磁性MAX相陶瓷研究进展

MAX 相陶瓷综合了陶瓷材料和金属材料的诸多优点,包括低密度、高模量、良好 的导电/导热性能、优异的抗热震性能、抗损伤性能以及优良的抗高温氧化性能等,已经获得研 究者的广泛关注。近年来,带有磁性的 MAX 相陶瓷相继被发现并被成功制备。本文结合国内 外在该领域的发展现状,重点介绍当前已被发现磁性 MAX 相陶瓷的合成和磁性特性。     

Yttrium incorporation in Cr2AlC: On the metastable phase formation and decomposition of (Cr,Y)2AlC MAX phase thin films

Herein we report on the synthesis of a metastable (Cr,Y)₂AlC MAX phase solid solution by co-sputtering from a composite Cr–Al–C and elemental Y target, at room temperature, followed by annealing. However, direct high-temperature synthesis resulted in multiphase films, as evidenced by X-ray diffraction analyses, room-temperature depositions, followed by annealing to 760°C led to the formation of phase pure (Cr,Y)₂AlC by diffusion. Higher annealing temperatures caused a decomposition of the metastable phase into Cr₂AlC, Y₅Al₃, and Cr-carbides. In contrast to pure Cr₂AlC, the Y-containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y-content dependent and was increased by ∼200°C for 5 at.% Y compared to Cr₂AlC. Calculations predicting the metastable phase formation of (Cr,Y)₂AlC and its decomposition are in excellent agreement with the experimental findings.     

High‐temperature oxidation and compressive strength of Cr2AlC MAX phase foams with controlled porosity

Cr₂AlC foams have been processed for the first time containing low (35 vol%), intermediate (53 vol%), and high (75 vol%) content of porosity and three ranges of pore size, 90‐180 μm, 180‐250 μm, and 250‐400 μm. Sacrificial template technique was used as the processing method, utilizing NH₄HCO₃ as a temporary pore former. Cr₂AlC foams exhibited negligible oxidation up to 800°C and excellent response up to 1300°C due to the in‐situ formation of an outer thin continuous protective layer of α‐Al₂O₃. The in‐situ α‐Al₂O₃ protective layer covered seamlessly all the external surface of the pores, even when they present sharp angles and tight corners, reducing significantly the further oxidation of the foams. The compressive strength of the foams was 73 and 13 MPa for 53 vol% and 75 vol% porosity, respectively, which increased up to 128 and 24 MPa after their oxidation at 1200°C for 1 hour. The increase in the compressive strength after the oxidation was caused by the switch from inter‐ to transgranular fracture mode. According to the excellent high‐temperature response, heat exchangers and catalyst supports are the potential application of these foams.     

Environmental resistance of Cr2AlC MAX phase under thermal gradient loading using a burner rig

Dense Cr₂AlC materials were tested under a gradient loading for the first time using a burner rig. The severe thermal cycling conditions consist of 500 short cycles at 1200°C, with an accumulative time at the maximal temperature of more than 29 hours. The samples showed no visible damage under these conditions due to the formation of an outer protective α‐Al₂O₃ layer, which shows a strong adhesion with the Cr₂AlC substrate. No cracks, delamination or damage were observed at the interface between the different layers. This excellent response under cyclic loading shows the excellent potential of Cr₂AlC compounds for high‐temperature applications.     

Oxidation resistance of Ti3AlC2 and Ti3Al0.8Sn0.2C2 MAX phases: A comparison

Ti₃AlC₂ and Ti₃Al_0.8Sn_0.2C₂ MAX phase powders are densified using Spark Plasma Sintering (SPS) technique to obtain dense bulk materials. Oxidation tests are then performed over the temperature range 800°C-1000°C under synthetic air on the two different materials in order to compare their oxidation resistance. It is demonstrated that, in the case of the Ti₃Al_0.8Sn_0.2C₂ solid solution, the oxide layers consist in TiO₂, Al₂O₃, and SnO₂. The presence of Sn atoms in the A planes of the solid solution leads to an easy diffusion of Sn out of the MAX phase which promote the formation of the nonprotective and fast growing SnO₂ oxide. Moreover, the small Al/Ti atom's ratio promotes the growth of a nonprotective rutile-TiO₂ scale as well. In the case of the Ti₃AlC₂ MAX phase, the oxide layer consists in a protective alumina scale; a few TiO₂ grains being observed on the top of the Al₂O₃ layer. The parabolic oxidation rate constants are about 3 orders of magnitude smaller for Ti₃AlC₂ compared to Ti₃Al_0.8Sn_0.2C₂.     

