Process for producing a composite of polycrystalline diamond and/or cubic boron nitride body and substrate phases

A process for producing a self-supporting composite consisting essentially of a polycrystalline body phase integrally bonded to a substrate supporting phase consisting essentially of providing a crystal-containing carbonaceous mass consisting essentially of at least a substantially uniform mixture of diamond and/or cubic boron nitride crystals 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, and mixtures thereof, said organic material being present in an amount sufficient on decomposition to produce on the crystal surfaces it coats at least a coherent continuous coating of elemental non-diamond carbon, providing a carbonaceous substrate consisting essentially of elemental non-diamond carbon, a solid organic substrate material, and mixtures thereof, 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 a preform consisting essentially of said crystal-containing carbonaceous mass in contact with said carbonaceous substrate forming an interface therewith and confining said preform therein, said preform being the form desired of said composite, said organic material and said solid organic substrate material decomposing at a temperature below 1400.degree. C. to elemental non-diamond carbon and gaseous product of decomposition and being present in an amount sufficient which on decomposition produces at least sufficient elemental non-diamond carbon to maintain said preform, associating said preform 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 does not have a significantly deleterious effect on said crystals, introducing said fluid silicon to said confined preform via said means for introducing fluid silicon into said cavity, and infiltrating said fluid silicon into said preform at least through said crystal-containing carbonaceous mass and at least into contact with the contacting surface of said substrate filling the pores through said interface so that it is at least substantially pore-free, said partial vacuum being at least sufficient to remove gas from said preform which blocks said infiltrating fluid silicon, said infiltrating silicon reacting with non-diamond elemental carbon forming silicon carbide, cooling the resulting composite in an atmosphere which has no significant deleterious effect on it, recovering the resulting composite of polycrystalline body phase bonded to substrate supporting phase, said polycrystalline body phase being substantially pore-free wherein the crystals ranging from about 1% by volume up to about but less than 80% by volume of the total volume of said polycrystalline body phase are bonded together by a bonding medium consisting essentially of silicon carbide and elemental silicon.