Airslip casting
As demonstrated in Part I, the pH value plays a critical role in the suspension stability and rheology. For this reason, the following results are referred to slips de¯occulated with TMAH at pH=111.30.2 for any composition and mixing procedure. Figure 2 shows the cast wall thickness evolution versus casting time of the three considered compositions prepared according to mixing route B, which presented the best characteristics from a rheological point of view. The casting rate of pure SN1 compositions is faster than that of slips containing sintering aids. This fact must be related with the diculties to maintain the desired pH. The long mixing time promotes adsorption of the TMAH but also some volatilization can occur. It has been proven that when pH is readjusted before casting the resulting kinetics is similar to that obtained for the other compositions. Similarly, Fig. 3 plots the slip casting curves of the slips prepared by homogenizing route C (centrifugal milling). In this case, a quite similar kinetics is obtained for all three compositions. Comparing with Fig. 2, it must be noted that the composition without additives SN1 exhibits the same casting rate for both mixing routes, and the dierences are only observed in the slips containing sintering aids. These compositions behave similarly in each case, but the casting rate is much lower when slips are prepared by route B. This is associated with the short mixing time in route C which is not enough to assure the particles to reach an equilibrium at the surface. The ®nal pH of the slips, just prior to casting, is summarized in Table 1. Slips prepared by route C maintain a higher pH, thus suggesting that adsorption occurs to a lower level. Taking into account the strong dependency of viscosity with pH, it could be expected that slips prepared by route C should present lower casting rates because of the higher pH. The fact that route B-slips have lower casting rates, even presenting lower pHs clearly demonstrates that a higher homogenization degree is achieved, which is in complete agreement with the rheological behaviour described in Part I. Homogenization route A was not considered for kinetics studies due to the lower reliability and the observation of bubbles during casting. The green densities of the slip cast samples are presented in Table 2. These values con®rm the rheological and casting behaviour described before. Route B produces once more better bodies with densities higher than those obtained by the other two routes. The maximum reachs a value of near 59% th, which is quite satisfactory. The lowest densities correspond to the slips prepared by route C, as could be predicted from the rheological studies shown in the ®rst part of this paper. The general morphology of samples cast from slips prepared according to the dierent mixing procedures was observed by optical microscopy on polished surfaces after sintering at 1750C lh-1 under N2 ¯ow. Figure 4(a) shows a general picture of a SN2 sample prepared by route A. As can be observed, big holes due to air bubbles are always Fig. 2. Slip casting kinetics of slips prepared according to mixing route B. Fig. 3. Slip casting kinetics of slips prepared by mixing route C. Table 1. Final pH values of slips after mixing according to routes B and C Composition Mixing route B C SN1 11 11.4 SN2 11.2 11.2 SN3 11.l 11.4 Colloidal ®ltration of silicon nitride aqueous slips, Part II. 51 present, as a consequence of the high mechanical shearing. These bubbles could not be removed even after a further low speed agitation. This fact explains why route A was rejected for the obtention of homogeneous, defect-free bodies. The same aspect is observed for the other compositions prepared by shear mixing. The aspect of sintered cast SN2 samples prepared by routes B and C can be seen in Fig. 4(b) and (c), respectively. As expected from the rheology and from the casting kinetics, route B allows the obtention of more uniform microstructures with smaller pore sizes for any composition. In samples prepared by route C occasional big pores can be also detected.
