3.1 Characterization of Alumina Powders and Slips:
XRD pattern corresponded to pure alpha Al2O3 phase as per alpha phase JCPDS (Match Entry C 96-100-0033) (Fig. 2). Average particle size distribution of alumina powders of grades MR-01, HIM-10 and the powder mix prepared from two grades of alumina are shown in the Fig. 3.
The two grades of the powders have shown an average particle size of 1.43 µm, 7 µm respectively while the average particle size of the powder mix is found to be about 3.16 µm. SEM micrographs showed irregular powder morphology (Fig. 4).
It is evident from the micrograph that the irregular morphology and the size variation as observed in the two grades (finer and coarser) alumina powders are desirable factors in achieving a higher packing fraction and interlocking of the particles one of the critical issues in addressing the collapse of the structure while de-molding the cast part under pressure.
3.2 Rheological behavior:
The parameters required in rheological assessment of the suspensions to achieve the cast bodies with desired slip properties. Among all the slips prepared, only those with ≥ 75 wt. % solid loading (powder mix ratio of 30:70) are observed to be stable and homogenous and studied for the rheology and the plot of Viscosity versus Shear rate as shown in Fig. 5. When a shear force (pressure) is applied on the slip, the particulates have to flow and fill all the cavities in the mold for which a shear thinning behavior is essential. The rheological behavior displayed by the present slip confirms that the lowering of viscosity value which is required for retaining the shape of the cast once pressure is removed. Under the pressure casting conditions, the slip in the polymer-based mold (with average pore size of about 10 µm) has to undergo a forced filtration resulting in the faster thickness built up. This observation is in agreement with finding of the requirement of low viscosity and low yield point for a good control of the casting rate in order to obtain high relative density of the cast bodies.
3.3 Casting of Specimens:
Alumina discs and spheres cast along with pressure casting cycle and sintered are shown in Fig. 6. The green samples immediately after the casting (PSC & CSC) were weighed accurately (± 0.01 g) and kept open in the ambient conditions at an average room temperature of 25oC and A typical pressure cast cycle followed for PSC is shown in the Fig. 6c).
The water content was estimated by the ratio between the weights of the wet body to that of dried one. The results are shown in Fig. 7 where it is evident that the maximum moisture content is 25% and 12% which can be completely removed by open room drying in 24 & 45 hrs. respectively for CSC an PSC samples. The low moisture content in PSC bodies can reduce the time involved in the drying step during the large-scale production with improved productivity and lower rejection rate.
3.4 PSC effect on thickness built up:
All the parameters of the PCS like feed rate, slip pressure, and pressure holding time were monitored carefully for each slip to achieve the maximum green strength. The thickness of the Alumina disc cast samples gradually increases with influence of holding time and under varying applied pressure conditions and is shown in Fig. 8. Thickness built up of the sample is found to be considerably faster till 200 seconds of holding time under given pressure applied on the slip. Then the thickness built up become much slower which is obviously due to the fact that the water finds it difficult to penetrate through as the thick sample layer formed on the internal walls of the polymer mold inhibits the direct path towards the pores. In order to study the maximum thickness built up that can be achieved in the pressure slip casting technique, five different pressures and as long as 300 seconds of (pressure) holding times were studied (Fig. 8).
A flat Alumina block of > 30 mm thick can be pressure slip cast using polymer split molds within 5 min under 35 bar pressure applied on the slip. Accordingly, by fine tuning both the machine and slip parameters it is possible to pressure cast green bodies as large as φ 60 mm (30 mm thick on each part of the split mold) without much difficulty. In the present study, solid Alumina spheres of φ 60 mm were pressure cast reproducibly and sintered to achieve > 98.5% density.
3.5 PSC influence on density and mechanical properties:
The influence of solid loading on the green densities of conventionally cast samples and the same in case of pressure cast samples as a function of the pressure applied on the slip is presented in Fig. 8. This graph highlights the fact that optimum solid loading and pressure to be applied in order to achieve the maximum green density are, 75–80 wt.% and 35 bar respectively. From literature while working on pressure casting of Al2TiO5 ceramics observed that green densities over the range 60–65% could be achieved under low pressure on the slip contrary to much higher pressures of > 100 MPa required for the same in CIP (cold isostatic pressing).This can be attributed to the fact that unlike dry pressing, slip with optimum solid loading of 75–80% on pressurization and holding condition selected in our laboratory through several experiments facilitates the re-arrangement of particles in the aqueous medium through rolling, twisting and interlocking finally leading to higher densities. Additionally, the average particle size of 1.43 µm and 7 µm of alumina used in the study is also expected to play a critical role in achieving the high packing factor. It is also evident that solid loading beyond the optimum results in the reduction of green density due to the restriction of movement due to higher particle density for rearrangements of the particles under pressure.
A similar trend is also observed with conventional slip casting due to the gravity settling of the particles rather than cast formation under high solid loading conditions. It is also obvious that due to the removal of excess water under pressure through the porous mold also reduces the drying time and rejection due to warpage and cracks resulting from the differential drying stresses generated during ceramic drying process.
3.5 PSC influence on Microstructural properties:
The microstructural study under FESEM on the sintered alumna samples produced from both the techniques are presented in Fig. 10a) and 10b). The comparison of the both SEM images clearly brings out the close packing and smaller grain size achieved in the pressure cast samples and also supports the higher sintered densities determined in the same. Distribution of smaller grains with an overall average size of 0.514 µm in the inter-particle spaces of coarser-grains leads to better coordination in the pressure cast samples resulting in higher densification in comparison to the conventionally cast samples.
An 22% increase in Vickers hardness is observed with PSC sample (14.92 ± 0.15 GPa) in comparison to CSC samples (11.77 ± 0.15 GPa). This can be attributed to the higher density as indicated by the close packed microstructure with relatively smaller grains. The fracture toughness, flexural strengths of the samples are presented in Table.1 along with densities for the both conventional and pressure cast samples. It is evident that enhancement of the flexural strength from 242.70 to 294.40 MPa, and the fracture toughness from 3.73 to 4.06 MPa.m½ in on application for PSC samples which can be attributed to the interlocking of elongated grains (with 3.786 µm major axis, 1.452 µm minor axis) with the smaller grains of average size of 0.514 µm.