Silicon nitride bonded silicon carbide (NBSC) can be produced by heating a mixture of silicon and powder and silicon carbide grains and firing to temperature usually in the range 1300-1450°C in nitrogen. SiAlON bonded silicon carbide refractories and ceramics can be formed in the same way, with addition of clay to the initial mixture.
Advantages of Clay Addition
The advantages of clay addition to the mix include:
• The clay lends plasticity to the mix, allowing complex shapes to be moulded.
• The aluminosilcates alloy with silicon nitride to for SiAlON bonding phases.
• Careful design of the mix can allow tailoring for a broad range of applications.
In the absences of silicon carbide, silicon reacts with nitrogen forming silicon nitride. The silicon nitride thus formed dissolves in the molten clay, with the subsequent precipitation of SiAlON phases.
In the presence of silicon carbide, the process is complicated by the fact that the silicon carbide tends to reduce the clay (in particular the silica SiO2 in the clay) and SiAlON via carbothermal reduction nitridation processes.
In the case of clays, mixed with silicon powder and silicon carbide, how the SiAlON bonding phase forms is not well understood. It is thought that the SiAlON boding phase forms as a result of both carbothermal and silicothermal reduction nitridation processes.
Applications for SiAlON bonded silicon carbide mainly revolve around refractories and include:
• Blast furnace refractory bricks
• Components for combustion furnaces
• Linings for the containment of non-ferrous molten metals
In these types of applications, clay based SiAlON boded silicon carbide materials offer better performance compared to NBSC.
Primary author: S.P. Swenson and Y.-B Cheng
Source: Abstracted from Journal of the Australasian Ceramic Society, Vol. 34, no. 2pp. 7-12 (1998).