Biodegradable cow dung for brake friction material: a preliminary investigation

The purpose of the study is to develop a biodegradable and non-asbestos/copper-free brake pad formulation. The possibility of using organic cow dung particles as an eco-friendly reinforcement in friction material for brake pads is investigated. Sodium hydroxide (NaOH) treated cow dung is Sun dried and ground to an average particle size of 200 microns. It is reinforced in epoxy resin in weight fractions of 5%, 10%, 15%, 20% and 25% along with other fillers and friction stabilizers. The composite samples are subjected to density, thermal conductivity, microhardness and tensile strength evaluation for mechanical characterization. Pin on disc testing is done to identify the coefficient of friction (CoF), wear coefficient and specific wear rate as a part of tribological characterization. The scanning electron micrographs and EDAX analysis of worn out surface is performed to study the wear mechanism. Promising results are seen with composite samples reinforced with 15% cow dung particles in terms of better microhardness, tensile strength, stable coefficient of friction and low wear. The investigation could guide industries working on brake pad materials. This could open up an era of low cost, organic and eco-friendly alternative to carcinogenic asbestos/copper in friction materials.


Introduction
The composite materials used in brake pads have reinforcements, lubricants, binders, fillers and abrasives. Generally the binder used is a resin and copper or asbestos fibres are used as reinforcements [1]. Though all the ingredients have explicit roles, hazardous copper particles suspended in the air can be harmful to the environment, while the asbestos dust in the air can be carcinogenic. Hence replacing the harmful constituents with eco-friendly materials becomes a prime requirement. Several natural fibres including jute, flax, coir, sawdust and hemp were suggested in the formulations for brake pad. However the properties observed were relatively poor with hemp fibres [2]. The use of rice husk for brake pads was also available in recent literature. Promising results were obtained with a renewable constituent like rice husk, which was harmless to the atmosphere [3]. Organic friction materials produced better fade and recovery characteristics along with an improved specific wear rate compared to asbestos based friction pads and steel based friction materials [4]. Generally the pin on disc test was used to mimic the wear characteristics and brake friction. The coefficient of friction (both static and kinetic) could be evaluated along with wear and fade characteristics using the pin on disc testing [5]. The usage of natural particles (agro-waste) of size 300 microns was observed to penalize the wear resistance without significant effects on the friction coefficient. Further the airborne emissions tend to increase in case of natural cocoa bean shells as an alternative ingredient [6]. The carbon fiber reinforced brake discs and nano coated brake discs displayed good compressive strength with a better and stable coefficient of friction [7,8]. The interface bond between matrix and fibres was important in producing an improved tribological performance. The tribological characteristics could be improved by proper treatment of fibres to ensure good Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
adhesion with the matrix [9]. The abrasive characteristics of periwinkle shell was used to develop a friction material with polyester resin. The wear rate was notably reduced with an increased content of polyester. The interfacial bonding among the constituents and matrix was found to be good with a minimal grain pull out and lesser distortion [10].
Renewable materials including peanut shell and aloe vera fibre were also used to toughen epoxy resin based composite. The post cured composites were observed to possess better mechanical properties [11]. Non asbestos based materials including zinc borate and flyash were tested as constituents of friction material. The fly ash reinforced material displayed better friction stability and fade characteristics [12]. In the process of developing the non-asbestos organic materials for brakes, steel slag with molybdenum disulphide was tried in formulations for brake lining with reasonable success [13]. Good improvement in fade and thermal stability were obtained in brake pads formulated with lignocellulosic banana fiber reinforced phenolic resin [14]. The fade and recovery features of the friction composites formed with banana peel powder were observed to increase significantly. The thermal stability was improved by the partial replacement of phenolic resin with banana peel powder [15]. The content of resin was observed to play a major role in tribological characteristics of brake pad formulations. The secondary binders could be vital in improving the mechanical properties of epoxy composites for engineering applications [16]. Cow dung is an organic, human-safe, biodegradable constituent used in the preparation of biofuels. The efficiency of anaerobic digester in the production of biogas was also investigated in open literature [17, , 18]. Experimental study related to usage of eco-friendly cow dung ash as fillers in highway pavement was also available in literature [19]. Cow dung is a cheap and easily available resource over the world. Friction composites reinforced with corn stalk fibres were tested using the pin on disc test method. These fibres were observed to produce stable friction coefficients and enhanced wear resistance [20]. Cow dung fibre was seen as a potential replacement for carbon fibres which were environmentally unsafe. The fatigue strength of cow dung fibres could also be improved by proper treatments [21].
