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.
Reaction bonded Si3N4 (RBSN), was made by carefully nitriding a silicon (Si) 'dough' to produce a formed piece of Si3N4. This RBSN body was porous and low strength, but it was refractory and could be formed in to relatively complex shapes.
Both these materials therefore had significant drawbacks. Then, in the early 1970s came a revolutionary breakthrough which made scientists and engineers alike sit up and take notice. This discovery was made independently at Newcastle University in the UK by Jack and Wilson (Ian Wilson was our late M.D.) and in Japan by Osana et al. This breakthrough was the discovery of sialon, an alloy of Si3N4. This was the first known man-made ceramic alloy and in 1972 the first Patent was granted for sialon materials under the names of Jack and Wilson.
The term 'sialon' describes silicon-aluminium-oxynitride alloys of silicon nitride. Further work on these alloys soon showed that the days of having to hot press or reaction bond Si3N4 to get a useful body were over. By making additions of rare earth oxides such as yttria (Y2O3), fully dense, pressureless sinterable materials could be produced.
Over the next two decades the sialon system was meticulously studied resulting in the modern day sialons as manufactured by International Syalons under the trade name Syalon.
This article can be viewed and downloaded as a PDF (opens in a new window) (0.16 Mb).