MATERIALS AND PROCESSING ROUTE FOR AIRCRAFT BRAKE SYSTEM

Transcription

1 International Journal of Mechanical and Materials Engineering (IJMME), Vol. 8 (2013), No. 1, Pages: MATERIALS AND PROCESSING ROUTE FOR AIRCRAFT BRAKE SYSTEM M.A. Maleque and U. Abdullahi * Department of Manufacturing and Materials Engineering, Kulliyyah of Engineering, International Islamic University, Malaysia, P.O.Box 10, Kuala Lumpur, Malaysia *Corresponding author s yabi78@yahoo.com Received 25 May 2012, Accepted 20 March 2013 ABSTRACT Research works are being done to substitute heavy material with a suitable lighter weight and cheaper material which has better properties for this particular application. Processing route plays a significant role in developing the material for any application with effective cost and environmental factors and careful selection of processing route is especially very important for brake disc material. In this paper, a comprehensive review has been done on material and processing route in view of selecting alternative lightweight and cheaper materials for aircraft brake disc application. The advantages and disadvantages between aluminium metal matrix composite and other materials as well as the advantages of powder metallurgy route with other processes are presented for better understanding of the materials and process selection. Finally, emphasis has given on lightweight and cheaper carbon nanotube aluminium matrix (CNT-Al) nano-composite material with specific processing route as a case study. This is mainly based on material selection using material selection process which showed CNT-Al as an excellent candidate material for brake disc application. Keywords: Aircraft brake disc, Powder metallurgy, Nano-composite. 1. INTRODUCTION Composite materials are multiple phase materials achieved through the artificial combination of dissimilar materials in order to get properties that the individual components by themselves cannot get. They are not multiphase materials in which the dissimilar phases are formed naturally by reactions, phase transformations, or other phenomena (Maleque et al., 2011). The main benefits that composite components offer are reduced weight and assembly simplification. The performance advantages coupled with reducing the weight of aircraft structural elements has been the major forward motion for military aviation composites development (Don and Wang, 2009). Composites are widely used in commercial aircraft, and the demand of safety and weight in aircraft are much more higher compared to other means of transportations (Manocha, 2003). Safety and weight can only achieved by using materials with high specific properties. Composites vary from conventional materials in that composite parts comprise two distinctly different components fibers and a matrix material (most often a polymer resin) that, when combined, remain discrete but function interactively to make a new material whose properties cannot be predicted by simply summing the properties of its components(aji et al., 2009; Chung, 2010). One of the most important advantages of the fiber/resin combination is its complementary nature. Thin glass fibers, for example, exhibit relatively high tensile strength, but are liable to damage. By contrast, most polymer resins are weak in tensile strength but are extremely tough and malleable. When combined, however, the fiber and resin each counteract the other s weakness, producing a material far more useful than either of its individual components (DeGarmo et al., 1979). From safety point of view brake system is the most important component in both automobile and aerospace industries even more emphasis is given on aircraft braking system. The material for brake systems should have stable and reliable frictional and wear properties at different conditions of load, speed, temperature and environment, and highly durable (Adebisi et al., 2011a). There are many factors to consider when selecting a brake disc material. The capability of the brake disc material to endure high abrasive wear and friction is of great consideration and also withstand high temperature generated due to friction. Weight, cost and processing method are also need to be taken in to consideration. The brake disc must be capable of absorbing thermal heat to prevent distortion and/or crack from thermal stress until the heat is dissipated. The frictional heat generated on the rotor surface can influence excessive temperature rise which in turn leads to undesirable effects, such as thermal elastic instability, premature wear, brake fluid vaporization and thermally excited vibrations (Adebisi et al., 2011b). Many research reported that a quite small amount of nanoscale reinforcement can have a notable effect on the macroscale properties of the composite due to the large amount of reinforcement surface area (Esawi et al., 2009; Morsi and Esawi, 2007; Noguchi et al., 2004). For example, adding carbon nanotubes enhanced the thermal conductivity and electrical properties, heat resistance or mechanical properties such as stiffness, strength and resistance to wear and damage. Generally, wear and frictional resistant, thermal conductivity, and cost of the final product made from a composite of aluminium matrix reinforced with small percentage of carbon nanotubes (CNTs) using different processing routes 14

