Operation of the Brake Disc in the Braking System - Coursework Example

MATERIALS By Student’s name Course code and name Professor’s name University name City, State Date of submission Section A Operation of the Brake Disc in the Braking System The operation of brake discs in modern motor vehicles’ typical disc brake system is meant to provide the physical force that should eventually put it to a stop. The break disc is sealed using rubber square cut O-ring seals that usually apply pistons on their bore. Disc brakes are considered effective since they do not require return springs in that when brakes are applied the O-rings distort to apply the energy required. Once the brakes are released, the seals retract to their initial position thereby pulling the piston back and releasing the rotor. Eventually, the brake disc glides on the rotor surface bringing the motor vehicle to a stop (Gilles, 2005, p.142). Operational Requirements that Influence Material Property of a Brake Disc According to Maluf et al. (2004), the operational requirements that determine the fundamental material properties of brake discs include the following: High temperatures of operation that call for high thermal conductivity. Humid and moisture conditions that require excellent corrosion strength. High levels of friction generate increasingly high noises that should be curbed by low noise material and low friction. Heavy usage requires long durability and a good price benefit ratio. The weight of the vehicle should not be affected by the installed disc thereby requiring the material used to be low weight. Section B Cast Iron For objectivity, the type of cast iron material chosen for analysis is Cylindrical –Vermicular graphite cast iron. The microstructure of this material is characterised by coarse crystals during the hyper-eutectic condition. The spheroidiser present in this material is 0.02% magnesium meant to neutralise sulphuric effects hence the half rounded graphite shape that is observed in its microstructure. The microstructure is further observed to contain a conjoint nucleation growth with the addition of magnesium to form a nodular structure. The resulting properties of Cylindrical –Vermicular graphite cast iron are: good machinability, low damping properties, good compressibility and finally wear resistance since it contains graphite which is deemed as a lubricant (Alp et al., 2005). Figure 1: An optical micrograph of Cylindrical –Vermicular graphite (Alp et al., 2005). Metal Matrix Composites For the purpose of this discussion, the metal matrix composite chosen is the short fibre reinforced aluminium matrix composites. In order to conduct a survey of the microstructural characteristics, the inter-fibre bond are sintered for temperature up to 1650°C. The microstructural properties of these preforms are usually observed using the identical fibre and filter-pressing method. Sintering these preforms depict enhanced fibre contact points thereby creating an observable morphology. The α-alumina grain size observed through this method shows a microstructural grain size that is larger than the original size at 1650°C in the order of 50nm. The resulting mechanical strengths are characterised by a decreased tensile strength and less reinforcing capability that is highly required for the purpose of brake discs manufacture (Peng et al., 2002). Figure 2: Fibre morphology observed in an experiment set up to investigate the microstructural characteristics of short fibre reinforced aluminium matrix composites (Peng et al., 2002). Further in their research, Peng et al. (2002) found out that heating the preforms has an adverse effect on the volume. Shrinkage of up to 20% was recorded in the microstructural experiments carried out on short fibre reinforced aluminium matrix composites at 1650°C as much as attrition cannot be avoided during the handling process. Traces of δ-alumina are observed in the α-alumina microstructure with a limited phase transformation with a heat treatment of up to 1200°C. Fibre strengthening is observed for microstructural temperatures below 1200°C which offer favourable operating condition for application for brake discs. On checking for mechanical properties, it is observed that the density is ultimately increased together with the compressive strength as the temperature rises leading to fibre strengthening. This comes with other beneficial properties such as a high thermal conductivity, porosity and specific heat capacity. The impact strength is also considerably increased. Carbon-Carbon Composite In order to demystify the properties of carbon-carbon composite materials, pitch-based carbon-carbon composites were investigated in a study by Appleyard et al. (n.d.). The crystallinity of the pitch-based carbon-carbon composites was noted to differ in crystallinity and texture as the temperature are drastically increased. The pore and microcracks also underwent a major change as the matrix and the fibre continued to interact at high temperature. Graphitisation of the outer microstructural turbostratic sheath increased gradually but