An Introduction to MAX Phases

A dozen years ago we synthesized and fully characterized, for the first time, the ternary compound, Ti₃SiC₂, and found it possess some of the best attributes of metals and ceramics.¹ A year later we showed that this compound was but one of over sixty phases,² most discovered and produced in powder form in the sixties by H. Nowotny and coworkers.³ In 1999⁴ we discovered Ti₄AlN₃ and realized that we were dealing with a much larger family of solids that all behaved similarly.

These layered, hexagonal carbides and nitrides have the general formula: M_n+1AX_n, (MAX) where n = 1 to 3, M is an early transition metal, A is an A-group (mostly IIIA and IVA, viz. groups 13 and 14) element and X is either C and/or N. Figure 1 lists the currently known MAX phases that have been reported in bulk form. If one includes MAX phases discovered in thin film form the list grows even larger.⁵

Figure 1. Elements in the periodic table (top) that react together to form the MAX phases. The red squares represent the M-elements; the blue, the A elements and black or X is C and/or N. Bottom boxes list the currently known 211, 312 and 413 phases. With the exception of the 413's the vast majority of these phases were discovered by Nowotny and co-workers in Vienna in the 1960's.3

Between 1996 when our first paper was published and today the MAX phase community has made tremendous progress in understanding the properties of these phases. These carbides and nitrides possess unusual and, sometimes, unique chemical, physical, electrical, and mechanical properties.^6-9 They are electrically and thermally conductive, most readily machinable (a manual hack-saw suffices), not susceptible to thermal shock, plastic at high temperatures, and exceptionally damage tolerant. Some, like Ti₃SiC₂ and Ti₂AlC, are also elastically rigid, lightweight, creep,¹⁰ fatigue,¹¹ oxidation¹² and corrosion¹³ resistant and maintain their strengths to high temperatures.¹⁴ Most of the MAX phases are better electric and thermal conductors than Ti.

Three interrelated characteristics distinguish how these phases deform in comparison to other solids in general, and layered solids in particular: the metallic-like nature of the bonding; basal dislocation slip, and only basal slip, is operative, and the unique combination of kink and shear band formation, together with delaminations of individual grains.^15,16 The pictures shown in Figure 2, demonstrate the uniqueness of the deformation modes of the MAX phases. Figure 3 illustrates their signature characteristics: machinability.

Figure 2. a) Typical scanning electron microscope micrographs of fractured Ti3SiC2 surface. Note fine scale of delaminations; in principle, every basal plane is a slip or delamination plane;3 b) Bridged crack in a coarse-grained Ti3SiC2 tested at room temperature.11,17

Figure 3. The MAX phases can he machined as easily as metals. They all can be machined using a manual hacksaw, despite the fact that some of them are three times as stiff as Ti metal and have the same density as Ti. The mechanism by which they machine, however, is not as one would scoop ice cream - i.e. by plastic deformation - but rather as one would shave ice. They can also be polished to a metallic luster because of their excellent electrical conductivities. Slide courtesy of Kanthal Corp. of Sweden who licensed the technology from Drexel University.

Interestingly, it is by characterizing the unique mechanical properties of the MAX phases - e.g.; polycrystalline Ti₃SiC₂ cylinders can be repeatedly compressed at room temperature, up to stresses of 1 GPa, and fully recover upon the removal of the load while dissipating 25% of the energy¹⁸ - that we discovered kinking non-linear solids and the micromechanism that is responsible for them, namely, incipient kink bands.¹⁹

Potential applications for the MAX phases include heating elements (Figs. 3a and b), gas burner nozzles in corrosive environments (Fig. 3c), high-temperature bearings²⁰ (Fig. 3d), diamond/Ti₃SiC₂ composites for dry drilling of concrete (developed with Hilti Corp.) (Fig. 4e), examples of very thin walled parts manufactured by slip casting (Fig. 4f).

Figure 4. Examples of potential MAX phase application courtesy of Kanthal Corp. and 3-ONE- 2, the company Dr. El-Raghy and I founded a few years ago to develop applications for the MAX phases.

