Sialons

Silicon Aluminum Oxynitride (SiAlON) Ceramics

Silicon aluminum oxynitride (SiAlON) ceramics are advanced ceramic alloys based on silicon (Si), aluminum (Al), oxygen (O), and nitrogen (N). Developed in the 1970s, these materials were created to overcome the fabrication challenges associated with silicon nitride (Si₃N₄).

As alloys of Si₃N₄, SiAlONs exist in three basic forms, each isostructural with one of the two common forms of Si₃N₄: beta (β) and alpha (α), plus silicon oxynitride. The relationship between SiAlON and Si₃N₄ is similar to that between brass and pure copper. Just as copper atoms are replaced by zinc to create a superior alloy, in SiAlON there is substitution of Si by Al with corresponding atomic replacement of N by O to satisfy valency requirements. The resulting solid solution (SiAlON) exhibits superior properties compared to pure silicon nitride.

The fundamental structural unit of Si₃N₄ is the SiN₄ tetrahedron, analogous to the SiO₄ structural units in silicates. These tetrahedra are linked together into a rigid three-dimensional framework by sharing corners. The Si-N bonds are short and exceptionally strong, giving rise to a strong, rigid, compact structure that accounts for many of the important properties of Si₃N₄.

β-SiAlON Ceramics

β-SiAlON is based on the atomic arrangement found in β-Si₃N₄. In this material, Si is substituted by Al with corresponding replacement of N by O. Up to two-thirds of the silicon in β-Si₃N₄ can be replaced by Al without causing structural changes. The chemical replacement involves changing Si-N bonds for Al-O bonds. While the bond lengths are approximately equal, the Al-O bond strength is significantly higher than that of Si-N. In SiAlON, aluminum is coordinated as AlO₄ rather than AlO₆ as in alumina (Al₂O₃). Therefore, in β-SiAlON, the Al-O bond strength is 50% stronger than in Al₂O₃, giving SiAlONs intrinsically superior properties compared to both Si₃N₄and Al₂O₃.

β-SiAlON has the general formula Si_6-zAl_zO_zN_8-z where z varies between 0–4.2. It requires a sintering additive such as yttria (Y₂O₃), magnesia (MgO), or a rare earth oxide to achieve densification. As a solid solution, β-SiAlON has a lower vapor pressure than Si₃N₄, resulting in more liquid formation at lower temperatures. This makes β-SiAlON more easily densified than Si₃N₄ using conventional sintering techniques. Additionally, the lower vapor pressure reduces decomposition at high temperatures, making SiAlON thermodynamically more stable than Si₃N₄.

Commercial β-SiAlONs typically use yttria as a sintering aid. When sintered above 1700°C, elongated hexagonal β-SiAlON grains precipitate and grow in the oxynitride liquid phase formed from sintering additives such as yttria, alumina, silica, and aluminum nitride. During cooling, the liquid phase forms a refractory intergranular glass. This microstructure, with its elongated β grains, is characterized by high strength and toughness.

The fine beta-sialon (β) grains surround small pockets of glass. Through controlled processing, it is possible to convert the intergranular glass to yttrium aluminum garnet (YAG), which is a refractory crystalline phase. This transformation provides better high-temperature properties, allowing the material to retain its strength at temperatures up to 1350°C.

α-SiAlON Ceramics

The second form of Si₃N₄ with which SiAlON is isostructural is α-Si₃N₄. The stacking structure in α-Si₃N₄ differs from β-Si₃N₄ in that the long channels running through the β structure are blocked at intervals, creating a series of interstitial holes. Each Si₁₂N₁₆ unit cell contains two interstitial holes. In α-SiAlONs, Si in the tetrahedral structure is replaced by Al with limited substitution of N by O. Valency requirements are satisfied by modifying cations occupying the interstitial holes. This allows cations such as yttrium (Y), calcium (Ca), lithium (Li), and neodymium (Nd) to be incorporated into the structure. Consequently, α-SiAlON has the general formula Me_xSi_12-(m+n)Al_m+nO_nN_16-n where x represents the cation content.

The α-SiAlON phase region can be illustrated using a Jänecke prism, which is the most convenient way to show the five-component SiAlON system (Me-Si-Al-O-N). The α-SiAlON phase region appears as a shaded region in the Si₃N₄-YN·Al₂O₃-AlN·Al₂O₃