THERMAL AND TRIBOLOGICAL CHARACTERISATION OF ASBESTOS-FREE PARTICULATE CERAMIC MATRIX COMPOSITES

Production of friction particulate ceramic matrix composites (PCMCs) by powder metallurgy method was carried out using iron 105 μm millscale, 50 μm silica sand, 80 μm magnesia and 53 μm bentonite clay as input materials. Different formulation of the blend of these materials were prepared and the composites were produced. Microstructural, frictional, thermal and wear characterisation were determined using standardised methods. The ceramic composites exhibited very good properties in terms of thermal stability (heat resistance), coefficient of friction (COF) and resistance to wear. The specific values of these properties exhibited by sample D having 12 wt. % iron millscale addition in terms of high resistance to thermal decomposition in the temperature region (0 – 1600 0 C) indicating thermal stability, appreciable high COF (0.59) and very low wear rate of 1.9093 x 10 g/m are desirable. The uniform dispersion of the particles as observed in the microstructure and strong bonding/adhesion contributed to the enhancement of the properties. These results are indication that the composite is very suitable for application in areas where high resistance to thermal stress and abrasive wear are required such as the brake assembly of automobiles, specifically the brake pads.


INTRODUCTION
The brake system is an important part of automobiles because of the vital role it plays and the brake pads are vital components of the brake system. Without the brake system, driving vehicles could possibly result in road accidents which can endanger the lives of drivers, passengers, and also pedestrians. The function of the brake is to retard or stop the motion of vehicles. It has been reported that an average automobile driver uses the brake about 75,000 times a year, making the brake one of the most important and overworked parts of vehicles (McPhee, 2007).
The ban and gradual phasing-out of asbestos brake pads in many parts of the world due to health reasons have sparked the onset of extensive research and development into safer alternatives.
Consumers demand and public awareness also led to extensive research and development of brake pads in the early 1990s to find suitable replacement for asbestos brake pads. These have promoted the production of different brake pad materials in the past decade of asbestos-free brake pad revolution. There are several categories of brake pads based on materials constituent such as metallic, semi-metallic and ceramic brake pads (Ma et al., 2012).
Despite the significance of the brake, it has its own challenges or problems. One of the reasons why automobiles brakes experience failure is due to the heat generated during braking causing heating of the brake pads which reduces braking efficiency. Accumulated heat within the brake system causes gradual thermal expansion and composition debonding of the brake pads resulting in impaired structural integrity and loss of friction. Further, these conditions aggravate the rate of wear of the brake pads surfaces which reduces its arresting capability resulting in brake-failure (Aza, 2014). The combination of these dysfunctional occurrences usually have adverse effect on brake pads in terms of performance and durability.
Attempts have been made to solve the problems associated with brake pads by using different types of materials to develop/produce brake pads. Some of these materials have problems associated with them. For instance, some less thermally stable resins in brake pads were identified as producing oily residues that contributed to brake fade (Tanaka et al., 1973). Certain phenolic resins were identified as evolving volatile gases during degradation causing a back pressure on the brake pad also resulting in brake fade (Herring, 1990). Of significant importance to the friction performance is the thermal stability and degradation reactions of the organic resin matrix in the brake pad (Bark et al., 1975). This highlights the necessity for thermally stable raw materials that will not evolve friction destabilising by-products during thermal degradation.
The friction between brake pads and discs converts kinetic energy of automobiles to thermal energy. The brake pads must be capable of absorbing heat to prevent distortion or cracking from Kufa Journal of Engineering, Vol. 11, No. 2, April 2020 51 thermal stress until the heat is dissipated (Adebisi et al., 2011). The coefficient of friction between pads and discs and wear behaviour of the pads must satisfy standard requirements (Jafar et al., 2012). The materials for brake systems should have stable and reliable frictional and wear properties at different conditions of load, speed, temperature, environment, and high durability (Maleque et al., 2010). Brake pads must satisfy several desirable properties such as wear rate, thermal stability and low noise making the selection of a proper formulation to require many experimental trials/attempts (Kchaou et al., 2013;Sellami et al., 2014).
There are many studies on the development of brake pads. Ademoh (2015 developed maize husks (asbestos free) based brake pad. The results of the study showed that coefficient of friction, abrasion resistance, thermal conductivity, etc of maize husk filler based brake pads compared favourably with commercial brake pads and those produced in past related research works on asbestos free brake pads. Ma et al., (2012) investigated the effect of zirconium silicate as an abrasive on the brake friction performance. They reported that by increasing weight percent of zirconium silicate, the coefficient of friction of brake pad was enhanced but the wear rate was depressed. Some studies focused on the environment issues related to diffraction of hazardous constituted materials of brake pads such as copper (Straffelini et al., 2015;Lee and Filip, 2013). Straffelini et al., (2015) reported that similar to other heavy metal emissions, the release of copper into the atmosphere may have important environmental and health effects. So they introduced some replacements for copper in brake pads such as graphite. Singh et al., (2015) used lapinus-wollastonite fibers in the friction composite materials and carried out wear test of samples which were different in the amount of both fibers. They concluded that the increased lapinus fiber content enhanced the friction stability whereas, increased wollastonite fiber enhanced the wear performance. Jafar et al., (2012) applied SAE J 661 to evaluate 10 formulations (15 types of materials) and select best formulation according to wear test results. The most important result of their study was dependency of wear results on materials constituted brake pads. That means, achieving proper wear properties needs several experiments with different chemical compositions to select the best formulation. Kchaou et al., (2013) carried out wear test of brake pads. They reported that in the high contact pressure, the friction coefficient was degraded relative to low contact pressure.
Thermal stress and wear being deleterious materials' performance phenomena, can be mitigated by the use of particulate ceramic matrix composites (PCMCs). Through the combination of certain materials composition modification and innovative processing, brake pads capable of 52 Durowaye et al. exhibiting enhanced thermal stability, low wear rate or high wear resistance can be developed.
However, without proper materials selection and formulation, this may not be possible. This work is aimed at producing a friction composite material by powder metallurgy method using iron millscale, silica sand, magnesia and bentonite clay that is suitable for application as automobiles brake pad.

