Silicon nitride (Si3N4) was developed in the 1960s and '70s in a search for fully dense, high strength and high toughness materials. A prime driver for its development was to replace metals with ceramics in advanced turbine and reciprocating engines to give higher operating temperatures and efficiencies. Although the ultimate goal of a ceramic engine has never been achieved, silicon nitride has been used in a number of industrial applications, such as engine components, bearings and cutting tools.
Silicon nitride has better high temperature capabilities than most metals combining retention of high strength and creep resistance with oxidation resistance. In addition, its low thermal expansion coefficient gives good thermal shock resistance compared with most ceramic materials.
Pure silicon nitride is difficult to produce as a fully dense material. This covalently bonded material does not readily sinter and cannot be heated over 1850oC as it dissociates into silicon and nitrogen. Dense silicon nitride can only be made using methods that give bonding through indirect methods, such as small chemical additions to aid densification. These chemicals are known as sintering aids, which commonly induce a degree of liquid phase sintering.
Types of Silicon Nitride
Since the material properties strongly depend on the fabrication method, silicon nitride cannot be considered as a single material. The three main types of silicon nitride are:
• Reaction bonded silicon nitride (RBSN)
• Hot pressed silicon nitride (HPSN)
• Sintered silicon nitrides (SSN)
Reaction bonded silicon nitride is made by direct nitridation of a compacted silicon powder, and because of the difficulty of ensuring complete reaction, it is hard to achieve a high component density. Usual densities are in the range 2300 - 2700kg.m-3 compared with 3200kg.m-3 for hot pressed and sintered silicon nitride. The higher density gives the HPSN and SSN materials better physical properties and means they are used in more demanding applications. The nitridation produces only a small volume change, which means that RBSN components do not need to be machined after fabrication and complex near net shapes can be produced in a single process stage.