April 2002
Materials Letters 53 (2002) 244 – 249 www.elsevier.com/locate/matlet
Copper matrix SiC and Al2O3 particulate composites by powder metallurgy technique S.F. Moustafa a, Z. Abdel-Hamid a,*, A.M. Abd-Elhay b a
Central Metallurgical Research and Development Institute, P.O. Box 87, Helwan, Cairo, Egypt b Faculty of Engineering, Helwan University, Cairo, Egypt Received 16 October 2000; received in revised form 28 June 2001; accepted 3 July 2001
Abstract Copper matrix reinforced with either Ni-coated or uncoated SiC and Al2O3 particulate composites were made by means of the powder metallurgy route. The reinforcement particles of SiC and Al2O3 were coated with a thin layer of nickel by electroless method. The coated or uncoated reinforcement particles of either SiC or Al2O3 were added to copper metal powders with nominal loading of 20 wt.%, mixed in a mechanical mixer having 360 rpm for a period of 10 min. Each mixture of the investigated powders was cold compacted at 600 MPa, and sintered at 900 BC, in hydrogen atmosphere. The electroless coating process of the investigated reinforcements is given. Micrographs of Ni-coated particles of SiC and Al2O3, and the microstructure of the sintered composites are also provided. Some physical and mechanical properties of the copper matrix Ni-coated and uncoated particulate composites are measured and explained with regard to its structure. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Copper matrix; SiC; Al2O3; Powder metallurgy technique
1. Introduction Metal matrix composites (MMCs), reinforced with particulate or discontinuous fibres of SiC or Al2O3, are potential candidate materials for a variety applications. The importance of these composites could be attributed to their high stiffness, superior room and elevated temperature strengths, improved wear resistance and low coefficient of thermal expansion [1– 3]. Several processing techniques could be used for the production of MMCs, which could be grouped into
*
Corresponding author. Fax: +20-2-5010-639. E-mail address:
[email protected] (Z. Abdel-Hamid).
two main routes depending on the state of matrix during the fabrication process, either liquid or solid routes [4 – 7]. The production of MMCs by liquid metal processing are receiving a great deal attention because of their low cost, but they suffer from the following drawbacks: (i) nonuniform distribution of ceramic particles or fibres due to the agglomeration and dendritic segregation, and (ii) undesirable chemical reaction at the interface due to the high temperature of the melt. These drawbacks could be minimized using powder metallurgy route. Powder metallurgy process (PM) lends itself well for economical mass production components. Different metal matrix composites are manufactured by this PM route [8 – 10]. However, relatively poor physical and mechanical properties
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are obtained due to the existence of a large amount of residual porosity due to the nonwetting characteristics between metal matrix and SiC and Al2O3 particles [11]. The objectives of the present investigation are: (i) to prepare Cu-matrix composites reinforced with 20 wt.% of either Ni-coated or uncoated powders of SiC or Al2O3, and (ii) to demonstrate the superiority of Nicoated powders reinforcements over the uncoated powders reinforced composites. The reason of selecting Ni as a coating material instead of Cu-coating is that Ni-coating has better adhesion on SiC and Al2O3 powders than Cu-coating. Also, nickel forms a solid solution with copper, i.e. nickel is compatible with both reinforcement materials and Cu-matrix.
2. Experimental 2.1. Materials The materials used in this investigation were highly pure SiC powders of a-type made by Carborundum, Perth Amboy, NJ, and Al2O3 powder of a-
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type of high purity made by Aldrich, Brockmann 1, STD grade, CA. The copper powders used in this investigation were of electrolytically precipitated powders and made by Ecko, Germany. The powder sizes were nominally minus 200 mesh. Powders of SiC and Al2O3 were etched in nitric acid, washed with distilled water and dried. 2.2. Coating of powders Powders of SiC and Al2O3 were coated separately with nickel by electroless method. The surface of the investigated powders were sensitized and activated prior to nickel coating. The treatment consists of the following steps: (1) cleaning the surface of powders in an organic solvent such as acetone for 15 min, (2) sensitization in an aqueous solution consists of 10 g/l SnCl2 H2O and 30 ml/l HCl for 15 min, (3) then activated in an aqueous solution of 0.25 g/l PdCl2 and 3 ml/l HCl for 15 min, and (4) rinsing the activated powders in boiling water, and drying [12]. The dried powders were gently dispersed in an electroless bath containing a solution of 45 g nickel chloride, 8 g sodium hypophosphite, 100 g sodium
Fig. 1. Variation of dimensional change as a function of sintering time of: Cu – 20% coated SiC, Cu – 20% SiC, Cu – 20% coated Al2O3, and Cu – 20%Al2O3 composites.
