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 individual 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 Tin 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 understanding. 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. Elastically, the MAX phases are quite stiff, with near-isotropic room temperature Young’s and shear moduli in the 178–362 GPa and 80–142 GPa ranges, respectively.7, 14 Because the (c) densities of some of the MAX phases are as low as 4–5 g/cm3, their specific stiffness values can be quite high. For example, the specific stiffness of

Ti3SiC2 is comparable to Si3N4 and roughly three times that of titanium metal.

Mechanical Properties (Credit: Barsoum;Drexel University.) Despite similarities between the Figure 5. Transmission electron microscopy of (a) dislocation wall consisting of basal physical properties of the MX and plane dislocations and (b) area containing kink band in Ti3SiC2 after compression at MAX phases, the differences between room temperature.35 (c) Schematic of the formation of incipient kink band, mobile dis- their mechanical properties is strik- location walls kink bands, and delaminations. Red grains are “hard” grains, and blue ing. The MX phases are some of the grains are “soft” grains with the basal planes favorably oriented for easy slip.6 hardest solids known. They are brittle, nonmachinable, damage intolerant, and The BPDs arrange themselves either permanent kink bands (Figure 3(c)). At susceptible to thermal shock. In sharp in walls (that is, high- or low-angle higher temperatures, the grain boundar- contradistinction, the MAX phases are grain boundaries (Figure 5(a)), in arrays ies are soft and the IKBs devolve into exceedingly damage tolerant and ther- or dislocation pileups (not shown) MDWs and KBs that lead to delamina- mal shock resistant, and most are read- parallel to the basal planes. Confining tion at the individual grain level and ily machinable. This stark difference the dislocations to the basal planes, in considerable plasticity. in behavior comes down to two words: turn, results in an important micro- Although the MAX phases are quite mobile dislocations. mechanism that is quite ubiquitous in stiff, they respond to cyclic loading, At this time is it fairly well estab- the MAX phases at all lengths scales, whether compression6 or tension5, with lished that basal plane dislocations viz., kink band (KB) formation (Figures spontaneous, fully reversible, strain- (BPD)—and only BPDs— are abun- 1 and 5(b)6, 9). When the MAX phases rate-independent hysteretic stress-strain dant, mobile, and able to multiply are loaded, initially the “soft” grains— loops (Figure 6(a). The shape and in the MAX phases at ambient tem- those with basal planes favorably ori- areas of these loops depend strongly on peratures.34 However, because the ented for easy slip (blue grains in Figure grain size (Figure 6(a) but weakly on dislocations are constrained to the 3(c))—deform and, in turn, cause the number of cycles. In other words, basal planes, the number of slip sys- the “hard” grains (red grains in Figure they are quite fatigue resistant. It fol- tems is fewer than the five needed for 5(c)), to develop incipient kink bands lows that a significant portion of the polycrystalline ductility. Therefore, (IKB). The latter are coaxial disloca- mechanical energy—about 25 percent tion loops that, as long as their ends are the MAX phases occupy an interest- at 1 GPa in the case of Ti3SiC2—dis- ing middle ground between metals and not sundered, are spontaneously and sipates during each cycle.6 At this ceramics, in that they are pseudoductile fully reversible. With further increase time, IKBs (Figure 5(c)) that form under confined deformations or high in applied load, if the polycrystal does during loading and annihilate during temperatures, but are brittle at room not fail by shear band formation or unloading are believed to account for temperature, especially in tension and fracture, the IKBs result in mobile dis- this nonlinear elastic (or hysteretic) thin form. location walls (MDW) and ultimately effect. Above the BPT temperature, the

American Ceramic Society Bulletin, Vol. 92, No. 3 | www.ceramics.org 23 MAX phases: Bridging the gap between metals and ceramics

(a) (b) 2–8 GPa. They are thus softer than most structural ceramics, but harder than most metals.1, 9 The room temperature

fracture toughness (KIc) values—that range from 5 to almost 20 MPa·m1/2—

Stress (MPa) are quite respectable when compared with other monolithic ceramics. The Engineering stress (MPa) MAX phases also exhibit

R-curve behavior, i.e., KIc increases with increasing Strain Engineering strain crack length. For exam-