Oxidation behaviour of V2AlC MAX phase coatings

We report on the oxidation behaviour of V2AlC coatings up to 800 °C, in air. The coatings were deposited at 580 °C using magnetron sputtering from a powder metallurgical composite V2AlC target and were subsequently oxidised for 5, 15 and 30 min. The microstructural evolution of the samples was investigated, and X-ray diffraction patterns were collected to track the formation of oxides. The first indications of oxidation appear after just 15 min at 500 °C, as V-based oxides grew on the surface of the coatings. Later, the presence of mostly V-based oxides and ternary (V, Al)-oxides was observed starting after 5 min at 600 °C. Further analyses confirmed outward diffusion of V and inward diffusion of O, while Al tends to sublimate. α-Al2O3 was only indexed after 5 min at 800 °C. Ex-situ electrical resistivity measurements allowed tracking the oxidation progress of the V2AlC coating.     

Multifunctional performance of Ti2AlC MAX phase/2D braided alumina fiber laminates

The processing and characterization of laminates based on Ti₂AlC MAX phase, as matrix, and triaxial alumina braids, as reinforcing phase, are presented. Ti₂AlC powders with a mean particle size below 1 µm are synthesized, while commercial 3M Nextel 610 alumina fibers are braided in a three-stage process consisting of spooling, braiding with an angle of 0° and ±60° and the separation to single-layer fabric. The laminates are processed by layer-by-layer stacking, where 3 two-dimensional alumina braids are interleaved between Ti₂AlC layers, followed by full densification using a Field-Assisted Sintering Technology/Spark Plasma Sintering. The multifunctional response of the laminates, as well as for the monolithic Ti₂AlC, is evaluated, in particular, the thermal and electrical conductivity, the oxidation resistance, and the mechanical response. The laminates exhibit an anisotropic thermal and electrical behavior, and an excellent oxidation resistance at 1200℃ in air for a week. A relatively lower characteristic biaxial strength and Weibull modulus (i.e., σ₀ = 590 MPa and m = 9) for the laminate compared to the high values measured in the monolithic Ti₂AlC (i.e., σ₀ = 790 MPa and m = 29) indicates the need but also the potential of optimizing MAX-phase layered structures for multifunctional performance.     

Molten salt dynamic sealing synthesis of MAX phases (Ti3AlC2, Ti3SiC2 et al.) powder in air

Since the synthesis of non-oxidized ceramic and alloy powders requires both high temperature and oxygen insulation conditions, here we demonstrate a cost-efficient molten salt sealing/shielded synthesis method with dynamic gas tightness. Compared to conventional synthesis method, it can prevent the loss of reaction materials at high temperature, cut off the connection between reacting material and outside air, and does not require long-time ball milling mixing treatment or provision of applied pressing before or during heating. Only low-cost salts (e.g., NaCl, KCl), a few minutes of raw material mixing, and regular heating molds are required to obtain high-purity (>96 wt%), micron-sized Ti₃AlC₂ and Ti₃SiC₂ powders with narrow size distribution, which significantly decreased the complexity and production costs in the synthesis process. The effect of temperature and raw material content on the products were investigated. The mechanism of diffusion reaction between reactants in molten salt environment was analyzed. The new method developed here was also applicable to Ti₂AlC, V₂AlC and Cr₂AlC MAX phases, as well as provided new ideas for the preparation of other MXenes precursors with certain stoichiometric ratios, air-sensitive materials and nanopowders.     

Cold spray deposition of Cr2AlC MAX phase for coatings and bond-coat layers

Highly pure Cr₂AlC powders were synthesized and deposited for the first time by cold spray technology on stainless steel substrates. The Cr₂AlC coatings were relative dense, up to 91%, and present high purity (> 98%) since only small traces of Cr₂Al, Al₂O₃ and Cr₂O₃ were detected by XRD, SEM and EDX. The microstructure of the coatings is homogeneous, although some preferential orientation in the basal plane was observed by XRD pole figures. The adhesion between the coating and the substrate is strong, and compressive residual stresses up to 300 MPa in the coating were determined by XRD. Furthermore, a conventional YSZ Thermal Barrier Coating (TBCs) was deposited by Atmospheric Plasma Spray (APS) on top of the cold sprayed Cr₂AlC coating in order to demonstrate the processing feasibility of Cr₂AlC MAX phases as a bond-coat layer.     

