Spheroidization of Titanium Carbide Powders by Induction Thermal Plasma Processing

Highly spherical particles of titanium carbide (TiC) have been produced by in-flight heat processing of irregularly shaped TiC powders in an aerosol reactor under argon-hydrogen and argon-helium induction thermal plasma. The spherical powders obtained by the plasma treatment consist of unagglomerated and uniform particles with mean diameters between 25 and 28.5 μm, which is smaller than the original TiC particle mean diameters (29.5 μm) because of partial evaporation of the particles during the plasma treatment. The spheroidization ratio of the treated TiC powders increases with the increase of hydrogen flow rate in plasma gases and the reduction of powder feeding carrier gas flow rate. Under certain processing conditions, the TiC powders have been completely spheroidized. The morphology and structure of individual spherical particles were examined and their formation mechanism was discussed based on calculation of heat transfer kinetics of the particles in the thermal plasma.     

Oxidation Resistance of Alumina–Titanium Nitride and Alumina–Titanium Carbide Composites

The presence of TiC or TiN paritcles in an Al₂O₃ matrix affects the thermal stability of the composites in oxidizing environments. In isothermic oxidation tests at 700°, 800°, 900°, 1000°, and 1100°C for up to 20 h, two different oxidation regimes have been observed at T < 900°C and at 900°C ≤ T ≤ 1100°C. At low temperatures ( T < 900°C), the oxidation follows a phase-boundary reaction; the reaction product initially consists of aggregates of submicrometer needlelike TiO₂ rutile crystals that subsequently grow and coalesce. When a continuous TiO₂ rutile layer is formed ( T ≥ 900°C), the oxidation kinetics change to parabolic, and the diffusion of O₂ through a thick TiO₂ layer is proposed as the governing step.     

Quick Analysis of Free Carbon in Metal Carbide by Plasma Oxidation

Plasma oxidation combined with emission monitoring was applied to the quantitative analysis of free carbon in ZrC powder. The emission was monitored with an optical color analyzer and was calibrated with standard samples of ZrO₂+ C mixtures. Oxidation rates of the free and the combined carbons are so different that it is possible to estimate the amount of the former from the emission. No chemical treatment is necessary.     

Microstructure and Oxidation Behavior of Silicon Carbide Fibers Derived from Polycarbosilane

Polycarbosilane-derived SiC fibers (CG Nicalon, Hi-Nicalon, and Hi-Nicalon type S) were exposed for 1–100 h at 1273–1673 K in air. Oxide layer growth and changes in tensile strength for these fibers were examined after exposure. The three types of SiC fibers decreased in strength as the oxide layer thickness increased. Fracture origins were located near the oxide layer–fiber interface. The Hi-Nicalon type S showed better oxidation resistance than the other polycarbosilane-derived SiC fibers after exposure in air at 1673 K for 10 h. This result was attributed to the nature of the silicon oxide layer on the surface of the SiC fibers.     

Microstructural Properties of Combustion-Synthesized and Dynamically Consolidated Titanium Boride and Titanium Carbide

Full-density TiB₂ and TiC have been fabricated by combustion synthesis reactions followed by dynamic compaction of the still hot, porous ceramic body. The relationship between the morphologies and purities of the precursor powders used and the ceramic product structures is presented. Intergrain bonding and residual porosity of the dynamically consolidated products are found to depend strongly on the impurity levels of the precursor powders. Analysis of the TiC indicates that density and microhardness increase as a function of the C/Ti ratio, with maximum values at the ratio of 1.0.     

Oxidation of Mullite-Zirconia-Alumina-Silicon Carbide Composites

The isothermal oxidation of mullite-alumina-zirconia-silicon carbide composites obtained by reaction sintering was studied in the temperature interval 800° to 1400°C. The kinetics of the oxidation process was related to the viscosity of the surface glassy layer as well as to the crystallization of the surface film. The oxidation kinetics was halted for T ≤ 1300°C, presumably because of crystallization.     

Formation of Titanium Carbide/Aluminum Oxide Nanocomposite Powder by High-Energy Ball Milling and Subsequent Heat Treatment

The preparation of titanium carbide/aluminum oxide (TiC/Al₂O₃) nanocomposite powders from a mixture of titanium, carbon, and Al₂O₃ powders via a high-energy ball milling process and subsequent heat treatment was investigated. The microstructure development of the powder mixtures was monitored by X-ray diffraction and transmission electron microscopy. The ball milling of an elemental carbon, titanium, and Al₂O₃ powder mixture at ambient temperature resulted in the formation of TiC within 15 h of milling. With further milling of up to 25 h, the resulting powder mixture was composed of nanosized TiC particles and nanocrystalline carbon, titanium, and Al₂O₃. The nanocrystalline titanium and carbon were transformed into nanosized TiC particles after subsequent heat treatment. The final product was composed of nanosized TiC and microcrystalline Al₂O_3. Most of the nanosized TiC particles were located within Al₂O₃ grains.     

