Processing of MAX phases: From synthesis to applications

2.1 Lightweight

The density of MAX phases varies typically between 4 and 6 g/cm³, although some compositions such as Ta₂GaC and Hf₂TlC present larger values, 13.05 g/cm³ and 13.65 g/cm³, respectively. Nevertheless, Ti₃SiC₂, Ti₂AlC, Ti₃AlC₂, and Cr₂AlC present relatively low densities, 4.52 g/cm³, 4.11 g/cm³, 4.51 g/cm³, and 5.23 g/cm³, respectively. These density values are comparable to most of the advanced structural ceramics such as Al₂O₃ (4.0 g/cm³), SiC (3.2 g/cm³), Si₃N₄ (3.2 g/cm³), ZrB₂ (6.0 g/cm³) or 8YSZ (6.1 g/cm³), and lower than high-temperature superalloys as EMP102 (9.2 g/cm³), Inconel 738 (8.1 g/cm³), RENE N5 (8.6 g/cm³), or RENE N6 (9.0 g/cm³). This factor is determinant in fields such as transportation, in particular for airplanes and space vehicles.

2.2 Mechanical properties

The mechanical response of MAX phases has been largely explored, and is probably the most intriguing property. Typically, layered solids lack the five independent slip systems needed for ductility.²⁴ However, in MAX phases, basal plane dislocations (BPD) are abundant, mobile and able to multiply at room temperature. Confining the dislocations to the basal planes provokes kink band (KB) formation, a characteristic of MAX phases (Figure 2). These mechanisms have been accepted to describe the mechanical response, but a new mechanism referenced as ripplocations has been recently proposed and generated substantial debate in the community.²⁸ In contrast to dislocations, ripplocations have no Burgers vector and no polarity, but are based on the buckling of surfaces in Van der Waals layers.^29-31 MAX phases present high Young´s modulus with values around 300 GPa, but are relatively soft with hardness between 3 and 7 GPa. Interestingly, MAX phases exhibit a nonlinear, hysteretic, elastic behavior, which is unusual for stiff compounds. For example, polycrystalline Ti₃SiC₂ loaded cyclically in compression up to 1 GPa at room temperature, fully recover on the removal of the load, while dissipating about 25% (0.7 MJm⁻³) of the mechanical energy.³² Fracture toughness (K_IC) is one of the properties that has attracted the most interest due to quite respectable values.^{20, 24} Typically, K_IC of polycrystalline MAX phases ranges from 5 to 8 MPa·m^1/2, although higher values have been reported for some compositions, such as Ti₃SiC₂ and Nb₄AlC₃.³³ The largest value reported so far is for textured Nb₄AlC₃, achieving K_IC of 17.9 and 11.5 MPa·m^1/2 when the load was applied perpendicular or parallel to the basal planes.³⁴

The mechanical response at elevated temperature is particularly interesting due to brittle to plastic transition (BPT) around 1000°C-1100°C, depending on the composition and microstructure. Below the BPT they are brittle as ceramics, but above it MAX phases are plastic, with strain failures up to 25% at low strain rates.^{24, 35, 36} Failure is mainly caused by damage accumulation in the form of cavitations, pores, microcracks, and delamination.^37-39 Creep resistance has been reported for Ti₃SiC₂,^{37, 40} Ti₂AlC,³⁸ Ti₃AlC₂,⁴¹ and recently for Cr₂AlC,⁴² and Ti₂AlN.⁴³ Primary, secondary, and tertiary creep take place in the temperature range between 1000 and 1300°C, with activation energies in the range of 400 to 550 kJ/mol, and stress exponents (n) of ~2, indicating a creep mechanism controlled by grain boundary sliding. TEM observations suggest the formation of numerous stacking faults, dislocations that are mainly confined in the basal planes, and original lenticular non-planar defects.⁴¹ Furthermore, MAX phases are not susceptible to thermal shock, where residual strength of as-quenched samples decreases gradually without any catastrophic failure. This effect has been observed for Cr₂AlC,^{44, 45} Ti₃SiC₂,^{5, 46, 47} Ti₃AlC₂,^{46, 48} Ti₄AlN₃,⁴⁹ V₂AlC,⁵⁰ and Ti₃GeC₂.⁵¹ Interestingly, an “abnormal” thermal shock behavior has been detected for most of these composition at temperatures higher than 900°C, where strength of the quenched samples increases with temperature. The mechanisms are still under debate but correlated to the self-healing effect of MAX phases and the compressive stresses on the specimen surface caused by the formation of an oxide layer.⁴⁸

2.3 Damage tolerance

MAX phases exhibit better damage tolerance than other advanced ceramics. This effect can be observed by performing Vickers indentation. Instead of crack formation and propagation, other mechanisms such as delamination, kinking of individual grains, and grain pullouts are activated in the nearby area of the indent (Figure 3). In addition, Weibull modulus of MAX phases is high, that is, 29 for polycrystalline Ti₃SiC₂,⁵² although more experiments are required.

2.4 Melting point

MAX phases melt incongruently, forming typically M-X specimens (carbides or nitrides) and A-rich liquids or intermetallics at high temperature. The temperature of this incongruent melting strongly depends on factors such as purity and environment. For example, Ti₃SiC₂ is quite refractory in vacuum, with a decomposition above 2300°C,^{7, 20} but that drops up to ~1000°C under oxidizing environments. The M_n+1X_n layers are stable, whereas the A layers are weakly bonded and mobile, so the “A” element typically diffuses out of the structure at elevated temperature. Nevertheless, this drawback at first glance causes one of the most interesting responses of MAX phases at elevated temperature when the “A” element is Al and Al₂O₃ is formed under oxidizing environments.

2.5 Oxidation resistance

In general, MAX phases oxidize at relatively low temperature (~600°C) according to the following reaction:

(1)

Consequently, most of them are not suitable for high temperature applications under oxidizing atmospheres. However, some compositions that contain Al as the “A” element are able to form an external and adherent α-Al₂O₃ layer, which protects against further inward oxygen diffusion at temperatures up to 1400°C.⁵³ Unfortunately, not all Al-based MAX phases form an external protective alumina layer due to the “competition” between the oxidation of the “M” and “A” elements.²⁵ Ti₂AlC, Ti₃AlC₂ and Cr₂AlC form protective Al₂O₃ scales, but V₂AlC,^{54, 55} Ta₂AlC⁵⁶ and Nb₂AlC⁵⁷ form M-oxides such as V₂O₅, Ta₂O₅ and Nb₂O₅, respectively, at low temperatures (600°C-900°C). Consequently, only Ti₂AlC, Ti₃AlC₂ and Cr₂AlC are considered to operate under aggressive environments at high temperatures (900°C-1400°C) for extended periods. After some years of discussions,²⁰ the alumina scale growth was shown to be controlled by grain boundary diffusion, leading to cubic oxidation kinetics instead of parabolic.⁵⁸ Nevertheless, recent investigations claim that only the formation of a continuous Al₂O₃ layer leads to cubic oxidation kinetics, meanwhile a discontinuous alumina scale entails parabolic kinetics since oxygen can continuously diffuse inwards.⁵⁹ Additionally, the oxidation response is strongly influenced by factors such as particle size, orientation, secondary phases (carbides and intermetallics as well as Si, Fe, or S elements in ppm), Al/M ratio, and surface roughness.^{20, 25, 60-63}

