Innovative processing of porous and cellular materials by chemical reaction

Innovative processing of porous and cellular materials by chemical reaction

Scripta Materialia 54 (2006) 521–525 www.actamat-journals.com Innovative processing of porous and cellular materials by chemical reaction N. Kanetake...

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Scripta Materialia 54 (2006) 521–525 www.actamat-journals.com

Innovative processing of porous and cellular materials by chemical reaction N. Kanetake, M. Kobashi

*

Department of Materials Processing Engineering, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received 15 August 2005; received in revised form 20 August 2005; accepted 24 October 2005 Available online 23 November 2005

Abstract Nickel aluminide foam and porous Ti composite (TiC, TiB(2)/Ti) were synthesized. A reaction between Ni and Al was used to make the nickel aluminide foam. Porous Ti composite was fabricated by reactions between (1) Ti and C or (2) Ti and B4C. Gas release from the elemental powders and initial porosity in the green powder compact were the formation mechanisms of the pores. Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Porous materials; Self-propagating high-temperature synthesis (SHS); Foaming; Intermetallic compounds; Metal matrix composites (MMC)

1. Introduction Porous materials, which contain large number of pores or cells, have been enthusiastically investigated recently because of their high energy absorbing capacity, reduced thermal conductivity, enhanced acoustic damping ability and so forth [1–3]. Recently a great deal of attention has focused on metallic foams. However, if metals could be substituted by ceramics, intermetallics or their composites, such foam materials could be used for applications in extremely severe environments, such as in thermal barrier coatings at high-temperatures and filter materials in severe corrosive environments. Furthermore, it is well-known that porous materials are suitable for surgical implants because the pores permit bone ingress and give a firm bond between the implants and the human bone [4]. The pore size is one of the most important characteristics for this purpose, and an optimum range of the pore size was reported to be roughly from 200 lm to 500 lm [5]. Pore morphology and porosity are also important characteristics, which determine physical properties of the porous materials. *

Corresponding author. Tel.: +81 52 789 3356; fax: +81 52 789 5348. E-mail address: [email protected] (M. Kobashi).

In general, chemical reactions which synthesize intermetallics or ceramics generate large amounts of heats of reaction [6,7]. Combustion synthesis is a process which makes use of the strong exothermic reactions. Fig. 1 shows a brief outline of the combustion process for the synthesis of porous materials. A compacted elemental (element X and Y) powder blend is prepared. Part of the powder compact is heated to trigger a reaction between the elemental powders (X + Y ! XY + DH). Once the reaction occurs at the heated zone, then the heat of reaction raises the temperature of the neighboring zone and triggers the reaction again. Hence the reaction propagates throughout the specimen spontaneously, and results in the formation of bulk intermetallics or ceramics [8]. In this research, intermetallic foams (nickel aluminide) and porous titanium matrix composites (TiB2, TiC/Ti) were fabricated by the combustion reactions shown below [9]. Nickel aluminide foam can be used at higher temperatures than aluminum foam and can therefore extend the range of applications, in areas such as fire walls or thermal barriers. Besides the bio-oriented applications, porous titanium composites are applicable for both tribological and filtering purposes.

1359-6462/$ - see front matter Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.10.063

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ger the combustion reaction. Porosity was measured by the Archimedes method. 2.2. Results and discussion

Fig. 1. Schematic illustration of the combustion reaction for synthesizing porous materials.

(1) 3Al + Ni! Al3Ni + 151 kJ, (2) 3Ti + B4C ! 2TiB2 + TiC + 761 kJ, (3) Ti + C ! TiC + 185 kJ.

