MAX Phase Powders Research Articles

Desilicidation of the Ti3SiC2 MAX Phase in a CO and SiO Atmosphere

We have studied the desilicidation of Ti3SiC2 MAX phase powder at 1600°C in a CO + xSiO (x = 0, 1, 1.5, 2, 3) gas atmosphere. The required gaseous atmosphere was generated using granulated powder mixtures containing commercial carbon, TiO2, SiO2, and SiC, which released CO and SiO gases during heating as a result of oxide reduction reactions. Ti3SiC2 desilicidation was sensitive to the gas phase composition and occurred only if the CO : SiO ratio exceeded 1 : 3 (x < 3). The Ti3SiC2 desilicidation process was shown to be reverse of TiC silicidation with SiO gas and could be described by the following chemical reaction scheme: Ti3SiC2 + CO(g) = 3TiC + SiO(g).

Influence of MAX-Phase Deformability on Coating Formation by Cold Spraying

As solid-state deposition technique avoiding oxidation, cold gas spraying is capable of retaining feedstock material properties in the coatings, but typically fails to build up coatings of brittle materials. Ceramic MAX phases show partial deformability in particular lattice directions and may thus successfully deposit in cold spraying. However, deformation mechanisms under high strain rate, as necessary for cohesion and adhesion, are not fully clear yet. A MAX-phase deposit only builds up, if the specific mechanical properties of the MAX phase allow for, and if suitable spray parameter sets get realized. To investigate the influence of material properties and deposition conditions on coating microstructure and quality, three MAX phases, Ti3SiC2, Ti2AlC and Cr2AlC, were selected. Up to ten passes under different spray parameters yielded Ti2AlC and Cr2AlC coatings with thicknesses of about 200-500 µm. In contrast, Ti3SiC2 only forms a monolayer, exhibiting brittle laminar failure of the impacting particles. In all cases, the crystallographic structure of the MAX-phase powders was retained in the coatings. Thicker coatings show rather low porosities (< 2%), but some laminar cracks. The deposition behavior is correlated with individual mechanical properties of the different MAX-phase compositions and is discussed regarding the particular, highly anisotropic deformation mechanisms.

Structural and morphological characterization of the layered carbide-derived-carbon nanostructures obtained by HF etching of Ti2AlC

In this study, we report a simple method for the synthesis of layered carbide-derived carbon (CDC) nanostructures by immersion of the Ti2AlC MAX phase powder in hydrofluoric acid (HF) solutions. The Ti2AlC MAX phase powders were synthesized by the ball-milling of the titanium, aluminum, and carbon powders. Then, the as-synthesized powders were immersed in the different concentrations of HF solutions (10 and 40 wt.%) at different times (2, 24, and 48 h). Results showed that the titanium and aluminum were extracted from the structure of Ti2AlC MAX phase by immersion in the HF solution. The structural and morphological investigation of the obtained CDCs were carried out by X-ray diffraction (XRD), Raman spectroscopy, and field emission scanning electron microscopy (FESEM). FESEM images proved the formation of the layered morphologies of the CDC nanostructures achieved after etching of the Ti2AlC MAX phase powder in HF solution. XRD and Raman spectroscopy results were in good agreement with the FESEM images. Also, results indicated that lower concentration of HF solution was more proper for the production of the layered CDC nanostructures, compared to the higher concentration of HF solution.

Obtaining of Ti&lt;sub&gt;2&lt;/sub&gt;AlC and Ti&lt;sub&gt;3&lt;/sub&gt;AlC&lt;sub&gt;2&lt;/sub&gt; MAX phases by SHS with reduction stage