Aqueous Processing of Titanium Carbide Green Sheets

TiC sheets were prepared by an aqueous tape-casting process. The zeta potential measurement showed that the isoelectric point for TiC powders in the absence of dispersant had a pH value of ∼3.3. According to the surface properties of TiC powders, a cationic polymer PEI was selected as dispersant. In the presence of dispersant, the isoelectric point increased to a pH value of ∼10.4. The slip stability was determined by visual observation of the fluidity of the slip as well as the settling of the powders. Results showed that the amount of dispersant required to achieve a minimum of viscosity for 50 vol% suspensions was equal to 1.2 wt%. In the absence and presence of dispersant, stable slips could be obtained in the pH ranges 7–9 and 11–12, respectively. The rheological measurements showed that with PEI as dispersant, TiC suspensions exhibited a small time dependent behavior. With polyvinyl alcohol as binder and glycerol as plasticizer, suspensions showed a thixotropic feature. As-cast tapes were dried in air at room temperature. The results showed that it was possible to fabricate homogeneous green tapes with smooth surfaces from these suspensions.     

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.     

Densification of Reaction-Synthesized Titanium Carbide by High-Velocity Forging

A novel process for the manufacture of dense titanium carbide is described. Titanium carbide is produced by the reaction synthesis method, while densification and near-net shaping is accomplished by a high-velocity forging step. Disks with 10-cm diameter were produced with densities over 96% of the theoretical density. The major problem encountered in this study has been thermal shock. Use of insulation and furnace cooling has decreased the severity of this problem. Optical and scanning electron microscopy observations of the resulting microstructure reveal equiaxed grains with an average size of 44 μ m. Quasi-static and high-strain-rate compressive strength measurements yield values greater than 1.7 and 2.2 GPa, respectively. The morphologies of thermally induced (slow) and rapidly propagating cracks were characterized and the fracture modes were found to be intergranular and transgranular, respectively. The addition of Ni (5 and 25 wt%) yielded a ceramic–metal composite with a favorable microstructure.     

放电等离子烧结致密单相钛三铝碳二的反应机理

以3Ti/1.1Al/1.9C混合粉末为原料,采用放电等离子烧结(SPS)技术,利用X射线衍射仪、扫描电子显微镜和透射电子显微镜等分析方法,研究了致密单相Ti3AlC2三元层状化合物的合成机理,详细探讨了烧结温度对产物合成的影响,提出了一种SPS制备致密单相Ti3AlC2的反应机理。结果表明:利用SPS技术,在1 350℃保温10min的条件下,可以获得致密度大于99%的层状致密单相Ti3AlC2材料。最终产物中TiC的残留与原料中C含量有密切关系,适当降低原料中C含量有利于最终产物中TiC的消除。致密单相三元层状化合物Ti3AlC2的合成过程中,AlTi3和TiAl是形成TiC和Ti2AlC的主要中间相,而Ti3AlC2是由TiC与Ti2AlC反应生成的。     

电感耦合等离子发射光谱测定钛白粉中的二氧化钛

用电感耦合等离子体发射光谱(ICP-AES)法测定钛白粉中的二氧化钛,试样用硫酸和硫酸铵加热分解,盐酸溶解盐类。通过与GB1706-2006结果对照,表明,方法的RSD为0.45%。     

Silicon Carbide/Silicon and Silicon Carbide/Silicon Carbide Composites Produced by Chemical Vapor Infiltration

Composites of SiC/Si and SiC/SiC were prepared from single yarns of SiC. The use of carbon coatings on SiC yarn prevented the degradation normally observed when chemically vapor deposited Si is applied to SiC yarn. The strength, however, was not retained when the composite was heated at elevated temperatures in air. In contrast, the strength of a SiC/C/SiC composite was not reduced after this composite was heated at elevated temperatures, even when the fiber ends were exposed.     

Pressureless Sintering of Alumina-Titanium Carbide Composites

The densification of Al₂O₃-TiC composites is detrimentally affected by chemical reactions between Al₂O₃ and TiC. These reactions must be suppressed in order to promote sintering. In this study, the specific reactions occurring in Al₂O₃-TiC composites were modeled, using thermodynamic calculations, and verified by experiments. The reaction between Al₂O₃ and TiC was suppressed by the use of specially prepared embedding powders allowing pressureless sintering to closed porosity. The Al₂O₃-TiC composites were subsequently hot isostatically pressed to > 99% of theoretical density without encapsulation. Typical flexural strength and fracture toughness of Al₂O₃-30 wt% TiC composites were 690 MPa and 4.3 MPa · m^1/2, respectively.     

