Si.sub.3 N.sub.4 formed by nitridation of sintered silicon compact containing boron

A process for producing a polycrystalline silicon nitride body by nitridation of a polycrystalline sintered body of silicon which comprises forming a particulate mixture of silicon powder and boron, said silicon powder and said boron having an average particle size less than 3 microns, said boron being present in an amount ranging from 0.1% by weight to 5% by weight of said silicon powder, shaping said mixture into a green body having a density of at least 30% of the theoretical density of silicon, an sintering the green body at a temperature ranging from 1250.degree. C to a temperature below the melting point of silicon in an atmosphere which has no significant deteriorating effect on the green body or the resulting sintered body to produce a sintered body having a density ranging from 60% to 75% of the theoretical density of silicon, said sintering densifying said green body in an amount of at least 2%, and nitriding said sintered body by reacting it in a gaseous nitrogen atmosphere ranging from subatmospheric pressure to superatmospheric pressure at a temperature ranging from 1100.degree. C to below the melting point of silicon to form a polycrystalline silicon nitride body having a density ranging from 79% to 92% of the theoretical density of silicon nitride and comprised of polycrystalline silicon nitride containing a boron nitride phase in an amount of 0.1% to 8% by volume of the solid phase of said silicon nitride body, and containing unreacted boron-doped silicon in an amount ranging from zero % to 10% by volume of the solid phase of said silicon nitride body.