Ti₂AlC is the most studied composition and has attracted high attention from industry. In fact, a Ti₂AlC heating element was already reported by Sundberg et al in 2004 due to excellent response after 8000 thermal cycles at 1350°C in air.⁶⁴ In this case, a 15 µm protective alumina layer was formed on the surface during use. The adhesion of the alumina scale is of significant importance for high temperature applications. High temperature alloys typically present high coefficients of thermal expansion (CTE),> 13 × 10^-6 K⁻¹, whereas α-Al₂O₃ has a lower value, 8-10 × 10^-6 K⁻¹. The CTE mismatch generates thermal stresses at the interface, leading to spallation of the oxide scale. These thermal stresses are notably reduced for Ti₂AlC since its CTE is 8.2 × 10^-6 K⁻¹. However, the main drawback of Ti₂AlC—as with Ti₃AlC₂—is the oxidation of the “M” element at temperatures as low as 600°C, forming rutile (TiO₂). As temperature increases the oxidation of the “A” element is favored, leading to the formation of an alumina scale that controls the oxidation kinetics. As a result, at temperatures between 1000°C-1300°C, the outer scale surface is composed of an alumina layer containing some TiO₂-rich nodules that generally do not seem to increase (Figure 4A). This configuration is quite stable up to temperatures around 1350°C for long times (>10³ hours), but at higher temperatures formation of Al₂TiO₅ and presence of cracks limit the exposure time.⁶⁵ Cr₂AlC exhibits also a good oxidation response at temperatures between 1000°C and 1300°C, following cubic kinetics. The main advantage of Cr₂AlC in comparison to Ti₂AlC is the limited oxidation of the “M” element. Consequently, a dense, continuous, adherent,⁶⁶ and protective alumina layer is formed at temperatures up to 1300°C under oxidizing atmospheres. On the other hand, the main drawback is the formation of a porous Cr₇C₃ layer between the alumina scale and the Cr₂AlC at temperatures higher than 1150°C (Figure 4B). This porous layer is critical although it survives hundreds of hours at 1300°C in air,⁶⁷ but a realistic operating temperature for Cr₂AlC under long exposure times should not exceed 1200°C. In addition, Cr₂AlC has a CTE of 12.0 - 13.3 · 10^-6 K⁻¹—the largest of the MAX phases—which generates more thermal stresses with the alumina layer, but opens the possibility to protect some refractory alloys against type I and II low temperature hot salt corrosion.^{68, 69}

Presence of water vapor at high temperature does not seem to greatly influence the scale formation and growth kinetics.^{62, 70} The oxidation resistance of Ti₂AlC and Cr₂AlC under more realistic environmental conditions using a burner rig has been reported.^{71, 72} Ti₂AlC was tested using a high pressure burner rig at 6 atm, 25 m/s, approx. 10% water vapor at 1100, 1200, and 1300°C. The cubic rate constants were approx. 20% of those measured under static conditions, which was cause by a small recession of the initial TiO₂ and TiAl₂O₅ on the alumina layer and their subsequent volatilization as TiO(OH)₂.⁷¹ Recently, dense Ti₂AlC was coated with porous YSZ as thermal barrier coating material, and tested in a jet fuel burner at high velocity of 100 m/s using 5h cycles.⁷³ No coating spallation or surface recession was detected after 500 hours, where the 20 µm alumina scale remained intact under the YSZ face, about twice in thickness that producing failure for TBC/superalloy systems. Regarding Cr₂AlC, samples were tested under severe thermal gradient conditions at 1200°C using a gas burner rig with gas velocity around 4-5 m/s. Cr₂AlC survived under these conditions thanks to the formation of a 5 µm-thick continuous, stable, and adherent alumina layer.⁷² The mechanical adhesion of the different interfaces formed, Cr₂AlC/Cr₇C₃, and Cr₇C₃/Al₂O₃, was characterized by micro-cantilever fracture tests, revealing that the excellent interlocking is caused by the combination of low internal strain due to similar CTEs and the convoluted interface.⁶⁶

2.6 Crack self-healing

The protective alumina layer, which is formed at temperatures between 900 and 1300°C under oxidizing environments, entails a volume expansion. Besides, alumina has similar CTE and Young´s modulus values compared to most MAX phases, thus they are suitable for crack self-healing. It was reported for the first time in 2008, when a crack with an approx. 7 mm length and an average width of 5 µm in Ti₃AlC₂ was healed at 1100°C for 2 hours. ⁷⁴ Other MAX phases, such as Ti₂AlC and Cr₂AlC,^{63, 75-78} have exhibited this response under similar conditions, but this effect was not observed for other phases such as Ta₂AlC.⁷⁹ Ann-Sophie Farle et al reported the different criteria for self-healing MAX phases as well as potential candidates, including the adhesion and cohesion energies, CTE and stresses, and Young´s modulus.⁸⁰ The healed cracks have sufficient mechanical integrity to make subsequent cracks from elsewhere upon reloading after the healing process.⁸¹ This effect has determinant consequences on the mechanical properties of MAX phases, such as after the impact of foreign objects ⁸² or exhibiting abnormal thermal shock behavior.^{44, 46}

2.7 Corrosion resistance

MAX phases have been evaluated under different corrosive environments and the response was good. Ti₃AlC₂ and Ti₂AlC were tested against molten Na₂SO₄ salt for 20 h hours at 900 and 1000°C resulting in weight gains of 0.15 and 0.45 kg·m⁻², respectively, meanwhile Cr₂AlC exhibited a weight gain of only 8 0.10^-4 and 1.5 · 10^-3 kg·m⁻² under the same conditions. ^83-85 Similar superior corrosion resistance was also reported for Cr₂AlC by K. Van Loo et al in a different corrosive medium.⁸⁶ In that work, Ti₂AlC, Ti₃SiC₂, Cr₂AlC, Nb₄AlC₃, (Nb,Zr)₄AlC₃, different SiC grades and a 40 vol% Fe/60 vol% (Nb,Zr)₄AlC₃ cermet were exposed to a KNO₃-NaNO₃ molten solar salt for 1000 hours at 600°C. Under these conditions, no alumina scale was formed leading to considerable corrosion of the materials except in the case of Cr₂AlC, which was protected by a micrometer-thin layer of Cr₇C₃. Furthermore, different MAX phases were screened with respect to their interaction with oxygen-poor (C_O ~ 5 × 10^-9 mass%), static and fast-flowing (v ~ 8 m/s) liquid lead-bismuth eutectic (LBE) for at least 1000 hours at 500°C.^{87, 88} No evidence of LBE dissolution attack was observed, despite the absence of a continuous external oxide scale. The local LBE interaction observed with the Zr-rich MAX phases consisted in the partial substitution of Al by Pb/Bi in the crystal structure and the in situ formation of (Zr,Ti)_n+1(Al,Pb,Bi)C_n solid solutions. Moreover the interaction of Zr-based MAX phases with static liquid LBE was accompanied by the dissolution of parasitic intermetallic phases, facilitating further LBE ingress into the bulk material. Interestingly, the selected MAX phases presented a better resistance to both dissolution corrosion and erosion in comparison to 316L stainless steels.⁸⁷ Surprisingly, only one work, involving Ti₂AlC, has recently been published about corrosion by calcium-magnesium-aluminum-silicate (CMAS) sand, despite the high potential of MAX phases for gas turbine applications.⁸⁹ Ti₂AlC pellets were exposed to molten CMAS at 1250°C for different hours and the melt was significantly suppressed by the alumina scale.

2.8 Coefficient of thermal expansion

MAX phases present a hexagonal crystal structure, with a consequent CTE anisotropy between a and c planes. Typically, the average CTE ranges between 8 × 10^-6 K⁻¹ and 10 × 10^-6 K⁻¹, although some compositions have lower or higher values.²⁰ For example, Nb₄AlC₃ presents an average CTE of 6.7 - 7.2 × 10^-6 K⁻¹,^{90, 91} meanwhile Cr₂AlC varies between 12.0 and 13.3 × 10^-6 K⁻¹.^{92, 93} Solid solutions modifies the CTE, as reported for Cr₂(Al_x,Ge_1-x)C in the 25°C-800°C temperature range.⁹² The thermal expansion of the a axis remains constant at 14 × 10^-6 K⁻¹, whereas the c axis decreases constantly from 17 × 10^-6 K⁻¹ for Cr₂GeC to approx. 12 × 10^-6 K⁻¹ for Cr₂AlC. The Cr₂(Al_0.75,Ge_0.25)C composition has an equal CTE in both axes, minimizing the thermal residual stresses. In general, the CTE of MAX phases lies between non-oxide ceramics and metals (Figure 5), and tend to be a good match for conventional Thermal Barrier Coatings (TBCs) such as YSZ (10-11 × 10^-6 K⁻¹) and the thermally grown oxide (TGO, α-Al₂O₃; 8.5-9.5 × 10^-6 K⁻¹). In fact, the good CTE match reduces around 1/7 the interfacial thermal stresses in comparison to conventional high temperature metallic systems. ^{94, 95}