Oxidation and hot corrosion behaviors of MAX-phase Ti3SiC2, Ti2AlC,Cr2AlC

Oxidation and hot corrosion behaviours of Ti₃SiC₂, Ti₂AlC and Cr₂AlC at 750 °C were investigated in this work. Ti₃SiC₂ and Ti₂AlC showed a linear increase in mass gain and a relatively poor oxidation resistance. This might be attributed to the porous TiO₂ scale. A dense α-Al₂O₃ layer was formed during the oxidation test. Cr₂AlC exhibited the best oxidation resistance. This dense oxide scale can effectively isolate the substrate from contact with oxygen leading to excellent oxidation resistance. In contrast to the oxidation test, Ti₃SiC₂ and Ti₂AlC showed relatively better resistance to hot corrosion, while Cr₂AlC showed inferior resistance to NaCl introduced hot corrosion. The hot corrosion mechanism of the MAX phases was analyzed. Due to the formation of Na₂TiO₃, Ti containing MAX phases showed a continuous increase in the mass gain. The corrosion products of Cr₂AlC were Al₂O₃, Cr₂O₃ and Na₂CrO₄. However, due to the volatilization of Na₂CrO₄, Cr₂AlC showed a mass loss during the hot corrosion test. The chemical reaction process of the MAX phase was also analyzed.     

High temperature compressive creep of dense and porous Cr2AlC in air

The creep behaviours of the dense and porous Cr₂AlC ceramics were investigated, being the first study on creep behaviour of not only Cr₂AlC but also porous MAX phase. Creep rates of rather dense (2% of porosity) and porous Cr₂AlC (53% and 75%) were measured in the temperature range 1073–1473 K during heating and subsequent cooling after high temperature exposure. The compressive creep tests were performed under stress range of 1–12 MPa. Creep rates of porous Cr₂AlC were higher than the ones of the rather dense material at lower temperature but lower at higher temperature, which can be attributed to effects of the formed oxide scale and some crack healing associated with the scale formation. A comparison of the creep rates with other refractory materials reveals favourable properties of the porous Cr₂AlC.     

Synthesis and physical properties of (Zr1−x,Tix)3AlC2 MAX phases

MAX phase solid solutions physical and mechanical properties may be tuned via changes in composition, giving them a range of possible technical applications. In the present study, we extend the MAX phase family by synthesizing (Zr_1?xTi_x)₃AlC₂ quaternary MAX phases and investigating their mechanical properties using density functional theory (DFT). The experimentally determined lattice parameters are in good agreement with the lattice parameters derived by DFT and deviate <0.5% from Vegard's law. Ti₃AlC₂ has a higher Vickers hardness as compared to Zr₃AlC₂, in agreement with the available experimental data.     

Cr2AlC MAX phase as bond coat for thermal barrier coatings: Processing,testing under thermal gradient loading,and future challenges

Cr₂AlC layers with thickness up to 100 µm were deposited by high-velocity-atmospheric plasma spray (HV-APS) on Inconel 738 substrates to analyze the potential of MAX phases as bond coat in thermal barrier coating systems (TBCs). The deposited Cr₂AlC layers showed high purity with theoretical densities up to 93%, although some secondary phases were detected after the deposition process. On top of this MAX phase layer, a porous yttria-stabilized zirconia (YSZ) was deposited by atmospheric plasma spraying. The system was tested under realistic thermal loading conditions using a burner rig facility, achieving surface and substrate temperatures of 1400°C and 1050°C, respectively. The system failed after 745 cycles mainly for three reasons: (i) open porosity of the bond coat layer, (ii) oxidation of secondary phases, and (iii) inter-diffusion. Nevertheless, these results show a high potential of Cr₂AlC and other Al-based MAX phases as bond coat material for high-temperature applications. Furthermore, future challenges to transfer MAX phases as eventual bond coat or protective layer are discussed.     

Bonding and oxidation protection of Ti2AlC and Cr2AlC for a Ni-based superalloy

Alumina forming, oxidation and thermal shock resistant MAX phases are of a high interest for high temperature applications. Herein we report, on bonding and resulting interactions between a Ni-based superalloy, NSA, and two alumina forming MAX phases. The diffusion couples Cr₂AlC/Inconel-718/Ti₂AlC were assembled and heated to 1000 or 1100 °C in a vacuum hot press under loads corresponding to stresses of either 2 MPa or 20 MPa. The resulting interfaces were examined using X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Good bonding between Cr₂AlC and NSA was achieved after hot pressing at 1000 °C and a contact pressure of only 2 MPa; in the case of Ti₂AlC a higher temperature (1100 °C) and pressure (20 MPa) were needed. In both cases, a diffusion bond, in which mainly Ni and Cr out diffused from the NSA into the MAX phase and a concomittant out diffusion of Al from the latter, was realized with no evidence of interfacial damage or cracking after cooling to room temperature. The reactions paths were determined to be: Cr₂AlC/Cr₇C₃/Cr₇C₃,β-NiAl/α-Cr(Mo)/NSA and Ti₂AlC/Ti₂AlC,Ti₃NiAl₂C/β-NiAl/α-Cr(Mo)/NSA. Twenty thermal cycles from room temperature to 1000 °C showed that Ti₂AlC is a poor oxidation barrier for Inconel-718. However, in the case of Cr₂AlC no cracks, delamination nor surface degradation was observed, suggesting that this material could be used to protect Inconel-718 from oxidation.     

