Microstructure and Properties of Nanosemicrystalline Si 3 N 4 Ceramics with Doped Sintering Additives: Part I. Microstru
- PDF / 474,280 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 47 Downloads / 228 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Microstructure and properties of nanosemicrystalline Si3 N4 ceramics with doped sintering additives: Part I. Microstructural characterization of nanosemicrystalline Si3 N4 powders K. H. Ryu and J-M. Yang Department of Materials Science and Engineering, University of California – Los Angeles, Los Angeles, California 90095-1595 (Received 23 June 1997; accepted 23 December 1997)
The characteristics of nanosized silicon nitride powders with doped Y2 O3 and Al2 O3 fabricated by a plasma-reacted chemical process were investigated. The chemical compositions of the powders were analyzed by wet chemical analysis. The morphology and the size distribution were determined by transmission electron microscopy (TEM). TEM with energy dispersive spectroscopy (EDS) was used to verify the existence of sintering additives in each individual particle. The crystal structure of the powders was identified by the selected area diffraction pattern (SADP). X-ray diffraction (XRD) technique was used for phase analysis and the measurement of degree of crystallinity. The characteristics of chemical bonding was analyzed by using Fourier transform infrared spectroscopy (FTIR).
I. INTRODUCTION
Silicon nitride is one of the leading engineering ceramics for high temperature structural applications in severe environments such as gas turbine engines. It possesses several attractive intrinsic properties, including high strength at elevated temperatures, excellent oxidation resistance, high strength-to-weight ratio, good wear, and thermal shock resistance.1 However, due to strong covalent bonds and low self-diffusivity of Si3 N4 , the classical solid-state sintering techniques are unable to densify the Si3 N4 . In order to densify Si3 N4 at low temperatures, appropriate sintering aids (such as MgO, Y2 O3 , and Al2 O3 ) are generally used to promote liquidphase sintering via solution-reprecipitation processes. During sintering, the oxide additives react with SiO2 layer on the powder surface to form a liquid phase which then dissolves the starting a –Si3 N4 powder and then precipitates b –Si3 N4 grains. Reprecipitated b –Si3 N4 grains can be grown to elongated shape. These elongated grains can be used to improve the fracture resistance of the Si3 N4 by deflecting or bridging the propagating cracks.2 Recently, considerable interest has been directed toward synthesizing and consolidating ceramics with ultrafine powders. This is strongly motivated by the excellent potential of low temperature sintering and low temperature superplastic deformation.3 The mechanism that causes enhanced sinterability and deformability in nanoceramics is attributed to fast diffusion in grain boundaries over short distances. Thus far, most of the work on nanoceramics has been focused on oxidebased ceramics such as TiO2 , ZrO2 , Y2 O3 , and Al2 O3 .4 2580
J. Mater. Res., Vol. 13, No. 9, Sep 1998
Only limited studies have been conducted on Si3 N4 nanoceramics using nanosized amorphous powders.5–9 Several different techniques for prod
Data Loading...
