Manufacturing nanostructured ceramic components could now become more industrially viable using processing techniques developed at Loughborough University, UK.

Scientists say they have overcome obstacles in slip casting, dry pressing and sintering nano-powders for application in power generation, biomedical devices, electronics and transport.

Nanomaterials offer improved strength, toughness and hardness, as well as other properties, including optical transparency and superelasticity at elevated temperatures.

Field emission gun scanning electron microscopic images of compacts pressed from sieved yttria partially stabilised zirconia spray freeze dried powders, (a) without Freon and Pressed at 380MPa, and b) with one volume per cent Freon and pressed at 250MPa. Using their methods, the researchers claim to have produced nanocrystalline yttria partially stabilised zirconia (YSZ) that is as strong as submicron YSZ (one gigapascal), more damage resistant (indentation toughness measurements of up to 14.5MPa m1/2), and that has immunity to hydrothermal ageing for up to two weeks at 245ºC and four bar. Conventional YSZ disintegrates within one hour.

Processing parameters

The difficulties in turning nanopowders into components lie in creating homogeneous high density green bodies with a nanograin size, explains Professor Jon Binner, at Loughborough.

His team, however, has managed to slip cast bodies that are ~52% dense and up to 60mm in diameter, without cracking, by controlling the slip rheology. The nanosuspension has been concentrated to ~30vol% solids, by evaporation at 60ºC and exposure to ultrasound to break up the agglomerates, while retaining a viscosity of <0.2Pa s.

Furthermore, rather than machining cavities directly into cast blocks of plaster of Paris, PVC inserts shape the moulds for better surface finishes that further inhibit cracking.

Dry pressing

However, more than wet forming, ‘industry prefers dry pressing’, says Binner. ‘But nanopowders do not flow or compact well, and require granulation – these [granules] can be too strong and don’t crush.’

The researchers have found that combining spray freeze drying with one to two volume per cent of the chemical Freon yields granules with flowability comparable to commercial submicron YSZ, and that crush at 250MPa. The latter is within the capabilities of existing ceramic processing equipment (see images above). Pressing these ‘granulated powders’ also does not cause sticking to the dies, delamination or capping.

The team says it can now produce homogeneous green bodies with a theoretical density of ~55%.

Firing ahead

This improvement is vital. ‘Due to poor green densities, massive grain growth can occur
[while heating], eliminating any possible advantages from starting with nanopowders,’ explains Binner. A pressureless sintering technique was needed to prevent this.

‘We can now [have] final mean grain sizes of 65nm from powders of ~18nm in diameter – an increase in diameter of less than four. [That] is unusual for pressureless sintering. We have achieved it by combining two-stage sintering with microwave sintering.’

A hybrid microwave/radiant sintering furnace applies a fixed level of microwaves, with a
varying level of radiant power according to the required temperature-time profile. The two-stage cycle involves heating to an initial temperature for about six seconds (T1) before cooling to a lower temperature. In the case of slip cast bodies, T1 is at 1,150ºC with three hours at 1,050ºC.

Technology transfer

While research remains to be done to improve the humidity drying times of slip cast products, the team is looking to license its technology.

CERAM, an expert in materials testing and analysis, based in Stoke-on-Trent, UK, has conducted an independent market survey for Loughborough. Speaking to Materials World, Stuart MacLachlan of PowdermatriX (a particulate engineering network run by CERAM) says, ‘Industry is eager to exploit nanostructured ceramics. It is important to give confidence that tangible improvements can be made, as shown at Loughborough. However, industry will also need to understand how the process can be scaled up from laboratory to factory’.

This is now the focus of the work.