Automotive, aerospace, and thermal management applications rely on ceramic-reinforced metal matrix composites for safety reasons. Despite the strength and toughness of these composites, the materials tend to lack ductility, which limits more widespread employment. Adding nanoparticles can overcome this limitation, but it has proven challenging to distribute the particles evenly throughout a metal matrix. Now, however, researchers think they have come up with a novel solution to the problem using graphene [Fadavi Boostani, A., et al., Composites: Part A (2014), doi: http://dx.doi.org/10.1016/j.compositesa.2014.10.010].
Ceramic SiC nanoparticles are wrapped in graphene nanosheets, rather like the layers of an onion, before being added into the metal matrix. Ball milling, an industrial process for grinding materials into very fine powders, is used to encase nanoparticles with highly flexible sheets of graphene. The encapsulation prevents the agglomeration of nanoparticles once incorporated into the molten alloy matrix. The coated particles also seem to resist the ‘push’ toward accumulation at grain boundaries.
The innovative approach has been developed by researchers at the Universities of Wollongong and Technology in Australia, Northeastern University in China, and Sahand University of Technology and Islamic Azad University in Iran.
“The most important novelty of this work is reaching toward a uniform distribution of nanoparticles in aluminum-based composites for the first time using the encapsulation capacity of graphene sheets,” researcher Zhengyi Jiang of the University of Wollongong told Materials Today.
The result is an improvement in yield strength and ductility of 45% and 84%, respectively, using just 1 vol.% of graphene nanosheets.
“The advantages of these composites are higher tensile properties and especially tensile elongation,” explains Zhengyi Jiang. “This work demonstrates a new roadmap for the implementation of graphene sheets in enhancing mechanical properties of metal matrix composites.”
The boost in tensile properties could be the result of more than one mechanism at work, suggest the researchers. The onion-like shells of graphene around the nanoparticles could reduce the susceptibility of SiC to cracking, which would in turn increase the threshold stress limit for the composite. The graphene could also block the movement of dislocations through the matrix, making propagation difficult. Fiber pull-out toughening, where growing cracks come across reinforcements in the matrix that require additional energy to move past, could also be having an effect.
The composites could be useful for aerospace applications where high tensile properties, combined with low weight, are highly desirable.
“Some modifications of this approach are needed before scaling up to mass production,” says Zhengyi Jiang. “But this is a completely practical approach to the production of advanced composites using a simple ball milling method.”