Abstract
Brake pads are vital parts of automobiles, where tribological properties are prime concern. Usually, asbestos brake pads were used, which were replaced by copper-based in later years. As brake lining materials have evolved, synthetic friction composites were explored as alternatives. Recent works suggest that, natural composites are promising friction material. However, poor thermal conductivity and frictional stability have limited their applications. Surface treatment of fibers and addition of secondary fillers have improved their possibilities. Thus, hybridization of reinforcements has expanded their range of applications. The present article attempts to review various aspects of brake pads and their testing with attention on natural composites. Their properties were studied to conform their ability to meet desired requirements. The article reviews the studies conducted in recent years on various materials properties related to brake pads. The future potential of natural components in the development of friction composite materials for automotive applications is highlighted.
Keywords
Introduction
In recent years, natural composites have emerged as promising alternative materials for engineering applications. Key concerns such as enviro-footprints, energy conservation, waste management, and recycling of agricultural by-products have drawn the attention of materials scientists and engineers. Compared to synthetic reinforcement types, the low density and low raw material cost of natural reinforcement materials have made them a candidate for components of engineering materials [1]. The reinforcement of natural materials in composites has paved the way for a new era of materials technology and development due to their improved physio-mechanical and tribological properties. However, poor bonding properties and friction-induced noise limit their extensive use [2]. The continuous and progressive research interest in this field has led to the development of natural composites with novel material reinforcements for tribological applications, such as automotive [3–5].
Automotive brake pads are significant components which are subjected to severe tribological conditions. The selection of suitable material for brake pads is vital. Commercial brake pads use asbestos, copper, and ceramic-based materials. Natural composite materials are under consideration in the past few years as brake pad materials owing to their superior mechanical and tribological properties. The available research review works have outlined published literatures on the brake performance characteristics of natural composites. However, the contribution in the past 5 years makes a significant mark in the development of natural composites. Hence, in this regard, the current review article attempted highlight remarkable advances in the field of natural resources as reinforcement materials in composites. The work focuses on the utilisation of natural fibre-reinforced composites as brake pads, which are an important tribo-component in the automotive industry. An attempt is made to summarise the current literature on the physio-mechanical and tribological behaviour of brake pads made of natural fibre-reinforced composites. The influence of process parameters on brake pad performance is also discussed. The future prospects for the development of sustainable brake pad materials and optimisation of brake pad composition are propounded.
Natural composites and their characteristics
Interest in sustainability and its significant impact on the environment has led materials scientists to utilise natural resources to develop materials for engineering applications ranging from spectacle frames to aerospace applications (Figure 1). It has led to an exponential increase in developed natural composites and their applications over the recent years Figure 2. The composites are categorised as natural composites if the matrix or reinforcement used is a naturally available resource. Reinforcement types i.e. fibres or particulates used are naturally available materials. They can be based on either mineral or biomass, as shown in Figure 3. The available literatures have paid attention on plant-based and animal-based reinforcements as substitutes for synthetic reinforcement materials. However, pre-processing of plant-based reinforcements, especially pre-treatment of fibres is necessary to obtain better properties that meet the requirements of engineering applications [6].
Natural composites presence in the US market [21]. Composition of natural composites.
In the available literatures, the properties of composites reinforced with plant fibres from hemp, coconut, flax, nettle, bamboo, ramie, jute, banana, tamarind, sisal, pineapple, Grewia optiva, bagasse, vetiver, kenaf, areca, oil palm, okara, Bauhinia vahlii, acacia, luffa, waste cotton, neem, abaca, sugarcane, etc., have been studied [7]. Some of them are illustrated in Figure 4 and mechanical properties of few natural composites studied in the past 5 years are summarised in Table 1. Particulates and whiskers extracted from plants were extensively studied. Sairkaya et al. [8] investigated the mechanical properties of epoxy natural composites reinforced with birch, palm, and eucalyptus fibres. Babu [9] evaluated the physio-mechanical properties of polyester composites reinforced with Caryota Urens Spadix (CUS) fibres. The increase in CUS fibre incorporation % has enhanced the tensile strength of composites at the cost of compressive strength and density. Natural fibre reinforcements have contributed significantly for the improvement of mechanical properties of natural composites. In addition, it helps to decrease the density of composites and improve other physical characteristics considerably. The resultant lighter composite is mainly due to the low density of natural fibres.
Mechanical properties of few natural composites studied in the past 5 years.
Fazio et al. [24] have shown that hemp fibre-impregnated epoxy composites are comparable to glass/epoxy and carbon/epoxy composites in bio-tribology applications. The advantages of hemp fibre impregnation were more pronounced at higher loading than at low loading. Rajini et al. [10] reported that Cyperus pangorei fibre-reinforced polymer composites offered better resistance to wear at a low coefficient of friction. Ugochukwu et al. [25] compared the effects of thermal ageing on scratch resistance of epoxy composites with jute, kenaf, and hemp fibres. The better scratch resistance such as higher critical load, fracture toughness, and scratch hardness was evident in the study compared to kenaf/epoxy and hemp/epoxy composites. The hybridisation of reinforcement has shown promising results compared to single type of reinforcement in natural composites. The improved characteristics of hybrid composites are remarkable at high and critical loading conditions.
