Formation of Titanium Nitride Whiskers by Reaction of Sodium Titanium Bronze with Molten Sodium Cyanide

The production of titanium nitride, TiN, whiskers by reaction of sodium titanium bronze, Na_xTiO₂ (STB), with excess sodium cyanide, NaCN, at 1000°C is reported. The solubility of Ti from a STB in molten NaCN has been estimated experimentally. The TiN whiskers obtained under different experimental conditions have been examined by scanning electron microscopy and analyzed by analytical electron microscopy.     

Microstructure of Silicon Carbide Whiskers Synthesized by Carbothermal Reduction of Silicon Nitride

The microstructure of silicon carbide whiskers synthesized by carbothermal reduction of silicon nitride has been studied using transmission electron microscopy. All of the whiskers examined are single crystals, and grow in the (111) crystallographic direction. Two different forms of stacking faults and microtwins were observed; in one the planar defects are normal to the whisker growth direction, and the other has the defect planes at an angle of about 70° to the growth axis, while both forms of the defects are on the [111] closed-packed planes. Without the addition of catalyst, droplets containing metallic impurities were not found at the tips of the whiskers synthesized by the present process. A core and outer regions were observed in the single-crystal whiskers, which may be evidence that the whiskers were formed by a two-stage mechanism.     

氮化硼晶须氮化工艺及其生长机理(英文)

通过氮化由三聚氰胺和硼酸为原料制备的氮化硼晶须前驱体制备出氮化硼晶须,并采用扫描电子显微镜、X射线衍射仪、红外光谱仪和热分析仪对氮化硼晶须氮化工艺进行研究,同时对氮化硼晶须的生长机理进行探讨。结果表明:当升温速率5℃/min、氮气流量0.2L/min、氮化温度1 700℃、保温时间30min时可制备出长径比高、性能优良的氮化硼晶须。氮化硼晶须的生长机理可归纳为前驱体脱水、聚合反应、无机化反应和晶相转变等过程。     

Texture in Silicon Nitride Seeded with Silicon Nitride Whiskers of Different Sizes

Silicon nitride ceramics seeded with 3 wt%β-Si₃N₄ whiskers of two different sizes were prepared by a modified tape casting and gas pressure sintering. The fine whiskers had a higher aspect ratio than the coarse whiskers. Quantitative texture analysis including calculation of the orientation distribution function (ODF) was used for obtaining the degrees of preferred orientation of sintered samples. The maximum multiples of random distribution (mrd) values of samples seeded with the fine and coarse whiskers were large, greater than 15 and 9, respectively. Meanwhile, the mrd value of a sample seeded with fine whiskers was only 9 when it was prepared by conventional tape casting. The microstructures and the XRD data revealed that the well-aligned whiskers grew significantly after sintering and dominated the texture. Differences among the degrees of preferred orientation of the samples were explained using Jeffrey's model on rotation of elliptical particles carried by a viscous fluid.     

Characterization of β-Silicon Nitride Whiskers

The physical, chemical, and structural properties of a commercially available β-Si₃N₄ whisker were characterized. Bulk chemical analysis indicated that the whiskers were close to stoichiometric silicon nitride, with oxygen and yttrium as the major impurities. Surface chemistry analysis by XPS analysis revealed that the surfaces consisted primarily of silicon nitride, with the oxygen and yttrium impurities concentrated at the surfaces. SEM and STEM studies indicated that the whiskers were dimensionally straight with relatively featureless surfaces, although some whiskers had Y-rich particles attached. The whiskers were also found to be of extreme crystallographic perfection, as determined by TEM analysis.     

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.     

Synthesis of SiC Whiskers from Silicon Nitride in Argon Atmosphere

SiC whiskers were synthesized by carbothermal reduction of silicon nitride. α-Si3N4 and β-Si3N4 powders were used as silicon sources, and graphite, active carbon and black carbon as carbon sources, as well as boron oxide as catalyst. The synthesized SiC whiskers were characterized by XRD and SEM. The results showed that the synthesizing temperature should be above 1 716 K; the decomposition of Si3N4 was the limited step in the synthesis of SiC whiskers; and catalyst not only offered the liquid condition, bu...     

Simultaneous Synthesis and Densification of Titanium Nitride/ Titanium Diboride Composites by High Nitrogen Pressure Combustion

Composites of TiN/TiB₂ were synthesized by a combustion process of BN, Ti in a nitrogen atmosphere. The effect of the BN/Ti ratio and the nitrogen gas pressure on the synthesis of these composites was investigated. Dense TiN/TiB₂ composites with relatively high hardness and toughness were fabricated by combustion synthesis from Ti and BN under a nitrogen pressure of 4.0 MPa. The Vickers microhardness of the products obtained from reactants with a BN/Ti mole ratio of 0.11 increased with an increase in nitrogen pressure and had a maximum value of ∼25 GPa. Fracture toughness, K _IC, of the products increased from 3.1 to 5.9 MPa·m^1/2 as the BN/Ti ratio increased from 0.11 to 0.20. However, products formed under nitrogen pressures higher than 6.0 MPa exhibited circumferential macrocracks due to thermal shock.     

