Fig xA The a XRD patterns and b

Graphene as one MPI-0479605 thick 2D layer of SP2 carbon arranged in a honeycomb lattice is obtained great attention [12]. Graphene with its combination of large specific surface area, two dimensional high aspect ratio sheet geometry and outstanding mechanical properties [13], [14], [15], [16] and [17] would contribute to application in ceramic matrix composites (CMCs). Moreover, graphene is considered to have super-electrical properties and very high thermal properties [18], [19] and [20]. All these unique properties of graphene make it a potential nanofiller in composites materials. Current studies indicated that significant improvement of mechanical properties of polymer based composites with relatively low graphene fillers loading. However, to our knowledge, graphene-ceramic composites are not well studied till now. In recent work by Walker et al. [21], they reported an improvement of 235% in fracture toughness with only a 1.5 vol% loading of graphene in Si3N4 matrix by SPS. Wang et al. [22] employed spark plasma sintering to prepare graphene nanosheet/Al2O3 composites with a 53 vol% increment in fracture toughness with a 2 wt% loading of graphene. Various toughening mechanisms including graphene platelets pulled out and bridging were observed. The addition of graphene nanofillers in Al2O3 matrix resulted in grain size refinement. Tapaszto et al. [23] prepared CNT-Si3N4 and graphene-Si3N4 composites at the same processing conditions, respectively. They found that an enhancement of 10–50 vol% in mechanical properties, such as fracture toughness, hardness, bending strength and Young’s modulus for graphene-Si3N4 composites compared to CNT-added composites with the same loading, but overall decreasing of both composites compared to monolithic Si3N4. Kun et al. [24] prepared graphene-Si3N4 composites by HIP with 1 and 3 wt% graphene addition. Study showed that graphene platelets may cause porosity in composites which resulted in decreasing both of Young’s modulus and bending strength with increasing the amount of graphene platelets. However, Kvetkova et al. [25] found a contrary achievement in a similar research. They prepared 1 wt% graphene-Si3N4 composites with different contents of nanofillers by HIP and indicated an improvement in fracture toughness for all composites compared to pure Si3N4 ceramics, but deceased in hardness because of increasing residual porosity except for multilayer graphene composites. Pull-out of GPLs, crack bridging and crack deflection had been observed which could help to enhancement of fracture toughness. Liu et al. [10] fabricated the GPL reinforced zirconia toughened alumina (GPL/ZTA) by using SPS. The addition of only a 0.81 vol% graphene nanofillers into ZTA composites resulted in a 40% increasing in fracture toughness. Similar to the former works showed above, various mechanisms such as pull-out, bridging and crack deflection were observed.