Decomposition Kinetics of MAX Phases in Extreme Environments

Decomposition Kinetics of MAX Phases in Extreme Environments

I.M. Low (Curtin University, Australia) and W.K. Pang (Curtin University, Australia & Tatung University, Taiwan)
DOI: 10.4018/978-1-4666-4066-5.ch002
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Abstract

MAX phases are remarkable materials but they become unstable at elevated temperatures and decompose into binary carbides or nitrides in inert atmospheres. The susceptibility of MAX phases to thermal dissociation at 1300-1550 °C in high vacuum has been studied using in-situ neutron diffraction. Above 1400 °C, MAX phases decomposed to binary carbide (e.g., TiCx) or binary nitride (e.g., TiNx), primarily through the sublimation of A-elements such as Al or Si, which results in a porous surface layer of MXx being formed Positive activation energies were determined for decomposed MAX phases with coarse pores but a negative activation energy when the pore size was less than 1.0 µm. The kinetics of isothermal phase decomposition at 1550 °C was modelled using a modified Avrami equation. An Avrami exponent (n) of < 1.0 was determined, indicative of the highly restricted diffusion of Al or Si between the channels of M6X octahedra. The role of pore microstructures on the decomposition kinetics is discussed.
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Introduction

MAX phases are remarkable materials that exhibit a unique combination of characteristics of both ceramics and metals with unusual mechanical, electrical and thermal properties (Barsoum, 2000; Barsoum & El-Raghy, 1996; 2001; Low, 1998; Low et al., 1998; Tian et al., 2007). These materials are nano-layered ceramics with the general formula Mn+1AXn (n = 1−3), where M is an early transition metal, A is a group A element, and X is either carbon and/or nitrogen. Similar to ceramics, they possess low density, low thermal expansion coefficient, high modulus and high strength, and good high-temperature oxidation resistance. Like metals, they are good electrical and thermal conductors, readily machinable, tolerant to damage, and resistant to thermal shock. The unique combination of these interesting properties enables these ceramics to be promising candidate materials for use in diverse fields which include automobile engine components, heating elements, rocket engine nozzles, aircraft brakes, racing car brake pads and low-density armour.

However, the high-temperature thermochemical stability in MAX phases has hitherto generated much controversy among researchers. For instance, several researchers have reported that Ti3SiC2 became unstable at temperatures greater than 1400ºC in an inert atmosphere (e.g., vacuum, argon or nitrogen), by dissociating into Si, TiCx and/or Ti5Si3Cx (Low & Oo, 2002; Low, 2004; Low et al., 2007, 2011; Oo et al., 2006; Pang et al., 2010). A similar phenomenon has also been observed for Ti3AlC2 whereby it decomposes in vacuum to form TiC and Ti2AlC (Pang et al., 2009, 2010; Low et al., 2011).

In other studies, Zhang et al. (2008) reported Ti3SiC2 to be thermally stable up to 1300ºC in nitrogen, but above this temperature drastic degradation and damage occurred due to surface decomposition. Feng et al. (1999) annealed the Ti3SiC2-based bulk samples at 1600 ºC for 2h and 2000 ºC for 0.5 h in vacuum (10-2 Pa) and found that TiCx was the only phase remaining on the surface. According to Gao et al. (2002) the propensity of decomposition of Ti3SiC2 to TiCx was related to the vapour pressure of Si, i.e., the atmosphere where the Ti3SiC2 exits. They believed that the partial pressure of Si plays an important role in maintaining the stability of Ti3SiC2 whereby it has a high propensity to decompose in N2, O2 or CO atmosphere at temperatures above 1400ºC. This process of surface-initiated phase decomposition was even observed to commence as low as 1000−1200ºC in Ti3SiC2 thin films during vacuum annealing (Emmerlich et al., 2007). The large difference in observed decomposition temperatures between bulk and thin-film Ti3SiC2 has been attributed to the difference in diffusion length scales involved and measurement sensitivity employed in the respective studies. In addition, Ti3SiC2 has also been observed to react readily with molten Al, Cu, Ni and cryolite (Na3AlF6) at high temperatures.

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