(Credit: Adapted from Barsoum, et. al., Ref. 6, 35.) ple, for coarse-grained Ti SiC , K increases from Figure 6. (a) Typical cyclic compressive stress–strain curves for Ti3SiC2 with two grain sizes. The loops 3 2 Ic overlap after one and one hundred cycles.6 (b) Engineering stress–strain curves for 2-mm cubes of 8.5 to 11 MPa·m1/2, with 37 highly oriented samples of Ti3SiC2. The inset cube shows a schematic sample with the chevron texture increasing crack size. and the orientation of the basal planes in individual grains depicted by thin lines.35 The high values of KIc and R-curve behavior result from the forma- stress-strain loops are open and strain as ideal plastic solids (Figure 6(b)) tion of plastically deformable bridging rate dependent but become smaller even at room temperature, with strain ligaments (Figures 7(a) and (b)) and 35 with increasing cycles, that is, cycling that exceeds 10 percent. By contrast, the crack-arresting properties of kink hardening takes place. The practical when the slip planes are parallel to the boundaries. The latter two mechanisms implication of these phenomena for applied load (loaded along the x-axis are unique to the MAX phases. structural applications cannot be over- in inset of Figure 6(b)) and deforma- Thus far, Ti3SiC2 is the only MAX estimated because the MAX phases tion by ordinary dislocation glide is phase on which cyclic fatigue stud- can dissipate a large portion of harmful suppressed, the sample yields at higher ies have been conducted. The studies structural vibrations or acoustic loads, stresses by KB formation. In this case, show that fatigue crack growth thresh- even at high temperatures. considerable strain softening occurs olds were comparatively higher than The room-temperature ultimate because the kink bands rotate basal those for typical ceramics and some compressive strengths of polycrystalline planes in such a way as to induce shear metals (e.g., 300-M alloy steel).37,38 35 MAX phases range from 300 MPa to 2 band formation. At 1,200°C, which is above the BPT, GPa and depend strongly on composi- the crack-growth rate versus stress tion and grain size. Like typical ceram- Softer than structural ceramics intensity curves show three distinctive ics, their room-temperature flexural and Unlike their MX counterparts, the regions emerging under the same condi- tensile strengths are lower than their MAX phases are relatively soft and tions, which suggests delamination or 1,7,14 compressive strength. For example, exceptionally damage tolerant. The grain-boundary decohesion as possible the compressive and tensile strengths Vickers hardness values of polycrystal- mechanisms. line MAX phases fall in the range of of Ti3SiC2, with 5-µm grains, are 1,050 All MAX phases tested to date go MPa and 300 MPa, respectively. At room temperature, they fail in a brittle manner. Nevertheless, they fail grace- fully—samples do not shatter but, rath- er, fail along planes inclined 30°–40° relative to compression axis. The stress–strain response of highly oriented (textured) microstructures loaded in compression is quite different from polycrystalline behavior, because (b) the former exhibit strong plastic anisot- (a) (Credit: Barsoum; Sci. Mater .) ropy. For example, when the basal (Credit: Barsoum; JACERS .) planes are oriented such that slip occurs Figure 7. SEM images of fatigue cracks in Ti3SiC2. The images also show bridging along the basal plane (along the z-axis ligaments, which plastically deform as a function of crack propagation. Arrow 36, 37 in inset of Figure 6(b)), they behave denotes direction of crack propagation.

24 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 3 through a BPT. The (a) (b) BPT temperature varies from phase to phase, but for many of them tested so far falls between 1,000°C and 1,100°C. Below BPT, the ultimate strengths of the exural strength (MPa) exural

