Thermal SiAlONs

Because of its high toughness and good thermal shock resistance, test results indicate the possibility of using square-, triangular-, and diamond-shaped tools of SiAlON for machining superaHoys in the intermediate speed range (ca 150 m /min) where only round tools are used currently with other ceramics. 

Materials made of siHcon nitride, siHcon oxynitride, or sialon-bonded siHcon carbide have high thermal shock and corrosion resistance and may be used for pump parts, acid spray nozzles, and in aluminum reduction ceUs (156—159). A very porous siHcon carbide foam has been considered for surface combustion burner plates and filter media. It can also be used as a substrate carrying materials such as boron nitride as planar diffusion source for semiconductor doping appHcations. 

SiAlON (Si3N4-Al203) ceramics reinforced with BN powder are used as break rings in continuous casting and for nozzles. The addition of BN provides better thermal shock resistance [127]. 

With increasing zvalue the density of P-SiAlON decreases linearly, similarly lowering Young s modulus, strength, thermal conductivity, hardness and fracture toughness.30 On the other hand, for low zvalues (z< 1) the hardness and fracture toughness increase and when z > 1 they decrease. It has been also depicted that glass-free microstructures could be obtained in P-SiAlON polycrystals with substitutional level z> 2.31... 

As with other ceramic composites, the combination of a- and/or P-sialon with reinforcement agents results in sialon composites. This simple and obvious statement encompasses many factors which must be taken into account for successfully fabricating composites with a designed microstructure and improved properties (Prewo, 1989). For sialon matrix composites, the most important factors are physical compatibility including Young s modulus, elastic strain (Kerans and Parthasarathy, 1991) and thermal expansion coefficient (Sambell etal., 1972a, b), and chemical compatibility between sialon matrix... 

Micro-crack induced by non-uniform distribution of carbon fibres and thermal mismatch between Cf and sialon in the Cf/a-sialon composite (reprinted from J. Eur. Ceram. Soc., 22(2), Yu Z B, Thompson D P and Bhatti A R, Synergistic roles of carbon fibres and Zr02 particles in strengthening and toughening Li-a-sialon composites, 225-235 (2002). Copyright 2002, with permission of Elsevier). 

It is worth pointing out that carbon fibre itself has anisotropic thermal expansion properties, and therefore this mismatch between the carbon fibres and the a-sialon matrix should be considered in both the radial and axial directions when carbon fibres are unidirectionally aligned in the composite. The thermal stress caused by thermal expansion differences between the carbon fibres and the matrix in the radial (cr) and axial (oa) directions can be estimated from the formulae (Chawla, 1993 Kerans and Parthasarathy, 1991) ... 

Thermal mismatch between SiC fibres and a sialon matrix is also an issue which can cause poor mechanical performance, even though there may be excellent chemical and physical compatibility between the fibres and the... 

Therefore, in SiC fibre-reinforced sialon composites, thermal treatment of fibres, thermal mismatch and chemical reactions between the fibre and the sialon matrix are significant factors affecting the interfacial bonding in these composites. The sintering additive plays an important role in controlling the nature of the interface and requires careful selection. 

Shen, Z.J., Ekstrom, T. and Nygren, M., (1996a), Homogeneity region and thermal stability of neodymium-doped a-sialon ceramics , J. Am. Ceram. Soc., 79(3), 721-732. 

The sialons are a group of ceramic compounds of increasing technological importance for their thermal, chemical and mechanical properties. These are similar to Si3N4 but can be more readily tailored to specific requirements because of the wide range of possible sialon compositions. 

The thermal development of amorphous sialon fibres from polyaluminocarbosilane gels at 1000°C and their crystallisation to p-sialon fibres at 1400-1500°C has been studied by Al and Si NMR (Soraru etal. 1993). The gel precursor shows the typical 3-peak Al spectrum, with resonances at 3.2, 32 and 57.4 ppm but the amorphous fibres formed at 1000°C contain only tetrahedral Al. On crystallisation of p-sialon, the tetrahedral Al resonance broadens due to the occurrence of mixed AIO4 - xNx sites (Soraru eta/. 1993). 

H. Yoshimatsu, M. Mitomo and H, Mihashi, Preparation of Sialon Powder from Mixture of Si02 and AI2O3-2H2O by Thermal Carbon Reduction, y. Ceram. Soc. Jpn., 95, 28-32 (1987). 

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