos, etc., produce brake friction materials. Each material formulation has specific parameters to be considered during the process, which influences the material formulation extensively. Brake composites are reinforced with various fibres, where steel, copper, and basalt fibres are most used as supporting materials. Other reinforcement fibres include aramid, glass and carbon fibres. The effect of various filler materials was reviewed, and the brake material has an optimum performance for each material. Therefore, a suitable filler material may be selected based on the application requirements. Also, the materials used as friction modifiers showed an appropriate change in the friction coefficient and significantly influenced the brake friction materials’ performance. Various studies on brake wear particles and PM emission were discussed. Several studies based on the brake dynamometer and pin-on-disc analysis were discussed along with the field studies.

Compared to grey cast iron or carbon/carbon, Ceramic Matrix Composites (CMC), reinforced with carbon fibres and silicon carbide matrices, exhibit better tribological properties. These Si-infiltrated carbon/carbon materials, also known as C/SiC or C/C-SiC composites, are potential options for high-tech friction systems due to their low density, strong thermal shock resistance and good abrasion resistance. Generally, the silicon carbide matrix enhances wear resistance compared to carbon/carbon, while the carbon fibres contribute to an improved damage tolerance compared to monolithic SiC [267].