Ti3AlC2 Ceramics Research Articles

Fabrication of textured Ti<sub>3</sub>AlC<sub>2</sub> by spark plasma sintering and their anisotropic mechanical properties

Textured, dense, polycrystalline Ti3AlC2 ceramics was fabricated by spark plasma sintering (SPS) of plate-like Ti3AlC2 powder synthesized by a reactive SPS-heat treatment of elemental Ti, Al and carbon black powders. The relative density was found to exceed 99% for samples sintered at 1573 K. The Lotgering orientation factor was determined to be f(00l) = 0.69. The crack propagation behavior near the indentation marks formed by a force of 49 N was observed. On the surface parallel to the SPS-loading direction, the toughening mechanism of crack deflection was observed.

Healing performance of Ti2AlC ceramic studied with in situ microcantilever bending

In situ microcantilever bending tests were carried out to evaluate the healing efficiency of pre-notched Ti2AlC ceramic after annealing at 1200 °C for 1.5 h. Microcantilevers of different orientations were fabricated with focused ion beam method at different locations, i.e. in a singular Ti2AlC grain, at a grain boundary or at the Ti2AlC–Al2O3 interface after healing. Ti2AlC microcantilever shows an anisotropic bending strength (ranging between 9.6 GPa and 4.6 GPa depending on the precise crystallographic orientations) that is closely related to the different atomic bonds in the layered structure. After healing, the Ti2AlC–Al2O3 microcantilevers exhibit almost the same strength of about 5.2 GPa, i.e. slightly higher than the cleavage strength (4.6 GPa) of the initial Ti2AlC microcantilevers. It suggests that the orientation of the matrix grain has no significant influence on the strength of healed microcantilevers. Furthermore, it turns out that the strength of the microcantilever with a healed grain boundary is at least twice the strength of the initial Ti2AlC cantilever with a grain boundary. It is concluded that the oxidation dominated self-healing mechanism of Ti2AlC ceramics can result in a perfect recovery of mechanical performance. The paper shows that the in situ microcantilever bending test provides a quantitative method for the evaluation of the strength of self-healing ceramics.

High temperature oxidation behaviour of Ti2AlC ceramic at 1200°C

The oxidation process of Ti2AlC ceramics in dry synthetic air at 1200°C was monitored with thermogravimetry. The microstructural evolution of the oxide scale with time was characterised by X-ray diffractometry, scanning electron microscopy, X-ray microanalysis and electron back scattering diffraction. The oxide scale is comprised of a continuous α-Al2O3 inner layer with isolated coarse TiO2 particles on top. At the early oxidation stage (roughly less than 0.5 h), α-Al2O3 and TiO2 are the main reaction products, while at longer reaction times the oxidation only leads to the formation of α-Al2O3. The α-Al2O3 grains in the oxide scale grow in size upon high-temperature oxidation with the grain size being uniform throughout the thickness of the scale. As diffusion of oxygen along the grain boundaries dominates the oxide scale growth, the change in grain size affects the oxide scale growth kinetics. A simple oxide scale growth model, that takes into account this change in fast diffusion paths, describes the experimentally observed oxide scale growth kinetics perfectly.

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A recent paper on “microstructural evolution during high-temperature oxidation of Ti2AlC ceramics” claims that the oxide product is solely TiO2 (rutile) when Ti2AlC is oxidized at 900 °C for 1 h. This comment presents experimental evidence and thermodynamic arguments confirming the formation of a primary Al2O3 layer instead of pure TiO2 scale under the same oxidation condition.

Effect of aluminum content on synthesis of Ti2AlC and Ti3AlC2 during treatment in a high-energy mill and hot pressing

Treatment of initial powders of titanium, carbon and aluminum in a high-energy mill and pressure sintering are used for synthesizing dense and single-phase Ti2AlC and Ti3AlC2 ceramics. The effect of the content of aluminum and of the sintering temperature on the synthesis process is studied. During treatment in the high-energy mill the initial Ti, Al and C react and yield TiC and TiAl x . In the course of subsequent pressure sintering and depending on the content of aluminum, the milled powders interact and form TiC/Ti2AlC, Ti3AlC2,Ti2AlC, and Ti2AlC/TiAl.

