Platelet-reinforced

Table 3. Platelet Reinforcements for Ceramic-Matrix Composites...

A problem arises in using platelet reinforcements if their naturally mechanically weak crystallographic direction is aligned perpendicular to the crack front. The platelets easily fracture in this orientation. Further research is needed to grow platelets with favorable crystallographic orientations. 

Platelet activating factor, 4 85 Platelet activation, 4 82, 84 Platelet aggregation, 4 82, 85 Platelet reinforcement, 5 554, 555... 

More recently, ceramic composite materials have been described that incorporate zirconium diboride platelet reinforcements in a zirconium carbide matrix [34, 35], These materials are prepared by the directed reaction of molten zirconium with boron carbide (B4C) to form a ceramic material composed of zirconium diboride platelets in a zirconium carbide matrix with a controlled amount of residual zirconium metal. 

The kinetics of formation of this zirconium diboride platelet reinforced zirconium carbide have been discussed, as have possible formation mechanisms [36] and detailed microstructural and orientation relationships between the phases [37]. These materials, in addition to being very refractory, are quite hard. Potential applications typically involve wear resistance, either at low to moderate temperatures or for short times at very high temperatures, such as in biomedical and rocket nozzle or rocket motor application [35]. 

W. B. Johnson, T. D. Claar, and G. H. Schiroky, Preparation and processing of platelet reinforced ceramics by the directed reaction by zirconium with boron carbide. Ceram. Eng. Sci. Proc. KK7-8), 588-598 (1989). 

W. B. Johnson, E. Breval, and A. S. Nagelberg, The kinetics of formation of a platelet reinforced ceramic composite prepared by the directed reaction of zirconium with boron carbide. J. Am. Ceram. Soc. 74 (9) 2093-2101 (1991). 

V. A. Ravi, T. D. Claar, J. A. Hornor, and W. B. Johnson, Platelet reinforced ceramics for severe thermal shock applications. In Composites for High Temperature Applications (V. A. Ravi, ed.), pp. 175-183. Metallurgical Society, Warrendale, PA, 1992. 

Fig. 8 A scanning electron microscope micrograph of microwave-joined MaCor and hydroxyapatite, joined at 1020°C for 20 min in a single-mode 2.45 GHz microwave cavity. (MaCor is a mica-platelet reinforced glass ceramic and HAP is a bioceramic material.) (From Ref. f Reprinted with permission of The American Ceramic Society, www. ceramics.org. Copyright 2003. All rights reserved.)...

In 1997, Lenk et. al. described a thermoplastic binder system consisting of polyethylene, paraffin, wax and surfactants p enk 97]. Various binder components for injection molding, extrusion and hot molding were tested for the fabrication of bars, tubes, discs, rings and balls of SiC-platelet-reinforced SiC composites. Good alignment of the SiC platelets in the matrix made it possible to improve the densification of the composite. 

Figure 1.6 Microstructure of a SiC platelet-reinforced alumina optical micrograph. (Courtesy of Matt Chou.)...

Platelets are particles that are constrained in one dimension. They are commercially important because this is the shape of clay particles and mica. Another example of platelets previously encountered is SiC, which forms as flat hexagonal crystals by the Acheson process. An in situ process has been developed to produce platelet-reinforced-intermetallic composites. The reaction is... 

Dense SiC platelet reinforced silicon carbide composites were recently fabricated [52]. The highest sintered densities of 97-98% TD were achieved with 20% platelet contents using hot molding as the thermoplastic forming technique [53]. 

FIGURE 5. The extrusion process for the fabrication of glass matrix composites with chopped fibre or platelet reinforcement ([42]). 

FIGURE 6. The dependence of the mechanical properties of alumina platelet reinforced borosilicate glass matrix composites on the platelet volume fraction (a) Young s modulus, (b) fracture strength and (c) fracture toughness... 

The variation of the elastic modulus, fracture strength and fracture toughness of a model alumina platelet-reinforced borosilicate glass matrix composite with the volume fraction of platelets is shown in Figure 6 [17,128]. The material exhibited a pore-free matrix, uniform distribution of the platelets and strong matrix/platelet interfacial bonding. In Figure 6 both... 

In the platelet reinforced glass composite referred to above, the thermal expansion coefficient of the inclusions (alumina, a,- = 8 10 /°C) is higher than that of the borosilicate matrix (a = 3.3 10 /°C), resulting in a hoop compression stress in the matrix around the platelets [128], Such a stress field will deflect the crack and cause it to travel around the platelet. These residual compressive stresses in the matrix will also contribute to the strengthening of the material (see Figure 6) since they will reduce the effect of any externally applied tensile stresses. 

R. I. Todd, A. R. Boccaccini, R. Sinclair, R. B. Yallee and R. J. Young, Thermal Residual Stresses and Their Toughening Effect in AI2O3 Platelet Reinforced Glass, Acta Materialia 47 3233-3240 (1999). 

A. R. Boccaccini, V. Winkler, Fracture Surface Roughness and Tou ness of A Os-Platelet Reinforced Glass Matrix Composites, Composites Part A IS, 125—131 (2002). 

Examples of fibrous reinforcements are glass fibres and carbon fibres. Examples of platelet reinforcements are mica and talc. 

When platelet reinforcements are discussed elsewhere, it is common practice to redefine the aspect ratio as d/l so that numbers mudi larger than 1 are obtained. 

The platelet reinforcements in common use are all minerals. Two in particular are notably successful talc and mica. Talc in a magnesium silicate, while mica is an aluminium silicate. 

The success of mineral platelet reinforcements is due to their desirable combination of cost and properties ... 

The shapes of the particles used in nanocomposites can be roughly spherical, fibrillar or platelets, and each shape will result in different properties. For maximum reinforcement, platelets or fibrillar particles would be used, since reinforcement efficiency is related to the aspect ratio (length/diameter. Lid). The most extensive research has been performed with layered silicates, which provide a platelet reinforcement [19]. 

When dense SiC platelet-reinforced SiC composites were fabricated by Lenk et al. [103], the highest sintered theoretical density (TD) achieved was 97-98%, with a 20% platelet content, using hot molding as the thermoplastic-forming technique [104]. Chou and Green [105-107] fabricated dense SiC platelet-reinforced... 

Equation (2.13) has also been used to predict the modulus of flake (platelet) composites containing planar oriented reinforcement for uniform arrays of flakes, Eq. (2.14), and for random overlap, Eq. (2.15) [10, 12, 14, 19]. Equations for the parameter u are somewhat different from those used for fibers, but thqr still contain the important parameters affecting the modulus of the composite, that is, aspect ratio, volume fraction, and flake/matrix modulus ratio. Equation (2.18) has also been used to predict the modulus of platelet-reinforced plastics [17, 21]. 

Keywords filler, aspect ratio, particle shape, calcium carbonate, talc, platelets, reinforcement, glass fiber, kaolin, particle size, particle size distribution, chemical composition, adhesion, interface, aggregation, specific surface area, flow-induced orientation, hardness, surface free energy, surface tension, surface treatment, mechanical properties, thermal properties. 

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