A reasonable amount of research is observed in brake friction materials and a recent flourish in use of organic, eco-friendly ingredients is also visualized. However the use of organic cow dung as a filler in brake pad material is limited in open literature. Hence the main objective of this preliminary investigation is to study and assess the potential of treated cow dung particles as a natural alternative brake pad ingredient. The density, thermal conductivity, microhardness and tensile strength of cow dung particles reinforced epoxy are studied along with the tribological characteristics including wear coefficient, specific wear rate and coefficient of friction. This could open up an era of low cost, organic, eco-friendly and biodegradable alternative to carcinogenic asbestos or copper in friction materials. The investigation will assist the industries working on brake pad materials.

Materials and methodology
The epoxy resin used in composite samples is Epidian 57, which is a homogenous combination of Epidian 5 and polyester resin. The Epidian 57 epoxy resin is a viscous liquid treated using HY-951 hardener to fabricate harder plastics. The properties of Epidian 57 epoxy compound and hardener is shown in table 1. The epoxy compound selected for investigation has better physical and mechanical properties as observed from literature [22]. Normally the fillers, binders, fibers and friction modifiers form the different constituents of a brake pad. The composite samples were designated with 10 constituents in which the parent base was made of eight constituents (35 wt%). Calcium hydroxide (Ca(OH) 2 ) and calcium carbonate (CaCO 3 ) were used as the space fillers to finetune the values of friction coefficient. These functional modifiers were observed to improve fade and noise as well [15]. The graphite and molybdenum disulphide (MoS 2 ) ensure stabilization of friction coefficient while playing the role of solid state lubricant in different temperature ranges [14]. Magnesium oxide (MgO) and calcium silicate (CaSiO 3 ) have a strong influence on coefficient of friction, while MgO can improve the heat transfer characteristics as well. Alumina (Al 2 O 3 ) and silicon carbide (SiC) additions could improve the base strength of the brake pad and decrease wear loss. The dung particles used as filler was obtained from cows fed with rice straw. The cow dung was Sun dried for a week after subjecting to alkaline treatment with sodium hydroxide (100 ml kg −1 ). It was done to remove odour and produce non-sticky fibres. The cow dung fibres used as fillers have a cellulose content of 33%, hemicellulose content of 16% and lignin content of 10%. Magnesium, calcium and potassium are also present as macronutrients along with traces of iron, cobalt and copper [18,19]. The composite samples were prepared by using the technique of powder metallurgy. A homogenous mixture of various constituents obtained by blending was poured in a stainless steel mould of dimension 600 mm×400 mm with a plate thickness of 3 mm. Wax was applied to prevent sticking of resin in the mould. A hydraulic press was used for hot compaction under a pressure of 15 MPa at 130°C [20,23]. Finally a curing time of three hours was ensured in an oven for post curing and composite samples of required dimensions were obtained using a tool room cutting and polishing machine. For each defined weight fraction of cow dung particles, five replicative samples were prepared for further investigation. The photograph of fabricated friction composite sample and SEM image displaying the reinforcements is shown in figures 2(a), (b). Cow dung particles of size 200 microns were reinforced in the epoxy compound base in weight fractions of 5%, 10%, 15%, 20% and 25% to prepare five different designated composite samples (table 2).
