Effects of N sub 2 gas pressure sintering on the surface and strength of Si sub 3 N sub 4

The flexural strength of Si{sub 3}N{sub 4} having an as-sintered surface was measured to investigate the effects of the sintering atmosphere on the surface. The as-sintered strength of Si{sub 3}N{sub 4} fires in 0.1 MPa N{sub 2} (pressureless sintering) degraded to 39% of its ground strength as a result of near-surface voids formed by thermal decomposition reactions between Si{sub 3}N{sub 4} and additive oxides. The as-sintered strength of Si{sub 3}N{sub 4} fired at 70 MPa N{sub 2} (gas pressure sintering), however, was shown to retain as much as 76% of its ground strength. This result seems to indicate that high-pressure N{sub 2} gas suppressed the void formation by reducing the decomposition reactions. Gas pressure sintering was effective in avoiding thermal decomposition and in maintaining a higher as-sintered strength.