US 4,289,503 A · Polycrystalline cubic boron nitride abrasive and process for preparing same in the absence of catalyst

Abstract

Cubic boron nitride has been made from powdered hexagonal boron nitride by a process which comprises vacuum firing of the HBN and conversion by high pressure-high temperature processing at 55-80 kilobars and 1600.degree. C. to the reconversion temperature. The high pressure reaction cell has a special design which prevents the entrance of impurities into the sample. This cell, referring to FIG. 2, comprises, for example, a carbon tube (8) enclosing concentric titanium sleeve (9). Within the cylinder defined by the tube and sleeve are: the HBN sample (4), carbon filler (3), shielding tantalum foil discs (2) and carbon end plugs (10). The vacuum firing is done at pressures of 10.sup.-5 --10.sup.-10 mm Hg, 1400.degree.-1900.degree. C., for 5 minutes--4 hours, and is believed to form a thin, free-boron coating on the HBN particles. The process works on both pyrolytic (turbostratic) and graphitic hexagonal boron nitride. Grinding grits formed by milling cubic boron nitride chunks recovered from the high pressure-high temperature process have resulted in higher grinding ratios than commercially available CBN.

Claims39

Dependent claims

An improved process for making cubic boron nitride from hexagonal boron nitride powder which comprises subjecting hexagonal boron nitride to the high pressure-high temperature process:

(i) at a pressure from 55-80 kilobars;
(ii) at a temperature of 1600.degree. C. to the reconversion temperature;
(iii) for a time sufficient to convert the hexagonal boron nitride to cubic boron nitride and sinter the cubic boron nitride;
and
(iv) in the absence of catalysts and impurities which interfere with the conversion to cubic boron nitride or the sintering of cubic boron nitride;
wherein the improvement comprises a pretreatment process consisting essentially of removing boric oxide from the surface of the hexagonal boron nitride powder--and generating a coating of boron on the surfaces of the oxide-free hexagonal boron nitride--before conversion to cubic boron nitride, which pretreatment is accomplished by a process carried out at a temperature in the hexagonal boron nitride thermal decomposition range and selected from vacuum firing and heating under inert atmosphere.

3 more elements

Dependencies are inferred from the claim text.

Polycrystalline cubic boron nitride abrasive and process for preparing same in the absence of catalyst

Life of this patent

Polycrystalline cubic boron nitride abrasive and process for preparing same in the absence of catalyst

Life of this patent

Abstract

Claims39