E D. Rodeghiero et al/ Materials Science and Engineering 4244(1998)11-21 EHT=10. 00 KV WD= 11 m 300nm Photo No =72 Detector= SE Fig. 7. Secondary electron SEM micrograph demonstrating ductile phase crack bridging in a 5/95 vol. Nif/,, metal-ceramic composite; retical predictions, suggesting that the porosity of the ment comes from classical ductile phase crack bridging composites(even though low) is indeed having a slight [26, 35, 36]. For the Sic/a-Al2O3 composites, added effect toughness comes from numerous reinforcement/crack Table 2 shows the same data for the SiC(whisker )/a- interactions including the widely accepted crack deflec Al,O3 composites synthesized by the sol-gel technique. tion, interface debonding, crack bridging, and whisker, In the case of these materials, however, two values of platelet pullout mechanisms [37, 38]. A secondary e Youngs modulus exist due to transverse isotropy. electron SEM micrograph exhibiting ductile phase Ey corresponds to the elastic modulus in the plane of crack bridging is depicted in Fig. 7. The figure shows a the whiskers (i.e. perpendicular to the hot-pressing di- 5/95 vol. Ni/ a-Al2O3 composite into which a crack rection) while E, corresponds to the elastic modulus has been introduced by performing a Vickers hardness perpendicular to the whiskers (i.e. parallel to the hot- indentation (not shown). Note the three Ni particles pressing direction). Note that, indeed, E>E for each which span the crack plane and hence increase the of the composites studied, as expected. In regard to the overall toughness of the composite. In related work, sintering of the composites listed in Table 2, notice that Cr,O3 doping of the Ni/a-Al2O3 system has been found the relative density of all the materials is quite high. to produce further toughness enhancement for reasons This is significant since the achievement of comparable related to a strengthening of the metal-ceramic inter- densities when using conventional powder mixing tech- face [39]. Likewise, microstructural evidence corrobo- niques requires both higher hot-pressing temperatures rating toughness enhancement in the ceramic-ceramic and pressures [33]. Hence, not only does the sol-gel composites has also been found. Fig. 8, showing a chnique result in better Sic dispersion, but also better secondary electron SEM image of the fracture surface overall consolidation in the alumina matrix of a 10/90 vol. SiC(whisker)/a-AL,O, composite, dis- a plot of the fracture toughness of various metal-ce- plays some of the toughening mechanisms mentioned ramic and ceramic-ceramic alumina based composites for the SiC-reinforced materials such as interface as a function of reinforcement volume fraction is shown debonding and whisker pullout in Fig6. a Kle value for alumina(≈4MPa√m)from Interestingly, the magnitude of the toughness the literature is also included [34]. Note that both the creases reflected in Fig. 6 for each of the different metal-ceramic composites and the Sic-reinforced ma- composites can, in fact, be quantitatively reproduced terials display enhanced toughness compared to pure quite well on the basis of the theoretical toughening alumina. In the cermet systems, this toughness enhance mechanisms/models mentioned above. However, theE.D. Rodeghiero et al. / Materials Science and Engineering A244 (1998) 11–21 19 Fig. 7. Secondary electron SEM micrograph demonstrating ductile phase crack bridging in a 5/95 vol.% Ni/a-Al2O3 metal–ceramic composite; (Ni=light contrast, a-Al2O3=dark contrast). retical predictions, suggesting that the porosity of the composites (even though low) is indeed having a slight effect. Table 2 shows the same data for the SiC(whisker)/a￾Al2O3 composites synthesized by the sol-gel technique. In the case of these materials, however, two values of the Young’s modulus exist due to transverse isotropy. E corresponds to the elastic modulus in the plane of the whiskers (i.e. perpendicular to the hot-pressing di￾rection) while EÞ corresponds to the elastic modulus perpendicular to the whiskers (i.e. parallel to the hot￾pressing direction). Note that, indeed, E \EÞ for each of the composites studied, as expected. In regard to the sintering of the composites listed in Table 2, notice that the relative density of all the materials is quite high. This is significant since the achievement of comparable densities when using conventional powder mixing tech￾niques requires both higher hot-pressing temperatures and pressures [33]. Hence, not only does the sol–gel technique result in better SiC dispersion, but also better overall consolidation in the alumina matrix. A plot of the fracture toughness of various metal–ce￾ramic and ceramic–ceramic alumina based composites as a function of reinforcement volume fraction is shown in Fig. 6. A KIc value for alumina (:4 MPa m) from the literature is also included [34]. Note that both the metal–ceramic composites and the SiC-reinforced ma￾terials display enhanced toughness compared to pure alumina. In the cermet systems, this toughness enhance￾ment comes from classical ductile phase crack bridging [26,35,36]. For the SiC/a-Al2O3 composites, added toughness comes from numerous reinforcement/crack interactions including the widely accepted crack deflec￾tion, interface debonding, crack bridging, and whisker/ platelet pullout mechanisms [37,38]. A secondary electron SEM micrograph exhibiting ductile phase crack bridging is depicted in Fig. 7. The figure shows a 5/95 vol.% Ni/a-Al2O3 composite into which a crack has been introduced by performing a Vickers hardness indentation (not shown). Note the three Ni particles which span the crack plane and hence increase the overall toughness of the composite. In related work, Cr2O3 doping of the Ni/a-Al2O3 system has been found to produce further toughness enhancement for reasons related to a strengthening of the metal–ceramic inter￾face [39]. Likewise, microstructural evidence corrobo￾rating toughness enhancement in the ceramic–ceramic composites has also been found. Fig. 8, showing a secondary electron SEM image of the fracture surface of a 10/90 vol.% SiC(whisker)/a-Al2O3 composite, dis￾plays some of the toughening mechanisms mentioned for the SiC-reinforced materials such as interface debonding and whisker pullout. Interestingly, the magnitude of the toughness in￾creases reflected in Fig. 6 for each of the different composites can, in fact, be quantitatively reproduced quite well on the basis of the theoretical toughening mechanisms/models mentioned above. However, the