Formation of a-sialons with dual modifying cations (Li + Y and Ca +Y)

摘要

a-Sialon (a-Si3N4 solid solution) can be derived from a-Si3N4 with partial replacement of Si-N by Al-N and Al-O. Charge balance is maintained by introducing a "modifying cation" (lithium, calcium, yttrium or many rare-earth elements). Modifiers can occupy up to two interstitial sites in the (Si, Al)-N network unit cell, resulting in a formula of M.v(Si, Al)i2(O, N)i6 with x less than or equal to 2 depending on the valency of the M ion [1]. a-Sialon possesses high hardness and strength. However, the fabrication of a-Sialon is more difficult than that of /3-Sialon because the higher nitrogen content in the a-Sialon compositions makes them difficult to densify completely. If two kinds of metal elements (e.g. yttrium and lithium, or yttrium and calcium) are used together as the "modifying cations", the formation and densification of such dual-cation a-Sialons would be completed more easily than single-cation a-Sialons, because the dual cations lower the eutectic temperature of the system. Some investigations of a-Sialons containing dual modifying cations have been reported [2-5]. Two mixed cations from lithium, calcium and neodymium can be accommodated in a-sialon structure [4]. A small amount of cerium can enter the large interstitial cages in a-Sialon structure with yttrium [2, 3]. A cation as large as strontium (ionic radius 0.113 nm) has entered the a-Sialon structure when mixed with yttrium and calcium [5].