The chemical treatment of fibres such as alkalisation, silanisation, etc., has been carried out to achieve enhanced characteristics of reinforced composites [26]. Ahmed et al. [3] reported that alkaline treatment was effective in enhancing the physio-chemical, mechanical, thermal and tribological characteristics of Areva javanica fibres reinforced phenolic composites. Liu et al. [8] showed that silane treatment of cellulose fibres from corn stalks improved the impact strength and fibre-matrix interfacial bonding of polymer composites. Rajan et al. [27] demonstrated that alkaline and silane treatment of Prosopis juliflora fibres leads to better mechanical and thermal properties of Prosopis juliflora/phenolic composites. The chemical treatment of fibres helps to achieve improvement of mechanical properties of natural composites compared to untreated fibres. The chemical treatments help to remove the hemicellulose and lignin content of the natural fibres. It improves the surface characteristics, crystallinity index and crystalline size of the fibres. In addition, the improved wettability of the fibres enables good interfacial bonding between the fibre and the matrix. Therefore, chemical treatment of plant-based fibres is necessary and/or an integrated processing step of the fibres before their reinforcement in polymer composites.
Hybrid composites with different fibres have shown promising results in the reported research compared to single fibre reinforcement. Hybridisation provides a balance between the cost, sustainability and performance characteristics of the different reinforced fibre types in the composite [6]. This leads to synergistic advantages and compensates for the disadvantages of the individual fibre properties. Kumar et al. [15] illustrated that physio-mechanical and tribological behaviour of G. Optiva/Bauhinia vahlii fibres reinforced epoxy hybrid composites was better compared to G. Optivia/epoxy composites. Megahed et al. [28] shown that carbon/flax hybrid natural fibre composites resulted in the best optimised designs with lightweight, and low-cost advantages compared to glass/carbon hybrid reinforcements. Jawaid et al. [29] succeeded in improving the thermal and thermo-mechanical characteristics of hybridisation of date palm/bamboo fibre-reinforced epoxy composites.
In addition, natural composites with organic-based fillers or particulates as reinforcements were developed by several researchers. The oil palm kernel shell [30], Chamaerops fruit shell [31], coconut shell [32] powder, etc., hazelnut dust [33], walnut dust, [34], etc., have been used as primary reinforcements in natural composites. Alfatah et al. [30] substantiated the literature reports on oil palm shell fillers in different plastics. The mechanical, thermal, and physio-mechanical characterisation studies of oil palm shell based natural composites were summarised. The study suggests that oil palm kernels are promising reinforcement materials for various engineering applications with surface modifications. Moreover, they can also play the role of secondary fillers and fibres in hybrid composites. Kannan et al. [35] analysed the mechanical behaviour of banana fibre-reinforced polyester composites filled with a varied volume fraction of flyash. The authors concluded that the addition of 3% flyash improved the mechanical properties. The peanut oil cake filler added as a secondary reinforcement to epoxy composites reinforced with pineapple/flax fibres, significantly enhanced the mechanical properties. The improved properties due to the addition of fillers and particles were possibly due to the increased crystallinity and enhanced compatibility of the natural composites [16]. Thus, the incorporation of organic fillers has led to the significant enhancement in the mechanical, thermal, physical, and tribological behaviour of natural composites.
Attempts were made to incorporate animal-based reinforcement materials in polymer composites. Bharath et al. [36] studied the failure behaviour and optimum toughness design of sheep/wool/epoxy composites. Lokesh et al. [13] investigated the effect of addition of fish scale and coconut shell powder on the mechanical properties of polymer composites. Tusnim et al. [37] illustrated that sheep wool with jute fibre reinforcement had the best mechanical properties compared to individual fibre reinforcement. The crab shell/sisal fibres/epoxy composites exhibited better fibre-matrix interfacial bonding resulting in enhanced mechanical properties [14]. Ganesan et al. [38] reported that addition of eggshell powder to jute fibre composites showed superior mechanical properties compared to jute/polyester composites. Sahoo et al. [39] demonstrated that chitosan rich content prawn shell powder can be an excellent filler material for natural composite to achieve better mechanical and physio-mechanical properties. In addition, reinforcement of crab shell powder [40], periwinkle and scallop shell powder [41], and cow bone particles [42] have shown promising results similar to those of plant based polymer composites. The calcium carbonate content in the shell powder owes to the enhanced properties of developed composites. The particulate reinforcements addition as secondary fillers is highly favourable compared to fibre-matrix interface to obtain better physical and mechanical properties. The particulate strengthening mechanism plays a crucial role in improvement of properties. However, the secondary particulate reinforcement percentage cannot exceed a threshold limit for a composite. The addition of filler above the certain limit results in deterioration of characteristics instead of improvement.
