Preform infiltration

By assuming a contact angle 0 = 0° [Einset, 1996 Ligenza Bernstein, 1951] the pore radius of the preform can be calculated if the height and time of infiltration are known. The rate of infiltration is determined from the slope in Fig. 5.2(a) and then from this slope the pore radius can be found. From the measurements, it was found that the pore radius of the alumina preform is 0.015 0.001 pm. Similarly the pore radius found from alumina preform infiltrated with TiCl4 is 0.018 0.002 pm [Manurung, 2001]. The errors in the radii only reflect the experimental uncertainty in the measured values for surface tension and viscosity. However, the measured pore radius is an order of magnitude smaller than the pore radius determined from porosimetry and SEM (Fig. 5.1). 

Precursors must have different properties " " " (1) a high content of the final elements (mostly aluminum, silicon, zirconium, titanium, phosphorus), (2) a low content of health hazardous elements and elements that corrode the equipment (e.g., chlorine, sulfur), (3) a viscosity adapted to the process low viscosity for preform infiltration, medium viscosity for spinning and coating, (4) a controlled precursor-ceramic transformation (bubbling is researched for foams but not for dense parts), (5) the ability to be mixed with other precursors or to be processed ( good hydrolysis rate), and (6) low cost. 

Fabrication by LPI A carbon-fiber preform infiltrated with resin (e.g., PCS) is pyrolyzed to form SiC. Infiltration and pyrolyzation are repeated a number of times until the pores are narrow enough so that further infiltration ceases. Finally, the body is heated to temperatures between 1000 and 1500 °C for crystallization of the SiC matrix. Densification takes less time and costs less than densification by CVI. Fabrication by LSI The LSI/MI processes can generally be subdivided in three main steps ... 

Preform infiltration This process usually involves the infiltration of a preform of fibers or particulate reinforcement with a liquid metal. Depending on the wetting characteristics and the type of alloy, either pressureless or pressure-assisted infiltration can be used. 

Continuously Reinforced MMCs For the production of continuous fiber-reinforced MMCs, the above-described methods of preform infiltration also apply. 

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. 

Reactive Hquid infiltration (45,68,90,93,94) is similar to the CVI process used to make RBSN. Driven by capillarity, a reactive Hquid infiltrates a porous preform and reacts on free surfaces. Reactive Hquid infiltration is used to make reaction bonded siHcon carbide (RBSC), which is used in advanced heat engines and as diffusion furnace components for semiconductor wafer processing. 

Recently, a new procedure was reported for the preparation of nanoporous polymeric spheres (NPSs) with well-controlled structure via the LbL infiltration and coating of MS spheres with preformed polymers (Figure 7.7) [69]. A main advantage of this approach is that it offers a general and versatile route to the preparation of NPSs of different and tailored compositions, as it is based on electrostatic assembly [69,109]. 

The majority of work done on VGCF reinforced composites has been carbon/carbon (CC) composites [20-26], These composites were made by densifying VGCF preforms using chemical vapor infiltration techniques and/or pitch infiltration techniques. Preforms were typically prepared using furfuryl alcohol as the binder. Composites thus made have either uni-directional (ID) fiber reinforcement or two-directional, orthogonal (0/90) fiber reinforcement (2D). Composite specimens were heated at a temperature near 3000 °C before characterization. Effects of fiber volume fraction, composite density, and densification method on composite thermal conductivity were addressed. The results of these investigations are summarized below. 

FIGURE 6.11 Diagram of the processing technique used to prepare Cu-Ce02-YSZ anodes for direct oxidation of hydrocarbon fuels by preparing a porous preform of YSZ and then infiltrating it with cerium nitrates to form ceria and then with copper nitrates to form metallic copper [84]. Reprinted from [84] with permission from Elsevier. 

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. 

In Section 3.4.2, we introdnced the concept of chemical vapor infiltration, CVI, in which a chemical vapor deposition process is carried out in a porous preform to create a reinforced matrix material. In that section we also described the relative competition between the kinetic and transport processes in this processing technique. In this section we elaborate npon some of the common materials used in CVI processing, and we briefly describe two related processing techniques sol infiltration and polymer infiltration. 

Chemical Vapor Infiltration (CVI). Recall from Section 3.4.2 that CVI is primarily nsed to create ceramic matrix composites, CMCs. Fabrication of CMCs by CVI involves a sequence of steps, the first of which is to prepare a preform of the desired shape and fiber architecture. This is commonly accomplished by layup onto a shaped form of layers from multifilament fibers using some of the techniques previously described, such as filament winding. 

The third step is to heat the preform in a sealed chamber and pass a mixture of gases into the chamber that will react when they contact the hot fibers to form and deposit the desired chemical constitnents of the matrix. The deposition rate is very slow and becomes even slower as the thickness of the deposit increases and the permeability of the preform decreases. To achieve high levels of densification, the partially densified part is removed from the CVl chamber, the surface is machined to reopen pore channels, and the part is returned to the chamber for further infiltration. This procedure is typically repeated a number of times to achieve a composite density of over 80% (<20% porosity). 

There are many methods to manufacture a CMC. Only a small selection is discussed in this section. Let us first have a look at the production process of a SiC matrix reinforced with SiC fibres. First a model is made of fibres, the so-called preform, then CVI (Chemical Vapour Infiltration) is applied to produce a coating on the fibres in order to ensure a better attachment to the matrix. The next step is resin infiltration. After pyrolysis (heating to a high temperature without oxygen) a network matrix of the porous carbon arises. Silicon which has first been melted in an oven is then introduced into this network it reacts with the carbon to form the following matrix ... 

Lanxide process (a) infiltration of preform (b) wicking of liquid metal along grain boundaries (reproduced by permission of Woodhead Publishing Limited)74. 

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 into dry porous materials occurs due to capillary action. The mechanism of infiltrating liquids into porous bodies has been studied by many researches in the fields of soil physics, chemistry, powder technology and powder metallurgy [Carman, 1956 Semlak Rhines, 1958]. However, the processes and kinetics of liquid infiltration into a powdered preform are rather complex and have not been completely understood. Based on Darcy s fundamental principle and the Kozeny-Carman equation, Semlak Rhines (1958) and Yokota et al. (1980) have developed infiltration rate equations for porous glass and metal bodies. These rate equations can be used to describe the kinetics of liquid infiltration in porous ceramics preforms, but... 

There are a number of formulae which are relevant for modelling the infiltration kinetics of a liquid into preforms. The first equation to calculate the height of infiltration against time was formulated by Washburn (1921) ... 

Mercury porosimetry measurements for a partially sintered alumina preform showed a bimodal pore size distribution with neck diameter Dn = 0.15 pm [Manurung, 2001], As a comparison with the pore sizes and distribution of the preform measured by porosimetry, SEM micrographs (Fig. 5.1) were taken before and after infiltration. Based on SEM examination, the pores in the preform before infiltration ranged in size from r 0.1-0.5 pm. Assuming an average pore radius of 0.3 pm, this radius is approximately four times larger than the pore-neck radius (Dn = 0.15 pm, so pore radius = 0.075 pm) determined by mercury porosimetry. 

In order to successfully model the infiltration kinetics in terms of the effects of presintering temperature, type of infiltrant, infiltration environment, and multiple infiltrations, the pore radius of alumina preform (presintered at 1000°C) was measured using water as infiltrant, since the viscosity and... 

SEM micrograph of the partially sintered alumina preform (1000°C) showing the pore microstructure (a) before and (b) after infiltration with TiCh [Manurung, 2001]. 

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