Silicon Nitride

Beta-SiAlON type ceramic

What are SiAION 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 iso-structural 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 later 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 of Si by Al with corresponding atomic replacement of N by O, to satisfy valency 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 Ceramics 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-SiAlON the bond strength is 50% stronger than in Al2O3. Thus SiAlONs intrinsically have better properties than both Si3N4 and Al2O3.

β-SiAlON has the general formula Si6-zAlzOzN8-z where z varies between 0-4.2 and requires a sintering additive such as yttria (Y2O3), magnesia (MgO) or a rare earth oxide in order to densify. 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. βSiAlON is thus more easily densified than Si3N4 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.

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Mechanical properties

The mechanical properties of {beta}-SiAlON ceramics joined using {beta}-SiAlON-glass-forming adhesives consisting of mixed Si{sub 3}N{sub 4}, Y{sub 2}O{sub 3}, Al{sub 2}O{sub 3}, and SiO{sub 2} powders are described. Use of adhesives with a {beta}-SiAlON:glass ratio of 60:40 gave an optimum joint strength of 650 MPa in four-point bending mode, i.e., 85% of that of unbonded material, when joining was carried out at 1,600-C for 10 min, under an applied uniaxial pressure of 2 MPa. Bonding pressures in excess of 2 MPa caused excessive compressive creep distortion during the joining operation. The strengths of postjoined HIPed material and HIPed, unbonded material, differed by only 4%, i.e., 975 and 1,010 MPa, respectively, which indicates that HIPing reduces the size of critical defects in the joint. Fracture toughness of the joint also improved upon HIPing.

(Mechanical properties of {beta}-SiAlON ceramics joined using composite {beta}-SiAlON-glass adhesives, OSTI.GOV, DOI:  10.1111/j.1151-2916.1995.tb08428.x)

There are more than 20 different chemical compositions Sialon ceramics.