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Grain Morphology and Intergranular Structure of Si3N4 and Al2O3 Based Ceramics

Håkan Björklund (Institutionen för fysik)
Göteborg : Chalmers University of Technology, 1996.

This thesis concerns factors controlling the grain morphology in Si3N4 and Al2O3 based ceramics. The microstructures of the ceramics were characterized by scanning electron microscopy in combination with quantitative microscopy and stereologic methods. Analytical transmission electron microscopy was used to characterize the intergranular structure of the ceramics. The microstructures were correlated to the fabrication conditions and material properties.

The main part of the thesis is focused on Si3N4 ceramics. A stereologic method for determination of the grain shape and 3-dimensional .beta.-Si3N4 grain size distribution from etched 2-dimensional sections through Si3N4 ceramics is presented.

SiC and .beta.-Si3N4 whisker reinforced Si3N4 densified by hot isostatic pressing were studied. These ceramics had been formed without metal oxide sintering additives or with 2.5 wt% Y2O3 and 0.2 wt% Fe2O3. Both metal oxide and whisker additions had a significant influence on the matrix microstructure. The addition of Y2O3 and Fe2O3 promoted the development of a fibrous microstructure. The presence of SiC whiskers suppressed matrix .beta.-Si3N4 grain growth during densification while an addition of .beta.-Si3N4 whiskers resulted in a coarser microstructure.

.beta.-Si3N4 grain growth in gas pressure sintered Si3N4 was studied. .beta.-Si3N4 grain growth took place by coalescence and possibly also by Ostwald ripening. Growth ledges on {101¯0 } facets of .beta.-Si3N4 grain facing curved grain boundaries did also indicate grain growth by coalescence. An increased Y2O3/Al2O3 ratio in the starting powder composition resulted in a higher .beta.-Si3N4 aspect ratio, and replacement of Y2O3 by Yb2O3 resulted in a further increased aspect ratio. During densification Al was incorporated into the Si3N4 and a dilute .beta.'-Si3N4 was formed.

The Al2O3 based ceramics were reinforced with 15 wt% SiC whiskers. Also here, SiC whiskers limit grain growth.

This work shows that grain morphology and the intergranular structure of ceramics can be controlled by the sintering additives, reinforcing agent and the processing parameters, and this will affect the properties.

Nyckelord: silicon nitride, aluminium oxide, ceramics, microstructure, grain growth, sintering, whiskers, electron microscopy, quantitative microscopy, stereology

Denna post skapades 2006-09-19.
CPL Pubid: 1116


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