Densification behavior and properties of Y{sub 2}O{sub 3}-containing {alpha}-SiAlON-based composites

Different SiAlON composites based on {alpha}{prime}-SiAlON are investigated, with respect to the phase relationships, densification behavior, and mechanical properties. The compositions are located on a phase-diagram line parallel to the Si{sub 3}N{sub 4}-Y{sub 2}O{sub 3}{center_dot}9AlN compound in the Si{sub 3}N{sub 4}-SiO{sub 2}-AlN-Al{sub 2}O{sub 3}-Y{sub 2}O{sub 3}-YN system. Analysis of the reaction sequences shows that the formation of the composites is associated with the transient appearance of Y{sub 4}Al{sub 2}O{sub 9} (YAM), yttrium-aluminum-garnet (YAG), melilite, and a nitrogen-rich liquid phase. The small shift of compositions on the Si{sub 3}N{sub 4}-Y{sub 2}O{sub 3}{center_dot}9AlN compound phase-diagram line toward the Al{sub 2}O{sub 3}-rich side offers the advantage of a higher sinterability and the removal of the melilite phase from a wide range of compositions containing {alpha}{prime}-SiAlON and polytypes. The {alpha}{prime}/{beta}{prime}-SiAlON composites show better mechanical properties in comparison to pure {alpha}{prime}-SiAlON and composites of {alpha}{prime}-SiAlON and polytypes. A post-heat-treatment causes the crystallization of YAG as a grain-boundary phase and leads to excellent strength retention up to temperatures of 1,350 C.