Fracture SiAlONs

Siaions consist of three-dimensional arrays of (Si—Al) (O.N)4 tetrahe-dra. These oxynitrides are traditionally fabricated with silicon nitride An example is beta-sialon, where the O and Si are partially replaced by N and Al, respectively. Advanced sialons are now being researched to enhance fracture toughness and iinpioved cieep properties,... 

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... 

Although fracture toughness can be increased, particle- or whisker-reinforced sialon composites generally show brittle behaviour and low damage tolerance this is in contrast to fibre-reinforced sialons which exhibit non-catastrophic failure. 

As shown in Table 18.2, both the volume content of Cf and the sintering temperature affect the properties and fracture behaviour of Cf/a-sialon... 

Micrographs of fracture surfaces of continuous SiCf/a-sialon composites sintered with (a) YAZ, (b) LAS, and (c) NLAS additives (reprinted from Composites Part A, 33(5), Yu Z B, Thompson D P and Bhatti A R, Fabrication and characterisation of SiC fibre reinforced lithium-a-sialon matrix composites, 621-629 (2002). Copyright 2002, with permission of Elsevier). 

Fig. 20. Room-temperature fracture toughness as a function of volume fraction of the second phase for SiAlON compositions containing garnet and cordierite. The specimens were hot-pressed [32],...

A kind of translucent but tough Dy-SiAlON was successfully prepared by HP-sintered at 1600-1650 °C for 60-90 min using LiF as a co-additive to reduce sintering temperature. The resultant samples possessed a higher transmittance of 65% and also kept a higher fracture toughness of 5.21 MPa m at the same time. 

Better understanding of the microstructural formation of the oc -SiAlON materials in the future will be necessary for the improvement of the mechanical properties, especially fracture toughness and strength, and the wider application of this group of materials, having higher hardnesses than the P-Si3N4 and P-SiAlON materials (see Fig. 16). 

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