2. Synthesis of Ni–Al foam 2.1. Experimental procedure The elemental powders used for this experiment were nickel (3–5 lm) and aluminum (<45 lm). Titanium powder (<44 lm) and boron carbide powder (B4C: 10 lm) were also used as foaming agents. The nickel and aluminum powders were blended at molar ratios in between 1:3 and 3:1. In some experiments, the foaming agent powders were mixed with the elemental powder blends. The blended powder was compacted (room temperature, porosity of the precursor: 22%) in a cylindrical shape (h = 15 mm, / = 16 mm) to make a precursor. The precursor was inserted into a chamber (Fig. 2). After the chamber was evacuated and backfilled with argon, the precursor was heated to trig-

2.2.1. Pore formation mechanism Fig. 3 shows the cross-sections of combustion synthesized Ni–Al intermetallics with different Ni/Al blending ratios (Ni3Al, NiAl and NiAl3). The porosity of NiAl3 (45%) was the highest among the three specimens, whereas Ni3Al contained the least porosity (<5%). The high porosity of NiAl3 was attributed to gas generation from the aluminum powder. The surface of aluminum is usually covered with hydrous oxide (Al2O3 Æ 3H2O) under air [10], and hydrogen is also dissolved in aluminum powder. When the reaction between nickel and aluminum takes place, hydrogen is released from both the surface of and inside the aluminum powder and becomes the source of the pores seen in the Al-rich specimens. The gas composition in the pore was analyzed and the result is shown in Table 1. Most of the gas element in the pore was confirmed to be hydrogen, which originated in the Al powder (Ar is the atmospheric gas). 2.2.2. Porosity control As described in the former section, the porous material was fabricated by blending Ni and Al (Al/Ni ratio: 3). However the porosity needs to be higher and also should be controlled in order to apply this material industrially. A foaming agent (Ti, B4C blended powder with mole blending ratio of 3:1) was used in order to increase the porosity. The foaming agent was mixed with the elemental powders prior to the combustion reaction. The role of the foaming agent is two-fold: (i) The reaction shown below occurred at the same time as NiAl3 was synthesized. 3Ti + B4 C ! 2TiB2 + TiC

ð1Þ

The heat of formation of the reaction (1) is much higher than that of NiAl3 formation. Therefore the addition of the foaming agent raised the combustion temperature, and the reaction between nickel and aluminum was enhanced. (ii) The fine ceramic particles (TiB2 and TiC, less than several microns) were formed by the reaction (1), and the particles raised the viscosity of molten NiAl3 during the reaction. Therefore the pores formed during the combustion reaction tended to be stable in molten NiAl3.

Fig. 2. Schematic illustration of the experimental set-up.

Figs. 4 and 5 show the porosity and cross-sections of NiAl3 foams as a function of foaming agent addition. It is clearly identified in both figures that the addition of the foaming agent effectively increased the porosity. The porosity reached about 80% and larger pores tended to be formed with the higher addition of the foaming agent.

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Fig. 3. Cross-sections of the combustion synthesized specimens.

Table 1 Chemical composition of gas elements in the cells of NiAl3 foam

3. Synthesis of porous Ti composite

Gas

H2

Ar

CO2, N2

Others

3.1. Experimental procedure

Volume %

68.7

8.7

3.0

19.6

3.1.1. Reaction between titanium and boron carbide The starting materials used in this experiment were titanium powder (<45 lm) and B4C powder (<10 lm). The powders were blended at molar ratios (Ti/B4C) ranging from 3 to 14.2. The blended powder was then compressed by different compacting pressures to make precursors with different porosities (18–60% porosity). The precursor was then heated to induce the combustion reaction, using induction heating in an Ar gas atmosphere. After the reaction, the cross-section of the specimen was observed using an optical microscope (OM) and scanning electron microscope (SEM) and was analyzed by an X-ray diffraction (XRD) method.

Fig. 4. Porosity of NiAl3 foam with different amounts of foaming agent additions.

3.1.2. Reaction between titanium and carbon Titanium powder (<45 lm) and carbon powder (<5 lm) were used as the starting materials. The starting powders were blended at molar ratios (Ti/C) ranging from 1.33 to 2.99. The same experimental procedure as shown in Section 3.1.1 was carried out.

Fig. 5. Cross-sections of NiAl3 foams with different additions of foaming agent.

