Cutting tool

A cutting tool formed of a silicon nitride-based sintered body that includes a matrix phase formed of silicon nitride or Sialon, a hard phase, and a grain boundary phase in which a glass phase and a crystal phase exist,

• wherein the silicon nitride-based sintered body
• contains yttrium such that a content of the yttrium is not less than 5.0 wt % and not greater than 15.0 wt % in terms of an oxide, and contains titanium nitride as the hard phase such that a content of the titanium nitride is not less than 5.0 wt % and not greater than 25.0 wt %;
• in an X-ray diffraction peak of the silicon nitride-based sintered body,
• a halo pattern appears at 2θ ranging from 25° to 35° in an internal region that is deep from a surface of the silicon nitride-based sintered body by a distance of greater than 1.0 mm;
• wherein regarding a maximum peak intensity A of the matrix phase and a maximum peak intensity B of the crystal phase in the grain boundary phase as defined below, a ratio B/A of the maximum peak intensity B to the maximum peak intensity A
• satisfies a relationship represented by Expression (1) in a surface region distant from the surface of the silicon nitride-based sintered body by 0.2 mm or less, and
• satisfies a relationship represented by Expression (2) in the internal region of the silicon nitride-based sintered body,
• 0.11≤B/A≤0.40 Expression (1)
• 0.00≤B/A<0.10 Expression (2),
• when the matrix phase is formed of a single kind of phase, the maximum peak intensity A is obtained as a maximum peak intensity of the phase, and, when the matrix phase is formed of a plurality of kinds of phases, the maximum peak intensity A is obtained as a sum of maximum peak intensities of the respective phases, and
• when the crystal phase in the grain boundary phase is formed of a single kind of phase, the maximum peak intensity B is obtained as a maximum peak intensity of the phase, and, when the crystal phase in the grain boundary phase is formed of a plurality of kinds of phases, the maximum peak intensity B is obtained as a sum of maximum peak intensities of the respective phases;
• and
• wherein when a cross-section of the silicon nitride-based sintered body is observed,
• a proportion of a number of particles having a maximum diameter of not greater than 0.5 μm among entire particles of the silicon nitride or the Sialon is not less than 50%, and
• a proportion of a number of particles having an aspect ratio of not less than 1.5 among particles, of the silicon nitride or the Sialon, having a minimum diameter of not less than 0.5 μm is not less than 55%.