Diamond sinter, high-pressure phase boron nitride sinter, and processes for producing those sinters

A process for producing a diamond or high-pressure phase boron nitride sinter comprising the steps of preparing coated diamond or high-pressure phase boron nitride particles and sintering said coated diamond or high-pressure phase boron nitride particles or mixtures containing such particles or mixtures of coated diamond or high-pressure phase boron nitride particles with bonding material,

• sintering said coated diamond or high-pressure phase boron nitride particles or said mixtures containing such particles at pressures no less than 2000 MPa and at temperatures in which diamond or high-pressure phase boron nitride is thermodynamically stable or metastable;
• or
• sintering said coated diamond or high-pressure phase boron nitride particles or said mixtures containing such particles at pressures of less than 2000 MPa and at temperatures not exceeding 1850.degree. C. under such sintering pressure and temperature conditions that diamond or high-pressure phase boron nitride is thermodynamically metastable if not stable;
• or
• sintering said mixtures of coated diamond or high-pressure phase boron nitride particles with a bonding material in a volume ratio of (1-90):(99-10), said bonding material being sinterable to a density of at least 85% at pressures less than 2000 MPa and at temperatures not exceeding 1850.degree. C. under such conditions that diamond and/or high-pressure phase boron nitride particles are thermodynamically metastable, at pressures less than 2000 MPa and at temperatures not exceeding 1850.degree. C. under such sintering pressure and temperature conditions that diamond and/or high-pressure phase boron nitride is thermodynamically metastable if not stable,
• wherein said coated diamond or high-pressure phase boron nitride particles are prepared by a method comprising the steps of charging a powder of diamond or high-pressure phase boron nitride core particles into a coating space and then permitting a precursor of a coat forming substance which is generated via the vapor phase and/or a precursor of a coat forming substance in a vapor-phase state to contact and/or impinge against the particles in said powder of core particles so that their surfaces are coated with the coat forming substance and
• said coated diamond or high-pressure phase boron nitride particles are prepared by coating means comprising:
• (A) a dispersing step in which the particles in the powder of core particles are dispersed by a group of means for high dispersion treatment of particles in a gaseous atmosphere to form a mixture of a gas and the particles in a powder, forming highly dispersed particles including agglomerating particles, the final treating means in said group of means for high dispersion treatment of particles being selected from between:
• (a) dispersing means for dispersing the particles in the powder of core particles in a gaseous atmosphere;
• and
• (b) means for selecting a mixture of a gas and the particles in a powder of highly dispersed particles which comprises:
• (b-1) means for selecting a mixture of a gas and the particles in a powder of highly dispersed particles that separates a portion of the powder that is composed of less highly dispersed particles including non-agglomerating single particles from the mixture of a gas and the particles in the powder of core particles in which the particles in the powder of core particles are dispersed in a gaseous atmosphere, thereby selecting a mixture of a gas and the particles in a powder of highly dispersed core particles in which the particles in the powder of core particles are present in the gaseous atmosphere mainly in a single-particle state;
• and
• (b-2) feedback means by which the portion of the powder that is composed of less highly dispersed particles and which has been separated by said means for selecting a mixture of a gas and the particles in a powder of highly dispersed particles is transported to the final dispersing means among the dispersing means in the group of means for high dispersion treatment of particles and/or treating means upstream of the final dispersing means;
• and
• (B) a coating step in which the particles in the powder of core particles that have been dispersed in the dispersing step (A) start to be coated by being allowed to contact and/or impinge against the precursor of the coat forming substance in the coating start region of the coating space as they are dispersed to satisfy the following conditions:
• the dispersity .beta. should be at least 70% if those particles have an average diameter of no more than 10 .mu.m in the frequency distribution by volume;
• the dispersity .beta. should be at least 80% if those particles have an average diameter in excess of 10 .mu.m but no more than 20 .mu.m in the frequency distribution by volume;
• the dispersity .beta. should be at least 90% if those particles have an average diameter in excess of 20 .mu.m but no more than 50 .mu.m in the frequency distribution by volume;
• the dispersity .beta. should be at least 95% if those particles have an average diameter in excess of 50 .mu.m but no more than 300 .mu.m in the frequency distribution by volume;
• the dispersity .beta. should be at least 97% if those particles have an average diameter in excess of 300 .mu.m but no more than 800 .mu.m in the frequency distribution by volume;
• the dispersity .beta. should be at least 99% if those particles have an average diameter in excess of 800 .mu.m in the frequency distribution by volume.