Slurry infiltration

The teehnique used for slurry infiltration is the same as that used for glass (Section 15.3.2.2) and to improve the densification, it is normal to adopt a multiple slurry impregnation process. The technique is most effective for glass ceramics, although recent attention has concentrated on SiC reinforcement. A limitation when coating continuous fiber is that the impregnated and dried tow has an excess of matrix particles, which persists after densification. 

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Schematic of the slurry infiltration process for making a fibre-reinforced glass and glass-ceramic composite (reproduced by permission of Woodhead Publishing Limited)74. 

To reduce residual stresses, careful control of the slurry infiltration processing parameters is necessary, but this may not be enough. Alternative procedures such as incorporating additions of Zr02 (Guo etal., 1982 Zhang and Thompson, 1995 Yu et al., 2002a) as well as introducing other phases into the matrix have been explored. 

In the sol gel process, smaller particles than in the slurry infiltration method are used and can be more easily homogenized into a system, which is nearly a monodisperse. This allows better penetration of the matrix into the fiber interstices, with less fiber damage and, moreover, is achieved at a lower working temperature. Unfortunately, the downside is that excessive shrinkage occurs, necessitating either repeat infiltrations or using a combination of sol gel and slurry methods [1]. 

In the fabrication process of three dimensional carbon fiber reinforced SiC matrix composite, Suzuki and Nakano [207] applied PCVI as the final densification process for the specimen, which was made by the joint process of slurry infiltration and organosilicon polymer pyrolysis. The open porosity and bulk density of the specimen changed from 5.3% and 2.63 g-cm (relative density of 94%) to 3.5% and 2.67 g cm (relative density of 95%) by the apphcation of PCVI (1173 1223K, total 90,000 pulses). The flexural strength of the specimen increased over 20% (mean value =153 MPa, maximum value = 174 MPa). 

Carbon fiber reinforced Si.Al.O.N composite materials were prepared by Dodds et al [219] using slurry infiltration of continuous carbon fiber tows, winding, filter pressing and uniaxial hot pressing using a graphite die in air. 

Hillig WB, Ceramic Composites by Infiltration, Ceramic Eng Sci Proc, 5(7 ), 674-683, 1985. Grenet C, Plunkett L, Veyret JB, Bullock E, Carbon fibre-reinforced silicon nitride composites by slurry infiltration, Evans AG, Naslain R eds., High-Temperature Ceramic-Matrix Composites II, Manufacturing and Materials Development, Ceramic Trans, American Ceramic Soc Inc., Santa Barbara, 58, 125-130, Aug 21 24 1995. 

Slurry infiltrated fibrous castable (SIFCA). A refractory castable containing up to 16% steel fibres. The fibres are filled into the mould first, followed by a fine-grained castable, this approach overcoming previous difficulties of mixing and placing which limited the fibre content to c. 2 vol%... 

A special type of this material is called SIFCA slurry infiltrated fiber-reinforced castable. This is a composite material of ceramic matrix with calcium aluminate cement and with aggregate made of aluminum oxide, mullite, zircon and calcined fireclay. The matrix is reinforced with stainless-steel fibres. SIFCA is used to made pre-cast elements for refractory structures, where temperature can rise up to 1100°C. Heat curing is often used during the pre-casting. Another kind of similar material is called SIMCON slurry infiltrated mat concrete in which arrays of single fibres are replaced by a system of steel mats for better and easier distribution of reinforcement (Murakami and Zeng 1998). 

As for other classes of composite materials, there are many processes that can be used to make CMCs. Key considerations in process selection are porosity and reactions among reinforcements, reinforcement coatings, and matrices. The most important processes for making CMCs at this time are chemical vapor infiltration, melt infiltration, preceramic polymer infiltration and pyrolysis (PIP), slurry infiltration, sol-gel, hot pressing, and hot isostatic pressing. In addition, there are a number of reaction-based processes, which include reaction bonding and direct metal oxidation ( Dimox ),... 

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