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Table 1 Values of relative density and porosity % of: Cu – 20% coated SiC, Cu – 20% SiC, Cu – 20% coated Al2O3, and Cu – 20% Al2O3 composites Material composition
Relative density
Porosity (%)
Cu – 20% Cu – 20% Cu – 20% Cu – 20%
0.95 0.87 0.975 0.92
5 13 2.5 8
coated SiC SiC coated Al2O3 Al2O3
citrate, 50 g ammonium chloride dissolved in 1 l distilled water [13]. After the nickel deposition, the powders were washed with cold water, dried in air, and weighed. 2.3. Composites fabrication Powders of 20 wt.% of SiC or Al2O3 (coated or uncoated) were each mixed with Cu-metal powder in conical flask using a mechanical stirrer of 360 rpm for 6 min. Four mixtures were obtained, namely Cu – 20% coated SiC, Cu – 20% SiC, Cu – 20% coated Al2O3, and Cu – 20% Al2O3. Each mixture was cold compacted in a floating die using hydraulic press at compaction pressure of 600 MPa. The compacts were in the shape of cylinders of 8 mm in diameter and 12 mm in length, with tolerances of 0.1 mm in length direction. The compacts were sintered in a horizontal tube furnace at 900 BC in pure hydrogen atmosphere.
Microstructural investigation was performed using optical and scanning electron microscopy. Property measurements included density and porosity. Compression strengths of these composites were measured using Schematzu testing machine at strain rate of 0.2 mm/mm min. Samples were compressed between graphite-lubricated steel disks at room temperature.
3. Results and discussion 3.1. Densification process Fig. 1 compares compact shrinkage as a function of time for Cu – 20% coated SiC, Cu – 20% SiC, Cu – 20% coated Al2O3, and Cu – 20% Al2O3 composites. It is observed that coated powders containing composites shrink during sintering and reach an almost dimension stability after about 24 min sintering time. However, in the case of uncoated powders containing composites the samples exhibit swelling during sintering. Dimensional stability during sintering of uncoated composites reached after about 60 min. It is evident that the densification of compacts made from Ni-coated powders are much faster than those made from regular powder mixtures by about 2.5 times. Lack of both wettability and solubility of SiC and Al2O3 (uncoated powders) with copper prevent densification by grain shape accommodation
Fig. 2. Micrographs of powders of: (a) Ni – coated SiC, (b) Ni – coated Al2O3.
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and solution-reprecipitate. Therefore, for such systems, the contribution to densification by coalescence of Cu-powder become very significant [14]. The uncoated powder composites are characterised by solid-state sintering between the Cu-particles during heating which results in a copper skeletal structure. Contrary to that, Ni-coated powders containing composites sintered by heterodiffusion mechanism. In such blends, the heterodiffusion takes place at the Cu – Ni contacts, and the Cu-skeleton shrinks because of the shrinkage at the Cu – Cu contacts is high. Table 1 reports the values of relative density and porosity
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content of the investigated composites. It is clear that uncoated powders of either SiC or Al2O3 have higher porosity content due to the lack of wettability. 3.2. Microstructure Fig. 2a and b shows micrographs of Ni – coated SiC and Ni– coated Al2O3 powders. The coating procedure used in this study resulted in a batched Ni-precipitation on the surfaces of either SiC or Al2O3 particles. The phosphorous concentration was very limited and it was less than 0.03%.
