MAX Phase Ceramics Research Articles

Toughening Mechanisms in Nanolayered MAX Phase Ceramics-A Review.

Advanced engineering and functional ceramics are sensitive to damage cracks, which delay the wide applications of these materials in various fields. Ceramic composites with enhanced fracture toughness may trigger a paradigm for design and application of the brittle components. This paper reviews the toughening mechanisms for the nanolayered MAX phase ceramics. The main toughening mechanisms for these ternary compounds were controlled by particle toughening, phase-transformation toughening and fiber-reinforced toughening, as well as texture toughening. Based on the various toughening mechanisms in MAX phase, models of SiC particles and fibers toughening Ti3SiC2 are established to predict and explain the toughening mechanisms. The modeling work provides insights and guidance to fabricate MAX phase-related composites with optimized microstructures in order to achieve the desired mechanical properties required for harsh application environments.

High-performance polymeric nanocomposites from phthalonitrile resin and silane surface–modified Ti3AlC2 MAX phase

In this work, Ti3AlC2 M n+ 1AX n (MAX) phase ceramic nanoparticles were prepared and used as new kind of reinforcement for a typical high-performance phthalonitrile (PN) resin. The synergistic combination of both phases led to nanocomposites with improved thermal and mechanical properties. For instance, the thermal conductivity and tensile properties of the neat resin were highly enhanced upon adding more nanofiller contents. Moreover, the PN resin toughness was ameliorated by 129% at the maximum nanoparticles loading of 15 vol%. The experimental investigations were also compared with predictions from series, Halpin–Tsai, and Kerner models, and a full discussion was provided. A high-resolution transmission electron microscope confirmed the ability of the MAX phase to create a conductive network, especially at high nanofiller amounts. Scanning electron microscope (SEM) analyses of the tensile fractured surfaces revealed positive changes in the morphology, such as an increase in the roughness and amount of hackling as well as the formation of multiple microcracks. The MAX phase also enhanced the thermal stability, stiffness, and glass transition temperature of the neat resin. This work confirms the superiority of the MAX phase ceramics over the traditional ones in enhancing the properties of the PN resin and opens the way for further research in the field.

Demonstrating the self-healing behaviour of some selected ceramics under combustion chamber conditions

Closure of surface cracks by self-healing of conventional and MAX phase ceramics under realistic turbulent combustion chamber conditions is presented. Three ceramics namely; Al2O3, Ti2AlC and Cr2AlC are investigated. Healing was achieved in Al2O3 by even dispersion of TiC particles throughout the matrix as the MAX phases, Ti2AlC and Cr2AlC exhibit intrinsic self-healing. Fully dense samples (>95%) were sintered by spark plasma sintering and damage was introduced by indentation, quenching and low perpendicular velocity impact methods. The samples were exposed to the oxidizing atmosphere in the post flame zone of a turbulent flame in a combustion chamber to heal at temperatures of approx. 1000 °C at low pO2 levels for 4 h. Full crack-gap closure was observed for cracks up to 20 mm in length and more than 10 μm in width. The reaction products (healing agents) were analysed by scanning electron microscope, x-ray microanalysis and XRD. A semi-quantification of the healing showed that cracks in Al2O3/TiC composite (width 1 μm and length 100 μm) were fully filled with TiO2. In Ti2AlC large cracks were fully filled with a mixture of TiO2 and Al2O3. And in the Cr2AlC, cracks of up to 1.0 μm in width and more than 100 μm in length were also completely filled with Al2O3.

Repeated crack healing in MAX-phase ceramics revealed by 4D in situ synchrotron X-ray tomographic microscopy.

MAX phase materials are emerging as attractive engineering materials in applications where the material is exposed to severe thermal and mechanical conditions in an oxidative environment. The Ti2AlC MAX phase possesses attractive thermomechanical properties even beyond a temperature of 1000 K. An attractive feature of this material is its capacity for the autonomous healing of cracks when operating at high temperatures. Coupling a specialized thermomechanical setup to a synchrotron X-ray tomographic microscopy endstation at the TOMCAT beamline, we captured the temporal evolution of local crack opening and healing during multiple cracking and autonomous repair cycles at a temperature of 1500 K. For the first time, the rate and position dependence of crack repair in pristine Ti2AlC material and in previously healed cracks has been quantified. Our results demonstrate that healed cracks can have sufficient mechanical integrity to make subsequent cracks form elsewhere upon reloading after healing.

