Low-Pressure Chemical Vapor Deposition of α-Si3N4 from SiF4 and NH3: Nucleation and Growth Characteristics

The crystal structure and surface morphology of Si₃N₄ prepared by LPCVD were characterized as a function of processing conditions. Temperature was the most dominant variable which affected the coating microstructure. Strongly faceted crystalline Si₃N₄ was deposited at temperatures above ∼ 1410°C. In the temperature range of 1300° to 1410°C, crystalline and amorphous phases were codeposited. The content of the crystalline phase rapidly decreased with decreased temperature. In this temperature range, the coating crystallinity was also influenced by kinetic factors such as deposition rate and reagent depletion. For example, Si₃N₄ became more crystalline as the deposition rate was decreased by either decreasing the flow rate or increasing the NH₃/SiF₄ molar ratio. At ∼ 1300°C, the coating surface appeared fully botryoidal, and the coatings were mostly amorphous. Changes in the orientation and size of Si₃N₄ crystallites were parametrically documented. As the temperature was increased, the Si₃N₄ grains generally became more preferentially oriented to the (102) and/or ( l 0 l ) where l = 1,2,3,., directions. The average facet size increased with coating thickness.     

Compressive Creep and Oxidation Resistance of an Si3N4 Material Fabricated in the System Si3N4-Si2N2O-Y2Si2O7

The compressive creep behavior and oxidation resistance of an Si₃N₄/Y₂Si₂O₇ material (0.85Si₃N₄+0.10SiO₂+0.05Y₂O₃) were determined at 1400°C. Creep re sistance was superior to that of other Si₃N₄ materials and was significantly in creased by a preoxidation treatment (1600°C /120 h). An apparent parabolic rate constant of 4.2 × 10⁻¹¹ kg²·m-⁴·s⁻¹ indicates excellent oxidation resistance.     

Role of Si2N2O in the Passive Oxidation of Chemically-Vapor-Deposited Si3N4

The results of two-step oxidation experiments on chemically-vapor-deposited Si₃N₄ and SiC at 1350°C show that a correlation exists between the presence of a Si₂N₂O interphase and the strong oxidation resistance of Si₃N₄. During normal oxidation, k _p for SiC was 15 times higher than that for Si₃N₄, and the oxide scale on Si₃N₄ was found by SEM and TEM to contain a prominent Si₂N₂O inner layer. However, when oxidized samples are annealed in Ar for 1.5 h at 1500°C and reoxidized at 1350°C as before, three things happen: the oxidation k _p increases over 55-fold for Si₃N₄, and 3.5-fold for SiC; the Si₃N₄ and SiC oxidize with nearly equal k _p's; and, most significant, the oxide scale on Si₃N₄ is found to be lacking an inner Si₂N₂O layer. The implications of this correlation for the competing models of Si₃N₄ oxidation are discussed.     

Hydrothermal Treatment of Si3N4 for the Improvement of Oxidation Resistance at 1400°C

The surface of Si₃N₄ ceramics was hydrothermally treated with HCl or H₂SO₄ using an autoclave. The thickness of the oxide layers formed on the Si₃N₄ samples decreased to one-fourth after oxidation at 1400°C by the treatment. The oxide layer of the treated samples was dense, and flaw formation in and beneath the layer did not occur at 1400°C. The avoidance of low melting Y-silicates by leaching Y₂O₃ is the reason for the improved oxidation resistance of the hydrothermally treated Si₃N₄, despite an increase in surface porosity through a 70 μm layer.     

Synthesis, Characterization, and Consolidation of Si3N4 Obtained from Ammonolysis of SiCl4

Thermal decomposition of silicon diimide, Si(NH)₂, in vacuum resulted in very-high-purity, fine-particle-size, amorphous Si₃N₄ powders. The amorphous powder was isothermally aged at 50° to 100° intervals from 1000° to 1500°C for phase identification. Examination of ir spectra and X-ray diffraction patterns indicated a slow and gradual transition from an amorphous material to a crystalline α-phase occurring at 1200°C for >4 h and/or 1300° to 1400°C for 2 h. As the temperature was increased to ≥1450°C for 2 h, the crystalline β-phase was observed. Phase nucleation and crystallite morphology in this system were studied by electron microscopy and electron diffraction combined with TG as functions of temperature for the inorganic polymer starting materials. Powders prepared in this manner with 4 wt% Mg₃N₂ added as a sintering aid were hot-pressed to high-density fine-grained bodies with uniform microstructures. The optimum hot-pressing condition was 1650°C for 1 h. Silicon concentration steadily increased as the hot-pressing temperature or time was increased. A method for chemical etching for high-density fine-grained Si₃N₄ is described. Electrical measurements between room temperature and ∼500°C indicated dielectric constant and tan δ values of 8.3±0.03 and 0.65±0.05×10⁻², respectively.     

