Researcher: Dr Daniel Glymond
Supervisor: Dr Luc Vandeperre
Sponsor: Microturbo
Powder pressing is industrially established but not well suited to the production of complex shaped turbine blades and components. However, pressing followed by green machining is applied successfully in space components and could potentially offer a way to produce intricate shapes while taking advantage of the well-developed pressing technology. Alternative shaping methodologies find their origin in clay based ceramics and are based on suspensions of ceramic powders: wet processing or colloidal techniques. A wide range of variants, often differing only in a limited way, exist. From these a number appear promising.
In gel-casting a gel former is added to the ceramic suspension so that it can be cast and subsequently gelled in a mould. The shape can then be demoulded, dried and sintered. Omatate from the Oak Ridge National Laboratory in the USA developed this methodology and used a radial-vane turbine rotor made from silicon nitride as a key example component [1]. However, drying of gelcast components can be cumbersome especially if the dimensions increase. Nevertheless, it seems obvious that this methodology must be part of the mix.
In later years, the use of binders from the food industry such as gelatine have been investigated and these have clear advantages over the polymer monomers in terms of toxicity [2]. A family of similar techniques are termed coagulation casting, in which a fluid suspension is made to solidify by removing the interparticle repulsive forces (for example by an homogeneous change in the suspension pH) once the object has been cast. An alternative is to engineer responsive particle surfaces that will promote particle dispersion in a suspension and subsequent assembly in response of an external trigger (e.g. pH). This could be done, for example, by grafting the particle surfaces with a pH responsive polymer such as a BCS polymer.
Finally, another way to produce ceramic parts with complex shapes is the use of solid-free-form fabrication techniques in which the part is printed in three dimensions following a computer design. We will focus on robotic assisted deposition, a continuous extrusion technique that can be used to print a sample layer by layer employing colloidal ceramic inks. The inks have to be pseudoplastic so they flow in the printing nozzle under pressure and after printing they can sustain the weight of the part without deformation. This can be achieved by, for example, using inks based on thermally reversible hydrogels that are fluid at low temperature allowing the homogeneous dispersion of ceramic particles and pseudoplastic at room temperature, allowing printing.
References:
1. Janney, M.A., et al., Development of Low-Toxicity Gelcasting Systems. Journal of the American Ceramic Society, 1998. 81(3): p. 581-91.
2. Vandeperre, L.J., A.M. De Wilde, and J. Luyten, Gelatin gelcasting of ceramic components. Journal of Materials Processing Technology, 2003. 135: p. 312-316.