Microstructure and Chemical Reaction in a Si3N4–TiC Composite

The effects of TiC addition to Si₃N₄ on microstructure and the chemical aspects of Si₃N₄–TiC interphase reaction were investigated in samples hot-pressed at 1800°C in Ar and N₂. Composition of a TiC_1–xN_x solid solution was predicted based on thermodynamic calculation, with titanium carbonitride taken to be an ideal solid solution. The predicted value of x = 0.7 is slightly higher than that derived from the measured lattice parameter and Vegard's law (x = 0.67). Four distinguishable areas were observed in samples hot-pressed in nitrogen atmosphere. They were identified as β-Si₃N₄, mixtures of TiC and titanium carbonitride solid solution, SiC with twins, and iron silicide. As the duration of hot-pressing increased, more titanium carbonitride was formed, while less TiC phase remained. Thermodymanic calculations indicate one source of nitrogen gas came from the decomposition of Si₃N₄. The TiC particles also became more irregular, and reactants were found inside or between TiC as the hot-pressing time was extended. Silicon carbide was not detected in samples which were hot-pressed in argon atmosphere; however, numerous pores were found around TiC.     

Synthesis of Titanium Carbonitride Phases by Reactive Milling of the Elemental Mixed Powders

Titanium carbonitride (TiC _x N_{1− x} ) powders were obtained by high-energy ball milling of elemental titanium and carbon (activated carbon or graphite) in a nitrogen atmosphere. The formation of the carbonitride phase was controlled by adequately choosing the experimental conditions of the milling process. The stoichiometry of TiC _x N_{1− x} powders was modified by adjusting the titanium-to-carbon molar ratio. The composition and cell parameters of the carbonitride phases were determined. Microstructural characterization of these phases showed a nanocrystalline nature.     

Auger Electron Spectroscopy Analysis of Cross-Section Surface of Oxidized Titanium Carbide Single Crystal

A TiC monocrystal was heated by three thermal cycles with isotherm at 1108 K while exposed to Ar/O₂ mixtures respectively with O₂ contents of 239, 1.0, and 324 ppm. The reactivity was detected with a homemade device based on two identical solid electrolyte oxygen sensors connected to a quadrupole mass spectrometer (QMS). The oxidized sample was cleaved by impact bending under high vacuum and the cross section investigated by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) techniques. Both area and profile AES spectra were acquired by using a 70 nm diameter beam. Spectral changes have been analyzed to identify chemical species present at the TiC/TiO₂ interface. A model for high-temperature oxidation of TiC has been proposed. It implies oxygen diffusion through a protective TiO₂ layer and the existence of two inner interfaces: TiC/TiC_{1− x} O _x and TiC_{1− x} O _x /TiO₂.     

Development of Reaction-Bonded Electroconductive Silicon Nitride-Titanium Nitride and Resistive Silicon Nitride-Aluminum Oxide Composites

Reaction-bonded Si₃N₄· TiN and Si₃N₄· Al₂O₃ composites were successfully fabricated by heating mixed powder compacts of Si and TiN or Si and Al₂O₃ in a nitrogen atmosphere. The former showed electrical conductivity, owing to the presence of TiN. An electrical resistivity of 2.6 × 10⁻⁵Ω· m was obtained for the Si₃N₄· TiN composite with 70 vol% TiN. The composite with 20 vol% TiN showed an electrical resistivity of 0.22 Ω· m and a bending strength of 460 MPa. On the other hand, the Si₃N₄· Al₂O₃ composite had insulating properties. The use of an appropriate amount of resin binder resulted in a higher green density and, consequently, a higher bending strength. Moreover, electroconductive Si₃N₄· TiN/resistive Si₃N₄· Al₂O₃ complex ceramics could be fabricated by heating green compacts composed of two different portions, one composed of mixed powders of Si and TiN and the other of Si and Al₂O₃. Attainment of such complex ceramics was attributed to the small dimensional change at the nitriding stage, under 0.3% and the similarity of the thermal expansion coefficients of the two composites.     

Titanium–Silicon–Nitrogen Composites with High Wear Resistance in Water and in Artificial Sea Water

Titanium–silicon–nitrogen (Ti-Si-N) composites were produced from a mixture of Si₃N₄, titanium nitride (TiN), and titanium by spark plasma sintering to develop wear-resistant materials with good environmental and biological compatibilities in water and sea water. The Ti-Si-N composite consisted of Si₃N₄, TiN, and titanium metal-rich particles coated with TiSi _x . Results of sliding tests in water and in artificial sea water showed that the wear resistance of the Ti-Si-N composite was much better than that of the titanium metal.     