Microstructural Evolution of Titanium Carbide–Chromium Carbide (TiC–Cr3C2) Composites Produced via Combustion Synthesis

A microstructural analysis of compounds produced by combustion synthesis coupled with hot pressing, for reactions between titanium, chromium, and carbon, was conducted. The reactions were aimed to produce composites of Cr₃C₂ and TiC at three different volume fractions of each carbide (25/75, 50/50, and 75/25). Large amounts of chromium and carbon were found to be in solution in the B1 rock-salt structure of TiC. The materials with 25 and 50 vol% of Cr₃C₂ consisted of 100% (Ti,Cr)C_ss solid solution, while the composition with 75 vol% Cr₃C₂ was formed by Cr₃C₂+ (Ti,Cr)C_ss. Some precipitation of Cr₃C₂ was achieved by annealing, but a minimum of 20 wt% was always in solution. The 50 vol% Cr₃C₂–50 vol% TiC composition was the most affected by the heat treatments. Discontinuous and general precipitation were observed, depending on the annealing conditions. A TTT-type diagram was plotted for this material.     

Processing and Microstructure Development of Titanium Carbide–Nickel Aluminide Composites Prepared by Melt Infiltration/Sintering (MIS)

A combined melt-infiltration/sintering (MIS) route has been described for the preparation of composites that are based on titanium carbide (TiC) with a ductile nickel aluminide (Ni₃Al) binder phase. This approach allows control of the Ni₃Al content, which can be varied readily in the range of 4–25 vol%. Densities of >98% of the theoretical density have been obtained for composites that have been prepared with Ni₃Al contents of ≥8 vol%. Preliminary examination indicates that the infiltration kinetics approximate to parabolic at 1300°C. Compositional analysis of the densified materials indicates only minimal titanium dissolution into the Ni₃Al alloy (<6 at.%), with an analyzed carbide composition of TiC_0.93. Cubic grain-growth kinetics are observed for TiC–16-vol% Ni₃Al, with an activation energy of ∼400 ± 60 kJ/mol.     

反应烧结碳化硅的显微组织及其导电性的研究

研究了液态硅参与下的反应烧结碳化硅的工艺参数、显微组织对其电阻率的影响.随着烧结气氛压力和成型压力增加,反应烧结碳化硅中游离硅量减少,电阻率增加.其烧结机理以碳的溶解及碳化硅的淀析过程为主.     

Joining of Silicon Carbide/Silicon Carbide Composites and Dense Silicon Carbide Using Combustion Reactions in the Titanium–Carbon–Nickel System

A new ceramic joining technique has been developed that utilizes an exothermic combustion reaction to simultaneously synthesize the joint interlayer material and to bond together the ceramic workpieces. The method has been used to join SiC/SiC composites and dense SiC ceramics using TiC-Ni powder mixtures that ignite below 1200°C to form a TiC-Ni joining material. Thin layers of the powder reactants were prepared by tape casting, and joining was accomplished by heating in a hot-press to ignite the combustion reaction. During this process, localized exothermic heating of the joint region resulted in chemical interaction at the interface between the TiC-Ni and the SiC ceramic that contributed to bonding. Room-temperature four-point bending strengths of joints produced by this method have exceeded 100 MPa.     

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.     

Effects of Intergranular Phase Chemistry on the Microstructure and Mechanical Properties of Silicon Carbide Ceramics Densified with Rare-Earth Oxide and Alumina Additions

Based on the processing strategy of improving the mechanical properties of liquid-phase-sintered materials by modifying the secondary phase chemistry, four rare-earth oxides (RE₂O₃, RE = La, Nd, Y, and Yb), in combination with alumina, were used as sintering aids for a submicrometer-size β-SiC powder. Doped with 5 vol% RE₂O₃+ Al₂O₃ additives, all specimens were hot-pressed to near full-densities at 1800°C, and they exhibited similar microstructures and grain size distributions. The SiC grains in all specimens revealed a core-rim structure after being plasma-etched, indicating that they were densified via the same solution-reprecipitation mechanism. It was found that a decrease in the cationic radius of the rare-earth oxides was accompanied by an increase in Young's modulus, hardness, and flexural strength of the SiC ceramics, whereas the fracture toughness was improved by incorporating rare-earth oxides of larger cationic radius. The changes in the mechanical properties were attributed to the difference in the chemistry of the intergranular phases in the four ceramics.