Whisker-reinforced composites

Toughening for whisker-reinforced composites has been shown to arise from two separate mechanisms frictional bridging of intact whiskers, and pullout of fractured whiskers, both of which are crack-wake phenomena. These bridging processes are shown schematically in Figure 13. The mechanics of whisker bridging have been addressed (52). The appHed stress intensity factor is given by ... 

Chu, C.Y., Singh, J.P. and Routbort, J.L. High-temperature failure mechanisms of hot-pressed Si3N4 and Si3N4/Si3N4-whisker-reinforced composites , J. Am. Ceram. Soc., 76[5] (1993) 1349-1353. 

Figure 3.25 shows a SEM micrograph of a well-consolidated 30 wt% SiC whisker-reinforced composite [10]. Table 3.18 gives the mechanical properties... 

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. 

For this mechanism to operate, debonding along the crack-whisker interface (often associated with crack deflection) must occur during crack propagation and allow the whiskers to bridge the crack in its wake. Examination of the fracture surfaces of whisker-reinforced composites reveals that they are microscopically rough with whiskers readily evident (Fig. 2.1). 

In both derivations of toughening behavior, increases in toughness for whisker-reinforced composites are dependent on the following parameters (1) whisker strength, (2) volume fraction of whiskers, (3) elastic modulus of the composite and whisker, (4) whisker diameter, and (5) interfacial fracture energies. 

As mentioned previously, the main body of research on whisker-reinforced composites was concerned with alumina, mullite, and silicon nitride matrix materials. None the less, selected work examined zirconia, cordierite, and spinel as matrix materials.16-18 The high temperature strength behavior reported for these composites is summarized in Table 2.5. As shown, the zirconia matrix composites exhibited decreases in room temperature strength with the addition of SiC whiskers. However, the retained strength at 1000°C, was significantly improved for the whisker composites over the monolithic. Claussen and co-workers attributed this behavior to loss of transformation toughening at elevated temperatures for the zirconia monolith, whereas the whisker-reinforcement contribution did not decrease at the higher temperature.17,18... 

Future Directions for Advanced High Toughness Whisker-Reinforced Composites... 

Improvement in the creep resistance of whisker-reinforced composites is modest compared to that obtained in particulate-reinforced composites. Creep... 

It seems unlikely that long-fiber ceramic matrix composites with strong bonds will find application because of their low temperature brittleness. However, for completeness, a model which applies to the creep of such materials can be stated. It is that due to Kelly and Street.21 It is possible also that the model applies to aligned whisker-reinforced composites since they may have strong bonds. In addition, the model has a wide currency since it is believed to apply to weakly bonded composites as well. However, the Mileiko18 model predicts a lower creep strength for weakly bonded or unbonded composites and therefore is considered to apply in that case. 

Testing of ceramic composites has been around since the earliest fabrication of these materials. For particulate- and whisker-reinforced composites, testing methods which are suitable for monolithic ceramics are generally used. These methods include three- and four-point flexure, uniaxial tension and compression, and many others. For fiber-reinforced ceramic composites, flexural testing was also used initially. However, as was recognized in the polymer composites area, flexural testing alone could not provide the type of... 

II. Whiskers and Ceramic Matrix Whisker-Reinforced Composites. 160... 

The bulk analysis of /3-SiC whiskers shows the least variation in chemistry. In some whiskers, the residual metals content can vary, most likely, as a result of additives that used as catalysts during synthesis. These include iron, cobalt, and chromium. Studies by Karasek et al. [56] have shown that the physical properties of silicon carbide whisker-reinforced composites do not correlate to the bulk properties of the whiskers significantly. This lack of significant correlation is mainly due to the fact that the important phase chemistry of the whisker-matrix interface is controlled by the matrix chemistry and the surface chemistry of the whiskers. There seems to be little impact of the diffusion of materials into or out of the bulk whisker material. 

The analytical expressions of micromechanics are generally most accurate at low volume fractions of the filler phase. The details of the morphology become increasingly more important at higher volume fractions. This fact was illustrated by Bush [64] with boundary element simulations of the elastic properties of particulate-reinforced and whisker-reinforced composites. The volume fraction at which such details become more important decreases with increasing filler anisotropy, as was shown by Fredrickson and Bicerano [60] in the context of analytical models for the permeability of nanocomposites. 

Difficult-to-process composites can be readily produced by thermal spray forming, and plasma spray is the process of choice for the most reactive matrix materials. Particulate-, fiber-, and whisker-reinforced composites have all been produced and used in various applications. Particulate-reinforced wear-resistant coatings such as WC/Co, Cr3C2/NiCr, and TiC/NiCr are the most common applications. Figure 8 illustrates the diverse forms of composites that can be thermally spray formed. Whisker particulates can be incorporated into sprayed deposits using so-called engineered powders, mechanical blending, or by... 

A comparison of the SiC whisker reinforced composite and matrix material revealed that the whisker reinforcement does not improve creep resistance at high stress. However, at lower stress there is a significant improvement. 

Whisker-reinforced composites can also be manufactured by this process, although a homogeneous composite structure can often be difficult to achieve with whiskers as they can form tangled agglomerates during the blending process. 

Fig. 8.80 Creep curves of whisker-free (monolithic) and whisker-reinforced (composite) hot-pressed silicon nitride at 1200 °C and 100 MPa. The tests were interrupted at 1000 h [40]. With kind permission of John Wiley and Sons...

Figure 10.5 TEM micrograph of creep cavitation sites in the 20 vol % SiC whisker-reinforced composite. ...

Jia et al. analyzed the influence of SiC whiskers with different L/D ratios on the stress distribution of a SiC resin composite material under a certain SiC whisker volume ratio by using the finite element method on the basis of the axisymmetric model of a resin matrix composite material reinforced by single orientation whiskers, and provided a theoretical basis for the optimal design of a whisker-reinforced composite material. 

In the theoretical prediction of axial tensile strength in whisker-reinforced composites, the following mix laws are usually applied to predict tensile strength ... 

For whisker-reinforced composite material, if the ultimate strength of the whisker is Ofu, the L/D ratio of the whisker must be higher than a certain critical value (k to reach Ofu, and is... 

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