Self-Propagating High-Temperature Synthesis of Ti3SiC2: Study of the Reaction Mechanisms by Time-Resolved X-Ray Diffraction and Infrared Thermography

Ti₃SiC₂ is synthesized by self-propagating high-temperature synthesis (SHS) of elemental titanium, silicon, and graphite powders. The reaction paths and structure evolution are studied in situ during the SHS of the 3Ti+Si+2C mixture by time-resolved X-ray diffraction coupled with infrared thermography. The proposed reaction mechanism suggests that Ti₃SiC₂ might be formed from Ti–Si liquid phase and solid TiC _x . Finally, the effect of the powders starting composition on the Ti₃SiC₂ synthesis is studied. For the investigated initial mixtures, TiC _x is always formed as a major impurity together with the Ti₃SiC₂ phase.     

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.     

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.     

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 Al4SiC4

The conditions necessary for synthesizing Al₄SiC₄ from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al₄C₃( s ) = Al₄SiC₄( s ) changed from positive to negative at 1106°C. SiC and Al₄C₃ formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al₄SiC₄ then formed by reaction of the SiC and Al₄C₃ at >1200°C. Al₄C₃, SiO₂, Al₂O₃, SiC, and Al₄O₄C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al₄SiC₄ formed from a reaction of those intermediate products at >1600°C.     

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.     

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.     

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.     

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.     

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.     

Processing and Mechanical Properties of Ti3SiC2: I, Reaction Path and Microstructure Evolution

In this article, the first part of a two-part study, we report the reaction path and microstructure evolution during the reactive hot isostatic pressing of Ti₃SiC₂, starting with titanium, SiC, and graphite powders. A series of interrupted hot isostatic press runs have been conducted as a function of temperature (1200°–1600°C) and time (0–24 h). Based on X-ray diffractometry and scanning electron microscopy, at 1200°C, the intermediate phases are TiC _x and Ti₅Si₃C _x . Fully dense, essentially single-phase samples are fabricated in the 1450°–1700°C temperature range. The time-temperature processing envelope for fabricating microstructures with small (3–5 μm), large (∼200 μm), and duplex grains, in which large (100–200 μm) Ti₃SiC₂ grains are embedded in a much finer matrix, is delineated. The microstructure evolution is, to a large extent, determined by (i) the presence of unreacted phases, mainly TiC _x , which inhibits grain growth; (ii) a large anisotropy in growth rates along the c and a directions (at 1450°C, growth normal to the basal planes is about an order of magnitude smaller than that parallel to these planes; at 1600°C, the ratio is 4); and (iii) the impingement of grains. Ti₃SiC₂ is thermally stable under vacuum and argon atmosphere at temperatures as high as 1600°C for as long as 24 h. The influence of grain size on the mechanical properties is discussed in the second part of this study.     

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.     

Experimental Investigation and Thermodynamic Calculation of the Titanium–Silicon–Carbon System

The 1100°C isothermal section and the isopleths at 5, 10, and 15 at.% C in the Ti–Si–C system were determined by DTA and XRD methods. Five invariant reactions (L (liquid) = Si + SiC + TiSi₂ at 1330°C, L = TiSi + TiSi₂+ Ti₅Si₃C _x at 1485°C, L + Ti₅Si₃C _x = Ti₃SiC₂+ TiSi₂ at 1485°C, L + Ti₃SiC₂= TiSi₂+ SiC at 1473°C, and L + TiC = bcc-(Ti) + Ti₅Si₃C _x at 1341°C) were observed. The transition temperature for L + TiC = Ti₃SiC₂+ SiC was measured by the Pirani technique. Optimized thermodynamic parameters for the Ti–Si–C system were then obtained by means of the CALPHAD (calculation of phase diagrams) method applied to the present experimental results and reliable literature data. The calculations satisfactorily account for most of the experimental data.     

Tribological Behavior of SiC-Reinforced Ti3SiC2-Based Composites under Dry Condition and under Lubricated Condition with Water and Ethanol

To evaluate the lubrication compatibility of SiC-reinforced Ti₃SiC₂-based composites, i.e., a Ti₃SiC₂/SiC composite, with water and alcohol, sliding experiments were conducted under dry condition and lubricated with water and C₂H₅OH. The friction and wear of the Ti₃SiC₂/SiC composite decreased in the order of dry condition, in water, and in C₂H₅OH. The worn surfaces indicated that oxide masses, which caused oxidation wear and abrasive wear, were formed by tribo-oxidation on the sliding surface under the dry condition and in water, while tribo-oxidation scarcely occurred in C₂H₅OH.     

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.     

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.     

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.     

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.     

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.