Preparation of Ti3AlC2 and Ti2AlC by self-propagating high-temperature synthesis

Abstract

Ti3AlC2 and Ti2AlC are members of a class of ternary carbides that recently have been shown to possess an unusual combination of properties. They combine the merits of both metals and ceramics. Like metals they are thermal and electrical conductive, easy to machine with conventional tools, and resistant to thermal shock; like ceramics they have high strength, high melting points and thermal stabilities. Especially, Ti3AlC2 is the only ceramic that exhibits some room-temperature compressive plasticity [1, 2]. Ti2AlC had been fabricated by mixing powders of Ti, Al4C3 and graphite, thereafter hot pressing (HP) the powders at 1600 ◦C for 4 h [2]. Tzeonov and Barsoum first prepared bulk polycrystalline samples of Ti3AlC2 by reactively hot isostatically pressing (HIP) a mixture of titanium, graphite, and Al4C3 powders at 1400 ◦C [1]. Additionally Tzeonov and Barsoum recently gave the first quantitative determination of the X-ray relative peak heights in Ti3AlC2 [1]. Because Al would melt about 650 ◦C, those methods used Al4C3 as aluminum source. It was reported by Hidekazu Tannka et al., however, that Ti, Al and graphite raw powders were mixed, followed by selfpropagating high temperature synthesis (SHS) reaction. Ti2AlC was detected in the obtained product [3, 4], which could be proved by X-ray pattern [3]. However, there were some peaks not interpreted in the pattern, which may be correspond to recently published X-ray pattern of Ti3AlC2. And more, TiC and Al were obtained from powders of Ti, C and Al by means of mechanical alloying (MA) [5], thermal explosion [6], and another SHS [7]. No ternary carbide could be detected in the obtained reaction products. The purpose of this paper is to report a new means of synthesizing Ti3AlC2 & Ti2AlC by SHS and put forward to a mechanism in elevated temperature reaction of elemental Ti, Al and C. All the work was conducted using powder mixtures of titanium (−300 mesh), Al (99.5% pure, −200 mesh) and carbon black. Typically, the powder mixtures were ball-milled with absolute alcohol for 24 h. The raw powders were mixed by Ti : C : Al atomic ratios = 2 : 1 : 1, The mixtures were cold-pressed into bars with dimensions of 50 mm × 10 mm × 10 mm, followed by the SHS reactions. The reactions ignited by reaction heat between titanium and carbon at one end of the bars, which were heated for several seconds by passing an electric current through a tungsten filament. To avoid the influence of oxygen, the reactions took place in vacuums. To compare SHS with HP reactions, the powders were cold pressed, and the green bodies were wrapped in graphite sheets that were presprayed with BN and placed graphite die, thereafter hot pressed in 1600 ◦C for 4 h under pressures of 25 MPa and atmosphere of pure argon. The mixtures were easy to ignite. After reactions, obtained specimens were porous. The XRD diffraction spectrum of their powders with 2θ = 20◦–80◦ was similar to that published by Hidekazu Tannka [3]. He considered the main products were Ti2AlC and TiC [3]. The peaks of Ti2AIC and TiC could be found in the spectrum. However, there were some weak peaks that could not be interpreted by Hidekazu Tannka [3]. If the peaks were compared with peaks of the new phase, Ti3AlC2 [1], the new phase might exist. In order to get the value proof, the powders were soaked in a 1 : 1 : 5 (by volume) solution of HCl, HNO3, H2O for 6 h to remove the possible impurities, TiAl and Ti2Al. Thereafter, XRD diffraction spectrum of the powder with 2θ = 5◦–75◦ was acquired again and shown in Fig. 1. We found that main peaks with 2θ = 20◦–75◦ were invariant after the powders soaked in acid. Therefore there was no TiAl or Ti2Al synthesized by this reaction. It was clear that there was a peak with 2θ = 9.4◦, which was particularly correspond to Ti3AlC2. Therefore, there must exist Ti3AlC2 in the SHS product. A typical scanning electron microscope (SEM) micrograph of the SHS product’s fracture surface was shown in Fig. 2, which confirmed the layered nature of Ti3AlC2 and Ti2AlC.