Polycrystalline cubic boron nitride and method for manufacturing the same

1. 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 8×10 15 /m 2 ,

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.

2. The polycrystalline cubic boron nitride according to claim 1 , wherein the dislocation density is 9×10 15 /m 2 or more.

3. The polycrystalline cubic boron nitride according to claim 1 , wherein the polycrystalline cubic boron nitride comprises 0.01% by volume or more of hexagonal boron nitride.

4. The polycrystalline cubic boron nitride according to claim 1 , wherein the polycrystalline cubic boron nitride comprises 0.01% by volume or more of compressed hexagonal boron nitride.

5. The polycrystalline cubic boron nitride according to claim 1 , wherein the polycrystalline cubic boron nitride comprises 0.1% by volume or more of wurtzite boron nitride.

6. The polycrystalline cubic boron nitride according to claim 1 , wherein the polycrystalline cubic boron nitride comprises a total content of an alkali metal element and an alkaline earth metal element of 10 ppm or less in terms of mass.

7. The polycrystalline cubic boron nitride according to claim 1 , wherein the dislocation density is calculated by using a modified Williamson-Hall method and a modified Warren-Averbach method.

8. The polycrystalline cubic boron nitride according to claim 1 , wherein the dislocation density is measured using synchrotron radiation as an X-ray source.

9. A method for manufacturing the polycrystalline cubic boron nitride according to claim 1 , 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, in the 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, 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 ≥−0.0037 T+ 11.301  Formula 1:

P ≤−0.085 T+ 117  Formula 2.

10. The method for manufacturing the polycrystalline cubic boron nitride according to claim 9 , wherein the entry temperature is 300° C. or less.

11. The method for manufacturing the polycrystalline cubic boron nitride according to claim 9 , wherein the second step comprises a step of holding the temperature and the pressure in the heating and pressurizing path for 15 minutes or more at a temperature and a pressure in the stable region of wurtzite boron nitride.

12. The method for manufacturing the polycrystalline cubic boron nitride according to claim 9 , wherein the second step comprises a step of, when the temperature is represented as T° C. and the pressure as P GPa, holding the temperature and the pressure in the heating and pressurizing path for 10 minutes or more at a temperature and a pressure in a region that simultaneously satisfies the following Formula 1, Formula 2, and Formula 3,

P ≥−0.0037 T+ 11.301  Formula 1:

P ≤−0.085 T+ 117  Formula 2:

P ≤−0.0037 T+ 11.375  Formula 3.

13. The method for manufacturing the polycrystalline cubic boron nitride according to claim 9 , further comprising a third step of, after the second step, holding the polycrystalline cubic boron nitride obtained in the second step for 10 minutes or more and 30 minutes or less under conditions of a temperature of 1500° C. or more and 2200° C. or less and a pressure of 10 GPa or more.

14. A method for manufacturing the polycrystalline cubic boron nitride according to claim 1 , 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, 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, 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 ≥−0.0037 T+ 11.301  Formula 1:

P ≤−0.085 T+ 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 ≥−0.085 T+ 125.5  Formula 5:

P ≤−0.085 T+ 151  Formula 6.

15. The method for manufacturing the polycrystalline cubic boron nitride according to claim 14 , wherein the entry temperature is 300° C. or less.

16. The method for manufacturing the polycrystalline cubic boron nitride according to claim 14 , further comprising a step C of, after step B, holding the polycrystalline cubic boron nitride obtained in step B for 10 minutes or more and 30 minutes or less under conditions of a temperature and a pressure in the final sintering region.