Extensive research work has been reported on the use of thermoset polymers such as epoxy, polyester, phenol-formaldehyde, and urea-formaldehyde resins as binders or matrix materials for natural composites. The excellent thermal, morphological, and physio-mechanical properties have proliferated the use of thermoset polymers. However, the utilisation of thermoplastic polymers and recyclable polymers still requires considerable progress to meet the desirable properties of engineering applications. Few notable works have used thermoplasts such as polypropylene, high-density polyethylene (HDPE), plasma modified polyethylene (PPE), etc. Garcia et al. [43] reported that addition of peanut shell powder to HDPE matrix increased the elastic modulus of HDPE. The addition of poplar wood, radiata pine, and rice husk in HDPE matrix was illustrated by Hao et al. [44]. The study proved that addition of natural reinforcements improves the mechanical properties such as flexural strength and impact strength of HDPE matrix material. A study by Nascimento et al. [45] revealed the superior yield strength and Young's modulus of polypropylene composites, when it was incorporated with natural fibres. Sari et al. [46] explored the possibility and potential of the PPE matrix in the development of natural composites. It was found that PPE has better resistance to water absorption compared to polyethylene (PE) composites and has superior mechanical properties. Thermal stability of thermoplastics is the major concern compared to thermoset polymer matrix. Kumar et al. [47] revealed that addition of coconut shell charcoal filler bamboo can improve the thermal stability of bamboo/PE composites. However, limited studies are available to validate the thermal stability characteristics at elevated temperatures. Further, progress in research seems necessary to enhance the thermal behaviour of thermoplastics. It enables engineers to consider thermoplastics as candidates for friction and high temperature applications.
Automotive applications include a large number of components with desired characteristics ranging from high strength to tribological properties. Brake pads in automotive applications are subjected to high levels of wear with travel distance ranging from 20,000 miles to 70,000 miles depending on friction material and friction coefficient of about 0.5 owing to very frequent contact with moving surfaces. Brake pads are mounted over rotating wheels to decelerate them (Figure 5). They consist primarily of a surface of optimum friction material that instigates the deceleration action required to stop automobiles (Figure 6). In addition, brake pads are friction components, that help to dissipate the kinetic energy generated due to applied brake. Therefore, the materials used for brake pads are categorised as friction materials. The desired characteristics of the material being considered for brake pads are summarised as follows [48]:
○ High friction coefficient ○ Low thermal conductivity, further minimum sensitivity of friction behaviour to temperature variation. ○ High resistance to wear under different environmental conditions ○ High mechanical strength ○ Resistance to water absorption ○ Low density ○ Resistance to heat fade ○ Recovery from fade ○ It should cause less damage to contact materials Brake pad and Disc assembly [59]. Elements of brake pad [60].
Although it is almost impossible to achieve no wear at a desired optimum friction, gradual efforts to minimise wear at optimum friction have been observed in recent years [49,50]. The asbestos and copper brake pads have covered the largest market. The main reason is being their suitability and the fact that they meet the desired characteristics to a great extent. However, concern about the health problems they pose has led to a decline in their applications over the past few years. The vapour of asbestos released due to decomposition of asbestos resulted from braking is carcinogenic. In addition, asbestos has been banned partially in the year 1989 and EPA proposed the ban on the current asbestos use including brake pads in April 2022 [51], because it can cause lung disorders. While copper is harmful to aquatic life on earth, hence its usage as a friction material is expected to be banned by 2025 [52]. Therefore in the last few years intensive research has been conducted to find alternative friction materials [53]. Composites reinforced with synthetic and metal fibre were successfully developed to replace asbestos and copper based brake pads [54,55]. Furthermore, natural fibre-reinforced composites have shown promising results as materials for brake pads [56–58]. They are categorised as non-asbestos organic (NAO) brake pads.
The common material ingredients or formulations of brake pads comprise binders, friction modifiers, base material, fibre reinforcements, abrasives, fillers, and lubricants. The reported literatures have employed different formulations to meet desired characteristics of brake pads. A summary of different types of tests employed for the property evaluation of brake pads is depicted in Figure 7. Seo et al. [85] have illustrated that contact plateaus play a vital role on the properties of brake pad surface. The wear resistance of brake pads strongly depends on the ingredients of friction materials and the braking environment. In addition, the volume fraction of reinforcement and filler materials as well as their types are of decisive importance and have a major influence on the number of contact plateaus. Thus, the composition of the brake pad affects its tribological properties.
Different types of tests are performed for brake pad characteristics evaluation [61].
Physical and mechanical properties of natural composite brake pad material.
