A friction brake is employed to stop a vehicle by converting the kinetic energy of the vehicle to heat energy, via friction, and to dissipate heat by conduction and convection. Some amount of heat is absorbed by physical and chemical reactions on friction interface. Additionally, friction brake materials should have resistance to corrosion, low wear rate, low noise, and stable friction. Friction brakes are classified as Metallic, Carbon, and Non-asbestos Organic (NAO) pads (Figure 1). Metallic based pads are of steel and copper, carbon based pads are of graphite, and non-asbestos organic pads are of non-ferrous metals like Kevlar and fiberglass. In olden days, asbestos was used as a reinforced fiber in friction brake as it has high melting point, mechanical strength, and high coefficient of friction. As it has low thermal conductivity, it tends to give less friction performance and increases wear. It is carcinogenic to human respiratory organs. As hardness of brake disk is more than plate, peaks on the surface of brake disk are pressed into the friction plate. Due to high speed of brake disk, some friction materials are removed due to shearing, which forms debris. This is adhered on the disc to form a friction film which has influences on the friction and wear of brake pair (Xiao et al., 2016). In this paper, the research progress on this field is studied and summarized.

Figure 1. Classification and Properties of Friction Brake Material

Algan and Kurt (2017) have analyzed the effect of metal fibers and borax powders to wear and friction performance on the organic based brake pads. They tested the material combination with 1%, 3%, and 5% borax powder, 1.5%, 3%, 4.5%, and 6% copper fibers, and 1.5%, 3%, 4.5%, and 6% bronze fibers by weight. Copper and bronze fibers were 1 mm in size. The addition of borax powder enhanced the wear resistance, but friction coefficient values are low. Friction test concludes low fade and normal friction coefficient values. Due to the addition of bronze and copper, fiber recovery and friction coefficient are increased while increasing the wear rate (Algan and Kurt, 2017).

Xiao et al. (2016) have focused on micro-contact on brake's friction interface, where loose granular films and dense sheet films are created during braking. Due to sliding, micro-asperities between friction pair are deformed and dropped to form some loose granular films. After Algan and Kurt (2017) have analyzed the effect of metal fibers and borax powders to wear and friction performance on the organic based brake pads. They tested the material combination with 1%, 3%, and 5% borax powder, 1.5%, 3%, 4.5%, and 6% copper fibers, and 1.5%, 3%, 4.5%, and 6% bronze fibers by weight. Copper and bronze fibers were 1 mm in size. The addition of borax powder enhanced the wear resistance, but friction coefficient values are low. Friction test concludes low fade and normal friction coefficient values. Due to the addition of bronze and copper, fiber recovery and friction coefficient are increased while increasing the wear rate (Algan and Kurt, 2017). Xiao et al. (2016) have focused on micro-contact on brake's friction interface, where loose granular films and dense sheet films are created during braking. Due to sliding, micro-asperities between friction pair are deformed and dropped to form some loose granular films. After increasing braking pressure and surface temperature, loose granular films are cut and get welded to form dense sheet film. Friction surface materials react with oxygen in air and forms oxidation film, which has low intensity, and is hard and brittle. Due to brittleness, it easily cracks and forms into debris. Due to oxidation, film friction coefficient decreases and the wear resistance increases. Due to thermal degradation, some gases like CO, CO₂ , CH₄ , and H₂ comes out and creates gas cushion film on the interface, which reduces the friction coefficient. Due to thermal decomposition of organic components which are present in friction materials, at high temperature liquid lubrication films generate that results in liquid lubrication between interfaces. Due to this, friction coefficient falls which is helpful to reduce wear. With the increasing braking pressure, initially friction coefficient increases then decreases, but wear rate keeps on increasing (Xiao et al., 2016).

Sharma et al. (2013) have worked on non-asbestos organic material with Nano- and Micro-Sized Copper Particles as Fillers. They developed microcomposite with 10% (wt) Cu with size of particles 400-600 μm and in the nano composite, only a part (2%) of the micro powder was replaced by nanopowder with size of nanoparticles 50 - 200 mm. They concluded that due to nano Cu powder, wear resistance, hardness, thermal conductivity, thermal diffusivity, and density were increased. The thermal conductivity of the composites plays an important role in the enhancement of the friction performance (Sharma et al., 2013).