2 depends on the homogeneously dispersal of CNTs in the matrix. The adhesive bond of the powder particles ensures the strength of the material as well. Kesheri and Agarwal, (2011), investigated the wear behaviour of plasma-sprayed carbon nanotubes reinforced aluminium oxide (Al 2 O 3 ) composite coatings, and they found lowest wear volume loss in the sea water as compared to dry environmental sliding at 25 and 600 O C. Zhang et al. (2009) prepared and studied the mechanical properties of carbon nanotube-alumina nano-composite by chemical vapour deposition and spark plasma sintering methods and achieved a moderate mechanical properties from the nano-composites. However, Liao et al. (2011) did the same work but claimed more improvement in the mechanical properties due to better dispersion and alignment of CNTs in the matrix as well as formation of Al 4 C 3 at the CNT/matrix interface. However, they also comment on the processing conditions of spark plasma sintering (SPS) that CNTs may have damaged or may have reacted with the matrix material. Lee et al. (2006) examined the tensile properties and microstructures of various Al alloys fabricated by pressureless infiltration method under a nitrogen atmosphere. The spontaneous infiltration of molten metal in to the powder bed occurred at a certain temperature within a specific time under nitrogen atmosphere. As a result confirmed that it was possible to fabricate Al Mg alloys reinforced with aluminium nitride (AlN) particles formed by in situ reaction. A considerable strengthening even in the control alloy occurred due to the development of in situ AlN particle even lacking an adding an artificial reinforcement. Strength values of the control alloy were improved with decreasing Al powders in bottom powders bed. In addition, tensile strength in Al Mg alloys was increased due to the presence of Mg content (Lee et al., 2006). Several researchers have explored the opportunity of deformation of powder compacts via shaping and forming process to achieve improved density and distribution and alignment of CNTs in the metal matrix composite (MMC). Kuzumaki et al. (1998) have synthesized CNT-Al composite through hot extrusion of powder compacts at 600 O C. It was found that the CNTs were aligned along the extrusion direction and were strong enough to withstand the extrusion load. However, inhomogeneous distribution of CNTs in the fractured surfaces of the composites suggested poor dispersion of CNTs in the matrix. Rolling process is expected to be a better alternative than extrusion for carbon nanotube reinforced metal matrix composite (CNT-MMC) materials such as CNT-Cu and CNT-Al in terms of alignment of CNT cluster in the matrix (Liao et al., 2011; Salimi et al., 2011). Good quality reinforcement of the composites was confirmed from the appearance of the bridging and pullout of the fracture surface with the improvement in wear and coefficient of friction. However, the assessment on the damage surface due to severe milling on CNT has been ignored. From the previous study, it is revealed that there are scopes to develop an alternative light weight nano-composite with simpler and cost effective processing route. Therefore, the main aim of this paper is to make a review survey on the materials and processing route for aircraft brake disc application, and also find an alternative light weight material through materials selection method with suitable and cheaper processing route for this application. 2. MATERIALS FOR BRAKE SYSTEMS The current materials used for aircraft brake disc are carbon-carbon composite, carbon silicon carbide composite, or carbon-carbon- silicon carbide composite. An anti-oxidant has to be used when processing these materials because they oxidizes readily in air and several precautions have to be taken in to consideration which makes the processing cost higher. The rate of inhomogeneity of those materials is high compared to carbon nanotube aluminium matrix composite. The potential candidates for aircraft brake disc application are described in the following section. 2.1 Carbon/Carbon Composites Carbon/carbon (C/C) composite is the engineering material used in aircraft and it performed excellently compared to other materials such as ceramics, metal, and plastic; it is light and strong and can withstand temperatures over 3000 without any loss in performance (Manocha, 2003). The low density value for C/C composites represents nearly four times the braking power of the forerunner steel brakes (Manocha, 2003). In order to prevent its oxidation, C/C composites need to be protected against oxidation either through matrix modification with silicon, zirconium or by multilayer oxidation protection coatings consisting of silicon carbide. This will lead to the development of other new materials, such as carbon-carbon-silicon carbide and carbon-silicon carbide composite materials. Carbon/carbon composites being of non-homogeneous structure consisting of fibres, matrix and pores, with the first two having a diversity of microstructures, estimation of their thermal transport properties become complex (Manocha et al., 2003). Another drawback is the inhomogeneity and high cost of the material. 