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The silicon carbide (SiC) whisker reinforcement of silicon nitride (Si3N4) improves fracture strength and toughness, hardness, and Young's modulus, resulting in higher resistance of the composites to sphere penetration and crack initiation at spherical impact. Sintered Si3N4 shows an elastic/plastic response and initiates median/radial cracks at 100 m/s impact velocity. SiC-whisker/Si3N4 composites, on the other hand, demonstrate an elastic response, with Hertzian cone crack initiation, only when impact velocity exceeds 280 m/s. The SiC-whisker/Si3N4 composites thus exhibit improved strength degradation versus critical impact velocity characteristics because of improved mechanical properties provided by the SiC whiskers.

Spherical-impact damage to two silicon nitrides is investigated. Gas-pressure-sintered silicon nitride exhibits an elastic response to impact by spherical partially stabilized zirconia particles, resulting in Hertzian cone-crack initiation in the sintered body. Pressureless-sintered silicon nitride, on the other hand, demonstrated an elastic/plastic response, with median/radical-crack initiation. These differences in behavior are due to their microstructural differences as well as to the different hardness values of the silicon nitrides in relation to those of the PSZ spheres. The postimpact bend strength of silicon nitrides is also degraded when crack length exceeds the inherent flaw size.

Conception d'un rotor en ceramique pour un systeme de suralimentation par turbocompresseur (Nissan). Amelioration de la reponse en acceleration

The R-curve evaluation was discussed for “Fractometer”, and was applied to the short bar specimens of sintered silicon nitride. The values of KR were increased slowly with crack extension. The KR value of crack extension less than about 2mm could not be determined, because of the “pop in” phenomenon prior to the initiation of the stable fracture. When the increment of the applied displacement was stopped, the load relaxation due to subcritical crack growth was observed. Thus, it is concluded that the measured KR values are likely to be the KI values of subcritical crack growth corresponding to the crack opening displacement rate of the specimens.

The effect of rare-earth oxide additives on the densification of silicon nitride by pressureless sintering at 1600° to 1700°C and by gas pressure sintering under 10 MPa of N2 at 1800° to 2000°C was studied. When a single-component oxide, such as CeO2, Nd2O3, La2O3, Sm2O3, or Y2O3, was used as an additive, the sintering temperature required to reach approximate theoretical density became higher as the melting temperature of the oxide increased. When a mixed oxide additive, such as Y2O3–Ln2O3 (Ln=Ce, Nd, La, Sm), was used, higher densification was achieved below 2000°C because of a lower liquid formation temperature. The sinterability of silicon nitride ceramics with the addition of rare-earth oxides is discussed in relation to the additive compositions.