Effects of packing powder on densification and properties of pressureless sintered silicon nitride ceramics

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Abstract
Pressureless sintering of Si 3 N 4 doped with 2 mol% Y 2 O 3 and 5 mol% MgO was carried out at temperatures ranging from 1700°C to 1770°C in three kinds of packing powders consisting of Si 3 N 4 and BN coupled with or without addition of Y 2 O 3 , MgO, and SiO 2 . Effects of packing powders on weight change, densification, and grain growth during sintering and properties (thermal conductivity, fracture toughness, and bending strength) of the sintered samples were studied. It was found that adding Y 2 O 3 and MgO into the packing powder resulted in improvement of densification, grain growth, thermal conductivity, fracture toughness, and bending strength of the sintered samples. However, further addition of SiO 2 into the packing powder could lead to suppressed grain growth and decrease in thermal conductivity, fracture toughness, and bending strength. The sample sintered at 1770°C in the packing powder containing Y 2 O 3 and MgO was fully densified, and it possessed well‐balanced properties: thermal conductivity of 76 W m −1 K −1 , fracture toughness of 7.82 MPa m 1/2 , and bending strength of 932 MPa.

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