A cutting insert of a substantially horizontal cylindrical segment shape and having top and bottom surfaces with a circular segment shape. The cutting insert has a convex side and a flat side, a top cutting edge which is formed where the convex side and the top surface meet, and a bottom cutting edge which is formed where the convex side and the bottom surface meet. The cutting insert includes a hole extending from the convex side towards the flat side. The hole is positioned at a center of a surface of the flat side.

A cutting tool includes a substrate of cemented carbide having hard constituents in a metallic binder. The hard constituents include WC. The WC content in the cemented carbide is 80-95 wt %. The cemented carbide has a Fe+Ni+Co+Cr content of 3-13 wt %, an atomic ratio of 0.05<Fe/(Fe+Ni+Co+Cr)<0.35, an atomic ratio of 0.05<Ni/(Fe+Ni+Co+Cr)<0.35, an atomic ratio of 0.05<Co/(Fe+Ni+Co+Cr)<0.35 and an atomic ratio of 0.05<Cr/(Fe+Ni+Co+Cr)<0.35. The crack resistance W measured on the rake face of the cutting tool is at least 25% higher than the W measured on a cross section of the bulk area of the cutting tool.

A cutting tool includes: a substrate; and a coating film disposed on the substrate, wherein the coating film includes an α-Al.sub.2O.sub.3 layer, the α-Al.sub.2O.sub.3 layer includes a plurality of α-Al.sub.2O.sub.3 crystal grains, and has a TC(006) of more than 5 in texture coefficient TC(hkl), and an elastic modulus E1 of the α-Al.sub.2O.sub.3 layer at a room temperature and an elastic modulus E2 of the α-Al.sub.2O.sub.3 layer at 800° C. represent a relation of the following expression B-1:

0<{(E1−E2)/E1}×100<10  Expression B-1.

A cutting tool includes: a substrate; and a coating film disposed on the substrate, wherein the coating film includes an α-Al.sub.2O.sub.3 layer, the α-Al.sub.2O.sub.3 layer includes a plurality of α-Al.sub.2O.sub.3 crystal grains, and has a TC (006) of more than 5 in texture coefficient TC (hkl), and a plastic deformation amount hp of the α-Al.sub.2O.sub.3 layer at 800° C. is 0 nm or more and 100 nm or less.

A cutting tool includes: a substrate; and a coating film disposed on the substrate, wherein the coating film includes an α-Al.sub.2O.sub.3 layer, the α-Al.sub.2O.sub.3 layer includes a plurality of α-Al.sub.2O.sub.3 crystal grains, and has a TC(006) of more than 5 in texture coefficient TC(hkl), and a hardness H1 of the α-Al.sub.2O.sub.3 layer at a room temperature and a hardness H2 of the α-Al.sub.2O.sub.3 layer at 800° C. represent a relation of the following expression A-1:

0<{(H1−H2)/H1}×100<60   Expression A-1.

An object of the invention is to provide a coated cutting tool whose tool life can be extended by having excellent wear resistance and fracture resistance. The coated cutting tool includes: a substrate; and a coating layer formed on a surface of the substrate, in which the coating layer includes a lower layer, an intermediate layer, and an upper layer in this order from a substrate side to a surface side of the coating layer, the lower layer includes one or more Ti compound layers formed of a specific Ti compound, the intermediate layer contains TiCNO, TiCO, or TiAlCNO, the upper layer contains α-type Al.sub.2O.sub.3, an average thickness of the lower layer is 2.0 μm or more and 8.0 μm or less, an average thickness of the intermediate layer is 0.5 μm or more and 2.0 μm or less and is 10% or more and 20% or less of a thickness of the entire coating layer, an average thickness of the upper layer is 0.8 μm or more and 6.0 μm or less, and in the intermediate layer, a ratio of a length of CSL grain boundaries to a total length 100% of a total grain boundary is 20% or more and 60% or less.

A coated tool includes a base and a coating layer on the base. The coating layer includes a first layer including Al.sub.2O.sub.3 particles, and a second layer on the first layer. The second layer includes, sequentially from the base, a first film, a second film in contact with the first film, and a third film in contact with the second film. The first to third films individually include Ti. The first film, the second film and the third film individually include at least one kind selected from C and N. The coated tool satisfies a relationship of a first N content>a third N content>a second N content, in which the first N content is an N content in the first film, the second N content is an N content in the second film, and the third N content is an N content in the third film.

A coated tool includes a base and a coating film on the base. The coating film includes an Al.sub.2O.sub.3 layer, and a surface layer on the Al.sub.2O.sub.3 layer. The surface layer has a first erosion rate of 0.1 μm/min or less. The first erosion rate is obtained from measurement by causing a liquid A, in which 3 mass % of amorphous Al.sub.2O.sub.3 particles having a mean particle diameter of 1.1-1.3 μm is dispersed in purified water, to collide with the surface layer. The surface layer has a second erosion rate of 2.0 μm/min or more. The second erosion rate is obtained from measurement by causing a liquid B, in which 3 mass % of spherical Al.sub.2O.sub.3 particles having a mean particle diameter of 2.8-3.2 μm is dispersed in purified water, to collide with the surface layer.

Provided is a cutting tool including a base material and a coating layer provided on the base material, the coating layer including a titanium carbonitride layer provided on the base material, an intermediate layer provided on the titanium carbonitride layer in contact therewith, and an alumina layer provided on the intermediate layer in contact therewith, the intermediate layer being composed of a compound made of titanium, carbon, oxygen, and nitrogen, the intermediate layer having a thickness of more than 1 μm, when P.sub.C1 atomic % represents an atomic ratio of the carbon in an interface between the intermediate layer and the alumina layer, and P.sub.C2 atomic % represents an atomic ratio of the carbon at a point A away from the interface by 1 μm on a side of the intermediate layer, a ratio P.sub.C1/P.sub.C2 of the P.sub.C1 to the P.sub.C2 being more than or equal to 1.03.

A coated cutting tool, comprising: a substrate; and a coating layer formed on a surface of the substrate, wherein the coating layer includes a lower layer and an upper layer in this order from a substrate side toward a surface side, and the upper layer is formed on a surface of the lower layer, the lower layer contains a compound having a composition represented by (Al.sub.xTi.sub.1-x)N, an average thickness of the lower layer is 1.0 μm or more and 15.0 μm or less, the upper layer contains an α-Al.sub.2O.sub.3 layer containing α-Al.sub.2O.sub.3, an average thickness of the upper layer is 0.5 μm or more and 15.0 μm or less, and in grains of the α-Al.sub.2O.sub.3 layer, a proportion of grains of which a grain size is 0.05 μm or more and less than 0.5 μm is 50% by area or more and 80% by area or less.