Effect of Vapor—Liquid—Solid and Vapor—solid Silicon Carbide Whiskers on the Effective Thermal Diffusivity/Conductivity of Silicon Nitride Matrix Composites

A study was conducted of the relative effect of vapor—liquid—solid (VLS) and vapor—solid (VS) SiC whiskers on the effective thermal diffusivity and conductivity of pressed-densified silicon nitride. It was found that VLS whiskers cause an increase in the thermal diffusivity/conductivity, whereas the opposite effect was found for the VS-SiC whiskers. Comparison with composite theory suggests that the VS-SiC whiskers have a thermal conductivity as low as 25 to 30 W/(m·K). In contrast the VLS-SiC whiskers appear to have a value for the thermal conductivity of at least about 100 W/(m·K) to as high as 250 W/(m·K). These large differences in thermal conductivity for these two types of SiC whiskers are attributed to the much larger density of structural defects in the VS-SiC whiskers, which act as phonon scatterers, thereby lowering the thermal conductivity.     

Synthesis and Crystallography of Sodium Titanium Bronze

An alternative reaction for the preparation of sodium titanium bronze (STB) based on the use of TiN as a reductant has been tested. Crystallographic data for STB available from the literature and from new preparations have been compiled and discussed.     

Creep of Duplex Alumina–Titanium Nitride Polycrystalline Microstructures

Compression creep results in an inert atmosphere are presented for fine-grained Al₂O₃–TiN composites deformed under a stress range of 10–50 MPa stress and a temperature range of 1250°–1400°C. The effects of TiN content (10–70 vol%) and the preparation route (i.e., reactive versus nonreactive hot pressing) are considered. Al₂O₃ and TiN both are efficient grain-growth inhibitors, relative to each other; thus, deformation develops in stable equiaxed microstructures. Except for low TiN contents (10 and 20 vol%), the processing route does not markedly influence the observed strain rates, at least up to a strain of −0.16. In all cases, deformation occurs mainly via interfacial/grain-boundary sliding, and its rate is enhanced in approximate proportion to the amount of Al₂O₃/TiN interfacial area. The deformation rate seems to be limited by either some interface reaction or a complex boundary diffusional mechanism that occurs at the Al₂O₃ grain boundaries.     

Al2O3/TC4合金钎焊接头中TiB晶须对接头组织结构及力学性能的影响

分别采用CuTi、CuTi+B和CuTi+TiB2钎料,在钎焊温度为930℃,保温时间为10min条件下,钎焊连接Al2O3和TC4合金。通过扫描电子显微镜、透射电子显微镜和压剪试验等方法,研究了接头中生成的TiB晶须对接头组织结构及力学性能的影响。结果表明:B或TiB2粉的添加均可在钎焊接头中原位自生TiB晶须。当钎料中添加B粉时,接头中原位自生的TiB晶须比添加TiB2时的尺寸小。原位自生的TiB晶须可将Al2O3/TC4合金钎焊接头界面分为5个区域,各区分布满足延性–刚性–延性结构,此结构有助于减小降低接头残余应力,提高接头抗剪强度。采用CuTi+TiB2钎料时,Al2O3/TC4合金钎焊接头的抗剪强度最大为143 MPa,比用CuTi钎料时所获接头强度提高了239%。     

Effect of Porosity on the Combustion Synthesis of Titanium Nitride

The effect of porosity on the self-propagating reaction between porous titanium and gaseous nitrogen was investigated. The relationship between total nitrogen uptake and porosity showed a maximum at about 44% porosity. High initial sample porosities lead to partial melting of the titanium and subsequently to a lower degree of conversion to the nitride. Low initial porosities limit the conversion through lower reaction interfacial areas and lower gas permeation. Wave velocity measurements, thermogravimetric determinations on the progression of the combustion front, and microstructural analyses demonstrated that the passage of the front is associated primarily with the formation of a surface layer of TiN_1-x with x being 0.10 and 0.06 for lowdensity (49%) and high-density (59%) samples. The product of combustion contained titanium nitride and primary and β-transformed α solid solutions. The relative abundance of the latter two phases was dependent on the initial relative density of the samples.     

Effect of Silicon Carbide Whisker and Titanium Carbide Particulate Additions on the Friction and Wear Behavior of Silicon Nitride

A Si₃N₄/TiC composite was previously demonstrated to exhibit improved wear resistance compared to a monolithic Si₃N₄ because of the formation of a lubricious oxide film containing Ti and Si at 900°C. Further improvements of the composite have been made in this study through additions of SiC whiskers and improved processing. Four materials—Si₃N₄, Si₃N₄/TiC, Si₃N₄/SiC_wh, and Si₃N₄/TiC/SiC_wh— were processed to further optimize the wear resistance of Si₃N₄ through improvements in strength, hardness, fracture toughness, and the coefficient of friction. Oscillatory pin on flat wear tests showed a decrease in the coefficient of friction from ∼0.7 (Si₃N₄) to ∼0.4 with the addition of TiC at temperatures reaching 900°C. Wear track profiles illustrated the absence of appreciable wear on the TiC-containing composites at temperatures above 700°C. Microscopic (SEM) and chemical (AES) characterization of the wear tracks is also included to deduce respective wear and lubricating mechanisms.     