2.2 Carbon reinforced silicon carbide (C/SiC) composite This is a development of new material from pure carbon carbon, and can be used in automotive and aerospace applications, such as components of brake systems namely the brake disc and brake pads. C/SiC utilizes silicon carbide with carbon fibre, and this composite is thought to be more long-lasting than pure carbon carbon (Maleque and Karim, 2005). 2.3 Aluminium alloys The Al-Si alloy is an alloy with moderate wear resistance, low thermal-expansion coefficient, fair corrosion resistance, and little improved mechanical properties at a wide range of temperatures. While Al-Be alloys consist of physical and mechanical properties that exceed those of standard aluminum alloys in such areas 15

3 Strength (MPa) as mechanical stability, dampening, thermal management and reduced weight (Maleque and Dyuti, 2010). The performance level of the brake disc will not be achieved with silicon and beryllium-aluminium alloys, though it can be considered for disc application. The main problem is lack of tribological properties which is the major concern in aircraft brake disc. 2.4 Monolithic ceramic materials Monolithic ceramic material can also compete with other candidate materials for aircraft brake disc application. Ceramic brake rotor is fabricated using this type of material which is considered for racing and luxury cars. The disc is produced in two halves from a blend of carbon fibres and liquid polymers thermally compressed in a mould. Once hardened, the rough discs are carbonized by pyrolysis in a furnace with a nitrogen atmosphere at temperature of approximately 1000 o C (Maleque et al., 2012). After that, the discs are siliconized at 1,800 o C under vacuum atmosphere for further improvement of wear and friction coefficient compared to other brake disc material. 2.5 Aluminium-nano-composite Pure aluminium (Al) is not found to possess high mechanical strength, and, hence alloyed with some other metals and reinforced with reinforcement to improve the mechanical properties (Singhal et al., 2012). Due to this reason, Al has a major potential in many weight sensitive and saving applications including aerospace. However, if Al is reinforced with reinforcement will offer much better benefits compared to many alloys in terms of greater strength, improved stiffness, reduced density (weight), high temperature properties, controlled thermal expansion coefficient, better thermal and/or heat management, enhanced and tailored electrical performance, improved abrasion and wear resistance, control of mass (especially in reciprocating applications), and improved damping capabilities (Singhal et al., 2012). Therefore, it is highly recommended to develop Al metal matrix composite with suitable reinforcement such as ceramic particles, fine fibres, and nano fillers such as CNT. Morsi et al. (2010) produced carbon nanotube reinforced aluminium (CNT-Al) matrix composites by a unique powder metallurgy route using spark plasma extrusion (SPE). Many researchers followed different processing routes to achieve uniform and homogeneous dispersion of CNT in the matrix in order to achieve strong bonding and enhance the strengthening of the composite. Therefore, in this paper, powder metallurgy route has been discussed with the intension to fabricate CNT-Al composite for the fabrication of aircraft brake discs. 2.6 Material selection Before selecting any suitable material it is highly recommended to consider the cost of the material. In this study, relative cost of the potential candidate material along with strength properties has been considered for the material selection. The material selection chart has been developed purposely using standard material selection method in order to find out the possible candidate material for brake disc application and details materials selection process can be found in Maleque and Dyuti, 2010) CNT-Al Al-Be alloy SiC Monolithic ceramic C/C Relative cost/ Unit volume (Mg/m 3 ) Figure1 Material selection chart: strength and relative cost of the candidate materials. Figure 1 is a logarithmic graph with strength at the vertical axis and relative cost along the horizontal axis. It shows the range of values for five different candidate materials which have already discussed in the earlier section for aircraft brake disc application with CNT-Al nano-composite as the strongest and cheapest material and carbon-carbon composite material as strong and the most expensive material. Aluminium nano-composite is cheapest compared to all materials because it falls within the highest range of value for strength and lowest value for the relative cost followed by aluminium-beryllium (Al-Be) alloy which is also cheaper but least strong among the candidate materials, however, silicon carbide (SiC) is the stronger and cheaper compared to monolithic ceramic and carbon-carbon composite materials. Though silicon carbide is cheaper and stronger but CNT-Al nanocomposite showed further cheaper and stronger and hence, this could be the most suitable material for aircraft brake disc application. 3. PROCESSING ROUTES FOR BRAKE DISC MATERIALS This section describes the available processing routes for the development of nano composite material emphasizing on the challenges in processing of carbon nanotube reinforced metal matrix composite (CNT- MMC). However, main challenges for CNT-nano composite materials are: Homogeneous dispersion of CNT throughout the metal matrix Interfacial bond strength between CNT and metal matrix Chemical and structural stability of CNT 16

