Improvement of mechanical properties of Si[subscript 3]N [subscript 4] using low cost materials and processes

Abstract

The difference in mass loss behavior of Si[subscript 3] N [subscript 4] between sintering in air atmosphere and sintering under controlled N [subscript 2]pressure was intensively studied and the results were systematically evaluated. In addition, the optimal composition and sintering condition for improving the mechanical properties of Si[subscript 3] N [subscript 4] using low-cost materials and processes were investigated. Low-cost Si[subscript 3]N[subscript 4] powder and additives were used as the starting materials. Sintering was performed both under air atmosphere and under N[subscript 2] atmosphere. The results showed that the mass loss of the Si[subscript 3] N [subscript 4] sintered under air atmosphere insignificantly changed with increasing sintering temperature, but significantly increased with the O[subscript 2] or SiO[subscript 2] contents in the composition before sintering. It was also affected by the pressure gradient of oxygen partial pressure (Po[subscript 2]) and the formation of the impermeable layer in the crucible during sintering. On the other hand, the mass loss during sintering increased with increasing the sintering temperature and soaking time when Si[subscript 3]N[subscript 4] was sintered under N [subscript 2] atmosphere. The optimal composition and sintering condition for obtaining the highest mechanical properties at room temperature were 96wt% Si[subscript 3]N[subscript 4] + 2wt% MgO + 2wt% AI[subscript 2] O [subscript 3] and sintering at 1850ํ C for 2 h under N[subscript 2] gas pressure of 1.0 MPa. The flexural strength, fracture toughness and hardness obtained were 544 MPa, 5.9MPa m[subscript 1/2] and 16 GPa, respectively. The mechanical property values are in the level of commercially available pressureless sintered Si[subscript 3] N [subscript 4] ceramics.