1. Field of the Invention
This invention relates to a production method of a sintered silicon nitride which has a high temperature strength and suitable for a material of so-called fine ceramic structural parts which are usable throughout a wide industrial field including automobiles, mechanical apparatuses, chemical apparatuses and aeronautical and astronautical apparatuses.
2. Description of the Prior Art
Sintered silicon nitrides whose main component is silicon nitride are chemically stable and high in mechanical strength at ordinary and high temperatures, and therefore are suitable for a material of sliding parts such as bearings and engine parts such as rotors of turbochargers. Since silicon nitride is difficult to be singly sintered, it is usually sintered upon adding a considerable amount of sintering assistants such as MgO, Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3. Such sintering method is disclosed, for example, in Japanese Patent Provisional Publication Nos. 49-63710, 54-15916 and 60-137873.
However, in the conventional sintered silicon nitride produced by adding a considerable amount of the sintered assistants such as MgO, Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3, there exists a considerable amount of oxides in grain boundary in the sintered silicon nitride. These oxides promote oxidation of silicon nitride at high temperatures. As a result, structual parts formed of such a sintered silicon nitride are unavoidably low in high temperature performance such as creep resistance, high temperature strength and oxidation resistance.