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     

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.     

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.     

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.     

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.     

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.     

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.     

Preparation and some properties of chemically vapour-deposited Si3N4-TiN composite

Chemically vapour-deposited (CVD) Si₃N₄-TiN composite (a plate with the maximum thickness of 1.9 mm) has been prepared on a graphite substrate using a mixture of SiCl₄, TiCl₄, NH₃ and H₂ gases. The CVD was carried out at deposition temperatures,T _dep, in the range of 1050 to 1450 ° C, total gas pressures,P _tot, from 1.33 to 10.7 kPa and gas flow rates of 136 (SiCl₄), 18 (TiCl₄), 120 (NH₃) and 2720 (H₂) cm³ min^–1. The deposits thus obtained appeared black. The Ti content in the composites ranged from 2.1 to 24.8 wt % and was found in the form of Tin. The structure of the Si₃N₄ matrices varied from amorphous (initially) to the- and-type, with increasingT _dep. Most of the- and-type deposits had a preferred orientation (001) parallel to the deposition surface. While the deposition surface of the amorphous deposits showed a pebble structure, the surfaces of the- and-type deposits were composed of various kinds of facets. The heat-treating experiment suggested that-Si₃N₄ obtained in the present work was formed directly via a vapour phase, and not from crystallization of amorphous Si₃N₄ or from transformation of-Si₃N₄.     

Preparation and dielectric properties of Si3N4/SiCw composite ceramic

Gelcasting was employed to fabricate Si₃N₄/SiC whisker (SiCw) composite ceramics, and the effects of heat-treatment temperature on the length-to-diameter ratio of the whiskers and SiCw content on microwave dielectric properties were studied. Compared with pure SiCw of spherical structure obtained at temperature of 1,750 °C(Ar), pure SiCw treated at 1,600 °C(Ar) showed rod-like structure, higher dielectric properties and more evenly distribution in Si₃N₄/SiCw composite ceramics. Both the real (ε′) and imaginary (ε″) permittivity of Si₃N₄/SiC whisker (SiCw) composite ceramics decreased with increasing frequency and increased as the whisker content raised owing to the interface and SiCw playing a role of dipole in the frequency range of 8.2–12.4 GHz. In addition, comparing the ceramics with lower content of SiCw, the reflectivity of the composite ceramics moved to a lower frequency; the maximum absorption peak reached ?22.4 dB at the whisker content of 15 wt%.     

Hot-pressed Si3N4 ceramics containing CaO-Al2O3-AlN modifying additives

We have studied the influence of SHS aluminum nitride content (5?C15%) and particle size on the fabrication conditions (hot pressing in a nitrogen atmosphere) of Si₃N₄-based ceramics containing CaO-Al₂O₃-AlN sintering aids. The results demonstrate that the reaction of Si₃N₄ with a eutectic calcium aluminate mixture during liquid-phase sintering leads to the formation of ??-sialon-based intergranular phases: Ca _m/z Si_{12 ? (m ? n)}Al_{(m + n)}O _n N_{16 ? n} . The addition of AlN to starting mixtures contributes to a more complete conversion of the calcium aluminate sintering aid to Ca-??-sialon and influences the relationship between ??-Si₃N₄ and ??-Si₃N₄ in the ceramics. We examine the influence of synthesis conditions and the percentages of added calcium aluminates and aluminum nitride on the density, porosity, and bending strength of the ceramics. Thermogravimetric analysis data demonstrates that the Si₃N₄ + Ca-??-sialon composites obtained are stable to oxidation in air up to 1300°C.     

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.     

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.     

Microstructure and mechanical properties of a directionally solidified Al2O3/Y3Al5O12/ZrO2 hypoeutectic in situ composite

Directionally solidified ternary Al₂O₃/Y₃Al₅O₁₂(YAG)/ZrO₂ hypoeutectic rod composites were successfully fabricated by the laser zone remelting technique. The microstructure and mechanical properties of the composite were investigated. The microstructure presented a complex three-dimensional network structure consisting of fine Al₂O₃ (41 vol.%) and YAG (49 vol.%) phases, with smaller ZrO₂ (10 vol.%) phases partially distributed at the Al₂O₃/YAG interfaces. The irregular growth behavior in the hypoeutectic was revealed. The hardness and fracture toughness at ambient temperature were measured to be 17.3 GPa and 5.2 MPa m^1/2, respectively. The toughness enhancement in comparison with previous binary Al₂O₃/YAG composites was mainly attributed to the refined microstructure, and crack deflection, branching and bridging. Moreover, the residual stresses, generated by different thermal expansion coefficients of the component phases, also importantly contributed to the improved toughness. Correlations between the addition of the third component ZrO₂ and the microstructure and properties were discussed as well.     

Phase transformation and densification during pressureless sintering of Si3N4 with MgO and Y3Al5O12 additives

The - transformation of Si₃N₄ during liquid-phase sintering appears to be controlled by the growth of the -Si₃N₄ grains in the direction perpendicular to thec-axis in the case of MgO additive. The diffusion through the liquid is the rate-controlling step in the case of the Y₄Al₅O₁₂ additive. The density of the sintered body at the solid skeleton stage was influenced by the change in the - transformation rate and/or by a change of the transformation mechanism. The indirect proportionality between the -phase content in the starting powder and the density at the solid skeleton stage was found. The microstructure of the sintered body is influenced by both the -phase content in the starting powder and the chemical composition of the additive. Fine, uniform microstructure with a high aspect ratio of -grains is obtained, when the -phase content in the starting powder is as small as possible and when the - transformation is controlled by grain growth.     

Microstructural modification to improve mechanical properties of a 70%Si3N4-30%BAS self-reinforced ceramic composite

Different microstructures of the 70%Si₃N₄-30%BAS self-reinforced composite, fine, coarse and bimodal, are obtained by pressureless sintering at 1920°C. Flexural strength, fracture toughness and crack-growth resistance (R-Curve) behavior of each microstructure are characterized by three-point bending, indentation and modified compact tension methods respectively, at room temperature. The crack deflection, whisker bridging and pullout are considered as major toughening mechanisms in the composite. It is found that coarsening -Si₃N₄ whiskers of this composite can improve the toughness/fracture resistance but deteriorate the strength. When limited large abnormally grown whiskers are introduced into the microstructure, the composite shows an improved toughness/fracture resistance behavior and concurrently sustains a high strength.     

Study on the structures of N-containing melilite Y2Si3O3N4 and Nd2Si2.5Al0.5O3.5N3.5

Rietveld refinements have been used to determine the structure of Y2Si3O3N4 from X-ray data and Nd2Si2.5Al0.5O3.5N3.5 from neutron powder diffraction data. The refinements show that in the melilite phase Y2Si3O3N4 and melilite solid solution Nd2Si2.5Al0.5O3.5N3.5 the distributions of cations and anions are almost identical. They are analogous to the akermanite (Ca2MgSi2O7) structure, with Si/Si,Al atoms at the origin and centre of the unit cell and with four N/N,O atoms forming the SiN4/(Si,Al)(N3.5O0.5) tetrahedra which share corners with SiO2N2/(Si,Al)O2.25N1.75 tetrahedra to form a continuous sheet structure. Each Y3+ or Nd3+ ion is surrounded by eight N/O atoms forming the coordination polyhedron in Y2Si3O3N4 and Nd2Si2.5Al0.5O3.5N3.5 respectively. The arrangement of Al, Si atoms in the tetrahedra in Nd2Si2.5Al0.5O3.5N3.5 structure is also discussed. This revised version was published online in November 2006 with corrections to the Cover Date.