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.     

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.     

Self-Propagating High-Temperature Synthesis of Ti3SiC2: I, Ultra-High-Speed Neutron Diffraction Study of the Reaction Mechanism

In situ neutron diffraction at 0.9 s time resolution was used to reveal the reaction mechanism during the self-propagating high-temperature synthesis (SHS) of Ti₃SiC₂ from furnace-ignited stoichiometric 3Ti + SiC + C mixtures. The diffraction patterns indicate that the SHS proceeded in five stages: (i) preheating of the reactants, (ii) the α→β phase transformation in Ti, (iii) preignition reactions, (iv) the formation of a single solid intermediate phase in <0.9 s, and (v) the rapid nucleation and growth of the product phase Ti₃SiC₂. No amorphous contribution to the diffraction patterns from a liquid phase was detected and, as such, it is unlikely that a liquid phase plays a major role in this SHS reaction. The intermediate phase is believed to be a solid solution of Si in TiC such that the overall stoichiometry is ∼3Ti:1Si:2C. Lattice parameters and known thermal expansion data were used to estimate the ignition temperature at 923 ± 10°C (supported by the α→β phase transformation in Ti) and the combustion temperature at 2320 ± 50°C.     

Fabrication and Microstructure Characterization of Ti3SiC2 Synthesized from Ti/Si/2TiC Powders Using the Pulse Discharge Sintering (PDS) Technique

Ti/Si/2TiC powders were prepared using a mixture method (M) and a mechanical alloying (MA) method to fabricate Ti₃SiC₂ at 1200°–1400°C using a pulse discharge sintering (PDS) technique. The results showed that the Ti₃SiC₂ samples with <5 wt% TiC could be rapidly synthesized from the M powders; however, the TiC content was always >18 wt% in the MA samples. Further sintering of the M powder showed that the purity of Ti₃SiC₂ could be improved to >97 wt% at 1250°–1300°C, which is ∼200°–300°C lower than that of sintered Ti/Si/C and Ti/SiC/C powders using the hot isostatic pressing (HIPing) technique. The microstructure of Ti₃SiC₂ also could be controlled using three types of powders, i.e., fine, coarse, or duplex-grained, within the sintering temperature range. In comparison with Ti/Si/C and Ti/SiC/C mixture powders, it has been suggested that high-purity Ti₃SiC₂ could be rapidly synthesized by sintering the Ti/Si/TiC powder mixture at relatively lower temperature using the PDS technique.     

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.     

Intermediate Phases in Ti3SiC2 Synthesis from Ti/SiC/C Mixtures Studied by Time-Resolved Neutron Diffraction

The reactive sintering of 3Ti/SiC/C to form the layered ternary carbide Ti₃SiC₂ was studied in situ by time-resolved neutron powder diffraction. A number of intermediate processes occur during the synthesis beginning with the α-β transition in Ti. Concurrent with the α-β transition, two intermediate phases, TiC _x and Ti₅Si₃C _x ( x ≤ 1), form. These phases account for almost the entire sample in the range 1500–1600°C beyond which they react with each other and a small amount of free C to form the product phase Ti₃SiC₂.     

Combustion Reaction During Mechanical Alloying Synthesis of Ti3SiC2 Ceramics from 3Ti/Si/2C Powder Mixture

Mechanical alloying (MA) synthesis of Ti₃SiC₂ from a stoichiometric elemental powder mixture of Ti, Si, and C was conducted by using a planetary mill with a specially designed MA jar, which enables the real-time measurement of temperature and gas pressure during the MA process. Sudden gas pressure and temperature rises were detected when the mixed powders were mechanically alloyed for a certain period, and consequently a large amount of Ti₃SiC₂ particles was synthesized. Using the Ti–Si–C system as an example, the present study confirmed the combustion reaction triggered by the ball-milling process for the first time.     

Effect of Vacuum Annealing on the Phase Stability of Ti3SiC2

The effect of vacuum annealing on the thermal stability and phase transition of Ti₃SiC₂ has been investigated by X-ray diffraction (XRD), neutron diffraction, synchrotron radiation diffraction, and secondary ion mass spectroscopy (SIMS). In the presence of vacuum or a controlled atmosphere of low oxygen partial pressure, Ti₃SiC₂ undergoes a surface dissociation to form nonstoichiometric TiC and/or Ti₅Si₃C _x that commences at ∼1200°C and becomes very pronounced at ≥1500°C. Composition depth profiling at the near surface of vacuum-annealed Ti₃SiC₂ by XRD and SIMS revealed a distinct gradation in the phase distribution of TiC and Ti₅Si₃C _x with depth.     

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.     

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.     

Reinforcement of Hydroxyapatite Bioceramic by Addition of Ti3SiC2

Ti₃SiC₂/HAp composites with different Ti₃SiC₂ volume fractions were fabricated by spark plasma sintering (SPS) at 1200°C. The effects of Ti₃SiC₂ addition on the mechanical properties and microstructures of the composites were investigated. The bending strength and fracture toughness of the composites increased with increasing of Ti₃SiC₂ content, whereas the Vickers hardness decreased. The bending strength and fracture toughness reached 252±10 MPa and 3.9±0.1 MPa·m^1/2, respectively, with the addition of 50 vol% Ti₃SiC₂. The increases in the mechanical properties were attributed to the matrix strengthening and interactions between cracks and the Ti₃SiC₂ platelets.     