The natural composites are fabricated by the hand lay-up method, vacuum bag moulding, hot compression, and resin transfer moulding process. Hot compression moulding process is widely adapted to produce brake pads. However, surface modification and treatment of natural fibres, especially organic-based fibres, is necessary before they can be reinforced. It is employed primarily to enhance their adhesion and mechanical properties. Chemical treatment results in the elimination of the cellulosic composition of the fibres. Surface treatment of the fibres reduces hydrophilic behaviour and improves the dimensional stability of the composite in which they are reinforced [90]. Once fibres are prepared through prior surface treatment, they are incorporated into the matrix to achieve desired properties according to application-specific requirements. The fibre-reinforced polymer (FRP) composites are initially fabricated by hand lay-up route. Fibres with bi-directional and unidirectional orientations are best suited for the hand lay-up process. Short fibres on the other hand require more attention and better workmanship to achieve proper blending and minimal voids during fabrication. The reported literatures have successfully prepared natural composites by hand lay-up technique [29,71,91]. The vacuum bag moulding process helps to overcome difficulties and limitations of hand lay-up process. Composites with minimum void volume fraction could be achieved through vacuum bag moulding process. Therefore, they are widely used in composite products for tribological applications such as brake pads [24,83].
Brake pads are made up of various ingredients whose reinforcement type varies from fibre to particles. Therefore, a proper blending of mixture plays a vital role in achieving the desired characteristics for brake pad application. The combination of hot press moulding and post-cure treatment process is suitable for the production brake pads, and results in reliable products. Therefore, the hot press moulding process is widely employed. Initially, ingredients such as friction modifiers, additives, fibres are properly blended using a mixer for 10–20 mins to obtain a homogenous mixture. Various literatures have employed mechanical mixture [92,93] and electric blender [73,94] to blend the ingredients. The resulting homogenous mixture is transferred into the die cavity. Compression of the mixture (or charge) is done through the application of gradual pressure. The applied pressure primarily aimed at eliminating gas gaps or air which may cause voids in composite material. Later, secondary pressure is applied to attain desired density of composite [92]. The pressed composite is cured at elevated temperatures in the range of 120°C–160°C with high pressure from 10 MPa to 80 MPa for a period of 7–30 min [95–98]. The curing time for hot press varies depending on the combination of temperature and pressure. Hot pressure specimen preparation is a widely used method for preparing brake pad specimens with a curing time of 7–10 min. Gas is released multiple times during the process, indicating that gas liberated by polymerisation is eliminated [98]. This step ensures the formation of solidified and hard composite sample with the desired shape according to the mould cavity. The cured samples were post cured in an oven in the temperature range of 100°C–200°C for 1–4 h. The post-curing process helps to improve the mechanical properties of the obtained composite samples [89]. The flow chart with steps involved in brake pad fabrication is illustrated in Figure 8. It was found in the literatures that, there is no standard temperature and pressure for fabrication. However, different temperature and pressure values were considered depending on various parameters such as matrix, filler, additives, friction modifiers, etc. The utmost care was also taken to improve mechanical properties and interfacial adhesion by eliminating voids, trapped gases, etc.
Brake pad fabrication flow chart (a) Plough shear mixture, (b) Brake die, (c) Compression moulding machine for performing and curing (d) Cured brake pad, (e) Post-curing oven, and (f) Prepared samples [3].
Physical characterisation
The physical characteristics such as density, flame resistance, water and oil absorption, ash content, acetone extraction, crystallinity index, and degradation analysis are vital for brake pad applications. The density and water absorption % of a few developed composites are presented in Table 2. The acetone extraction study of a composite is the measure of the uncured resin it contains. It is estimated as per ASTM D 494 standard or its equivalent [61]. The Soxhlet extraction apparatus is the usual equipment for the determination. The admissible percentage of acetone extraction is less than 1.5 to confirm better curing. However, a maximum value of 3% is considered for non-asbestos brake pads. Studies have shown that natural fibre-reinforced brake pads can be fabricated with permissible percentage of acetone extraction according to OEM standards [64,70,99]. The larger porosity forms when coarser particles are reinforced compared with fine particles. It facilitates proper curing of the matrix or binder due to better heat flow [100]. Crystallinity index (CI) is the parameter used to determine the amount of crystalline material in the given sample by Segal's empirical method. Similarly, resistance to water absorption is also significant for natural reinforcements considered for friction materials. Crystallinity index of a material has a direct effect on its resistance to water absorption and chemical attack [70]. Palai and Sarangi [76] demonstrated that the CI of natural fibres increases with chemical treatment, such as alkali and silane treatment. Besides, silane treated Eichhornia crassipes fibres exhibited the highest CI value indicating better fibre stiffness and contributing to the increase in mechanical strength. Kumaran et al. [101] showed that alkali treatment of Sansevieria ehrenbergii fibres drastically increased their CI value. Similarly, CI values of Prosopis juliflora [27] and Cyperus pangorei fibres [79] also improved after chemical treatment. Thus, it helps to remove impurities and amorphous content of the fibres and enhances the mechanical properties when fibres are used as reinforcement in composite applications.