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.     

Mechanism and Kinetics for the Formation of Uranium Mononitride by the Reaction of Uranium Dioxide with Carbon and Nitrogen

The mechanism and kinetics of UN formation by reaction of a pellet of mixed UO₂ and C with N₂ were studied for temperatures of 1420° to 1750°C. The reaction followed the first-order rate equation; the activation energy was 83 kcal/mol. Only UN_1−xC _x was produced. The lattice parameter variation of UN_1−xC _x had a minimum and a maximum during reaction; at the maximum, UN_1−xC _x + C + N₂ were in equilibrium. The overall reaction was divided into four stages: (1) formation of UN_1−xC _x from UO₂, (2) decarburization of UN_1−xC _x , (3) formation of UN_1−xC _x with the equilibrium composition, and (4) pure UN formation. The lowest reaction rate was in stage (4).     

Reactions Which Form Both Silicide and Nitride Layers

Reactions of Ti which form both a silicide and a nitride after rapid thermal annealing have been characterized using Auger depth profiling, X-ray diffraction, and transmission electron microscopy. Reactions of Ti with Si annealed in a nitrogen ambient at temperatures above 700°C form a thin TiN layer on TiSi₂ (C54). A similar structure is formed by annealing Ti deposited on thin Si₃N₄ films on Si in Ar at 1000°C. The reaction of nitrogen directly with TiSi₂ begins at temperatures above 950°C, but at temperatures greater than 1050°C, TiSi₂ completely reacts to form TiN. The reaction of Ti with thick Si₃N₄ films was studied in greater detail. At lower temperatures in Ar, mostly Ti₅Si₃ is formed at the interface, with some contaminant oxygen and nitrogen released from the reaction, uniformly dissolved throughout the unreacted Ti. Also, a very thin TiN layer exists at the Ti₅Si₃/Si₃N₄ interface. At higher temperatures, a three-layered structure, TiN/TiSi _x /TiN/(unreacted Si₃N₄) develops, allowing a conducting layer to be formed on an insulator. A mechanism explaining the formation of the three-layered structure is discussed. The important reactions are the fast dissolution of nitrogen into the unreacted Ti and the thermal instability of the Ti silicide/Si₃N₄ interface.     

Fabrication of Al-Added TiN Materials by the Combination of Double Self-Propagating High-Temperature Synthesis and Pulsed Electric-Current Pressure Sintering

Crystalline phases of Al-added TiN, denoted as (Ti_{1− x} Al _x )N _y (0≤ x ≤0.10, 0.8< y <1.0), prepared from a mixture of Ti and Al powders by self-propagating high-temperature synthesis (SHS) in a nitrogen atmosphere, have been investigated. By repeating SHS twice, in the region of 0.0< x ≤0.02 cubic (Ti_{1− x} Al _x )N _y solid solutions, and in the region of 0.02< x ≤0.10 composites consisting of (Ti_{1− x} Al _x )N _y and hexagonal Ti₂AlN were formed. After powder characterization, they were consolidated to dense materials (>97% of theoretical) by pulsed electric-current pressure sintering. With increasing Al addition, the optimum sintering temperatures were lowered, followed by reduction of grain size. Their mechanical properties, that is, three-point bending strength σ_b, Vickers hardness H _v, and fracture toughness K _{I C} were evaluated as a function of Al content.     

Development and Characterization of Y2O3-Stabilized ZrO2 (Y-TZP) Composites with TiB2, TiN, TiC, and TiC0.5N0.5

Up to 50 vol% of TiB₂, TiC_0.5N_0.5, TiN, or TiC was added to Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (Y-TZP) and hot pressed under vacuum. The influence of the type of secondary phase on the microstructure and mechanical properties was studied, as a function of the hot-pressing temperature. The influence of the secondary-phase content on the mechanical properties was studied by varying the TiB₂ content up to 50 vol%. Fully dense Y-TZP-based composites with very high toughness (up to 10 MPa·m^1/2), excellent bending strength (up to 1237 MPa), and increased hardness, with respect to ZrO₂ (Vickers hardness up to 1450 kg/mm²), were obtained.     

