Abstract
Effects of carbon impurity on HIP'ed silicon nitride under 60 and 200MPa at 1823, 1873 and 1973K were studied. The density, hardness, fracture toughness, α-β phase ratio, heat capacity and thermal diffusivity were evaluated. The results were compared with those of silicon nitride prepared by the normal and hot press sintering. Carbon behavior of grain boundaries was investigated by X-ray diffraction and Auger electron spectroscopy (AES).
(1) Silicon carbide was not formed during HIP'ing of silicon nitride powder with carbon film. Thermodynamic analysis showed that carbon is stable as graphite under the HIP'ing conditions studied. Silicon carbide can be produced from silicon nitride during the normal and hot press sintering. The formation of silicon carbide is explained by the free energy change of the related reaction under high pressure (high pressure Ellingham diagrams).
(2) Silicon nitride was HIP'ed to full density. But carbon retards α to β phase transformation of silicon nitride in HIP sintering in contrast to normal or hot press sintering.
(3) The mechanical properties are not influenced by the carbon content, unlike the cases for normal or hot press sintering. This can be also explained the pressure of gas phase during sintering.
(4) Carbon decreases the thermal diffusivity and thermal conductivity. This may be due to the fact that carbon reduces the α to β phase transformation of silicon nitride in HIP'ing. Heat capacity is not influenced by the carbon content for HIP'ed silicon nitride.
(5) The thermal conductivity data showed that carbon dissolves into silicon nitride up to 1000ppm between 1823 and 1973K.
