No more scratches

Materials World magazine
,
1 Nov 2017

A double discovery could lead to scratchproof cars and a cheaper alternative to diamonds used in electronics, as Ellis Davies reports. 

Graphene and boron nitride are two materials with much potential in the world of composites, nanoelectrochemical systems, electronics and sensing. But how do the mechanical properties of these materials change when multiple layers are applied? This is an area of little coverage, according to Dr Elton Santos of Queen’s University Belfast, UK, who, along with a team of international researchers from Australia, China, USA, Japan and Korea, studied this change and discovered two key developments – firstly, graphene develops superlubricity, and secondly, boron nitride becomes one of the strongest electrically insulating materials, on a par with diamond.

The two discoveries have practical applications. The superlubricity of bilayered graphene could lead to scratchproof paint for cars, whereas boron nitride has applications in electronics, aerospace, sports and civil engineering through integration with polymers and compounds reinforcement. Santos explained the main difference between the two elements, ‘The main difference between them is that graphene gets weaker as more layers are put together. This is the opposite of boron nitride, which remains almost with the same mechanical properties.’ 

Superlubricity is a concept in which all friction vanishes from the surface of a material. The team found that bilayered graphene entered this superlubricity state, which is related to atomic orbitals that compose carbon atoms. Santos said, ‘Normally, to generate friction, some orbitals must overlap and heat, or some other form of energy must be released. Surprisingly, our research shows that graphene does not require this – it spontaneously slides on top of other layers, but does not release heat. This means that graphene, which is 300 times stronger than steel, becomes mechanically weaker and can easily break.’

Santos added that this property places graphene in a group of just a few materials. Because the Young modulus – a measure of the ability of a material to withstand changes in length when under tension or compression – is still high for a thin material – a few nanometres thick – it is still useful as a bilayered material.

Testing under strain revealed that bilayered graphene experiences sliding of layers, but the identically layered boron nitride resists. This sliding concentrates stresses to the lowest layer bonded, effectively shielding the other layers, which would stop the appearance of a scratch.

Cheaper than diamonds

‘Boron nitride becomes very stable with more layers. It is comparable to diamond in terms of strength, but is cheaper, more flexible and lighter,’ Santos told Materials World. Diamonds are used in a range of electronic devices, although companies are beginning to integrate boron nitride and graphene into prototype smart-devices. Boron nitride is also a common lubricant used in several automotive and industrial applications.

The team discovered that boron nitride layers can be easily integrated into tiny electronic circuits, or can be used to reinforce structures, as boron nitride is robust against shocks or mechanical stress. ‘A number of people use it as a substrate for electrical circuits. You can use inks or solutions to print circuits with boron nitride in specific positions, which can be covered in graphene,’ said Santos. 

The team is currently looking for combinations of 2D crystals to be used for similar applications. ‘The future is bright for 2D materials because of the development, progress and research currently being performed worldwide,’ summarised Santos.