The need
A new class of ultra-lightweight and high strength metal composites could potentially revolutionise the use of engineering materials within harsh environmental conditions. But, because metal composites are currently more expensive than common engineering metal alloys, they are limited to certain uses where the cost benefit can be realised. A significant breakthrough in the cost competitiveness and availability of these metal composites is required for unleashing innovation across many industries to help achieve UK government’s ambition to cut emissions by 2035.
TISICS, a company which integrates metals with a ceramic reinforcement, creates metal composites to give long-lasting, increased mechanical and thermal performance at a fraction of their original mass. These novel metal composites have the potential to reduce CO2 emissions in even the most challenging of industries to decarbonise, such as aviation. An estimated 3.7 million tonnes of CO2 emissions could be saved annually by applying this technology across the landing gear of wide-body aircraft fleets.
TISICS’ metal composites are reinforced by silicon carbide (SiC) monofilaments, high-strength ceramic fibres with the same width as human hair, to give the new components their extraordinary properties. The monofilaments are produced using reactors that push the limits of chemical vapour deposition (CVD). In this CVD process, SiC is deposited using highly reactive gases onto a 1,000 °C tungsten wire surface.
To meet the growing demand for lightweight metal composites in green applications, TISICS required further innovations to increase the production rate within the manufacturing process of their monofilament. However, trials to date have shown that accelerating the deposition speed has a detrimental effect on the monofilament’s mechanical performance.
Current methods of analysing monofilaments, including confocal Raman spectroscopy, have been unable to explain this deterioration in the monofilament’s properties during the accelerated production process, possibly because measuring the microstructural changes is beyond the analytical capability of this technique. Without this detailed knowledge of the structure at the nanoscale, further work on increasing the production rates was limited to development by trial-and-error.
The solution
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As part of NPL’s Measurement for Recovery programme (M4R), TISICS was given access to NPL’s expertise, unique facilities and their cutting-edge technique, Tip Enhanced Raman Spectroscopy (TERS). TERS pushes the chemical analysis capability of standard Raman spectroscopy to the nanoscale by combining the technique with the high spatial resolution of atomic force microscopy (AFM). The unparalleled capability of TERS can measure the composition and structures of amorphous or crystalline features that are only a few tens of nanometres in size.
The use of TERS generated a breakthrough in measuring the changes to the monofilament’s material properties. The nanoscale resolution of the TERS analysis successfully revealed the subtle microstructural changes that occurred when increasing the production rate. The analysis revealed there was a transition from stoichiometric SiC to a gradient structure of SiC and C across the monofilament’s coatings. Changes observed in this gradient structure at higher production speeds explained the detrimental impact on the mechanical properties.
The observed differences in the coatings were used to suggest changes in the TISICS manufacturing process, such as altering the reactant gas concentration and adjusting the thermal gradients within the reactor, to produce monofilaments at a higher production speed with the same properties as the original monofilament.
The impact
As a result of the support provided by NPL, the novel technique improved TISICS’ understanding of the complex coating deposition conditions that occurred in their manufacturing process and has revealed new opportunities for improvement.
TISICS found that the processing speed could be increased by 20% without needing to increase the amount of the reactive silane gas used for depositing SiC. Optimising the feedstock usage and production rates will lower the manufacturing costs to produce monofilaments, and subsequently economise the fabrication of MMCs. This enables composites to be used in more applications where the mass savings can offer significant benefits.
These manufacturing improvements will be pivotal for the mass manufacture of SiC monofilaments, as TISICS transitions from a pilot plant to a full-scale commercial green production factory over the coming years.
Potential applications where TISICS metal composites could provide unrivalled benefits over other materials include greener aircraft, spacecraft and electric vehicles. TISICS’ application of metal composites into ultra-lightweight landing gear components for airplanes is currently supporting steps towards the helping the aviation sector to become a net zero emitter by 2050.