The Effect of Pores on the Grain Growth Rate of Sintered Alumina

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Abstract
The effect of pores on the grain growth rate of alumina was investigated using specimens of various green densities in the range from 20 to 94% of theoretical one, which were prepared in four different methods in graphite molds, a) hot-pressing at 1600°C, b) hot-pressing at 1200°C, c) pressing at a room temperature and d) filling at a room temperature. After the forming pressure was removed graphite molds were heated inductively up to 1800°, 1850° or 1900°C and kept at the temperatures for 5, 10 or 60 min. The forming and heating atmosphere was a mixture of nitrogen and carbon monooxide gas. By these experiments the rate of grain growth: dD/dt, the average grain diameter: D, and the volume fraction of pores: f and the average diameter of pores: d, were determined. Among them next relationship was found to exist: dD/dt=K(1/D-2f/d) The same experiments as above were carried out with 0.16% addition of magnesia, and the significant retarding effect of magnesia on the grain growth of alumina was observed. In this case next relationship was found to exist: dD/dt=K(1/D-2f/d-K′) K′ represents the retarding effect of magnesia, the value of which was found to be constant among specimens of the same heating temperature and time, and larger than the value of 2f/d corresponding to 16 vol.% of pores. The exponent of time, m, in the equation D2-D02=ktm was found to increase with decreasing green density of specimens. The growth of pores in specimens was observed and added magnesia was found also to have an effect on the pore growth.

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