Internal Phase Changes in Dense Si3N4 Associated with High-Temperature Oxidation

The effects of oxidation at 1400°C for 100 h on both surface and internal composition of commercial and laboratory hot-pressed Si₃N₄ with MgO or ZrO₂ additives as well as chemically vapor deposited (CVD) Si₃N₄ were studied using X-ray diffraction. Samples were also compared to the same temperature treatments in Ar. The results indicate the grain boundaries act as rapid diffusion paths for the transport of oxygen.     

Silicon Nitride/Molybdenum Disilicide Composite with Superior Long-Term Oxidation Resistance at 1500°C

The long-term high-temperature cyclic oxidation (100 cycles, 10⁴ h, 1500°C) of a Si₃N₄ material and a Si₃N₄/MoSi₂ composite, both fabricated with Y₂O₃ as a sintering additive, was studied. Both materials exhibited similar oxidation rates because of surface SiO₂ formation described by an almost parabolic law and a total weight gain of 3–4 mg/cm² after 10⁴ h. As a consequence of oxidation processes in the bulk, microstructural damage was found in the Si₃N₄ material. These effects were not observed in the composite. The remarkable microstructural stability observed offers the high potential of Si₃N₄/MoSi₂ composites for long-term structural applications at elevated temperatures up to 1500°C.     

Oxidation Behavior of Hot-Pressed Si3N4

The high-temperature chemical stability of hot-pressed Si₃N₄ was studied between 600° and 1450°C. Reactions were followed by X-ray diffraction and scanning electron microscopy. In air, this material begins to oxidize at 700° to 750°C; a distinct amorphous siO₂ surface layer results after 24 h at 750°C-Concomitant formation of cristobalite occurs, depending on exposure time, and is enhanced as temperature is Increased. Magnesium and calcium magnesium silicates form above 1000°C. The data suggest that impurities, e.g. Mg, Ca, and Fe, greatly lower the oxidation resistance of Si₃N₄ in air.     

High-Temperature Chemistry and Oxidation of ZrB2 Ceramics Containing SiC, Si3N4, Ta5Si3, and TaSi2

The effect of Si₃N₄, Ta₅Si₃, and TaSi₂ additions on the oxidation behavior of ZrB₂ was characterized at 1200°–1500°C and compared with both ZrB₂ and ZrB₂/SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB₂ was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si₃N₄-modified ceramics increased with increasing Si₃N₄ content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB₂, with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta₅Si₃, 30 vol% TaSi₂, 35 vol% Si₃N₄, or 20 vol% Si₃N₄ with 10 mol% CrB₂. These materials exceeded the oxidation resistance of the ZrB₂/SiC ceramics below 1300°–1400°C. However, the ZrB₂/SiC ceramics showed slightly superior oxidation resistance at 1500°C.     

Oxidation and Strength Retention of Monolithic Si3N4 and Nanocomposite Si3N4-SiC with Yb2O3 as a Sintering Aid

The oxidation behaviors of monolithic Si₃N₄ and nanocomposite Si₃N₄-SiC with Yb₂O₃ as a sintering aid were investigated. The specimens were exposed to air at temperatures between 1200° and 1500°C for up to 200 h. Parabolic weight gains with respect to exposure time were observed for both specimens. The oxidation products formed on the surface also were similar, i.e., a mixture of crystalline Yb₂Si₂O₇ and SiO₂ (cristobalite). However, strength retention after oxidation was much higher for the nanocomposite Si₃N₄-SiC compared to the monolithic Si₃N₄. The SiC particles of the nanocomposite at the grain boundary were effective in suppressing the migration of Yb³⁺ ions from the bulk grain-boundary region to the surface during the oxidation process. As a result, depletion of yttribium ions, which led to the formation of a damaged zone beneath the oxide layer, was prevented.     

