MAX Phases: Bridging the Gap Between Metals and Ceramics Figure 1

MAX phases: Bridging the gap between metals and ceramics Figure 1. Scanning electron microscopy of the fractured surface –1 in Ti2AlC after dynamic testing of at a strain rate of 2400 s showing typical laminated nature and deformation of individu- al grains by kinking. bulletin cover story MAX phases: Bridging the gap between metals and ceramics By Miladin Radovic and Michel W. Barsoum he term “MAX phases” was coined T in the late 1990s and applies to a family of 60+ ternary carbides and nitrides that share a layered structure as illustrated in Figures 1 and 2. They are so called because of their chemical formula: Mn+1AXn —where n = 1, 2, or 3, where M is an early transition metal, A is an A-group element (specifi- cally, the subset of elements 13–16), and X is 1 (Credit: Credit: Radovic and Benitez; TAMU.) carbon and/or nitrogen, Figure 2. Nowotny and coworkers2, 3 discovered most of these phases in powder form roughly 40 years ago. However, Barsoum and El-Raghy’s4 report The MAX phases are a new and exciting class of carbides in 1996 on the synthesis of phase-pure bulk and nitrides that bridge the gap between properties typical of metals and ceramics, while offering fundamentally new Ti3SiC2 samples and their unusual combina- directions in tuning the structure and properties of ceramics tion of properties catalyzed renewed interest for emerging applications. in them. Since then, research on the MAX phases has exploded. According to ISI, to date around 1,200 papers have been published on one MAX phase alone, Ti3SiC2, with roughly half of those published in the past six years. 20 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 3 The growing interest results from the unusual, often unique, properties of the MAX phases. Like their correspond- ing binary carbides and nitrides (MX), the MAX phases are elastically stiff, good thermal and electrical conductors, resistant to chemical attack, and have relatively low thermal expansion coefficients.1 Mechanically, however, they cannot be more different. They are relatively soft and most are readily machinable, thermal shock resistant and damage tolerant. Moreover, some are fatigue, creep, and oxidation resistant. At room temperature, they can be compressed to stresses as high as 1 GPa and fully recover on removal of the load, while dissipating approximately 25 percent of the mechanical energy.6 At higher temperatures, they undergo a brittle-to-plastic transition (BPT), above which they are quite plastic even in tension.5 This article gives an overview of the salient properties of the MAX phases and of the status of our current under- standing. Some of their potential applications also are highlighted. For a thor- ough review of the large body of work on MAX phases, the reader is referred to a recently published book1 and a number of excellent review articles.7–15 Crystal structure and atomic (Credit: Credit: Radovic; TAMU.) Figure 2. Unit cells of the M AX phases for (a) n = 1 or M AX, (b) n = 2 or bonding in the MAX phases n+1 n 2 M AX , and (c) n = 3 or M AX phases, and (d) M, A, and X elements that form The MAX phases are layered hex- 3 2 4 3 agonal crystal structures (space group the MAX phases. P63/mmc) with two formula units per unit cell, as illustrated in Figure 2, for complex stacking sequences, such as Interestingly, some of solid solu- structures with n equal 1 to 3. The M5AX4, M6AX5, and M7AX6 also have tions exist even when one of the end been reported.8,16 members does not. The number of unit cells consist of M6X-octahedra with the X-atoms filling the octahedral In addition to the “pure” MAX MAX phases and their solid solutions sites between the M-atoms, which are phases that contain one of each of the continues to expand. The discovery of identical to those found in the rock M, A, and X elements highlighted in new phases has advanced significantly salt structure of the MX binaries. The Figure 2(d), the number of possible through the combination of experi- octahedra alternate with layers of pure solid solutions is quite large. Solid solu- mental and theoretical density func- 1,18–20 A-elements located at the centers of tions have been processed and charac- tional theory (DFT) approaches. 