Designing with Ceramics

This guide is aimed at engineers and designers wishing to incorporate advanced technical ceramics, such as silicon nitride and sialonsaluminazirconiaor silicon carbide into their design.

Often an engineer unfamiliar with using ceramics will want a direct copy of a component that was originally metal, for example, made in ceramic. Very often this is not the best solution and can unnecessarily increase manufacturing costs and even result in the component not working as desired.

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Development of Syalons

Silicon nitride (Si3N4) was discovered in the mid-nineteenth century and caused no excitement among the engineering fraternity of the day. The problem was that Si3N4 does not lend itself to ease of fabrication.

Over the next century two principle routes for fabricating Si3N4 ceramics were developed: hot pressing and reaction bonding.

Hot pressed Si3N4 (HPSN), was made by adding a flux (usually magnesia, MgO) to a fine Si3N4 powder and then pressing the powder in a graphite die at high pressure and temperature. The resultant HPSN body, although fully dense and extremely strong, could only be made in to simple shapes which would then require expensive diamond grinding to obtain the desired profile.

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What are Sialon Ceramics?

SiAlONs are ceramic alloys based on the elements silicon (Si), aluminium (Al), oxygen (O) and nitrogen (N) and were developed in the 1970s to solve the problem of silicon nitride (Si3N4) being difficult to fabricate.

As alloys of Si3N4, SiAlONs exist in three basic forms. Each form is isostructural with one of the two common forms of Si3N4, beta (β) and alpha (α) and with silicon oxynitride. The relationship between that of SiAlON and Si3N4 is similar to that between brass and pure copper. In the latter case, copper atoms are replaced by zinc to give a better and stronger alloy than the mother metal. In the case of SiAlON, there is substitution for Si by Al with corresponding atomic replacement of N by O, to satisfy valancy requirements. The resulting 'solution' (SiAlON) has superior properties to the original pure solvent (silicon nitride).

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