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.     

Mechanical Properties of Fine-Grained Substoichiomebic Titanium Carbide

An attempt was made to lower the ductile-to-brittle transition temperature (DBTT) of polycrystalline TiC and to prevent the premature intergranular fracture noted in stoichiometric Tic at high temperatures by producing four substoichiometric compositions chosen from the single-phase TiC phase field. Each desired composition was prepared by blending and vacuum hot-pressing the appropriate mixture of Ti powder and stoichiometric TiC powder. Each billet was characterized for density, hardness, lattice parameter, and microstructure. The actual bulk compositions were determined by averaging electron probe microanalysis data collected randomly from a polished section of each billet. Specimens cut from the billets were strength-tested in four-point bending from room temperature to 1400°C and in compression from room temperature to 1200°C. A qualitative determination of the material's ductility was obtained from a load vs deflection plot and by optical microscopy of polished surfaces after deformation. Both the hardness and strength dropped with decreasing C/Ti atom ratios. Billets produced at the lower C/Ti atom ratios showed a significant deviation from linearity of the load/ deflection curve at temperatures as low as 1200°C in bending, with little or no drop in strength, and as low as 600°C in compression.     

Synthesis of Nanostructured Silicon Carbide through an Integrated Mechanical and Thermal Activation Process

Changes of crystal structures and microstructures of SiO₂ and graphite powder mixtures induced by high-energy milling, the effect of these changes on the reactivity of reactants, and the mechanism of enhanced SiC formation have been studied using a variety of analytical instruments, including X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, solid-state ²⁹Si nuclear magnetic resonance, and nitrogen adsorption (i.e., the BET method). High-energy milling before carbothermic reduction leads to substantial changes in the structural and energy states of the reactants, which in turn increases the reactivity of the reactants and enhances the formation of nanostructured SiC particles. Furthermore, the structural and energy-state changes contribute to the enhanced SiC formation through the increased reaction kinetics as well as the increased reaction driving force.     

Synthesis of Submicrometer Titanium Carbide Powders

A novel synthesis process, based on a carbothermal reduction of titania, has been developed for producing high–purity, submicrometer, and nonagglomerated titanium carbide powders. The process utilizes a carbothermic reduction reaction of novel coated precursor powders that has potential as a high–quality (submicrometer and high–purity) powder synthesis route. The precursor is derived from a titania (TiO₂) and a hydrocarbon (C₃H₆) gas and provides high contact area between the reactants. This yields a better distribution of carbon within the titania and inhibits the agglomeration among the titania particles, resulting in a more complete reaction and a purer product at a comparatively low temperature. The TiC powders produced at 1550°C for 4 h under argon gas flow have oxygen content of 0.6 wt% and total carbon content of 22.9 wt%, a very fine particle size (,0.1μm) (surface area of about 20 m²/g), uniform shape, and loose agglomeration.     

Chemical, Mechanical, and Tribological Properties of Pulsed-Laser-Deposited Titanium Carbide and Vanadium Carbide

The chemical, mechanical, and tribological properties of pulsed-laser-deposited TiC and VC films are reported in this paper. Films were deposited by ablating carbide targets using a KrF (λ= 248 nm) laser. Chemical analysis of the films by XPS revealed oxygen was the major impurity; the lowest oxygen concentration obtained in a film was 5 atom%. Oxygen was located primarily on the carbon sublattice of the TiC structure. The films were always substoichiometric, as expected, and the carbon in the films was identified primarily as carbidic carbon. Nanoindentation hardness tests gave values of 39 GPa for TiC and 26 GPa for VC. The friction coefficient for the TiC films was 0.22, while the VC film exhibited rapid material transfer from the steel ball to the substrate resulting in steel-on-steel tribological behavior.     

Impurities in the Combustion Synthesis of Titanium Carbide

The evolution of solid and gaseous impuritics from powders and their effects during the combustion synthesis (self-propagating high-temperature synthesis) of titanium carbide have been studied. First, the volatiles from the precursor powders, released during bakeout in a vacuum furnace, were measured by residual gas analysis. Second, both condensable and non-condensable materials emitted during the reaction of the Ti + C system were analyzed. The results indicate that the major noncondensing species in both cases were water vapor, hydrogen, carbon monoxide, carbon dioxide, and hydrocarbons. Significant amounts of oxygen and TiO₂ were also enitted during the reaction, but were not observed during bakeout. These impurities are shown to affect not only the reaction process, but also the microstructure of the reaction product.     

Fabrication of Titanium Carbide Ceramics by High-Pressure Self-Combustion Sintering of Titanium Powder and Carbon Fiber

Titanium carbide ceramics have been fabricated from the reactants titanium powder and carbon fiber by the highpressure self-combustion sintering method (HPCS) under two pressure conditions (65 MPa and 3 GPa). Porous TiC with a density of about 50% of theoretical was obtained under a pressure of 65 MPa. A possible model accounting for the formation mechanism of TiC is proposed on the basis of observation of the microstructure of the products formed from the incomplete reaction. On the other hand, dense TiC (>95% of theoretical) was fabricated under 3 GPa. The mean grain size varied, depending on the mixing molar ratio of the reactants. The influence of differences in the carbon (powder and fiber forms) on the structure of TiC compacts is discussed.     

