Brake pads are steel-backed plates with friction material bonded to the surface that faces the disk brake rotor. Brake pads convert the kinetic energy of the vehicle to thermal energy through friction. The major component in the brake pads is the lining materials, which are made up of structural materials, binder, filler and frictional additives and modifiers based on function they perform apart from controlling friction and wear performance. The structural materials provide the structural reinforcement to the composite matrix, while binder holds the ingredients together, to maintain structural integrity of the brake lining, and filler make up the free volume of the brake lining while friction modifiers stabilize the coefficient of friction and wear. These components perform synergistically in controlling friction and wear performances of the brake pad [1]. Asbestos in fibrous form has been used as reinforcing material in brake pads since 1908, when English inventor Herbert Frodo came up with a combination of asbestos, brass wire, and resins for use as a friction lining. It was most preferred filler materials in friction liners up to 1989 due to its desirable mechanical and tribological properties, but its use has been averted due to its carcinogenic nature which is hazardous to health [1]. Consequently, researchers have struggled to come up with equally efficient alternative friction materials to develop asbestos-free brake pads. pads such as rubber and cashew dust are usually incorporated into brake pads for the purpose of reducing brake noises due to their superior viscoelastic characteristics [2], and also as under- layer material because their low thermal conductivity prevents heat from transmitting to the backing plate of the brake friction material [3]. However, these particles, especially cashew; fall off the friction surface easily, leaving behind large pores that eventually crack [4].Inorganic particles are always associated with the enhancement of mechanical properties of polymer composites, which have been widely investigated in the past decades. Typical inorganic fillers such as barium sulphate, mica, vermiculite, and calcium carbonate possess a relatively high melting point [5, 6]. Some are able to suppress low-frequency brake noise due to its plane netlike structure [7, 8]. However, their stratified structure results in a low interlayer strength which causes interlayer splitting of the friction lining, especially at high braking loads [8]. Other researchers, such as Kesavan and Burmester [9], have also proposed the use of alkali metal titanates (such as sodium titanate) for use as fillers, claiming that they promote the stability of the friction coefficient.

Several researches are now focusing on ways of utilizing industrial and agricultural wastes as a source of raw materials in the area of development of asbestos free brake pads. The utilization of these wastes would help control health hazards associated with them and also help in reducing rate of depletion of natural resources. Abundance of these wastes at very low or zero cost is also an attraction that motivated awareness of exploring their possible incorporation into friction composite of brake pad. Some researchers like Mohanty and Chugh [10] and Malhotra et al. [11] have studied the use of combustion waste particle as fillers. These particles, such as fly ash, when used in friction braking application exhibit high temperature resistance and provide good integrity or compatibility with the resin, thereby enhancing the friction and wear performance of the composite materials.

The possibility of utilizing industrial and agricultural wastes has been studied by some researchers. Ikpambese et al. [12] and Ibhadode et al. [13] used palm kernel shells (PKS), an agro waste material, along with other ingredients to developed friction lining material for brake pads. Results showed that PKS can be suitable for replacement of asbestos brake pads with epoxy resin as a binder. IDris et al. [14] studied the morphology, physical, mechanical and wear properties of the brake pad produced using banana peels waste to replaced asbestos and phenolic resin as a binder. The samples containing 25wt% in un-carbonized banana peels (UNCBp) and 30wt% carbonized (CBp) gave superior properties. The use of periwinkle shells as filler in the production brake pads lining was investigated by Yawas, et al. [15]. In the work, periwinkle shell brake pad material was characterized and its morphology and properties were determined. The sun dried periwinkle shell was milled and sieved using a set of +710, +500,+355, +250, and +125μm sieve aperture. The formulation included periwinkle shell powder, phenolic resin (phenol formaldehyde), engine oil (SEA 20/50), and water. They concluded that periwinkle shell can be effectively used as replacement for asbestos in brake pad friction lining. Ruzaidi et al. [16], Ghazali et al. [17] and Ghazali et al. [18] utilised palm slag, agricultural waste material, as filler material along with calcium carbonate and dolomite in the production of brake pad lining material. The mechanical properties and wear behaviour were studied under varying processing pressures. Results showed that the compactness of the palm slag brake pad composite plays significant role in enhancing the mechanical and wear properties of the product. Palm slag and calcium carbonate brake pad composite shown better wear properties than dolomite and comparable with the conventional asbestos based brake pads.

