The Control of Microstructural and Crystallographic Orientation via Ceramic Forming Methods for Improved Sintered Transparency
Transparent alumina is a candidate material for ballistic applications where visible or infrared wavelength transmission is required. However, the transparency of polycrystalline alumina can be limited due to the rhombohedral crystal structure being inherently birefringent. Birefringence causes light scattering at grain boundaries and is detrimental to the transparency. It has been shown experimentally that the application of a high magnetic field during processing can lead to crystallographic alignment and the reduction of birefringent light scattering. This alignment method is effective but is limited in terms of scalability. This research addresses these limitations through the use of simple and cost-effective shear and elongational forming processes such as uniaxial warm pressing and direct ink writing (DIW) for the improvement of final sintered transparency. To further support the improvement of these processes as alternatives and to evaluate the possibility of using powder ratios to improve the alignment, this research will also investigate the sintering behavior during hot-pressing of equiaxed and platelet powders.
Platelet ceramic-filled thermoplastic blends were developed and formed into sheets through uniaxial warm pressing. The solids loading (30 – 40 vol.%) and platelet diameter (1.2 and 11μm) were varied to compare effects on viscosity, percent reduction, and final alignment. All ceramic- filled thermoplastic polymer blends exhibited pseudoplastic behavior. Crystallographic alignment of green body samples was quantified by the orientation parameter (r) and grain misalignment angle (full width at half maximum, FWHM) obtained from rocking curve analysis. Blends with 11μm diameter platelets displayed a higher temperature sensitivity constant, better flow properties, and higher alignment compared to blends with 1.2μm diameter platelets. Optimal samples produced with blends containing 30 vol.% of 11μm diameter platelets demonstrated an alignment of r = 0.251 +/- 0.017; FWHM = 11.16° +/- 1.16°. A sample with optimal alignment was hot-pressed to transparency and obtained an in-line transmission of 70.0% at 645nm. The final alignment of this pre-aligned hot-pressed sample (r = 0.254 +/- 0.008; FWHM = 11.38° +/- 0.54°) improved when compared to a non-pre-aligned sample (r = 0.283 +/- 0.005; FWHM = 13.40° +/- 0.38°).
Additionally, the use of direct ink writing, an additive manufacturing technique, as a viable alignment process for producing transparent alumina was investigated. Highly loaded (> 54 vol.%) equiaxed alumina suspensions were developed with platelet additions ranging from 0-20vol.% of the total solids loading. An increase in the amount of platelet powders from 5-20vol.% increased the dynamic yield stress from 104Pa to 169Pa and decreased in the equilibrium storage modulus from 17,036Pa to 13,816Pa. It was found that the DIW process significantly increased the alignment in one orientation when compared to samples cast from the same suspensions and this behavior may be connected to the rheological properties. Lastly, an optical analysis showed that sample developed with 5vol.% platelet suspensions had higher in-line transmission values across the visible spectrum when compared to samples developed with 20vol.% suspensions. A sample cast from a 5vol.% platelet suspensions had the lowest grain alignment but possessed an in-line transmission of 42.8% at 645nm, which was the highest of the samples produced in this study. An optical loss analysis showed, that this sample has the lowest backwards scattering losses due to residual porosity and this result was supported by the density data. It is suggested that the alignment of the DIW samples is more complex and a more advanced texture analysis will need to be conducted to properly characterize the grain alignment.
Lastly, the densification behavior of equiaxed and platelet powder ratios with no intentional pre-alignment was investigated. An initial sintering investigation identified the optimum maximum pressure selected for the hot-pressing process as 20MPa. Under the selected hot- pressing parameters, the effects of 0, 25, 50, 75, and 100wt.% equiaxed powder additions on the sintering behavior, optical properties, and grain alignment was investigated. The data showed that an increase in the amount of equiaxed powders decreased the initial powder compact displacements rate. Additionally, an increase in the wt.% equiaxed powders from 0wt% to 75wt% decreases the in-line transmission from 70.9% to 40.2%, respectively at 645nm. Lastly, an increase in the wt.% equiaxed powders from 0wt% to 75wt decreased the alignment from (r = 0.321 +/- 0.005; FWHM = 16.26° +/- 0.40°) to (r = 0.509 +/- 0.022; FWHM = 34.63° +/- 2.61°), respectively.