However, the high-temperature thermochemical stability in MAX phases has hitherto generated much controversy among researchers. In addition, MAX phases such as Ti 2AlC and Ti 3SiC 2 have been shown to exhibit sufficient damage tolerance to irradiations which renders them as promising materials for high-temperature nuclear applications. 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 armor. Like metals, they are good electrical and thermal conductors, readily machinable, tolerant to damage, and resistant to thermal shock. Similar to ceramics, they possess low density, low thermal expansion coefficient, high modulus and high strength, and good high-temperature oxidation resistance. These materials are nano-layered ceramics with the general formula M n +1 AX n ( n = 1–3), where M is an early transition metal, A is a group A element, and X is either carbon and/or nitrogen. MAX phases exhibit a unique combination of characteristics of both ceramics and metals and have unusual mechanical, electrical and thermal properties. Other controlling parameters that also promote decomposition or degradation as reported in the literature are also briefly reviewed and these include effects of pressure and ion irradiations. Ironically, the understanding of phase decomposition via exfoliating or selective de-intercalation by chemical etching formed the catalyst for the sensational discovery of Mxenes in 2011. Arrhenius Avrami equations were used to determine the activation energy of phase decomposition and to model the kinetics of isothermal phase decomposition. Ti-based MAX phases tend to decompose readily above 1400 ☌ during vacuum annealing to binary carbide (e.g., TiC x) or binary nitride (e.g., TiN x), primarily through the sublimation of A elements such as Al or Si, forming in a porous MX x surface layer. A critical overview of the various parameters, such as annealing atmospheres, pore microstructures, and pore sizes, that are critical in controlling the decomposition kinetics of Ti-based MAX phases is given in this paper.
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