FABRICATION OF HIGH VOLUME FRACTION ALUMINA/ALUMINUM COMPOSITES VIA REACTION SINTERING ROUTE

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1 Journal Keyvan of Chemical Piri, Mahdi Technology Shafiee and Afarani, Metallurgy, Ghodratollah 52, 1, 2017, Roudini FABRICATION OF HIGH VOLUME FRACTION ALUMINA/ALUMINUM COMPOSITES VIA REACTION SINTERING ROUTE Keyvan Piri, Mahdi Shafiee Afarani, Ghodratollah Roudini Department of Materials Engineering Faculty of Engineering University of Sistan and Baluchestan Zahedan, Iran Received 14 April 2016 Accepted 29 September 2016 ABSTRACT А new kind of alumina-aluminum ceramic matrix composites was successfully fabricated by reaction bonding process. Тhe mixed alumina powders of different aluminum concentrations were initially packed by vibration and uniaxial pressing methods. The packed samples were sintered at 1300 C and 1400 C for 2.5 h. The effect of the powder compaction methods, the aluminum concentration (mass %) and the sintering temperature on the porosity, density, compressive strength and flexural strength was investigated. Structural and microstructure analyses were performed by X-ray diffraction and scanning electron microscopy techniques, respectively. The results showed that the aluminum content increase up to 50 mass % increased significantly the compressive strength the up to 50 MPa. Keywords: ceramic matrix composite, reaction sintering, compressive strength, alumina, aluminum. INTRODUCTION Processing of ceramics is performed by the consolidation of loose powders to form polycrystalline samples. Most of ceramics are sintered using solidstate or liquid-phase sintering [1, 2]. Ceramic powders sintering with the application of conventional methods often causes problems of distortion and residual stresses in the sintered parts. These problems can be reduced significantly by methods such as reaction sintering [3]. The latter refers to a chemical reaction of the starting powder materials in the course of a thermal treatment leading to production of new materials. It can be used for fabrication of both single phase and composite materials [1]. To form a composite body, a mixture of at least two kinds of powders is required for the reaction. The most important advantage of this process is the production of a uniform structure and composite with minimal shrinkage [4-7]. The reaction sintering method is a reliable method for producing alumina composites based on aluminaaluminum powders mixture oxidation. It is referred to as reaction bonded aluminum oxide (RBAO) production [8-10]. The sintering parameters [11-15] and the properties of alumina bodies sintered [16-18] by RBAO method has been investigated, but the method s green effect has received less attention. This is considered in the present work along with aluminum additives and sintering temperature effects on the alumina-aluminum composites production and properties. EXPERIMENTAL High purity α-al 2 primary particles (WDR4, Indal Chemical, d 50 = 1 μm, BET surface area of 1.5 m 2 g -1 and a purity of 99.4 %) and Aluminum powder (Khorasan Metallurgy, d 50 = 56 μm and purity of 95 %) were used as starting materials. The alumina and aluminum powders of three different concentrations of aluminum 10 mass 81

2 Journal of Chemical Technology and Metallurgy, 52, 1, 2017 %, 30 mass % and 50 mass % were initially mixed gently. Green compacts were formed using uniaxial pressing and vibration methods. The powders used for subsequent pressing were uniaxially pressed in a steel die at pressure of 4.5 MPa and 9 MPa, while those used in case of vibration application were poured in graphite molds and then vibrated for 5 min with 50 Hz shaker. Cylindrical specimen of 10 mm diameter and 50 mm height were produced for compressive tests. Rectangular compacts with mm 3 in size were fabricated for flexural test. Finally, the green compacts samples were sintered at 1300 C and 1400 C. Heating rates from ambient to 1100 C and from 1100 C to the sintering temperature were 10 C min -1 and 5 C min -1, respectively. The apparent density of the sintered specimens was calculated using their dimensions and weight values. Three point flexural strength measurements were performed with a mechanical testing machine (Instron model 4208). The structure and microstructure analyses of the samples were performed using XRD (Unisantis XMD400) and FE-SEM (HITACHI, S4160) techniques, respectively. All experiments were performed at least 3 times. The values obtained refer to the mean one ±SD. a) b) RESULTS AND DISCUSSION The preliminary experimental results show that the vibration is not an appropriate method for the preparation of alumina/aluminum composites when compared to pressing. This is due to the weak attachments of Al and Al 2 particles. As illustrated by the macro-image shown in Fig. 1, the composites formed by vibration are more porous than the pressed one. Moreover, these composites have weaker and a smaller number of connections than the pressed ones. Besides, the fibrous Fig. 1. Macro-image of samples formed with a) vibration; b) pressing methods. Fig. 2. Microstructure of samples sintered at 1400 C formed with applied pressing load 9 MPa: a) vibration, b) pressed. shaped solidified aluminum is not strong enough to keep tightly the alumina particles (Fig. 2). The theoretical and experimental density values of the composites obtained at different sintering temperatures are listed in Table 1. The composites have a lower theoretical density than that obtained experimentally due to residual porosities in the sintered samples. The continuous trend of apparent density increase and porosity decrease can be related to the oxidation reaction during the sintering process. Thus the pressing method of different applied loads was selected for the further investigation. 82

