The present disclosure relates to a polycrystalline cubic boron nitride and a method for manufacturing the same. This application claims priority from Japanese Patent Application No. 2019-036262 filed on Feb. 28, 2019, and International Application PCT/JP2020/001437 filed on Jan. 17, 2020. The entire contents of the Japanese patent application and the international application are incorporated herein by reference.

TECHNICAL FIELD

Background Art

Cubic boron nitride (hereinafter, also referred to as “cBN”) has a hardness that is second only to diamond, and also has excellent thermal stability and chemical stability. For this reason, cubic boron nitride sintered bodies have been used as a material for tools.

As the cubic boron nitride sintered body, a sintered body including about 10 to 40% by volume of a binder has been used. However, the binder causes the strength and thermal diffusivity of the sintered body to decrease.

To solve this problem, there has been developed a method in which sintering is conducted while simultaneously directly converting a hexagonal boron nitride into cubic boron nitride at ultrahigh pressure and high temperature without using a binder to obtain a binder-free cubic boron nitride sintered body.

Japanese Patent Laying-Open No. 11-246271 (Patent Literature 1) discloses a technique for obtaining a cubic boron nitride sintered body by directly converting and sintering a low-crystallinity hexagonal boron nitride into a cubic boron nitride sintered body under ultrahigh temperature and high pressure.

CITATION LIST

Patent Literature

PTL 1: Japanese Patent Laying-Open No. 11-246271

SUMMARY OF INVENTION

A polycrystalline cubic boron nitride of the present disclosure is a polycrystalline cubic boron nitride comprising 96% by volume or more of cubic boron nitride, wherein

the cubic boron nitride has a dislocation density of more than 8×1015/m2,

the polycrystalline cubic boron nitride comprises a plurality of crystal grains, and

the plurality of crystal grains have a median diameter d50 of an equivalent circle diameter of less than 100 nm.

A method for manufacturing a polycrystalline cubic boron nitride of the present disclosure is a method for manufacturing the above-described polycrystalline cubic boron nitride, comprising:

a first step of preparing a hexagonal boron nitride powder having a d90 of an equivalent circle diameter of 0.3 μm or less; and

a second step of heating and pressurizing the hexagonal boron nitride powder to a temperature greater than or equal to 1500° C. and less than or equal to 2200° C. and to a pressure greater than or equal to 10 GPa, with the temperature and the pressure passing through a temperature and a pressure in a stable region of a wurtzite boron nitride, to obtain the polycrystalline cubic boron nitride, wherein

the stable region of wurtzite boron nitride is, when the temperature is represented as T° C. and the pressure as P GPa, a region that simultaneously satisfies the following Formula 1 and Formula 2,
P≥−0.0037T+11.301  Formula 1:
P≤−0.085T+117  Formula 2:

in a heating and pressurizing path of the second step, an entry temperature into the stable region of wurtzite boron nitride is 500° C. or less, and

the second step comprises a step of holding the temperature and the pressure in the heating and pressurizing path for 10 minutes or more at a temperature and a pressure in the stable region of wurtzite boron nitride.

A method for manufacturing a polycrystalline cubic boron nitride of the present disclosure is a method for manufacturing the above-described polycrystalline cubic boron nitride, comprising:

a step A of preparing pyrolytic boron nitride; and

a step B of heating and pressurizing the pyrolytic boron nitride to a temperature and a pressure in a final sintering region, with the temperature and the pressure passing through a temperature and a pressure in a stable region of a wurtzite boron nitride, to obtain the polycrystalline cubic boron nitride, wherein

the stable region of wurtzite boron nitride is, when the temperature is represented as T° C. and the pressure as P GPa, a region that simultaneously satisfies the following Formula 1 and Formula 2,
P≥−0.0037T+11.301  Formula 1:
P≤−0.085T+117  Formula 2:

the final sintering region is, when the temperature is represented as T° C. and the pressure as P GPa, a region that simultaneously satisfies the following Formula 4, Formula 5, and Formula 6,
P≥12  Formula 4:
P≥−0.085T+125.5  Formula 5:
P≤−0.085T+151  Formula 6:

and in the heating and pressurizing path of step B, an entry temperature into the stable region of wurtzite boron nitride is 500° C. or less.