Density of a composite material is highly significant and depends on the density of its constituents, the dimensions of the powder, moulding techniques, and heat treatment process employed. Typically, Archimedes’ principle is utilised in accordance with ASTM B962 for the determination of composite materials. The density of brake pads is highly significant and affects the weight of an automobile. Moreover, increasing the density of brake pad materials is not conducive to commercial applications [102]. In recent years since 2012, research interest has strongly shifted to natural fibres as an alternative to synthetic fibres [103]. The reason for their selection is their low density, low cost, and biodegradability [104]. Increasing the amount of walnut shell dust has increased the density of the composite [34]. Ali et al. [105] depicted that coconut fibres as reinforcement in organic pads increased the density. In addition, chemical treatment of the fibres increases the density of fibres and thus the reinforced brake pads. It owes to the fact that, chemical treatment fills the surface irregularities such as voids and pores with their graft molecules [76].
Inorganic reinforcements are incorporated into brake pads to increase hardness or act as solid lubricants, depending on their nature. They also affect the density of the product. The addition of graphite marginally increased the density of phenolic brake pads based on lapinus fibre and aramid fibres [106]. Brake pads reinforced with banana fibre also exhibited similar results [94]. The addition of wallostonite in the form of fibres and particles resulted in a decrease in density as it replaced barium sulphate due to the difference in density. In addition, the study revealed that there was a difference in density when wallostonite was used in different forms. It is due to the presence of voids in fibre reinforced composites [80]. Jeganmohan et al. [57] studied the density variations at varied volume fractions of calcium sulphate (0–15%) and barite (20–5%). It was found that the studied sample with 15% barite and 5% calcium sulphate had higher density and better tribological characteristics, since the barite density was higher than that of calcium sulphate. The density of the sample increases with the increase of the high-density ingredient of the brake pad. Further, the manufacturing parameters also play a vital role in the resulting density. Yusubov [107] attempted to determine the optimum parameters of the hot compression process for better density of polymer composite brake pads. The study shown that pressure is the significant influencing parameter. The combination of high pressure (25.5 MPa), level 2 heating time (35 min) and plasticiser content of 10% yields better value for density. Thus, the selection of optimum parameter combination and the percentage of fibre reinforcement yields desired density of the friction material.
The characterisation of water and oil absorption capacity of brake friction material is significant. A higher absorption capacity suggests that given material possesses higher porosity, thereby resulting in poor tribological behaviour [108]. Akincioglu et al. [61] developed eco-friendly brake pads with hazelnut powder as reinforcement, whose water and oil absorption capacity is comparable to commercial brake pads. Polymer composite brake pads reinforced with palm kernel fibres exhibited higher water and oil absorption capacity compared to asbestos based brake pads [109]. In addition, increasing the percentage of fibre increases the water and oil absorption capacity [66,67]. It owes to the fact that reinforcement of natural fibres increases the porosity, thereby increases the tendency to absorb more water and oil. However, this depends on several other parameters such as the temperature of the immersion medium, type and volume fraction of reinforcement, permeability of the sample, voids, etc. The chemical treatment of organic reinforcement increases the water and oil absorption resistance of organic composite brake pads [27]. Hence, alkali treatment and thermal treatment of fibres before the production of composite materials is employed extensively by various researchers to reduce the water and oil absorption characteristics.
Thermal characterisation
The brake of automotives generates an elevated temperature due to friction and dissipation of kinetic energy. The friction material contained in it should possess better thermal conductivity to absorb thermal energy. It should also be stable at elevated temperatures since it influences significantly the structural integrity of the brake pads. Thermogravimetric analysis (TGA) is used to characterise the thermal stability of friction composite materials. Ash content retention assessment is also used to investigate the thermal conductivity of brake pads. The commonly used phenolic resins decomposes at temperature greater than 350°C. The excellent chemical bonding between phenolic resins and natural fibres has improved the thermal stability of the material and thus its decomposition temperature. The lignin content present in natural fibres increases the decomposition temperature of friction materials up to 760°C [94,110]. Ali et al. [105] reported that natural composites with coconut fibres have better thermal stability at elevated temperature about 300°C. Recently, basalt fibre reinforced composites have been explored as high temperature stable reinforcements [111]. Thus, natural fibres have demonstrated that they can be a promising sustainable friction material for brake pads in the future.
Mechanical characterisation
A better understanding of the mechanical properties of natural composites is very vital for brake pad applications. When the brake is applied, two pads are pressed together against the disc to slow and stop the rotor. The materials used as friction materials are subjected to compressive stress. Therefore, they should have high compressibility, high modulus, and high hardness to resist wear. Hence, a suitable reinforcement type and composition impart the desired compressive strength. The presence of hydrophilic material in natural fibres limits their potential applications. The mechanical properties of composite brake pad materials in the available literature published in past 5 years are summarised in Table 2. However, the development of surface treatment techniques, improvement of interfacial bonding and advanced composite manufacturing techniques in conjunction with hydrophilic eliminated fibres have resulted in improved properties. In addition, natural reinforcements have shown promising results. Singh et al. [64] showed that increasing ramie fibre content in the natural composite brake pads has enhanced its compressive strength. The greater compressibility about 36% achieved due to higher content of ramie fibre (20%). Madnasri et al. [112] compared the mechanical properties of brake pads reinforced with pineapple leaf, coconut, and areca fibres. The study illustrated that hardness decreased with increasing fibre content. Further, composite brake pads reinforced with coconut fibres of 45° orientation exhibited better hardness, tensile strength and modulus characteristics than other fibres with random and perpendicular orientation of reinforcement. Thus, fibre content and fibre orientation are the key parameters to achieve the desired mechanical properties of natural fibre composite brake pads.
