Polycrystalline diamond and/or cubic boron nitride body and process for making said body

A process for producing a polycrystalline body of predetermined shape and size which consists essentially of providing at least a substantially uniform mixture of crystals selected from the group consisting of diamond, cubic boron nitride and combinations thereof and a carbonaceous material wherein none of the surfaces of said crystals are exposed significantly and wherein at least a substantial amount of said crystals are enveloped and separated from each other by at least a coherent continuous coating of said carbonaceous material on said crystals, said carbonaceous material being selected from the group consisting of elemental non-diamond carbon, an organic material which completely decomposes at a temperature below 1400.degree. C. to elemental non-diamond carbon and gaseous product of decomposition, and mixtures thereof, said organic material being present in an amount sufficient on decomposition to produce on the crystal surfaces it coats a coherent continuous coating of elemental non-diamond carbon, providing a mold with a cavity of desired size and shape and means for introducing fluid silicon into said cavity and means for maintaining a partial vacuum in said cavity, filling said cavity with said mixture of crystals and carbonaceous material and confining said mixture therein, associating said mixture in said filled cavity with a mass of solid silicon via said means for introducing fluid silicon into said cavity, providing the resulting associated structure with a partial vacuum wherein the residual gases have no significant deleterious effect on said associated structure, heating said associated structure in said partial vacuum to a temperature above 1400.degree. C. at which said silicon is fluid and which has no significant deleterious effect on said crystals, introducing said fluid silicon to said confined mixture via said means for introducing fluid silicon into said cavity and infiltrating said fluid silicon throughout said confined mixture, said partial vacuum being at least sufficient to remove gas from said confined mixture which blocks said infiltrating fluid silicon, said infiltrating silicon reacting with non-diamond elemental carbon forming silicon carbide, cooling the resulting infiltrated mass of crystals in an atmosphere which has no significant deleterious effect on said infiltrated mass, and recovering the resulting polycrystalline body of said predetermined shape and size wherein the crystals are bonded together by a bonding medium comprised of silicon carbide and elemental silicon and wherein the bonded crystals range from about 1% by volume up to about but less than 80% by volume of the total volume of said body, said body being pore free or containing pores up to less than about 5% by volume of said body.