R. L. Satet

Three-dimensional organization of rare-earth atoms at grain boundaries in silicon nitride

Used in the preparation of Si3N4 components, rare-earth elements promote the growth of needlelike grains essential to elevated toughness; evidently, La is significantly more effective than Lu. To explore this difference, we determine the three-dimensional organization of rare-earth atoms in the amorphous phase near prismatic interfaces in La- and Lu-containing Si3N4 using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and image processing. Evidence is presented for substantial atomic structure in notionally amorphous volumes. While the atomic arrangement in the amorphous phase conforms to the periodicity of the terminating crystal plane in both cases, the attachment sites are very different.

Arrangement of rare-earth elements at prismatic grain boundaries in silicon nitride

The arrangement of rare-earth atoms at {100} prism planes of La- and Lu-containing polycrystalline Si3N4 specimens is studied using high-angle annular dark-field scanning transmission electron microscopy. For both systems, the attachment sites of rare-earth atoms are well-defined and largely conform to the periodicity of the terminating plane of the Si3N4 grain. We observe significant differences between the structural arrangement of La and Lu atoms at the interface.

Indentation moduli and microhardness of RE–Si–Mg–O–N glasses (RE=Sc, Y, La, Sm, Yb and Lu) with different nitrogen content

Abstract The indentation moduli and microhardness of RE–Si–Mg–O–N glasses (RE=Y, Sc and lanthanides La, Sm, Yb and Lu) containing 20–24 eq.% of nitrogen were found to depend on RE type and nitrogen content as well. The Lu-containing glasses were approximately 15–28% harder than La-glasses with the same nitrogen content and 4% increase in nitrogen content results in 1 to ∼10% increase of microhardness. The effectiveness of the RE elements with regard to their ability to increase selected mechanical properties of the oxynitride glasses rises from La, Y/Sc/Sm to Yb and Lu. The properties studied increase approximately linearly with cationic field strength (CFS) of the corresponding lanthanide cation in the glass network. The properties of Y- and Sc-containing glasses do not follow this dependence. These changes cannot be explained in terms of CFS and additional effects have to be considered to characterize relationships between composition, structure, and properties in the glasses containing non-lanthanide elements.

Impact of the Intergranular Film Properties on Microstructure and Mechanical Behavior of Silicon Nitride

Oversaturated oxynitride glasses as well as bulk ceramics were designed in order to investigate the impact of 6 different metallic elements that form Me 3+ cations of varying radius on grain growth anisotropy and mechanical behavior of silicon nitride ceramics. We show that grain growth anisotropy and toughness increase with increasing cation size, which seems to be the result of a preferred segregation of large Me 3+ cations on the prism planes of Si3N4 grains. Furthermore, the experiments reveal that the electronic configuration of the metallic element seems to have an impact on the behavior of the Me 3+ in the intergranular film.

Measuring Electrostatic Potential Profiles across Amorphous Intergranular Films by Electron Diffraction

Amorphous 1-2-nm-wide intergranular films in ceramics dictate many of their properties. The detailed investigation of structure and chemistry of these films pushes the limits of todays transmission electron microscopy. We report on the reconstruction of the one-dimensional potential profile across the film from an experimentally acquired tilt series of energy-filtered electron diffraction patterns. Along with the potential profile, the specimen thickness, film orientation with respect to the grain lattice and specimen surface, and the absolute specimen orientation with respect to the laboratory frame of reference are retrieved.

Rheological Properties of the Rare-Earth Doped Glasses

RE-Si-Mg-O-N glasses with RE = Sc, Y, La, Yb, Sm, or Lu with two different nitrogen contents were investigated to understand the effects of rare-earth elements and nitrogen on microhardness, indentation modulus, fracture toughness and viscosity. The microhardness and indentation modulus of glasses containing lanthanide dopants increase approximately linearly with the increase of the cationic field strength (CFS) of the corresponding RE element, whereas fracture toughness is almost unaffected. Nitrogen content increase shifts the dependencies to higher values. The properties of the glasses with non-lanthanide dopants, Y and Sc, deviate from linear dependence. At elevated temperatures, variations of the softening temperature Tg from linearity were found in Yband Sc-containing glasses. Additional differential thermal analysis experiments on Yb-containing glass revealed the possibility of changes in glass structure at elevated temperatures. CFS concept seems to be insufficient to describe all the effects of rare-earth dopants on the properties of the oxynitride glasses.

A simple model of fully-faceted grain growth and coarsening with non-linear growth laws

Abstract A numerical simulation of the growth and coarsening of completely faceted particles in a two-dimensional closed system is described. The particles grow from a supersaturated solution with driving forces including an anisotropic-Gibbs–Thompson effect. Linear and non-linear growth laws were incorporated. This allows comparison to data obtained from physical experiments of Si3N4 growing from a glass matrix with additions of various rare earths (La, Y, Lu) where particle growth is more or less anisotropic (depending on the particular rare earth dopant). Simulations explore the ranges of kinetic parameters for which particle shapes tend to be dictated by thermodynamic and/or kinetic anisotropy. Comparison to data taken from a series of experiments indicates that La probably has a non-linear growth effect whereas Lu is less so.