The addition of secondary fillers or additives to the natural fibre-reinforced composites further improves their mechanical properties. Flyash addition to banana fibre up to 3% resulted in better mechanical properties such as tensile, flexural and impact strength, but further increase to 5% deteriorated the properties [35]. The improved tensile strength is about 45-50% compared to neat polyester and 25% greater than banana fibre/polyester composite. The greater rigidity of fillers relative to fibre or matrix constituents of composite chiefly contributes to enhance the mechanical properties of composite materials. The addition of graphite particles has increased hardness at the cost of flexural and compressive strength [113]. Baryte based composite brake pads exhibited low density, good hardness and shear strength [114]. The smaller particle size of additives tends to agglomerate and weaken the adhesive bond at fibre matrix interface. Similar observations were reported by various researchers for higher percentage of particulate reinforcement. It causes debonding of fibre and matrix, thereby results in the reduction of desired properties.
Mineral fibres and particles aid to manipulate and achieve the desired properties of brake pads. Basalt fibres have demonstrated the desired better brake pad properties. Compared to ceramic brake pads, brake pads loaded with basalt fibres exhibited higher shear strength at the cost of compressive strength [111]. Further, Zhao et al. [102] depicted that optimal content of basalt fibres as reinforcement is necessary to obtain desired mechanical properties of composite materials. Basalt fibres exhibited excellent properties at elevated temperature about 400°C. It is one of the basic requirements for brake pad materials. Because the higher temperature is attained due to friction and energy transfer phenomena during braking conditions. Thus, it is necessary for the candidate material to retain its mechanical and exhibit tribological characteristics at temperature greater than 300–350°C.
Tribological characterisation
Evaluating the tribological properties of the desired material is vital to conforming its suitability as friction material. The tribological characterisation of brake pad material encompasses evaluation of friction and wear behaviour. The coefficient of friction is evaluated to understand frictional behaviour of materials. Whereas, wear behaviour is studied using various response parameters such as material loss, specific wear rate, fade percentage, and recovery percentage. Minimum material loss, specific wear rate and higher recovery rate are significant and vital factors for an ideal brake material. In addition, worn surface morphology was studied to understand the wear mechanisms and to develop a material that has better wear resistance combined with optimal COF. Material parameters such as reinforcement type and percentage, its orientation and so on were reported in detail. However, the process parameters considered in the literature depend on the type of brake pad testing technique employed. The friction and wear testing facilities utilised to characterise brake pads are pin-on-disc as per ASTM G99 [115], chase brake dynamometer SAE J-991 [116], Krauss test machine according to ECE R-90 [117], FAST (friction assessment and screening test) device [70], and full-scale inertial dynamometer as per JASO C 406 standards [118]. Kchaou et al. [119] presented detailed report on the various the test procedures employed for brake lining material assessment. The effect of parameters such as sliding distance, normal load, sliding speed, temperature at the interface, duration of sliding, braking time, braking cycle, etc on tribological characteristics of brake lining is discussed.
The natural composite brake pads are developed from distinct types of natural ingredients evaluated at different testing facilities to ensure that they are comparable to commercial brake pads. The frictional behaviour of composite brake pads with 3% palm seed powder was superior compared to unfilled, 6% and 9% palm seed powder added composites [65]. Singaravelu et al. [40] studied the tribological behaviour of chemically treated, thermally treated, and 0% crab shell powder-filled brake pads. Kumar et al. [120] investigated the tribological performance of brake pads incorporated with bamboo fibres and synthetic polyacrylonitrile (PAN) based carbon fibres. It was noticed in the reported literatures that surface-treated natural fibres reinforced phenolic resin brake pads shown promising results with optimum COF, quicker recovery rate coupled and minimum wear loss (Figure 9) [76]. Secondary fillers such as bagasse ash and flyash in phenolic based eco-friendly composite brake pads enhances the tribological performance at elevated temperature [83]. Nogueira et al. [115] illustrated the untreated and heat treated rice husk incorporated phenolic brake pads exhibits comparable friction coefficient and low wear rate with reduced airborne particulate matter emissions. Similar observation was reported for rick husk based brake pads by Gehlen et al. [121]. The study also showed that 6% rice husk added composite exhibited better and superior properties over unfilled, 3% and 12% rice husk reinforcement. The heat treatment of rice husk has shown better frictional property coupled with optimum wear rate [115]. The larger plateaus formed on the surface of heat-treated rice husk reinforced composites are responsible for the resultant frictional coefficient.
COF values, fade rate and recovery rate, and weight loss and thickness loss of untreated, alkali-treated and silane-treated Eichhornia crassipes fibre-based composite brake pads [76].
