Tribophysical Properties of Polycrystalline Bulk Ti3AlC2

A nearly pure, dense, polycrystalline bulk Ti₃AlC₂ sample was prepared by reactively hot pressing the element titanium, aluminum, and graphite powders. The tribophysical properties were investigated by sliding a Ti₃AlC₂ block dryly against a low-carbon steel disk. It was found that the friction coefficient is as low as ∼0.1, and the wear rate of Ti₃AlC₂ is only ∼2.5 × 10⁻⁶ mm³/N·m for the highest sliding speed of 60 m/s and the largest normal pressure of 0.8 MPa. These unusual properties are attributable to the presence of a compact self-generating film, which covers uniformly over the friction surface of Ti₃AlC₂ with a thickness of ∼0.5 μm.     

Synthesis of Ti3SiC2 Powders: Reaction Mechanism

Titanium silicon carbide (Ti₃SiC₂) and Ti₃SiC₂-based composite powders were synthesized by isothermal treatment in an inert atmosphere as a function of initial compositions (mixtures). A high content of TiC was obtained in the final product when the initial mixtures contained free carbon. The use of TiC as a reagent was unsuccessful in obtaining Ti₃SiC₂. High Ti₃SiC₂ conversion was found for the initial mixtures containing SiC as the main source for silicon and carbon. An initial mixture with a large excess of silicon, 3Ti/1.5SiC/0.5C, was needed to obtain high-purity Ti₃SiC₂. A reaction mechanism, where Ti₃SiC₂ nucleates on Ti₅Si₃C crystals and grows by long-range diffusion of Ti and C, is proposed. The reaction mechanism was proposed to be based on silicon loss during the formation of Ti₃SiC₂.     

Pressure-Assisted Combustion Synthesis of Dense Layered Ti3AlC2 and its Mechanical Properties

A near-single-phase Ti₃AlC₂ ternary carbide was synthesized from 3Ti–1.1Al–1.8C powder blend, both by the wave propagation and thermal explosion (TE) modes of self-propagating high-temperature synthesis. The application of a moderate (28 MPa) pressure immediately after TE at 800°C (reactive forging) yielded a 95% dense material containing, in addition to Ti₃AlC₂, an appreciable amount of TiC_{1− x} . By adjusting the starting composition, a 99% dense material containing up to 90 vol.% Ti₃AlC₂ was obtained. The material had a fine-layered microstructure with Ti₃AlC₂ grain size not exceeding 10 μm. The samples were readily machinable and had a high compressive strength of ∼800 MPa up to 700°C.     

Near-Net-Shape Fabrication of Ti3AlC2-Based Composites

A porous ceramic preform was fabricated by printing a powder blend of TiC, TiO₂, and dextrin. The presintered preforms contained a bimodal pore size distribution with intra-agglomerate pores ( d ₅₀≈0.7 μm) and inter-agglomerate pores ( d ₅₀≈30 μm), which were subsequently infiltrated by aluminum melt spontaneously in argon above 1050°C. A redox reaction at 1400°C resulted in the formation of dense Ti–Al–O–C composites mainly composed of Ti₃AlC₂, TiAl₃, Al, and Al₂O₃, which attained a bending strength of 320 MPa, a Young's modulus of 184 GPa, and a Vicker's hardness of 2.5 GPa.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel on Ti3AlC2 Substrate

A MgAl₂O₄ (MA) spinel layer was synthesized on Ti₃AlC₂ substrate through the molten salt synthesis (MSS) method. The Ti₃AlC₂ substrate was immersed in MgCl₂·6H₂O powders and treated at 800°, 850°, and 900°C for 4 h in air. A continuous and 10-μm-thick MgAl₂O₄ layer was obtained at 900°C, by which the surface hardness of Ti₃AlC₂ can be effectively improved. The combined scanning electron microscopy observations and crystal morphology simulation further revealed that the as-formed MgAl₂O₄ presents tetragonal bipyramids morphology with (400)-orientation.     

