Abstract

Self-reinforced silicon nitride ceramics were synthesized by introducing 5 wt% previously prepared β-Si₃N₄ seeds into a mixture of α-Si₃N₄ and 8 wt.% of sintering additive. The microscopic evidence indicates that β-Si₃N₄ seeds play an important role in microstructural development and mechanical properties for silicon nitride ceramics. The growth of β-Si₃N₄ grains is initiated from the seeds, resulting in a core/shell microstructure observed in the abnormal grain growth. The internal dislocation, π-boundary, and Moire fringes are present in Si₃N₄ grains, which were formed to compensate the mismatch and compositional differences between the two grains. Coalescence can occur at the final stage of sintering.

Abstract

Composite materials based on Si₃N₄ were synthesized by introducing previously prepared β-Si₃N₄ seeds into a mixture of α-Si₃N₄ and 10 wt% of sintering additive. The densification process during pressureless sintering was studied in the presence of CeO₂ additive, which is less frequently used as sintering aid to Si₃N₄. The effect of seed concentration and sintering time on the densification and fracture toughness was followed. The results showed that the addition of seeds retarded densification. On the other hand, fracture toughness passes through maximum at the content of 3 wt% seeds.

Abstract

Silicon nitride materials with distinct microstructures have been prepared by hot-pressing mixtures of α-Si₃N₄ powders plus oxide additives and β-Si₃N₄ seeds. These seeds were β crystals obtained by the SHS technology. The presence of these β-Si₃N₄ particles retarded the densification rate. The influence of the holding time at the sintering temperature on the microstructure and mechanical properties has been analysed in the β-seeded Si₃N₄ materials, in comparison with the non-seeded materials. These β-seeds were effective in enhancing α→β transformation and the general coarsening of the microstructure. The toughness has been discussed as a function of microstructural parameters, such as average grain size, average aspect ratio and area fraction of elongated grains.

Abstract

The effect of seeding on the microstructural development and mechanical properties of sintered reaction-bonded silicon nitride was investigated by the use of β-Si₃N₄ particles. Seeding of an appropriate amount of the β-Si₃N₄ particles gave rise to the promotion of density and the resulting increase in fracture strength and hardness. The fracture toughness was also increased due to the development of elongated grains. On the other hand, addition of a large amount of seed particles showed lower strength and fracture toughness owing to the coalescence of large elongated grains. By seeding of 2 wt.% β-Si₃N₄ particles on Si powders of 7 μm, high fracture strength of 1100 MPa and fracture toughness of 7.2 MPa·m^1/2 were obtained in sintered reaction-bonded silicon nitride ceramics.

Abstract

Crack-bridging mechanisms can provide substantial increases in toughness coupled with strength in ceramics. Herein, we describe the various bridging mechanisms, their toughening contributions and how they are affected by microstructure in silicon nitride ceramics, which are a classic example where both high strength and toughness are achieved. Crack growth resistance curves (R-curves) for seven different silicon nitrides doped with various metal oxides and with different microstructures were measured, and bridging stress distributions were calculated for each. Based on an analysis of those results combined with results of the relative interfacial toughness of two of the ceramics, a new mechanistic theory for the evolution of the R-curve is proposed. The initial steep rise in toughness is attributed to the formation of elastic bridges that experience no debonding. This mechanism has not previously been recognized in the literature and the high strength of these materials (up to 1 GPa) is here attributed primarily to that mechanism. As those bridges begin to fracture and the mechanism becomes saturated, a change in the R-curve slope is observed and the more traditional mechanisms of partially debonded elastic and fully debonded frictional bridges dominate the continuing rise in toughness. Furthermore, the saturation of each mechanism is associated with a change in slope of the R-curve. The results of this study provide a fundamental insight into how to optimize silicon nitride microstructures for high strength and toughness.

Abstract

Large scale optical inhomogeneities were analysed in different Si₃N₄ ceramics containing different additive ratios and densified by hot pressing, gas pressure sintering and spark plasma sintering.

In all analysed materials showing these optical inhomogeneities (“snow flakes”) non-filled triple junctions were locally found which could be correlated with the “snow flake” structure. The internal tensile stress in the amorphous grain boundary phase is the most likely reason for this phenomenon. These stresses are caused by thermal mismatch between the grain boundary phase and the Si₃N₄ skeleton, or the volume change during crystallisation of the grain boundary phase.