β-sialon is produced by International Syalons using yttria (Y2O3) as a sintering aid and marketed under the trade name Syalon 101. During sintering at temperatures above 1400°C, the oxides react to form an yttrium-silicon-aluminium-oxynitride liquid which is necessary for densification. This then forms an intergranular glass on cooling. Syalon 101 is a fully dense ceramic characterised by high strength and toughness.
What are Sialon Ceramics?
Sialons are ceramic alloys based on the elements silicon (Si), aluminium (Al), oxygen (O) and nitrogen (N) and were developed in the 1970s to solve the problem of silicon nitride (Si3N4) being difficult to fabricate.
As alloys of Si3N4, sialons exist in three basic forms. Each form is isostructural with one of the two common forms of Si3N4, beta (β) and alpha (α) and with silicon oxynitride. The relationship between that of sialon and Si3N4 is similar to that between brass and pure copper. In the latter case, copper atoms are replaced by zinc to give a better and stronger alloy than the mother metal. In the case of sialon, there is substitution for Si by Al with corresponding atomic replacement of N by O, to satisfy valancy requirements. The resulting 'solution' (sialon) has superior properties to the original pure solvent (silicon nitride).
The fundamental structural unit of Si3N4 is the SiN4 tetrahedron, which is analogous to the SiO4 structural units in silicates. The tetrahedra are linked together into a rigid three dimensional framework by sharing corners. The Si-N bonds are short and they are very strong. This strong, rigid, compact structure is responsible for many of the important properties of Si3N4.
β-sialon is based upon the atomic arrangement existing in β-Si3N4. In this material, Si is substituted by Al with corresponding replacement of N by O. In this way up to two-thirds of the silicon in β-Si3N4 can be replaced by Al without causing a change in structure. The chemical replacement is one of changing Si-N bonds for Al-O bonds. The bond lengths are about the same for the two cases but the Al-O bond strength is significantly higher than that of Si-N. In sialon the Al is co-ordinated as AlO4 and not as AlO6 as in alumina (Al2O3). Therefore, in β-sialons the Al-O bond strength is 50% stronger than in Al2O3. Thus sialons intrinsically have better properties than both Si3N4 and Al2O3.
Micrograph showing the microstructure of Syalon 101. The fine beta-sialon (β) grains surround small pockets of glass (g).
As a solid solution, the vapour pressure of β-sialon is lower than that of Si3N4 and as a result the sialon will form more liquid at a lower temperature with Y2O3. Sialon is, thus more easily densified using normal sintering techniques. Furthermore, it should be noted that the lower vapour pressure of sialon reduces decomposition at high temperatures so that the sialon is thermodynamically more stable than Si3N4.
The second form of Si3N4 with which sialon is isostructural is α-Si3N4. The stacking structure in α-Si3N4 is different from β-Si3N4 in that the long 'channels' which run through the β structure are blocked at intervals. This gives rise to a series of intersticial holes. In α-sialons, Si in the tetrahedral structure is replaced by Al with limited substitution of N by O. Valancy requirements are satisfied by modifying cations occupying the intersticial holes. In this way cations of yttrium (Y), calcium (Ca), lithium (Li) and neodymium (Nd) for example can be incorporated into the structure.
International Syalons market an α-sialon under the trade name Syalon 050. α-sialons are intrinsically hard materials. Hardness can be simplistically related to bond energy density, which for α-sialons is high, giving extreme hardness. In addition, during sintering and subsequent heat treatment of α-sialons such as Syalon 050, the intergranular phase is taken up into the structure resulting in a dense, hard ceramic which is almost free of a grain boundary phase. This results in the materials properties being retained at up to 1400°C, with improved oxidation resistance.
The final form of sialon, O-sialon, is isostructural with silicon oxynitride (Si2N2O). The structure of Si2N2O consists of layers of Si3N4 rings joined by Si-O-Si bonds. In O-sialon, Al and O replace some Si and N atoms.
International Syalons don't manufacture an O-sialon as, although they have good refractory properties, they are low in strength and toughness and therefore are not bracketed as advanced ceramics. They are effectively a refractory with no benefits over β-sialon for example, other than cost, although the much superior properties of α- and β-sialon provide much greater service life, thus offsetting any supposed cost benefits.
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