Three-Dimensional Printing of Nanolaminated Ti3AlC2 Toughened TiAl3–Al2O3 Composites

Nanolaminates with a layered M _{N +1}AX _N crystal structure (with M: transition metal, A: group element, X: carbon or nitrogen, and N =1, 2, 3) offer great potential to toughen ceramic composites. A ternary Ti₃AlC₂ carbide containing ceramic composite was fabricated by three-dimensional printing of a TiC+TiO₂ powder mixture and dextrin as a binder. Subsequent pressureless infiltration of the porous ceramic preform with an Al melt at 800°–1400°C in an inert atmosphere, followed by reaction of Al with TiC and TiO₂ finally resulted in the formation of a dense multiphase composite of Ti₃AlC₂–TiAl₃–Al₂O₃. A controlled flaw/strength technique was utilized to determine fracture resistance as a function of crack extension. Rising R -curve behavior with increasing crack extension was observed, confirming the operation of wake-toughening effects on the crack growth resistance. Observations of crack/microstructure interactions revealed that extensive crack deflection along the (0001) lamellar sheets of Ti₃AlC₂ was the mechanism responsible for the rising R -curve behavior.     

Surface Chemistry, Dispersion Behavior, and Slip Casting of Ti3AlC2 Suspensions

The surface chemistry and dispersion properties of aqueous Ti₃AlC₂ suspension were studied in terms of hydrolysis, adsorption, electrokinetic, and rheological measurements. The Ti₃AlC₂ particle had complex surface hydroxyl groups, such as ≡Ti–OH,=Al–OH, and −OTi–(OH)₂, etc. The surface charging of the Ti₃AlC₂ particle and the ion environment of suspensions were governed by these surface groups, which thus strongly influenced the stability of Ti₃AlC₂ suspensions. PAA dispersant was added into the Ti₃AlC₂ suspension to depress the hydrolysis of the surface groups by the adsorption protection mechanism and to increase the stability of the suspension by the steric effect. Ti₃AlC₂ suspensions with 2.0 dwb% PAA had an excellent stability at pH=∼5 and presented the characteristics of Newtonian fluid. Based on the well-dispersed suspension, dense Ti₃AlC₂ materials were obtained by slip casting and after pressureless sintering. This work provides a feasible forming method for the engineering applications of MAX-phase ceramics, wherein complex shapes, large dimensions, or controlled microstructures are needed.     

Mechanical-Alloying-Assisted Synthesis of Ti3SiC2 Powder

Mechanical alloying (MA) has been used to synthesize Ti₃SiC₂ powder from the elemental Ti, Si, and C powders. The MA formation conditions of Ti₃SiC₂ were strongly affected by the ball size for the conditions used. MA using large balls (20.6 mm in diameter) enhanced the formation of Ti₃SiC₂, probably via an MA-triggered combustion reaction, but the Ti₃SiC₂ phase was not synthesized only by the MA process using small balls (12.7 mm in diameter). Fine powders containing 95.8 vol% Ti₃SiC₂ can be obtained by annealing the mechanically alloyed powder at relatively low temperatures.     

The Design of Crystalline Precursors For the Synthesis of Mn−1AXn Phases and Their Application to Ti3AlC2

A methodology to allow the deliberate design of solid precursors to affect the solid-state synthesis of materials has proven elusive. We have designed a conceptual synthesis route for M _{n +1}AX _n phases that does not involve the usual intermediate phases. Instead, it is proposed that the common structural units within a solid-state precursor M _{n +1}X _n containing vacancy ordering should be the basis for direct synthesis of the desired M _{n +1}AX _n phase. The method is demonstrated to be successful in producing titanium aluminum carbide (Ti₃AlC₂) by the rapid intercalation of Al into TiC_0.67 at 400°–600°C below the conventional synthesis temperature. Time-resolved neutron diffraction at 1 min time-resolution has confirmed the reaction sequence. The vacancy ordering in TiC_0.67 occurred simultaneously to, and appeared to be greatly facilitated by, the ingress of aluminum. There is considerable scope for adaptation of the method to other M _{n +1}AX _n phases.     