4 3.1 Powder metallurgy route Powder metallurgy (PM) is considered to be the most common and reliable production technique for nanocomposite, which can be characterized by high-quality dimensional and geometrical precision as well as good mechanical properties (Bui et al., 2011). Using PM process, several properties of metal matrix nanocomposite can be improved, such as increase in hardness, wear resistance, mechanical durability, thermal durability, and thermal conductivity and decrease the density of the material. By these improvement of metal matrix nano-composite properties, the material become good quality and can be used in many applications which required good type of materials such as in automotives and in aerospace applications. Powder metallurgy route is sub divided in to four different methods depending on the approach. The basic process steps consist of mixing of nano filler a particles powder by mechanical alloying or grinding, followed by consolidation by compaction and sintering, cold isostatic pressing, hot isostatic pressing, or spark plasma sintering (De Garmo and Sullivan, 1979). However, green composite compacts are subjected to post-sintering deformation processes such as rolling, equi-channel angular processing, extrusion (Haghighi et al., 2012). Irrespective of the process steps, the major focus is on obtaining excellent reinforcement, by achieving homogeneous dispersion of CNT in the metal matrix and good bonding at the metal/cnt interface. Fig 2 shows a typical process steps for powder metallurgy route. Metal powder (matrix) Reinforcement, binder, lubricants Mixing / Blending (also known as precompaction step) Compaction Sintering Finish P/M product Figure 2 Typical powder metallurgy process steps. 3.2 Melting and solidification route This is the most predictable processing route for MMCs, however, has also been identified for synthesizing certain types of nano-composites. However, nano-composite does not allow high temperature which might damage certain nano fillers or formation of chemical reaction product at the interface. Michalowski et al., (2011) used chemical vapour infiltration (CVI) methods to manufacture C/C composites, and also studied the influence of impregnation process as porous 2D carbon fibre substrate with resin and pyrocarbon deposited by CVD technique on mechanical properties of formed composites. The results indicated that using CVI method large pores remain in the matrix resulting in lower mechanical strength. The values of moduli of composites obtained by chemical vapour deposition (CVD) and chemical vapour infiltration (CVI) methods do not differ significantly. Comparison of two methods of fabrication for C/C composite showed that much better strengths can be achieved by forming the carbon matrix in solid state condition. The main shortcoming of the CVI technique is the lengthy processing time of infiltration, usually in the range of a hundred hours. This is coupled with a very slow rate of deposition associated with a relatively low conversion efficiency of the precursor. As mention earlier, CVI technique is disadvantageous due to the long processing time of infiltration. 3.3 Thermal spray Thermal spraying involves spraying of molten or semimolten particles on to a substrate to form a coating and/or deposit by means of impact and solidification. It offers the advantage of cooling rates during solidification. This technique is mostly used to produce coatings on structural materials. Such coatings provide protection against high temperatures, corrosion and wear; they can also change the appearance, electrical or tribological properties of the surface or replace worn material (Bakshi et al., 2010). 3.4 Electrochemical route This method is primarily used for formation of thin composite coatings. It has also been used for coating of CNTs with metals to produce one-dimensional (1D) composites such as nano-wire (Bakshi et al., 2010). However, both electro deposition and electroless deposition can be used for CNT metal matrix nanocomposite fabrication. 3.5 Molecular level mixing This method is capable of producing composite particles or 1D nanostructure of CNT coated with metal (Bakshi et al., 2010). The process requires CNTs to be acid treated and functionalized before introducing them into the metal-salt bath, thus aiding the CNT suspension and surface metal deposition on their surface to produce CNT-metal composite powder (Bakshi et al., 2010). The detailed process sequence shown in Figure 3. Research has been done on these unique processing routes in order to produce CNT-Cu nano composite (Bakshi et al., 2010). Some of these routes are improvisation of the conventional processes whereas others are indigenous and novel techniques, and confirmed to be used in the processing of CNT-MMC materials. 17

