PRESSURELESS

Alternatively, tows of fibers can be passed through a Hquid metal bath, where the individual fibers are wet by the molten metal, wiped of excess metal, and a composite wine is produced. A bundle of such wines can be consoHdated by extmsion to make a composite. Another pressureless Hquid metal infiltration process of making MMCs is the Prim ex process (Lanxide), which can be used with certain reactive metal alloys such as Al—Mg to iafiltrate ceramic preforms. For an Al—Mg alloy, the process takes place between 750—1000°C ia a nitrogen-rich atmosphere (2). Typical infiltration rates are less than 25 cm/h. 

The fermentation in these big vessels (max 3—5 brews) is normally performed pressureless, but several investigations have been made on fermentation under pressure and using somewhat higher temperatures in order to reduce the time needed still further. Some brewers agree with the statement that no yeast yet known can tolerate this extra stress and at the same time ferment a beet of equivalent excellent quaUty, but new selected yeast strains (genetic engineering) in the future may be better suited to these conditions. 

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. 

Lanxide A process for making composites of metals with oxides. A molten metal reacts with an adjacent oxidant and is progressively drawn through its own oxidation product so as to yield a ceramic/metal composite. Fibres or other reinforcing materials can be placed in the path of the oxidation reaction and so incorporated in the final product. The Lanxide Corporation was founded in 1983 in Newark, DE, to exploit this invention. In 1990 it formed a joint venture with Du Pont to make electronic components by this process. Variations are Dimox (directed metal oxidation), for making ceramic metal composites, and Primex (pressureless infiltration by metal), for making metal matrix composites. 

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. 

---