Materials
The materials used in this study are iron millscale, silica sand, magnesia, and bentonite. Iron millscale was sourced from Universal Steel Nigeria Limited located in Ogba, Lagos. Silica sand was obtained from the beach of the Lagos Atlantic Ocean (bar beach). Magnesia and bentonite clay powders were obtained from a local vendor within the chemicals supplier trade group registered in Nigeria but were manufactured in China and Wyoming, USA respectively. The photographs of these materials and their chemical composition determined using an X-ray spectrometer model ARL9400XP+ Thermo are shown in Fig. 1 and Table 1 respectively.

Production of the Composites
The as-received silica sand and iron millscale were milled using a steel ball mill (model A50 43, Mashine, France) and sieved to 50 μm and 105 μm particle sizes respectively. These materials were manually blended with 80 μm magnesia and 53 μm bentonite according to the formulation presented in Table 2 to obtained many formulations of samples A, B, C, D and E.
Appropriate quantity of water was added to each blend. Thereafter, each of the wet-mixed blends was poured into the fabricated metallic mould. Green samples were obtained by uniaxial cold pressing at 0.33 MPa using a hydraulic press (Type P100EH, Model No. 38280, Weber Hydraulik, Germany) to enhance surface smoothness of the samples. Little lubricant (Mobil engine oil XHP) was rubbed on the inner part of the moulds as a releasing agent for easy discharge of the samples from the moulds after compaction. The samples were sun dried (30-32 0 C) for 24 hours followed by drying under a controlled atmosphere in an oven at 110 0 C for 24 hours to remove any moisture left in the samples and to avoid cracking during high sintering temperature employed. Gradual sintering of the samples at 1200 0 C was carried out in a muffle furnace pre-set at heating rate of 10 0 C/min. After 3 hours of sintering, the samples were removed and cooled at room temperature. Sintering is heating the samples below the melting point of the materials but high enough to develop significant solid state diffusion. This is to enhance bonding of the particles for properties enhancement. This production process is similar to the one used by . The photographs of some of the composite samples and brake pads produced are shown in Fig. 2.

