C. Reynaud et al. International Journal of Refractory Metals Hard Materials 19(2001)425-435 tween the geometric level of porosity calculated from the weight and dimensions of sintered specimens and bulk porosity measured by mercury porosimetry is attained On the other hand, similar trends between bulk porosity free PFA and PFA content are obtained. Linear relationships fuly densification between porosity and PFa with a slope less than I have partial densification-98% been obtained for low PFA concentrations(see Table 3) For higher PFA contents, the porosity values are almost constant and maximal. a higher maximum of porosity has been obtained by using starch. Only laminates using corn starch present a level of porosity higher than the X金 theoretical level of porosity required to ensure crack deflection The porosity distribution between open and closed porosity is presented in Fig. 9. Results of only two PFA which represent the two behaviours observed are plotted for clarity reasons. Firstly, the volume fractions of closed porosity for low PFA content (5-10 vol%)are lower than usually expected. Corbin et al. [ 12] have seen of volumetric shrinkage during sintering with the this phenomenon and have attributed it to an insufi- green ske nsity for the ditterent PFA nature and content tested. cient homogenisation of the PFa into the slurry.Sec ondly, PA-C and PA-B have an irregular behaviour; whereas, the trend of the variation of open and closed porosity for corn starch and PA-A is similar to the po- rosity distribution usually observed on starch(geometric density 3.4. Microstructure of monolithic block Homogeneous microstructure has been obtained Figs. 10(a)and(b) shows optical micrographs of the different pore structures elaborated. Pore size measured by mercury porosimetry versus PFA content is plotted in Fig. 11. A similar trend in- dependent of the PFA type was observed(sigmoidal e ). For low PFA content (rA<15 vol%), the PA-A pore diameter is quite constant. Pores left after burn out of the PFa are apparently isolated but in fact they are connected by small channels according to Figs. 9 and 10; the porosity is mainly open. Few pores are connected together to form pairs. When the PFA content reaches a value comprised between 15 and 25 vol%o, the pore diameter increases due to the achieve- ment of the percolation threshold. Then for PFA content higher than 25 vol%, the pore diameter is quite Fig. 8. Volume fraction of porosity versus PFA content. The maximal constant, due to the fact that all values of porosity reached for each PFA are indicated by a double line. themselves, and its value depends on the PFA size Indeed, on the one hand, for random packing of in- clusions into a matrix. when the inclusion volume predictions densification lines. For high contents in PA- fraction reaches 0. 16, one network of percolated in- A (PA>40 vol%), significant greater volumetric clusions connects one side to the other. For an inclu shrinkages are obtained than theoretical predictions as sion volume fraction equals 0. 10, many pairs of seen previously, and suggested some pores left after inclusions are created. And on the other hand, every burn-out of pa-a have been annihilated inclusion is a part of the same continuous network if Fig. 8 shows the porosity of sintered specimens versus the inclusion volume fraction is higher than 0. 22 [15] PFA content On the one hand a good agreement be Fig 12 which shows the cumulative volume fraction of  M         
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