Low-Temperature Synthesis of Nanocrystalline Titanium Nitride via a Benzene–Thermal Route

A benzene–thermal reaction of TiCl₄ and NaN₃ at 350°–380°C was conducted for the preparation of nanocrystalline TiN. Powder X-ray diffractometry patterns indicated that the powder was cubic-phase TiN with a lattice constant a = 4.23656 Å. Transmission electron microscopy images showed the TiN powders consisted of uniform spherical particles with an average diameter of 50 nm. The binding energies of Ti 2 p 3/2 and N 1 s core levels at the positions of 454.85 and 397.1 eV, respectively, and the Ti:N atomic ratio of 1.08:1.00 were detected by X-ray photoelectron spectra. A possible formation mechanism of TiN was proposed.     

Processing Effects on Microstructural Charcteristics and Properties of Reactively Ion-Plated Titanium Nitride Coatings

The effects of key processing parameters on the resulting microstructure and mechanical properties of thin TiN coatings, reactively ion-plated on polished aluminum and steel substrates, were investigated experimentally. A deposition procedure was established and a processing scheme for reactive ion plating was developed. Optimization of processing conditions was achieved by means of controlling the principal ion-plating parameters to prevent the growth of species different from TiN, evaporant poisoning, and variations in the coating thickness. Emphasis was placed on identifying the effects of critical independent and dependent processing parameters, such as the rf power density, the deposition rate, and the coating thickness, on the TiN microscopic morphology, crystal structure, surface roughness, microhardness, and deformation behavior arising under static and sliding contact loads. Results from various testing and characterization methods revealed that deposition of hard, strongly adherent, and relatively smooth TiN coatings possessing dense microstructures of fine equiaxed grains with a preferential (200) texture can be accomplished under certain ionplating conditions.     

Oxidation of a Silicon Nitride-Titanium Nitride Composite: Microstructural Investigations and Phenomenological Modeling

The high-temperature oxidation of a silicon nitride-titanium nitride (Si₃N₄–TiN) composite has been investigated via scanning electron microscopy and energy-dispersive and wavelength-dispersive spectrometry. At 1150°C, the oxidation of both the silicon nitride and titanium nitride phases takes place. Several oxidation processes act simultaneously and/or successively. First, the oxidation of the titanium nitride occurs and leads to the formation of a continuous titanium oxide (TiO₂) crystal layer at the surface. Next, the TiO₂ formation takes place in the sublayer at the same time as the Si₃N₄ oxidation. The oxidation of this last phase leads to the formation of vitreous silica (SiO₂). For long a duration of oxidation (>50 h), a continuous layer of SiO₂ is formed under the outer TiO₂ scale. Large pores grow in this layer and deform the outer oxide layers, whereas the oxidation occurs in the material. Based on these results and bibliographical data, a phenomenological model is proposed to describe the stages of the high-temperature oxidation of Si₃N₄–TiN materials.     

In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders

Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β-SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si₃N₄ could be completely converted to SiC with molar ratio Si₃N₄:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.     

Synthesis of Nanocrystalline Titanium Nitride Powders by Direct Nitridation of Titanium Oxide

Nanocrystalline TiN powder has been synthesized by the direct nitridation of nanocrystalline TiO₂ powder. Powder XRD patterns indicated that the TiN nanocrystalline powder could be obtained by nitridation at 800°C for 5 h. TEM micrographs showed that the synthesized TiN powders consisted of uniform spherical particles with an average diameter of ∼20 nm. The effect of the nitridation temperature and holding time on the powder properties is discussed.     

Preparation of Titanium Nitride: Microwave-Induced Carbothermal Reaction of Titanium Dioxide

Carbothermal reduction and nitridation of TiO₂ was performed in a 2 kW, 2.45 GHz microwave furnace. Carbon, generally added for the removal of oxygen from TiO₂ lattice also served as the low-temperature susceptor in these experiments. At temperatures >1200°C, the mixtures started reacting vigorously (self-burn), which was never observed with the respective pure compounds. In the self-burning state, the minimum duration required for complete titanium nitride transformation was ∼20 min with stoichiometric amounts of carbon. With excess C, the transformation duration dropped to just 1 min. CO removal and subsequent Ni fixation occur more directly in microwave-induced reactions than in conventional nitridation procedures. Intermediate formation of successive Magneli phases (Ti_{2 n} O_{2 n −1}) was not found in the microwave-induced reactions. The combination of microwave processing and combustion makes this route of economical interest.