Tribological Properties of Polycrystalline Ti3SiC2 and Al2O3-Reinforced Ti3SiC2 Composites

Tribological properties of Ti₃SiC₂ and Al₂O₃-reinforced Ti₃SiC₂ composites (10 and 20 vol% Al₂O₃) were investigated by using an AISI-52100 bearing steel ball dryly sliding on a linear reciprocating athletic specimen. The friction coefficients were found varying only in a range of 0.1 under the applied loads (2.5, 5, and 10 N), and the wear rates of the composites decreased with increasing Al₂O₃ content. The enhanced wear resistance is mainly attributed to the hard Al₂O₃ particles nail the surrounding soft matrix and decentrale the shear stresses under the sliding ball to reduce the wear losses.     

High-Resolution Transmission Electron Microscopy of Ti4AlN3, or Ti3Al2N2 Revisited

The structure and chemistry of what initially was proposed to be Ti₃Al₂N₂ are incorrect. Using high-resolution transmission electron microscopy, together with chemical analysis, the stoichiometry of this compound is concluded to be Ti₄AlN_3-delta (where delta = 0.1). The structure is layered, wherein every four layers of almost-close-packed Ti atoms are separated by a layer of Al atoms. The N atoms occupy ∼97.5% of the octahedral sites between the Ti atoms. The unit cell is comprised of eight layers of Ti atoms and two layers of Al atoms; the unit cell is hexagonal with P 6₃/ mmc symmetry (lattice parameters of a = 0.3 nm and c = 2.33 nm). This compound is machinable and closely related to other layered, ternary, machinable, hexagonal nitrides and carbides, namely M₂AX and M₃AX₂ (where M is an early transition metal, A is an A-group element, and X is carbon and/or nitrogen).     

Interdiffusion Between Ti3SiC2–Ti3GeC2 and Ti2AlC–Nb2AlC Diffusion Couples

In this work, we report on the interdiffusion of Ge and Si in Ti₃SiC₂ and Ti₃GeC₂, as well as that of Nb and Ti in Ti₂AlC and Nb₂AlC. The interdiffusion coefficient, D _int, measured by analyzing the diffusion profiles of Si and Ge obtained when Ti₃SiC₂–Ti₃GeC₂ diffusion couples are annealed in the 1473–1773 K temperature range at the Matano interface composition (≈Ti₃Ge_0.5Si_0.5C₂), was found to be given by
D _int increased with increasing Ge composition. At the highest temperatures, diffusion was halted after a short time, apparently by the formation of a diffusion barrier of TiC. Similarly, the interdiffusion of Ti and Nb in Ti₂AlC–Nb₂AlC couples was measured in the 1723–1873 K temperature range. The D _int for the Matano interface composition, viz. ≈(Ti_0.5,Nb_0.5)₂AlC, was found to be given by
At 1773 K, the diffusivity of the transition metal atoms was ≈7 times smaller than those of the Si and Ge atoms, suggesting that the former are better bound in the structure than the latter.     

Tribological Properties of Ti3SiC2

The present contribution reports the unlubricated friction and wear properties of Ti₃SiC₂ against steel. The fretting experiments were performed under varying load (1–10 N) and the detailed wear mechanism is studied using SEM-EDS, Raman spectroscopy, and atomic force microscopy. Under the selected fretting conditions, Ti₃SiC₂/steel tribocouple exhibits a transition in friction as well as wear behavior with coefficient of friction varying between 0.5 and 0.6 and wear rate in the order of 10⁻⁵ mm³·(N·m)⁻¹. Raman analysis reveals that the fretting wear is accompanied by the triboxidation with the formation of TiO₂, SiO₂, and Fe₂O₃. A plausible explanation for the transition in friction and wear with load is proposed.     