Szlichting et al. (2016) have focused on the influence on a kind of composite material on morphology and composition of its surface layer and tribological properties. They have suggested that due to tribochemical phenomena, there is possibility of reduction of the wear of brake pads. The tribochemical phenomena should be considered for both elements of the friction pair (Szlichting et al., 2016).

Craciun et al. (2017) have worked on composite material for friction and braking applications. In this paper, they have developed the composite material by adding coconut fiber reinforcement in aluminum matrix with different percentages (0%, 5%, 10%, and 15%). Friction performance is good in case of higher percentage of coconut fiber (Craciun et al., 2017).

Alemani et al. (2017) have investigated the sliding behavior of a low-steel friction material against a cast iron disc at different applied loads, to investigate the effect of the temperature rise induced by frictional heating. Oxidation of kinetics of the disc increases with temperature. Iron oxides become important components of the friction layer, which reduces the abrasive component of wear although wear rate is enhanced by tribo-oxidation of the disc due to decomposition of the phenolic binder (Alemani et al., 2017).

Fan et al. (2011) have studied the wear mechanisms of C/SiC brake materials. They fabricated C/SiC brake materials’ chemical vapor infiltration with composition as 65 wt % C, 27 wt% SiC, and 8 wt % Si. Due to local high temperature, oxidation-abrasion happened, which causes adhesive wear and weakens the carbon fiber strengthening. Silicon is the significant factor in adhesive wear. High wear rates observed due to adhesive wear can cause high wear rates and a large unstable friction coefficient. So, Si available in the brake materials should be removed (Fan et al., 2011).

Singh et al. (2015) have studied the effect of nano-filler in brake materials. The brake friction materials were containing nanoclay and multi-wall carbon nanotube. The result shows that multi-wall carbon nanotube enhances the friction and fade performance, but decreases the wear performance, whereas nanoclay affects the reverse as that of multi-wall carbon nanotube. In nanoclay filled friction composites, recovery response is more (Singh et al., 2015).

Wang et al. (2015) investigated the tribological properties of metro brake shoe materials. Velocity and pressure of braking have major effect on the friction coefficient. With increasing velocity and pressure, wear loss of brake shoe rings increases (Wang et al., 2015).

Verma et al. (2016) studied friction layer in friction pair at high-temperature. Cast iron and friction material were the friction pair, tested at 25 °C, 170 °C, 200 °C, 250 °C, 300 °C, and 350 °C. Thermal degradation of phenolic resin has major influence on tribological behavior of friction material. At temperature 170 °C to 200 °C, transition in wear rate is observed from low to severe (Verma et al., 2016).

Polajnar et al. (2017) studied the friction and wear performance of functionally graded ductile iron for brake pads. During braking, after removal of surface material, thickness due to friction wear resistance increases and friction coefficient gets stabilized. On ductile-iron pads, graphite (solid lubricant) nodules were formed and on the carbon-reinforced ceramic disc, a patchy and layered transfer film was formed (Polajnar et al., 2017).

Liew and Nirmal (2013) studied the frictional performance of non-commercial asbestos brake pad and non-asbestos brake pad materials. The non-commercial asbestos brake pad and non-asbestos brake pad materials were tested and compared. Friction layer generated on the friction surface has major effect on the frictional performance which enhances stability of friction coefficient and hence improves the fade resistance. Non-asbestos brake pad materials showed highest friction coefficient among all friction materials. Stabilization of the friction coefficient has occurred after 7 km sliding distance when it slides against cast iron disc (Liew and Nirmal, 2013).

Kchaou et al. (2013) studied the friction characteristics of a brake friction material with brass fiber at different sliding speeds and nominal contact pressure. At low contact pressure, coefficient of friction increases from 0.35 to 0.38 and at high contact pressure, it varies from 0.22 to 0.24. Initially at sliding velocity 3 m/s and contact pressure 0.6 MPa, stable friction film was generated on the surface, which provided excellent friction stability along with reduced wear. At sliding velocity 6 m/s and contact pressure 1.2 MPa, the friction coefficient values show a slight discontinuity and the fibers were found to be more agglomerated. Agglomeration reduces the wear performance and also deterioration of the pad surface was observed due to oxide film formation at high temperature conditions (Kchaou et al., 2013).