Synthesis of Nanosized Titanium-Chromium Oxynitride Powders by Ammonolysis of Coprecipitated TiO2/Cr2O3 Precursors

Interstitial titanium-chromium oxynitrides in the solid solution series Ti_{1− z} Cr _z (O _x N _y ) ( z = 0.2, 0.4, 0.5, 0.6, 0.8) have been obtained by ammonolysis of the TiO₂/Cr₂O₃ precursors resulting from the coprecipitation method. The precursors and the resulting oxynitrides were characterized by auger electron spectroscopy, X-ray diffraction analysis, electron probe microanalysis, transmission electron microscopy, and BET surface area techniques. Compounds in the Ti_{1− z} Cr _z (O _x N _y ) series are prepared as single phases by nitridation at 1073 K for 8 h. The as-synthesized oxynitride powders contain only Ti_{1− z} Cr _z (O _x N _y ) with cubic structure and the particle size is in the nanometer scale.     

Preparation of Titanium Nitride Films from Amide Precursors Synthesized by Electrolysis

A TiN precursor solution was synthesized by galvanostatic electrolysis of Ti metal and isopropylamine at a current density of 50 mA·cm⁻² at room temperature. TiN films were prepared by dip-coating of the precursor solution on a Si wafer, followed by two-stage heat treatment at 400°C and a fixed temperature of 800–1200°C in flowing N₂, N₂/NH₃, or NH₃ gas. The TiN films were characterized by XRD, chemical analysis, XPS, and electrical resistivity measurements. The TiN films were composed of uniform grains 20 to 200 nm in size with thicknesses ranging from 300 to 400 nm at temperatures of 800–1200°C. The effect of the heat treatment atmosphere (N₂ and NH₃) on the impurity content, crystallinity, particle size, and electrical resistivity is discussed.     

Effects of Heat Treatment on the Surface Microstructure and Mechanical Properties of MoSi2-TiC0.7N0.3 Composites

MoSi₂-TiC_0.7N_0.3 composites were prepared by hot-pressing under vacuum, and MoSi₂–TiC_0.7N_0.3 composites were heat-treated in air at various temperatures. SEM analysis showed that the surface microstructure of the MoSi₂-TiC_0.7N_0.3 composite changed because of the oxidation of MoSi₂ and TiC_0.7N_0.3 and that many TiO₂ whiskers appeared on the surfaces of the composites. Compared with the non-heat-treated MoSi₂-TiC_0.7N_0.3, the bending strength of the heat-treated MoSi₂-TiC_0.7N_0.3 was significantly improved.     

Preparation, Structure, and Selected Catalytic Properties of the System LaMn1-xCuxO3-y

Samples of LaMn_1-xCu_xO_3-y in the range 0≤x≤0.8 were prepared from freeze-dried solutions of the nitrates. Samples with x≤0.6 were single-phase perovskites. At higher values of x , the samples contained La₂CuO₄ and CuO as well as the perovskite phase. Samples of LaMn_1−x,Cu_x,O_3−y supported on ceramic monoliths or when mixed with powdered A1₂O₃ exhibit catalytic activity for the oxidation of CO. Greatest activity is shown for 0.4≤x≤0.7. Although the catalysts are severely poisoned by SO₂, 2% H₂O in the gas stream causes only slight deactivation. Activities of other oxide catalysts were also measured and compared. Rate constants per unit surface area at 200° to 400°C follow the order Co₃O₄>Pt>LaMn_1−xCu_xO_3−y (0.4≤x≤0.7)>copper chromite>La_1−xSr_x,MnO₃≤ other substituted LaMnO₃ materials, CuO, or La₂CuO₄. The perovskite catalyst is more stable than Co₃O₄ or copper chromite when heated in 10% H₂+ 90% N₂.     

Fabrication of TiN/Si3N4 Ceramics by Spark Plasma Sintering of Si3N4 Particles Coated with Nanosized TiN Prepared by Controlled Hydrolysis of Ti(O-i-C3H7)4

TiN-coated Si₃N₄ particles were prepared by depositing TiO₂ on the Si₃N₄ surfaces from Ti(O- i -C₃H₇)₄ solution, the TiO₂ being formed by controlled hydrolysis, then subsequently nitrided with NH₃ gas. A homogeneous TiO₂ coating was achieved by heating a Si₃N₄ suspension containing 1.0 vol% H₂O with the precursor at 40°C. Nitridation successfully produced Si₃N₄ particles coated with 10–20 nm TiN particles. Spark plasma sintering of these TiN/Si₃N₄ particles at 1600°C yielded composite ceramics with a relative density of 96% at 25 vol% TiN and an electrical resistivity of 10⁻³Ω·cm in compositions of 17.5 and 25 vol% TiN/Si₃N₄, making these ceramics suitable for electric discharge machining.     

