Oxidation Studies of Aluminum-Implanted NBD 200 Silicon Nitride

The effects of aluminum-ion-implantation on the oxidation behavior of NBD 200 Si₃N₄ were investigated over an implant concentration range of 0–30 at.%, at 800°–1100°C, in 1 atm dry O₂. Oxidation of both unimplanted and implanted samples follows a parabolic rate law. The parabolic rate constant decreases and the activation energy increases with aluminum concentration. Smooth and crack-free oxides are formed under the combination of high implant concentrations and low oxidation temperatures. Outward diffusion of Mg²⁺ from the bulk of NBD 200 to the oxide layer remains the rate-limiting step for aluminum-implanted samples. The enhancement of the oxidation resistance of NBD 200 by aluminum implantation is attributed to the retardation of the outward diffusion of Mg²⁺.     

Improved Oxidation Resistance of Silicon Nitride by Aluminum Implantation: II, Analysis and Optimization

In the preceding paper, it was shown that aluminum ion implantation significantly improves the oxidation resistance of Si₃N₄ ceramics under the influence of sodium. Not only is the oxidation rate reduced by up to an order of magnitude, the phase and morphological characteristics of the oxides grown on aluminum-implanted samples are improved as well. The role of aluminum in negating the detrimental effect of sodium on the oxidation resistance of Si₃N₄ ceramics can be interpreted on the basis of network modification of the oxide layers by sodium and aluminum cations. The degree of improvement in the oxidation resistance does not, however, necessarily increase with the aluminum concentration. A simple quantitative analysis is presented which correlates the optimum aluminum implant concentration and the sodium content in the gas phase for the optimization of the oxidation resistance of Si₃N₄ ceramics.     

Oxidation Behavior of NBD 200 Silicon Nitride Ceramics

The oxidation behavior of NBD 200 Si₃N₄ containing 1 wt% MgO sintering aid was investigated in oxygen at 900°-1300°C. The oxide growth followed a parabolic rate law with an apparent activation energy of 260 kJ/mol. The oxide layers were enriched with sodium and magnesium because of outward diffusion of intergranular Na⁺ and Mg²⁺ cations in the ceramics. The 2-4 orders of magnitude higher oxidation rate for NBD 200 Si₃N₄ than for other Si₃N₄ ceramics with a similar amount of MgO could be attributed to the presence of sodium. The oxidation process was most likely rate limited by grain-boundary diffusion of Mg²⁺.     

Corrosion Kinetics of Silicon Nitride in Dry Air Containing Sodium Nitrate Vapors

The gaseous alkali corrosion kinetics of Si₃N₄ were systematically investigated from 950° to 1100°C in dry air containing 0.98 vol% sodium nitrate vapors. The linear reaction rates observed at all temperatures indicate that the alkali corrosion of Si₃N₄ is interface-controlled with an activation energy of 199 kJ/mol. The overall reaction involves a complex absorption–dissolution–oxidation process and the rate-controlling step appears to be the interfacial oxidation of Si₃N₄ to SiO₂. A comparison of the reaction kinetics and mechanism between the oxidation of Si₃N₄ in pure oxygen and in the alkali-containing atmosphere is presented.     

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.     

Tribooxidational Effects on Friction and Wear Behavior of Silicon Nitride/Tool Steel and Silicon Nitride/Gray Cast Iron Contacts

Unlubricated pin-on-disk wear tests of Si₃N₄ against tool steel and gray cast iron were performed at 5 N of normal load, 0.5 m/s of sliding speed, and environmental temperature in the range 22°-600°C. The friction coefficient of Si₃N₄ sliding against tool steel and gray cast iron had maximum values of 0.88-0.98 for tests at 100°C. The friction coefficient of Si₃N₄ sliding against gray cast iron couples had minimum values of 0.48-0.57 at 400°C. Because of the increased third-body protection, the wear coefficient of the Si₃N₄ pins of the Si₃N₄/gray cast iron couples decreased by 1 order of magnitude from 1.6 10^-5 mm³/(Nm) at room temperature to 1.3 10^-6 mm³/(Nm) at 600°C. Fe₂O₃ and Fe₃O₄ resulting from tribooxidation of the metallic disks were the main constituents of the wear debris and adherent tribolayers. Activation energy values (6.3-13.7 kJ/mol) were comparable to those of oxidation wear of steel (7.3-11.8 kJ/mol) but were much lower than the activation energy for oxidation of iron alloys in static conditions. Calculations of the activation energy of the oxidation wear corroborate the morphological observations of a sacrificial action of the metallic surface protecting the ceramic material.     

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.     

