Fibre whisker

Fibre-whisker- and particulate-reinforced ceramic composites... 

Incorporation of fibres, whiskers or particles in a ceramic/glass-ceramic matrix can result in a tough ceramic material. This happens because... 

Reinforcements in the form of continuous fibres, short fibres, whiskers or particles are available commercially. Continuous ceramic fibres are very attractive as reinforcements in high-temperature structural materials. They provide high strength and elastic modulus with high temperature-resistant capability and are free from environmental attack. Ceramic reinforcement materials are divided into oxide and non-oxide categories, listed in Table 3.1. The chemical compositions of some commercially available oxide and non-oxide reinforcements are given in Table 3.2 and Table 3.3. 

Over the last decade, considerable efforts have been committed to the toughening of sialons and substantial progress has been achieved using various reinforcements. According to the form of reinforcement, sialon composites can be classified as either particle reinforced, discontinuous fibre (whiskers/ short fibres) reinforced, or continuous fibre reinforced. 

Figure 11.7 Scanning electron microscopy images of marine beachrock and meteoric cayrock cements. (A) Blades of high-magnesium calcite on constituent particles in beachrock. Northeast Sapodilla Cay, southern Belize Barrier Reef. (B) Dissolution cavity is lined with blockyand bladed low-magnesium calcite in cayrock. Cay Bokel, Turneffe Islands, Belize. (C) Low-magnesium calcite needle fibre (whisker) cement in cayrock. Harry Jones Point, Turneffe Islands, Belize.

Ceramic particles, chopped fibres, whiskers and platelets have been used as discontinuous reinforcement in glass and glass-ceramic matrices. 

Natural fibres can be obtained from various parts of a plant, as shown in Table 13.2. Natural fibres can be continuous fibres, short fibres, whiskers or particles, as shown in Figure 13.4. Types of polymer composite based on shape and structure are divided into particle, fibre and structural. 

Carbon Formation. Steam reforming involves the risk of carbon formation by the decomposition of methane and other hydrocarbons or by the Boudouard reaction (reactions (7) -(10)). Reactions (7) - (8) are catalyzed by nickel (Rostrup-Nielsen, 1984a). The carbon grows as a fibre (whisker) with a nickel crystal at the tip. The methane or carbon monoxide is adsorbed dissociatively on the nickel surface (Alstrup, 1988). Carbon atoms not reacting to gaseous molecules are dissolved in the nickel crystal, and solid carbon nucleates at the non-exposed side of the nickel crystal, preferably from Ae dense (111) surface planes. Reaction (10) results in pyrolytic carbon encapsulating the catalyst. 

BORON FIBRE CARBON FIBRE--IRON WHISKER-... 

More recently, Stanicioiu, Chinta Hartner (1959) attempted to reinforce the cement with glass fibres, but this was not successful. The most serious study on the reinforcement of dental silicate cement was made by J. Aveston (in Wilson et al., 1972). Silicon carbide whiskers, carbon fibres and alumina powder were introduced into the cement mix. Unfortunately, the glass powder/liquid ratio had to be reduced, and the strength gained by reinforcement was thereby lost. It is clear that dental silicate cement cannot be strengthened by fibre or particulate reinforcement. 

The reactants were ultrasonically dispersed in ethylendiamine, then transferred to a Teflon-lined autoclave and gradually heated up to 160°C. The products were filtered out, washed with absolute alcohol, normal HC1 solution and water and finally dried in vacuum. In the product nano-fibres and near single-crystal whiskers were obtained. 

The graph shown in Figure 6.7 plots the strength versus the modulus of some typical fibres. Note the very special performance of the whiskers (single-crystal fibres). 

Whiskers are single-crystal fibres. They are very expensive and difficult to produce with only a few specific applications, for example, submicron gears or connectors. 

Experimental results. Some carbon fibre specimens reveal several orders of 001 particularly in electron diffraction patterns Figure 15 shows a plot of (3 against l2, equation (3), for an electron diffraction pattern from the skin region of a high-modulus material. L(oOl)> usually referred to as Lc, is 3.5 nm and a = 2%. A full description of electron-diffraction analysis in several similarly heterogeneous carbon fibres has been published (23). Figure 15 also includes a plot from the 001 electron diffraction profiles of a carbon whisker, an exceptionally perfect graphite material. This specimen, with an Lc of 10 nm, has zero distortion, and represents the only case where we have found no distortion in a fibrous specimen. 

It is well known that the Young s modulus of a composite can be calculated by the rule of mixtures for long-fibre reinforced material. In the case of whiskers, the rule of mixture is also applied to estimate the change of modulus (conventionally, reinforcements are added to improve the stiffness of a material, though for ceramic matrix composites this is not always the primary concern). 

Commercially available non-oxide ceramic reinforcements are in three categories continuous, discontinuous, and whiskers. The great breakthrough in the ceramic fibre area has been the concept of pyrolysing polymers under controlled conditions, containing the desired species to produce high-temperature ceramic fibres. Silicon carbide fibre is a major development in the field of ceramic reinforcements. 

Whiskers are normally obtained by vapour phase growth. They are monocrystalline, short fibres with extremely high strength because of their high aspect ratio (50 to 10 000). They have a diameter of a few microns, but they do not have uniform dimensions and properties. 

Whisker-reinforced glass-ceramic matrices are expected to find several applications in automotive components, metal forming, cutting tools, etc., due to their low thermal expansion, high thermal shock resistance, high reliability and low material and processing costs. Some industrial applications for continuous fibre-reinforced ceramic matrix composites (CMCs) are listed below. 

Briggs, A. and Davidge, R.W., in Whisker-and Fibre-toughened Ceramics, ASM International, Materials Park, OH, p. 153 (1988). 

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. 

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