Hardness, tensile strength and wear behaviour are important to check the suitability of cow dung composite as replacement for asbestos in brake pads. Hence the designated samples with defined cow dung fractions were prepared for measuring Vickers microhardness (HV) as per ASTM E384 standard. An optical Vickers microhardness tester (model: ASI-VHT) with a motorised loading/unloading set up and built in projection  screen was used for measurements. A pyramidal diamond indenter was used to create an indentation whose diameter was finally measured to arrive at the microhardness value. Microhardness measurements were carried out by applying a load of 300 gm for a dwell period of 10 s. Five indentations were made on the top and bottom side of each sample and the values were averaged as final Vickers microhardness of the sample. Tensile strength evaluation was performed in Instron (series: 3369) tension testing machine using ASTM D3039 standard. The computerised tester can generate plots for observing tensile properties. Two samples of similar designation were tested in tension and the averaged result was presented as tensile strength.
The Electronic pin on disc (PoD) wear tester (Brand: CE-Liangong-Model-MRS-10B) with a rotating disc diameter of 120 mm was used to observe the tribological characteristics of various samples in dry sliding condition ( figure 3(a)). The tester equipped with an inbuilt control unit (figure 3(b)) could generate wear graphs as shown in figure 3(c). The disc was made of grey cast iron (240 HV30) with a surface roughness of 2 μm, while the wear pin sample was prepared as a cylinder of length 30 mm and diameter 15 mm. During the wear test, the pin arm was ensured to be parallel to the disc and measurements were made as per ASTM G99-05 standard. A contact pressure of 1 MPa and a persistent sliding velocity of 1.5 m s −1 was ensured during wear testing at 750 rpm, for a sliding distance of 1000 m. This resembles a mild wear condition reminiscent to one in a real braking condition [6,23,24].
The coefficient of friction (COF) was continuously documented by an inbuilt software with the help of a load cell. The resulting COF was an averaged value from three replications [6]. The COF was expressed by equation (1), where A r is the real area of contact, F is the normal force and m t is the shear stress is governed by the strength of adhesive bonds.
A precision balance with a sensitivity of 10 -4 g was used to find the weight loss by measuring the weight of samples before and after each test run. The wear volume calculations from the weight loss and density were used along with the sliding distance (s) and contact force (F N ) to arrive at the wear coefficient 'K' using equation (2).
The specific wear rate (SWR) of composite sample was studied using equation (3), where ρ is the composite density, V s is the sliding velocity, Δm is the wear loss of composite, F is the applied load and t is the testing time [23].  4(a)). An improvement in microhardness value by 44% compared to the 90E/5CD/5C sample  /5CD/5C  60  5  5  5  10  3  3  3  4  2  85E/10CD/5C  55  10  5  5  10  3  3  3  4  2  80E/15CD/5C  50  15  5  5  10  3  3  3  4  2  75E/20CD/5C  45  20  5  5  10  3  3  3  4  2  70E/25CD/5C  40  25  5  5  10  3  3  3   having least microhardness (40HV) could be due to reduction in sample porosity, bringing about a desired restriction to the penetration of diamond indenter [20,21]. The microhardness values were observed to increase with the weight fraction of cow dung particles but beyond a weight fraction of 15%, the microhardness values tend to decrease. The Vickers microhardness value for the third composite sample (80E/15CD/5C) was the best, proving the better performance of cow dung particles as fillers in enhancing the surface hardness. The quadratic (polynomial) equation of third order (y = 0.0833x 3 −6.8929x 2 + 41.024x+5.2) was generated for Vickers microhardness curve shown in figure 4(a) and its fitness was proved by the R-square value of 0.96. A similar trend was observed in case of tensile strength as well. The reduction in tensile strength value of 70E/25CD/5C composite (11.6 MPa) by 40.81% compared to the 80E/15CD/5C composite sample (19.6 MPa) having highest tensile strength could be due to an increased concentration of cow dung fibres. Though addition of cow dung particles improved the tensile strength till a weight fraction of 15%, a further rise backfires as a reduction in strength was observed. Similar results were reported in literature [20]. Strength improvement could be due to the interfaces which reduce the movement of dislocation (Orowan Strengthening) and hence restricting the plastic flow of matrix [25]. Microhardness and tensile strength which form an important part of mechanical characterization was in favour of composite sample designated as 80E/15CD/5C, signifying the role of cow dung particles ( figure 4(b)). A polynomial equation of third order (y = 0.0417x 3 − 1.4964x 2 + 6.8619x + 9.12) was formed for the tensile strength curve shown in figure 4(b) and its fitness was substantiated by the