5 Table 1 Advantages and disadvantages matrix of powder metallurgy route over other manufacturing routes. Powder metallurgy route Other manufacturing routes Advantages Disadvantages Advantages Disadvantages Difficult to store and handle The material is not the powders necessarily in powder Resource materials are available The compaction dies last longer form High cost of compaction die Furnaces, presses and other equipment last longer Raw materials may not be available for process High cost of maintenance Minor or no scrap Any addition/reduction of material may affect the final product Excess use of material has no effect on the final product High amount of scrap Ability to create composite, mixed and different microcrystalline structure The process requires lubricants, binders, additives Alloying is easy using casting method Difficult to produce high purity material Can attain homogeneous and uniform microstructures Due to porosity, specified mechanical properties are difficult to be obtained Damages of the material may occur Relatively low processing temperature Control temperature need to be ensure Temperature requirement depends on the process and the material use in casting High melting point material processing is difficult Applicable to all classes of material Not economical materials to some Limited to particular class of materials High energy demand and uneconomical to some materials Production of precise part with close tolerances Sintering is compulsory for the green body The produced component may not require heat treatment Machining and finishing need to be done after casting High volume production rates with minimum energy requirement Not economical for small scale production Can suite different volume of production he energy demand may be high Applied to major industries and technology Not economical other industries to some Application depends on the industry or technology Not favourable to all CNT Metal anions Table 2 Raw material utilization and energy requirement for various manufacturing process. CNT CNT-metal salt Salt Drying & Cu-anions calcinations CNT precursor Manufacturing process Melting and solidification Raw material utilization (%) Required energy per kg of finished part (Kw/kg) Powder metallurgy Cold spraying CNT-MMC powder Reduction CNT-metal oxide Hot drop forging Machining process Figure 3 Process of molecular level mixing (Bakshi et al., 2010). 18

6 4. ADVANTAGES OF POWDER METALLURGY ROUTE OVER OTHER PROCESSING ROUTES A wider range of products can be obtained from powder processes to a great extent than from direct alloying of fused materials. The Table I explored the advantages and disadvantages of powder metallurgy route over other processing routes. Powder metallurgy products are nowadays used in a wide range of industries, from automotive and aerospace applications to power tools and household appliances due to the less demands of energy and scrap. From the Table 2, it can be seen that powder metallurgy is a promising processing route for the development of new and advanced materials such as nano-composites which requires less energy with more raw material utilization. 5. CONTRIBUTION TO KNOWLEDGE This study contributed to the areas of advanced materials with high tech processing routes and found the most suitable material and processing route for aircraft brake disc. The idea of selecting the material is clearly explained and presented both graphically and in tabular form in this paper. From the graphical representation of the materials, it is clearly shown that CNT-aluminium nano-composite is the strongest and cheapest among all other five composite materials which fall within the highest range of strength and lowest range of relative cost. The paper immensely provides a better understanding and the advantages of powder metallurgy route over other processes as presented in Table 1 with critical thinking. The table mainly presents the advantages of powder metallurgy over other processes, though it has some short comings as listed in the table such as lubricant and control temperature, sintering of the green body and economic consideration for a small scale production. Other routes can serve as an alternative to overcome the drawback of powder metallurgy, but those methods have their own short comings which are more challenging than that of powder metallurgy. Some of the challenges are excess use of raw material that leads to high amount of scrap, high cost of maintenance, and not favorable to all type of industries and technology. It also help to know the conventional process of producing metal matrix nano-composites apart from powder metallurgy and to discover the material and methods of producing metal matrix nano-composites. Moreover, the paper highlighted the applications of nano-composites in aircraft brake system based on the critical literature survey. It is clearly shown that aluminium nano composite has a higher resistant to wear, less dense and cheaper compared to other candidate materials. 6. SUMMARY The materials and processing routes described are suitable for aircraft brake system development. 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