Processing of bulk nanostructured ceramics
journal contributionposted on 17.03.2008, 10:01 by J.G.P. Binner, Vaidhy VaidhyanathanVaidhy Vaidhyanathan
Conventional ceramic forming routes have been adapted for the processing of ~16 nm, 3 mol% yttria stabilized zirconia nanopowders leading to the production of ~99% dense nanostructured ceramics that display average grain sizes as fine as ~65 nm. The precursor material is in the form of ~5 vol% solids content nanosuspensions produced commercially; these can now be concentrated up to ~37 vol% whilst retaining the viscosity at ~0.05 Pa s. A patent application has been submitted related to the process. The concentrated suspensions have then been used to produce granulated powders suitable for dry forming via spray-freeze drying. Whilst powders have been produced that will yield green bodies with densities of ~50% of theoretical, currently the powders suffer from either poor flow and low fill densities or granules that are too strong to crush during pressing, even at pressures up to 500 MPa. The same suspensions have also been slip cast into extremely homogeneous green bodies with densities of ~54% of theoretical after drying using a humidity drier. Higher densities are currently blocked by cracking of the samples during drying and/or burnout of the organics if the solids content of the suspensions exceeds ~20 vol%. Radiant and hybrid pressureless sintering experiments have been performed on the dry and wet processed green bodies using both conventional single step and two-step sintering cycles. Whilst densities >98% of theoretical were achievable by all combinations, a nanostructure could only be retained using the two stage sintering approach. With hybrid heating the average grain sizes for die pressed samples were in the range 70 – 80 nm whilst for the more homogeneous slip cast samples a final average grain size of just 64 nm was achieved for a body with a final density of ~99.5%. It is believed that the primary advantage offered by hybrid heating is the ability to use a much faster initial heating rate, 20 versus just 7oC min-1, without risking damage to the samples. Whilst detailed characterisation of the properties of these nanostructured ceramics has begun, preliminary results have suggested that the toughness is lower and hardness roughly equivalent to submicron grain-sized 3-YSZ, although the resistance to wear and hydrothermal ageing may have been improved. As a result of detailed crystallographic characterisation this is believed to be due to a grain size dependent shift in the phase boundary composition for nano YSZ ceramics leading to ‘over stabilisation’ at any given yttria content. Current work is focused on investigating the effect of both yttria content and average grain size on the properties of these new materials.
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