Microstructural evolution during high-temperature oxidation of Ti 2AlC ceramics

Microstructural development during high-temperature oxidation of Ti 2AlC below 1300 °C involves gradual formation of an outer discontinuous TiO 2 layer and an inner dense and continuous α-Al 2O 3 layer. After heating at 1400 °C, an outer layer of mixed TiO 2 and Al 2TiO 5 phases and a cracked α-Al 2O 3 inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO 2 grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO 2 grains due to thermal expansion mismatch of the phases (TiO 2, α-Al 2O 3 and Al 2TiO 5) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO 2 grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al 2TiO 5 and the volume changes associated with the reactions during Ti 2AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O 2− and outward diffusion of Al 3+ and Ti 4+. The weakly bound Al leaves the Al atom plane in the layered structure of Ti 2AlC, and diffuses outward to form a protective inner α-Al 2O 3 layer between 1100 and 1300 °C. However, the α-Al 2O 3 layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.

Ultra-high temperature ablation behavior of Ti2AlC ceramics under an oxyacetylene flame

The linear and mass ablation rates of Ti2AlC ceramics under an oxyacetylene flame at a temperature up to 3000°C were examined by measuring the dimensions and weight change of the ablated samples. The linear ablation rate was decreased from 0.14μms−1 for the first 30s of the ablation to 0.08μms−1 after 180s. Ti2AlC ceramics gained small amounts of weight upon ablation, which is attributed to the formation of oxidation products on the ablated surface. The ablation surface exhibits a two-layer structure: an oxide outer layer, consisting mainly of α-Al2O3 and TiO2 and some Al2TiO5, and a porous sub-surface layer containing Ti2Al1−xC and TiCxOy. With increasing ablation time, the content of TiO2 and Al2TiO5 in the outer layer increased, and more pores developed in the sub-surface layer. The thermal oxidation of Ti2AlC under the flame and scouring of the viscous oxidation products by high-speed flow of gas torch are the main ablation mechanisms.

Early stages of oxidation of Ti3AlC2 ceramics

Abstract The nucleation and growth of oxide scale at the early stages of oxidation of Ti3AlC2 ceramics was studied via oxidizing at 1100 °C in air for short times (≤900 s). The nucleation of nanosized Al2O3 particles mainly occurs at the ledges of the fractured lamellar Ti3AlC2 grains as well as on the {0 0 0 1} basal surfaces. The Al2O3 nuclei mainly grow along these ledges to form oxide strings, and then spread on the terraces and the {0 0 0 1} basal surfaces. An oxide layer consisting predominantly of nanosized α-Al2O3 forms after oxidizing for 180 s. The formation of lenticular hexagonal pores in Ti3AlC2 grains is attributed to the faster consumption of Ti, Al and C atoms along 〈 1 1 2 ¯ 0 〉 direction than along 〈0 0 0 1〉 direction. With further oxidation, rutile-TiO2 particles form on top of the α-Al2O3 layer, and grow to form a rutile-TiO2 layer. Further oxidation leads to the formation of pores underneath the primary α-Al2O3 layer. In this porous layer both Al2O3 and TiO2 were present with a preference for Al2O3 to stay adjacent to the inward moving interface of Ti3AlC2 substrate.

Fracture Behavior and Microstructure of Ti <sub>3</sub>AlC<sub>2</sub> Ceramics Prepared by SHS/PHIP Method

Fracture behavior and the microstructure of Ti3AlC2 ceramics prepared by SHS/PHIP method were studied. Stress-strain curves at different temperature with a strain rate of 1×10-3 s-1 were obtained. Fracture toughness, flexural strength, crack propagation behavior and compressive deformation of specimens were investigated. Results show that the microstructure of the large size Ti3AlC2 ceramics prepared by SHS/PHIP method has typical layered feature of ternary carbide compound. The bridge-link phenomena induced by the flaky grains occurred in three-point bending test. It restrained the crack propagation and improved the fracture toughness of the material. Cylindrical specimens under axial compression usually smashed into chips for most of the ceramics materials, however, for the Ti3AlC2 ceramics prepared by SHS/PHIP, shear fracture along 45º incline of the specimen occurred at room temperature, and bulging deformation without any crack exhibited when temperature was high. It is concluded that the Ti3AlC2 ceramics prepared by SHS/PHIP has better fracture-resistance properties.

Synthesis and microstructure of layered-ternary Ti 2AlC ceramic by high energy milling and hot pressing

Fully dense and single-phase Ti 2AlC ceramic was successfully synthesized by a high energy milling and hot pressing using Ti, C and Al as starting materials. The effects of composition of the initial elemental powders and sintering temperatures on the purity and formation of Ti 2AlC were examined. The formation mechanism for the single-phase Ti 2AlC ceramics was investigated by XRD in details, which could be described as follows: the most of initial elements reacted to form TiC and Ti–Al intermetallics; the intermetallics and the residual Ti and Al transformed to TiAl phase; and finally the TiAl intermetallics and the TiC reacted to yield Ti 2AlC.

