Isothermal Oxidation of Ti2SC in Air

The oxidation behavior of fully dense Ti2SC was studied thermogravimetrically in air in the 500–800°C temperature range. The oxidation product was a single-layer of rutile in all cases. At 800°C, the oxide layer was not protective and the oxidation kinetics were rapid. At 600 and 700°C, and up to ~50 h, the kinetics were parabolic before they became linear. It was only at 500°C that the weight gain reached a plateau after a 50 h initial parabolic regime. Mass spectrometry of the gases evolved during oxidation confirmed that both CO2 and SO2 are oxidation products. The overall oxidation reaction is thus Ti2SC + 4O2 → 2TiO2 + SO2 + CO2. On the basis of this and previous work, we conclude that oxidation occurs by the outward diffusion of titanium, sulfur, and carbon, the latter two either as atoms or in the form of CO2 and SO2 and, most probably, the inward diffusion of oxygen. Mesopores and microcracks were found in all rutile layers formed except those formed at 500°C. The presence of these defects is believed to have led to significantly higher oxidation rates as compared to other rutile-forming ternary carbides, such as Ti3SiC2. Disciplines Engineering | Materials Science and Engineering Comments Suggested Citation: Amini, S., McGhie, A.R. and Barsoum, M.W. (2009). "Isothermal oxidation of Ti2SC in Air." Journal of the Electrochemical Society. 156 © The Electrochemical Society, Inc. 2009. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in Journal of the Electrochemical Society, Volume 157, Issue 7, 2009, pages P101-P106. Publisher URL: http://scitation.aip.org/JES/ This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/196 Isothermal Oxidation of Ti2SC in Air Shahram Amini, Andrew R. McGhie, and Michel W. Barsoum Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA The oxidation behavior of fully dense Ti2SC was studied thermogravimetrically in air in the 500–800°C temperature range. The oxidation product was a single-layer of rutile in all cases. At 800°C, the oxide layer was not protective and the oxidation kinetics were rapid. At 600 and 700°C, and up to 50 h, the kinetics were parabolic before they became linear. It was only at 500°C that the weight gain reached a plateau after a 50 h initial parabolic regime. Mass spectrometry of the gases evolved during oxidation confirmed that both CO2 and SO2 are oxidation products. The overall oxidation reaction is thus Ti2SC + 4O2 → 2TiO2 + SO2 + CO2. On the basis of this and previous work, we conclude that oxidation occurs by the outward diffusion of titanium, sulfur, and carbon, the latter two either as atoms or in the form of CO2 and SO2 and, most probably, the inward diffusion of oxygen. Mesopores and microcracks were found in all rutile layers formed except those formed at 500°C. The presence of these defects is believed to have led to significantly higher oxidation rates as compared to other rutile-forming ternary carbides, such as Ti3SiC2. © 2009 The Electrochemical Society. DOI: 10.1149/1.3117348 All rights reserved. Manuscript submitted December 26, 2008; revised manuscript received March 17, 2009. Published May 6, 2009. The Mn+1AXn MAX phases, where M is an early transition metal, A is an A-group element, and X is C or N are layered hexagonal solids with two formula units per unit cell, in which near close-packed layers of M are interleaved with layers of a pure group A element, with the X atoms filling the octahedral sites between M layers. It is fairly well established that these phases have an unusual and sometimes unique combination of properties. They are excellent electrical and thermal conductors, thermal shock resistant, and damage tolerant. Despite being elastically quite stiff, they are all readily machinable with nothing more sophisticated than a manual hacksaw. Moreover, some of them are fatigue, creep, and oxidation resistant. Titanium sulfur carbide Ti2SC is a member of this class of ternary carbides. Like the others, its unit cell is hexagonal space group D6h 4 –P63/mmc . At 11.22 Å, its c-lattice parameter is the lowest of all MAX phases. Because of this low value, partially attributable to the small diameter of S, it was postulated that Ti2SC could exhibit unusual mechanical properties as compared to the other known MAX phases. Elsewhere we have reported on its mechanical, electronic, thermal, magnetic, and elastic properties. With an average Vickers hardness of 8 2 GPa in the 2–300 N range, this hardness is the