2.9 Radiation tolerance

Ti₃SiC₂, Ti₂AlC, Ti₃AlC₂, Cr₂AlC, V₂AlC, Zr₂AlC, Zr₃AlC₂, and Nb₄AlC₃ have been proposed as accident tolerant fuel (ATF) claddings in third-generation light-water reactors (LWRs) and future fourth-generation fission plants due to the good radiation tolerance at room and high temperature. Ti₃SiC₂ was the first MAX phase to be investigated using heavy ions of different energies at room temperature to understand and differentiate between the nuclear and electronic interactions.⁹⁶ Elastic collisions lead to the amorphization of the Ti₃SiC₂, sputtering of the grain boundaries, and preferential sputtering as a function of the crystal orientation, whereas the electronic interactions induce the amorphization of only Ti₃SiC₂ and the expansion of the unit cell along the c axis. Expansion of the c-lattice parameter, as well as irradiation-induced hardness that could be annealed at 800°C, was also observed by Liu et al after irradiation to a maximum dose of 3.25 dpa (displacement per atom) of Al-doped Ti₃SiC₂ with high-energy Kr and Xe ions.^{97, 98} Ti₂AlC and Ti₃SiC₂ show good irradiation resistance up to 0.1 dpa, providing evidence for the MAX phases´ dynamic recovery at temperatures as low as 695°C.⁹⁹ Interestingly, elevated temperatures provide sufficient energy to allow migration and recombination of point defects, annealing the damage during irradiation.¹⁰⁰ Ti₃AlC₂ also shows excellent tolerance to irradiation damage over a wide fluence range. No amorphization occurs upto 31 dpa, although the nanolamellar structure disappeared through the formation of antisite defects and a phase transformation from α- to β Ti₃AlC₂ phase.¹⁰¹ Similar phase transitions from hcp to fcc structures were found in Ti₂AlN, Ti₂AlC, and Ti₄AlN₃, where the formation of intermediate γ-phases is driven by the production of cation antisite defects and a corresponding rearrangement of anions.¹⁰² The occurrence of extended defects, including stacking faults, contributes to the transition from the intermediate γ-phases to fcc phases. Cr₂AlC has been also irradiated under different ions and temperatures, detecting a structural transition for irradiations above 1 dpa, although the structure is stable up to 5.2 dpa without obvious lattice disorder.¹⁰³ The structural transition and irradiation effects saturation are ascribed to irradiation-induced antisite defects (Cr_Al and Al_Cr) and C interstitials, with a potential transition from the initial phase to γ-Cr₂AlC.^{103, 104} Furthermore, Zr- and Nb-based MAX phases have been recently considered due to the small neutron cross-section of Zr and the favorable refractory metal properties of Nb.¹⁰⁵ The post-irradiation examination revealed that Zr₃AlC₂ and (Zr_0.5,Ti_0.5)AlC₂ have a superior ability for defect-recovery above 400°C, meanwhile the Nb₄AlC₃ does not show any appreciable defect recovery below 600°C. In addition, DFT calculations have shown that 413-MAX phases in general may not perform well under low temperature irradiation conditions due to the preference for M:A antisites and C Frenkel pairs at the M-2 layer.¹⁰⁵

2.10 Tribological properties

The layered structure suggests a good tribological response. The first work published in 1996 reported a lubricious feel,⁵ and a few years later an ultra-low coefficient of friction (µ) was measured for Ti₃SiC₂ basal planes (2-5 × 10^-3) using a lateral force microscope.¹⁰⁶ However, the results reported later in polycrystalline samples have less attractive values. An exhaustive review was published by Gupta et al,²⁶ who also performed several investigations.^107-110 In general at room temperature, the wear rate (WR) was relatively high and almost linearly dependent to sliding distance, meanwhile the initial µ was low (< 0.2) but increased rapidly to larger values (>0.4). Certainly, the tribological response depends on several factors of the microstructure (grain size, purity, porosity, etc), the counter-body (metal, ceramic, hardness, etc), and the testing conditions (temperature, atmosphere, velocities, pressure, lubrication, etc).^{26, 111-113} Additionally, tribological response of polycrystalline MAX phases is controlled by complex tribochemical reactions.²⁶ At high temperatures, the tribological response is better, where initial µ is high (>0.6) but decreases with time to steady-state values (0.4-0.6) and WRs are typically low (<10^-5 mm³/Nm).^{108, 109}

Tribological performance has been improved by adding secondary phases such as Al₂O₃, BN, B₄C, SiC, chopped SiC fibers, Ti₅Si₃, and Ag.^114-118 Ceramic particles improve the wear resistance due to their higher hardness and decentralization of shear stresses under the counter-body, meanwhile the addition of metallic particles modifies the tribochemistry reactions. Additionally, MAX phases have been recently added as reinforcing particles in metals ^{119, 120} and polymers.^{121, 122} The composites present lower µ and WRs due to the mitigation of the abrasive wear and modification of the tribochemistry reactions.

2.11 Electrical properties

MAX phases are metallic electrical conductors in which the resistivity increases linearly with temperature.²⁰ In most of the cases, the electrical resistivity at room temperature varies between 0.02 and 2.00 µΩ m.^{23, 49} Interestingly, some MAX phases present lower electrical resistivities than their metallic counterpart, that is, Ti₂AlC and Ti₃AlC₂ (0.20-0.35 µΩ m) and Ti (~ 0.4 µΩ m). Electrical conductivity is affected by particle size and by secondary phases/impurities, and solid solutions which lead to scattering. Regarding the Seebeck coefficient, MAX phases present small values—even negligible,¹²³ with no particular temperature dependence. Electrical resistivity, Hall Coefficients, Magnetoresistance, transport mechanisms, and Seebeck coefficients are explained in detail elsewhere.²⁰ Electrical conductivity of MAX phases is several order of magnitude higher than advanced ceramics, which has an important consequence for machinability and opens new solutions for electrochemical reactions and electrically heating-catalyst (EHC).^{124, 125}

2.12 Thermal properties

MAX phases are considered good thermal conductors, with conductivities between 12 and 60 W m⁻¹K⁻¹.²³ The thermal response is extensively described elsewhere,²⁰ including the electron and phonon contributions as a function of temperature, rattler effect, role of defects, phonon density of states, heat capacities, and Debye temperatures. As with the electrical response, some MAX phases exhibit larger thermal conductivities than their corresponding transition metals.²⁰ The thermal response is the sum of the electron (Ƙ_e) and phonon (Ƙ_ph) conductivities, where each contribution depends on several factors such as temperature, composition (mostly the “A” element), grain size, secondary phases/impurities, and defect concentration.^{20, 126, 127} Overall, MAX phases are good phonon conductors because of the M-X bonds, and particularly composition containing Al due to the lower scattering since it is light and mobile. Furthermore, phonon contribution is sensitive to the “n” value, where the distribution is wider as follows 211 > 312 > 413. Regarding the respond at high temperature (1300 K), Ti₂AlC presents one of the highest thermal conductivities thanks to its high Ƙ_ph. These experimental data are in good agreement with the calculation of Ƙ_ph using the Debye theory.¹²⁸

2.13 Machinability

MAX phases can be cut using a handsaw or a lathe due to the combination of low hardness, excellent damage tolerance, and good thermal shock resistance. Furthermore, they can be precisely machined by electrical discharge machining (EDM) thanks to their high electrical conductivity.¹²⁹ This is of special importance when compared to most of the advanced ceramic materials, where machinability is difficult, time consuming, and expensive due to their high hardness and brittleness. Melting and decomposition were found to be the material removal mechanisms during EDM machining of Ti₃SiC₂. As expected, material removal rate was enhanced by increasing discharge current and working voltage, but microcracks on the surface and loose grains in the subsurface were detected, degrading the strength and reliability.¹³⁰ The cutting resistance and roughness of Ti₃SiC₂ are lower when compared with a middle-carbon steel, SM45C.¹³¹

