Cubic boron nitride polycrystal, cutting tool, wear-resistant tool, grinding tool, and method of producing cubic boron nitride polycrystal

Abstract

A cubic boron nitride polycrystal includes cubic boron nitride, the cubic boron nitride having an average grain size of not more than 150 nm, a ratio b/a being not more than 0.085 in measurement of Knoop hardness at 23° C.±5° C. under a test load of 4.9 N, the ratio b/a being a ratio between a length a of a longer diagonal line and a length b of a shorter diagonal line of a Knoop indentation.

What is claimed is: 
     
         1 . A cubic boron nitride polycrystal comprising cubic boron nitride,
 said cubic boron nitride having an average grain size of not more than 150 nm,   a ratio b/a being not more than 0.085 in measurement of Knoop hardness at 23° C.±5° C. under a test load of 4.9 N, said ratio b/a being a ratio between a length a of a longer diagonal line and a length b of a shorter diagonal line of a Knoop indentation.   
     
     
         2 . The cubic boron nitride polycrystal according to  claim 1 , wherein said cubic boron nitride has an average grain size of not more than 100 nm. 
     
     
         3 . The cubic boron nitride polycrystal according to  claim 1 , wherein said cubic boron nitride polycrystal includes not less than 0.01 volume % of wurtzite type boron nitride. 
     
     
         4 . The cubic boron nitride polycrystal according to  claim 1 , wherein said cubic boron nitride polycrystal includes 0.01 to 0.5 volume % of compressed hexagonal boron nitride. 
     
     
         5 . The cubic boron nitride polycrystal according to  claim 1 , wherein for X-ray diffraction in said cubic boron nitride, a ratio I (220) /I (111)  of X-ray diffraction strength I (220)  at a (220) plane to X-ray diffraction strength I (111)  at a (111) plane is not less than 0.1 and not more than 0.3. 
     
     
         6 . A cutting tool comprising the cubic boron nitride polycrystal recited in  claim 1 . 
     
     
         7 . A wear-resistant tool comprising the cubic boron nitride polycrystal recited in  claim 1 . 
     
     
         8 . A grinding tool comprising the cubic boron nitride polycrystal recited in  claim 1 . 
     
     
         9 . A method of producing a cubic boron nitride polycrystal, comprising the steps of:
 preparing, as a starting material, hexagonal boron nitride powder having a grain size of not more than 0.5 μm; and   converting said hexagonal boron nitride powder into cubic boron nitride and wurtzite type boron nitride and sintering said cubic boron nitride and said wurtzite type boron nitride under temperature and pressure satisfying the following conditions:
     P≧ 0.0000122 T   2 −0.0531 T+ 65.846,
 
     T≦ 2200, and 
     P≦ 25, 
   
       where the pressure is represented as P (GPa) and the temperature is represented as T (° C.).