independently on each constituent composite. Interface stress effects lead to the alignment that is observed in the microstructure as a source of fibre matrix coupling or shrinkage which comes with continued composite carbonization. Figure 3: Microstructure illustration of pitch-based carbon-carbon composite (Appleyard et al., n.d.). Fibre damage is also observed at high temperatures as the relative texture continues to dominate throughout the microstructural form. The texture differs from one phase to the other with adverse effects drawn from the organic matrix precursor fibre-matrix interface effects and the organic matrix precursor. In the anisotropic phase, the microstructural layers are observed to grow with a considerable derivation from the pitches which allows for spheres to shoot at the mesophase stage hence a coarse microstructure. Finer microstructures may be achieved if plastic deformation occurs at mesophase as a result of pyrolytic products, a process similar to carbonization (Appleyard et al., n.d.). Isotropic matrices on the other side have been observed to cause graphitisation at extreme temperatures in an effect rather known as concentration of stress. A combination of fibre-matrix interface and matrix densification causes thermal mismatch due to the arrangement of carbon atoms that are arranged in planar layers that are parallel to the fibre surface. Forces acting on the disc-like molecules lead to strong anchoring effect which induces a magnetic field. The anisotropy of these materials is therefore inclined to the ability to control its own texture throughout the phases. Further microstructural changes have been observed in the anisotropy when graphitisation occurs leading to fold sharpening and eventual polygonalization which is also responsible of the coarse structure (Appleyard et al., n.d.). The mechanical properties that are attached to pitch-based carbon-carbon composites include the flexural strength that is influenced by the fracture behaviour i.e. "brittle-catastrophic" to "pseudo-ductile in accordance to the suggested constituent mix ratio. Matrix cracking due to brittle failure mechanism is also a norm in this material owing to the densification of the HMU composite. This material is also characterised by graphitisation and low temperature oxidation due to the reaction of fibre matrix and the fibre which leads to debonding and sliding. Pitch-based carbon-carbon composites also possess a greater strain capability for large loads making it suitable for brake discs manufacture (Appleyard et al., n.d.). Ceramics-Carbon Composites For ceramics-carbon composites, silicon carbide is discussed to portray the larger picture of this group of composites with respect to their applicability in brake disc manufacture. Good dispersion is evident in silicon carbide carbon nanotubes; a property that offers flexural strength and fractural toughness. Sintering the solid state of silicon carbide to temperatures approximately 2100°C densifies this material. The microstructure of this material is characterised by a coarse values of up to 400Mpa that is brought about by the reduced flaw tolerance. Sintering silicon carbide in its liquid state provides the material with properties that are easily densified at temperatures lower than 2000°C. Metal oxides act as transport media in the sintering stage which eventually leads to liquid phase formation (Sciti & Bellosi, 2000). Figure 4: Surface microstructure of silicon carbide (Guo & Yang, 2005). The silicon carbide properties that result from the structure depend on the starting poweders, the sintering method, atmospheric gases and liquid phase composition. The densification behaviour is however controlled by the fabrication parameters. The intergranular microstructure that remains as a residue of the sintering process softens with increase in temperatures. Densification of silicon carbide results to a tougher and denser microstructure for improved properties in the brake disc manufacture industry (Sciti & Bellosi, 2000). Section C Justification of Brake Disc Manufacturing Methods The manufacture of brake disc is entirely dependent on materials that possess the characteristics which include high temperatures of operation, low noise and friction, durable and low weight. The route that shall be used for the justification of the methods used in the manufacture of brake discs is therefore in accordance to these properties. To begin with, silicon carbide which is a ceramic-carbon composite that has been previously patented by Heine et al. (2000) is prepared through use of short graphite fibres that are meant to impart the quasi-ductile behaviour that is highly desired in this invention. The fact that this process is carried through the impregnation method ensures that no cracks are imparted on the final product. Carbonizing is carried out through the compression method leads to compaction of the material thus lengthening its life. Manufacture of brake discs from aluminium matrix composites is justifiable due to the properties that are achieved while tuning its