References

1. Barsoum, M.W. & El-Raghy, T., Synthesis and Characterization of a Remarkable Ceramic: Ti₃SiC₂. J. Amer. Cer. Soc. 79 (7), 1953-1956 (1996).
2. Barsoum, M.W., Brodkin, D., & El-Raghy, T., Layered Machinable Ceramics For High Temperature Applications. Scrip. Met. et. Mater. 36, 535-541 (1997).
3. Nowotny, H., Struktuchemie Einiger Verbindungen der Ubergangsmetalle mit den elementen C, Si, Ge, Sn. Prog. Solid State Chem. 2, 27 (1970).
4. Barsoum, M.W. et al., High-Resolution Transmission Electron Microscopy of Ti₄AlN₃, or Ti₃Al₂N₂ Revisited. J. Amer. Cer. Soc. 82 (9), 2545-2547 (1999).
5. Eklund, P., Beckers, M., Jansson, U., Högberg, H., & Hultman, L., The M_n+1AX_n phases: Materials science and thin-film processing. Thin Solid Films 518, 1851-1878 (2010).
6. Barsoum, M.W., The Mn+1AXn Phases: a New Class of Solids; Thermodynamically Stable Nanolaminates. Prog. Solid State Chem 28, 201-281 (2000).
7. Barsoum, M.W. Physical Properties of the MAX Phases in Encyclopedia of Materials Science and Technology, edited by K. H. J. Buschow et al. (Elsevier, Amsterdam, 2006).
8. Barsoum, M.W. & Radovic, M., Mechanical Properties of the MAX Phases in Encyclopedia of Materials Science and Technology, edited by R. W. Cahn K. H. J. Buschow, M. C. Flemings, E. J. Kramer, S. Mahajan and P. Veyssiere (Elsevier, Amsterdam, 2004).
9. Barsoum, M.W., The MAX Phases and Their Properties in Ceramics Science and Technology, Vol. 2: Properties,, edited by R. R. Riedel & I.-W. Chen (Wiley-VCH Verlag GmbH & Co, 2010), Vol. 2.
10. Radovic, M., Barsoum, M.W., El-Raghy, T., & Wiederhorn, S.M., Tensile Creep of Coarse-Grained (100-300 µm) Ti₃SiC₂ in the 1000-1200 °C Temperature Range. J. Alloys and Compds. 361, 299-312 (2003).
11. Gilbert, C.J. et al., Fatigue-crack Growth and Fracture Properties of Coarse and Finegrained Ti₃SiC₂. Scripa Materialia 238 (2), 761-767 (2000).
12. Sundberg, M., Malmqvist, G., Magnusson, A., & El-Raghy, T., Alumina Forming High Temperature Silicides and Carbides. Ceramics International 30, 1899-1904 (2004).
13. Jovic, V.D., Barsoum, M.W., Jovic, B.M., Gupta, S., & El-Raghy, T., Corrosion Behavior of Select MAX Phases in NaOH, HCl and H₂SO₄. Corr. Science 48, 4274-4282 (2006).
14. El-Raghy, T., Barsoum, M.W., Zavaliangos, a., & Kalidindi, S.R., Processing and mechanical properties of Ti₃SiC₂: II, effect of grain size and deformation temperature. J. Amer. Cer. Soc. 82 2855-2860 (1999).
15. Barsoum, M.W. & El-Raghy, T., Room Temperature Ductile Carbides. Metallurgical and Materials Trans. 30A, 363-369 (1999).
16. Barsoum, M.W., Farber, L., El-Raghy, T., & Levin, I., Dislocations, Kink Bands and Room Temperature Plasticity of Ti3SiC2. Met. Mater. Trans. 30A, 1727-1738 (1999).
17. Chen, D., Shirato, K., Barsoum, M.W., El-Raghy, T., & Ritchie, R.O., Cyclic Fatigue- Crack Growth and Fracture Properties in Ti3SiC2 Ceramics at Elevated Temperatures. J. Amer. Cer. Soc. 84, 2914 (2001).
18. Barsoum, M.W., Zhen, T., Kalidindi, S.R., Radovic, M., & Murugahiah, A., Fully Reversible, Dislocation-Based Compressive Deformation of Ti₃SiC₂ to 1 GPa. Nature Materials 2, 107-111 (2003).
19. Barsoum, M.W. & Basu, S., Kinking Nonlinear Elastic Solids in Encyclopedia of Materials Science and Technology, edited by R. W. Cahn K. H. J. Buschow, M. C. Flemings, B. Ilschner, E. J. Kramer, S. Mahajan and P. Veyssiere (Elsevier, Oxford, 2010), pp. 1-23.
20. Gupta, S., Filimonov, D., Zaitsev, V., Palanisamy, T., & Barsoum, M.W., Ambient and 550 degrees C tribological behavior of select MAX phases against Ni-based superalloys. Wear 264, 270-278 (2008).