Characterisation of the Composite
Two of the samples were subjected to qualitative analysis using an X-ray Diffractometer (XRD). They were sample A which did not have iron millscale addition and sample D representing samples with iron millscale addition. 10 g of sample was placed in the sample holder and was bombarded by a Cu Kα radiation (40 kV and 40 mA) within the X-ray tube and the intensities of the reflected X-rays were recorded. The scan range, scan step size, time per step were evaluated simultaneously. The scan ranges were selected based on strong peak positions and limiting interference from excipients which lead scan range set to 10 0 to 70 0 (2 Theta) in a step scan mode of 0.05 0 , speed 2 0 /min, time/step 0.025 0 , wavelength 1.54056Å (Cu).
The microstructure of the test samples was examined using an ASPEX 3020 model variable pressure scanning electron microscope (SEM) equipped with Noran-Voyager energy dispersive X-ray spectroscope (EDS). Thermal stability of the samples was determined by the flame resistance method using a Bunsen burner and also by thermogravimetric method using a differential thermal analyser model NETZSCH

Microstructure
As presented in Fig. 3, mullite (3Al2O3·2SiO2) has the major phase in the X-ray diffraction

Thermal Stability
The mass (weight) losses of decomposition of the ceramic composite samples A, B, C, D and E were evaluated using thermogravimetric method as illustrated in Fig. 10(ae). The figures show the weight loss percentage as a function of temperature. Sample A which was not blended with iron millscale exhibited the highest weight loss percent of 0.9 while sample D which was blended with 12 wt. % of iron millscale exhibited the lowest weight loss percent of 0.2. There was no loss in weight of the samples from temperature 0 -1600 0 C. However, beyond 1600 0 C to the peak (1800 0 C), there was weight loss of 0.9 %, 0.7 %, 0.5 %, 0.2 % and 0.3 % in samples A, B, C, D and E respectively. The increase in weight loss of sample E when compared to sample D could be due to weak bonding of the particles. The bonding of the particles of sample E during sintering may be less than that of sample D thereby resulting in higher weight loss.
The onset of thermal decomposition of the samples occurred in the temperature region 1600 -1800 0 C. These temperatures were quite high indicating that the composites exhibited high 60 Durowaye et al. ability to absorb heat before disintegration implying that they are thermally stable. This may be due to the refractoriness of the ceramic materials coupled with the strong interfacial bonding of the particles. The strong interfacial bonding of the particles effectively acted as barriers to decomposition of the samples during heating, thereby enhancing the thermal stability of the composites. This is similar to what was reported by Subasinghe et al., (2016).
As shown in Table 3 When compared with the result of this study, the ceramic composites were more thermally stable than the asbestos based commercial brake pad.

Coefficient of Friction of the Ceramic Composites
Coefficient of friction (COF) is a dimensionless property indicating the friction quality of one material sliding over another. As illustrated in Fig. 11, samples A suggests that the samples could be more effective because a higher coefficient of friction would offer a better performance. Brake pads must ensure a stable friction with a sufficiently high coefficient of friction. They must not decompose or break down in such a way that the friction Kufa Journal of Engineering, Vol. 11, No. 2, April 2020 61 coefficient with the brake disc is not compromised at high temperatures (Djafri et al., 2014;Ikpambese et al., 2016).
The values of the COF of the composite samples is also within the industrial standard of 0.3 -0.6 (Dagwa and Ibhadode, 2006;Aigbodion and Akadike, 2010) recommended for automobiles brake pads.

