Preparation of High-Density Si3N4 by a Gas-Pressure Sintering Process

Si₃N₄ compacts, containing ≅7 wt% of both BeSiN₂ and SiO₂ as densification aids, can be reproducibly sintered to relative densities >99% by a gas-pressure sintering process. Nearly all densification takes place via liquid-phase sintering of transformed β-Si₃N₄ grains at T =1800° to 2000°C. Compacts with high density are produced by first sintering to the closed-pore stage (≅92% relative density) in 2.1 MPa (20 atm) of N₂ pressure at 2000°C and then increasing the N₂ pressure to 7.1 MPa (70 atm) where rapid densification proceeds at T = 1800° to 2000°C. The experimental density results are interpreted in terms of theoretical arguments concerning the growth (coalescence) of gas-filled pores and gas solubility effects. Complex chemical reactions apparently occur at high temperatures and are probably responsible for incomplete understanding of some of the experimental data.     

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.     

Diffusional Reaction-Bonding of Si3N4 Ceramics Under High Pressure

Diffusional reaction-bonding of Si₃N₄ ceramics was accomplished by maintaining surnples at 3.0 GPa and 1800°C for 1 h. Interfaces of bonded samples were examined by scanning electron microscopy, Vickers microhardness measurements, and electron probe microanalysis. These results indicate that strong bonding between sintered ceramic bodies is achieved by a diffusional reaction at high temperature and pressure .     

Reaction-Bonding Preparation of Si3N4/MoSi2 and Si3N4/WSi2 Composites from Elemental Powders

Si₃N₄/MoSi₂ and Si₃N₄/WSi₂ composites were prepared by reaction-bonding processes using as starting materials powder mixtures of Si-Mo and Si-W, respectively. A presintering step in an At-base atmosphere was used before nitriding for the formation of MoSi₂ and WSi₂; the nitridation in a N₂-base atmosphere was followed after presintering with the total stepwise cycle of 1350°C × 20 h +1400°C × 20 h +1450°C × 2 h. The final phases obtained in the two different composites were Si₃N₄ and MoSi₂ or WSi₂; no free elemental Si and Mo or W were detected by X-ray diffraction.     

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.     

Crystallization of Ultrafine Amorphous Si3N4 During Sintering

Ultrafine amorphous Si₃N₄ powders were synthesized from laser-heated gases and cold-pressed into pellets for sintering experiments. At temperatures >1300°C, the powders crystallized with a concurrent, linearly proportional decrease in surface area. These powders densified on a local scale without additives or pressure.     

Hot Isostatic Pressing of Si3N4 Powder Compacts and Reaction-Bonded Si3N4

Hot isostatically pressed silicon nitride was produced by densifying Si₃N₄ powder compacts and reaction-bonded Si₃N₄ (RBSN) parts with yttria as a sintering additive. The microstructure was analyzed using scanning electron microscopy, X-ray diffraction, and density measurements. The influence of the microstructure on fracture strength, creep, and oxidation behavior was investigated. It is assumed that the higher amount of oxygen in the Si₃N₄ starting powder compared with the RBSN starting material leads to an increased amount of liquid phase during densification. This results in grain growth and in a larger amount of grain boundary phase in the hot isostatically pressed material. Compared with the hot isostatically pressed RBSN samples therefore, strength decreases whereas the creep rate and the weight gain during oxidation increase.     

Combustion Synthesis of Si3N4–SiC Composite Powders

Fine Si₃N₄-SiC composite powders were synthesized in various SiC compositions to 46 vol% by nitriding combustion of silicon and carbon. The powders were composed of α-Si₃N₄, β-Si₃N₄, and β-SiC. The reaction analysis suggested that the SiC formation is assisted by the high reaction heat of Si nitridation. The sintered bodies consisted of uniformly dispersed grains of β-Si₃N₄, β-SiC, and a few Si₂N₂O.     

Properties of Si3N4 Produced by Nitriding Slip-Cast Si Containing Si3N4 Grog Additions

The properties of Si₃N₄ compositions produced by nitriding slip-cast Si bodies containing up to 16% Si₃N₄ grog were determined. The introduction of grog consistently lowered the densities, the room- and high-temperature strengths, and the resistance to oxidation. The open structure of the grog-containing mixes favored low-temperature gas-phase reactions leading to α-Si₃N₄ formation. In higher-density compositions containing predominantly Si, gas-liquid-solid reactions at higher temperatures produced a relatively greater content of the β phase.     

Surface Modification of Si3N4 Powders by Coprecipitation of Sintering Aids

Two commercial Si₃N₄ powders were coated with sintering aids by coprecipitation. Lanthanum and yttrium nitrates were used as sintering aid precursors. Electrokinetic sonic amplitude measurements and X-ray photoemission spectroscopy analysis were used to investigate electrokinetic behavior and surface properties, respectively. Coprecipitation produced different effects on the composition of the coating layer depending on the actual features of the starting Si₃N₄ powders. The electrokinetic behavior of aqueous suspensions with coated powders depended strongly on the additives, their solubility, and the rate of oxidation of the coated layer. The coprecipitation conditions had to be carefully controlled to obtain reproducible composition and morphology of the coating layers. Treatments of the starting powder, pH, and washing volumes were optimized to tailor the coating layer and improve the coprecipitation yield.     

Comparison of Tribological Behavior Between α-Sialon/Si3N4 and Si3N4/Si3N4 Sliding Pairs in Water Lubrication

We report here the study on tribological behavior of α-Sialon in aqueous medium. The results derived from a wide range of test conditions are briefly discussed. A reduction in friction coefficient from 0.7 to 0.03 and a decrease in wear rate by two orders of magnitude were achieved under low load (9.8 N) and high speed (>0.54 m/s) conditions. The tribological behavior of α-Sialon/Si₃N₄ ceramics was then compared with Si₃N₄/Si₃N₄ tribopairs.     

Volatilization Associated with the Sintering of Polyphase Si3N4 Materials

Weight loss which occurs while sintering composited Si₃N₄ powders requires the loss of one or more of the end-member constituents through a volatilization reaction. By plotting the direction of the compositional change on the appropriate equivalence phase diagram, the principal volatilization reaction can be determined. For a particular composition in the system Si-Y-N-O sintered at 1750°C, the principal volatilization reaction was Si₃N₄(s) +3SiO₂(s)→6SiO(g) +2N₂(g).     

Sintering of Si3N4 with the Addition of Rare-Earth Oxides

The effect of rare-earth oxide additives on the densification of silicon nitride by pressureless sintering at 1600° to 1700°C and by gas pressure sintering under 10 MPa of N₂ at 1800° to 2000°C was studied. When a single-component oxide, such as CeO₂, Nd₂O₃, La₂O₃, Sm₂O₃, or Y₂O₃, was used as an additive, the sintering temperature required to reach approximate theoretical density became higher as the melting temperature of the oxide increased. When a mixed oxide additive, such as Y₂O₃–Ln₂O₃ (Ln=Ce, Nd, La, Sm), was used, higher densification was achieved below 2000°C because of a lower liquid formation temperature. The sinterability of silicon nitride ceramics with the addition of rare-earth oxides is discussed in relation to the additive compositions.