SiAION ceramics

SiAlONs is a general name for a large family of the ceramic alloys based on silicon nitride.13 They were first discovered independently at about the same time (1971-72) in the United Kingdom at the University of Newcastle-upon-Tyne (by Jack and Wilson14) and in Japan (by Oyama15). 

1 The p-Si3N4 structure is obtained by ABAB... stacking of silicon and nitrogen atoms. 

Silicon nitride is a highly covalent bonded compound with self-diffusion coefficient of the nitrogen atoms of 6.3 x 10 2(1 cm2/s at 1400°C.22 Therefore, densification without any sintering additives is nearly impossible. In 1961, Deeley and Herbert.23 was the first to report that Si3N4 ceramics could be 

Pressureless sintering can be employed to fabricate complex shapes, and significant stabilization can be achieved by using a protective powder bed while the products generally indicate low density and the process requires large amounts of additives for densification.24 

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Another method is to apply isostatic pressure and hot-isostatic pressing (HIP), now being another established technique. This technique is a very attractive because it offers possibilities of making dense SiAION ceramics with a negligible residual glassy grain boundary phase and hence better high-temperature properties. However, HP and HIP techniques are very costly. 

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. 

Chen, L., Kny, E., Groboth, G. (1998), SiAION ceramics with gradient microstructures , Surface and Coating Technology, 100-101, 320-323. 

C. F. Chen and T. Y. Tien, High temperature mechanical properties of SiAION ceramics Microstructural effect. Ceram. Eng. and Sci Proc. 8 (7-8), 778-795 (1987). 

Translucent and Toughened Dy-a-SiAION Ceramics with LiF as Sintering Additive... 

TRANSLUCENT AND TOUGHENED Dy-a-SiAION CERAMICS WITH 303 LiF AS SINTERING ADDITIVE... 

I-W. Chen and A. Rosenfianz, A tough SiAION ceramic based on a-Si3N4 with a whiskerlike microstructme . Nature, 389, 1997, 701-704. 

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