3 Keyvan Piri, Mahdi Shafiee Afarani, Ghodratollah Roudini Fig. 3. Density vs Al concentration (mass %) for samples sintered at different temperatures formed with applied pressing loads of a) 4.5 MPa; b) 9 MPa. Fig. 4. SEM micrographs of the sample with 50 % Al sintered at 1400 C formed with applied pressing loads of a) 4.5 MPa, b) 9 MPa. Figs. 3a and 3b show the density as a function of Al concentration (mass %) for samples sintered at 1300 C and 1400 C formed with applied pressing loads of 4.5 MPa and 9 MPa, respectively. As illustrated, the increase of the aluminum concentration and the green compaction pressure brings about a slight increase of the composites density. The compaction methods affect the oxidation reaction rate and the resulting aluminum oxide obtained. At a higher pressure the contacts number between aluminum and alumina increases, the rate of the reaction between the particles increases in the course of sintering and consequently less porosity is reached (Fig. 4). Moreover, two competitive mechanisms can be observed with sintering temperature increase: (1) samples density decrease due to oxidation of Al and formation of alumina of a lower molar volume; (2) progress of liquid phase sintering resulting in density increase. The second effect can be attributed to the formation of a greater amount of a liquid phase. The experimental observation illustrates that all samples are expanded after sintering. Figs. 5a and 5b illustrate the samples diameter increase versus Al concentration. Several factors can affect the dimensions of the sintered samples. These are the green density, the oxidation as well as the sintering progress. As mentioned earlier, the oxidation of Al and alumina formation lead to expansion. In case of a higher aluminum concentration, the expansion occurs due to increase of alumina 83

4 Journal of Chemical Technology and Metallurgy, 52, 1, 2017 Fig. 5. Expansion vs Al concentration (mass %) for samples sintered at different temperatures formed with applied pressing loads of a) 4.5 MPa; b) 9 MPa. Fig. 6. Compressive strength versus Al concentration (mass %) for samples sintered at different temperatures formed with applied pressing loads of a) 4.5 MPa; b) 9 MPa. Fig. 7. Flexural strength versus Al concentration (mass %) for samples sintered at different temperatures formed with applied pressing loads of a) 4.5 MPa; b) 9 MPa. 84

5 Keyvan Piri, Mahdi Shafiee Afarani, Ghodratollah Roudini c) Fig. 8. SEM micrographs of the samples sintered at 1400 C formed with applied pressing load 9 MPa: a) 10 % Al, b) 30 % Al, c) 50 % Al. Fig. 9. XRD patterns of samples sintered at different temperatures formed with applied pressing load 9 MPa: a) 50 % Al, b) 30 % Al. formation in the course of the oxidation reaction and decrease of aluminum liquid phase. In the other words, the volume expansion due to oxidation can overcome volume shrinkage during sintering. At higher temperatures, the progress of the oxidation reaction leads to further expansion. But in presence of a low concentration of aluminum, sintering dominates the oxidation. Compressive strength and flexural strength results of the composites at different sintering temperatures of 1300 C and 1400 C are shown in Figs. 6 and 7, respectively. The strength of the composites is related to the initial green density, the composition and the sintering temperature. The strength of composites of a constant composition increases with temperature increase due to porosity decrease. The strength of the samples increases also with aluminum concentration increase. This effect is attributed to the greater volume fraction of molten aluminum in the composite during in the course of sintering and consequently to improved contacts between the particles and more uniform distribution of pores (Fig. 8). Figs. 9a and 9b show X-ray diffraction patterns of samples containing 30 % and 50 % of Al sintered at different temperatures. It is seen that alumina (Card No: ) and aluminum (Card No: ) are the two main phases present. As expected, alumina formation is facilitated with sintering temperature and aluminum concentration increase. CONCLUSIONS Green compaction of samples plays a fundamental role in fabrication of alumina/aluminum composites. Vibration is found an inappropriate method in this respect. It is found that density increases while porosity decreases as a result of the sintering and the oxidation reaction proceeding. No significant changes in the density and porosity are observed in the range of sintering temperatures selected. On the other hand, the expansion due to oxidation can overcome the shrinkage determined by sintering and the volume of samples increases, while 85