The paper focuses on obtaining Ti 2 AlC and Ti 3 AlC 2 MAX phase powders by self-propagating high-temperature synthesis (SHS) from oxide raw materials using magnesium-thermal reduction. The source of titanium was its oxide TiO 2 with magnesium used as a reducing agent. Cleaning from magnesium oxide was conducted in hydrochloric acid solution with a concentration of 1:3 at t = 70 °C. The yield of the target product in magnesium thermal reduction is 35–40 %. It was found that the synthesis product consisted of Ti 2 AlC, MgAl 2 O 4 and TiC after chemical leaching in hydrochloric acid at the stoichiometric ratio of components. MgAl 2 O 4 spinel was formed due to the lack of magnesium reducing agent in the green mixture, while some part of aluminum reacted with titanium oxide reducing it and forming Al 2 O 3 . It led to MgO·Al 2 O 3 formation. An increase in the excess magnesium content in the green mixture from 20 wt.% to 30 wt.% leads to the complete reduction of titanium from its oxide by magnesium with the formation of Ti 2 AlC MAX phase and titanium carbide. A decrease in carbon content by 10 wt.% in the green mixture leads to a decrease in titanium carbide content to 4 %. With an excess content of soot from 20 % to 35 %, a product containing Ti 3 AlC 2 , Ti 2 AlC and TiC MAX phases is formed, and the mass fraction of Ti 3 AlC 2 increases from 86 % to 89 %, respectively. The resulting powders are agglomerates consisting of thin plates of 70–100 nm thick MAX phases. 87 % of such agglomerates are less than 65 μm in size.

Microstructure and Tribological Behavior of In Situ TiC-Ni(Si,Ti) Composites Elaborated from Ni and Ti3SiC2 Powders

Herein, we study the effect of Ti3SiC2 on the microstructures and tribological properties of an in situ TiC reinforced Ni(Si, Ti) composites elaborated from Ni and Ti3SiC2 MAX phase powders against steel (100Cr6). Pressureless sintering at 1080 °C for 4 h of Ni and Ti3SiC2 powders was used to elaborate these composites with 10, 20 and 30 wt.% of Ti3SiC2. The microstructures of the composites were investigated by scanning electron microscopy (SEM), x-rays diffraction and Raman spectroscopy. Standard ball-on-disk friction wear tests under different applied loads were conducted on the composites surfaces at room temperature. For the three elaborated composites, Ti3SiC2 was totally decomposed and transformed to TiC phase, while the released Si and Ti atoms from Ti3SiC2 diffused into Ni matrix forming Ni(Si, Ti) solid solution. As compared with reference (Ni) sinter, the addition of 20 wt.% Ti3SiC2 in the Ni matrix improved the hardness by ~ 250%. The addition of Ti3SiC2 particles also had a beneficial effect on the tribological performance of these composites against steel. The worn surfaces of the elaborated composites under all applied loads are characterized by the presence of a lubricious Fe3O4-αFe2O3 tribofilms. The effect of chemical compositions and different applied loads on the wear mechanisms of the three elaborated composites is discussed.

Fabrication of Cr2AlC powder by molten salt electrolysis at 850 °C with good oxidation resistance

The Cr2AlC is a typical ternary intermetallic who possesses unique properties such as good high temperature oxygen resistance, excellent mechanical strength, and so on, making it a potential ceramic candidate to be utilized under a relatively strict circumstance. In this paper, a straightforward and cheaper process was proposed to synthesize high quality and uniform micron-size Cr2AlC, via electrochemical reduction of Cr2O3/Al2O3/C mixed powder at only 850 °C. It is demonstrated that Cr2AlC was formed by the following three steps. Firstly, Cr2O3 was reduced to metallic Cr and react with carbon to form CrnCm. Then formed CrnCm will react with reduced alumina to form Cr2AlC. The SEM-EDS images confirmed the homogeneous distribution of Cr, Al and C in micron-size Cr2AlC powder. And the TEM and TG results further demonstrate the excellent layered structure and good oxidation resistance of Cr2AlC powder. This work realizes and provides a low-cost and scalable low temperature technique to prepare homogeneous MAX phase powders for commercial applications.