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.     

Oxidation Behavior of Chemically-Vapor-Deposited Silicon Carbide and Silicon Nitride from 1200° to 1600°C

The isothermal oxidation of pure CVD SiC and Si₃N₄ has been studied for 100 h in dry, flowing oxygen from 1200° to 1600°C in an alumina tube furnace. Adherent oxide formed at temperatures to 1550°C. The major crystalline phase in the resulting silica scales was alpha-cristobalite. Parabolic rate constants for SiC were within an order of magnitude of literature values. The oxidation kinetics of Si₃N₄ in this study were not statistically different from that of SiC. Measured activation energies were 190 kJ/mol for SiC and 186 kJ/mol for Si₃N₄. Silicon oxynitride did not appear to play a role in the oxidation of Si₃N₄ under the conditions herein. This is thought to be derived from the presence of ppm levels of sodium impurities in the alumina furnace tube. It is proposed that sodium modifies the silicon oxynitride, rendering it ineffective as a diffusion barrier. Material recession as a function of oxide thickness was calculated and found to be low. Oxidation behavior at 1600°C differed from the lower temperatures in that silica spallation occurred after exposure.     

The System Si3N4-SiO2-Y2O3

Subsolidus phase relations were established in the system Si₃N₄-SiO₂-Y₂O₃. Four ternary compounds were confirmed, with compositions of Y₄Si₂O₇N₂, Y₂Si₃O₃N₄, YSiO₂N, and Y₁₀(SiO₄)₆N₂. The eutectic in the triangle Si₃N₄-Y₂Si₂O₇-Y₁₀(SiO₄)₆N₂ melts at 1500°C and that in the triangle Si₂N₂O-SiO₂-Y₂Si₂O₇ at 1550°C. The eutectic temperature of the Si₃N₄-Y₂Si₂O₇ join was ∼ 1520°C.     

Pressureless Sintering of Dense Si3N4 and Si3N4/SiC Composites with Nitrate Additives

The effect of aluminum and yttrium nitrate additives on the densification of monolithic Si₃N₄ and a Si₃N₄/SiC composite by pressureless sintering was compared with that of oxide additives. The surfaces of Si₃N₄ particles milled with aluminum and yttrium nitrates, which were added as methanol solutions, were coated with a different layer containing Al and Y from that of Si₃N₄ particles milled with oxide additives. Monolithic Si₃N₄ could be sintered to 94% of theoretical density (TD) at 1500°C with nitrate additives. The sintering temperature was about 100°C lower than the case with oxide additives. After pressureless sintering at 1750°C for 2 h in N₂, the bulk density of a Si₃N₄/20 wt% SiC composite reached 95% TD with nitrate additives.     

Phase Relations in the System Si3N4-SiO2-MgO and Their Interrelation with Strength and Oxidation

Phase relation studies of Si₃N₁, SiO₂, and MgO have established three important subsolidus tie lines, viz. Si₃N₄-MgO, Si₃N₄-Mg₂SiO₄, and Si₂N₂O-Mg₂SiO₄ for nonoxidizing fabrication conditions. Strength measurements at 1400°C show that optimum strengths are obtained for compositions approaching the Si₃N₄-MgO and Si₃N₄-Si₂N₂O tie lines and that inferior strengths are obtained for compositions approaching the Si₃N₄-Mg₂SiO₄ tie line. Oxidation measurements at 1375°C show that the oxidation kinetics depend on the content of MgO and Mg₂SiO₄ phases. Optimum oxidation resistance is observed for compositions approaching the Si₃N₄-Si₂N₂O tie line. Strength and oxidation results are discussed with regard to phase equilibrium considerations.     

Fabrication and Microstructures of MoSi2 Reinforced–Si3N4 Matrix Composites

Details of the fabrication and microstructures of hot-pressed MoSi₂ reinforced–Si₃N₄ matrix composites were investigated as a function of MoSi₂ phase size and volume fraction, and amount of MgO densification aid. No reactions were observed between MoSi₂ and Si₃N₄ at the fabrication temperature of 1750°C. Composite microstructures varied from particle–matrix to cermet morphologies with increasing MoSi₂ phase content. The MgO densification aid was present only in the Si₃N₄ phase. An amorphous glassy phase was observed at the MoSi₂–Si₃N₄ phase boundaries, the extent of which decreased with decreased MgO level. No general microcracking was observed in the MoSi₂–Si₃N₄ composites, despite the presence of a substantial thermal expansion mismatch between the MoSi₂ and Si₃N₄ phases. The critical MoSi₂ particle diameter for microcracking was calculated to be 3 μm. MoSi₂ particles as large as 20 μm resulted in no composite microcracking; this indicated that significant stress relief occurred in these composites, probably because of plastic deformation of the MoSi₂ phase.     

