Process Development for Silicon Carbide Based Structural Ceramics

摘要

The objective of this program is to develop a process for making shaped silicon carbide based ceramic materials with reduced microstructural flaw size by in situ reaction of silicon with fine, ultra-uniform pored carbon skeletons that are produced from liquid polymer solutions without particulate additions. A very uniform siliconized microstructure has been made from a carbon skeleton with average pore size of approx. 1.9 micrometers and apparent density of approx. 85 gm/cc. This material had a room temperature, four point Weibull characteristic strength of 714 MPa which exceeds commercial reaction bonded SiC (NC433) by approx 100, sintered alpha -SiC by approx 85, and approximately equals hot pressed SiC (NC203) when tested under identical conditions. The fracture toughness, KIC, measured by Vickers indentation testing was approximately the same as hot pressed Silicon carbide (NC203). Efforts to produce finer structured materials have not yet reproducibly achieved uniform microstructures free from silicon viening. The materials thus far produced give Weibull moduli in the range 5 to 7 compared with 10-15 for the commercial materials. Fracture analysis indicates the lower values are due to machining defects and undesirable microstructures at the surfaces which were incompletely removed in sample preparation.