Weight Loss and Wear Rate of the Composites
As illustrated in Fig. 12a, there was weight loss in all the composite samples due to plastic deformation of materials on the friction surface as a result of increase in the shear force between asperities and the ploughing force of asperities. Similar observation was also reported by Djafri et al., (2014) and Yawas et al., (2016). The wear rate of the samples shown in Fig. 12b is similar to the trend observed in Fig. 12a because wear rate is directly related to weight loss due to wear.
Samples B to E which contain millscale in their formulation (Table 2) exhibited lower weight loss and wear rate than sample A. This indicates the effect of concentration of millscale addition in reducing wear. The lower weight loss and wear rate exhibited by samples B -E could be attributed to the strong interfacial bonding between the particles as shown in the microstructure ( Fig. 6 -9) which reduced the possibility of particle pull out. The decrease in wear rate was also enhanced by the decrease in the number of microstructural flaws which are stress concentrators needed for nucleation and propagation of cracks resulting in removal of materials (Hussainova, 2005). Also, decrease in wear rate of the composites was due to the high hardness and compressive strength values of the composites.
The wear rate of commercial conventional asbestos based brake pad is 3.8 x 10 -6 g/m (Olabisi et al., 2016) while the range of wear rate of all the composites obtained in this study is (1.909 -2.705) x 10 -6 g/m. This is much lower than that of asbestos based conventional brake pad and is an indication that the composites have the potential of better performance when used as brake pads.

Fractography of Worn Surfaces of the Composites
As shown in Fig. 13(ae), there was formation of grooves on the surfaces of the worn samples as a result of abrasion. The surfaces of the worn samples showed a typical abrasive wear mode in which abrasive particles (debris), scratches, shallow, and deep grooves were formed. The wear grooves are indication of plastic deformation. A similar observation was reported by Djafri et al., (2014). The findings of this study are also supported by the fact that abrasive wear in contacting surfaces resulted in scratching, furrows, work hardening, or grooves (Holmberg and Matthews, 2009;Kumar and Singh, 2011 The microstructure of worn surface of sample A (Fig. 13a) shows more groove defects than that of Fig. 13(be). The worn surfaces appeared to be rough on the microscopic scale. During the wear process, wear debris were generated and they were mainly fine powders that were either released into the environment or trapped between the contact areas. Occasionally, some of the wear debris may also be trapped in any available porous or void area of the samples. The wear debris accumulated and piled up against the contact areas. This phenomenon resulted in the contact areas increasing in size to form a friction layer and patches due to the compaction of wear debris trapped between the sliding surfaces. The debris in the microstructures enhanced the rate of wear of the samples. Similar observations were also reported by Sloan, (2008) and Ruzaidi et al., (2013).
The microstructures of the worn samples revealed that abrasion, adhesion, and grooves formation are the wear types that were in operation during the wear process. Abrasion is always characterised by the presence of shallow grooves of ploughed marks on the friction layers. This wear mechanism was observed in all the microstructures and a similar observation was reported by Talib et al., (2008). This phenomenon could be due to the hard particles of the ceramic materials which existed during the wear process that ploughed into the worn surfaces. Wear of friction materials should be minimized as much as possible. A higher wear rate means shorter friction material life and thus, the incurring of more material and maintenance costs. A lower wear rate would increase the life of the brake pad and a higher friction coefficient would offer a better performance.

CONCLUSIONS
In this study, the development via powder metallurgy method and characterisation of particulate ceramic matrix composites (PCMCs) have been successfully carried out. The conclusions from the study are highlighted as follows: 1. The ceramic composites exhibited very good properties in terms of high thermal stability (heat resistance), stable coefficient of friction and resistance to wear.
2. The specific values of the properties exhibited by composite sample D in terms of resistance to thermal decomposition in the temperature region (0 -1600 0 C), appreciable and high coefficient of friction (0.59) and very low wear rate (1.9093 x 10 -6 g/m) are desirable.
3. The uniform dispersion of the particles as observed in the microstructure and strong bonding/adhesion of the particles contributed to the enhancement of the properties.
4. These are indications that the composite is very suitable for application in areas where high resistance to thermal stress and abrasive wear are required such as the brake assembly of automobiles, specifically the brake pads.