The silicon nitride sintered body according to the present disclosure contains silicon nitride particles and yttrium silicon oxynitride particles and has a porosity of 14% or less.

A cutting tool (1) formed of a silicon nitride-based sintered body (2) including a matrix phase (3), a hard phase (4), and a grain boundary phase (10) in which a glass phase (11) and a crystal phase (12) exist. The sintered body (2) contains yttrium in an amount of 5.0 wt % to 15.0 wt % in terms of an oxide, and contains titanium nitride as the hard phase (4) in an amount of 5.0 wt % to 25.0 wt %. In an X-ray diffraction peak, a halo pattern appears at 2 ranging from 25 to 35 in an internal region of the sintered body (2). A ratio B/A of a maximum peak intensity B to a maximum peak intensity A satisfies 0.11B/A0.40 . . . Expression (1) in a surface region of the sintered body (2), and satisfies 0.00B/A0.10 . . . Expression (2) in the internal region of the sintered body (2).

A preparation method for a copper plate-covered silicon nitride ceramic substrate is provided. The structure of the copper plate-covered silicon nitride ceramic substrate includes a silicon nitride ceramic substrate, copper sheets disposed on the upper and lower sides of the silicon nitride ceramic substrate and soldering layers disposed between the copper sheets and the silicon nitride ceramic substrate; the composition of the silicon nitride ceramic substrate comprises a silicon nitride phase (more than or equal to 95 wt %); and a grain boundary phase (containing at least three elements (Y, Mg and O) and less than or equal to 5 wt %, and the content of a crystalline phase in the grain boundary phase is more than or equal to 40 vol %); and the sintering aids are Y.sub.2O.sub.3 and MgO. The two-step sintering process comprises: in a nitrogen atmosphere, performing low-temperature heat treatment and high-temperature heat treatment in sequence.

The present disclosure relates to a batch sintering method for a high-property silicon nitride ceramic substrate. The batch sintering method includes: (1) silicon nitride ceramic substrate green bodies are stacked and put into a boron nitride crucible, and a layer of boron nitride powder is applied between adjacent silicon nitride ceramic substrate green bodies; (2) after step-by-step vacuumization, debinding is performed in a nitrogen atmosphere or a reducing atmosphere at 500 C. to 900 C.; (3) gas pressure sintering is then performed in a nitrogen atmosphere at 1800 C. to 2000 C., completing the batch preparation of the high-property silicon nitride ceramic substrate.