MAX phases
The MAX phases are layered, hexagonal carbides and nitrides have the general formula: Mn+1AXn, (MAX) where n = 1 to 3, M is an early transition metal, A is an A-group (mostly IIIA and IVA, or groups 13 and 14) element and X is either carbon and/or nitrogen. The layered structure consists of edge-sharing, distorted XM6 octahedra interleaved by single planar layers of the A-group element.
Ti3InC2, V3AlC2,
Ti3SiC2,
Ti3GeC2,
Ti3SnC2,
Ta3AlC2,
Zr3AlC2
V4AlC3,
Ti4GaC3,
Ti4SiC3,
Ti4GeC3,
Nb4AlC3,
Ta4AlC3,
In the 1960s, H. Nowotny and co-workers discovered a large family of ternary, layered carbides and nitrides, which they called the 'H' phases, now known as the '211' MAX phases (i.e. n = 2), and several '312' MAX phases. Subsequent work extended to '312' phases such as Ti3SiC2 and showed it to have unusual mechanical properties. In the 1990s, fully dense and phase pure Ti3SiC2 was synthesized by Barsoum and characterized and co-workers for the first time and revealed it to possess a distinct combination of some of the best properties of metals and engineering ceramics. In 1999 they also synthesized Ti4AlN3 (i.e. a '413' MAX phase) and realized that they were dealing with a much larger family of solids that all behaved similarly. Since 1996, when the first paper was published on the subject, tremendous progress has been made in understanding the properties of these phases. Since 2006 research has focused on the fabrication, characterisation and implementation of composites including MAX phase materials. Such systems, including aluminium-MAX phase composites, have the ability to further improve ductility and toughness over pure MAX phase material.
The synthesis of ternary MAX phase compounds and composites has been realized by different methods, including combustion synthesis, chemical vapor deposition, physical vapor deposition, arc melting, hot isostatic pressing, self-propagating high-temperature synthesis (SHS), reactive sintering, spark plasma sintering, and mechanical alloying.
These carbides and nitrides possess unusual combination of chemical, physical, electrical, and mechanical properties, exhibiting both metallic and ceramic characteristics under various conditions. These include high electrical and thermal conductivity, thermal shock resistance, damage tolerance, machinability, high elastic stiffness, and low thermal expansion coefficients. Some MAX phases are also highly resistant to chemical attack (e.g. Ti3SiC2) and high-temperature oxidation in air (Ti2AlC, Cr2AlC, and Ti3AlC2). They are useful in technologies involving high efficiency engines, damage tolerant thermal systems, increasing fatigue resistance, and retention of rigidity at high temperatures. These properties can be related to the electronic structure and chemical bonding in the MAX phases. It can be described as periodic alteration of high and low electron density regions. This allows for design of other nanolaminates based on the electronic structure similarities, such as Mo2BC and PdFe3N.
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