C. Reynaud et al. International Journal of Refractory Metals Hard Materials 19(2001)425-435 the pore coordination number is higher than the crit ical value Nc, the pore is thermodynamically stable, the pore surface curvature is concave and it will grow △ Corn starch For lower coordination number, the pore is unstable and will shrink(annihilation) due to its convex shape For a dihedral angle value of 120(typical value ob- served for solid phase sintering), the critical number n extende analysis in space, carried out by Kingery and francois [17 indicates a value of Nc= 12. If we adapt those 。o results to our liquid phase sintering, pores left after burnout of the pfa for PA-B. PA-c and corn starch △△△ are thermodynamically stable owing to the high ratio between the resulting pore diameter(PA-C: 12 um, Corn starch: 9 um, cf. Table 5)and Sic grain size (1.2 um). For PA-A(diameter=3.8 um), some pores may PFA content(vol% The pore filling is connected to the contact angle tween ceramic grain and liquid phase. For a total ig. 11. Mode of pore size distribution with different PFA contents. For a given PFA content, the mode of pore size distribution corre- wetting, the pores filling occurs when the liquid menis sponds to the pore diameter for which the largest fraction of pores is cus radius is equal to the pore radius according to [18]: pore filling is easier for low contact angles. However low liquid phase volume fraction and large pores limit this phenomenon due to the preferential localisation of the o 5vol% liquid phase along the smaller spaces between higher capillary pressure). The pore filling must be limited in our materials. Moreover, no refilled pore has been observed 25wc% 3.6. Microstructure of multilayered composites () Layered structure: Layered structures have been 9E>9 obtained with parallel layers and uniform thickness um; porous layer: 80 um)(Fig 13(a)). No migrations of pores from porous to dense 1.88金aege layers have been observed. Excepted for alternate laminates using PA-A as PFA, no defects and no del aminations have been shown These defects for PA-a ight be attributed to internal stresses due to the mis- match between the shrinkage of a dense and a porous re diameter(um) layer or caused by an excessive and sudden gas emis- Fig. 12. Cumulative volume of porosity versus pore diameter for dif- sion during burnout(Fig. 13(b)) Trent corn starch contents (i) Grain Size: Image analysis was performed on three different multilayered specimens with a stack ing of dense and porous layers. The PFA volume Table 4 Comparison between the plateau value reached by the mode of the contents of porous layer were: 45 vol% of corn pore size distribution with theoretical prediction calculated according arch. 45 and 25 vol% of PA-A. the values mea the Eq 3 derived by Zok et al. (15 sured are plotted in Fig. 14 for the two multilayered PFA type of the Mode of the composites with porous layers containing 45 vol% of materia distribution(um) PFA. No influence of PFA type and content on grain size is shown. Dense layers and skeletons in porous layers are constituted by the same homo PA-C geneous, fine and equiaxed microstructure (mean Corn starch Ceramic grain size has been taken as I um pea 1 25 um, mean aspect ratio= 1.7)(see Figs. 13(c)and (d))            *                         , ,  ,          , 3 '  +    1           " ! '     *        +        &  A  ,  % 1           N     ;"7<        "  E ,       &            
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