In the 90s, the research work of Barsoum et al. from Drexel University (Philadelphia, USA) led them to focus on Ti₃SiC₂ (Barsoum et al., J. Am. Ceram. Soc., 1996). They demonstrated that this compound is stiff, lightweight, machinable, made from relatively inexpensive raw materials, resistant to oxidation and thermal shock, and capable of remaining strong up to temperatures above 1,300°C in air. They synthesized around fifty compounds with similar promising properties. Due to their composition, Barsoum et al. referred to these remarkable materials as M_n+1AX_n phases, where:
– M is a transition metal from the groups 3, 4, 5 or 6
– A is an element from the groups 12, 13, 14, 15 or 16
– X is carbone C and/or nitrogen N.
n is an integer from 1 to 3. Therefore the structures M₂AX, M₃AX₂ and M₄AX₃ are respectively called 211, 312 and 413.

MAX phases have a laminated structure with a hexagonal lattice. The primitive cell can be described as a stacking of n M6X octahedron layers with one layer of A element. Furthermore, measurements of lattice parameters with various methods reveal that MAX phases exhibit high crystalline anisotropy. The c/a ratio is generally higher than 3. MAX phases synthesized by powder metallurgy are polycrystalline bulk samples with random grain orientations. It is commonly observed that during synthesis, grains grow in a platelet shape. Due to the high crystalline anisotropy, platelet surfaces are parallel to basal planes. Therefore, projections on the surface are observed as rectangles with a high aspect ratio.

The technological impact of these ternary compounds will not derive from any single property, but rather they achieve a unique combination of properties. They combine properties of both ceramics, such as refractory, high stiffness, low density (e.g. 4.5 g/cm³ for Ti₃SiC₂), and low ductility at room temperature, and metals, such as high thermal and electrical conductivity, thermal shock resistance, low hardness, and mechanical resistance. This combination of properties makes them suitable for a wide range of high-temperature applications, such as aerospace, automotive, biomedical, and energy storage.

MAX phases are also the precursors for synthesizing one of the latest and largest 2D materials reported to date: MXenes. Similar to graphene, these materials consist of a few-atom-thick layers but unlike graphene, the layers are made of transition metal carbides, nitrides, or carbonitrides. Therefore, MXenes open several new promising research tracks in the field of 2D materials, due to their unique properties and potential applications. MXenes are being explored for their electrical and thermal conductivity, their high surface area, and their high mechanical strength. These properties make them attractive for use in fields such as energy storage, electronics, and electrocatalysis.

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Some of my publications on MAX phases

Dislcoation modelling in Ti2AlN MAX phase based on the Pierls-Nabarro model.
K. Gourriet, P. Carrez, P. Cordier, A. Guitton, A. Joulain, L. Thilly, C. Tromas
PHILOSOPHICAL MAGAZINE, 2015, 95 (23), 2539–2552
DOI: 10.1080/14786435.2015.1066938

Evidence of dislocation cross-slip in MAX phase deformed at high temperature.
A. Guitton, A. Joulain, L. Thilly, C. Tromas
SCIENTIFIC REPORTS, 2014, 4 (6358)
DOI: 10.1038/srep06358

Effect of microstructure anisotropy on the deformation of MAX polycrystals studied by in-situ compression combined with neutron diffraction.
A. Guitton, S. Van Petegem, C. Tromas, A. Joulain, H. Van Swygenhoven, L. Thilly
APPLIED PHYSICS LETTERS, 2014, 24 (241910)
DOI: 10.1063/1.4884601

Pressure-enforced plasticity in MAX phases: from single grain to polycrystal investigation.
G.P. Bei, A. Guitton, A. Joulain, V. Brunet, S. Dubois, L. Thilly, C. Tromas
PHILOSOPHICAL MAGAZINE, 2013, 93 (15), 1784–1801
DOI: 10.1080/14786435.2012.755272

Dislocation analysis of Ti2AlN deformed at room temperature under confining pressure.
A. Guitton, A. Joulain, L. Thilly, C. Tromas
PHILOSOPHICAL MAGAZINE, 2012, 92 (36), 4536–4546
DOI: 10.1080/14786435.2012.715250

Deformation mechanisms of MAX phases: a multiscale experimental approach.
A. Guitton
PhD Thesis, Université de Poitiers, 2013
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