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.     

Effect of Addition of Silicon on the Microstructures and Bending Strength of Continuous Porous SiC–Si3N4 Composites

The microstructures and mechanical properties of continuous porous SiC–Si₃N₄ composites fabricated by multi-pass extrusion were investigated, depending on the amount of Si powder added. Si powder with different weight percentages (0%, 5%, 10%, 15%, 20%) was added to SiC powder to make raw mixture powders, with 6 wt% Y₂O₃–2 wt% Al₂O₃ as sintering additives, carbon (10–15 μm) as a pore-forming agent, ethylene vinyl acetate as a binder, and stearic acid (CH₃(CH₂)₁₆COOH) as a lubricant. In the continuous porous SiC–Si₃N₄ composites, Si₃N₄ whiskers like the hairs of nostrils were frequently observed on the wall of the pores. In this study, the morphology of Si₃N₄ whiskers was investigated with the nitridation condition and silicon addition content. In composites containing an addition of 10 wt% Si, a large number of Si₃N₄ whiskers were found at the continuous pore regions. In the sample to which 15 wt% Si powder was added, a maximum value of about 101 MPa bending strength and 57.5% relative density were obtained.     

Effect of Crystallizing the Grain-Boundary Glass Phase on the High-Temperature Strength of Hot-Pressed Si3N4 Containing Y2O3

The effects of fabrication variables on the high-temperature strength of hot-pressed Si₃N₄ containing 5 wt% Y₂O₃ were studied. Materials containing a crystalline grain-boundary phase, formed as a consequence of a high-temperature presintering heat treatment and identified as Si₃N₄·Y₂O₃, had high-temperature strengths significantly superior to those observed for materials containing a glass phase.     

Solid-Liquid Reaction in the Si3N4-AlN-Y2O3 System under 1 MPa of Nitrogen

The melting behaviors of selected compositions in the Si₃N₄-AlN-Y₂O₃ system were determined under 1 MPa of nitrogen. The phase diagrams of the ternary and their binary systems are presented. The lowest melting composition of the ternary system contains 15 mol % Si₃N₄, 25 mol % AIN, and 60 mol % Y₂O₃ and has a melting temperature of 1650°C. The binary eutectic compositions and temperatures are 15 mol % Si₃N₄ and 85 mol % Y₂O₃ at 1720°C, and 20 mol % AIN and 80 mol% Y₂O₃ at 1730°C.     

Synthesis of Porous Si3N4 Ceramics with Rod-Shaped Pore Structure

Porous Si₃N₄ ceramics were synthesized by pressureless sintering of green compacts prepared using slip casting of slurries containing Si₃N₄, 5 wt% Y₂O₃+2 wt% Al₂O₃, and 0–60% organic whiskers composed of phenol–formaldehyde resin with solids loading up to 60 wt%. Rheological properties of slurries were optimized to achieve a high degree of dispersion with a high solid-volume fraction. Samples were heated at 800°C in air and sintered at 1850°C in a N₂ atmosphere. Porosities ranging from 0% to 45% were obtained by the whisker contents (corresponding to 0–60 vol% whisker). Samples exhibited a uniform pore distribution. Their rod-shaped pore morphology originated from burnout of whiskers, and an extremely dense Si₃N₄ matrix.     

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.     

Subsolidus Phase Relationships in Part of the System Si,Al,Y/N,O: The System Si3N4─AIN─YN─Al2O3─Y2O3

The subsolidus phase relationships in the system Si,Al,Y/N,O were determined. Thirty-nine compatibility tetrahedra were established in the region Si₃N₄─AIN─Al₂O₃─Y₂O₃. The subsolidus phase relationships in the region Si₃N₄─AIN─YN─Y₂O₃ have also been studied. Only one compound, 2YN:Si₃N₄, was confirmed in the binary system Si₃N₄─YN. The solubility limits of the α'─SiAION on the Si₃N₄─YN:3AIN join were determined to range from m = 1.3 to m = 2.4 in the formula Y _{m /3}Si_{12- m} Al _m N₁₆. No quinary compound was found. Seven compatibility tetrahedra were established in the region Si₃N₄─AIN─YN─Y₂O₃.     

Effects of Y2O3 and ZrO2 on the Densification of Si3N4-Zr(Y)O2 Composites

The effects of ZrO₂ and Y₂O₃ on the densification of hotpressed Si₃N₄-Zr(Y)O₂ composites have been studied. High density could not be obtained by the addition of pure or 3-mol%-Y₂O₃-doped ZrO₂ in this composite; however, nearly full density (>97%) could be obtained in Si₃N₄ using 6- and 8-mol%-Y₂O₃-doped ZrO₂. It is concluded that Y₂O₃ diffusing out from the added Zr(Y)O₂ promoted the densification and that ZrO₂ also had some role in the formation of an oxynitride glass.     

Sintering of Si3N4-Y2O3-Al2O3

Sintering kinetics of the system Si₃N4-Y2O₃-Al₂O3 were determined from measurements of the linear shrinkage of pressed disks sintered isothermally at 1500° to 1700°C. Amorphous and crystalline Si₃N₄ were studied with additions of 4 to 17 wt% Y₂O₃ and 4 wt% A1₂O₃. Sintering occurs by a liquid-phase mechanism in which the kinetics exhibit the three stages predicted by Kingery's model. However, the rates during the second stage of the process are higher for all compositions than predicted by the model. X-ray data show the presence of several transient phases which, with sufficient heating, disappear leaving mixtures of β ' -Si₃N₄ and glass or β '-Si₃N₄, α '-Si₃N₄, and glass. The compositions and amounts of the residual glassy phases are estimated.     

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.     