The performance of brake friction materials is categorised as its performance at room temperature before braking called as cold performance, at higher temperature during braking called as fade performance and at reduced temperature after braking due to cooling effect called recovery performance [122]. Manoharan et al. [100] attempted to characterise the various graphite forms based on friction materials using fade and recovery performance studies. The characteristic curves were plotted against the number of braking events and temperature variations to determine the frictional stability and its value of the studied samples (Figure 10). Identical plots were widely used in several literatures for similar investigations of different materials [49,52,94,123]. Rajan et al. [124] depicted that low fade rate, better frictional stability, and highest friction occurred with slag waste content of 60% and coir fibre content of 5%, while brake pads with slag content of 45% and coconut fibre content of 20% had quick recovery performance of brake pads. Naidu et al. [62] compared the tribological performance of phenolic composite brake pads reinforced with hemp fibre, hemp with basalt fibres, and hemp with aluminium strands. The hemp-phenolic composite brake pads showed superior friction and wear performance than other composites.
Fade and recovery frictional performance against braking application number and temperature [100].
Different types of brake pads tribological experiments with parameters and responses studied.
The morphology of worn surface studied with the help of scanning electron microscopy (SEM) image provides information about the mechanism and causes of wear. The tribological testing of brake pads with countersurface reveals the irregular surface characteristics of the sample. Brake pad surface wear is characterised by the formation of primary and secondary plateaus on the contact surface. Further, the formation of plateaus depends on the operating conditions of the brake. The characteristics of the formed plateaus are closely related to the surface integrity of the contact surfaces and the performance parameters during braking [132]. It is also reported that one large plateau is more likely to result in stick slip amplitudes than multiple small plateaus. It is always better to produce multiple small plateau with surface to achieve better friction characteristics [133]. Therefore, great efforts have been made in previous studies to understand the formation of contact plateaus on the worn surface. Figure 11 illustrates the worn surface morphology of banana peel based phenolic brake pads. It demonstrates various details on worn surface. The distinction between secondary and primary plateau is shown in Figure 12. The primary plateaus also known as contact plateaus, are formed by the interaction of contacting surfaces i.e. drum or counterpart and significantly thermally stable ingredients of friction material such as fibres, secondary fillers, and friction modifiers. While the secondary plateaus are formed on the surface of thermally less stable substances such as loose wear debris particles, polymeric compositions, etc., which lead to wear phenomenon [95]. The nucleation of plateaus primarily depends on the probability of the occurrence of a lubricating film at the contact zone. The higher the possibility of lubrication, the lower the formation of contact plateaus, which improves the wear resistance. On contrary, friction coefficient also gets affected, hence selection of optimum braking conditions and material composition are necessary criterion while choosing friction modifiers. However, during fade and recovery conditions, elevated temperature causes the agglomeration of loose debris around the primary plateau resulting in hard surface. The generated tribofilm becomes unstable at this temperature. Thus, it results in higher friction coefficient [41]. In the natural composites, these plateaus are observed more on the composite surface with high fibre content. It owes to the greater irregular surface resulted from poor bonding between fibres and binders or matrix and improper mixing. In addition, higher fibre content results in a heterogenous structural composite product, which causes higher wear loss [64]. The possible governing wear mechanisms of natural composite brake pads can be summarised as follows: fibre pull out, wear debris formation, spalling, pit formation, crater formed due to displacement of materials and cracks due to plastic deformation.
Illustrating various worn surface morphology characteristics [128]. Table depicting primary plateau and secondary plateau [54].
The composite brake pad is an important part of automotive applications. Asbestos and copper-based brake pads are the most commonly used types. However, due to rising health concerns about the carcinogenic effects of asbestos and the harmful effects of copper on aquatic ecosystems, their applications are limited. In addition, sustainable friction materials in engineering applications are increasing rapidly. Therefore, the interest among the research community has increased to employ natural composites as friction materials. Many efforts have been made in the published literatures to examine the different types of formulated composites to meet the desired properties of brake pads.
The biomass materials such as fibres and particles extracted from different parts of plant ranging from stem to fruits, animal wool, etc., are reinforced in the natural composite brake pads. The examined studies have also added industrial wastes such as flyash, cement dust, etc. and natural mineral reinforcements such as graphite, calcium carbonate, barytes, basalt fibres, and wollastonite powder as secondary fillers for the further improvement in desired brake features. Besides, the chemical and thermal treatments of plant-based fibres are necessary to impart better tribological properties of the composites through elimination of cellulosic content present in them. The alkali treatment and silane treatment have shown promising results to meet desired properties. Composite brake pads were manufactured through hand lay-up technique, compression moulding process, and vacuum bag moulding process. Compression moulding process, followed by heating in oven upto 120°C for a specific curing period of about 10 min to 1 h was employed extensively. Hot air oven curing is mainly carried out to achieve minimum porosity and high resistance to water absorption and oil absorption, which is a significant limitation of natural composite materials.