3.1 Physical vapor deposition

Eklund et al published in 2010 an exhaustive review about thin-film processing of MAX phases, including PVD and other techniques, nucleation and growth mechanisms as well as resulting properties.¹³² In general, three main methods have been considered to synthesize thin films with high purity and density: magnetron sputtering, cathodic arc deposition, and pulse laser deposition (PLD). Depositions have been mainly performed at substrate temperatures between 800 and 1000°C, which are high for some sensitive substrates such as certain steels.¹³² Consequently, effort have been focused on the reduction of the deposition temperatures of some compositions such as Cr₂AlC (450°C),¹³³ V₂GeC (450°C),¹³⁴ or V₂AlC (600°C).¹³⁵ Magnetron sputtering is the most used PVD method thanks to the easy processing, high flexibility, good control over phase purity and composition. Individual targets, typically three for “M”, “A”, and “X” (graphite) elements, are commonly used and preferred for the individual and flexible control of elements.¹³⁶ On the other hand, a single target, either a MAX phase compound target or a composite target of “M”, “A”, and “X” elements in the desired MAX phase stoichiometry, is mostly considered for industrial processes due to simplicity and repeatability. In the case of MAX-nitrides, reactive sputtering using nitrogen gas as N source is typically used.¹³⁷ Synthesis can be performed in one step, which involves heating the substrate during the deposition and often leads to columnar growth. Another option is to deposit first a film with the desired MAX phase stoichiometry, and annealing treatment at high temperature to crystallize the MAX phase. In general, highly dense and pure thin films with thicknesses from few nanometers to several micrometers can be obtained by sputtering. Cathodic arc deposition has been less studied in comparison to sputtering techniques due to the higher complexity of the process and equipment. The process is based on the vaporization from typically three cathode targets (M, A, and X elements) caused by a pulsed high current and low voltage arc. The high energetic arc generates a high degree of ionization of the deposition flux, almost 100%, which might reduce the synthesis temperature.^{138, 139} This process leads to a fine control of the final composition and in some specific cases even lower synthesis temperature, such as for Ti₂AlN.¹³² Thick films can be obtained due to the high degree of ionization, but the powerful energies produce a negative effect, which is the presence of macroparticles embedded into the film. The amount of macroparticles can be reduced using filters, but their presence is a common drawback of this technique. PLD has been less explored than other PVD techniques despite the high potential to deposit MAX phases at temperatures below even 300°C.¹⁴⁰ A high-power pulsed laser beam strikes a MAX phase single target in a high-vacuum chamber, vaporizing the MAX phase and depositing it on a substrate kept at a low temperature (25°C-800°C). PLD leads to high purities, but in the case of MAX phases, some carbides are typically detected between the nanometric MAX grains.¹⁴¹ The ion-beam has several interesting effects on film thickness and composition, concentration gradients within the films and on conductivity and hardness.¹⁴¹ However so far, no clear model can be presented, but the high potential of PLD appeals for further investigations.

3.2 Liquid/solid state reaction

Liquid/Solid state reaction (SSR) is the most used method to synthesize MAX phases. This method is based on the reactions at high temperature between the elemental constituent starting powders or other precursors such as carbides (MX) or hydrides (MH). The synthesis temperature typically ranges between 1100°C and 1700°C and is carried out under protective atmosphere to avoid the oxidation. As “M” source elemental powders such as Ti for Ti₂AlC and Ti₃AlC₂, or Cr for Cr₂AlC are more commonly used, although carbides such as TiC and Cr₃C₂ are also good options. Another alternative is to use metal hydrides, such as TiH₂ or ZrH₂,¹⁴² since they seem to be beneficial for compositions that are difficult to synthesize such as Zr₂AlC or Zr₃AlC₂.^{143, 144} However, the use of metallic hydrides might be dangerous because of the liberation of H₂ during the thermal process, so safety has to be strongly considered for large quantities. As “A” source, elemental powders such as Al or Si are extensively used due to the low prices, and an excess up to 20 at. % is added to compensate their loss due to evaporation at high temperature. As “X” source, graphite is used for carbide-MAX phases due to its low price, although carbides can also be employed. In general, substoichiometric quantities of carbon, ranging between 1.0 and 0.9 at.%, reduces the formation of secondary phases, and, consequently, increasing the yield. In the case of nitride-MAX phases, nitrides are used as starting powders since nitrogen is a gas under standard conditions. In terms of processing, SSR is simple. Starting powders are homogenously mixed in ethanol, followed by drying and pressing of the powders. Afterwards, pellets are heated up under protective atmosphere for the synthesis. Several reactions take place during the thermal process,^{145, 146} so MAX phases typically coexist with carbides or nitrides and some intermetallic compounds. Among the different synthesis methods, pressureless synthesis (PLS), self-propagating high-temperature synthesis (SHS), hot-pressing (HP), hot isostatic pressing (HIP), spark plasma sintering (SPS), and microwave (MW) are briefly explained.

3.2.1 Pressureless synthesis

PLS is one of the most common synthesis methods due to its simplicity.^{145, 147-151} The pellets are just placed in a crucible and heated under vacuum or argon atmosphere to temperatures around 1400°C for few hours. The formation of MAX phases starts around 1000°C, but the highest yield is achieved around 1300°C-1400°C. At higher temperatures some carbides or nitrides start to be detected due to the degradation of MAX phases. PLS leads to highly pure, around 95 wt%, and porous pellets with relative densities up to 90%–92%. This process is commonly used to produce powders, and is, therefore, often followed by a milling step. Figure 6 shows an example of a 500g batch of Cr₂AlC synthesized by PLS, followed by milling to obtain fine powders. In general, PLS is simple, cost-effective, flexible in terms of precursors, scalable to large quantities, and leads to relatively high purities. The limitations are related to the long thermal cycles, the inability to produce fully dense components, and the necessity of a milling step to obtain powders.

3.2.5 Spark plasma sintering

Sample preparation is similar to HP but the main difference resides in the thermal cycle. SPS allows faster heating rates (50-200°C/min) and shorter dwell times (<10 minutes) than HP, limiting grain growth.¹⁶³ Furthermore, lower temperatures can be used so dense samples are obtained between 1100 and 1400°C.^164-167 Conventional samples are discs of 4-5 mm in height and 20-30 mm in diameter, although larger samples of 100 mm in diameter are also possible (Figure 7A).^, The main advantage of SPS is the full densification of highly pure MAX phases with controlled microstructural features, from fine (<10 µm) to coarse grains. However, the main limitation is again the simple geometry of the produced samples.

3.3 Molten processes

Reactive melt infiltration (RMI) and molten salt (MS) are categorized as molten processes because a liquid phase is involved. Perhaps, RMI and MS are not the most attractive methods to synthesize some MAX phase ceramics for the fundamental studies, but they might contribute substantially to the processing of complex components and synthesis of MAX phase powders.

4.1 Dense bulk samples

Full densification of MAX phases cannot be achieved by PLS—maximum relative densities around 90%. Thus, pressure-assisted sintering techniques are required, limiting the sample geometry mostly to discs and plates. Dense bulk specimens with high purity produced by SPS typically have fine grains (around 3-15 µm) while those produced by HP have elongated coarse grains (>15 µm). Most of the time, reactive sintering (synthesis and sintering in the same thermal cycle) is performed due to simplicity and precise control of purity. However, sintering of pre-reacted MAX phase powders by SPS often leads to a better control of the microstructure. For example, Badie et al have recently synthesized submicron Ti₂AlC powders by MS3, followed by densification by SPS at 1200°C for 10 minutes to produce dense Ti₂AlC with an average grain size below 1 µm, which is the finest microstructure reported for a dense MAX phase to date.¹⁹⁵

Pressure assists densification but can promote texturing, where basal planes of grains are oriented perpendicular to the applied force. As expected, texturing is mostly observed in HPed samples due to the long sintering times and coarse microstructure, meanwhile SPSed samples present practically isotropic microstructures or slight texturing. Texturing can be magnified to tailor the microstructure and properties. MAX phase powders were slip casted and oriented under strong magnetic fields (around 10-12 T), followed by SPS. Lotgering orientation factors for the top and side surfaces in the range of 0.6 to 0.7 have been reported for Ti₃SiC₂, Nb₄AlC₃, and Ti₃AlC₂.^{34, 61, 196, 197} Another approach is referenced as edge-free spark plasma sintering (EFSPS) and is based on the plastic deformation near the brittle-to-plastic transition temperature.¹⁹⁸ In this case, a dense MAX phase disc with a smaller diameter than the graphite die was heated in a SPS system under low pressure near to the brittle-to-plastic temperature. At this temperature high pressure was applied, deforming the sample until its diameter was equal to the inner graphite die and promoting texturing.¹⁹⁸ Uniaxial pressure usually precludes the sintering of complex structures in SPS, but a recent strategy based on a carbon bed leads to sinter near net-shapes.^{199, 200} The green sample with a complex structure was surrounded completely by graphite powders, which was placed in a graphite die and between two graphite punches (Figure 7B,C). Then, a conventional SPS cycle was run, where mechanical load was transferred to the sample to produce pseudo-isostatic conditions.^{199, 200} This strategy has been performed on a Ti₂AlC gear wheel processed by injection molding and sintering at 1350°C and 50 MPa of uniaxial pressure (Figure 7B,C).²⁰¹ The gear wheel had a relative density of 98.2%, while only 89.6% was achieved by pressureless sintering at the same temperature.