Strength Retention in Hot-Pressed Si3N4 Ceramics with Lu2O3 Additives after Oxidation Exposure in Air at 1500°C

The effect of oxidation exposure on room-temperature flexural strength was examined in 3.33- and 12.51-wt%-Lu₂O₃-containing hot-pressed Si₃N₄ ceramics exposed to air at 1500°C for up to 1000 h. After oxidation exposure, the room-temperature strength of the ceramics was degraded, and strength retention decreased with time at temperature, dependent on the amount of additive. The retention in room-temperature strength displayed by the two compositions after 1000 h of oxidation exposure was 75%–80%. The degradation in strength was attributed to the formation of new defects at and/or near the interface between the oxide layer and the Si₃N₄ bulk during oxidation exposure.     

Controlled Crystallization in Self-Reinforced Silicon Nitride with Y2O3, SrO, and CaO: Crystallization Behavior

The controlled crystallization of the amorphous grain boundary phase has been examined in a series of self-reinforced Si₃N₄ materials with added Y₂O₃, SrO, and CaO. The effects of time, temperature, atmosphere, glass content, glass chemistry, and matrix Si₃N₄ on the crystallization have been investigated. The stability of the crystallized product, the crystallization kinetics ( T-T-T curve), and crystallization mechanisms have also been examined. Crystallization produced an oxynitroapatite containing Y, Sr, and Ca over a broad range of heat-treatment conditions and glass compositions. The oxynitroapatite was compatible with Si₃N₄ and remained stable up to 1600°C. At low temperatures (<1350°C), the rate-limiting crystallization mechanism was oxygen diffuson in the glass, and at higher temperatures (>1350°C) the rate-limiting crystallization step changed to either the formation of new Si₃N₄ grains or solute diffusion in the glass.     

High-Temperature Failure Mechanisms of Hot-Pressed Si3N4 and Si3N4/Si3N4-Whisker-Reinforced Composites

The high-temperature flexural strength of hot-pressed silicon nitride (Si₃N₄) and Si₃N₄-whisker-reinforced Si₃N₄-matrix composites has been measured at a crosshead speed of 1.27 mm/min and temperatures up to 1400°C in a nitrogen atmosphere. Load–displacement curves for whisker-reinforced composites showed nonelastic fracture behavior at 1400°C. In contrast, such behavior was not observed for monolithic Si₃N₄. Microstructures of both materials have been examined by scanning and transmission electron microscopy. The results indicate that grain-boundary sliding could be responsible for strength degradation in both monolithic Si₃N₄ and its whisker composites. The origin of the nonelastic failure behavior of Si₃N₄-whisker composite at 1400°C was not positively identified but several possibilities are discussed.     

Preparation of Silicon Nitride-Titanium Nitride and Titanium-Titanium Nitride Composites from (CH3)3SiNHTiCl3-Coated Si3N4 and Ti Particles

[(Trimethylsilyl)amino]titanium trichloride, (CH₃)₃-SiNHTiClj, was isolated as a red-orange crystalline solid in 58% yield from the reaction of TiCl₄ with [(CH₃)₃Si]₂NH in 1:1 molar ratio in dichloromethane at —78°C. Pyrolysis of (CH₃)₃SiNHTiCl₃ at 600°C furnished titanium nitride. This precursor is suitable for the preparation of composites and was employed to prepare Si₃N₄-TiN and Ti-TiN powders by adding Si₃N₄ particles or titanium powders to a solution of (CH₃), SiNHTiCl₃ in dichloromethane, drying and pyrolyzing the resulting solid. This precursor also has been used as a binder to prepare Si₃N₄-TiN and Ti-TiN bodies. High-resolution transmission electron microscopic studies of the Si₃N₄-TiN composite showed that titanium nitride is concentrated on the surface of the Si₃N₄ particles.