Origin of anomalous laminar cracking, volume expansion and weight increase of Ti2AlC MAX phase powders at 600 °C

Planar cracks tend to form in Al-containing Mn+1AXn phase materials after oxidation at an intermediate temperature range between 550 and 650 °C, although they have outstanding oxidation resistance beyond 1200 °C. Herein, Ti2AlC Mn+1AXn phase powders are employed as a model example to investigate such laminar cracks formation at 600 °C, accompanying with continuous volume expansion and weight increment with oxidation duration. Coupled with the crystallinity structure development during oxidation, density function theory calculation has been performed to elucidate the origin of such anomalous behavior at 600 °C for Ti2AlC, which can be applicable to other Mn+1AXn phase materials.

Molten salt shielded synthesis (MS3) of Ti2AlN and V2AlC MAX phase powders in open air

Synthesis of high-temperature carbide and nitride MAX phases requires an inert atmosphere to prevent oxidation. Energy and cost expensive conventional synthesis processes are not useful for large scale productions. Molten salt synthesis is simple but generally carried out in argon atmosphere. Molten salt shielded synthesis (MS3) is carried out in the air but needs salt encapsulation to prevent oxidation below melting point of the salt as reported earlier. In the present work, MS3 method without salt encapsulation in open air has been explored by using low melting eutectic salt mixture to provide oxidation shielding of the metal precursors at high temperature as well as to produce the required phase at lower temperature with shorter soaking time. In this paper, V2AlC and Ti2AlN have been synthesized using MS3 to obtain satisfactory phase purity. Therefore; this method can be extended to large scale industrial production through its eco-friendliness, sustainability and energy-efficient nature.

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

AbstractTi3AlC2 and Ti3Al0.8Sn0.2C2 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 Ti3Al0.8Sn0.2C2 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 Ti3Al0.8Sn0.2C2.

Synthesis of Ti3SiC2 MAX phase powder by a molten salt shielded synthesis (MS3) method in air

Titanium silicon carbide (Ti3SiC2) powder was synthesized by molten salt shielded synthesis route of elemental reactants. Potassium bromide (KBr) was used for gas-tight encapsulation of the consolidated reaction mixture for further high temperature processing. The synthesis of Ti3SiC2 powder was carried out in air, the salt cladding and molten salt pool provided for the protection of the material against oxidation both at low and high temperature. The process yielded free flowing Ti3SiC2 powders without the need of a milling step. Al addition to the reaction mixture resulted in a high purity (96 wt. %) of Ti3SiC2 at a synthesis temperature of 1250 °C. The synthesized micro-metric Ti3SiC2 can be milled to nano-metric powders.

The Influence of Oxygen Concentration during MAX Phases (Ti₃AlC₂) Preparation on the α-Al₂O₃ Microparticles Content and Specific Surface Area of Multilayered MXenes (Ti₃C₂Tx).

The high specific surface area of multilayered two-dimensional carbides called MXenes, is a critical feature for their use in energy storage systems, especially supercapacitors. Therefore, the possibility of controlling this parameter is highly desired. This work presents the results of the influence of oxygen concentration during Ti3AlC2 ternary carbide—MAX phase preparation on α-Al2O3 particles content, and thus the porosity and specific surface area of the Ti3C2Tx MXenes. In this research, three different Ti3AlC2 samples were prepared, based on TiC-Ti2AlC powder mixtures, which were conditioned and cold pressed in argon, air and oxygen filled glove-boxes. As-prepared pellets were sintered, ground, sieved and etched using hydrofluoric acid. The MAX phase and MXene samples were analyzed using scanning electron microscopy and X-ray diffraction. The influence of the oxygen concentration on the MXene structures was confirmed by Brunauer-Emmett-Teller surface area determination. It was found that oxygen concentration plays an important role in the formation of α-Al2O3 inclusions between MAX phase layers. The mortar grinding of the MAX phase powder and subsequent MXene fabrication process released the α-Al2O3 impurities, which led to the formation of the porous MXene structures. However, some non-porous α-Al2O3 particles remained inside the MXene structures. Those particles were found ingrown and irremovable, and thus decreased the MXene specific surface area.