4.2 MAX phase composites

MAX phases have been reinforced with ceramic particles to improve their mechanical and tribological performance, and they have also been used as reinforced phase in metallic and polymeric matrices. In general, the precursors are mixed in precise quantities to attain the maximum MAX phase yield, minimizing the content of carbides or nitrides. However, the content of these secondary phases can be controlled through the quantities of the starting powders to develop Ceramic Matrix Composites (CMCs). Ti₃SiC₂ composites containing 30 vol% of SiC particles or 30 vol% of TiC particles were sintered by HIP at temperatures up to 1600°C modifying the amounts of TiH₂, α-SiC, and graphite.¹⁶² Similar strategy was used by Tian et al,²⁰² where Ti, Si, and TiC were used as starting powders to synthesize Ti₃SiC₂ using SPS at 1400°C, as well as to develop Ti₃SiC₂/TiC composites through the addition of excess TiC. The incorporation of TiC particles increases the hardness from 5 GPa (nominally pure Ti₃SiC₂) to approx. 11 GPa (Ti₃SiC₂ containing 40 vol% TiC). A similar trend was observed for the flexural strength, which increased from ~350 MPa to ~575 MPa with the incorporation up to 40 vol% TiC.²⁰² Ti₃SiC₂ composites containing TiB₂ and TiC particles were also sintered using HP at 1500°C for 2 hours through the adjustment of the starting TiH₂, Si, graphite, and B₄C precursors.²⁰³ In this work, oxidation response up to 1200°C was improved with the incorporation of TiB₂ particles. Cr₂AlC containing up to 20 vol% Al₂O₃ particles were processed by in-situ reaction of Cr₃C₂, Cr, Al, and Cr₂O₃ powders in HP at 1400°C.²⁰⁴ Hardness and flexural strength increase continuously with the alumina content, meanwhile the fracture toughness increases with contents up to 11 vol% Al₂O₃ followed by a decrease for higher alumina contents. This strategy is simple and controls properly the second phase content, but unfortunately does not tailor the morphology of the particles.

Control of morphology can only be attained through the incorporation and dispersion of the second phase into the MAX phase powders, followed by consolidation at high temperature. This was the case for developing Ti₃SiC₂ reinforced with cubic-BN,²⁰⁵ and MAX phase composites reinforced Al₂O₃ and SiC fibers. Carbon fibers react with MAX phases—at least with Ti₃SiC₂, Ti₂AlC, Ti₃AlC₂, and Cr₂AlC- so the composites will contain a strong reaction zone at the matrix/fiber interface that grows and consumes the fibers at high temperature. Nevertheless, Ti₃SiC₂ reinforced with C fibers has been processed,^{206, 207} showing a reaction zone at the interface that was minimized through the fast sintering using SPS.²⁰⁷ Reactions also take place between Ti₂AlC and Ti₃AlC₂ with SiC fibers, forming TiC and Ti-Al compounds.²⁰⁸ However, the reaction is hindered using a titanium barrier layer between the commercial SiC fibers and the Ti₃AlC₂ powders, enhancing the flexural strength and promoting a non-catastrophic fracture behavior.²⁰⁹ No reactions have been reported for Ti₃SiC₂ and Cr₂AlC with SiC fibers at high temperature,^{116, 208} as well as for Ti₂AlC, Ti₃AlC₂, and Cr₂AlC with Al₂O₃ fibers.^{210, 211} In the case of Ti₃SiC₂ reinforced with SiC fibers or SiC whiskers, the creep resistance at temperatures up to 1300°C was improved up to two orders of magnitude.^{212, 213} Furthermore, addition of 10wt% of short SiC fibers into Cr₂AlC enhanced the tribological response, specifically a reduction of 20% in the friction coefficient and up to 80% in the wear rate.¹¹⁶

MAX phases have also been added into metallic matrices to improve some properties. For example, Cu/Ti₃SiC₂ composites were processed by powder metallurgy to develop a new electrofriction material with a self-lubricated response.²¹⁴ Copper was also mixed with Ti₃AlC₂ particles and consolidated by HP, and the composites present superior flexural strength than copper without a remarkable loss of fracture toughness.²¹⁵ In addition, Cr₂AlC particles were used to tailor the electrical performance of copper, in particular to improve the arc erosion ability.²¹⁶ Aluminum has been also reinforced with Ti₃AlC₂ particles, increasing the yield stress up to 100% over the temperature range of 20°C-500°C.²¹⁷ Mg-alloy (AZ91D) composites containing Ti₂AlC particles were fabricated by stir casting technology to increase the yield strength, Vickers hardness, ultimate compressive strength, and elastic modulus.²¹⁸ Another alternative is to process a highly porous MAX phase structure (porosity around 40-60 vol%), which can then be infiltrated with a molten metal at high temperature. Mg/Ti₂AlC,²¹⁹ Al alloy/Ti₂AlC,²²⁰ NiTi/Ti₃SiC₂, and NiTi/Ti₂AlC,²²¹ were processed using this method. The Mg/Ti₂AlC composites showed a reversible, reproducible, hysteretic stress-strain loops during uniaxial cyclic compression tests, as well as a superior ultimate compressive (700 MPa) and tensile strength (380 MPa) in comparison to other Mg composites.²¹⁹

4.3 Porous architectures

MAX phase porous structures present an excellent combination of properties such as low density, good corrosion and oxidation resistance, high surface area, machinability, superior mechanical properties, and electrical conductivity. Content and size of the porosity can be precisely controlled by the processing route, which are categorized in three main groups: sacrificial template, replica method, and direct foaming (Figure 8).²²² Additionally, the incomplete densification obviously entails a porous material, but in this case the size of the porosity cannot be controlled (Figure 9).^{223, 224}

4.4 Coatings

Highly pure and dense thin films can be deposited by PVD at fairly low temperatures, which certainly might be determinant to coat sensible substrates. The thickness of these films ranges commonly between few nm to ~50 µm, but for thicker coatings thermal spray technologies are preferred. Briefly, thermal spray deposition is based on the ejection and acceleration of heated and/or molten particles towards a substrate. Coatings of up to some millimeters can be obtained. High gas temperatures (> 10.000°C) are typically required to accelerate (100-500 m/s) the particles and to heat/melt them. However, MAX phases might oxidize and degrade due to these high gas temperatures and their inherent incongruent melting, even under the low particle flight-times (<10^-2 s). This was the case during the deposition of commercial Ti₂AlC powders (initial purity of 68.3%) by plasma spray, where coatings were underdense and presented only 26% Ti₂AlC purity.²³⁷ One alternative was to use high gas temperature to synthesize MAX phases in-situ during spraying. Feedstock composed of Ti, Si, and graphite powders were deposited by plasma spray, but the very short time (~ 10^-4 s) limited the Ti₃SiC₂ formation, achieving purities lower than 20%.²³⁸ Similar results were obtained after the deposition of Cr, Al, and graphite powders by plasma spray, where the yield of Cr₂AlC was only 1.6 wt%, which increased up to 23.7% after an annealing process at 800°C. ²³⁹

The best alternative is to avoid high temperatures during spraying, such as High velocity oxy fuel (HVOF) and cold spray (CS), that use lower gas temperatures, but higher velocities to provide enough kinetic energy to the particles. Gas temperatures are strongly reduced to 2000-3000°C and 300-1000°C for HVOF and CS, respectively, whereas the particle velocity ranges from 400 to 700 m/s and 700 to 900 m/s. Highly dense, 200 µm thick and adherent Ti₂AlC coatings were deposited on Inconel substrates by kerosene-fueled HVOF, although purity was only 53 wt%.²⁴⁰ Comparable results were reported for other HVOF depositions using commercial Ti₂AlC powders, where highly dense coatings that were 100-200 µm thick contained TiC and Ti_xAl_y particles.^241-243 In spite of the high density, thickness, and adhesion of the HVOF coatings, the gas temperature still causes the MAX phases to decompose into carbides and intermetallics. The degree of degradation of MAX phases is decreased when powders are deposited by CS. Most ceramics cannot be deposited by CS, but the shear response and high deformability of MAX phases allows for this possibility (Panel A). Approx. 90 µm thick Ti₂AlC coatings with high density and good adhesion were deposited onto Zircaloy-4 substrates.²⁴⁴ In general, purity is maintained by CS, although coatings typically have inter-splat cracks.^{245, 246} Highly pure Cr₂AlC powders were deposited by CS (Figure 10 A,B), obtaining highly pure coatings (>98% by Rietveld) with preferentially oriented particles.²⁴⁷ As for other MAX phase coatings processed by CS, some inter-splat cracks were detected, leading to a final relative density up to 91%. CS is certainly a good technique to develop MAX phase coatings since purity is kept and large thicknesses and relatively high densities can be attained. However, optimization of the powder features is required to increase the density of coatings.