Bond Energetics at Intergranular Interfaces in Alumina-Doped Silicon Nitride

In Si₃N₄ ceramics sintered with Al₂O₃, the interfacial strength between the intergranular glass and the reinforcing grains has been observed to increase with increases in the aluminum and oxygen content of the epitaxial β-Si_{6- z} Al _z O _z N_{8– z} layer that forms on the Si₃N₄ grains. This has been attributed to the formation of a network of strong bonds (cross bonds) that span the glass-crystalline interface. This proposed mechanism is considered further in light of first-principles atomic cluster calculations of the relative stabilities of bridge and threefold-bonded atomic fragments chosen to represent compositional changes at the glass/Si₃N₄ grain interface. Calculated binding energies indicate Al-N binding is favorable at the Si₃N₄ grain surface, where aluminum occupancy can promote the growth of SiAlON, further enhancing the cross-bonding mechanism of interfacial strengthening.     

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 of Silicon Nitride in Wet Air and Effect of Lutetium Disilicate Coating

Oxidation behavior of silicon nitride (Si₃N₄) was investigated in flowing air (2.45 cm/s) containing 10%–50% H₂O at a total pressure of 1.8–10 atm at 1300°–1500°C for 100 h. The oxidation of Si₃N₄ progressed with volatilization of the SiO₂ scale; it was more enhanced at a high partial pressure of H₂O rather than at high temperature. The total pressure had little effect on the oxidation. In order to avoid the oxidation, Si₃N₄ substrate was coated with lutetium disilicate (Lu₂Si₂O₇) layer through the intermediate SiO₂-rich phase. While the coating layer well suppressed the oxidation in case of small amount of water vapor, it was not sufficiently effective to suppress the oxidation when the water vapor was rich. SiO₂ volatilization was observed between the layer and substrate. The flexural strength of the coated Si₃N₄ at room temperature was somewhat increased after the oxidation in wet air, while that of the uncoated one was almost unchanged. This increase was attributable to crack healing of the substrate during the oxidation.     

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.     

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.     

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.     

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.     

Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

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.     

Oxidation Behavior of Silicon Yttrium Oxynitride

The oxidation behavior of the silicon yttrium oxynitride Y₁₀Si₇O₂₃N₄, so-called H-phase, in the temperature range 700–1400°C has been investigated. A nitrogen retention phenomenon in the oxidation product Y_4.67(SiO₄)₃O (O-apatite) is discussed. The H-phase is one of the four quaternary compounds identified in hot-pressed Si₃N₄ materials fabricated within the Si₃N₄–SiO₂–Y₂O₃ pseudoternary system.     

Kinetics of Thermal Oxidation of Silicon Nitride Powders

The kinetics of thermal oxidation of H. C. Starck M-11 and Ube SN-E10 Si₃N₄ powders was evaluated in the temperature range 650°-1200°C using isothermal and nonisothermal thermogravimetric analysis, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Between 700° and 1200°C, the isothermal kinetics was modeled equally well using the Ginstling-Brounshtein and Zuravlev-Lesokhin-Tempeľman equations. Despite their similar particle sizes and surface areas, the two powders exhibited different oxidation kinetics. The activation energies for oxidation of the M-11 and SN-E10 powders were determined to be 400 and 540 kJ/mol, respectively, between 1000° and 1200°C, and 230 and 260 kJ/mol, respectively, between 700° and 1000°C. The parabolic rate constants for oxidation of the two powders were comparable to those reported in the literature for monolithic, chemically-vapor-deposited Si₃N₄ at the higher temperatures. At lower temperatures, the oxidation kinetics of the M-11 powder was nearly linear, whereas the kinetics of the SN-E10 powder remained power-law dependent.     

Strength and Creep Behavior of Rare-Earth Disilicate–Silicon Nitride Ceramics

The flexural strength and creep behavior of RE₂Si₂O₇–Si₃N₄ materials were examined. The retention in room-temperature strengths displayed by these ceramics at 1300°C was 80–91%, with no evidence of inelastic deformation preceding failure. The steady-state creep rates, at 1400°C in flexural mode, displayed by the most refractory materials are among the lowest reported for sintered Si₃N₄. The creep behavior was found to be strongly dependent on residual amorphous phase viscosity as well as on the oxidation behavior of these materials. All of the rare-earth oxide sintered materials, with the exception of Sm₂Si₂O₇–Si₃N₄, had lower creep strains than the Y₂Si₂O₇–Si₃N₄ material.     

Oxidation of CVD Si3N4 at 1550° to 1650°C

A thermo gravimetric study of the oxidation behavior of chemically vapor-deposited amorphous and crystalline Si₃N₄ (CVD Si₃N₄) was made in dry oxygen (0.1 MPa) at 1550° to 1650°C. The specimens were prepared under various deposition conditions using a mixture of SiCl₄, NH₃, and H₂ gases. The crystalline CVD Si₃N₄ indicated a parabolic oxidation kinetics over the whole temperature range, whereas the amorphous CVD Si₃N₄ changed from a parabolic to a linear law with increased temperature. The oxidation mechanism is discussed in terms of the activation energy for the oxidation and the microstructure of the formed oxide films.