MAX phases depend Stress (MPa) f weakly on temperature and deformation rate.1,5

Above BPT, their Retained stress–strain response depends strongly on temperature and, more –3 importantly, deforma- Temperature (k [log t (h)+20]3 10 ) Quench temperature (°C) tion rate. More spe- (Credit: Barsoum; Wiley .) cifically, when loaded Figure 8. (a) Creep properties of select metallic, intermetallic alloys, and Ti3SiC2 plotted as stress-to-rupture above their BPT tem- versus the Larson–Miller parameter. The solid black line represents compression results, and the dashed 1 7 peratures at high defor- line, tension results. (b) Postquench ﬂexural strength versus quench temperature of select MAX phases. mation rates, they fail in from temperatures as high as 1,200°C patented, and widely used. For exam- a brittle manner. However, when loaded into ambient-temperature water (see ple, Sandvik Materials Technology slowly, they can be plastically deformed Figure 8(b)). (Hallstahammar, Sweden) has manu- at 1,200°C in air—to strains greater Lastly, arguably the most charac- factured Ti SiC and Ti AlC powders than 25 percent even in tension—before 3 2 2 and parts since the late 1990s under its failing in a graceful manner.5 Because K teristic trait of the MAX phases and Ic MAXthal brand (Figure 9(b)). drops above the BPT temperature,39 we what truly sets them apart from other MAX phases in any form usually are can categorically rule out the activation structural ceramics or high-temperature fabricated from elemental powders and/ of additional slip systems.37 A sufficient alloys is the ease with which they can or binary carbides, and, thus, their price condition needed to explain the BPT is be machined (Figure 9(a)). The MAX is determined mostly by the price of the onset of a temperature-dependent phases can be readily machined with those powders. Currently, Sandvik sells grain-boundary decohesion-strength, regular high-speed tool steels or even manually with a hacksaw. Ti SiC and Ti AlC powders at around delamination strength, or both. 3 2 2 $500 per kg. This price is signiﬁcantly Although the MAX phases are higher than the price of Al O , SiC, considered good candidate materi- Potential applications 2 3 Before discussing potential applica- and Si N powders used to make other als for high-temperature applications, 3 4 tions, availability and cost have to be structural and high-temperature ceram- there are only a few published reports put in perspective. There are many ics. However, pressureless sintering in on their creep response. The few that methods for processing the MAX phas- inert atmospheres can yield fully dense exist for Ti SiC and Ti AlC suggest 3 2 2 es as bulk materials, powders, porous MAX phase parts without using sinter- that creep is independent of grain size, foams, coatings, and thin films.1,8,14 ing aids. More importantly, fully dense resulting from dislocation creep togeth- Some of the methods are quite mature, MAX phases are readily machined to er with significant accumulation of voids and microcracks.40 Nevertheless, (a) (b) creep resistance of the MAX phases is quite good when compared with other known creep-resistant materials (Figure 8(a)), and they offer great promise for future improvements. Another important property of the MAX phases is their exceptional thermal shock resistance. Unlike typical

ceramics, the MAX phases not only (Credit: Sandvik Materials Technology, Sweden.) do not shatter after quenching, and, Figure 9. (a) A MAX phase billet machined by a lathe. (b) Ti2AlC and Ti3SiC2 powders in some cases, their residual flexural and parts fabricated by Sandvik Heating Technology, Sweden, and commercially avail- strengths increase even after quenching able under the trade name MAXthal 211 and MAXthal 312.

American Ceramic Society Bulletin, Vol. 92, No. 3 | www.ceramics.org 25 MAX phases: Bridging the gap between metals and ceramics very high tolerances, which should ren- cations.43 The recent intense interest in References: der the cost of final parts competitive the MAX phases indicates that applica- 1M.W. Barsoum, MAX Phases: Properties compared with other structural ceram- tions are forthcoming. This is impor- of Machinable Carbides and Nitrides. Wiley ics. Furthermore, the price of powders tant—applications keep a research field VCH, 2013. should drop as demand increases. The vital. 2H. Nowotny, “Struktuchemie einiger development of reaction synthesis The understanding we have achieved verbindungen der ubergangsmetalle mit den methods from less-expensive precursor to date not withstanding, there remain elementen C, Si, Ge, Sn,” Prog. Solid State Chem., 2, 27–62 (1970). powders, such as TiO2 instead of pure outstanding scientiﬁc questions to titanium and TiC would constitute a answer and technological hurdles to 3H. Nowotny, J.C. Schuster, and P. Rogl, major breakthrough. overcome. Questions under exploration “Structural chemistry of complex carbides Given the remarkable set of prop- by more than a dozen research groups and related compounds,” J. Solid State erties that the MAX phases exhibit, around the world include Chem., 44, 126–33 (1982). especially their high-temperature stabil- • Can we extend the number of 4M. W. Barsoum and T. El-Raghy, “Synthesis ity, thermal shock resistance, damage known MAX phases to M, A, and X and characterization of a remarkable tolerance, good machinability, and elements not shown in Figure 2(c)? ceramic: Ti3SiC2,” J. Am. Ceram. Soc., 79, the exceptional oxidation resistance of • What are the effects of the lattice 1953–56 (1996). some of them, it is not surprising that defects on thermal and electrical prop- 5M. Radovic, M.W. Barsoum, T. El-Raghy, they were first targeted for high-tem- erties? (As well as the related question S.M. Wiederhom, and W.E. Luecke, “Effect perature applications. The most promis- of non-stoichiometry and its effect on of temperature, strain rate, and grain size on the mechanical response of Ti SiC in ing MAX phase for high temperature properties?) 3 2 tension,” Acta Mater., [Apr.] 1297–306 applications is Ti AlC because of the • To what extent can properties be 50 2 (2002). relatively low cost of raw materials tailored by solid solutions or by control- 6 needed, low density, superb oxidation ling the microstructure? M.W. Barsoum, T. Zhen, S.R. Kalidindi, M. Radovic, and A. Murugaiah, “Fully revers- resistance (that is immune to thermal • Why are they so thermal shock ible, dislocation-based compressive deforma- cycling), and crack-healing capabili- resistant? tion of Ti SiC to 1 GPa,” Nat. Mater., 2 41 3 2 ties, among others. This combina- • What determines their critical [Feb.] 107–11 (2003). tion of properties together with good resolved shear stresses? 7M.W. Barsoum and M. Radovic; pp. 195– electrical conductivity led Kanthal to • Can they be processed using more 227 in Annual Review of Materials Research, evaluate heating elements made from affordable precursors? Vol. 41. Edited by D.R. Clarke and P. Fratzl.

