D. Rodeghiero et al/ Materials Science and Engineering 4244(1998)11-21 Density and Youngs modulus data of sol-gel derived Ni/o-AlO3 metal-ceramic composites Ni/a-Al2O3 volume Relative density Young's modulus orrected Youngs modulus, (g cm-3) Eo (GPa) 9988 353 44555 9992 891b 214° The critical stress intensity factors were determined can be added simultaneously. A secondary approach through a work of fracture analysis. This involved was used for the synthesis of the Fe/a-Al2O3 com- neasuring the area under the load /displacement curve posites. In this case, a coprecipitation technique involv of each composite beam through integration and then ing the addition of Naoh to an aqueous solution of dividing by twice the fracture area to obtain the work aluminum and ferric nitrates was utilized. The benefits of fracture, ywof. Then, assuming plane strain, Kiwor was of this approach are simplicity and low cost calculated using the following equati To prepare the SiC-reinforced alumina materials, a K1wo={(2E)/(1-v2) imilar yet more complex route had to be utilized. First an appropriate amount of Sic whiskers or platelets was where E is the Youngs modulus of the composite and dispersed in a stirred aluminum isopropoxide solution. v is Poissons ratio(both determined through the acous- Enough water to gel the solution was next added tic testing). In the case of the SiC/a-Al,O3 composites, However, due to the relatively large mass of the Sic more than one value of e and v exist for reasons particulates, the gel initially formed was typically not previously discussed. Furthermore, it is not at all clear viscous enough to support the whiskers or platelets and which set of the anisotropic elastic values should be prevent settling. Hence, through stirring the mixture at substituted into Eq 20]. However, since the degree elevated temperature(&70oC), the viscosity of the gel of elastic anisotropy in the SiC/a-Al2O3 composites was was raised until the SiC particulates could no longer found to be rather small anyway, the value of e in the settle. Finally, drying was performed at a 100oC. This plane of the Sic whiskers and platelets (i.e. parallel to overall procedure was devised and first performed by the length of the chevron-notched beams)was the one J.J. Lannutti et al. in 1984 [21]. However, to our ed to determine the kiwor values knowledge, no high temperature consolidation or me- chanical property data was ever subsequently reported) The primary attribute of this sol-gel technique is its 3. Results and discussion excellent ability to homogeneously disperse the SiC particulates in a stirring liquid and then freeze' them To produce the metal-ceramic composites, our most into position, resulting in a very high degree of final widely used technique consisted of first preparing a composite uniformity and the complete elimination of metal alkoxide solution as previously discussed. To this SiC agglomerates, a challenge which is almost insur was then added an aqueous solution of a metal salt mountable using conventional powder mixing tech- which resulted in spontaneous hydrolysis and conden- niques sation of the ceramic precursor and incorporation of Fig. I depicts the typical crystallographic evolution le metal salt into the growing ceramic gel at or near of an alumina-based metal-ceramic composite. The the molecular level. Attributes of this approach are its XRD pattern in Fig. 1(a) corresponds to a 20/80 vol. mplicity, its very high homogeneity, and its high Ni/a-Al2O3 precursor gel in the dried state just after degree of chemical flexibility due to the large selection grinding. While the absence of ceramic phase reflections of metal alkoxides and metal salts available. This tech- is to be expected for the air-dried materials, the fact nique is also very favorable for doping, since small that there are also no apparent XRD features at amounts of additional metal alkoxides or metal salts ibutable to the nickel formate salt has significant16 E.D. Rodeghiero et al. / Materials Science and Engineering A244 (1998) 11–21 Table 1 Density and Young’s modulus data of sol–gel derived Ni/a-Al2O3 metal–ceramic compositesa Ni/a-Al Relative density Young’s modulus, Porosity corrected Young’s modulus, 2O3 volume Density E0 (g cm (% of theoretical) E (GPa) (GPa) −3 composition ) — 390c 0/100 3.97b — 5/95 4.18 99.2 388 379 10/90 4.41 98.7 339 353 15/85 4.55 355 96.6 320 20/80 4.77 96.2 344 306 30/70 5.12 93.9 288 348 33/67 5.23 93.2 251 310 40/60 5.54 352 93.2 285 50/50 5.97 313 92.8 250 100/0 8.91b — 214c — a Adopted from [15]. b Reference [29]. c Reference [30]. The critical stress intensity factors were determined through a work of fracture analysis. This involved measuring the area under the load/displacement curve of each composite beam through integration and then dividing by twice the fracture area to obtain the work of fracture, gwof. Then, assuming plane strain, KIwof was calculated using the following equation: KIwof={(2E · gwof)/(1−n 2 )}1/2 (1) where E is the Young’s modulus of the composite and n is Poisson’s ratio (both determined through the acous￾tic testing). In the case of the SiC/a-Al2O3 composites, more than one value of E and n exist for reasons previously discussed. Furthermore, it is not at all clear which set of the anisotropic elastic values should be substituted into Eq. (1) [20]. However, since the degree of elastic anisotropy in the SiC/a-Al2O3 composites was found to be rather small anyway, the value of E in the plane of the SiC whiskers and platelets (i.e. parallel to the length of the chevron-notched beams) was the one used to determine the KIwof values. 3. Results and discussion To produce the metal–ceramic composites, our most widely used technique consisted of first preparing a metal alkoxide solution as previously discussed. To this was then added an aqueous solution of a metal salt which resulted in spontaneous hydrolysis and conden￾sation of the ceramic precursor and incorporation of the metal salt into the growing ceramic gel at or near the molecular level. Attributes of this approach are its simplicity, its very high homogeneity, and its high degree of chemical flexibility due to the large selection of metal alkoxides and metal salts available. This tech￾nique is also very favorable for doping, since small amounts of additional metal alkoxides or metal salts can be added simultaneously. A secondary approach was used for the synthesis of the Fe/a-Al2O3 com￾posites. In this case, a coprecipitation technique involv￾ing the addition of NaOH to an aqueous solution of aluminum and ferric nitrates was utilized. The benefits of this approach are simplicity and low cost. To prepare the SiC-reinforced alumina materials, a similar yet more complex route had to be utilized. First, an appropriate amount of SiC whiskers or platelets was dispersed in a stirred aluminum isopropoxide solution. Enough water to gel the solution was next added. However, due to the relatively large mass of the SiC particulates, the gel initially formed was typically not viscous enough to support the whiskers or platelets and prevent settling. Hence, through stirring the mixture at elevated temperature (:70°C), the viscosity of the gel was raised until the SiC particulates could no longer settle. Finally, drying was performed at :100°C. (This overall procedure was devised and first performed by J.J. Lannutti et al. in 1984 [21]. However, to our knowledge, no high temperature consolidation or me￾chanical property data was ever subsequently reported). The primary attribute of this sol–gel technique is its excellent ability to homogeneously disperse the SiC particulates in a stirring liquid and then ‘freeze’ them into position, resulting in a very high degree of final composite uniformity and the complete elimination of SiC agglomerates, a challenge which is almost insur￾mountable using conventional powder mixing tech￾niques. Fig. 1 depicts the typical crystallographic evolution of an alumina-based metal–ceramic composite. The XRD pattern in Fig. 1(a) corresponds to a 20/80 vol.% Ni/a-Al2O3 precursor gel in the dried state just after grinding. While the absence of ceramic phase reflections is to be expected for the air-dried materials, the fact that there are also no apparent XRD features at￾tributable to the nickel formate salt has significant