Microstructural Evolution of Gas-Pressure-Sintered Si3N4 with Yb2O3 as a Sintering Aid

Microstructural evolution of gas-pressure-sintered Si₃N₄ with Yb₂O₃ as a sintering aid was observed. Microstructures typical for in situ toughened Si₃N₄, i.e., large elongated grains randomly distributed in a fine matrix, were observed. However, the size of the elongated grains near the surface was much larger than that at the center, resulting in two distinct regions: an inner region and an outer region. The smaller the amount of Yb₂O₃ added, the larger the difference in the size of the elongated grains between the outer and inner regions. The difference between microstructures was diminished when 16 wt% Yb₂O₃ was added. The microstructural change with Yb₂O₃ content was attributed to the evaporation of Yb-containing liquid phase from the surface.     

Phase Evolution in Heat-Treated Si3N4 with Additions of Yb2O3

The heat treatment of silicon nitride (Si₃N₄) ceramics with additions of 8, 12, and 16 wt% Yb₂O₃ was carried out at different temperatures and the evolution of grain boundary (GB) phase was investigated systematically by X-ray diffraction (XRD) as well as scanning electron and transmission electron microscopic analyses. XRD results reveal that the extent and the ease of GB crystallization increase with increasing the Yb₂O₃ content, and that high heat-treatment temperatures in general favor crystallization of the quaternary compounds such as the Yb₄Si₂O₇N₂ phase. These results provide an insight into the GB phase evolution in the Yb-system Si₃N₄ ceramics subjected to a postsintering heat treatment.     

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.     

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.     

Strength Retention in Hot-Pressed Si3N4 Ceramics with Lu2O3 Additives after Oxidation Exposure in Air at 1500°C

The effect of oxidation exposure on room-temperature flexural strength was examined in 3.33- and 12.51-wt%-Lu₂O₃-containing hot-pressed Si₃N₄ ceramics exposed to air at 1500°C for up to 1000 h. After oxidation exposure, the room-temperature strength of the ceramics was degraded, and strength retention decreased with time at temperature, dependent on the amount of additive. The retention in room-temperature strength displayed by the two compositions after 1000 h of oxidation exposure was 75%–80%. The degradation in strength was attributed to the formation of new defects at and/or near the interface between the oxide layer and the Si₃N₄ bulk during oxidation exposure.     

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 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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.