The use of two-dimensional nanomaterials as fillers in ceramic composites is significantly increasing in the last decade for several reasons. Due to their high specific surface area, 2D fillers can enhance the mechanical performance of structural ceramics introducing certain microscopic mechanisms, but they can also provide them with new stunning functionalities such as electrical conductivity in otherwise isolating ceramics. Although the number of studies on ceramic composites with 2D fillers is growing at a good pace, there are still open issues regarding their strength and fracture toughness mechanisms, crack growth resistance, electrical conduction mechanisms or suitability for electrical-discharge machining, among others. This work presents an overview of recent advances on tetragonal yttria-zirconia composites with graphene based 2D fillers.
Zirconia is a well-known advanced ceramic with structural and biomedical applications such as milling balls, pieces in turbines, cutting tools, implants, and dental crowns, but there are still points to improve such as hydrothermal degradation and resistance to crack propagation in zirconia ceramics. Graphene nanosheets (GNS) possess an outstanding Young Modulus and extraordinary electrical conductivity. In these composites, the type, content, shape, size, crystallinity (structural integrity) and distribution of the 2D nanofiller in the ceramic matrix are key factors for their performance. Therefore, relevant mechanical properties such as flexural strength, fracture toughness and crack growth resistance, as well as wear resistance and functional properties like electrical conductivity can be evaluated in terms of the filler type, the processing parameters, (including different powder processing routines and spark plasma sintering temperatures) and microstructural features.
One of the main findings is the optimum filler content in the composites according to the pursued functionality. For structural applications, there is a critical graphene content which suppresses hydrothermal degradation and enhances fracture toughness and flexure strength (1-2.5 vol.% depending on the graphene nanostructure) while these properties significantly decrease for contents higher than 5 vol.%. For this critical optimum content (2.5 vol.%) reduced graphene oxide and few layer graphene inclusions have demonstrated an intrinsic reinforcement effect which enhances crack growth resistance during stable crack propagation in the composites. The crack growth resistance has been rarely explored in graphene ceramic composites up to date and gives an unambiguous proof of the reinforcement effect of the fillers, since no transformation toughening was detected. Concerning the electrical conductivity, contents as low as 1 vol.% few layered graphene are enough to attain electrical percolation when the sintering temperature allows an optimum recrystallization, which maximizes electrical conductivity in the composites. Thorough Raman spectroscopy studies have shown that high energetic powder processing techniques such as planetary ball milling under certain conditions can produce permanent structural defects in the graphene nanofillers impeding a complete recrystallization upon sintering and decreasing the composites’ electrical conductivity. High graphene filler content (20 vol.%) allows electrical-discharge machining of the composites at low electrode energy, producing a good surface finish and opening a new path for applications in which small parts with complex shapes are needed.