Microstructure and Mechanical Properties of Fine-Grained Magnesia-Partially-Stabilized Zirconia Containing Titanium Carbide Particles

Fine-grained Mg-PSZ has been fabricated by adding TiC particles. The average cubic grain size was smaller by more than an order of magnitude than without TiC when the TiC content was over 2.5 vol%. Pressurelessly sintered specimens contained numbers of relatively large pores while hot-pressed ones were fully dense. For hot-pressed specimens, addition of TiC particles did not affect the growth behavior of tetragonal precipitates during annealing. With increasing TiC content, the bend strength of hot-pressed specimens increased while the fracture toughness decreased. The bend strength and the fracture toughness of fine-grained Mg-PSZ containing 5 vol% TiC were 980 MPa and 8.2 MPa·m^1/2, respectively.     

Carbon Dioxide laser Synthesis of Ultrafine Silicon Carbide Powders from Diethoxydimethylsilane

Ultrafine SiC powders have been synthesized from diethoxydimethylsilane by laser-induced vapor-phase reactions. The powders produced under different flow rates of the reactant vapor have been characterized by chemical analysis, transmission electron microscopy, infrared spectros-copy, and X-ray diffractometry. The results show that the powders are less than 30 nm in average particle diameter, uniformly distributed, spherical in shape, and amorphous. The oxygen and free carbon of the powders vary with the vapor flow rate of the reactant. Annealing of the powders at 1873 K in nitrogen for 1 h results in a complete transformation from amorphous SiC to β-SiC and a small amount of α-SiC.     

Solid-State Reaction Between Titanium Carbide and Titanium Metal

The solid-state reaction between titanium and titanium carbide to form substoichiometric titanium carbide was studied by annealing single-crystal diffusion couples at T = 1350° to 1525°C. The thickness of the epitaxially grown carbide layer was almost an order of magnitude greater than that predicted from published values for chemical diffusion of carbon through titanium carbide. A value of 424 exp[–368 kJ/mol/RT] cm²/s is calculated for the chemical diffusivity of carbon from the diffusion-couple data of the present study. This anomalously rapid diffusion of carbon is associated with short-circuit diffusion along platelets of Ti₂C which develop parallel to {111} planes in TiC during the reaction. The Ti₂C platelets extend from the original Ti-TiC interface to the moving reaction front and also grow from the original interface into the original carbide crystal.     

Pore Destruction Resulting from Mechanical Thermal Expression

Mechanical thermal expression (MTE) is a dewatering technology ideally suited for the dewatering of internally porous biomaterials. For such materials, the combined application of temperature and compressive force in the MTE process enhances the collapse of the porous structure, resulting in effective water removal. In this article, a comparison of the dewatering of titanium dioxide, which is an ideal incompressible, non-porous material, and lignite, which is a porous plant-based biomaterial, is presented. The comparison is based on the parameters critical to dewatering, namely the material compressibility and the permeability. With the aid of mercury porosimetry results, a detailed discussion of the pore destruction of lignite resulting from MTE processing is presented. It is illustrated that there is a well-defined relationship between the pore size distribution after MTE dewatering and the MTE temperature and pressure. The discussion is extended to an investigation of the effects of MTE processing conditions on the effective and non-effective porosity. The effective porosity is defined as the interconnected porosity, which contributes to flow through the compressed matrix, while the non-effective porosity is the remaining porosity, which does not contribute to flow. It is illustrated that there is a linear relationship in both the effective and non-effective porosity with the total porosity. The linear relationship is independent of the processing conditions. It is also shown that MTE processing collapses the effective and non-effective pores at roughly the same rate.     

Pore Destruction Resulting from Mechanical Thermal Expression

Mechanical thermal expression (MTE) is a dewatering technology ideally suited for the dewatering of internally porous biomaterials. For such materials, the combined application of temperature and compressive force in the MTE process enhances the collapse of the porous structure, resulting in effective water removal. In this article, a comparison of the dewatering of titanium dioxide, which is an ideal incompressible, non-porous material, and lignite, which is a porous plant-based biomaterial, is presented. The comparison is based on the parameters critical to dewatering, namely the material compressibility and the permeability. With the aid of mercury porosimetry results, a detailed discussion of the pore destruction of lignite resulting from MTE processing is presented. It is illustrated that there is a well-defined relationship between the pore size distribution after MTE dewatering and the MTE temperature and pressure. The discussion is extended to an investigation of the effects of MTE processing conditions on the effective and non-effective porosity. The effective porosity is defined as the interconnected porosity, which contributes to flow through the compressed matrix, while the non-effective porosity is the remaining porosity, which does not contribute to flow. It is illustrated that there is a linear relationship in both the effective and non-effective porosity with the total porosity. The linear relationship is independent of the processing conditions. It is also shown that MTE processing collapses the effective and non-effective pores at roughly the same rate.     