highest of any MAX phase characterized to date. The room-temperature compressive stress was 1.4 0.2 GPa; the failure mode was brittle. Its Young’s modulus, also one of the highest for a M2AX phase measured to date, was 316 2 GPa. Unlike all other MAX phases known to date, wherein their loading–unloading stress–strain curves outline nonlinear, fully reversible, strain-rate–independent, reproducible, closed hysteretic loops, there was no evidence for the formation of incipient kink bands during simple compression and the behavior was nearly fully elastic until failure. The room-temperature thermal conductivity 60 W/mK is also one of the highest of any MAX phase measured to date. The Young’s, shear, and bulk moduli, determined from ultrasonic measurements, are 290, 125, and 145 GPa, respectively. Its electrical conductivity is metallic-like and equal to 1.9 106 −1 m−1 at room temperature. The Debye temperature is 765 K. The bulk modulus, calculated using the Birch–Murnaghan equation of state, is 191 3 GPa, which is comparable to those of Ti2AlC, Nb2AlC, and V2AlC. In another recent paper, we have shown that the tribological properties of Ti2SC against alumina are excellent over the 25–550°C range. Elsewhere, we reported on the thermal expansion and stability of Ti2SC powders in air and Ar atmospheres using high-temperature X-ray diffraction. Ti2SC is stable in Ar atmosphere up to 400°C; above this temperature, it dissociates into TiS2. With little anisotropy in thermal expansion, its volumetric thermal expansion was calculated to be 25.2 10−6 °C−1. In air, at 400°C, the powders start to oxidize into anatase, which, in turn, transforms into rutile at higher temperatures. This recent work was qualitative and was carried out on powders. In this report on the oxidation of bulk samples, it is useful to briefly review the oxidation kinetics and morphology of the oxide phases that form after long-term oxidation of other Ti-containing MAX phases in air. When polycrystalline samples of Ti3SiC2, Ti3SiC2–30 vol % TiC, and Ti3SiC2–30 vol % SiC are oxidized in air in the 900–1400°C range, the scales that formed were dense, adherent, and resistant to thermal cycling. The oxidation mostly resulted in a duplex oxide layer an inner TiO2/SiO2 layer and an outer rutile layer ; the kinetics are initially parabolic, but become linear at longer times. The oxidation of Tin+1AlCn ternaries in the 800–1100°C range resulted in a rutile layer in which some Al is dissolved. It has also been shown that subtle changes in chemistry can result in the formation of pure Al2O3 layers. For example, Sundberg et al. 20 have shown that Ti2AlC forms an excellent Al2O3 layer that is exceedingly protective, even under intense thermal cycling. Wang and Zhou also reported the formation of dense protective alumina layers on the surfaces of Ti3AlC2 and Ti2AlC, in Ref. 21 and 22, respectively, when oxidized in air. When the Ti2AlC and Ti2AlC0.5N0.5 solid solution are oxidized in air, the oxidation products are comprised of a duplex oxide layer of rutile-based solid solution and Al2O3. In the 1000–1100°C temperature range and for short times 20 h , the oxidation kinetics are parabolic. At 900°C, the kinetics are quasi-linear, and up to 100 h, the outermost layers that form are almost pure rutile, dense, and protective. In Ti4AlN2.9 and Ti3AlC2, at short times 10 h , the oxidation kinetics are parabolic in the 800–1100°C temperature range but become linear at longer times. The scales that form are comprised mainly of a rutile-based solid solution and some Al2O3. 19 In Ti3GeC2 and Ti3 Ge0.5Si0.5 C2, at 800°C and higher, the oxide layers formed are not protective and the oxidation kinetics are linear. At higher temperatures, GeO2 whiskers, visible to naked eye, form on the surface of the Ti3GeC2. 23 Herein, we report on the isothermal oxidation behavior of bulk Ti2SC samples in air in the 500–800°C temperature range. From a technological point of view, it is crucial to determine whether the oxidation kinetics remain parabolic or become linear. More importantly, given that the oxidation products of both C and S are gases, it was postulated that its oxidation behavior may be considerably different from other Ti-containing MAX phases. z E-mail: [email protected] Journal of The Electrochemical Society, 156 7 P101-P106 2009 0013-4651/2009/156 7 /P101/6/$25.00 © The Electrochemical Society P101 Downloaded 21 Dec 2010 to 130.91.117.41. 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