High velocity atmospheric plasma spray (HV-APS) and liquid plasma spray (LPS) have been used to deposit Cr₂AlC and Ti₃AlC₂.^{248, 249} Cr₂AlC coatings with thicknesses up to 100 µm and relative densities up to 93% were deposited on stainless steel and Inconel 738 by HV-APS (Figure 10 C). Coatings exhibited good adhesion, but Cr₂AlC was partially decomposed. Additionally, Cr₂AlC coatings were used as bond-coat, between an Inconel substrate and a conventional porous YSZ TBC layer. The Inconel/Cr₂AlC/YSZ system was tested in a burner rig at 1400°C (surface temperature) and survived 745 cycles, indicating that Cr₂AlC has a high potential as bond-coat.²⁴⁸ Regarding the deposition of Ti₃AlC₂ powders by LPS,²⁴⁹ coatings were 10-20 µm thick with high density and purity despite an expected temperature higher than 2200 K during deposition. The solvent and the pH play a determinant role, but more investigation is required to confirm and improve these promising results. Finally, electrophoretic deposition (EPD) has briefly been explored to develop MAX phase coatings, however, porosity issues were observed in the coatings. Ti₃Si(Al)C₂ powders were deposited at 3V on indium-tin-oxide substrates from a suspension with 1 vol% solid loading at pH 9 without any dispersant.²⁵⁰ The coatings were not dense and exhibited a (00l) preferred orientation. In addition, porous Ti₃SiC₂ coatings were deposited by EPD at 10V using a 4.3 wt% solid loading at pH 9.²⁵¹ Afterwards, the thin coating was rapidly densified using a 3D printing laser, but only some Ti₃SiC₂ remains after sintering.

4.5 Near net shaping

Different approaches, with respect to the complexity of the structure, have been performed to produce near net shape components. Surprisingly, the number of investigations is rather low, which has reduced the interest of potential industrial partners.

Slip casting is a simple and inexpensive method to manufacture large and thin walled complex parts. The first investigation was performed by Ansell Healthcare and Drexel University to make latex products.²⁰ Subsequently, Sun et al investigated the rheological properties, revealing that Ti₃AlC₂ suspensions with 2.0 dwb% PAA at pH 5 present an excellent stability and a characteristic Newtonian behavior.²⁵² The relative densities of green bodies were 61% and PLS samples was 96%. Ti₃AlC₂ particles can be dispersed in basic pH, particularly at pH 10 and using 1 wt% of Arabic gum as dispersant.²⁵³ The green compacts were densified at 1450°C for 90 minutes using an Al₄C₃ powder bed, achieving 95.3% relative density and a low content of impurities. Recently, Ti₃SiC₂ tubes have been processed by slip casting as a proof of concept for nuclear fuel cladding.²⁵⁴ The suspension contained 49.5% dry weight basis (dwb) Ti₃SiC₂, 46% dwb water, 4% dwb polyethyleneimine, and 0.5% dwb. methylcellulose, and it was sintered at 1450°C for 2h under vacuum. The sintered tubes had a relative density of 89% with some silicon loss at the outer edges.

Injection molding is widely used in industry to shape small items due to the high precision, simplicity, velocity and low cost.²⁵⁵ The first step is the preparation of the feedstock, which is a mixture of powders, wax, thermoplastic binder, and additives. The feedstock is injected into a mold at temperatures around 100°C-200°C, and the near net shapes are debinded first in a non-polar solvent to remove the wax, and then in an oven to remove the rest of the binders. Finally, the components are sintered. Commercial Ti₂AlC and Ti₃SiC₂ powders were mixed with paraffin 65 as wax, to ensure the mechanical stability and stearic acid to lubricate.²⁵⁶ Feedstocks contained 62 vol% of solid content and 38 vol% of the binder system, and were injected into a gear wheel mold with a flow rate of 15 cm³/s, injection pressure of 1150 bars, and injection and mold temperatures of 175°C and 90°C, respectively (Figure 7C). Ti₂AlC-Al composites have been also processes using commercial Ti₂AlC powders.²⁵⁷ The feedstock contains 83 wt% of hydrophobized Ti₂AlC, 15 wt% of paraffin and 2 wt% of carnauba wax, and was injected into building block molds with dimensions of 4.6 mm × 1.5 mm × 1.5 mm. Afterwards, the Ti₂AlC building blocks were sintered in argon at 1350°C, and jointed to brick-and-mortar structures using aluminum as filler.

AM is an excellent approach for small samples with a high degree of complexity, although presents two main challenges for MAX phases: full densification of the 3D structure—as in general for ceramics—and the inherent incongruent melting. As a result, AM techniques that utilize melting, such as selective laser melting, should be avoided to retain phase purity. One alternative might use the focalized high temperature to synthesize MAX phases, but the extremely fast kinetics reduce the phase yield.²⁵⁸ The first publication was reported in 2002 by Sun et al,²⁵⁹ where fully dense (> 99%) and complex Ti₃SiC₂ structures were obtained by a three-stage fabrication process, based on 3D layered printing, cold isostatic pressing and sintering. The green parts were built by deposition of Ti₃SiC₂ powder by binder spray layer, using layers of ~90 µm and ~180 µm thick. In spite of this promising outcome, the most used approach for MAX phases so far is the combination of 3D printing of a porous carbide, followed by liquid silicon infiltration or reactive melting infiltration with aluminum to synthesize Ti₃SiC₂ and Ti₃AlC₂, respectively.^{176, 260-262} Densities are high, but the main problem is the low yield of the synthesis reaction that ranges between 50 and 90 wt%. A similar concept was performed to produce gears by laminated object manufacturing.²⁶³ Tapes with thickness of approx. 100 µm, composed of TiC and SiC, were processed by tape casting followed by drying and lamination. Afterward, green tapes were processed via Laminated Object Manufacturing to fabricate a 3D gear structure, consolidated at 1600°C, and infiltrated with silicon at 1450°C. The gears were fully dense but Ti₃SiC₂ coexisted with other phases such as TiSi₂. The best alternative to achieve high purity is to manufacture 3D structures using MAX phase powders, followed by their consolidation at high temperature. Robocasting and direct ink writing—based on the same principle - have been used to produce complex 3D structures based on Cr₂AlC (Figure 9) and Ti₂AlC, respectively.^{126, 236} The processing was described above in Section 4.3 (porous architectures).

5.1 High-temperature structural material

The most likely potential application for Al₂O₃ scale-forming MAX phases is as components that operate under aggressive environments at temperatures between 1100°C and 1400°C over long period of times. In addition to heating elements and gas nozzles, Ti₂AlC, Ti₃AlC₂ and Cr₂AlC are potential candidates to replace Ni/Co superalloys in the hottest part of a gas turbine engine to enable operation at higher temperatures and thereby increase efficiency. Ni/Co superalloys have maximum operating temperature around 1100°C-1150°C, which could be increased by ~100°C using MAX phases. Combustor liners, blades and vanes withstand temperatures above 1600°C, oxidizing atmospheres, entry of corrosive particles such as CMAS and foreign objects, and high thermal and mechanical stresses. Superalloys are protected from these conditions by TBCs, which are based on an external porous oxide layer and an intermediate bond-coat.²⁶⁴ MAX phases have an excellent CTE match (Figure 5) with standard TBC materials and the thermally grown oxide (TGO) α-Al₂O₃ at high temperature. Consequently, the thermal stresses are reduced by nearly an order of magnitude compared to conventional high temperature alloys (CMSX-4, CMSX-10, Rene 80 or Inconel 738), ^{53, 94, 95} which can extend the life of the whole system. Additionally, bond-coats are not any longer required due to the strong adhesion, compatibility and formation of TGO, simplifying the processing.^{73, 94, 95} Furthermore, Ni/Co superalloys are composed of and typically doped with strategic elements that have limited supplies and/or under serious threat, that can also be toxic to the environment and human health. Meanwhile, elements such as Ti, Al, and C that compose MAX phases are abundant in high quantities, nontoxic, and cheap.

