Abstract

Silicon nitride ceramics are used in numerous applications as high-temperature structural components, cutting tools or wear parts. The materials are densified by liquid phase sintering using oxide additives to obtain a multiphase microstructure with elongated silicon nitride grains surrounded by an amorphous or partially crystalline grain boundary phase.

The first part of the presentation describes the influence of different rare earth (RE) additives on the growth anisotropy of the silicon nitride crystals. Model experiments, in which Si₃N₄ particles can grow freely in an RE-Si-Mg-oxynitride glass matrix, show that, with increasing ionic radius of the additive, grain anisotropy increases due to non-linear growth kinetics. Cyclic heat treatments of the samples revealed furthermore that growth anisotropy is controlled by the adsorption behaviour of the cation at the interface between grain and intergranular film. The use of elements of the same valence (Sc, Y), but different electronic structures than the rare-earth elements (Lu, Yb, Sm and La) showed that the electronic structure has a major role on the adsorption behaviour of the Me³⁺ cation.

The mechanical behaviour of silicon nitride ceramics has been proven to be not only a function of the grain size and aspect ratio but to a large extend to depend on the characteristics of the grain boundary phase. To probe the relevance of the grain boundary composition and therefore strength on the macroscopic mechanical behaviour, bulk samples were prepared with different additives and sintered so as to obtain equivalent grain sizes. Samples of equivalent microstructures then yield an increasing toughness with increasing ion size of the RE³⁺, reflecting an increasingly intergranular crack path. These samples are also strong and flaw tolerant, but the trends of strength and toughness raise the issue that the toughest material is not the strongest. R-curve measurements show that the R-curve rises slower but to higher levels for materials containing larger rare earth elements. Moreover, crack kinking experiments on particles in bulk glasses enable a semi-quantitative measurement of the interfacial strength.

Curriculum vitae -- Michael J. Hoffmann