Some aspects of processing, microstructure and properties of the various types of silicon nitride are discussed. Special emphasis is placed on the relationships between powder properties, process conditions, densification and microstructure, as well as the interdependence between microstructure and properties. After summarizing the areas of crystal structure and thermodynamic properties, and processing of the different types of Si3N4, the state-of-the-art of dense and reaction-bonded silicon nitride is given. For both types the formation mechanisms and microstructure, relationships between powder properties, additives (in the case of dense Si3N4), process conditions, and densification and microstructure, as well as data and microstructural effects of various mechanical, thermal and thermo-mechanical properties, are outlined. Advanced processing techniques, such as sintering, gas-pressure sintering, post-sintering, and the different routes of hot-isostatic pressing (starting with powder compacts, reaction-bonded Si3N4 or pre-sintered Si3N4 and the resulting properties, are discussed.

Relationships between processing, microstructure and properties of dense and reaction-bonded silicon nitride

Review: Fabrication of engineering ceramics by injection moulding. I. Materials selection

The fabrication and intergranular-phase devitrification of silicon nitride densified with rare-earth (RE) oxide additives has been investigated. The additions of the oxides of Sm, Gd, Dy, Er, and Yb, having high melting points and behaving similarly to Y2O3, were compositionally controlled to tailor a microstructure with a crystalline secondary phase of RE2Si2O7. The lanthanide oxides were found to be as effective as Y2O3 in densifying Si3N4, resulting in identical microstructures and densities of 98–99% of theoretical density. The crystallization behavior of all six disilicates was similar, characterized by a limited nucleation and rapid growth mechanism resulting in large single crystals. Complete crystallization of the intergranular phase was obtained with the exception of a thin residual amorphous film which was observed at interfaces and believed to be rich in impurities, the cause of incomplete devitrification.

Fabrication and Secondary‐Phase Crystallization of Rare‐Earth Disilicate–Silicon Nitride Ceramics

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.

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

Ytterbium oxide (Yb2O3) was used as a sintering aid to enhance the mechanical properties of silicon nitride (Si3N4) ceramics. The amount of Yb2O3 had significant effects on microstructural evolution and the composition of secondary phases at the grain boundary. When the Yb2O3 added was less than 8 wt%, small homogeneous grains were formed. At the grain boundary, crystalline Yb2Si2O7 was formed along with a glassy phase. As the amounts of Yb2O3 were increased to higher than 8 wt%, large elongated grains were developed in the fine matrix. In those cases, the grain boundary crystalline phase was changed from Yb2Si2O7 to Yb4Si2O7N2. Mechanical properties were influenced by these changes in microstructure and grain boundary phase. The fracture toughness increased with the Yb2O3 content up to 8 wt% and decreased slightly thereafter. The increase in fracture toughness was apparently due to the formation of the large elongated grains. When more than 8 wt% of Yb2O3 was added, interfacial debonding energy between the elongated grains and grain boundary phase became too large, resulting in a decrease in the fracture toughness. The room-temperature flexural strength was not significantly affected by the Yb2O3 content or the microstructure, other than in the case of 2 wt% addition. The high-temperature strength in nitrogen, however, increased steadily with Yb2O3 content. The highest strength, 870 MPa at 1400°C, was observed when 16 wt% of Yb2O3 was added. The increase in the high-temperature strength with Yb2O3 content was attributed to the formation of crystalline Yb4Si2O7N2 phase at the grain boundary.

Microstructural Evolution and Mechanical Properties of Si3N4 with Yb2O3 as a Sintering Additive

A graphite resistance furnace was used. Si/sub 3/N/sub 4/ specimens were sintered at 1800 to 1950/sup 0/C under N/sub 2/ pressures of 250 to 300 psi with only minimal degradation and 1 to 2% weight loss. CeO/sub 2/ was used as sintering additive. Modulus-of-rupture values up to 63 ksi were obtained. (DLC)

Sintering of Si3N4 Under High Nitrogen Pressure

Sintering studies of Si3N4-Y2O3/Al2O3 compositions indicated that increased densification could be produced using a dual N2 pressure process. RT modulus of rupture values and microstructures were found to be similar to hot-pressed material. The technique should be adaptable to complex shaped Si3N4 bodies where high density and strength are required.

G. E. Gazza

Papers

Sintering of Si3N4 Under High Nitrogen Pressure

Densification of Si3N4‐Y2O3/Al2O3 by a Dual N2 Pressure Process