Sintering process of Y2O3 and Al2O3-doped Si3N4

The sintering process of Y₂O₃- and Al₂O₃-doped Si₃N₄ has been investigated by dilatometry and microstructural observations. The densification progressed through three processes. The bulk density increased to 85% theoretical without the formation of -Si₃N₄ in the initial process. The densification once terminated after the second process. The / transformation of Si₃N₄ and the related formation of prismatic grains reduced the densification rate in the second process, although the grain size and the aspect ratio were very small. The final process was the densification of -Si₃N₄, where the fibrous grains grew remarkably. The kinetic order for the densification of -Si₃N₄ indicated a diffusion-rate controlling mechanism with the activation energy of 244 kJ mol^–1 (<1450 ° C) and 193 kJ mol^–1 (>1450 ° C). The influence of heating rate on the grain growth was characterized by a parameter derived from kinetic parameters. The relationships between grain growth and densification behaviour have also been discussed.     

Pressureless sintering of Si3N4 with Y2O2 and Al2O3

Pressureless sintering of Si₃N₄ with Y₂O₃ and Al₂O₃ as additives was carried out at 1750°C in N₂ atmosphere. Si₃N₄ materials which had more than 92% relative density were obtained with 20wt% addition of additives. The flexural strength of as-sintered materials containing 5 to 8.6wt% Al₂O₃ and 15 to 11.4wt% Y₂O₃ was in the range of 480 to 560 MPa at room temperature. The glassy grain-boundary phase of as-sintered materials crystallized to 3Y₂O₃ · 5Al₂O₃ (YAG), Y₂O₃ · SiO₂ (YS), Y₂O₃ · 2SiO₂ (Y2S) and 10Y₂O₃ · 9SiO₂ sd Si₃N₄ (NA) by heat-treatment at 1250° C for 3 days. A specimen containing 15wt% Y₂O₃ and 5wt% Al₂O₃ sintered at 1750° C for 4 h was heat-treated at 1250° C for 3 days to precipitate YAG and YS. The nitrogen concentration of the grain-boundary glassy phase of the specimen was found to be very high, and therefore the flexural strength of the crystallized specimen scarcely decreased at elevated temperatures (the flexural strength of this specimen is 390 MPa at room temperature and 360 MPa at 1300° C). Resistance to oxidation at 1200° C of the specimen was good as well as the flexural strength, compared with that of as-sintered materials.     

Effect of additive-oxide amount on sintering of Si3N4 with Y2O3 and Nd2O3

The effect of additive amount on the gas-pressure sintering of silicon nitride is investigated. Silicon nitride containing 0.5 to 10 mol % (SN10) of equimolar Y₂O₃-Nd₂O₃ is fired at 1600 to 1900 °C for 4 h in 10 M Pa N₂ gas. A small amount of oxide (1 mol %; SN1) is effective for densification as well as a larger amount of oxide (6–10 mol %) when fired at 1900 °C. Composition analysis of sintered specimens indicates that SN1 densifies through a small amount of SiO₂-rich liquid-phase, whereas SN10 densifies by way of a large amount of additive-oxide-rich liquid phase.     

Slip-casting properties of Si3N4 with Y2O3 and Al2O3 as sintering additives

The effects of particle charging and powder–liquid suspension stability on the slip-casting properties of Si3N4 powder were examined. Y2O3 and Al2O3, used as sintering additives, were seen to affect the dispersion stability of the base material (Si3N4). The zeta potentials of the three powders and the rheological behaviour of the 55 wt% solids-loaded slips, involving known concentrations of a polymeric deflocculant (Dolapix PC33), showed that the multicomponent system can be dispersed stably within the pH range 9–11. Green compacts, obtained by casting these slips into plaster moulds, were found to give densities in the range 50–61% of the theoretical value. © 1998 Chapman & Hall     

Creep behaviour of Si3N4/Y2O3/Al2O3/AIN alloys

The compression creep behaviour of pressureless sintered Y₂O₃/Al₂O₃/AIN-doped Si₃N₄ was studied between 1473 and 1673 K, under stresses ranging from 100–300 MPa. Strain rate versus stress and temperature analysis give a stress exponent n1 and an activation energy Q=860 kJ mol^–1. Microstructural change was investigated by transmission electron microscopy. The observed strain whorls, the stress exponent and the activation energy are indicative of a solution-diffusion-precipitation accommodated grain-boundary sliding where the diffusion through the glass is rate controlling.     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.     