The reinforcement of natural reinforcements primarily reduces the density of resultant composite material owing to their low density. Silane and alkali-treated biomass reinforcements increased marginally the density of composites compared to untreated reinforcements. Since their lighter content cellulose is removed during chemical treatment. However, the presence of only lignin content significantly improves the physical characteristics such as acetone extraction, low water absorption %, low oil absorption %, ash content %, etc. In addition, compressibility %, compressive strength, shear strength, tensile strength, hardness, and impact strength were also improved. The obtained mechanical properties are close to the desired standards of brake pad materials. The desired hardness of OEM standard brake pads is in the range of a minimum 80–90 HRR, while the natural composite brake pad were reported to possess >85 HRR. Similarly, the available literatures reported the compressibility % as less than 2.5% for natural fibre and particulate-reinforced composite brake pads. Besides, the thermal stability of plant-based fibres is satisfactory at temperature ≤300°C. Above 300°C, their thermal degradation occurs, hence they are less suitable for very high friction conditions which generates greater heat energy. The mineral reinforcements such as basalt fibres, basalt particles, wollastonite particles, etc reinforced composites have exhibited satisfactory stable mechanical properties at higher temperature about 760°C. However, they possess relatively higher density compared to biomass-based reinforcements and also economically costlier. Hence, appropriate evaluation and analysis based on the reinforcement type and their volume fraction to achieve optimum physio-mechanical properties of composite brake pad materials.
Tribological behaviour of brake pads is studied as a function of wear loss, wear rate, coefficient of friction, fade, and recovery characteristics. The optimum coefficient of friction and high resistance to wear are the desired characteristics of material for brake pad applications. The natural fibre-reinforced composites exhibited optimum friction coefficient and high wear resistance with higher fibre volume fraction. Whereas fade and recovery performance of composites with low fibre content was better, which is a characteristic feature of brake pads. Similar results were observed for natural particulate reinforced composites. It was reported in the available literatures that higher particle content % tends to agglomerate leading to decline in the properties. The low fibre content composite revealed the formation of primary plateaus consisting of fibres and load bearing elements, which sustains during friction fluctuations. Further, greater friction fluctuations are reported for higher reinforcement percentage compared to optimum % of reinforcement in composites owing to formed secondary plateaus due to fibre debonding. Therefore, optimum formulation of brake pad ingredients is necessary to obtain desired properties.
The presented literature survey reveals that several types of natural fibres were examined for their suitability as reinforcement material in composite brake pads. The biomass fibres and particulates are investigated by good number of researchers. However, limited studies are available on mineral fibres and particulate reinforcements. Hence, there is a large scope to examine the suitability and characteristics of mineral based reinforcements for brake pad composites. Limited efforts have been made to develop prediction models and optimise the formulation of composite ingredients and tribological parameters. The optimum range of parameters and composite formulations can be suggested for the longer service life of friction materials. Statistical techniques and evolutionary algorithms can be effectively utilised to build the data sheet encompassing the information on optimum value of the process parameters and materials properties. It helps the practising material scientists and design engineers of automotive brake pad in industries. The optimum formulation also helps the automotive industries to provide low cost, lighter material brake pads for their consumers. The advantage of additive manufacturing can be used in the brake pad manufacturing process. Also, the challenges exist in the additive manufacturing process of brake pads needs great attention [134]. The advancements in the electric vehicles production are demanding a novel friction material for brakes [135]. The sustainable solutions to meet the advancements in friction materials of automotives is anticipated to take paradigm shift in the near future.
Conclusion
The current review paper provides an overview of significant contributions on natural composite brake pads reported in the past 5 years i.e. accounted majority from 2018. It was attempted to examine the research works on the materials being employed and characteristics studied. The governing wear mechanism has been discussed with the help of worn surface morphologies using SEM. The properties of materials are substantiated to validate their ability to meet desired properties of friction materials and specifically for automotive brake pads.
The broad literature survey demonstrates that natural composites are undoubtedly candidates for friction materials. They have shown promising tribological, mechanical, physical, and thermal properties. Several types of natural fibres, non-organic natural fillers, bio-fillers are extensively used as ingredients of natural composites for friction applications. However, mineral fibres such as wollastonite, basalt fibres, etc., are utilised as reinforcement materials to a limited extent. Hence there is a wide scope for mineral fibres, since they possess superior thermal stability and better tribological characteristics.
The review on the effect of natural reinforcements on the physio-mechanical and tribological properties is presented. The optimum content of natural reinforcements in composite results in better friction and wear behaviour with superior fade and recovery characteristics during braking. Therefore, optimisation of ingredients is vital to attain better features of brake pad material. However, investigations on the implementation of statistical techniques such as response surface methodology, Taguchi technique, etc and evolutionary algorithms such as fuzzy logic, genetic algorithms for optimisation of material composition and process parameters is limited available. Overall, substantiate material information is required which can help engineers and material scientists in the real-time application of natural composites as brake pads. Therefore, lot of research results are needed to obtain such significant data for design and development of friction materials. It also helps to understand the wear mechanism of composite materials. Furthermore, there is lot of scope to adopt advanced manufacturing techniques such as additive manufacturing techniques for the brake production.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the authors.