Effect of sintering method on the microstructure of pure Cr<sub>2</sub>AlC MAX phase ceramics

Effect of sintering method on the microstructure of pure Cr<sub>2</sub>AlC MAX phase ceramics

High temperature ion irradiation effects in MAX phase ceramics

The family of layered carbides and nitrides known as MAX phase ceramics combine many attractive properties of both ceramics and metals due to their nanolaminate crystal structure and are promising potential candidates for application in future nuclear reactors. This investigation examines the effects of energetic heavy ion (5.8 MeV Ni) irradiations on polycrystalline samples of Ti3SiC2, Ti3AlC2, and Ti2AlC. The irradiation conditions consisted of midrange ion doses between 10 and 30 displacements per atom at temperatures of 400 and 700 °C, conditions relevant to application in future nuclear reactors and a relatively un-explored regime for this new class of materials. Following irradiation, a comprehensive analysis of radiation response properties was compiled using grazing incidence X-ray diffraction (XRD), nanoindentation, scanning electron microcopy (SEM), and transmission electron microscopy (TEM). In all cases, XRD and TEM analyses confirm the materials remain fully crystalline although the intense atomic collisions induce significant damage and disorder into the layered crystalline lattice. X-ray diffraction and nanoindentation show this damage is manifest in anisotropic swelling and hardening at all conditions and in all materials, with the aluminum based MAX phase exhibiting significantly more damage than their silicon counterpart. In all three materials there is little damage dependence on dose, suggesting saturation of radiation damage at levels below 10 displacements per atom, and significantly less retained damage at higher temperatures, suggesting radiation defect annealing. SEM surface analysis showed significant grain boundary cracking and loss of damage tolerance properties in the aluminum-based MAX phase irradiated at 400 °C, but not in the silicon counterpart. TEM analysis of select samples suggest that interstitials are highly mobile while vacancies are immobile and that all three materials are in the so-called point defect swelling regime between 400 and 700 °C. All results are consistent with previous work involving traditional and MAX phase ceramics. Results show the aluminum MAX phases are not fit for application near 400 °C and that the silicon MAX phase is more damage tolerant at 400–700 °C.

Molten salts electrochemical synthesis of Cr2AlC

The unique combination of electrical, mechanical and chemical properties of MAX phase ceramics makes them attractive for energy and structural industrial applications. Here, an inexpensive and simple electrochemical deoxidation process is used to produce high quality, homogeneous micron-size particles of one of the most widely studied MAX phases, Cr2AlC. The process used cathode precursors made of cheap and available oxides of both chromium and aluminum and applied a constant potential difference in molten NaCl–CaCl2 as the electrolyte. It was found that the reduction mechanism took an unusual pathway through the formation of sub-stoichiometric oxides and oxycarbide phases that did not necessarily involve reactions between the cathode and the electrolyte. The Cr2AlC powder produced could be easily pressed into a monolithic bulk that have 4.8±0.8GPa hardness and showed excellent oxidation resistance at temperatures below 1100°C. The method is quite generally applicable to the preparation of other MAX phases. Implications are drawn considering the industrial scale-up of this method for the low-cost production of MAX ceramics.

Macro-porous Ti2AlC MAX-phase ceramics by the foam replication method

This paper describes the fabrication of porous Ti2AlC MAX-phase ceramics using the foam replication technique. The influence of heat-treatment cycle and sintering time on phase composition, microstructure and type of defect is discussed. Compression testing indicates that strength is related to defects that are introduced during the foam replication stage, as observed by optical and electron microscopy. The manufacturing process is adapted by employing additional coating stages to produce electrically conductive macro-porous ceramics with a range of strengths (0.2–6.3MPa) and macrostructures. Such materials are of interest for use as a high surface area electrode for harsh environments or microbial fuel cells.

