IMPROVEMENT ON THE TRIBOLOGICAL CHARACTERISTICS OF PARTICULATE COPPER SILICON CARBIDE COMPOSITES

Transcription

1 IMPROVEMENT ON THE TRIBOLOGICAL CHARACTERISTICS OF PARTICULATE COPPER SILICON CARBIDE COMPOSITES David Esezobor 1 and Atinuke Oladoye Department of Metallurgical & Materials Engineering, University of Lagos, Nigeria. esezobordave@yahoo.com 1, modesolawal@yahoo.co.uk 2 Keywords: Tribology, Hybrid, Particulate, Stir cast, Copper, Silicon carbide, Composite Abstract Particulate Copper Silicon carbide (Cu-SiCp) composites find application as wear and heat resistant materials in electrical sliding contacts such as in homopolar machines, railway overhead current collector systems where high electrical/thermal conductivity combined with good wear properties is required. However, challenges occur during machining due to the presence of hard reinforcement in the matrix. This may lead to high turnover of tool wear and poor surface finish. The adoption of near-net shape technology to produce such hard-to-machine metal matrix composites and subsequent finish machining with its attendant cost has reported limited success. This paper critically appraises the challenges and opportunities in the improvement of tribological characteristics of particulate Cu-SiC composites and identifies low cost reinforcement material that could be used to improve its tribological characteristics. Introduction Tribology consideration is increasingly becoming a decisive factor in the design and selection of materials for applications involving interacting surfaces in relative motion. The consideration is mainly on the combined effects of friction and lubrication on material s wear integrity and performance amongst other. Annual losses due to tribology in the US industries are estimated at between 1-2% of the Gross National Income [1], in Russia it is about 1-4% of the GDP [2]. Thus, the attributes of tribology has become imperative in modern machinery especially those applications involving sliding and rolling surfaces, and electrical contacts [3-8].These are readily found in homopolar machines, internal combustion engines, aircraft engines, gears, cams, seals and railway overhead current collector systems [9-12]. In these systems, the dissipation of frictional heat arising from the interacting surfaces as quickly as possible imposes constraints on the choice of materials for such application to prevent softening, plastic and shear flow and the transfer of metal as wear debris onto the interacting surfaces. If these possibilities occur, system performance is compromised and premature failure may result. Materials for these systems are expected to possess high electrical and thermal conductivities, low coefficient of thermal expansion, good corrosion resistance and high melting point [3-8, 13]. Copper metal/alloy fits perfectly into these requirements. However, it exhibits low hardness, low tensile and poor creep strength at both low and elevated temperatures. It thus manifests very poor wear properties in all conditions [9-12]. Based on this, monolithic copper alloys are rarely used, rather copper metal matrix composite (Cu-MMC) in which hard ceramics materials such as SiC, 827

2 Al 2 O 3,TiB 2, boric acid, B 4 C (2-20) are incorporated as reinforcement have been reported with improved strength and wear resistance at low and elevated temperature. MMCs are generally classified according to the nature of reinforcements which could take the form of either discontinuous (particulates, platelet, whiskers or chopped fibres) or continuous (long fibres form) [13]. MMCs are characterised by high stiffness, high strength, good creep strength, high hardness and low coefficient of thermal expansion. Recently, they have come into prominence due to low cost of processing and isotropic nature of properties with acceptable performance [5,10-11].Among the disperoid particles used in reinforcing MMCs, SiC is widely reported [3-12], it is derived from rice hulls and can be produced in large quantities [13]. Particulate Cu-SiC composites find applications as wear and heat resistant materials in electrical sliding contacts such as in homopolar machines [5-11], railway overhead current collector systems where high electrical/thermal conductivity combined with good wear properties is required [6,19]. Cu-MMCs may be applied in supersonic speed vehicles, like rockets and aircraft, where they can be used as the attack edges of wings and the facing material in combustion chambers, as well as heat sink materials for fusion energy applications. Cu-MMCs are also candidate materials for the next generation of thermonuclear reactors [18]. Inspite of these attractive properties and potentials of particulate Cu-MMC in tribological applications, challenges occur during machining due to the presence of hard reinforcement in the matrix. This may lead to high turnover of tool wear and poor surface finish. The adoption of near-net shape technology to produce such hard-to-machine metal matrix composites and subsequent finish machining with its attendant cost has recorded limited success. Further more, SiC has been reported to deteriorate the thermal conductivity of Cu-MMCs despite its many advantages [4-5, 21].Recently, the incorporation of solid lubricants into the Cu-SiC particulate composites has resulted in improved wear resistance.however; this has been at the expense of thermal and mechanical properties [ 20, 22-3]. The mechanism and science of this composite is still under investigation. This paper critically appraises the challenges and opportunities in the improvement of tribological characteristics of particulate Cu-SiC composites and identifies low cost reinforcement material that could be used to improve its tribological characteristics. Particulate Copper Silicon Carbide Composite. Cu-SiC p can be produced by liquid state, solid state and deposition processes. Although, the method adopted for Cu-SiC p production may vary, an important factor to be considered irrespective of production route is the wettability of Cu and SiC reinforcement asides the cost and complexies involved in production. Liquid metal infiltration, pressure infiltration, squeeze casting, diffusion bonding, electro-deposition, powder metallurgy, and stir-casting have been of wide application. However, other methods of production like in-situ casting has not been explored [3-9, 18, 24-26]. 828