Effect of Nitrate Salts as Sintering Additives during the Ball-Milling Process of Silicon Nitride Powders

A chemical adsorption method in a Si₃N₄ slurry that contained a nitrate solution was studied during ball milling, with particular interest in increasing the oxide layer in the Si₃N₄ powder and improving the distribution homogeneity of the sintering additives. The nitrate salts Al(NO₃)₃·9H₂O and Y(NO₃)₃·6H₂O were selected as sintering additives. The following characterization techniques were used: oxygen–nitrogen analysis, X-ray photoelectron spectroscopy, high-resolution electron microscopy (coupled with energy-dispersive X-ray spectroscopy), and X-ray imaging (using wavelength-dispersive X-ray spectroscopy). The thickness of the amorphous layer and the oxygen content of the Si₃N₄ powder were greater for samples that were milled with nitrate additives, which were heat-treated at 600°C, than those of powders that were milled with oxide additives. The chemical composition of the oxygen-containing layer—that is, the amorphous layer that formed and/or changed on the Si₃N₄ surface—was similar to Si₂N₂O in heat-treated Si₃N₄ powder with nitrate additives, whereas the composition of heat-treated Si₃N₄ powder with oxide additives was similar to SiO₂. Furthermore, a homogeneous distribution of the additives was achieved via the incorporation of aluminum and yttrium into the amorphous layer on the Si₃N₄ surface. The metal ratio (Y:Al) of the adsorbates was somewhat higher than that of the additives.     

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.     

Chemical Interaction between Silicon Nitride and Titanium Carbide Powder

Chemical interaction within the system Si₃N₄-TiC was investigated in the present study by using thermodynamic calculations and kinetic analyses. The thermodynamic stabilities of such Si₃N₄-TiC composites as Si₃N₄-TiN-C and Si₃N₄-Ti(C,N)-C, and SiC-Ti(C,N) stability regions were defined and related to temperature and nitrogen partial pressures. Kinetic analyses were performed by constructing a relative weight-loss analysis of various Si₃N₄:TiC molar ratios reacted at temperatures from 1300° to 1750°C in an argon atmosphere. The reaction rates increased with the decreases in the Si₃N₄:TiC ratio and with increases in temperature. The reaction products consisted mainly of SiC and Ti(C,N) phases. The overall chemical interaction observed in the present study is attributable to chemical reactions between Si₃N₄ and TiC and to the diffusion of carbon and nitrogen through the reaction layer after a dense reaction product layer had covered the titanium carbide.     

Melilite Formation in a Samarium-Stabilized α-Sialon Ceramic during Postsintering Heat Treatments

The formation of the melilite solid solution phase (M'), Sm₂Si_3−xAl_xO_3+xN_4−x, in an α-sialon sample of overall composition Sm_0.6Si_9.28Al_2.69O_1.36N_14.76, was studied as a function of time in the temperature interval 1375–1525°C. The alpha-sialon ceramic contained only minor amounts of the 21R sialon polytype and some residual grain-boundary glass before heat treatment. In situ studies by high-temperature X-ray diffraction were combined with postsintering heat treatment followed by quenching. The M'-phase was found to be formed by two different mechanisms: either crystallization of the residual grain-boundary liquid or a direct decomposition of the α-sialon phase. The liquid crystallized during the first 10–15 min of heat treatment, yielding a rapid M'-phase formation, and further formation of M'-phase continued at a much slower rate, related to the decomposition of α-sialon.     

Controlled Crystallization of Amorphous Si3N4by Ti and CI Additives

Thin films of amorphous Si₃N₄ were prepared by the rf-sputtering method, and the effects of titanium and chlorine additives on its crystallization were examined. When Ti-doped amorphous Si₃N₄ was heated, TiN precipitated at >1100°C; the TiN precipitates promoted the conversion of amorphous Si₃N₄ to β-Si₃N₄. Chlorine led to preferential conversion of amorphous Si₃N₄ to α-Si₃N₄.