5.2 High temperature coatings

MAX phase coatings are being considered as protective coatings for refractory alloys and as bond-coat layers in TBC systems. In both applications, CTE is critical to reducing stresses and avoiding spallation of the coating. Cr₂AlC is more suitable for metallic systems due to the larger CTE (12.0-13.3 × 10^-6 K⁻¹), meanwhile Ti₂AlC and Ti₃AlC₂ (8.2-9.0 × 10^-6 K⁻¹) match better with TBC compounds. For example, advanced turbine disks can be protected against Type I and II Low Temperature Hot salt Corrosion (LTHC).^{68, 69} Cr₂AlC coatings deposited on a low γ´solvus and a high refractory content alloy—referenced as LSHR and developed at NASA—prevented hot corrosion pitting and improved the low cycle fatigue resistance up to 90%. Recently, Cr₂AlC bond-coats have been deposited on Inconel 738 substrates, followed by deposition of a porous yttria-stabilized zirconia as external TBC.²⁴⁸ The system was tested under thermal loading conditions similar to a gas turbine environment, achieving surface and substrate temperatures of 1400°C and 1050°C, respectively. The system failed after 745 cycles (conventional bond-coat systems under similar conditions around 1000-1200 cycles) mainly for three reasons: (i) open porosity of the Cr₂AlC bond-coat layer, (ii) oxidation of secondary phases, and (iii) inter-diffusion. Furthermore, future challenges to specifically transfer MAX phase as bond-coats are described elsewhere.²⁴⁸ In this case, Cr₂AlC was tested, but Ti₂AlC presents even more potential because of better CTE match.

5.3 Nuclear

Phases such as Ti₃SiC₂, Ti₂AlC, Ti₃AlC₂, Cr₂AlC, V₂AlC, Zr₂AlC, Zr₃AlC₂ and Nb₄AlC₃, that combine excellent radiation tolerance with oxidation and corrosion resistance, mechanical properties and chemical stability have attracted considerable attention as potential accident tolerant fuel (ATF) claddings in third-generation light-water reactors (LWRs) and future fourth-generation fission plants. Zirconium alloys fulfill the characteristics to operate as cladding materials in nuclear power plants, but they perform poorly in the event of Loss of Coolant Accidents (LOCAs) as happened in Chernobyl (1986) and Fukushima (2011). MAX phases could replace or coat zirconium alloy claddings due to their superior oxidation and corrosion resistance. For this specific case, dense, highly pure, and nontextured coatings based on MAX phases should be processed at low temperature to avoid damage and/or any modification of the zircaloy substrates. Furthermore, one of the six Generation IV nuclear reactor concepts under development is based on a lead-cooled design.²⁶⁵ This design is commonly referred as lead-fast reactor, since fast neutrons (>1 MeV) are used for the fission reaction. This high energy neutron spectrum results in higher fuel efficiency, enabling the usage of depleted uranium as nuclear fuel and transmutation of long-lived nuclear waste.²⁶⁶ The good radiation tolerance, oxidation and corrosion resistance, mechanical stability and excellent compatibility with Pb-alloy coolants of some MAX phases is very promising.⁸⁷ Specifically under certain operating conditions most of the nuclear steel grades suffer from liquid metal embrittlement and heavy liquid metal attack (HLM), necessitating the search for alternative materials.²⁶⁷

5.4 Electrical contacts

Ti₃SiC₂ has been proposed as ohmic contacts on 4H-SiC due to its linear current-voltage behavior. Furthermore, Ti₃SiC₂ can easily be processed in a single-step by sputter-deposition of Ti on SiC substrates.²⁶⁸ This approach puts forward the possibility to directly synthesize oxygen-barrier capping layers after the main contact is deposited, without exposing the device to air, avoiding potential oxidation, contamination or any need for a cleaning step, thus, increasing the long-term stability of the device. Furthermore, Ti₂AlN has also been considered as ohmic contact for GaN-based devices due to the ohmic behavior with contact resistivity in the range of 10^-4 Ω·cm².²⁶⁹

5.5 Heat exchangers

One more application that has been considered is to use MAX phases as heat exchangers for higher temperatures (~850°C) than conventional metallic systems,²⁷⁰ where ceramics are the only alternative. Unfortunately, their poor mechanical properties, difficult machinability, low thermal conductivity, and CTE have limited their use. MAX phases can operate at high temperature (up to 1400°C), have better thermal shock response, mechanical properties, chemical stability and machinability than SiC and other advanced ceramics. However, the main limitation is the intermediate thermal conductivity (12 and 60 W m⁻¹K⁻¹), which is acceptable, but lower than SiC compounds (up to 125 W m⁻¹K⁻¹).

5.6 Concentrated solar power (CSP)

CSP systems convert solar energy into electric power without greenhouse emission and offer the possibility of thermal energy storage. CSP designs concentrate solar radiation into a receiver using mirrors, and the heat is transported by a heat transfer fluid (HTF) to a steam turbine to produce electricity.²⁷¹ As for all the thermal processes, efficiency of energy conversion increases with higher temperatures and combined cycle systems.²⁷² The temperature of CSP combined cycles should be in the range of 1000-1500 K, and heat is carried by HTFs such as air, molten salts, minerals, or synthetic oils. Other nonoxide ceramics such as SiC and ZrB₂^{273, 274} have also been proposed as solar receivers due to their higher degradation temperatures.²⁷⁵ Particularly, porous SiC volumetric receivers have attracted a lot of attention due to the enhancement of heat transfer per unit volume combined with a low drop in pressure.²⁷⁶ MAX phases have potential in two different CSP sections, the solar receiver and the storage tank. The solar receiver should have excellent oxidation resistance, and features such as high absorptance (α_s) in the ultraviolet-visible-near infrared (UV-vis-NIR) region and low thermal emission in the infrared (IR) region.²⁷⁷ Unfortunately, not enough research has been performed to confirm the potential of Ti₂AlC and Cr₂AlC for CSP applications.²⁷⁸ Both MAX phases have excellent resistance to high concentrated flux (527-917 kWm⁻²), although Ti₂AlC shows higher light scattering due to surface oxidation. The thermal performance of Ti₂AlC and Cr₂AlC varies in the range of 0.56-0.68 and 0.60-0.67, respectively, for the selected flux levels. Regarding the volumetric receivers with open and controlled porosity, MAX phase foams have a high potential due to the low oxidation and the excellent thermal shock resistance in comparison with SiC, but so far they have not been tested. Furthermore, they have potential as structural material for storage tanks because of their excellent compatibility with molten solar salts. K. Van Loo et al have recently reported and tested the compatibility of different MAX phases, SiC grades and reference metallic structural materials with a molten salt (40 wt% KNO₃ and 60 wt% NaNO₃) for 1000 hours at 600°C.⁸⁶ In this study Cr₂AlC shows a superior corrosion resistance due to the formation of a micrometric-thin and protective layer of Cr₇C₃.