The high temperature creep deformation of Si3N4-6Y2O3-2Al2O3

The creep properties of silicon nitride containing 6 wt % yttria and 2 wt% alumina have been determined in the temperature range 1573 to 1673 K. The stress exponent, n, in the equation ⁿ was determined to be 2.00±0.15 and the true activation energy was found to be 692±25 kJ mol^–1. Transmission electron microscopy studies showed that deformation occurred in the grain boundary glassy phase accompanied by microcrack formation and cavitation. The steady state creep results are consistent with a diffusion controlled creep mechanism involving nitrogen diffusion through the grain boundary glassy phase.     

Sintering process of Y2O3-added Si3N4

The sintering process of Y₂O₃-added Si₃N₄ has been investigated by dilatometry and microstructural observations. Densification was promoted above 1440 ° C by the formation of eutectic melts in the Y₂O₃-SiO₂-Si₃N₄ triangle. However, the dilatometric curves indicated no shrinkage corresponding to the rearrangement process, despite liquid-phase sintering. The kinetic order for The Initial-stage sintering was 0.47 to 0.49. These values indicated that the phase-boundary reaction was rate controlling. The apparent activation energy (323 kJ mol^–1) was smaller than the dissociation energy for the Si-N bond (435 kJ mol^–1). ESR data and lattice strain indicated that the disordered crystalline structure of the Si₃N₄ starting powder promoted the reaction of Si₃N₄ with eutectic melts. After a period of initial-stage sintering, the formation of fibrous -Si₃N₄ grains resulted in interlocked structures to interrupt the densification.     

Hydrolytic Stability of Y2.5Nd0.5Al5O12-Based Garnet Ceramics under Hydrothermal Conditions

Inorganic Materials - We have studied the hydrolytic stability of Y2.5Nd0.5Al5O12-based garnet ceramics produced by spark plasma sintering. The ceramics have been tested under hydrothermal...     

Hot isostatic pressing of Si3N4 with Y2O3 additions

The effect of Y₂O₃ additive on the properties of hot isostatically pressed silicon nitride was studied. The influence of small additions of Y₂O₃ on the densification of silicon nitride was investigated. The density and elastic moduli of the product increase with increasing of the Y₂O₃ additions. The hot isostatically pressed pure silicon nitride consists of -Si₃N₄, -Si₃N₄and Si₂N₂O; phase content of the hot isostatically pressed silicon nitride with 10 wt % Y₂O₃addition consists of -Si₃N₄, yttrium silicate and Y₂Si₃O₃N₄. The effect of the outgassing of the specimens prior to hot isostatical pressing on the properties of the final material is discussed.     

Microstructure and mechanical properties of a Si3N4/Al2O3 nanocomposite

A nanocomposite material fabricated by hot pressing in the form of nanometre-sized Si3N4 particles dispersed in an Al2O3 matrix has been shown to exhibit enhanced mechanical properties compared with monolithic matrix material. It was observed by transmission electron microscopy (TEM) for the first time that the alumina grains were in the shape of elongated columns with aspect ratios in the range 2.5–4. The presence of liquid phase during sintering was found to be responsible for the appearance of columnar grains. Regular hexagon-shaped larger -Sialon grains formed during sintering were mainly situated at grain boundaries of the matrix material while irregular smaller dispersoids were trapped within the alumina grains. The improvement in the mechanical properties of the nanocomposite is attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the self-reinforcement effect arising from the elongated columnar grains of the matrix, as well as the pinning effect due to the existence of intergranular -sialon particles. It was revealed that the trapped particles have an -Al2O3 structure with partial sites of aluminium and oxygen atoms substituted by silicon and nitrogen atoms, which is also likely to lead to the strengthening of the composite.     

Sintering of Si3N4 in the presence of additives from Y2O3-SiO2-Al2O3 system

The sintering process of Si₃N₄ in the presence of a liquid phase from the Y₂O₃-SiO₂-Al₂O₃ system was investigated. The starting composition of liquid phase was varied according to data in the phase diagram of the Y₂O₃-SiO₂-Al₂O₃ system, in order to lower the temperature of liquid formation because it might exhibit an influence on the sintering behaviour of Si₃N₄. Densification as well as phase analysis were followed as a function of composition and the amount of liquid phase, both in the sintered and in hot pressed samples.