Coconut fibers were used as filler along with aluminum composite with phenolic resin as binder to developed brake friction lining via powder metallurgy technique by Maleque et al. [19]. The superior properties in terms of higher density, lower porosity and higher compressive strength were obtained from 5 and 10% coconut fiber composites. It was equally found that the coconut fibre well distributed to the matrix and acts as filler in the friction materials. Also, coconut shells based brake pad was produced by Bashar et al. [20]. The formulation included ground coconut shells (filler), epoxy resin (binder), iron chips (reinforcement), iron and silica (abrasives), and brass (friction modifier) among other materials. The pulverized filler (coconut shells) was sieved to 710μm size. From results, it was concluded that the higher percentage of ground coconut filler the lower the breaking strength (brittleness), hardness, compressive strength, and impact. Ademoh and Adeyemi [21] developed composite brake pad friction liner using maize husks filler particulate size of 300μm along with epoxy resin as the binder. Based on the analytical tests conducted, it was shown that reducing the filler content in the formulation increased the tensile strength, compressive strength, and hardness, wear rate and thermal conductivity of the developed brake pad, whereas water and oil absorption, density and coefficient of friction increased with increase in the filler content.

Fillers are used to maintain the overall composition of the friction material, as well as to improve physical, mechanical and tribological properties of brake pad [22]. Fillers are also included in a brake pad lining in order to improve its manufacturability and also to reduce the overall cost of the brake pad [23]. The amount of filler is one of the highest constituents in a brake pad composition. Thus, to some extent, fillers affect the final properties of brake performance, especially in terms of resistance to heat, abrasion, and strength. Therefore, the essence of present study is to employed sawdust as fillers in a typical brake pad formulation together with phenolic resin as binder, silicon carbide as abrasive component, steel dust as lubricant and carbon black as reinforcing material. Sawdust used in this study was found as waste from the processing of hardwood: mahogany (swietenia macrophylla) and Iroko (milicia excels) trees in sawmill. Sawdust is by-product from milling of wood, and it is usually spread on the ground and disposed indiscriminately. Therefore, utilization of sawdust as alternative filler to asbestos in brake pad formulation will help to prevent environmental degradation and pollution.

Materials

The materials needed for the production of the brake pad lining include binder, filler, reinforce, abrasive and lubricant. Phenolic resin was selected as the binder and this was obtained from a of retail shops in Ojota, Lagos, Nigeria. Hardwood (from mahogany and Iroko trees, and botanically refer to as swietenia macrophylla and milicia excels, respectively) sawdust used in the research was industrial waste material collected from the saw mill industry in Ikorodu, Lagos. Carbon black was used as reinforcer, it also helps to conduct heat and reduce thermal damage [24], and it was gotten from retail shop in Ojota, Lagos, Nigeria. Silicon carbide was used as the abrasive because of its very good chemical, thermal and mechanical properties, and this was bought from retail shop at Mushin, Lagos, Nigeria. Steel dust from steel slag was selected as lubricant for the production of the braking pad lining because of its moderate water absorption (less than 3%), high bulk specific gravity, high bearing strength, good abrasion resistance and good heat capacity. The steel dust was bought from steel mill in Ikorodu, Lagos, Nigeria.

Formulation of the Friction Composite

The formulation used in the brake pad friction lining in this study was chosen based on the typical formulation suggested by Blau [1], and as also typically used in most of the sample formulation of commercial brake pad. This combination of the composite materials is shown in Table 1.

Table 1: Weight percentage of materials of the samples.

Preparation of materials

Sawdust, an industrial waste, was collected from sawmill and delivered in different solid sizes, and similarly, steel slag, silicon carbide and carbon black were purchased in pellet form. The materials need were crushed and sieved into smaller sizes before used to serve the intended purpose. For each of the materials, crushing was done using a jaw crusher (Model PE 250 X 400) and thereafter, high speed grinding was used to obtain the smaller sizes material. The different sizes of the ground material were then segregated into 100μm and 250μm sizes materials using sieve shaker to sieve the crushed and ground materials.

