There is a plethora of possible MAX phases, but from what i have seen, papers tend to focus mainly on those three. What about all the other ones? Are they experimentally not feasible?
I can think of several reasons:
1. 211 MAX phases wherein A = Al are the most numerous.
2. Cr2AlC and Ti2AlC have found the highest number of prospective applications due to better high-temperature stability (e.g. oxidation resistance and oxidative self-healing) and relatively low density. For example, Ta2AlC and Nb2AlC are even more dense than steel, if I'm not mistaken, which goes against the concept of using ceramics for light-weighting typical metallic components. No one has yet made Zr2AlC with high phase purity, despite several papers on the subject.
3. Ti, Cr, and Al as precursors are much easier to work with and/or cheaper compared to several others (e.g. Pb, Cd, Ga, In, S, Ta). Zr poses a higher flammability risk, while V is prone to oxidation , which can complicate processing.
4. From an ethical standpoint, Ta and Sn are derived from "conflict minerals". Local laws might make those harder to obtain. I'm not sure if this really a contributing factor for academic research, but it could lead to problems for industrial adoption.
Indeed there are myriads of possible MAX phase compositions. However much of the research tends to focus on processing methods through which enhanced properties can be achieved, in particular the mechanical properties of MAX phase composites. These mechanisms are relevant for all MAX phase compositions, so it makes sense to focus on a limited number of commonly studied compositions and examine the role of synthesis and structure.
Actually these three 211 MAX phases could be synthesized very conveniently using the normal initial materials with low cost. Also, the oxidation resistance of them is good enough for preparing the coatings.
I can think of several reasons:
1. 211 MAX phases wherein A = Al are the most numerous.
2. Cr2AlC and Ti2AlC have found the highest number of prospective applications due to better high-temperature stability (e.g. oxidation resistance and oxidative self-healing) and relatively low density. For example, Ta2AlC and Nb2AlC are even more dense than steel, if I'm not mistaken, which goes against the concept of using ceramics for light-weighting typical metallic components. No one has yet made Zr2AlC with high phase purity, despite several papers on the subject.
3. Ti, Cr, and Al as precursors are much easier to work with and/or cheaper compared to several others (e.g. Pb, Cd, Ga, In, S, Ta). Zr poses a higher flammability risk, while V is prone to oxidation , which can complicate processing.
4. From an ethical standpoint, Ta and Sn are derived from "conflict minerals". Local laws might make those harder to obtain. I'm not sure if this really a contributing factor for academic research, but it could lead to problems for industrial adoption.
Ti2AlC, Ti3AlC2 and Cr2AlC are very commonly studied for the reasons well explained by Sankalp Kota, Chunfeng Hu, and Dorian Hanaor above. However, the archetype Ti3SiC2 is probably even more studied.
Generally, the nitrides are not as commonly investigated; the syntehsis is typically more difficult than for the carbides. Ti2AlN is the most commonly investigated MAX nitride, but not nearly as much as the carbides.
Al-containing MAX phases have shown remarkable anti-oxidation and anti-corrosion properties by the formation of a protective Al2O3 layer.
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