Dependence of Silicon Carbide Product Morphology on the Degree of Mechanical Activation

Mechanical activation before carbothermic reduction can substantially enhance the formation of SiC from SiO₂and carbon mixtures. However, the morphology (e.g., particles or whiskers) of SiC formed from mechanically activated SiO₂and carbon mixtures is dependent of the degree of mechanical activation and the condition of the subsequent carbothermic reduction. These phenomena are investigated and rationalized based on the increased reactivity of the reactants and SiC formation mechanisms.     

Thermal Conductivity of Titanium Carbide, Zirconium Carbide, and Titanium Nitride at High Temperatures

The thermal conductivity of titanium carbide, zirconium carbide, and titanium nitride was measured by a variety of techniques in the temperature range 200° to 2000° C. The titanium nitride results are a factor of four higher than the previously published values at 1000°C and show a positive temperature coefficient for the thermal conductivity rather than the negative coefficient previously reported in other work. Recent thermal conductivity data on Tic confirm earlier data obtained by the writers which differed from previous literature values. Contemporary theory fails to explain the temperature dependency of the phonon and/or electron contribution to the heat conduction in the refractory interstitial compounds.     

Microstructure of Silicon Carbide/Carbon Composites and Relationships with Mechanical Properties

Three different 2-D SiC/C composites, analyzed by TEM, have mechanical performances directly controlled by the interfacial structure resulting from processing. Localized attachment of the fibers to a brittle matrix constituent is shown to be responsible for composite weakening by impeding interface debonding and sliding. Moreover, the particulate phase used in the matrix is found to have a strong influence. There is a consequent sensitivity of composite strength to the particulate properties. Finally, it is demonstrated that the hindrance of debonding and sliding caused by the brittle matrix phase can be suppressed by using a C fiber coating.     

Oxidation of Hafnium Carbide and Titanium Carbide Single Crystals with the Formation of Carbon at High Temperatures and Low Oxygen Pressures

The isothermal oxidation of the 200 face of HfC and TiC single crystals was performed at temperatures of 700°—1500°C and at oxygen pressures of 0.08—80 kPa for 4 h. The weight gain by oxidation of the two crystals was followed using an electromicrobalance. A polished cross section of the oxidized crystals was observed using backscattered electron imaging in a scanning electron microscope. Quantitative chemical analysis for Hf, Ti, O, and C was performed by wavelength-dispersive X-ray microanalysis. The early-stage oxidation kinetics of HfC crystals were described by the contracting volume equation, followed by slowed reaction in the latter stage, whereas the same equation was applied to the oxidation of TiC over the entire oxidation time. The preferred {200} orientation of monoclinic HfO₂ occurred on the oxidized surface of the HfC crystal. The oxide scale on the HfC crystal consisted of a compact and pore-free black inner scale (zone 1) and a white/gray outer scale that contained many pores (zone 2). Zone 1 contained ∼25 at.% unoxidized carbon, and zone 2 contained 6—11 at.% carbon. The oxide scale of TiC was composed of an inner dense lamella subscale (zone 1) with a carbon content of 7—23 at.% and an outer region with laminations that was separated by pores and cracks (zone 2). The Ti₃O₅ phase, which exhibits a strong 020 line, was formed at depths of ≥40 μm in the scale obtained at 1500°C. Treatment with a concentrated HF solution allowed zone 1 to be separated from the HfC crystal in the form of carbon-containing films, which were characterized using Raman spectroscopy and transmission electron microscopy.     

Synthesis and Kinetics for Nanocrystalline Titanium Carbide upon Metallothermic Reduction of Liquid Chlorides

Nanosized titanium carbide particles were synthesized by the reaction of liquid magnesium and vaporized TiCl₄ + C_xCl₄ (x = 1, 2) solution. Fine titanium carbide particles about 50 nm in size were successfully produced by combining Ti and C atoms released by the chloride reduction of magnesium, and a vacuum was then used to remove the residual phases of MgCl₂ and excess Mg. Small amounts of impurities such as O, Fe, Mg, and Cl were detected in the product, but such a problem can be solved by more precise control of the process. The lattice parameter of the product was 0.43267 nm, which is near the standard value. With respect to the reaction kinetics, the activation energy for the reactions of TiCl₄ + C₂Cl₄ and Mg was found to be 69 kJ/mol, which was about half of the value of the reaction with TiCl₄ + CCl₄, and the higher reactivity of the former contributed to an increase in the stoichiometry to the level of TiC_0.96 and to a decrease in the free carbon content to below 0.3 wt %.Original English Text Copyright © 2005 by Fizika i Khimiya Stekla, Lee, Alexandrovskii, Tolochko, D. Kim, B. Kim.This article was submitted by the authors in English.