Characterization of materials

Characterization of the raw materials was very important in the development of multi-component composites like brake pad composite materials. This helps to develop a comprehensive base of knowledge concerning the raw materials used in the formulations and their impacts on the performance of the products. The five raw materials used in this study were tested in order to study and understand their individual properties. The chemical composition of the sawdust was determined by using a MiniPAL 4 standardless mode ED-XRF spectrometer. The morphologies of the raw materials were determined using a JEOL JSM-6460LA scanning electron microscope (SEM). Test specimens were handled carefully to prevent any contamination or damage that would affect the results. Furthermore, samples of sawdust, silicon carbide, carbon black and steel dust particles were subjected to thermogravimetric analyses (TGA) to determine changes in the weight of the respective samples as a function of temperature. The TGA test procedures were performed on each sample during the course of a single experiment, and separate on combinations of sawdust with the phenolic resin (binder). Nitrogen was used as the inert gas during the experiments, and the samples were monitored over the temperature range of 30 - 1000°C.

Mixing

The raw materials were mixed under dry conditions by using a two-roll ball mill machine having a pair of rollers with vertical ‘nips’ between them. Sawdust was mixed with the other filler ingredient. i.e., phenolic resin, steel dust, silicon carbide, carbon black and the mixtures were milled using a two-roll ball mill for 3 hours at 273 rpm with 50 balls in the mill jar to ensure that the final mixtures were homogeneous.

Compaction

After the milling process, the loose powder was compressed at room temperature, and it was molded into a shape through cold press technique (powder metallurgy method). The die cavity that is closed on one end (vertical die, bottom end closed by a punch tool) is filled with the milled homogeneous mixture ingredients of the brake pad. Prior to the filling, a lubricant (WD-40) was sprayed on the mold to lubricate it and prevent the powder from sticking to the surface of the mold. The homogeneous mixture placed in the mold was then compacted at room temperature. The compacted brake pad lining sample was ejected from the die cavity (mold). Sample produced was of cylindrical shape with diameters of 10 mm, and this was compacted further and cured to produce brake pad lining samples.

Hot Press Curing

The produced samples were cured in a hot press at five different melding pressure loads of 10 Mg, 20 Mg, 40 Mg, 60 Mg and 80 Mg, and at temperature of was 160 °C. The curing process took 5 minutes, after which the samples were allowed to cool at room temperature. Then the samples were post cured in an oven with air circulating at temperature of 160 °C for 4 hours to allow for full cure. The weights and diameters of the samples were measured before and after the curing process. The measurements were used to calculate the percentage of change in weights dimensions of the samples. Samples of the produced brake pad lining are shown in Figure 1.

Characterization of the Brake Lining Samples

Hardness Test

Hardness value describes the durability of the friction material. In this research the Rockwell type E hardness values of the samples were obtained using a digital Rockwell hardness tester. The tests were conducted using a 12 mm diameter steel ball indenter with a load of 100 kg on the samples.

Figure 1: Laboratory samples of the produced brake lining.

Compressive Strength Test

The compressive strengths of the brake pad samples were measured based on ASTM D695 standard using an Instron Universal Testing Machine (Figure 2). The samples were placed in such a manner that compressive load was applied at a crosshead speed of 5mm/minute. The machine used the load at which failure occurred to calculate compressive strength. For each sample, the test was repeated five times.

Density

Density measures the relative “heaviness” of objects with a constant volume. The density of composite samples was obtained using Archimedes principle in accordance with Malaysian Standard MS 474: 2003 test procedures.

Morphology

The morphology of the surfaces of the brake pads also was observed by using a scanning electron microscope (SEM) JEOL JSM-6460LA. The samples were scanned with a high-energy beam of electrons in a raster scan pattern and the images of the surfaces of the sample of brake pads with different percentage of filler were obtained before and after the wear test.

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