Preparation and properties of Ln-Si-Al-O-N glasses

Oxynitride glass phases which occur at the grain boundaries of silicon nitride based ceramics can impair subsequent high temperature mechanical properties. Bulk glasses in the Y-Si-Al-O-N system have undergone extensive investigation since Y₂O₃ and Al₂O₃ are generally used as sintering additives for Si₃N₄ and sialon ceramics. The use of lanthanides as densification aids has been explored, but it has been mainly confined to La₂O₃, Ce₂O₃ and Nd₂O₃ additions because of cost. In this paper, the preparation details of bulk glasses in various Ln-Si-Al-O-N systems (Ln = Nd, Sm, Dy, Ho & Er) containing 10 and 17e/o nitrogen are reported. The properties of these oxynitride glasses are examined in detail. Changes in hardness, density and glass transition temperature as a function of rare earth metal oxide additives are reported. Further, the effects of cationic field strength on properties of Ln-Si-Al-O-N glasses are presented. The compositional dependence on the properties is discussed and the implications of lanthanide additions as densification aids for silicon nitride based ceramics are also assessed.