Hans-Joachim Kleebe

Model experiments concerning abnormal grain growth in silicon nitride

Abstract Model experiments were designed to study abnormal grain growth in Si 3 N 4 -based ceramics. Experiments relating inhomogeneous crystalline secondary-phase distribution to exaggerated grain growth conclusively showed that abnormal grain growth is not governed by secondary-phase distribution, because a rapid homogenization of locally formed liquid occurs via capillary forces. Further investigations were focussed on intrinsic properties of the α-Si 3 N 4 starting powders. The influence of: (i) β-Si 3 N 4 -grain morphology; (ii) β-Si 3 N 4 -nuclei density, and (iii) β-Si 3 N 4 . grain-size distribution of the powder blends on microstructural development were analyzed. The results revealed that a large basal plane of β-Si 3 N 4 seeds energetically and kinetically favours grain growth. However, this effect is only partly responsible for abnormal grain growth. The formation of elongated Si 3 N 4 grains, such as in in situ reinforced Si 3 N 4 materials, strongly depends on the amount and grain-size distribution of β-Si 3 N 4 nuclei present in the α-Si 3 N 4 -starting powder.

SiC and Si3N4 materials with improved fracture resistance

Abstract The SiC and Si3N4 materials studied were densified to near theoretical density via liquid-phase sintering. All materials have been investigated by means of transmission electron microscopy (TEM). The observed characteristic microstructural features were correlated to fracture resistance. Toughness measurements were performed using the chevron notch configuration. Both SiC and Si3N4 materials showed an improved fracture resistance compared with commercially available grades. The toughness values obtained from load/displacement records were 6–7 √m for SiC and 9–11 √m for the Si3N4 grades. Additional high-resolution and analytical electron microscopy studies (HREM and AEM) were performed to relate characteristic interfacial features to the resulting mechanical properties. TEM observations revealed marked differences between the two non-oxide ceramic materials analyzed. The major difference regarding the overall microstructures of SiC and Si3N4 is twofold: (i) SiC shows matrix grains with a more globular morphology, while in Si3N4 large elongated grains are present with a grain diameter of up to 3 μm and a length of ≥ 30 μm; (ii) no amorphous intergranular films were observed in the SiC material with improved toughness, while in Si3N4 materials thin amorphous grain- and phase-boundary films were always present. From HREM observations it can be concluded that the presence or absence of amorphous intergranular films governs the resulting fracture resistance and the overall toughening mechanism.

HREM and AEM studies of Yb2O3-fluxed silicon nitride ceramics with and without CaO addition

Abstract The grain-boundary microstructure and interface chemistry of Yb2O3/Al2O3-fluxed sintered Si3N4 materials with and without the addition of CaO has been investigated by means of high-resolution and analytical electron microscopy. Each Si3N4 grade examined, exhibited a thin amorphous intergranular layer at both homophase and heterophase boundaries. The presence of calcium within this amorphous layer is confirmed in the CaO-doped materials. Furthermore, no solid solution of CaO within the Si3N4-matrix grains could be detected. The observed variability in the grain-boundary film thickness appears to be closely related to changes in the glass composition, in particular to the Ca content. The addition of CaO resulted in a widening of the intergranular film of up to 0·4 nm. Moreover, postsintering heat-treatment, which induces crystallization of the secondary phase pockets, led to only small changes in the intergranular film thickness of 0·1 nm when compared to the as-sintered specimens, for both CaO-doped and undoped materials.

The structure of special grain boundaries in α-Al2O3

Abstract With high-resolution transmission electron microscopy we have studied the structure of the (1104) twin boundary in pure α-Al2O3. Two twin boundaries of this orientation were prepared in the form of diffusion-bonded bicrystals. In the second of these specimens, one of the surfaces to be bonded was predoped with Al. Two distinct boundary structures (labelled B and C) were identified in this specimen, one of which (C) was also observed in the first specimen. The commonest structure (C) exhibited continuous lines of intensity maxima through the boundary, whereas in the other structure (labelled B) these were broken by a parallel translation of the grains. The boundary was then simulated by atomistic relaxation, which also resulted in two mechanically stable structures similar to B and C. Finally an atomistic structure was estimated by fitting simulated images to the experimental micrograph. The theoretical calculations were consistent with some features of the micrographs, notably the two translation states observed, but significant discrepancies remain. We discuss the likely sources of error.

High-Resolution Electron Microscopy Observations of Grain-Boundary Films in Silicon Nitride Ceramics

Characterization of silicon nitride ceramics by transmission electron microscopy (TEM) provides structural and compositional information on intergranular phases necessary to elucidate the factors that can influence the presence and thickness of grain-boundary films. Different TEM techniques can be used for the detection and determination of intergranular-film thickness, however, the most accurate results are obtained by high-resolution electron microscopy (HREM). HREM studies were applied, in conjunction with analytical electron microscopy, to investigate the correlation between intergranular-phase composition and film thickness. Statistical analyses of a number of grain-boundary films provided experimental verification of a theoretical equilibrium film thickness. Model experiments on a high-purity Si 3 N 4 material, doped with low amounts of Ca, suggest the presence of two repulsive forces, a steric force and a force produced by an electrical double layer, that may act to balance the attractive van der Waals force necessary to establish an equilibrium film thickness.

Compositions and Thicknesses of Grain Boundary Films in Ca-Doped Silicon Nitride Ceramics

Nanobeam analytical electron microscopy and high-resolution electron microscopy have been used to characterize both the local composition as well as the thickness of the amorphous intergranular silicate-rich film in high-purity Si3N4 ceramics doped with different levels of Ca impurities. Grain boundary films could be detected in all ceramics. The films always contained Si (cation) and O and N (anions). Ca was detected at both grain boundary junctions and triple junctions for all materials doped with different levels of Ca. The thickness h of the intergranular films depends sensitively on the Ca content. In undoped materials h is 1.0±0.1 nm. With increasing Ca content the thickness h decreased for a dopant of 80 ppm Ca but increased with further additions of Ca. Variations in film thickness and composition can be semi-quantitatively understood in terms of different long-range interatomic forces acting normal to the film.

Heat treatment effects on domain configuration and strain under electric field in undoped Pb[Zr x Ti 1−x ]O 3 ferroelectrics

Undoped Pb[ZrxTi1-x]O3 (PZT) ceramics from the tetragonal and rhombohedral side of the morphotropic phase boundary (MPB) were examined. Temperature dependent changes in domain configuration were observed by in situ hot-stage transmission electron microscopy (TEM). In corresponding macroscopic ex situ strain measurements an enhanced influence of the electric field was detected after annealing.