Infiltration processing

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. 

Zhou et al. obtained nitrogen-doped titanium dioxide replicas via a two-step infiltration process with natural leaves as templates [220]. The replicas inherited the hierarchical structures of the natural leaf at the macro-, micro-, and nanoscales. These materials showed enhanced light-harvesting and photocatalytic hydrogen evolution activities. The photocatalytic water splitting activity of the artificial leaf structures was eight times higher than that of titanium dioxide synthesized without templates. 

Table 1 Typical tensile properties of isothermal chemical vapor infiltration processed C/SiC...

Reactive fibers, 9 486-489 Reactive flame retardants, 11 474-479 brominated, ll 475-477t Reactive gases, 13 456 Reactive groups, types of, 9 178 Reactive hot melt butyl sealants, 22 44 Reactive hot melt polyurethanes, 22 37-38 Reactive hot melt silicones, 22 35 Reactive ion-beam etching (RIBE), 22 184 Reactive ion etching (RIE), 20 278 22 183 of lotus effect surfaces, 22 120 Reactive lead alloys, 14 779 Reactive liquid metal infiltration process, 16 168... 

Fig. 4.21 gives mean values for some organic substances in the Glatt infiltration system. The figure illustrates that tetrachlorethylene is not eliminated during the infiltration process. In the case of 1,4-dichlorbenzene and 1,3-dimethylbenzene (m-xylene) elimination occurs but this elimination is caused by biodegradation. 

In this chapter, we examine the various mechanisms that influence chemical redistribution in the subsurface and the means to quantify these mechanisms. The same basic principles can be applied to both saturated and partially saturated porous media in the latter case, the volumetric water content (and, if relevant, volatilization of NAPL constiments into the air phase) must be taken into account. Also, such treatments must assume that the partially saturated zone is subject to an equilibrium (steady-state) flow pattern otherwise, for example, under periods of heavy infiltration, the volumetric water content is both highly space and time dependent. When dealing with contaminant transport associated with unstable water infiltration processes, other quantification methods (e.g., using network... 

Other than in polymer matrix composites, the chemical reaction between elements of constituents takes place in different ways. Reaction occurs to form a new compound(s) at the interface region in MMCs, particularly those manufactured by a molten metal infiltration process. Reaction involves transfer of atoms from one or both of the constituents to the reaction site near the interface and these transfer processes are diffusion controlled. Depending on the composite constituents, the atoms of the fiber surface diffuse through the reaction site, (for example, in the boron fiber-titanium matrix system, this causes a significant volume contraction due to void formation in the center of the fiber or at the fiber-compound interface (Blackburn et al., 1966)), or the matrix atoms diffuse through the reaction product. Continued reaction to form a new compound at the interface region is generally harmful to the mechanical properties of composites. 

Let us consider two hypothetical phases in our composite, A and B, without specifying their physical state. They conld be a polymer melt and a glass fiber reinforcement during melt infiltration processing, a metal powder and ceramic powder that are being snbjected to consolidation at elevated temperatnre and pressure, or two immiscible polymer melts that will be co-extruded and solidified into a two-phase, three-dimensional object. In any case, the surface that forms between the two phases is designated AB, and their individual surfaces that are exposed to their own vapor, air, or inert gas (we make no distinction here) are labeled either A or B. The following three processes are defined as these surfaces interact and form ... 

Figure 3.35 Classification of chemical vapor infiltration processes. From Carbide, Nitride, and Boride Materials Synthesis and Processing, A. W. Weimer, ed. p. 563. Copyright 1997 by Chapman Hall, London, UK, with kind permission of Kluwer Academic Publishers.

Figure 4.14 Schematic diagram of the melt infiltration process for flow past a unidirectional fiber assembly.

Whiskers can be incorporated into the metallic matrix using a number of compositeprocessing techniques. Melt infiltration is a common technique used for the production of SiC whisker-aluminum matrix MMCs. In one version of the infiltration technique, the whiskers are blended with binders to form a thick slurry, which is poured into a cavity and vacuum-molded to form a pre-impregnation body, or pre-preg, of the desired shape. The cured slurry is then fired at elevated temperature to remove moisture and binders. After firing, the preform consists of a partially bonded collection of interlocked whiskers that have a very open structure that is ideal for molten metal penetration. The whisker preform is heated to promote easy metal flow, or infiltration, which is usually performed at low pressures. The infiltration process can be done in air, but is usually performed in vacuum. 

This polymer precursor (1) requires relatively inexpensive starting materials, (2) is quite stable in air, (3) offers good processability for polymer infiltration processing of composites, (4) provides excellent SiC ceramic yields and (5) high purity with controllable microstructures. However, one important drawback is the use of costly LiAlHzt. This polymer is now available commercially (Starfire Inc., NY). 

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

In this chapter, we describe the synthesis and characterisation of the microstructure and properties of layered-graded alumina-matrix composites through liquid infiltration. This approach is relatively simple and offers excellent control over the depth of the graded layer. The presence of a graded dispersion of reinforced particles in the alumina matrix has a profound influence on the physical and mechanical properties of the composites. An overview of the infiltration kinetics and the use of the infiltration process as a new philosophy for tailoring novel graded ceramic systems are also presented. 

Liquid-phase infiltration of preforms has emerged as an extremely useful method for the processing of composite materials. This process involves the use of low-viscosity liquids such as sols, metal- or polymer-melts. Using this infiltration process, it is possible to design new materials with unique microstructures (e.g. graded, multiphase, microporous) and unique thermomechanical properties (graded functions, designed residual strains and thermal shock). 

Liquid infiltration processing of layered-graded alumina-matrix composites. 

The abundance of mullite in the mullite/ZTA system increased with increasing infiltration time [Low et al., 1993]. The density (p) and mullite content of the sintered sample as a function of infiltration time are shown in Table 5.2. The results suggest that the infiltration process was time (t) dependent and diffusion-controlled with the infiltration front travelled as a function of t112. The content of mullite was greatest near the surface and decreased sharply towards the core of the sample. 

Marple, B.R. Green, D.J. (1991) Mullite/alumina particulate composites by infiltration processing III, Mechanical properties. J. Am. Ceram. Soc. 74, 2453-2459. 

Pratapa, S. Low, I.M. (1998) Infiltration-processed functionally-graded AT/alumina-zirconia composites II, Mechanical properties. J. Mater Sci. 33, 3047-3053. 

Singh, M. Low, I.M. (2002b) Layered and graded alumina-based hybrid composites by infiltration processing. Key Engineering Materials 224-226, 493 4-98. 

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. 

Figure 13.5 Schematic representation of chemical vapour infiltration process.

---