microstructure. Approaching this issue from a wider perspective, the MMCs are particularly applied in the manufacture of break discs due to thermal characteristics and durability. The thermal properties that are achieved during the manufacture of this material are approximately 5.3 times as compared to SG iron’s specific heat capacity. For railway braking systems this material is actually overrated by a whopping 50°C since the achievable temperatures at the maximum speed is 350°C. The wear characteristics that are impacted by the manufacturing method used in this MMC also ensure that the brake disc lasts 3 times longer (The A to Z of Materials and AZojomo, 2013). Carbon-carbon composites were invented by Duval et al. (2001) for application in brake disc manufacture. This is usually done through superposing of fibre structure layers which are then bonded together through needling. Preforms are however formed through compression of liquid agent for purposes of densifying the fibres. The idea of the inventors was to come up with materials that are totally compatible with the extreme working conditions of brake pads. It is justifiable since these materials juggle with properties of the constituent compounds thereby achieving lightweight, heat resistant and durable brake pads. The inventors of this method of brake pads manufacture aimed at a higher fatigue life with an acoustical insulation coupled with low vibration thereby making the entire method justifiable. The Cylindrical –Vermicular graphite cast iron is usually compacted in order to give the best results within the engineering industry. This manufacturing process renders the microstructure of cast iron highly tensile and hard which ensures that wear is eliminated. Considering this statement, compaction is considered as the best method through which to manufacture cast iron brake discs. An addition of phosphorous in this material strikes a balance to this material bringing the combination to 400Mpa while brake discs only require 300MPa. This difference is highly desirable in order to sustain the operation of the brake discs making the compaction process acceptable. Effects of manufacturing process microstructure and properties The deductions made from the discussion above are that the materials meant for brake disc manufacture are altered in terms of properties in order to ensure they befit the working conditions of brake disc. For example, the compaction of Cylindrical –Vermicular graphite cast iron introduces a tensile property of up to 400Mpa that is not readily available in any other cast iron material. Compaction ensures that braking properties are introduced in this material by altering the microstructure to an acceptable level. The microstructure is affected in that it is made coarse in order to sustain the frictional requirements of stopping a moving vehicle or train. The densification process on the other hand gives the material coarse properties especially in the metal carbon composites. The fact that this process is carried out at high temperatures is meant to instil permanent properties. This is done together with the introduction of short fibres in the material to ensure durability of the material as per the properties requirement of the brake discs. This process also introduces redundant properties towards extreme heat that is produced by friction. The specific heat capacity is also enhanced to the advantage of braking properties connected to porosity and coarseness. Achievement of high temperatures during treatment makes the materials to obtain a stable microstructure that is sustainable to temperatures achieved while breaking. List of References Alp, T., Wazzan, A.A. & Yilmaz, F., 2005. Microstructure - Property Relationships in Cast Irons. The Arabian Journal for Science and Engineering, 30(2B), pp.1-13. Appleyard, S.P., Rand, B. & Ahearn, C.E., n.d. Processing, Structure and Properties of Pitch- based Carbon-Carbon Composites. pp.260-64. Gilles, T., 2005. Automotive Chassis: Brakes, Suspension, and Steering. New York: Cengage Learning. Guo, X.-z. & Yang, H., 2005. Sintering and microstructure of silicon carbide ceramic with Y3Al5O12 added by sol-gel method. Journal of Zhejiang University Science B, 6(3), pp.213–218.. Heine, M. & Neusass, G., 2000. Silicon Carbide Articles Reinforced with Short Graphite Fibers. United States Patent. Maluf, O. et al., 2004. Development of Materials for Automotive Automotive Disc Brakes. Minerva, 4(2), pp.149-58. Peng, H.X., Fan, Z., Mudher, Z. & Evans, J.R.G., 2002. Microstructures and mechanical properties of engineered short fibre reinforced aluminium matrix composites. Materials Science and Engineering A, 335, pp.207-16. Sciti, D. & Bellosi, A., 2000. Effects of additives on densification, microstructure and properties of liquid-phase sintered silicon carbide. Journal of Materials Science , 35(15), pp.3849- 55. The A to Z of Materials and AZojomo, 2013. MMCs, an Alternative Material for Railway Car Braking Systems. [Online] Available at: http://www.azom.com/article.aspx?ArticleID=518 [Accessed 20 December 2013]. Read More