Wear and friction behaviour
The values of coefficient of friction and specific wear coefficient calculated for different composite samples is shown in table 4 and progression of COF for different samples is represented graphically in figure 5. The ratio defining COF relates the tangential force for maintaining motion with the actual normal force. The tangential force is dependent on the shear stress necessary to dismantle the surface asperities and interfaces [22,23]. A minimal amount of local plastic deformation could offer the necessary assistance as well. Generally the value of static coefficient is larger than the kinetic COF and the proportionality between area and normal force described for metals hold good for composites as well [6]. The contact pressure alters the real area of contact which includes the regions within plateaus. The temperature changes could trigger pressure changes and hence the area of contact [24]. The cow dung additives might increase the surface energy and hence adhesion interaction between mating surfaces, resulting in higher value of shear stresses and increased COF values (figure 6(a)). Hence the COF value was largest at cow dung weight fraction of 20% (75E/20CD/5C sample). However a further increase in cow dung particles results in a reduced bond strength at interface of matrix and filler and hence a reduction of COF values [24]. The enhancement in COF values could be due to improvement in stability of binding at appropriate levels of cow dung particles [20]. More forces resisting the slide between mating surfaces involving 75E/20CD/5C sample compared to other composite samples had resulted in a larger value of COF. A quadratic equation was generated for the COF curve shown in figure 6(a) (y = −0.0158x 3 + 0.119x 2 − 0.0331x − 0.0487) and its fitness was demonstrated by the R-square value of 0.9707.
The variation of wear coefficient for different composite samples is represented graphically for better interpretation in figure 6(a). The value of 'K' was the least for 80E/15CD/5C sample displaying the lowermost wear rate compared to samples with higher weight fraction of cow dung particles. The wear coefficient was calculated using the volume loss in terms of weight of pins before and after wear test. The least wear coefficient value identified for composite sample with 15% cow dung particles was the most desired in terms of friction characteristics for brake pad [20,24]. A polynomial equation of third order (y = 0.1583x 3 −0.9321x 2 + 1.4095x +3.56) was generated for the wear coefficient curve shown in figure 6(b). Its fitness was demonstrated by the R-square value of 0.9998.
The relation between the coefficient of friction and wear coefficient for different composite samples is shown in figure 7. A third order polynomial equation was used to obtain a good fit with the experimental data (y = 153.6-97.708x+20.151x 2 −1.3275x 3 ). A higher value of R-square (0.9182) had proved the closeness of data to    regression curve and hence the fitness of equation [6]. The generation of friction was possible only by abrasive and adhesive interaction among the pin and disc. The wear characteristics were dependent on the friction layer morphology existing in the sliding interfaces [24]. The samples containing carbon from base material as well as organic filler could generate graphite on a micro scale in the friction layer hence improving the friction performances [3]. This could create a stabilized friction coefficient due to compact micro friction layers and a decrease in wear rate. The wear rate for various composite samples is shown in figure 8. The specific wear rate (Ws) for the pin found out using equation (3) is listed in table 5. This variation could be observed from figure 9, which displays the influence of cow dung fibres on the specific wear rate of composite samples. The SWR was improved with addition of cow dung fibres up to a weight fraction of 15%, beyond which a significant decline was observed. The deterioration of SWR value for 70E/25CD/5C sample by 31%, compared to the composite sample with better   SWR (80E/15CD/5C) could be due to inclusion of a higher fraction of cow dung particles which worsen the even adherence of friction film in the mating surfaces [23].