Synthesis and Properties of Ternary Carbide Ti<sub>3</sub>AlC<sub>2</sub> Ceramics

Polycrystalline bulk samples of ternary carbide Ti3AlC2 ceramics were fabricated by reactively hot-pressing a mixture of Ti, Al, and graphite powders with and without Sn additive. The effects of sintering temperature, time and addition of Sn on the purity, mechanical properties and microstructure of Ti3AlC2 were investigated. The result showed that the TiC content was strongly influenced by sintering temperature for the Ti3AlC2 samples without Sn additive, and the most suitable sintering temperature to create the lowest TiC content was 1400°C. The addition of Sn additive led to a distinct decrease in TiC content. The flexural strength of the testing materials had close relation with the TiC content and sintering time. A certain content of TiC second phase and longer sintering time were helpful to improving the flexural strength. The sample sintered at 1400°C for 2 h possessed the highest flexural strength.

Microstructural characterization of bulk Ti3AlC2 ceramics

The microstructural characteristics of the bulk Ti3AlC2 ceramic, synthesized by a solid–liquid reaction and simultaneous in-situ hot pressing, have been studied by electron diffraction and high-resolution transmission electron microscopy. It was found that the as-synthesized ceramic is fully dense and free from amorphous phase at the grain boundaries. The ceramic is predominantly a single phase of hexagonal Ti3AlC2 with a minor volume of tetragonal Al3Ti and cubic TiC. The Al3Ti grains are crystallographically independent of the Ti3AlC2, whereas TiC is intergrown with Ti3AlC2, forming thin platelets and maintaining a close orientational relationship with the Ti3AlC2 matrix.

Shear strength and shear failure of layered machinable Ti3AlC2 ceramics

Shear-induced failures in uniaxial compression and Hertzian contact damage of Ti3AlC2 fabricated by a solid–liquid reaction synthesis were investigated, and shear strength was evaluated using double notched samples and punch hole tests respectively. Compressive strength of 749 MPa and the slip angle of 38° between the slip plane and the loading direction were measured when the applied load was parallel to the hot-pressing direction of the material; whereas the compressive strength of 841 MPa and the angle of 26° were obtained when the applied load was perpendicular to the hot-pressing direction. The shear strength obtained using double-notched sample was 96 MPa and that from punch-shear test was 138 MPa, respectively. SEM fractographs of specimens failed in shear and compression tests indicated a combination of intergranular and transgranular fracture and also an evidence of friction in the slip plane during compression failure process. Punch-shear test was developed and confirmed to be a simple and feasible method for determining the shear strength of layered machinable ceramics.

Evaluating high-temperature modulus and elastic recovery of Ti3SiC2 and Ti3AlC2 ceramics

Elastic modulus and elastic recovery of both Ti3SiC2 and Ti3AlC2 at various temperatures were evaluated using cyclic loading–unloading in three-point bending tests and comparing relative slopes. It was found that the elastic moduli of Ti3SiC2 and Ti3AlC2, 310 and 288 GPa, were nearly constants in the temperature range from 20 to 1000 °C and then dramatically declined with further increasing of temperature. Elastic recovery as a function of temperature exhibited quite similar curve as the elastic modulus. The simple testing method was demonstrated convenient for estimating the elastic modulus and plastic deformation of ceramics at high temperature.

Synthesis and mechanical properties of Ti3AlC2 by spark plasma sintering

In this paper, spark plasma sintering (SPS), after hot isostatically pressing (HIP) method was reported as a new approach to prepare bulk polycrystalline samples of Ti3AlC2. The ternary carbide was fabricated by spark plasma sintering (SPS) at a pressure of 22 MPa and temperature of 1250°C. The raw materials, elemental powders of Ti, Al and activated carbon, were pretreated in the following different ways prior to SPS: one way was to obtain porous Ti3AlC2 by self-propagating high-temperature synthesis (SHS) from mixture of Ti, Al and C, and then densify the product by SPS; the second way was to synthesize Al4C3 from Al and C firstly, and then mix powders of Ti and C with synthesized Al4C3 to fabricate bulk Ti3AlC2 by SPS. Obtained polycrystalline Ti3AlC2 ceramics had excellent mechanical properties: density was 4.24 ± 0.02 g/cm3, flexural strength was 552 ± 30 MPa and fracture toughness (KIC) was 9.1 ± 0.3 MPa · m1/2. It could be concluded that SPS method was a useful method to synthesize bulk Ti3AlC2 with excellent properties in a very short time and easily sintering process. The optimal conditions to synthesize Ti3AlC2 were also discussed.