1988 Volume 96 Issue 1110 Pages 127-133
The relationship between carbon in the silicon nitride grain boundaries and mechanical properties has been studied. Silicon nitride powder was coated with carbon and hot pressed at 1873K and 20MPa. Vickers hardness, fracture toughness and flexural strength were determined. Behavior of carbon in the grain boundary interface was investigated by X-ray diffraction and Auger electron spectroscopy. The results can be summarized as follows:
(1) The amount of β-silicon nitride phase increases with increasing carbon content.
(2) The mechanical properties depends on the carbon content. The toughness improves above certain amount of carbon.
(3) During hot pressing of Silicon nitride powder with carbon film, the following chemical reaction is proposed at the grain boundaries. SiO2+3C→SiC+2CO Ellingham diagram shows that the reaction is favorable over 1630K. Carbon mono-oxide becomes stable at the oxygen partial pressures above 10×10-10Pa. The grain boundary glass phase is reinforced by silicon carbide precipitates.
