Recently, carbon nanotubes (CNTs) are attracting significant attention as potential toughening agents in micro-grained ceramics due to its excellent properties, such as low density, high aspect ratio (1000–10000), tensile strength (150 GPa), and an elastic modulus of 1 TPa [9]. Initial investigations on toughness of CNT/ceramic composites indicated little or no improvement in fracture toughness due to difficulties such as dispersion of CNT in matrix, and methods used for determining fracture toughness that failed to uncover the actual failure mechanism. Zhan et al. have observed three fold increase in fracture toughness for 10 vol% CNT-Al2O3 compared to pure alumina [10]. Recently, CNT reinforced ceramics composites were processed using powder metallurgy routes where dispersion of CNT in the matrix were achieved by ball milling using high energy ball mills. It should be noted that conventional hot pressing was avoided while compacting these materials as hot pressing might damage CNTs and degrade their properties [10]. It was observed that conventional sintering techniques like hot pressing were not helpful in compacting these composites as sintering this composites at higher temperatures for longer time results in damaging of CNTs. In a recent investigation, Ma et al. reported only incremental (10 %) increase in toughness for CNT reinforced SiC composites processed using conventional hot pressing [11]. While significant progress has been made towards using chemical, mechanical, and ultrasonic methods to enhance the dispersion of the CNTs in various matrices, the non-uniform distribution of CNTs is still a serious issue. High quality carbon nanotubes dispersed effectively in the ceramic matrix is very essential in order to carry loads and transfer stress which results in toughening of the ceramic. Graphene is a one atom thick 2-D layer of sp2 carbon arranged in a honeycomb lattice [12, 13]. It is considered the building block of different forms of carbon such as fullerene, carbon nanotube and graphite. While fullerene and carbon nanotube can be visualized as graphene rolled into spherical and cylindrical shape, graphite is basically graphene sheets stacked together to form a 3-D structure. Graphene or few layer of graphene possess a combination of unique set of electrical, optical, and mechanical properties. Specially, these properties provide a way to overcome the shortcomings of other materials that are currently being used or worked on. Mechanical properties of graphene has been much less investigated compared to its electronic and optical properties until recently. Like other allotropes of carbon (CNT), graphene also possess excellent mechanical properties. Recent investigation of suspended graphene sheet using AFM nanoindentation by Lee et al. revealed that graphene is the strongest material ever measured with elastic stiffness of 340 N/m and breaking strength of 42 N/m [14]. These values translates to 1.0 TPa Young’s modulus and 130 GPa intrinsic strength for bulk graphite. Moreover, graphene can sustain more than 20 % local strain before fracture. In a separate study, it was shown that intrinsic strength of graphene film reduces slightly as the number layers is increased from single (130 GPa) layer to three layers (101 GPa) [15]. One of the possible ways to utilize these unique properties of graphene is to use it as nanofillers in nanocomposites. Even though graphene/polymer nanocomposite system is well studied for quite some time now, graphene/ceramic composite system has not been well understood. Most of the studies published so far indicate the possibility of improvement in properties due to addition of graphene in ceramic matrix. It has been reported that graphene nanosheet reinforced Al2O3 showed a percolation threshold of 3 vol%, however, for 15 vol% graphene nanosheet reinforcement electrical conductivity was increased to 5,709 S/m which is 170 % higher while compared with the result available for CNT reinforced Al2O3 [16]. This improvement in electrical conductivity was attributed to network like structure of graphene in the composite. It was also observed that graphene nanosheet restrained grain growth of Al2O3 in the composite during spark plasma sintering process [17]. Excellent improvement (53 %) in fracture toughness of alumina by graphene reinforcement has also been observed by Wang et al. [17]. This improvement in fracture toughness was mainly due to crack bridging and nanosheet pulling out. Liu et al. reported toughening of a multi-component system of ZrO2-Al2O3 by graphene nanoplatelet [18]. Addition of graphene platelet increased fracture toughness by 40 % by various extrinsic toughening mechanisms. Walker et al. reported a remarkable 235 % increase in fracture toughness of Si3N4 by only addition of 1.5 vol% [19]. This increase in fracture toughness was attributed to proper processing of the composite and a new toughening mechanism (out of plane crack deflection) that was observed for these composites. In their work, dispersion of the graphene was achieved by colloidal processing and compaction was obtained by spark plasma sintering. Recently, graphene-Si3N4 system has also been studied for tribological properties [20]. It was found that at lower wt% addition graphene bonds strongly to the matrix and does not provide any wear resistance. However, 3 wt% graphene addition reduced wear rate by 60 % compared to monolithic Si3N4. In our previous work, we have observed strengthening of SiC matrix with graphene addition due to better dispersion [21]. In the current investigation, a novel approach is presented that combines the techniques of polymer precursor processing and spark plasma sintering (SPS). This process can yield bulk SiC with uniform dispersion of graphene nanoplatelets with the possibilities of retaining grain sizes down to the sub-500 nm range while still being able to fabricate net-shape dense forms. The aim of the study is to establish a novel processing technique to enable fabrication of fine-grained SiC reinforced with uniform graphene dispersion, investigate the microstructure and the mechanical properties as a function of graphene content. 166 A. Rahman et al.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==