Functionally graded SiAION ceramics

Functionally graded SiAION ceramics can be produced by lamination (powder and tape), powder bed, controlling the sintering conditions, infiltration, slip casting and so on. Some of these production techniques will be discussed in the following sections. 

In compositional design of functionally graded SiAION ceramics, selection of cations and a P phase ratios are very important parameters. Type of cations affects the densification and diffusion between the layers. They also control the thermomechanical properties of SiAION ceramics. 

Both XRD and SEM studies support the results of hardness measurement, as illustrated in Fig. 6.13. Hardness values decreased gradually from 19 to 15 GPa, over a distance from a-SiAION to P-SiAlON rich sides for each functionally graded SiAION ceramic. 

The results obtained revealed that the gradient layer was longer for the B3-A1 laminate than for the B2-A1 laminate.50 This can be explained by the fact that the larger amount of liquid phase in the B 3 layer facilitates the diffusion between the layers. The resulting functionally graded SiAION ceramics may be potential candidates for wear applications. 

In the literature, there has been no report about production of functionally graded SiAION ceramics by tape casting. The main advantage of this method with respect to others is that continuous change in composition, microstructure and mechanical properties can be obtained by stacking controlled layer thicknesses of different tape compositions. 

In another study, Mandal and co-workers produced functionally graded SiAION ceramics using the powder bed method.50 In their study, P-SiAlON compacts were embedded in two different homogeneously mixed powder bed compositions, a-SiA10N (100 wt%) and A1N BN (50 50 wt%). The effects of powder bed composition and pressure on the formation of a-SiAlON on the compact surface were investigated. 

As a result, it is obvious that the powder bed technique is not as effective for the production of functionally graded SiAION ceramics as the lamination method. 

Jalifl, N., Kara, A., Kara, F., Mandal, H. (2004), Development of laminar type functionally graded SiAION ceramics , Key Eng. Mater., 264-268, 1095-1098. 

Jalifl, N. (2002), Functionally graded SiAION ceramics , BSc Thesis, Anadolu University, Turkey. 

---