A revolutionary technique for testing nanomaterial strength

A UCL nanoscience research team has developed a new technique for testing the strength of nanomaterials, the miniscule make-up of all sorts of everyday objects. The secret is a process much more rapid and generic that anything previously imagined.

Nanomaterials are everywhere. In phones, computers, atmospheric pressure sensors. We’re often unaware of them, yet they’re part of our daily lives, mainly in the form of nanowires, nanotubes, thin films, nanoparticles and nanoribbons. Which is why their strength is important. Measuring it, however, isn’t easy, given their smaller than microscopic size. A UCL research team led by Profs. Thomas Pardoen and Jean-Pierre Raskin has nevertheless created a mechanical testing technique to study these materials from strain to fracture. The technique has saved scientists a lot of time.

Nanomaterials: cameras and smartphones

Concretely, nanomaterials are currently used in the form of layers between a micron (a millionth of a meter) and a nanometre (a billionth of a meter) thin. Thus it’s an extremely fine material that can be applied, for example, to glass to give it special properties, such as scratch-resistance or infrared or ultraviolet light filtration. To do this, layers of a few or dozens of nanometres are applied. They’re invisible until between 100 and 200 nanometres, when they become slightly opaque. Nanomaterials are also in microelectronics: they’re necessary for producing electronic components and integrated circuits. They’re found in cameras, smartphones and airbag sensors.

‘Nanomaterials make complex electromechanical functions possible’, Prof. Raskin says. ‘To understand what’s possible, we need to know their properties on the scale at which they’re used.’

Indeed, while it is current practice to conduct mechanical testing on macroscopic technology to verify the materials can withstand the rigours of their intended purpose, measuring the strength of nanoscopic materials is more complicated. ‘The properties of an aluminium rod one centimetre in diameter are different from those of a rod ten nanometres in diameter. While properties vary little between one meter and one millimetre, for example, we see big differences in the mechanical strength of materials smaller than one micron. Transferring and adapting data from one to the other isn’t reliable. Experimental trials are necessary to learn how this comes about at the nanometric scale. Because handling objects of this size is very difficult.’

Ten years of research

One of the most common tests in materials mechanics is uniaxial traction. It involves testing fracture resistance by pulling on the material to reach its distortion and breaking points relative to an applied force. But how is this done at this scale? Researchers opted for Micro Electromechanical System (MEMS) techniques. ‘But it’s horribly complicated’, Prof. Raskin says. ‘It won’t make science advance quickly.’

And so, for some ten years, UCL researchers studied another method, a traction technique for testing nanomaterial strength by harnessing residual stresses. ‘There are always residual stresses in an applied material’, Prof. Raskin says. ‘For example, varnish applied to furniture cracks after it dries too quickly. Why? Because every applied material has internal stress. We don’t know whether the material will react well to a given strain. The scientific community sees this as a major problem. As for us, we told ourselves that we had to put these stresses to good use.’

In other words: use a material whose behaviour is known in order to study another. For example, place silicon nitride, whose properties are partially known, on aluminium, to test the latter’s strength. The internal stress of the first will stretch the second. Under both materials, a sacrificial layer is etched with hydrochloric acid to create a ‘photograph’ of the test. ‘We take advantage of the internal stress of one material to distort another’, Prof. Raskin explains. ‘In accordance with the element’s geometry, we apply a certain force, which distorts and then breaks it. The idea is to multiply the number of samples in order to draw conclusions.’

International collaboration and funding

Since 2005, this technique has been under constant study by doctoral and postdoctoral researchers, as well as the subject of numerous international collaborations. In addition, UCL researchers, with their ULB and University of Antwerp colleagues, just published an article in the prestigious journal Nature Communications, in which they take stock of this revolutionary nanomaterials characterisation technique developed and patented by laboratories in Louvain-la-Neuve. This research is made possible by support from, among others, the Walloon Region Directorate General of Research and Energy Technologies, the FNRS and Action de Recherche Concertée de la Fédération Wallonie-Bruxelles.

Anne-Catherine De Bast

A Glance at Thomas Pardoen's bio