Ti2AlC. (Figure 5(a)). The company • What benefits can be gained by Annual Reviews, Palo Alto, Calif., 2011. also tested MAX phases for gas burner combining the MAX phases with met- 8P. Eklund, M. Beckers, U. Jansson, H. nozzles and industrial die inserts. Other als44 or ceramics in composite materials? Hogberg, and L. Hultman, “The M(n+1)AX(n) evaluated applications—such as high- phases: Materials science and thin-film temperature foil bearings, glove and Acknowledgments processing,” Thin Solid Films, 518 [Feb.] condom molds, tooling for dry drilling This work was partially funded by 1851–78 (2010). of concrete (3-ONE-2, LLC), and non- grants from the NSF (DMR-0503711) 9M.W. Barsoum and M. Radovic; pp. 1–11 stick cookware—took advantage of low and the ARO (W911NF-07-1-0628 in Encyclopedia of Materials Science and friction and good wear resistance of the and W911NF-11-1-0525) to Drexel Technology. Edited by R.W. Cahn et al. MAX phases and their composites.1 University and grants from the AFOSR Elsevier, Amsterdam, 2004. Besides high-temperature applica- (FA9550-09-1-0686) and NSF (CMMI- 10 M.W. Barsoum, “Physical properties of the tions, there may be electrical applica- 1233792) to Texas A&M University. MAX phases”; in Encyclopedia of Materials tions. For example, the first commercial Science and Technology. Edited by K.H.J. Buschow, R.W. Cahn, M.C. Flemings, application of Ti3SiC2 was as sputter- About authors ing targets for electrical contact deposi- Miladin Radovic is associate profes- E.J. Kramer, S. Mahajan, and P. Veyssiere. Elsevier, Amsterdam, 2006. tion (Impact Coatings, Sweden). They sor at the Department of Mechanical also were investigated for electrochemi- Engineering and Materials Science and 11X.H. Wang and Y.C. Zhou, “Layered cal chlorine production electrodes.42 Engineering Program at Texas A&M machinable and electrically conductive Ti AlC and Ti AlC ceramics: A review,” J. University, College Station, Texas. 2 3 2 Mater. Sci. Tehnol., 26, 385–416 (2010). The way forward Michel W. Barsoum is distinguished 12 Our understanding of the structure professor, Department of Materials J.Y. Wang and Y.C. Zhou; pp. 415–43 in Annual Review of Materials Research, Vol. 39. and properties of the MAX phases Science and Engineering, Drexel Annual Reviews, Palo Alto, Calif., 2009. has come a long way in less than two University, Philadelphia, Pa. Contact: 13M.W. Barsoum, “The M AX phases: decades. Typically, it takes between 10 [email protected] or barsoumw@ (n+1) (n) A new class of solids; Thermodynamically and 20 years from “discovery” to appli- drexel.edu. stable nanolaminates,” Prog. Solid State

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