Particle processing technology

In recent years, there has been strong demand for the development of novel devices and equipment that support advanced industries including IT/semiconductors, the environment, energy and aerospace along with the achievement of higher efficiency and reduced environmental impact. Many studies have been conducted on the fabrication of innovative inorganic materials with novel individual properties and/or multifunctional properties including electrical, dielectric, thermal, optical, chemical and mechanical properties through the development of particle processing. The fundamental technologies that are key to realizing such materials are (i) the synthesis of nanoparticles with uniform composition and controlled crystallite size, (ii) the arrangement/assembly and controlled dispersion of nanoparticles with controlled particle size, (iii) the precise structural control at all levels from micrometer to nanometer order and (iv) the nanostructural design based on theoretical/experimental studies of the correlation between the local structure and the functions of interest. In particular, it is now understood that the application of an external stimulus, such as magnetic energy, electrical energy and/or stress, to a reaction field is effective in realizing advanced particle processing[13].This special issue comprises 12 papers including three review papers. Among them, seven papers are concerned with phosphor particles, such as silicon, metals, Si3N4-related nitrides, rare-earth oxides, garnet oxides, rare-earth sulfur oxides and rare-earth hydroxides. In these papers, the effects of particle size, morphology, dispersion, surface states, dopant concentration and other factors on the optical properties of phosphor particles and their applications are discussed. These nanoparticles are classified as zero-dimensional materials. Carbon nanotubes (CNT) and graphene are well-known one-dimensional (1D) and two-dimensional (2D) materials, respectively. This special issue also includes two papers on the fabrication of mechanically reliable nanocomposites by dispersing graphene into a ceramic matrix, and on supercapacitors with high energy densities in a Co(OH)2 system decorated with graphene and carbon nanotubes. As a novel preparation method of oxide films, the fabrication of alumina films with laminated structures by ac anodization is reviewed. Moreover a new type of nanosheet has been fabricated by the exfoliation of layered, ternary transition-metal carbide and nitride compounds, known as Mn 11AXn phases (or MAX phases) where M is an early transition metal, such as Ti or Nb, A is an A group element, such as Si or Al, X is carbon and or nitrogen and n 113[4]. Among the MAX phases, those containing Mo have been theoretically calculated by first-principles calculations to be a source for obtaining Mo2C nanosheets with potentially unique properties. As an example of improving bulk ceramic properties, texturing by using a high magnetic field[5] and sintering by the electric current activated/assisted sintering (ECAS) technology[6] have been demonstrated for ultra-high temperature ceramics with high-temperature strength.A project on the development of materials and particle processing for the field of environment and energy has been ongoing at the National Institute for Materials Science since April 2011. This project employs various core competence technologies for particle processing such as ion beam irradiation for nanoparticle fabrication[7], fullerene nanomaterial processing using liquidliquid interface precipitation[8], a gas reduction nitridation process to obtain Si3N4-based phosphor materials[9], advanced phosphors via novel processing[10, 11], ultra-high pressure technology for processing and in situ analysis[12, 13], colloidal processing in a high magnetic field to obtain laminated, textured ceramics[1, 3, 5], the ECAS process for nanostructuring ceramics[6] and so forth. Here, I would like to introduce some research achievements that are not covered in this special issue.(1) The evolution of hydrogen by the reaction of fine metal particles such as Al[14] and Mg[15] with water; the specific surface area and surface modification are important factors.(2) The realization of new carbon related materials with 1D and 2D structures consisting of fullerenes prepared by liquid liquid interface precipitation: alkaline-doped superconductive nanotubes consisting of fullerenes[16], application to solar cells of fullerene/cobalt porphyrin hybrid nanosheets[17], etc.(3) The fabrication of textured films and bulk materials with excellent functional properties by colloidal processing methods such as slip casting[5], gel casting[18] and electrophoretic deposition[3, 19], in a high magnetic field, and with subsequent heating; examples of such materials include dye-sensitized TiO2 solar cells, thermoelectric materials and cathode materials for solid state Li-ion batteries and dielectric ceramics.(4) The fabrication of high-strength and high-toughness MAX phase ceramics[20, 21] inspired by the nacreous structure[22].(5) The modeling and development of the ECAS process[6]. This involves two-step pressure application[23] and high-pressure application above 400 MPa to fabricate transparent oxides[2426], and rapid heating to obtain dense nanocomposites of ceramic–CNT[27] and diamonds[28].(6) The contraction of ternary phase diagrams for oxide ion conductor systems such as zirconia[29] and apatite systems[30], leading to an increased understanding of the stability of such systems and assisting the search for high oxygen ion conductors.

Oxide-scale growth on Cr2AlC ceramic and its consequence for self-healing

Outward growth of oxide scale on Cr2AlC surface during multiple annealing at 1100 degrees C was investigated with ex situ microscopy. A dense alumina scale was formed on the surface of Cr2AlC samples after annealing for 40 h. A wedge-shaped specimen with varying thickness of oxide scale was prepared by focused ion beam slicing for further annealing. It is concluded that the outward diffusion of Al ions via grain boundaries dominates the growth of oxide scale during repeated annealing. This result is relevant to the self-healing kinetics of exposed cracks in MAX phase ceramics via cyclic heating. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Investigation of MAX phase/c-BN composites

Abstract We have successfully fabricated novel MAX phase/c-BN composite materials, which have the potential to improve significantly on current grinding and cutting materials. The synthesis conditions include elevated temperatures and moderate pressure, which allows for good bonding of the carrier to the abrasive. Preliminary results show that MAX phase ceramics offer outstanding benefits as carrier materials for c-BN abrasive particles.

Filling of surface cracks in MAX phase ceramics using cathodic electrophoretic deposition

It is well known that surface defects, such as cracks, are electrochemical more active than a smooth surface and usually work like seeds for the deposition processes. The present work used this enhanced activity to locally deposit surface-charged Al2O3 and TiC micro and nano-particles into surface cracks in Ti2AlC substrates. The formed deposit was compact and adhered well to the crack walls. A crack of about 30μm could be filled with alumina particles in 10min when applying 15V and using a 5wt% alumina suspension in ethanol. The deposition of TiC was more challenging due to coarseness of the particles. Grinding these particles to 1–2μm yielded a very stable suspension and an improvement in the crack filling behaviour.

Textured Ti<sub>3</sub>SiC<sub>2</sub> by EPD in a Strong Magnetic Field

We present a method for fabrication of textured MAX phase ceramics, particularly, Ti3SiC2; by EPD in a strong magnetic field (12T). Ti3SiC2 was dispersed in cationic polyelectrolyte-Polyethylenimine (PEI). Addition of 0.3-1dwb PEI resulted in high zeta potential values and suspension was found to be stable and of good fluidity. The optimized suspension parameters for EPD were determined as 10vol% Ti3SiC2 and 1dwb PEI in 50 % ethanolic water at pH ~ 7. X-ray diffraction analysis of the textured samples revealed that the preferred orientation of Ti3SiC2 grains parallel to the magnetic eld direction was along the a,b-axis. The Lotgering orientation factors on the textured top surface and textured side surface were determined as f (hk0) = 0.35 and f (00l) = 0.75, respectively.