Mahale et al. (2017) studied the performance of NAO brake-pads having nano-potassium titanate particles in the pad. Composite without potassium titanate particles and two composite with 3% micro and 3% nanopotassium titanate particles were studied. Results showed that inclusion of 3 wt% potassium titanate in friction material decreased the overall friction coefficient, which in turn shows its lubricating property. But wear and fade also decreases. Composites with nano-potassium titanate particles show better performance than its counterpart with micro-potassium titanate particles (Mahale et al., 2013).

Djafri et al. (2014) studied the tribological behavior of the brake disc materials based on humidity and corrosion. Cast iron, chromium bearing steel and aluminum based composite were used for this study. Results showed, as humidity increased, water film was formed which acts as lubricant. In case if cast iron friction coefficients increased with increasing humidity from 20 to 40%, it also decreases in between 40 to 90% humidity, whereas in case of chromium steel, it gets decreased. In case of aluminum composites, there is no such influence of humidity on friction coefficient. Wear rate decreases in all cases (Djafri et al., 2014).

Kumar and Bijwe (2011) studied the role of copper, its shape and amount on wear behavior of non-organic friction composites. They developed the NAO friction composites with addition of copper particles and fiber with 0, 10, and 20% by weight. Coefficient of friction decreases with speed and pressure in composites. Wear resistance slightly increases due to 10% Cu fibers, whereas higher content of Cu powders are more beneficial (Kumar and Bijwe, 2011).

Ho et al. (2005) studied the effect of steel, brass, cellulose, and ceramic fibers on mechanical and tribological properties of friction material. A lowest wear was observed in Steel fiber-added material. Copper fiber added material shows the stable coefficient of friction as well as low wear (Ho et al., 2005).

Jang et al. (2004) studied the effect of addition of Copper, Steel, and Aluminum fibers on the friction performance of automotive brake friction materials. Friction material with Copper and Steel fibers when rubbed on cast iron disk shows more speed sensitivity than that of aluminum disk. Fade resistance at high temperature was improved with friction material containing copper and steel fibers rubbed on cast iron disk, but heat due to friction is more. Best fade resistance is observed in case of Aluminum Disk. Steel fiber friction material was rubbed with Aluminum Metal Matrix Composite (AMMC) disk at elevated temperature, which shows erratic friction behavior (Jang et al., 2004).

Eriksson and Jacobson (2000) studied tribofilm formation on the disc surface and nanoindentation; scanning electron microscopy, energy dispersive X-ray analysis, and three-dimensional profilometry was done. The contact plateaus formation was observed because of wear debris.

Wear resistant components cause the primary plateaus from which secondary plateaus was formed due to nucleation sites. The primary plateaus contents are iron, and the secondary plateaus are iron and oxygen. Sulphur is mainly found in the place which is surrounded by the plateaus (Eriksson and Jacobson, 2000).

Due to addition of Borax powder in friction material, coefficient of friction is low as compared to copper fiber. Due to copper fiber, wear rate is increased. Due to thermal degradation, gas cushion film occurs on the interface, which reduces the friction coefficient. Due to Nanocopper powder, wear resistance, hardness, thermal conductivity, thermal diffusivity, and density were also increased. The wear performance reduces due to agglomeration. In case of C/SiC brake materials, wear mainly occurs due to adhesive wear which is because of the Silicon present in it, where the coefficient of friction is also unstable. The inclusion of 3 wt% potassium titanate in friction material also decreased the overall friction coefficient, but wear and fade also decreased. There is no such influence of humidity on aluminum composites friction material as compared to cast iron or chromium steel friction material. Friction coefficients in case of cast iron increased, whereas in chromium steel, it gets decreased.

It is necessary to do the study on frictional surfaces by using different techniques like nano-indentation, scanning electron microscopy, energy dispersive X-ray analysis, and three-dimensional profilometry.

With the view of all researchers, during braking initially coefficient of friction increases, after certain temperature it decreases. To stabilize the friction coefficient, different materials with addition of different types of filler either in micro or nano form can be developed. Bonding strength between fiber and matrix may improve the wear resistance, fade resistance, and mechanical properties. Newly developed friction material should lower the emissions and increase the fuel efficiency. Thermal conductivity of friction material can play a major role in the performance of friction pair. With consideration of different materials for friction pad, focus on dimensional analysis of it can be done in future.