Worn surface morphology
The study of worn surfaces would give a clear understanding of the wear mechanism. A scanning electron microscope (model: MA-15/EVO18-Zeiss) with superior resolution (4 nm) was used in low vacuum to generate SEM images ( figure 10). The worn composite sample (80E/15CD/5C) with better friction and mechanical characteristics was subjected to morphology study. The surface irregularities (pit formation) due to sliding friction were seen in figure 10(a). The pile up of organic debris was also seen well anchored and quite dense at instances, while appearing weakly compacted at few spots as well. The large area seen in figure 10(b) was covered by wear fragments with an impression indicating thermal swelling because of an increased temperature. These could constitute the friction layer with primary and secondary plateaus, with microscopic cracks appearing in the primary surface [6,23]. Both micro and macro cracks on the worn surface covered by less compact debris appear to propagate in the sliding surface ( figure 10(c). The possibility of the organic debris acting along with plateaus as a third body and enhancing the COF due to rolling abrasion could never be ignored [3]. A few treated particles of cow dung were also seen among the pile up of fragments on worn surface. The good interfacial bond due to hydrophilic nature of epoxy and the cellulose of cow dung produce better brake pad characteristics [20].
Lignin also plays a key role in polymerization resulting in a good interfacial bond [13]. The brittle wear cracks were visible at higher magnifications along with the internal fracture due to shear stress. The slight movement of matrix layers due to intensity of wear test and consequent thermal fatigue could also be visualized in figure 10(d).
These brittle cracks were preferred over ductile deformation which could distort the brake pad during heavy braking [25]. These brittle cracks display sufficient thermal stability and heat dissipation during braking [24].
The scanning electron microscope furnished with EDX system was used to obtain the EDAX scan image ( figure 11). It displays the elemental composition (spectrum 49) of the worn surface ( figure 10(a)). The cumulative composition results indicated the highest percentage of constituent carbon by weight (52.13%), followed by oxygen (33.16%) due to presence of aluminium oxide.

Conclusions
A preliminary investigation was conducted to check the viability of cow dung particles as an organic, ecofriendly and biodegradable constituent in friction material for brake pads. This could assist in replacing the carcinogenic constituents. The following inferences and main results were summarized.
2. The Vickers microhardness of 80E/15CD/5C composite sample was largest (72HV), indicating a reasonable level of difficulty in penetrating the material surface. The improvement in microhardness value was 44%, compared to the sample (90E/5CD/5C) with least value of microhardness (40HV). The third order polynomial regression for microhardness was expressed as y = 0.0833x 3 −6.8929x 2 + 41.024x+5.2, with R-square value of 0.96.
3. The largest tensile strength value was observed for 80E/15CD/5C composite (19.6 Mpa). It was 40.81% higher, compared to the 70E/25CD/5C sample with least strength (11.1 MPa). Hence microhardness and tensile strength which form an important part of mechanical characterization was in favour of composite sample 80E/15CD/5C signifying the role of cow dung particles. The third order polynomial regression for tensile strength was expressed as y = 0.0417x 3 −1.4964x 2 + 6.8619x+9.12, with R-square value of 0.8186.
5. Organic wear fragments, surface irregularities and cracks were observed in worn surface morphology due to severe shear and thermal stresses.
The investigation proves the vigour of treated cow dung particles as a human-friendly and biodegradable ingredient for brake pad. The 80E/15CD/5C composite specimen displayed good binding ability of cow dung particles (15 wt%) with the resin. The mechanical and tribological properties were observed to lie inside the acceptable range for brake pad material hence proving its fitness for application as a brake pad material. These investigation findings could open up an era of low cost, organic and eco-friendly alternative to carcinogenic asbestos or copper in friction material. The work augments indispensable design recommendations to industries working on brake pad materials.