1 trigonal prisms that are slightly larger, terized with substitution on For example, ab-initio studies recently and thus more accommodating of • M sites, e.g., (Nb,Zr)2AlC, extended the family of the MAX phases the larger A-atoms. When n = 1, the (Ti,V)2AlC, (Ti,Nb)2AlC, to compounds with magnetic proper- A-layers are separated by two M-layers (Ti,Cr)2AlC, (Ti,Hf)2InC, and ties that contain later transition-metal (Figure 2(a)). When n = 2, they are (Ti,V)2SC; substitutions on the M sites, such as 21 • A-sites, e.g., Ti3(Si,Ge)C2, and (Cr,Mn)2AlC. separated by three layers (M3AX2 in Figure 2(b)). When n = 3, they are Ti3(Sn,Al)C2; and A large body of work devoted to 17 • X-sites, e.g., Ti2Al(C,N) and DFT calculations of the electronic separated by four layers (M3AX2 in Figure 2(c)). MAX phases with more Ti3Al(C,N)2. structures and chemical bonding in the American Ceramic Society Bulletin, Vol. 92, No. 3 | www.ceramics.org 21 MAX phases: Bridging the gap between metals and ceramics equal, too.10 Several MAX phases, most notably Ti3SiC2, have very low thermoelectric or Seebeck coefficients.10,29 Solids with essentially zero thermopower can, in principle, serve as reference materials in thermoelectric measurements, for example, as leads to measure the abso- lute thermopower of other solids. Resistivity (µΩ·m) Resistivity The optical properties of the MAX phases are dominated by delocalized Thermal conductivity (W/m·K) electrons.30 Magnetically, most of them are Pauli paramagnets, wherein the (a) (b) susceptibility is, again, determined by Temperature (K) Temperature (K) the delocalized electrons and, thus, (Credit: Adapted from Ref. 31, 32.) Figure 3. Temperature dependence of (a) electrical conductivity31 and (b) thermal is neither very high, nor temperature 31 conductivity of select MAX phases.32 dependent. Thermally, the MAX phases share 22-28 MAX phases shows that much in common with their MX • Similar to the MX phases, MAX (a) counterparts, that is, they are good phase bonding is a combination of thermal conductors because they are metallic, covalent, and ionic bonds; good electrical conductors. At room • The M and X atoms form strong temperatures their thermal conductivi- directional covalent bonds in the M-X ties (Figure 3(b)) fall in the 12–60 W/ layers that are comparable to those in (m·K) range.1,10 The coefficients of 22, 27, 28 the MX binaries; thermal expansion (CTE) of the MAX • M–d–M–d metallic bonding domi- phases fall in the 5–10 µK–1 range and nates the electronic density of states at are relatively low as expected for refrac- the Fermi level, N(E ); and F tory solids.15 The exceptions are some • In most MAX phases, the M–A chromium-containing phases with bonds are relatively weaker than the CTEs in the 12–14 µK–1 range. M–X bonds. At high temperatures, the MAX Given the similarities between some phases do not melt congruently but (b) aspects of the atomic bonding in the decompose peritectically to A-rich MX and MAX phases it is not surpris- liquids and Mn+1Xn carbides or nitrides. ing they share many common attributes Thermal decomposition occurs by the and properties, such as metal-like elec- loss of the A element and the forma- trical conductivities, high stiffness val- tion of higher n-containing MAX ues, thermal stability, and low thermal phases and/or MX. Some MAX phase, expansion coefficients. such as Ti3SiC2, are quite refractory with decomposition temperatures above Physical properties 2,300°C.1 Most of the MAX phases are excel- Because of their excellent electrical, lent electrical conductors, with electri- thermal and high-temperature mechan- cal resistivities that mostly fall in the ical properties, some MAX phases narrow range of 0.2–0.7 µΩ·m at room currently are being considered for 1,10 temperature. Like other metallic structural and nonstructural high-tem- conductors, their resistivities increase perature applications. Their oxidation with increasing temperatures (Figure resistance, however, determines their (Credit: Sandvik Materials Technology, Sweden.) 3(a). Ti SiC and Ti AlC conduct 3 2 3 2 usefulness in air. In most cases, MAX better than titanium metal. Even more phases oxidize according to Eq (1). Figure 4. (a) Ti2AlC-based heating element resistively heated to 1,450°C in interesting and intriguing, many of the M AX +bO = air. (b) Micrograph of the Al O oxide MAX phases appear to be compensated n+1 n 2 2 3 (n+1)MO +AO +X O (1) layer after 10,000 thermal cycles up to conductors, wherein the concentra- x/n+1 y n 2b-x-y 1,350°C showing no spallation or crack- tions of electrons and holes are roughly ing of the oxide layer.33 equal, but their mobilities are about Consequently, their oxidation resis- 22 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 3 tance depends on nature of the oxides (a) (b) that form. The most oxidation-resistant MAX phase is Ti2AlC, because it forms a stable and protective Al2O3 layer that can withstand thermal cycling up to 1,350°C for 10,000 cycles without spallation or cracking (Figure 4).33 The oxidation resistance of Cr2AlC also is superb because it also forms a protective Al2O3 layer, however, the oxide spalls off during thermal cycling.

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