Fig. 3. Microstructures of: (a) Cu – 20% coated SiC, (b) Cu – 20% SiC, (c) Cu – 20% coated Al2O3, and (d) Cu – 20% Al2O3 composites.
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The microstructures developed in the sintered composites reveals the differences in their processing parameters. Fig. 3a – d show micrographs of Cu – 20% coated SiC, Cu – 20% SiC, Cu – 20% coated Al2O3, and Cu – 20% Al2O3 composites, respectively. Fig. 3a shows a very good adhesion between SiC particles and Cu-matrix, but uncoated SiC graph, Fig. 3b, reveals that the adhesion between SiC particles and Cu-matrix is poor, due to the presence of gaps and porosity between SiC and Cu-matrix. Almost no detectable porosity and very good adhesion between particles and Cu-matrix are observed in Cu-coated Al2O3 composite, as indicated in Fig. 3c. In the case of Cu-uncoated Al2O3 composite, Fig. 3d, gaps between Al2O3 particles and Cu-matrix are observed. Metallographic observation of composites produced by uncoated reinforcement of both cases of SiC and Al2O3, as those shown in Fig. 3b and d, revealed that both closed and interconnected porosity are present. The presence of porosity in the case of Cu-uncoated reinforcement composites could be attributed to the masking or at least partially masking of Cu powder with fine dust of SiC or Al2O3 which was generated during the mixing process of uncoated reinforcement and Cu due to the wear abrasion of reinforcements during their contacts with each other or with the wall of the mixer. When these Cu-powders which covered with dust were investigated using scanning electron microscope, the dust powders were seen at the start of investigation, but after few seconds they fly like ashes and disappeared. The emitted electrons of the microscope hit the dust, and due to the absence of adhesion or bonding between copper powders and the dust of SiC or Al2O3, they fly in the evacuated chamber of the microscope. During sintering, the Cu/ Cu contacts became less, but SiC/SiC or Al2O3/ Al2O3 contacts increased due to the partially covered Cu powder with reinforcement dust, which resulted a weak interfacial bonding between SiC or Al2O3 and Cu-matrix. 3.3. Compression strength Compression strengths were determined at 20 BC on samples of Cu – 20% coated SiC, Cu – 20% SiC, Cu –20% coated Al2O3, and Cu – 20% Al2O3 composites. The yield stress measured as 0.2% proof stress for all composites, either coated or uncoated ones. It
Table 2 Compression properties of: Cu – 20% coated SiC, Cu – 20% SiC, Cu – 20% coated Al2O3, and Cu – 20% Al2O3 composites Materials composition
0.2% proof stress (MPa)
Fracture strength (MPa)
Elongation (%)
Cu – 20% Cu – 20% Cu – 20% Cu – 20%
83 16 62 14.1
344 135 285 112
43.6 30.4 48.4 33.2
coated SiC SiC coated Al2O3 Al2O3
was noticed that all compressed samples, either coated or uncoated, were fractured at an inclined angles of 45B, which means that fracture is taking place by a shear mode. The results compiled in Table 2 demonstrate that the yield and breaking (fracturing) strengths of the coated composites are much higher than those of uncoated composites. Also, it is noted that the compression strains of coated composites are higher than those of uncoated composites. The improvements of compression properties of the coated composites could be attributed to the good adhesion between coated particles and the Cu-matrix, in addition to the higher density and lower porosity.
4. Conclusions This investigation provides insights into densification and compression properties in Cu – 20% coated SiC, Cu – 20% SiC, Cu – 20% coated Al2O3, and Cu – 20% Al2O3 composites. The Cu-composites fabricated by mixing uncoated powders of SiC or Al2O3 exhibit swelling during sintering at constant temperature of 900 BC. However, Ni-coated powders containing composites shrink during sintering. The densification of compacts fabricated from coated powders is much faster with 2.5 times than those made from uncoated powders. The Cu matrix Nicoated reinforced composites have higher relative density and lower porosity content than the uncoated composites, due to the good adhesion between the reinforcements and the Cu-matrix. Yield and breaking compression strengths of coated reinforcement powders containing composites are superior than those of uncoated ones.
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