Phase Relationships in the Si3N4–SiO2–Lu2O3 System

Phase relationships in the Si₃N₄–SiO₂–Lu₂O₃ system were investigated at 1850°C in 1 MPa N₂. Only J-phase, Lu₄Si₂O₇N₂ (monoclinic, space group P 2₁/ c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si₃N₄–SiO₂–Lu₂O₃ system. The Si₃N₄/Lu₂O₃ ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si₃N₄, and a new phase of Lu₃Si₅ON₉, having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si₃N₄–LuN–Lu₂O₃ system. The phase diagram suggests that Lu₄Si₂O₇N₂ is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications.     

Solubility Limits of α'-SiAION Solid Solutions in the System Si,Al,Y/N,O

The solubility limit of α'-SiAION solid solutions on the Si₃N₄─YN:3AIN composition join in the system Si₃N₄─YN─AIN has been determined at 1800°C. The end members of these solid solutions are Y_0.43Si_10.7Al_1.3N₁₆ and Y_0.8Si_9.6Al_2.4N₁₆. Unit-cell dimensions of the α'-SiAION solid solutions in the system Si,Al,Y/N,O can be expressed as follows: a _o(Å) = 7.752 + 0.045 m + 0.009 n , c _o(Å) = 5.620 + 0.048 m + 0.009 n , where the α'-SiAION solid solution has the formula Y _x Si_{12-( m+n )}Al _m+n N_{16- n} O _n . The single-phase boundary of the solid solution α'-SiAION on the composition triangle Si₃N₄─YN:3AIN─AIN:Al₂O₃ is delineated. The present paper also reports the phase relationships involving α'-SiAION.     

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.     

Sintering Silicon Nitride Ceramics in Air

Silicon nitride (Si₃N₄) ceramics, prepared with Y₂O₃ and Al₂O₃ sintering additives, have been densified in air at temperatures of up to 1750°C using a conventional MoSi₂ element furnace. At the highest sintering temperatures, densities in excess of 98% of theoretical have been achieved for materials prepared with a combined sintering addition of 12 wt% Y₂O₃ and 3 wt% Al₂O₃. Densification is accompanied by a small weight gain (typically <1–2 wt%), because of limited passive oxidation of the sample. Complete α- to β-Si₃N₄ transformation can be achieved at temperatures above 1650°C, although a low volume fraction of Si₂N₂O is also observed to form below 1750°C. Partial crystallization of the residual grain-boundary glassy phase was also apparent, with β-Y₂Si₂O₇ being noted in the majority of samples. The microstructures of the sintered materials exhibited typical β-Si₃N₄ elongated grain morphologies, indicating potential for low-cost processing of in situ toughened Si₃N₄-based ceramics.     

Silicon Nitride Joining

Hot-pressed Si₃N₄ was joined using an Mgo-A1₂O₃-SiO₂ glass composition chosen to approximate the oxide portion of the grain-boundary phase in the ceramic. After it has been heated at 1550° to 1650°, the interface of the joined ceramic is an interlocking mixture of Si₂N₂O, β-Si₃N₄, and a residual oxy-nitride glass. The kinetics of reactions between Si₃N₄ and the molten joining composition were studied by X-ray diffraction analysis of the phases present in Si₃N4 powder-glass mixtures quenched after varied heat treatments. Analytical transmission electron microscopy of the composition and micro-structure of the reaction zone in joined specimens, together with the X-ray diffraction results, suggests that the driving force for joining is the lowering of the Si₃N₄ interfacial energy when it is wet by the molten silicate, augmented by the negative Gibbs energy for the reaction SiO₂( l ) + Si₃N₄= 2Si₂N₂O.     

Molten-Salt Corrosion of Silicon Nitride: I, Sodium Carbonate

Corrosion of Si₃N₄ under thin films of Na₂CO₃ was investigated at 1000°C. Pure Si₃N₄ and Si₃N₄ with various additives were examined. Thermogravimetric analysis and morphology observations lead to the following detailed reaction mechanism: (I) decomposition of Na₂CO₃ and formation of Na₂SiO₃, (II) rapid oxidation, and (III) formation of a protective silica layer below the silicate and a slowing of the reaction. For Si₃N₄ with Y₂O₃ additions, preferential attack of the grain-boundary phase occurred. The corrosion of pure Si and SiC was also studied for comparison to Si₃N₄. The corrosion mechanism generally applies to all three materials. Silicon reacted substantially faster than Si₃N₄ and SiC.     

Qualitative and Quantitative Evaluation of Silicon Nitride Grain-Boundary Phase by Cryogenic Specific Heat Measurements

Three types of Si₃N₄ have been hot isostatically pressed at 1700°C under 60 MPa for 1 h in a capsule. The quantity and quality (i.e., crystalline or glassy structure) of the grain-boundary phase of the three types of Si₃N₄ can be evaluated by measuring cryogenic specific heats. The specific heats of Si₃N₄ ceramics with Al₂O₃ and Y₂o₃, additives have been measured at different temperatures between 10 and 40 K. The temperature dependency of measured cryogenic specific heat provides quantitative and qualitative information of the grain-boundary phases. This method is very useful for evaluating small changes in the amount and crystalline structure of grain-boundary phases and can clarify different heat histories of the sintering processes in Si₃N₄ ceramics. This proposed method is nondestructive, and the sensitivity can be extremely high. This method eventually leads to a new quality control method of ceramics.     