6 Journal of Chemical Technology and Metallurgy, 52, 1, 2017 the density decreases. The strength of the composites increases with temperature increase because of porosity decrease brought about by the sintering progress. REFERENCES 1. M.N. Rahaman, Ceramic Processing and Sintering, 2nd edn., 2003, Marcel Dekker, New York. 2. M.W. Barsoum, Fundamentals of Ceramics, McGraw- Hill, New York, V. Hlavacek, J.A. Puszynski, Chemical Engineering Aspects of Advanced Ceramic Materials, Ind. Eng. Chem. Res., 35, 1996, N. Claussen, T. Le, S. Wu, Low Shrinkage Reaction- Bonded Alumina, J. Eur. Ceram. Soc., 5, 1989, S. Wu, N.A. Travitzky, N. Claussen, Processing of Reaction-Bonded Al 2 (RBAO) Composites, in Structural Ceramics Processing, Microstructure and Properties, ed. by J.J. Bentzen, J.B. Blide-Sorensen, N. Christiansen, A. Horsewell, B. Ralph, Riso National Laboratory, Roskilde, Denmark, 1990, J.S. Haggerty, Y.N. Chiang, (1990) Reaction-Based Processing Methods of Ceramics and Composites, Ceram. Eng. Sci. Proc., 11, 1990, P. Griel, S. Seibold, Active Filler-Controlled Pyrolysis (AFCOP) - A Novel Fabrication Route to Ceramics Composites Materials, in Ceramic Transactions. 19, Advanced Composite Materials: Processing, Microstructure, Bulk and Interface Properties, Characterization Methods and Applications. ed. by M.D. Sachs, American Ceramic Society, Westerville, 1991, A.J. Gesing, G. Burger, E. Luce, N. Claussen, S. Wu, N.A. Travitzky, Preparation and Characterization of RBAO - Matrix SiC Particular Filler Composites, Ceram. Eng. Sci. Proc., 11, 1990, S. Wu, A.J. Gesing, N.A. Travitzky, N. Claussen, Fabrication and Properties of Al-Infiltrated RBAO - Based Composites, J. Eur. Ceram. Soc., 7, 1991, N. Claussen, Processing, Reaction Mechanisms and Properties of Oxidation Formed Al 2 Matrix Composites, Journal de Physique, 3, 1993, J. Luyten, J. Cooymans, P. Diels, J. Sleurs, RBAO Used as Part of a Membrane Configuration. In: Third Euro-Ceramics, 1, ed. by P. Duran, J.F. Fernandez, Faenza Editrice Iberica, S.L. Spain, 1993, J. Luyten, J. Cooymans, C. Smolders, S. Vercauteren, E.F. Vansant, R. Leysen, Shaping of multilayer Ceramic Membranes by Dip Coating, J. Eur. Ceram. Soc., 17, 1997, S. Wu, D. Holz, N. Claussen, Mechanism and Kinetic of Reaction-Bonded Aluminum Oxide Ceramics, J. Am. Ceram. Soc., 76, 1993, M. Parlier, M.H. Ritti, A. Jankowiak, Potential and Perspectives for Oxide/Oxide Composites, Journal Aerospace Lab., Issue 3, November, N. Claussen, N.A. Travitzky, S. Wu, Tailoring of Reaction-Bonded Al 2 (RBAO) Ceramics, Ceram. Eng. Sci. Proc., 11, 1990, N. Claussen, S. Wu, Processing and Properties of Reaction-Bonded Al 2 (RBAO) and Mullite Composites, Ceramic Powder Science, American Ceramic Society, Westerville, 22, 1991, S. Wu, N. Claussen, Fabrication and Properties of Low-Shrinkage Reaction-Bonded Mullite, J. Am. Ceram. Soc., 74, 1991, S. Banerjee, S.K. Roy, Net-shape Forming of Bi- Continuous Al 2 /Al composite by Displacement Reaction, Materials Chemistry and Physics, 67, 2001,