Synthesis and Characterization of a Remarkable Ceramic: Ti3SiC2

Polycrystalline bulk samples of Ti₃SiC₂ were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10⁻⁸ and 2 × 10⁻⁵ kg²-m⁻⁴.s⁻¹. The activation energy for oxidation is thus =300 kJ/mol. The room-temperature electrical conductivity is 4.5 × 10⁶Ω⁻¹.m⁻¹, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 × 10⁻⁶°C⁻¹, 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti₃SiC₂ does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.     

Toughening of SiC with Ti3SiC2 Particles

Composites in the SiC–TiC–Ti₃SiC₂ system were synthesized using reactive hot pressing at 1600°C. The results indicate that addition of Ti₃SiC₂ to SiC leads to improved fracture toughness. In addition, high microhardness can be retained if TiC is added to the material. The best combination of properties obtained in this study is K _{I c} =8.3 MPa·m^1/2 and H _v=17.6 GPa. The composition can be tailored in situ using the decomposition of Ti₃SiC₂. Ti₃SiC₂ decomposed rapidly at temperatures above 1800°C, but the decomposition could be conducted in a controlled manner at 1750°C. This can be used for synthesis of fully dense composites with improved properties by first consolidating to full density a softer Ti₃SiC₂-rich initial composition, and then using controlled decomposition of Ti₃SiC₂ to achieve the desired combination of microhardness and fracture toughness.     

Solid-State Synthesis of Ultrafine BaTiO3 Powders from Nanocrystalline BaCO3 and TiO2

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO₂ (70 nm) with BaCO₃ of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO₃ in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO₃ already leads to formation of the single-phase BaTiO₃ after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d ₅₀=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO₃ and TiO₂ represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.     

Dislocations and Stacking Faults in Ti3SiC2

The ternary carbide Ti₃SIC₂ fabricated by a reactive hot-press route is investigated by transmission electron microscopy. The material consists mainly of large elongated grains with planar boundaries, and is characterized by a low defect density. Dislocations are observed in the grains and at grain boundaries. Perfect dislocations with b = 1/3<1120> lying in (0001) basal planes are present. These basal plane dislocations are mobile and multiply as a result of room-temperature deformation. All of the stacking faults observed lie in the basal planes.     

Synthesis and Reaction Mechanism of Ti3SiC2 by Mechanical Alloying of Elemental Ti, Si, and C Powders

Formation of titanium silicon carbide (Ti₃SiC₂) by mechanical alloying (MA) of Ti, Si, and C powders at room temperature was experimentally investigated. A large amount of granules less than 5 mm in size, consisting of Ti₃SiC₂, smaller TiC particles, and other silicides, have been obtained after ball milling for only 1.5 h. The effect of excess Si in the starting powders on the formation of Ti₃SiC₂ was studied. The formation mechanism of Ti₃SiC₂ was analyzed. It is believed that a mechanically induced self-propagating reaction is ignited during the MA process. A possible reaction mechanism was proposed to explain the formation of the final products.     

Preparation of Ti3SiC2 by Electron-Beam-Ignited Solid-State Reaction

This paper describes a novel way to prepare the ternary phase Ti₃SiC₂ in a single-step procedure that we call electron-beam-ignited solid-state reaction (EBI-SSR). The preparation route is discussed by means of an isothermal section of the Ti-Si-C phase diagram. Properties such as the Vickers hardness and the electrical resistivity of the resulting samples are presented. Our property data compare well to those that have been published. The main advantages of this preparation method are the controllability of process parameters such as heating rates, temperatures, and times, as well as the short duration of the overall sample preparation. However, a disadvantage is the presence of second phases (typically in amounts of <8%) that must be reduced via further optimization of the process.