3 In the production of Cu-SiC p, a low contact angle between copper and SiC is required for effective bonding. A contact angle of C is reported at C [25, 31]. At higher temperatures, interfacial reactions and formation of sillicides have been reported. The infiltration of active elements such as Titanium decreased the contact angle of the Cu/SiC system by a reduction in the interfacial energy [25]. Coating of the reinforcement by electro-less deposition has also been employed for effective bonding of the reinforcement to the matrix. Metallic powders including Cu [11-12, 17-18], Ni [6, 12-13], Mo [3-4, 8, 21], W [24], Ti [27] and Diamond [3-4, 8] have been reported to be effective. An effective bonding between the reinforcement and the matrix is essential in tribological application to avoid the pull-out of reinforcement during service thereby resulting in increased wear during use of such appliances. Copper based matrix composites are used as heat sinks in thermal applications. For effectiveness, a high thermal conductivity is expected but at temperatures needed for the production of Cu- SiCp, Silicon attacks Copper as depicted in equation 1 [3-4,8].SiC is not stable when in contact Cu SiC Cu( Si) C [1] at elevated temperatures ( C). Silicon is partially dissolved in copper forming a copper (silicon) solid solution while pure carbon remains at the Cu/SiC interface [3-4, 8, and 22]. Dissolved silicon reduces dramatically the thermal and electrical conductivities of Cu/SiCp composites (Table I). Therefore, diffusion barriers are necessary to prevent the detrimental interfacial reaction between copper and SiC. Table I: Effect of SiC addition on the thermal conductivity of Cu [29] Reinforcement Matrix Thermal Conductivity,W/mK Coefficient of Thermal Expansion(ppm/k) Density --- Cu SiC Cu SiC Al The machining of Cu based composite has been a major challenge limiting its wide use in electronic packaging where complex shapes are needed. In most cases, it is been replaced with Al/SiC even though Cu/SiCp composites possess higher thermal conductivity than Al based MMCs [Table I]. The machining of Al/SiC is widely reported in the literatures [28-9]. However, studies on the machining of Cu MMCs are yet to be explored. The incorporation of solid 829

4 lubricants to improve the wear and friction properties of Copper based composites is widely reported. Boric acid [18] and graphite [18, 21, 23-24] have been incorporated into copper matrix to improve the wear resistance and machining of Cu MMC. These second generation of composites are called hybrid copper based composites. Tribology of Cu-SiC p The principal tribological parameters that control the friction and wear performance of Cu based composites and MMCs generally irrespective of production route can be classified into two categories: 1. Material factors which are intrinsic to the material undergoing surface interaction. These include the nature of reinforcement, the size and distribution of reinforcement, the reinforcement shape, the reinforcement / matrix interfacial bond and the reinforcement volume fraction [5, 31-33]. 2. Mechanical and physical factors which are extrinsic to the material undergoing surface interaction such as applied load, the sliding velocity, the sliding distance, the environmental temperature, the surface finish and the counterpart [6,31-33]. Generally, volume fraction and particle size of the reinforcing particles are very important factors that affect the wear characteristics of composites. An increase in volume fraction and a decrease in particle size of the reinforcement will result in an increase in wear resistance of the composites even at high temperatures [8]. The effect of wearing conditions (distance, and applied load, and sliding speed) on MMCs is summarised by the Archard equation of wear. [41]. kwx V [2] H where, V is the wear volume, W is the normal load, x is the sliding distance, H is the hardness of the flat material, and K is the probability of formation of a wear particle during a particular asperity interaction called wear coefficient. From the above expression, it is clear that the wear volume is directly proportional to applied load and sliding distance and inversely proportional to the hardness. Studies involving the effect of these factors abound in the literatures [6, 10-12]. The fabrication and wear performance of Cu/SiCp is strongly influenced by the reinforcementmatrix interface. Proper bonding between Cu and the reinforcement can attain good load transfer between the phases. This is achieved by the electro-less coating of reinforcement with metallic powders resulting in an improved wear resistance. Coated Copper based composites were reported to exhibit better mechanical properties than uncoated ones [7, 12, 13, 18, 22]. This was attributed to stronger interfacial bonding between the reinforcement particles and the matrix. 830

