A method of producing a boron nitride polycrystal includes: a first step of obtaining a thermally treated powder by thermally treating a powder of a high pressure phase boron nitride at more than or equal to 1300° C.; and a second step of obtaining a boron nitride polycrystal by sintering the thermally treated powder under a condition of 8 to 20 GPa and 1200 to 2300° C.

An insert of the present disclosure includes a boron nitride sintered body including a first surface. In a transmission X-ray diffraction of a cross section of the boron nitride sintered body vertical to the first surface, X-ray intensity at a top of a 111 diffraction peak of cubic boron nitride in a direction vertical to the first surface is IcBN(111)v. X-ray intensity at a top of a 002 diffraction peak of compressed boron nitride is IhBN(002)v. X-ray intensity at a top of a 111 diffraction peak of the cubic boron nitride in a direction parallel to the first surface is IcBN(111)h. X-ray intensity at a top of a 002 diffraction peak of the compressed boron nitride is IhBN(002)h. A compressed boron nitride content value obtained from these X-ray intensities is larger than 0.005. A cubic orientation value is larger than 0.5, and a compressed boron nitride orientation value is larger than the cubic orientation value.

A cutting part of a cutting insert may include a first surface including a corner, a first side, a first region, a second region and a third region. The first region may be located along the corner and the first side. The second region may be located at a more inner part than the first region. The third region may be located at a more inner part than the second region. A boundary between the corner and the first side may be a first point. A boundary between the first region and the second region may be a second point in a cross section that passes through the first point and is orthogonal to the first side. An imaginary straight line passing through the first point and the second point may be a first imaginary straight line. The first imaginary straight line may intersect with the third region.

A surface-coated cutting tool includes a substrate and a coating formed on a surface of the substrate, the coating including one or two or more layers, at least one of the layers being an Al-rich layer including hard particles, the hard particle having a sodium chloride type crystal structure, and including a first unit phase in a form of a plurality of lumps and a second unit phase interposed between the lumps of the first unit phase, the first unit phase being composed of a nitride or carbonitride of Al.sub.xTi.sub.1-x, the first unit phase having an atomic ratio x of Al of 0.7 or more and 0.96 or less, the second unit phase being composed of a nitride or carbonitride of Al.sub.yTi.sub.1-y, the second unit phase having an atomic ratio y of Al exceeding 0.5 and less than 0.7.

A coated cutting tool includes a body and a hard and wear resistant coating on the body. The coating has at least one NbN layer with a thickness between 0.2 m and 15 m, wherein the NbN layer includes a phase mixture of a cubic phase, c-NbN, and a hexagonal phase, h-NbN.

A cutting tool according to an aspect of the present disclosure includes a cutting edge portion which contains at least one of cubic boron nitride and polycrystalline diamond. The cutting edge portion includes a flank face, a negative land contiguous to the flank face, and a cutting edge formed by a ridgeline between the flank face and the negative land. At least one of the negative land and the flank face is provided with a plurality of recesses and a projection. The projection is formed by arranging the edges of adjacent recesses in contact with each other.

A surface-coated cutting tool includes a substrate and a coating formed on a surface of the substrate, the coating including one or two or more layers, at least one of the layers being an Al-rich layer including hard particles, the hard particle having a sodium chloride type crystal structure, and including a first unit phase in a form of a plurality of lumps and a second unit phase interposed between the lumps of the first unit phase, the first unit phase being composed of a nitride or carbonitride of Al.sub.xTi.sub.1x, the first unit phase having an atomic ratio x of Al of 0.7 or more and 0.96 or less, the second unit phase being composed of a nitride or carbonitride of Al.sub.yTi.sub.1y, the second unit phase having an atomic ratio y of Al exceeding 0.5 and less than 0.7.

A coated cutting tool includes a body and a hard and wear resistant coating on the body. The coating has at least one NbN layer with a thickness between 0.2 m and 15 m, wherein the NbN layer includes a phase mixture of a cubic phase, c-NbN, and a hexagonal phase, h-NbN.

A method of producing a boron nitride polycrystal includes: a first step of obtaining a thermally treated powder by thermally treating a powder of a high pressure phase boron nitride at more than or equal to 1300 C.; and a second step of obtaining a boron nitride polycrystal by sintering the thermally treated powder under a condition of 8 to 20 GPa and 1200 to 2300 C.

A cutting tool according to an aspect of the present disclosure includes a cutting edge portion which contains at least one of cubic boron nitride and polycrystalline diamond. The cutting edge portion includes a flank face, a negative land contiguous to the flank face, and a cutting edge formed by a ridgeline between the flank face and the negative land. At least one of the negative land and the flank face is provided with a plurality of recesses and a projection. The projection is formed by arranging the edges of adjacent recesses in contact with each other.