The phase and morphological evolution of Ti3SiC2 MAX phase powder after HF treatment

The aim of this study is to investigate the phase and morphological evolution of the prepared Ti3SiC2 MAX phase powder after immersion in hydrofluoric acid (HF) solution for 24, 72, 96 and 120 h. The phase and morphological evolution of Ti3SiC2 was analyzed using X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). XRD results revealed that after 24 h HF treatment the diffraction peaks of Ti3SiC2 were fully disappeared and this treatment led to the decomposition of Ti3SiC2 to TiC. XRD and EDS findings demonstrated that de-intercalation of Si layer occurred after HF treatment. Moreover, SEM images showed that the morphological shape of TiC particles significantly changed as a function of HF treatment times. In summary, the shape of TiC particles changed from the polyhedron to hexagonal, then to truncated octahedron, octahedron and cubic.

Influence of porosity on elastic properties of Ti2AlC and Ti3SiC2 MAX phase foams

MAX phase foams could have various applications where tailored functional and mechanical properties are required. In this study, Ti2AlC and Ti3SiC2 MAX phase foams with controlled porosity and pore size were produced and characterized. The foams were produced from MAX phase powders by powder metallurgy method using crystalline carbohydrate as a space holder. Foams with overall porosity up to approximately 71 vol% and pore size from 250 μm to 1000 μm were successfully produced; micro-porosity and macro-porosity was characterized. Poisson's ratio and elastic moduli of the foams were measured by resonant ultrasound spectroscopy (RUS) and analyzed as a function of porosity and pore size. Different models were used to fit the experimental data and interpret the effect of pore size and amount of porosity and on elastic properties. It was found that the amount and type of porosity has a larger influence on the elastic properties than the pore size.

Kerosene-fuelled high velocity oxy-fuel (HVOF) spray of Ti2AlC MAX phase powders

Kerosene-fueled high velocity oxy-fuel (HVOF) spray was employed to deposit commercial Ti2AlC MAX phase powders onto Inconel 600 substrates. After optimizing three process parameters including flow rates of kerosene and oxygen as well as spray distance, a dense Ti2AlC coating with a very low decomposition was obtained. Using Rietveld phase analysis of the X-ray diffraction pattern, the weight percentage of Ti2AlC in the coating was determined to be 53.0%, which is not only far better than the coating deposited by plasma spray, but also is comparable with the optimum Ti2AlC coatings sprayed by H2-fueled HVOF and cold spray. This optimized Ti2AlC coating was further analyzed by a variety of techniques to probe its surface chemistry, morphology, cross-sectional microstructure, mechanical properties and salt fog corrosion resistance.

Synthesis and sintering of Ti3GeC2 MAX phase powders

Ti3GeC2 is one of the potential MAX phase compounds, for which, production and processing methods are yet to be optimized. There are very few reports on the synthesis of this compound, which are mostly in bulk form and contain large impurities. Present investigation attempts to synthesize Ti3GeC2 powder with improved phase purity and to understand phase evolution sequences. Synthesis was characterized as a liquid-solid reaction process; initiated after melting of Ge particles. At low temperature (~1200°C), yield was found to be very low, and the product had several intermetallics and carbides. At relatively higher temperature (at 1500°C), the conversion was found to be very high and particles were mostly finer than 10µm. Phase purity was further increased (over 96wt%) during sintering stage, as observed through x-ray diffraction analysis where impurity peaks were very small. Ti3GeC2 powder was found to exhibit poor densification behavior during pressureless sintering; however, it showed slow grain growth characteristics too, a highly desired feature for structural applications.

Plasma spray of Ti2AlC MAX phase powders: Effects of process parameters on coatings' properties

Plasma spraying was employed to spray commercial Ti2AlC MAX phase powders onto Inconel 600 substrates. Effects of different process parameters, including arc current, flow rates of primary Ar, secondary H2 and carrier Ar, powder feedrate, spray distance as well as presence of cooling air on the backside on the substrate were systematically screened by evaluating the content of Ti2AlC in the coating through X-ray diffraction and Rietveld phase analysis. The highest content of the remaining Ti2AlC in the coating was achieved to be 26.0%. The coating with 26.0wt% of Ti2AlC was further analyzed by a variety of techniques to probe its surface chemistry, morphology, cross-sectional microstructure, mechanical properties and salt fog corrosion resistance.

