Oxidation of Si3N4 Alloys: Relation to Phase Equilibria in the System Si3N4-SiO2-MgO

The oxidation resistance at 1400°C of silicon nitride alloys hot-pressed with magnesia was investigated as a function of the phase content of the alloys. The extent of oxidation, the products of oxidation, their stratification, and the continuity of the oxide scale are all dependent on the initial composition. Also, the compositions of the alloys themselves change during oxidation. The findings can be interpreted on the basis of compositional paths in the equivalence phase diagram. Reasons for the outward diffusion of Mg and impurities are given and the effect of the secondary phase on oxidation kinetics is discussed.     

Phase Equilibria in the System Si3N4-SiO2-BeO-Be3N2

The 1780°C isothermal section of the reciprocal quasiternary system Si₃N₄-SiO₂-BeO-Be₃N₂ was investigated by the X-ray analysis of hot-pressed samples. The equilibrium relations shown involve previously known compounds and 8 newly found compounds: Be₆Si³N₈, Be₁₁Si₅N₁₄, Be₅Si²N₆, Be₉Si₃N₁₀, Be₈SiO₄N₄, Be₆O₃N₂, Be₈O₅N₂, and Be₉O₆N₂. Large solid solubility occurs in β-Si₃N₄, BeSiN₂, Be₉Si₃N₁₀, Be₄SiN₄, and β-Be₃N₂. Solid solubility in β-Si₃N₄ extends toward Be₂SiO₄ and decreases with increasing temperature from 19 mol% at 1770°C to 11.5 mol% Be₂SiO₄ at 1880°C. A 4-phase isotherm, liquid +β-Si₃N₄ ( ss )Si₂ON₂+ BeO, exists at 1770°C.     

Phase Relations and Stability Studies in the Si3N4-SiO2-Y2O3 Pseudoternary System

Composite powders were hot-pressed to determine the phase relations in the Si₃N₄-SiO₂-Y₂O₃ pseudoternary system. Four quaternary compounds, Si₃Y₂O₃N₄, YSiO₂N, Y₁₀Si₇O₂₃N₄, and Y₄Si₂O₇N₂, were identified. Studies of polyphase and single-phase materials in this system showed that these 4 compounds are unstable under oxidizing conditions. Materials within the Si₃N₄-Si₂N₂O-Y₂Si₂O₇ compatibility triangle precluded the unstable compounds, and are extremely resistant to oxidation.     

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.     

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.     

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.     

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.     

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.     

Effects of Oxidation and Oxidation Under Load on Strength Distributions of Si3N4

The room-temperature strength distributions of a sintered and a hot-pressed Si₃N₄ were examined in the as-machined condition, after oxidation at 1370°C and after oxidation under load at 1370°C. The strength-controlling flaw populations were highly transient in nature. Both the duration of oxidation and the magnitude of the applied load were observed to effect changes in strength. This dynamic situation is related to both strengthening and weakening processes, which at times may occur simultaneously in the same strength distribution.     

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.     

Phase Relations in the Si3N4-Rich Portion of the Si3N4–AlN–Al2O3–Y2O3

The phase relations in the Si₃N₄-rich portion of the Si₃N₄–AlN–Y₂O₃ rystem were investigated using hot-pressed bodies. The one-phase fields of β3 and α, the twophase fields of β+α, β+M (M=melilite), and α+M, and the three-phase fields of β+α+M were observed in the Si₃N₄-rich portion. The α- and β-sialons are not two different compounds but an allotropic transformation phase of the Si–Al–O–N system, and an α solid solution expands and stabilizes with increasing Y₂O₃ content. Therefore, the formulas of the two sialons should be the same.     

Phase Relations in the ZrO2-MgO System

Phase relations were studied in the system ZrO₂-MgO with emphasis on the range 1350° to 1600°C. A phase relation was determined from samples, using precision lattice parameters, X-ray diffraction line intensities, and petrographic observations, and from dynamic observations of the phases present using high-temperature X-ray diffraction techniques. Limits were established for the solubility of MgO in tetragonal ZrO₂ and for the range of the cubic solid solution. The phase relations below 1240°C were complicated by hysteresis in the monoclinic to tetragonal inversion of ZrO₂.     

A New Si3N4 Material: Phase Relations in the System Si-Sc-O-N and Preliminary Property Studies

A new polyphase silicon nitride alloy has been developed using Sc₂O₃ as a densification aid. Subsolidus phase relations in the system Si-Sc-O-N are reported together with preliminary oxidation and compressive creep results of a representative composition in the Si₃N₄-Si₂N₂O-Sc₂Si₂O₇ phase field. Microstructural observations of the material are also presented.     

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.     

Effect of Microstructure on Strength of Si3N4-SiC Composite System

The Si₃N₄-SiC composite system was investigated to better understand the effect of microstructure on the strength-controlling factors, i.e. fracture energy, elastic modulus, and crack size. Silicon carbide dispersions with average particle sizes of 5, 9, and 32 μm were used to form 3 composite series within this system, each containing 0.10, 0.20, 0.30, and 0.40 vol fraction of the dispersed phase. These composites were fabricated by hot-pressing. Fracture energy and strength values were measured for each composite. A linear relation between the elastic modulus of the two phases was assumed. The crack size was calculated for each composite using the appropriate property values. The strength behavior of the 9- and 32-μm series was controlled by the crack size, which, in turn, was controlled by the particle size and volume fraction of the SiC phase. Particle size and volume fraction did not affect the crack size of the 5-μm series, in which strength was controlled by both fracture energy and elastic modulus. Strengths measured at 1400°C and thermal conductivity measurements indicate that several of these composites are promising as high-temperature structural materials.     

Phase Relations and Ordering in the System ZrO2-MgO

The Phase relationships in the system ZrO₂-MgO were reinvestigated over a wide range of temperatures and compositions. The extent of the cubic solid solution field was determined with precise lattice parameter measurements and a high-temperature X-ray furnace using analyzed samples. DTA results show that the addition of MgO to ZrO₂ decreases the transition temperature for monoclinic ⇌ tetragonal ZrO₂ and 1 mol% of MgO is soluble in the monoclinic zirconia at ∼1070°C.The invariant eutectiod point is at 13.5 ± 0.3 mol% MgO at 1406°± 7°C, which is in fair agreement with previous results by Grain. The ordered phase Mg₂Zr₅O₁₂ (δ-phase) can form metastably in cubic solid solutions at temperatures as low as 800°C after prolonged annealing. Evidence for the existence of the ordered phase MgZr₆O₁₃(γ-phase) was obtained by electron diffraction technique. Conditions for the formation of this phase are described. The ordered phases in this system are metastable and their formation is an intermediate step in the eutectoid decomposition of the cubic phase.