Formation of N-phase and Phase Relationships in MgO–Si2N2O–Al2O3 System

Formation of N-phase in the system Mg,Si,Al/N,O was studied. Its composition was confirmed to be MgAl₂Si₄O₆N₄ (2Si₂N₂OMgAl₂O₄). Subsolidus phase relationships in the MgO–Si₂N₂O-Al₂O₃ system were determined. The results are discussed by comparing with two similar systems, CaO-and Y₂O₃–Si₂N₂O–Al₂O₃.     

Microstructure of Y2O3 Fluxed Hot-Pressed Silicon Nitride

Detailed microstructural analysis of a 10 mol% Y₂O₃ fluxed hot-pressed silicon nitride reveals that, in addition to the yttrium-silicon oxynitride phase located at the multiple Si₃N₄ grain junctions, there is a thin boundary phase 10 to 80 Å wide separating the silicon nitride and the oxynitride grains. Also, X-ray microanalysis from regions as small as 200 Å across demonstrates that the yttrium-silicon oxynitride, Y₂Si(Si₂O₃N₄), phase can accommodate appreciable quantities of Ti, W, Fe, Ni, Co, Ca, Mg, Al, and Zn in solid solution. This finding, together with observations of highly prismatic Si₃N₄ grains enveloped by Y₂Si(Si₂O₃N₄), suggests that densification occurred by a liquid-phase "solution-reprecipitation" process.     

Characteristics of ZrO2-Dispersed Si3N4 without Additives Fabricated by Hot Isostatic Pressing

Dense, ZrO₂-dispersed Si₃N₄ composites without additives were fabricated at 180 MPa and ∼1850° to 1900°C for l h by hot isostatic pressing using a glass-encapsulation method; the densities reached >96% of theoretical. The dispersion of 20 wt% of 2.5YZrO₂ (2.5 mol% Y₂O₃) in Si₃N₄ was advantageous to increase the room-temperature fracture toughness (∼7.5 MPa˙m^1/2) without degradation of hardness (∼15 GPa) because of the high retention of tetragonal ZrO₂. The dependence of fracture toughness of Si₃N₄–2.5YZrO₂ on ZrO₂ content can be related to the formation of zirconium oxynitride because of the reaction between ZrO₂ and Si₃N₄ matrix in hot isostatic pressing.     

Sintering of Si3N4-Y2O3 Under Nitrogen Pressure

This study shows that the amount ofAl₂O₃ needed to form high density Si₃N₄-15Y₂-O₃ samples can be reduced by using high surface area Si₃N₄ powder and high N₂ overpressure (high sintering temperatures) during the sintering process. The reduction in AI₂O₃ content results in improved oxidation resistance of the sintered samples.     

Formation of α-Si3N4 Solid Solutions in the System Si3N4-A1N-Y2O3

The subsolidus phase diagram of the quasiternary system Si₃N₄-AlN-Y₂O₃ was established. In this system α-Si₃N₄ forms a solid solution with 0.1Y₂O₃: 0.9 AIN. The solubility limits are represented by Y_0.33Si_10.5Al_1.5O_0.5N_15.5 and Y_0.67Si₉A1₃ON₁₅. At 1700°C an equilibrium exists between β-Si₃N₄ and this solid solution.     

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.     

Properties of Isostatically Hot-Pressed Silicon Nitride

Hot isostatic pressing was studied for densification of reaction-bonded Si₃N₄ containing various levels of Y₂O₃. Near-theoretical density was achieved for com positions containing 3 to 7 wt% Y₂O₃. An Si₃N₄-5 wt% Y₂O₃ composition had a 4-point flexural strength at 1375°C of 628 MPa and survived 117 h of stress rupture testing at 1400°C and 345 MPa .     

Change in Oxygen Content of Y2O3-Nd2O3-Doped Silicon Nitride During Firing

The oxygen content of silicon nitride with 1 mol% Y₂O₃—Nd₂O₃ additive was measured after firing to determine the compositional change during gas-pressure sintering. Oxygen content decreases from 2.5 to 0.94 wt% during firing for 4 h at 1900°C and 10-MPa pressure in N₂. This decrease in oxygen results from the release of SiO gas generated by a thermaldecomposition reaction between Si₃N₄ and SiO₂. The resultant sintered silicon nitride material contains less than 1 wt% oxygen.     

Hot Isostatic Pressing of Silicon Nitride with Boron Nitride, Boron Carbide, and Carbon Additions

Si₃ N₄ test bars containing additions of BN, B₄C, and C, were hot isostatically pressed in Ta cladding at 1900° and 2050°C to 98.9% to 99.5% theoretical density. Room-temperature strength data on specimens containing 2 wt% BN and 0.5 wt% C were comparable to data obtained for Si₃ N₄ sintered with Y₂O₃, Y₂O₃ and Al₂O₃, or ZrO₂. The 1370°C strengths were less than those obtained for additions of Y₂O₃ or ZrO₂ but greater than those obtained from a combination of Y₂O₃ and Al₂O₃. Scanning electron microscope fractography indicated that, as with other types of Si₃N₄, roomtemperature strength was controlled by processing flaws. The decrease in strength at 1370°C was typical of Si₃N₄ having an amorphous grainboundary phase. The primary advantage of non-oxide additions appears to be in facilitating specimen removal from the Ta cladding.     

Hot-Pressing of Si3N4 with Y2O3 and Li2O as Additives

The rates of densification and phase transformation undergone by α-Si₃N₄ during hot-pressing in the presence of Y₂O₃, Y₂O₃−2SiO₂, and Li₂0−2Si0₂ as additives were studied. Although these systems behave less simply than MgO-doped Si₃N₄, the data can be interpreted during the early stages of hot-pressing as resulting from a solution-diffusion-reprecipitation mechanism, where the diffusion step is rate controlling and where the reprecipitation step invariably results in the formation of the β-Si₃N₄ phase.