5.7 Catalysis

catalyst performance has been less explored, but has high potential due to the massive number of compositions and solid solutions in “M”, “A”, and “X” sites. MAX phases can act both as catalyst and support. Cr₂AlC powders have been tested as solid catalyst to reduce the formation of carbon monoxide in catalytic wet peroxide oxidation (CWPO) processes.²⁷⁹ The CO concentration in the fenton process reaches a maximum of 6651 mg/Nm³, which is notably reduced to 187 mg/Nm³ by the presence of Cr₂AlC, and particularly noteworthy when compared to CO values (2454 mg/Nm³) of conventional activated carbon-Fe catalysts. Ti₃AlC₂ increases the reversible hydrogen storage properties of MgH₂,²⁸⁰ as well as the oxidative dehydrogenation of n-butane.²⁸¹ Furthermore, Ti₃SiC₂, Ti₂AlC, and Ti₃AlC₂ show excellent chemoselectivity with 100% selectivity for the hydrogenation of organic compounds with low contents of Palladium (0.0005 wt%).²⁸² As catalyst substrate, reticulated porous Ti₃AlC₂ was coated with nanocrystalline CeO₂ for gas exhaust devices.¹²⁵ Around 50%-90% of the total emission of hydrocarbons from modern cars with three-way catalysts are released during the cold start of the engine when the catalyst activity is still low. One solution is the preliminary electric heating of the system, limiting the use of ceramic foams but opening the possibility of MAX phases due to their good electrical conductivity, thermal stability, and high mechanical strength.

5.8 Joining

Joining is a critical technology to manufacture large, complex assemblies through the controlled integration of smaller, simpler parts. Joining is particularly challenging for ceramic components, such as monolithic SiC and SiC/SiC composites, as well as for the integration of ceramic parts into metallic systems. Among the different methods, most of them use an inert or reactive filler materials at the interface. In the particular case of MAX phases, they can be directly joined by solid-state diffusion without the incorporation of any filler compound. Ti₃SiC₂/Ti₃SiC₂, Ti₃AlC₂/Ti₃AlC₂, and Ti₃SiC₂/Ti₃AlC₂ have been joined at temperatures <1300°C and pressures between 10-30 MPa.^{283, 284} The joining is strong and attributed to the interdiffusion of Si and Al at the interface. MAX phase tapes—mainly Ti₃SiC₂—with thicknesses between 30 and 100 µm have been proposed as a filler material to join SiC at high temperatures. At temperatures around 1300°C, the joining is successful with bending strength of ~80 MPa due to a dense interface.²⁸⁵ However, at higher temperatures, 1500°C-1900°C,²⁸⁶ the strength increases to almost 170 MPa due to a chemical reaction at the interface and the partial or complete decomposition of Ti₃SiC₂.^{287, 288} Decomposition might promote a strong joint between SiC parts since a uniform, strong, and dense TiC/SiC interface without cracks is obtained.²⁸⁹ The residual thermal stress of SiC/Ti₃SiC₂/SiC joints were also calculated by finite element analysis, showing strong stress reduction and high bending strength (157 MPa) when the system is post-annealed at 900°C.²⁹⁰

MAX phases can be also joined to metallic components, although some reactions at the interfaces have been detected. Cr₂AlC coupons were vacuum diffusion bonded to an advanced turbine disk alloy at 1100°C.²⁹¹ The interface revealed an inner diffusion zone of approx. 10 µm of β-Ni(Co)Al, decorated with γ´(Ni,Co)₃Al, Ta(Ti,Nb)C, and W(Cr,Mo)₃B₂ precipitates. On the Cr₂AlC side, a porous Cr₇C₃ layer of ~40 µm was detected, whereas on the superalloy side, enhanced carbide precipitation developed over a depth of ~80 µm. Dense Cr₂AlC samples were also bonded to a Ni-based superalloy (Inconel 718) at 1000°C, without evidence of cracking and/or delamination at the interface, but containing a multiphase diffusion zone of approx. 20 µm. ²⁹² On the Cr₂AlC side Ni migrates and Al depletes forming Cr₇C₃, meanwhile the other side is mainly composed of α-Cr(Mo) phase due to outward Ni diffusion. Nevertheless, the system survived 20 thermal cycles from room temperature to 1000°C without cracks, delamination nor surface degradation. Higher temperature and pressure were required to achieve full bonding without cracks or delamination between Ti₂AlC and Inconel 718. The diffusion zone was 60 µm with presence of different zones and compounds, and the system failed after 20 thermal cycles. Interfacial reactions have been also detected between MAX phase coatings (mostly Cr₂AlC) and the metallic substrates. In general the “A” element diffuses into the metal, meanwhile element such as Co, Ti, Mo, Zr or Ni are detected in the MAX phase side.^{248, 293, 294} Diffusion of Al from Cr₂AlC into Zr substrates has been even detected during the annealing treatment at 800°C in vacuum to crystallize the coatings.²⁹⁵ The diffusion can be suppressed using ZrN and AlN diffusion barriers, which was thermodynamically calculated and experimentally validated for one hour at 1000°C.

5.9 Precursors of MXenes

MXenes are a new type of 2-Dimensional material that were discovered by Naguib et al in 2011.²² They are produced by selective chemical etching of the “A” element of MAX phases, followed by a delamination process. In spite of their novelty, MXenes have been attracted massive attention due to the high electrical conductivity, hydrophilicity, huge surface area, and biocompatibility, which lead them to be proposed as cathodes for Li- and Na- batteries, electromagnetic interference shielding, energy storage, water purification and desalination, filtration, sensors, CO catalysts, biosensors, and antibacterial agents.^{296, 297} Several research groups and industries are interested in MXenes, but so far MAX phase powders are the only precursors. The current huge explosion and demand of these materials might trigger the industrial production of MAX phase powders.

6.1 MAX phases and solid solutions

Multiple phases and their structural characterization have been explored, therefore no major issues are left to be addressed in this field. However, investigating new solid solutions and/or doping with catalyst elements such as Pd, Pt, Ni, Mo, Ru, etc would be of interest. In fact, the huge demand and interest in MXenes during the last years have relaunched the synthesis of new phases, such as containing double A-layer (ie Mo₂Ga₂C) and compositions with noble metals as “A” element (ie Ti₂AuN).^{298, 299} These MAX phases with novel layer-structure/element might exhibit new properties or applications, such as superconductivity or unusual magnetic response. Another aspect is the development of phase diagrams for systems that are difficult to synthesize such as Zr-Al-C.

6.2 Properties

Most of the work in the last two decades has been focused on the characterization of properties. In general, mechanisms are well understood, although some open questions remain, ie, the existence of ripplocations, Weibull Modulus, and/or creep mechanisms. The next challenge is to investigate the response under realistic environmental conditions and long exposures (> months). This is particularly determinant for structural materials and coatings for high temperature such as CSP systems and nuclear cladding, but tests require complex infrastructure and are expensive. However, individual parameters (ie water vapor, corrosive particles, CMAS, foreign object impacts, or long exposure) can be readily measured. As mentioned before, catalytic activities have not been explored sufficiently relative to the high potential, number of phases, and vast number of solid solutions/doping. This is an enormous field with many possibilities for future studies.

6.3 Interfacial reactions

MAX phases have excellent chemical stability with oxide and non-oxide ceramics such as YSZ, Al₂O₃, and SiC. Interactions with superalloys seem to be problematic as some reactions have been detected at interfaces. Unfortunately, the number of experiments, systems and conditions are low. More systems and conditions should be tested to unveiling mechanisms and diffusion rates, but most importantly, different strategies such as the diffusion barriers should be investigated.

6.4 Synthesis

The main limiting factor to transfer MAX phases to the market is the development of a suitable synthesis process to produce large quantities of highly pure MAX phase powders at affordable costs. If this problem is overcome and high-purity commercial powders are readily available, then components based on MAX phases can be produced for market. Synthesis of MAX powders by SHS method was shown to successfully produce industrial quantities at reasonable prices. However, the purity is not sufficient, particularly if the components must operate at elevated temperatures and in oxidizing environments. Some international companies are seeking large amounts of powders (in the order of tons) but the lack of reliable producers is hindering progress. More efforts and attention are required to synthesize and scale-up MAX phase powders. In that sense, molten salt processes might be an excellent alternative, as they can be used to produce kilograms of powders per batch. Furthermore, novel strategies and different precursors (ie intermetallics, or oxides to reduce costs) have started to be investigated.

6.5 Structures

Thin films have been largely investigated and they present a huge potential in several applications. More difficulties and clearly less investigations have been performed to obtain thick, dense and highly pure coatings by spray technologies. Investigations should focus on the optimization of MAX powders (ie size, morphology, flowability) and deposition parameters such as gas temperature, particle velocity, spray distance, gas flows, etc Once again, lack of commercial powders has limited the number of groups working on that topic. Similar problems occur for processing of complex shapes, where research groups and industries demand commercial powders with specific characteristics to further investigate.