三元层状可加工导电MAX相陶瓷研究进展

三元层状可加工导电陶瓷是一类键合具有明显各向异性的层状碳化物或氮化物,它通常又被称为MAX相陶瓷。MAX相陶瓷具有优良的可加工性,良好的导电、导热能力,可观的高温强度,同时还具有良好的热稳定性、抗氧化性、抗热震性和耐腐蚀性能。本文介绍了MAX相陶瓷的结构、性能以及制备方法和应用前景。     

Mechanical and oxidation behavior of textured Ti2AlC and Ti3AlC2 MAX phase materials

Highly textured Ti₂AlC and Ti₃AlC₂ ceramics were successfully fabricated by a two-step fabrication process, and the Lotgering orientation factors for {00l} planes of textured Ti₂AlC and Ti₃AlC₂ were calculated as 0.82 and 0.71, respectively. The effect of texturing was evaluated in terms of elastic modulus and hardness by macro- and micro-indentation. Moreover, the oxidation behavior of the MAX phases was investigated at 1300 °C in air, revealing that the oxidation was markedly anisotropic, where the textured side surface exhibited much better oxidation resistance, resulting from the rapid diffusion of Al element within its basal planes to form a protective Al₂O₃ scale on it. Furthermore, Ti₂AlC had larger difference regarding oxidation behavior between the top and side surface than Ti₃AlC₂, correlated to its higher Al ratio, leading to higher texturing degree and more diffusion pathways to the outer surface to produce an Al₂O₃ layer already at the initial oxidation stage.     

Thermal stability and selective nitridation of Cr2AlC in nitrogen at elevated temperatures

The thermal stability and selective nitridation of Cr₂AlC under a high–temperature nitrogen atmosphere were studied. Cr₂AlC began to decompose beyond 1073 K in N₂, and the activation energy was estimated to be 108.93 kJ·mol^?1. Initially, the selective nitridation led to the generation of an AlN layer, which shielded the underlying Cr₂AlC and the process was dominated by a surface reaction. The final products were AlN, Cr₇C₃ and Cr₃C₂, and the weaker Cr–Al bonds in Cr₂AlC facilitated the rapid diffusion of aluminum from the interior outwards. As the reaction proceeded, micropores were observed on the grain surface, as well as a loose structure, which facilitated the diffusion of N₂ and thus accelerated the reaction. Finally, the intensive reaction involving Cr₂AlC and N₂ could be attributed to the gas diffusion channels caused by improved temperature and continuous escape of vaporized aluminum.     

Phase formation and microstructure evolution of plasma sprayed Cr2AlC MAX phase coatings under post annealing

In this study, thick Cr₂AlC coatings were first synthesized via plasma spraying of Cr₃C₂–Al–Cr agglomerated powders and post annealing. The microstructure evolution and mechanical properties of the Cr₂AlC coatings annealed at 500–1000 °C were investigated. The as-sprayed coatings exhibited a lamellar structure, primarily consisting of Cr₂AlC, Cr₇C₃, Cr₂₃C₆, and (Cr, Al)C_x solid solutions. The short residence time during spraying led to incomplete reactions in the Cr₃C₂@Al–Cr agglomerates, resulting in the formation of (Cr, Al)C_x. Post annealing provided sufficient energy for the transition of (Cr, Al)C_x → Cr₂AlC. With an increase in the annealing temperature (<900 °C), gradual transition of the (Cr, Al)C_x phase led to a slight increase in the Cr₂AlC content, and thus, the as-annealed coatings maintained high hardness (>1000 HV_0.2) with improved fracture toughness. Higher annealing temperatures (>900 °C) promoted clear enhancement of the Cr₂AlC content, thus reducing the coating hardness. The transition phase (Cr, Al)C_x and high temperature annealing were the primary factors to promoting the formation of the Cr₂AlC phase in sprayed coatings. This study indicates that the Cr₃C₂@Al–Cr agglomerates can be effective alternatives to expensive MAX phase powders as feedstock for plasma spraying of Cr₂AlC coatings.     

A combined experimental and modeling study revealing the anisotropic mechanical response of Ti2AlN MAX phase

Ti₂AlN MAX phase with a hexagonal crystal structure exhibits great potential as structural material for operation under harsh environments due to its excellent mechanical performance. For a reliable application, a comprehensive understanding of the mechanical behavior, and in particular of the anisotropic properties is needed. Thus, in this study, we combined nanoindentation and electron-backscatter diffraction experiments to correlate elastic modulus and hardness of Ti₂AlN to the crystallographic orientation. We used two different modeling approaches to better understand, validate, and in the long run to predict the anisotropic mechanical behavior of MAX phase materials. While we observed consistent trends in both experiments and modeling, elastic modulus and hardness showed different dependencies on the crystal orientation.