Si3N4 differs from other ceramics because it doesn't melt, but between 1800 and 1900 centigrade degree it decomposes and sublimates into Silicon and Nitride.howerver, stabilized forms can be made as requried.
Silicon Nitride is light,hard,and has low thermal expansion coefficient,It has high mechanical strength, fracture toughness,and it is resistant to deformation at room temperature as well as at elevated temperature.
a-Si3N4 & β-Si3N4.
Regarding the two type, different technics,process,facility.
(Si powder + N2).
a-Si3N4, temperature 1100~1300 centigrade degree.
β-Si3N4,above 1400 centigrade.