Comprendre la microstructure du nitrure de silicium

Silicon nitride is a solid compound which can be formed by direct reaction between silicon and nitrogen. It appears in three distinct phases characterized by their crystalline structure, namely:

• The α and β phases, which are the most common, display hexagonal crystal structures (corner-sharing, tetrahedral SiN₄ with strong Si-N bonds resulting in a rigid, layered 3D framework). Both these phases are synthesized at normal pressures; the α phase forms at low temperatures with the transition into the β phase occurring between 1,400°C and 1,600°C.
• The γ phase, which only forms at high temperatures and pressures.

Sintering Additives

Sintering is a heat treatment process commonly applied to powders. In order to control the formation of microstructures — i.e., the formation of covalent bonded structures resulting in material toughness — the introduction of sintering additives during production is crucial. Rare earth and metal oxides such as Yttrium oxide (Y₂O₃) and Aluminum Oxide (Al₂O₃) are common sintering additives.

In one study, it was observed that:

1. Despite fractional increases of β-Si₃N₄ (from 7.67 to 57.34 wt%) during the initial α to β phase transformation, average grain size and hardness (ca. 20 GPa) remain unchanged. Thereafter, grains adopt a rod-like structure accompanied by a substantial increase in fracture toughness (from 3.36 to 7.11 MPaM^1/2).
2. During second stage sintering, β-Si₃N₄ reaches 100.00 wt% accompanied by a rapid increase in grain size. This results in a fractional increase in fracture toughness (from 7.11 to 7.61 MPaM^1/2) against a noticeable reduction in hardness (from 20 GPa to 16.80 GPa).

It has been shown that introducing a high-hardness second phase — such as silicon carbide (SiC), titanium carbide (TiC), or titanium nitride (TiN) — improves the fracture toughness and hardness of silicon nitride engineering materials. This shows that bimodal microstructured ceramics (α- and β-phases) can be tailored for the production of optimally performing materials.

The hardness of silicon nitride advanced materials is linearly proportional to their strength. As previously mentioned, mechanical properties can be significantly improved through the introduction of sintering additives. In industrial applications, silicon nitride-engineered tools display superior cutting performance on materials such as ductile iron, malleable cast iron, and gray cast iron because of their outstanding high-temperature mechanical properties.

In another study, silicon carbide ceramic materials comprising 1 wt % Co and 10 wt % WC displayed optimal performance at a sintering temperature of 1,650 °C, achieving fracture toughness of 7.26 MPaM^1/2, room temperature bending strength of 1,132 MPa, and hardness of 16.96 GPa. Furthermore, the microstructure did not exhibit any abnormal growth. Silicon nitride materials produced by Saint-Gobain display:

• Superior flexural strength (850 MPa)
• High fracture toughness (7 MPaM1/2)
• Low density (3.17 g/cm³)

By tailoring compositions and sintering parameters, silicon nitride high-temperature materials can be introduced into a wide range of industrial applications.

Silicon nitride ceramics combine superior thermal, mechanical, and chemical properties for a wide range of industrial applications, surpassing the performance of metals and polymers. Hot isostatic pressing (HIP) and spark plasma sintering (SPS) are common methods employed in the commercial production of silicon nitride materials.

Due to their high strength and fracture toughness, and superior oxidation, corrosion, and wear resistance, silicon nitride ceramics are being considered for a wide range of structural applications in some of the toughest operating environments in aerospace, automotive, oil and gas, and more, including:

• Next-generation power devices.
• Turbocharger rotors.
• Surfaces within pumps (such as seals, impellers, shafts).
• Seal face components (within pressurized water pumps, boiler water reactor pumps, and others).
• Diesel engine components.
• Gas turbine engines.

With a firm grasp of the complexities associated with manufacturing high-performance materials, Saint-Gobain High-Performance Ceramics & Refractories produce high-grade, hot-pressed, and sintered silicon nitride ceramics with minimal requirement for post-production finishing.