Near-net shaping of dense ceramics via freeze-casting

Abstract

At present, producing dense complex-shaped ceramic parts require time-consuming and labour intensive processes, that are unsuitable for large-scale mass production. This results in prohibitively expensive part costs, in turn limiting the range of practical applications for complex-shaped dense ceramic materials. With process modifications, the freeze-casting process has potential as a suitable near-net shaping process for this application. In freeze-casting, a well-dispersed suspension of particles, such as a dispersion of ceramic powder in a carrier solvent, is frozen in a mould to produce a solid part. The frozen solvent can then be removed by sublimation in a freeze dryer, to produce a ceramic green body. Sintering this part enables us to produce a dense ceramic part retaining the shape of the mould.

Current research in the field of freeze-casting elsewhere is primarily focused on tailoring the freezing process, to create solvent-crystals of various morphologies, for use as pore-forming templates in the production of highly porous components. In this thesis, freeze-casting was performed with highly loaded non-aqueous alumina ceramic suspensions, primarily with submicron alumina in cyclohexane carrier solvent, with the end-goal of producing dense ceramic components. Previous progress in this research area has been hindered by the samples exhibiting severe internal cracking.

In this thesis the mechanisms and key factors responsible were identified for crack formation within samples, and applied this new understanding to explore selected avenues for mitigating the formation of cracks through changes to the process and to the suspension composition. In this work, the role of each of the primary freeze-casting process steps (freezing, freeze-drying and pressureless sintering) were systematically explored relating to both microstructure development and crack formation. A range of techniques were used to examine internal sample microstructures, including micro computed-tomography (micro-CT) and SEM imaging, with specific attention being given to the effects of freeze-drying conditions on samples.

The key conclusions are as follows. By performing dense freeze-casting with highly loaded suspension, and by maximising the freezing rate (using pre-chilled moulds to minimise the templated pore size), it was demonstrated that dense and complex-shaped objects can be formed via this process. The formation of undesirable internal cracks in this process was identified and proven to occur during the freeze-drying step, with cracks being absent in the frozen state, and present prior to sintering. It was found that the rate of sublimation drying influences both the distribution and configuration of the crack network, with fast drying rates (rapid sublimation under vacuum) resulting in both external radial ‘spoke’ cracking and internal discontinuous cracking, while slow drying rates (self-sublimation of samples in a freezer at ambient pressure) resulted in ‘onion-like’ concentric ring cracks. While cracks occur during drying, it was identified that the development of cracks occurs towards the start of drying, and significantly, that crack development occurs independently to the progress of drying and solvent removal. Addition of an acrylic binder to the suspension was shown to significantly increase the mechanical green body strength, and this reduced but did not eliminate cracks. With regards to crack mitigation, the combination of both using slow drying rates, and adding an acrylic binder to the suspension, was demonstrated to successfully mitigate crack formation. Finally, the use of cyclooctane as an alternative carrier solvent also resulted in reduced cracking, confirming that the solvent selection is integral to the effectiveness of the dense freeze-casting process.

In this way, the viability of a dense freeze-casting process for rapid production of complex shaped ceramic components was critically evaluated, and potential limitations of this process were identified and considered.