Sialon composites

Over the last decade, considerable efforts have been committed to the toughening of sialons and substantial progress has been achieved using various reinforcements. According to the form of reinforcement, sialon composites can be classified as either particle reinforced, discontinuous fibre (whiskers/ short fibres) reinforced, or continuous fibre reinforced. 

Toughening mechanisms in a/p-sialon composites are similar to those operative in second-phase particle reinforced composites, but, rather than the deliberate addition of a second phase, a/P-sialon composites are fabricated by simultaneous crystallisation of the two solid solutions a- and P-sialon from a eutectic composition liquid. This requires careful design of the starting composition which is usually located within the (a + P)-sialon region of the a-sialon plane as illustrated in Fig. 18.1. 

The a/p phase ratio can be controlled just by changing the overall composition in most sialon systems. P-Sialon grains normally grow in elongated shape with a high aspect ratio (Lange, 1979 Wotting et al., 1986), and this 

1 Schematic illustration of a- and P-sialon phase regions on the a-sialon plane (M metal cation with a valence of 3+). 

Different starting powders and chemical compositions result in different phases, grain boundary states, and microstructures (see Table 18.1). For 

---

AS3. Ekstrom T (1997) a-SiAlON and a// -SiAlON composites Recent research. In Babini GN, Haviar M, Sajgalik P (eds) Engineering Ceramics 96 Higher Reliability through Processing, Kluwer Academic Publications, The Netherlands, p 147... 

Zhao, Z., Johnson, M., Shen, Z. (2002), Microstructure and mechanical properties of titanium carbonitride whisker reinforced P-sialon composites , Materials Research Bulletin, 37, 1175-1187. 

Section 18.2 gives a very brief introduction to sialons. Section 18.3 outlines the challenges to be overcome in order to make toughened and strengthened sialon products. Progress in developing sialon composites forms the main part of this chapter and is discussed in section 18.4, which deals with a/p-sialon composites, particle/whisker-reinforced sialons, and fibre-reinforced sialons. In the final section, several conclusions and suggestions for future work are summarised. 

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... 

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. 

Table 18.2 Effect of sintering temperature on the properties of Cf/sialon composites...

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. 

Table 18.3 Properties of carbon fibre/a-sialon composites with and without added Zr02...

Effect of heat treatment of SiC fibres on SiCf/sialon composites... 

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. 

There is also scope for the development of new techniques such as chemical vapour infiltration (CVI) (Caputo and Lackey, 1984 Caputo et al., 1985), normal chemical reaction bonding processes, laminar sialon composites, etc. More recently, laminated composites in non-oxide and sialons have demonstrated very promising results for strengthening (Goto and Kato, 1998) and even achieved a non-brittle failure behaviour accompanied by high damage tolerance (Yu and Krstic, 2003 Yu et al., 2005). 

Ekstrom, T., (1997), a-Sialon and a/(3-sialon composites recent research , in Babini, G.N., et al., Engineering Ceramics 96 Higher Reliability Through Processing, Dordrecht, Kluwer Academic, 147-167. 

Yu, Z.B. and Thompson, D.P., (1998), Preparation of carbon fibre reinforced Li-alpha-sialon composites , in Gibson, G., Consolidating New Applications, Seventh International Conference on Fibre Reinforced Composite, Cambridge, UK, Woodhead, 264-270. 

---