5 The interfacial bonding between Cu and SiC is a weak one characterised by a high contact angle of C. TEM images of the uncoated Cu-SiC composites revealed thin layer of fissure between Cu and SiC [8, 21]. On the introduction of an intermediate layer of Mo, the TEM images (Figure 1b) showed a good adhesion of the nano-crystalline Mo coating to the SiC particles. Figure 1 (a) TEM images of an interfacial region in copper composite with uncoated SiC particles. (b) Interfacial region in copper composite with an intermediate layer of Mo. (TEM specimen prepared using standard procedure involving (a)dimpling and ion polishing (b) and focused ion beam milling) [8] An extensive study on the coating of reinforcement is therefore needed. It is pertinent to note that optimum thickness and uniformity of coating on the reinforcing particles has not been achieved by any of the studies. Moreover, the cost implication of both the powder and the coating process need to be considered. Tribology of Hybrid Cu-MMCs Hybrid particulate copper silicon carbide composites containing graphite have reduced coefficient of friction and increased wear resistance as such they have higher lifespan than mono reinforced composites [6, 11, 12, 13, and 21] and find applications in devices where liquid lubrication is to be avoided. Zhan and Zhang [22] provided the foundation for exploration into hybrid Cu-SiC p by incorporating graphite, since then a lot of research has gone into hybrid copper silicon graphite composites (Cu-SiC-Gr).Graphite improves the wear resistance and anti-friction properties of Cu-SiC p by forming a graphite rich mechanically mixed layer (MML) thus reducing the direct contact area between the composite and the counter face [23]. However, a major set back has been the deterioration of mechanical properties of these hybrid composites, hence the need for reinforcement that will improve the tribological behaviour of copper composites without having any adverse effect on the mechanical properties. 831

6 Fly ash, a waste product generated by combustion of coal in thermal power plants is a good reinforcement to be considered. Fly ash possess certain characteristics that make it suitable for use in friction composites which include fine size particles (mean size mm), uniform physical and engineering characteristics, low specific gravity in the range of 2 3 as compared to other reinforcements used in metal matrix composites. Fly ash particles are typically generated at very high temperatures, i.e C upwards hence, they should provide a thermally stable bulk for high-temperature environments that friction composite experiences, also it is readily available for negligible cost (Figure 2) as coal-fired power plants all over the world generate huge amounts of fly ash each year, in which only a small percentage of the fly ash is beneficially used in applications such as concrete, embankments, road sub base, and land filling etc. [37-40] SiC Al2O3 Graphite 3246 Fly Ash Series1 Figure 2: The price of commonly used reinforcements in MMCS [39] The incorporation of fly ash into aluminum has produced a new generation of Al-MMCs known as aluminum alloy--fly-ash (AL FA) composites; these have been reported to have low density, adequate properties for several automotive applications, enhanced wear and corrosion resistance with increasing volume of fly ash [36-40]. It is proposed that the incorporation of fly ash into Cu/SiCp composites will decrease the density and coefficient of thermal expansion of Cu as obtained in Aluminium. These properties are required for use of Cu composites as thermal management materials. Also a reduction in energy content required in producing Cu composites castings is expected. Conclusion Hybrid particulate Cu- SiC-Gr composites have increased wear resistance with increasing volume of graphite thus, enhancing its use in tribological applications. However, these composites have poor mechanical properties which further enhance failure in service. The incorporation of fly ash in Cu matrix is anticipated to result in enhancement of wear and corrosion resistance as well as reduction of the cost of production of Cu-MMCs. T he resultant conservation of materials is anticipated to reduce energy consumption and pollution. References 1. Karl-Heinz Zum Gahr, Microstructure and Wear of Materials, Tribology Series, 10 (Amsterdam; Elsevier science Publishers, 1987),

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