Thermodynamic Assessments and mechanically activated synthesis of ultrafine Cr2AlC MAX phase powders

This work attempts to produce ultrafine powders and studies the effect of mechanical activation on reaction kinetics and thermodynamics of Cr2AlC formation, which are essential to design the processing parameters. Cr2AlC formation was found to be exothermic in nature, which occurred as soon as Al started melting. Milling energies (1.6kJ/g and 3.2kJ/g after milling for 15h and 30h respectively) were found to enhance the reactivity of chromium carbide powders with Al, and lowered the activation energy of reaction significantly. The enthalpy of reaction was found to be changed from −53kJ/mol to −66kJ/mol and the change in Gibbs free energy of reaction was decreased from −107kJ/mol to −139kJ/mol, when milling duration was increased from 15h to 30h. Thus the mechanical activation provides more favorable condition for the conversion and nearly pure Cr2AlC powder with ultrafine (<400nm) size were produced at relatively lower temperature (at 800°C).

Probing the oxidation behavior of Ti2AlC MAX phase powders between 200 and 1000 °C

Oxidation of commercial Ti2AlC MAX phase powders at 200–1000°C has been investigated by XRD, XPS, SEM, STA and TGA coupled with FTIR. These powders are a mixture of Ti2AlC, Ti3AlC2, TiC and Ti1.2Al0.8. Oxidation at 400°C led to disappearance of carbide phases from Ti 2p, Al 2p and C 1s XPS spectra. At 600°C, powders changed from dark grey to light grey with a significant volume increase due to crack formation. Powders were severely oxidized by detecting rutile with minor anatase TiO2. At 800°C, α-Al2O3 was detected while anatase transformed into rutile TiO2. The cracks were healed and disappeared. At 1000°C, the Ti2AlC powders were fully oxidized into rutile TiO2 and α-Al2O3 with a change of powder color from light grey to yellow. FTIR detected the release of C as CO2 from 200°C onwards but with additional CO above 800°C.

Sintering mechanisms of ultrafine Cr2AlC MAX phase powder

This investigation aims to understand the sintering mechanisms of high purity ultrafine Cr2AlC MAX phase powder. Green compacts were sintered using vertical dilatometer system and the sintering data were analyzed through different sintering models to determine possible dominating mechanism. Activation energies and frequency factors estimated through the grain boundary diffusion models were found to be unusually higher than normally reported values on most polycrystalline materials. Activation energies (301 and 419kJ/mol) and frequency factors obtained through the volume diffusion mechanism were found to be closure to that of self-diffusion of chromium as well as chromium diffusion in many multi-elements systems; indicating volume diffusion to be a possible sintering mechanism, where chromium seems to play a decisive role. However, it should be noted that the present result needs to be verified, once the diffusion data on pure Cr2AlC phase would be available in future.

Formation of Ti3SiC2 Interphase of SiC Fiber by Electrophoretic Deposition Method

Due to its stability at high temperature and its layered structure, Ti3SiC2 MAX phase was considered to the interphase of SiC f /SiC composite. In this study, Ti₃SiC₂ MAX phase powder was deposited on SiC fiber via the electrophoretic deposition (EPD) method. The Zeta potential of the Ti₃SiC₂ suspension with and without polyethyleneimine as a dispersant was measured to determine the conditions of the EPD experiments. Using a suspension with 0.03 wt.% ball milled Ti₃SiC₂ powder and 0.3 wt.% PEI, Ti₃SiC₂ MAX phase was successfully coated on SiC fiber with an EPD voltage of 10 V for 2 h. Most of the coated Ti₃SiC₂ powders are composed of spherical particles. Part of the Ti₃SiC₂ powders that are platelet shaped are oriented parallel to the SiC fiber surface. From these results we expect that Ti₃SiC₂ can be applied to the interphase of SiC f /SiC composites.