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.     

Machinable Ti3SiC2/Hydroxyapatite Bioceramic Composites by Spark Plasma Sintering

In this paper, we report a machinable Ti₃SiC₂/hydroxyapatite (HAp) composite prepared by spark plasma sintering. The experimental results of a drilling test demonstrated that the composites exhibit excellent machinability when the Ti₃SiC₂ content is higher than 20 vol%, which can be attributed to the improvement in the mechanical and machinable properties of the composites by addition of Ti₃SiC₂ phase, which possessess unique mechanical and machinable properties and energy-absorbing mechanisms. The superior mechanical and machinable properties of Ti₃SiC₂/HAp composites suggest that the composite system could be attractive for practical applications of novel biomaterials.     

A New Ternary Nanolaminate Carbide: Ti3SnC2

We report on the synthesis of Ti _{n +1}SnC _n (MAX phases) by hot isostatic pressing starting with titanium, tin, and carbon powders. In addition to the already known Ti₂SnC compound (211 MAX phase), a new 312 MAX phase, Ti₃SnC₂, is formed. Its lattice parameters, deduced from Rietveld analysis of X-ray diffraction patterns, are found to be a =0.31366 nm and c =1.8650 nm.     

Titanium Silicon Carbide Pest Induced by Nitridation

The thermal stability of bulk Ti₃SiC₂ in high-purity nitrogen was investigated. It was surprising to observe that Ti₃SiC₂ underwent rapid and catastrophic disintegration above 1300°C, although this material was thermally stable below this temperature. This degradation was unexpected and extremely serious, and has been termed "Ti₃SiC₂ pest." This phenomenon was related to the volume change associated with the formation of mixtures of TiC _x , Ti(C, N) _x , and TiN, which caused internal tensile stresses and cracked the resulting layers. "Ti₃SiC₂ pest" could be prevented by increasing oxygen partial pressure in nitrogen.     

In Situ Processing and High-Temperature Properties of Ti3Si(Al)C2/SiC Composites

Ti₃SiC₂ has many salient properties including low density, high strength and modulus, damage tolerance at room temperature, good machinablity, and being resistant to thermal shock and oxidation below 1100°C. However, the low hardness and poor oxidation resistance above 1100°C limit the application of this material. The poor oxidation resistance at temperatures above 1100°C was because of the absence of protective layer in the scale and the presence of TiC impurity phase. TiC impurity could be eliminated by adding a small amount of Al to form Ti₃Si(Al)C₂ solid solutions. Although the high-temperature oxidation resistance was significantly improved for the Ti₃Si(Al)C₂ solid solutions, the strength at high temperatures was lost. One important way to enhance the high-temperature strength is to incorporate hard ceramic particles like SiC. In this article, we describe the in situ synthesis and simultaneous densification of Ti₃Si(Al)C₂/SiC composites using Ti, Si, Al, and graphite powders as the initial materials. The effect of SiC content on high-temperature mechanical properties and oxidation resistance were investigated. The mechanisms for the improved high-temperature properties are discussed.     

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.     

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.     

Thermal Expansion of Polycrystalline Ti3SiC2 in the 25°–1400°C Temperature Range

We measured the volume thermal expansion of Ti₃SiC₂ from 25° to 1400°C using high-temperature X-ray diffraction using a resistive heated cell. A piece of molybdenum foil with a 250 μm hole contained the sample material (Ti₃SiC₂+Pt). Thermal expansion of the polycrystalline sample was measured under a constant argon flow to prevent oxidation of Ti₃SiC₂ and the molybdenum heater. From the lattice parameters of platinum (internal standard), we calculated the temperature by using thermal expansion data published in the literature. The molar volume change of Ti₃SiC₂ as a function of temperature in °C is given by: V _M (cm³/mol)=43.20 (2)+9.0 (5) × 10⁻⁴ T +1.8(4) × 10⁻⁷ T ². The temperature variation of the volumetric thermal expansion coefficient is given by: α_v (°C⁻¹)=2.095 (1) × 10⁻⁵+7.700 (1) × 10⁻⁹ T . Furthermore, the results indicate that the thermal expansion anisotropy of Ti₃SiC₂ is quite mild in accordance with previous work.     

Synthesis of Titanium Silicon Carbide

Synthesis of bulk titanium silicon carbide (Ti₃SiC₂) from the elemental Ti, Si, and C powders has been accomplished for the first time, using the arc-melting and annealing route. The effects of various parameters on the phase purity of the Ti₃SiC₂ have been examined, including the starting composition of the powders, compaction technique, arc-melting of the samples, and temperature and time of anneal. The best bulk samples, containing about 2 vol% TiC as the second phase, were made from Si-deficient and C-rich starting compositions. Based on electron probe microanalysis data from a number of bulk samples, it appears that Ti₃SiC₂ exists over a range of compositions; the Ti-Si-C ternary section has been modified to reflect this. The purest samples of the ternary phase were obtained by leaching powders of silicide-containing samples in diluted HF, and contained over99vol%Ti₃SiC₂.