الفهرس 
AbstractOnly 14 pages are availabe for public view
 |  | from | 299 |  |  |
| | | from | 299 | | |
Abstract
Fracture analysis was performed on bend specimens of two grades of silicon nitride : sintered silicon nitride (S-SiaN 4) and hot isostatical1y pressed -SigN 4 (HIP - SigN 4). The main objective of the fracture analysis was to find the fracture initiation point, to describe the nature of flaw leading to fracture and to. compare between the measured bending strength and local fracture strength derived from the fracture mechanical evaluation. The stress intensity factor (KIC) was calculated for the flaws supposed to be the fracture origins and compared with the fracture toughness evaluated using the Vickers indentation technique. In addition, further material characterization was performed including : quantitative¬structural analysis, X-ray diffraction analysis, bulk element analysis and microprobe analysis to determine element stribution
Examination of the fracture surface of S-SigN 4 specimens revealed the presence of three types of flaws. These were pores, porous regions and inclusions. The most frequent flaw type was the porous region. In most cases rod-like and prismatic like
graIns of B-SiaN 4 were well developed in the pores. The presence of large porous regions suggests that full densification of the bulk material did not occur during sintering. The porous regions were originated in the green body microstructure as density inhomogenity and may have evolved during sintering into its final state. Therefore, the properety improvement of S-SiaN 4 can be achieved through green. microstructure control
(achieving density homogenity).
Moreover, further improvements could be obtained through control of other flaw types, such as metallic incl usions.
Examination of fracture surface of hot isostatical1y pressed-Si aN 4 specimens revealed the presence of four types of
flaws. These were surface flaws, edge flaws, porous regions and inclusions. The most frequent flaw type was the surface flaws. Therefore, the property improvements of HIP-Si aN 4 can be achieved through surface finsihing improvements and control of other flaw types. The stress intensity factor calculated for the flaws, I supposed to be the fracture origins, increased with the flaw size for both materials (S-SiaN 4 HIP- Si3N 4)’ The increase of the