Synthesis of polycrystalline cubic boron nitride

A method for preparing a sintered polycrystalline compact with substantial intercrystalline bonding of high pressure boron nitride which comprises the steps of:

• forming a mixture comprising high pressure boron nitride with an amount of low pressure boron nitride, an aluminum containing adjuvant material, and cobalt aluminide having a melting temperature lower than the melting temperature of cobalt phase;
• subjecting the mixture to elevated temperature in a non-oxidizing environment sufficient to remove boron oxide;
• placing the mixture onto a substrate comprising tungsten carbide cemented with cobalt;
• subjecting the mixture to elevated temperature and pressure conditions sufficient to melt cobalt phase in the substrate, and at which conditions high pressure boron is thermodynamically stable;
• and
• maintaining the elevated conditions for a time sufficient to allow for substantial intercrystalline bonding of cubic boron nitride to thereby sinter the compact.

A method for preparing a sintered polycrystalline compact of cubic boron nitride comprising the steps of:

• combining cubic boron nitride crystals with hexagonal boron nitride, aluminum nitride and cobalt aluminide having a lower melting point than cobalt phase, the cubic boron nitride comprising in the range of from 45 to 65 percent by weight of the mixture, the hexagonal boron nitride comprising in the range of from 30 to 45 percent by weight of the mixture, the aluminum nitride comprising in the range of from 1 to 7 percent of the mixture, and the cobalt aluminide comprising in the range of from 1 to 3 percent by weight of the mixture;
• compacting the mixture onto a preform;
• subjecting the preform to a heat treatment in a vacuum, ammonia, or hydrogen atmosphere at a temperature of at least 600.degree. C.;
• placing the preform onto a tungsten carbide substrate cemented with cobalt;
• loading the substrate and mixture into a closed chamber;
• subjecting the mixture and substrate to elevated temperature and elevated pressure conditions sufficient to melt cobalt phase in the substrate, at which conditions the cubic boron nitride is thermodynamically stable, whereby cobalt phase infiltrates the mixture causing intercrystalline bonding;
• and
• maintaining the elevated temperature and pressure conditions for a time sufficient to allow for substantial intercrystalline bonding to thereby sinter the compact.