Pressureless sintered

The properties of siHcon carbide (4—6) depend on purity, polytype, and method of formation. The measurements made on commercial, polycrystalline products should not be interpreted as being representative of single-crystal siHcon carbide. The pressureless-sintered siHcon carbides, being essentially single-phase, fine-grained, and polycrystalline, have properties distinct from both single crystals and direct-bonded siHcon carbide refractories. Table 1 Hsts the properties of the hiUy compacted, high purity material. 

K. J. D. MacKenzie and R. H. Meinhold, Additive-assisted pressureless sintering of carbothermal p -sialon an X-ray and solid-state MAS NMR study. /. Mater. Chem., 1996, 6,821-831. 

The most common densification method is the gas-pressure sintering which guarantees better reproducibility and improved properties by only moderate increase in production costs compared to pressureless sintering. The first dense Si3N4 ceramics were produced by hot pressing [13]. Nowadays it is used for the preparation of specimens in materials development or for some applications where components of simple geometries and low quantities are required. 

Fig. 17a-d. Microstructures of //ss ceramics (plasma etched), a from a-rich starting powder, b, c, d from //-rich starting powders a and b pressureless sintered c hot pressed, and d sample c additionally heat treated... 

Yonezawa, T., Saitoh, S.-I., Minamizawa, M. and Matsuda, T. Pressureless sintering of silicon nitride composites , Composites Sci. Tech., 51 (1994) 265-269. 

Unlike fibre- or whisker-reinforced composites, particulate composites have the advantage of being compatible with conventional powder processing, and in many cases can be pressurelessly sintered. As with other ceramic microstructures, a myriad of other ingenious fabrication routes have also been reported, but these are too numerous and system-specific to describe here. This section merely outlines the main points of powder processing where the production of composites in chemically compatible systems (i.e. those in which the components do not react chemically with one another) differs from that of monolithic ceramics. 

This striking property of the Al203/SiC interface can be understood in terms of the observation of Ashby and Centamore [14] that the more refractory of two phases at an interface (the covalently bonded SiC in this case) controls the interface reaction because in general atoms in both phases must be involved in the reaction. The majority of the Al203/SiC interfaces in the nanocomposites have been observed to be free of any glassy phase, the presence of which would presumably allow alumina to be removed or deposited at the interface without the involvement of the SiC, and consequently much more rapidly. The introduction of an interfacial layer may be the source of the ability of sintering aids such as Y203 to enable these materials to be pressurelessly sintered [15, 16] (Fig. 4.2). 

Jeong, Y.K. and Niihara, K. Microstructure and mechanical properties of pressureless sintered Al203/SiC nanocomposites , NanostructuredMaterials 9 (1997) 193-196. 

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... 

For the powder bed method, a composition rich in P-SiAlON (named B2) was selected as a compact composition to observe compact-powder bed interaction. Two different powder bed compositions, a-SiA10N (100 wt%) and A1N BN mixture (50 50 wt%), were prepared. P-SiAlON-rich pellets were embedded into the powder bed compositions, as schematically shown in Fig. 6.21. Both green and sintered pellets were embedded into the same powder bed composition in order to compare the effect of presintering on the interaction. Sintering of the pellets was carried out under 22 bar nitrogen gas pressure at 1800°C for 1 hour. To understand the effect of pressure on the interaction zone, pressureless sintering was also carried out for comparison. 

Xu, X., Mei, S., Ferreira, J.M.F. (2003), Fabrication of a-SiAlON sheets by tape casting and pressureless sintering , Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, Portugal, www.mrs.org/publications/jmr/jmra/2003/ jun/012.html. 

Surface morphology of A Oa/LaPC composites by pressureless sintering. 

SEM images of fracture surface for 40 wt% AI203/LaP04 composite by pressureless sintering. 

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