The Characterization of α’ - Sialons and the α - β Relationships in Sialons and Silicon Nitrides

Abstract

The preparation and characterization of sialons with structures based on α-Si₃N₄ extends the science and technology of “ceramic alloying” and also suggests possible relationships between the α and β structures in both sialons and silicon nitrides. The α’ structure occurs in M-Si-Al-O-N systems and is derived from the Si₁₂N₁₆ unit cell by partial replacement of Si⁴⁺ by Al³⁺. Valency compensation is by modifier cations (Li, Ca, Y and all the rare-earth elements except La and Ce) occupying the interstices of the (Si, Al)-N network. Where a modifier oxide is used (e.g. CaO), some O may also replace N, and because there are only two available interstitial sites per unit cell, the α’-phases have the general composition M_x(Si, Al)₁₂(O, N)₁₆where x ⊁ 2 Minimum and maximum observed values of x are Y_0.3 and Ca_{1, 6}. The limit of replacement of nitrogen by oxygen is probably not more than one atom per unit cell e.g. CaSi₉Al₃ON₁₅.

The transformations α ⇌ β’ occur by chemical reactions. By analogy, the relationships between α’ and β’ support earlier proposals that α-silicon nitride is a defect structure with a range of composition that can accommodate small amounts of oxygen. This accounts for the observed variation in density of α, the relatively wide variation in unit-cell dimensions, the marked differences in properties between α produced by CVD and by SiO-N₂ interaction, and also for the thermodynamics of the Si-O-N system.