Fabricating complex-shaped components by room-temperature injection molding of aqueous ceramic suspension gels

Valerie Wiesner, Purdue University

Abstract

Water-based ceramic suspension gels (CeraSGels) effectively produced dense, near-net shape ceramic parts by room-temperature injection molding, a novel processing method based on traditional ceramic injection molding. This alternative method eliminated the need for heating and cooling feedstocks to process, as is required in conventional injection molding, through control of the rheological response of the CeraSGels by simply varying polymer content without the use of any harsh crosslinking or curing agents or further chemical processes. The development of room-temperature injection molding initially focused on forming CeraSGels based on alumina, a readily available model material, in order to optimize suspension preparation and processing prior to tailoring the process to zirconium diboride (ZrB2), an ultra-high temperature ceramic (UHTC) system. CeraSGels were highly loaded (>50 vol.%) with alumina powder and had a minimal amount (≤5 vol.%) of water-soluble polyvinylpyrrolidone (PVP). Rheological study of alumina CeraSGels using parallel plate rheometry revealed that suspensions containing PVP as a viscosity modifier behaved like yield-pseudoplastic fluids at room temperature. Because understanding flow properties of CeraSGels was vital to enable fabrication of ceramic components into useful geometries, estimations of the interparticle interactions, which qualitatively reflected the colloidal stability, of the alumina-PVP suspensions were modeled to correlate with experimental rheological observations. Alumina ring-shaped samples were successfully injection molded at room temperature using CeraSGels containing 1 to 5 vol.% PVP with molecular weight of 55,000 g/mol. Alumina samples could not be produced by room-temperature injection molding using suspensions without PVP additions. Specimens prepared using PVP-containing alumina CeraSGels exhibited high green body strength and machinability prior to binder removal and sintering, and increasing PVP content was observed to enhance green machinability. Alumina rings with high green densities (60-63% true density (TD)) as found using Archimedes technique were obtained after binder removal, which was accomplished by heating specimens at a rate of 2°C/min to 700ºC with an isothermal hold for 1h. In a subsequent heat treatment, specimens were pressureless sintered by heating at a rate of 5°C/min to 1620ºC and holding for 1.5h. Bulk density of sintered samples using the Archimedes technique was found to reach 98%TD with linear shrinkage of <16%. Microstructural characterization revealed minimal pore formation within sintered samples regardless of initial PVP content, and average grain sizes were ~3.4 µm. Ultimate strength of the sintered alumina samples was determined using the ASTM C1323-10 compressive C-ring test, and C-strength values were comparable to values found in literature for specimens prepared by conventional processing methods. A minimum strength value of 192 ± 27.2 MPa was found for specimens prepared with CeraSGels containing 5 vol.% PVP, and a maximum of 261 ± 57.6 MPa was obtained for rings produced using CeraSGels with 2.5 vol.% PVP, suggesting it was the optimal concentration. Alumina CeraSGels containing 2.5 vol.% PVP of varying molecular weights of 10,000, 360,000 and 1,300,000 g/mol were prepared to determine if molecular weight affected the resulting ring-shaped specimens. Despite similar mechanical and microstructural properties, the favorable flow properties as well as high green and sintered densities distinguished CeraSGels containing 2.5 vol.% PVP with a molecular weight of 55,000 g/mol from all other suspensions examined in this study as the optimal PVP molecular weight and concentration to be incorporated into alumina suspensions for room-temperature injection molding. The room-temperature injection molding technique was later adapted to ZrB2, a leading UHTC material that was difficult to fabricate into complex shapes due to its high melting temperature (>3000°C) and sensitivity to impurities during sintering. ZrB2-based CeraSGels containing 3.5 wt.% boron carbide (B4C) and 10.5 wt.% tungsten carbide (WC) sintering aids as well as 1 to 3 vol.% PVP with molecular weight 10,000 g/mol exhibited a time-dependent rheological response. Although the shear stress required to initiate flow for ZrB2-based CeraSGels was observed to decrease with increasing PVP content, all suspensions flowed at room temperature to successfully yield dense ring-shaped specimens after binder burnout and sintering. The polymer binder and impurities were removed by a series of heat treatments, which involved heating at a rate of 4°C/min to 600ºC held for 1h in medium vacuum (~10-5 Torr) followed by heating at 10ºC/min to 1650ºC for a 1-h hold at which point argon was flowed into the system and a final ramp at 10ºC/min to 1850ºC for 1.5h that was performed in an inert argon environment, to facilitate pressureless sintering of ZrB2/B4C/WC rings to full density (>98%TD). Microstructural and elemental analysis suggested that the binders did not affect the resulting composition or microstructure of the ZrB2-based specimens as no oxide-containing phase were present. Room-temperature mechanical properties were determined using the ASTM C1323-10 compression test. C-strength values ranged from 30.7 ± 12.0 MPa to 75.1 ± 26.7 MPa for specimens prepared by CeraSGels containing 1 and 3 vol.% PVP, respectively. Further processing improvements are speculated to enhance the resulting mechanical and microstructural properties of ZrB 2 specimens prepared using CeraSGels. Room-temperature injection molding of CeraSGels proved to be a viable, environmentally friendly processing alternative to fabricate dense, complex-shaped ceramic components.

Degree

Ph.D.

Advisors

Youngblood, Purdue University.

Subject Area

Engineering|Materials science

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