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3.2. Results and discussion 3.2.1. Observation of microstructure Microstructures of the composites made by the combustion reaction from the Ti/B4C powders with different blending ratios (Ti/B4C = 3.0, 7.6 and 9.1) are shown in Fig. 6. When Ti/B4C ratio was 3.0, TiC and TiB2 were formed and titanium was not detected. But when the Ti/ B4C ratios were 7.6 and 9.1, the titanium matrix was visible. The size of the ceramic phase became smaller by increasing the Ti/B4C ratio. Fig. 7 shows the microstructures of the composites made by the combustion reaction of the Ti/C powders with different blending ratios (Ti/C = 2.33 and 1.88). When the Ti/C ratio was 2.33, both titanium and TiC were clearly detected, whereas TiC and residual carbon were detected when the Ti/C ratio was 1.88. Titanium carbide has a cubic NaCl-type structure which has a wide composition range between TiC0.49–TiC0.95 [11] and the composition of nonstoichiometric TiCx can be predicted from the Ti–C phase diagram. Therefore, when the Ti/C ratio was 2.33, the formation of TiC0.49 and the remnant of titanium were expected, and the formation of the duplex-phase material (Ti/TiC composite) was confirmed experimentally.

3.2.2. Pore morphology The following two parameters were varied to control pore morphology: Parameter 1: Blending ratio of the starting powder. This parameter controls the fraction of the metal phase (titanium). During the combustion process, the metal phase melts and increases fluidity. Therefore increasing the titanium blending ratio contributes to the high mobility and deformation ability of pores. Parameter 2: Porosity of the precursor. This parameter is directly connected to the pore morphology, especially when the titanium blending ratio is low (the mobility of pores during the combustion reaction is low due to the small fraction of liquid titanium). Considering the two parameters indicated above, an open-pore structure was expected to be obtained with a low titanium blending ratio and high porosity of precursor. Fig. 8(a) shows the pore morphology of the porous Ti composite (Ti/B4C = 3, precursors porosity = 60%). An

Fig. 6. Microstructures of the composites made from the Ti/B4C powders using different blending ratios: (a) 3.0, (b) 7.6 and (c) 9.1.

Fig. 7. Microstructures of the composites made from Ti/C powders using different blending ratios: (a) 1.88 and (b) 2.33.

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Fig. 8. Pore morphology of the porous titanium/ceramic composite: (a) Ti/B4C = 3, precursors porosity = 60%; (b) Ti/B4C = 10, precursors porosity = 25%.

Fig. 9. Pore morphology of the porous titanium/ceramic composite: (a) Ti/C = 1.88, (b) Ti/C = 2.33, precursors porosity = 25%.

open-pore structure was identified. On the other hand, spherical closed pores were formed at a higher Ti/B4C ratio. Fig. 8(b) shows the pore structure of the Ti composite with a high blending ratio (Ti/B4C = 10, precursors porosity = 25%). The high Ti/B4C ratio makes the fluidity of the Ti composite higher during the reaction, and resulted in the formation of spherical pores driven by the surface tension of liquid titanium. The same tendency was observed when Ti/C blended powder was used (Fig. 9).

between titanium and non-metal powder (carbon or B4C). 5. The blending ratio of elemental powders (material composition) was an important factor for the cell morphology of porous titanium composite. A ceramic-rich composition was suitable for obtaining an open-cell material. Spherical closed cells were formed from a metal-rich composition. 6. The porosity of the precursor had an effect on the porosity of porous titanium composites.

4. Conclusion Nickel aluminide foam and porous titanium composite (TiC, TiB(2)/Ti) were fabricated by chemical reactions and the following results were obtained: 1. Nickel aluminide foam was fabricated by reaction between nickel and aluminum. The Al/Ni blending ratio was an important parameter in controlling the porosity. 2. The porosity of Al3Ni foam increased through adding the blowing agent (Ti + B4C blended powder) and reached about 80% by adding 5 vol.% blowing agent. 3. About 70 vol.% of the gas in the pore of Al3Ni foam was proved to be hydrogen. 4. Titanium composites (TiC/Ti composite and TiC, TiB(2)/Ti composite) were fabricated by reactions

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