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.