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.     

Pressureless Sintering of Si3N4 Ceramic Using AlN and Rare-Earth Oxides

Using intermediate, liquid-forming compositions in the (Y,La)₂O₃-AlN system as additives, fully dense Si₃N₄ ceramics with high strength at high temperature have been obtained by pressureless sintering. The ceramics contain rod-shaped β-Si₃N₄ with M' or K' solid solutions as grain-boundary phases. The strength of these ceramics is 1150 MPa at 1200°C, and the room-temperature toughness is maintained at }7 MPa·m^1/2. Phase relations that are pertinent to the new additive compositions are delineated to rationalize their beneficial effects on sinterability and mechanical properties.     

Microstructural Changes in β-Silicon Nitride Grains upon Crystallizing the Grain-Boundary Glass

Crystallizing the grain-boundary glass of a liquid-phase-sintered Si₃N₄ ceramic for 2 h or less at 1500° led to formation of δ-Y₂Si₂O₇. After 5 h at 1500°, the δ-Y₂Si₂O₇ had transformed to β-Y₂Si₂O₇ with a concurrent dramatic increase in dislocation density within β-Si₃N₄ grains. Reasons for the increased dislocation density are discussed. Annealing for 20 h at 1500° reduced dislocation densities to the levels found in as-sintered material.     

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.     

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.     

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.     

Grain-Boundary-Phase Crystallization and Strength of Silicon Nitride Sintered with a YSlAlON Glass

Densifying silicon nitride with a YSiAlON glass additive produced 99% dense materials by pressureless sintering. Subsequent heat-treating led to nearly complete crystallization of the amorphous intergranular phase. Transmission electron microscopy revealed that for heat treatments at 1350°C, only β-Y₂Si₂O₇ was crystallized at the grain boundaries. At a higher temperature of 1450°C, primarily YSiO₂N and Y₄Si₂O₇N₂ in addition to small amounts of Y₂SiO₅ were present. Al existed only in high concentrations in residual amorphous phases, and in solid solution with Si₃N₄ and some crystalline grain-boundary phases. In four-point flexure tests materials retained up to 73% of their strengths, with strengths of up to 426 MPa, at 1300°C. High-strength retention was due to nearly complete crystallization of the intergranular phase, as well as to the high refractoriness of residual amorphous phases.     

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.     

The Role of Cerium Orthosilicate in the Densification of Si3N4

The existence of compounds between Si₃N₄-CeO₂ and Si₃N₄-Ce₂O₃ was investigated for firing temperatures of 1600° to 1700°C. The two new monoclinic compounds found were Ce₂O₃·2Si₃N₄ with lattice parameters a = 16.288, b = 4.848, and c =7.853 Å and β=91.54° and Ce₄Si₂O₇N₂ with lattice parameters a = 10.360, b = 10.865, and c =3.974 Å and β=90.33°. Cerium orthosilicate (Ce 4.67 (SiO₄)₃O) is present during firing as a glassy intermediate phase which promotes sintering and densification and then reacts with silicon nitride to form cerium silicon oxynitrde (CeSiO₂N).     

Preparation of β-Silicon Nitride Seeds for Self-Reinforced Silicon Nitride Ceramics

This paper describes a method for the preparation of silicon nitride (Si₃N₄) seeds that have an average aspect ratio of ∼4. The seeds are prepared via heat treatment of a powder mixture that contains alpha-phase-rich Si₃N₄ and 0.5 wt% Y₂O₃ at a temperature of 1800°C and a nitrogen pressure of 35 kPa. A Y-Si-O-N liquid forms during heat treatment; this liquid acts as a flux for seed precipitation. During cooling, the Y-Si-O-N liquid transforms to a thin intergranular grain-boundary phase and causes strong agglomeration of the seeds. The seeds can be isolated by dissolving the grain-boundary phase in hot phosphoric acid, followed by an ultrasonic treatment (for 30 min). The method can be used to produce large quantities of seeds.     

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.     

Fabrication of Reaction-Sintered Sialon

A factorial design experiment was conducted to determine the major parameters affecting the densification of mixtures having the composition (in mol%) 50 Si₃N₄, 25 A1₂O₃, and 25 A1N. Improved densities of the resulting sialon were achieved using a longer milling time and SN-502 Si₃N₄ powder. However, sialon samples prepared from the SN-502 powder contained some of the X-phase, whereas those fabricated from AME Si₃N₄ powder did not. This difference is considered to be due to the higher SiO₂ content of the SN-502 powder and its greater susceptibility to partial hydrolysis during the milling process. Reaction-sintered sialon (97.2% theoretical density) fabricated using AME powder had a room-temperature bend strength of 352
MN/m² (51,000 psi) and exhibited slow crack growth at 1300° and 1400°C. It is suggested that this is not an inherent property of sialon but is due to the presence of a calcium-containing grain-boundary phase, as in hot-pressed Si₃N₄.