21.2 Material Fabrication As a first step amorphous–nanocrystalline SiC powder was prepared from a preceramic polymer, allylhydridopolycarbosilane (AHPCS) (Starfire Systems Inc., Malta, NY). The powder preparation process was started by heating the liquid polymer precursor to 650 ∘ C, at1 ∘ C/min, under an inert atmosphere and then holding it at 650 ∘ C for 10 min. This initiated the crosslinking of the polymer precursor. For complete conversion to amorphous–nanocrystalline SiC, the heating was continued till 1,400 ∘ C, at 3 ∘ C/min. The material was held at the final temperature for 1 h to ensure thermal equilibrium and complete processing. Finally, the material was cooled down to room temperature, at 5 ∘ C/min. Due to the release of hydrogen gas during the polymer to ceramic conversion, the final material contained large voids. This material was ground using a hand grinder until the particles passed through a colander of mesh size 12 followed by subsequent milling into fine powder ( 0.5 μm) using a high energy ball mill (Pulverisette, Fritsch GmbH). Ball milling was performed using a ball-to-powder mass ratio of 5:1 with tungsten carbide (WC) balls as grinding media for 15 min with 750 rpm. Note that the starting powder was amorphous and the purpose of milling was to decrease the particle size without inducing any phase transformation. The amorphous/nanocrystalline powder was subsequently used in spark plasma sintering for processing bulk SiC. Exfoliated graphene nanoplatelets, xGnP1-M-5 grade (99.5 % carbon) with an average diameter of 5 μm were obtained in dry powder from XG Sciences, Inc. (East Lansing, MI). Two sets of powder were prepared for this study. For preparing SiC-graphene powder mixture, controlled weight fraction (2 and 5 wt%) of graphene nanoplatelets was mixed with AHPCS polymer and ball milled using a planetary ball mill (PM-100, Retsch GmbH, Haan, Germany) for 30 min. Ball milling was performed using a ball-to-powder mass ratio of 10:1 with tungsten carbide (WC) balls as grinding media for 30 min with 300 rpm. Subsequently, this mixture was pyrolyzed using the similar procedure mentioned earlier. The pyrolyzed amorphous/nanocrystalline SiC reinforced with graphene nanoplatelets was carefully milled using parameters mentioned earlier such that uniform mixture without significant damage of graphene nanoplatelets can be achieved. It has been reported that controlled ball mill can aid in exfoliation of graphene nanoplates due to shear component of the applied stress [22]. SiCgraphene composite powder mixtures with varying levels of graphene reinforcements (2 and 5 wt%) was prepared for subsequent SPS densification. Another set of powder was prepared for comparison. In this case, AHPCS was pyrolyzed first to prepare SiC powder. Then, 2 and 5 wt% graphene nanoplatelets were mixed with SiC powder (prepared from AHPCS) and ball milled in isopropanol as dispersing media using the parameters mentioned previously. For ease of identification, the first set of powder will be referred as AHPCS-2 wt% C, AHPCS-5 wt% C and the second set of powder will be referred as SiC-2 wt% C, SiC-5 wt% C. Note that these are essentially SiC-graphene composite powder prepared in different way. Spark plasma sintering (SPS) was used to consolidate SiC and SiC-graphene powder using an SPS system (Model 10-3, Thermal Technology, LLC., Santa Rosa, California, USA). The DC pulse cycle for the system was 25 ms on and 5 ms off. The ball milled powder was loaded in a graphite die with an internal diameter of 20 and 10 mm of wall thickness. Graphite felt with a thickness of 4 mm was wrapped around the graphite die to avoid thermal loss during sintering. Temperature of the sample was monitored during sintering using an optical pyrometer through a hole of 2 mm diameter and 5 mm depth in the graphite die. Samples were sintered at 2,000 and 2,100 ∘ C, using a heating rate of 150 ∘ C/min under argon atmosphere. Sintering pressure of 70 MPa and a soak time of 10 min were employed for all the samples. 21.3 Experimental Procedure Phase analysis of the sintered compacts was performed using Philips Norelco X-ray diffractometer operating with Cu Kα (λ ¼1. 54178 A˚ ) radiation at 45 kV and 40 mA. The 2θ diffraction angle was varied between 10 ∘ and 90 ∘ at a step increment of 0.02 ∘ with a count time of 1 s. WITec alpha300 R Raman system with a 532 nm laser excitation was used for detailed investigations on fragmentation and exfoliation of graphene in the SPS sintered compacts. The buoyancy method was used for determining bulk density and porosity of the samples using a density measurement kit along with a highresolution analytical balance following the ASTM C830–00 test method [23]. Vickers hardness of the polished samples were obtained using a microhardness tester (Tukon microhardness tester, Page–Wilson corporation, Bridgeport, Connecticut, USA) at a load of 49 N applied for 30 s. At least five readings were collected for each sample and an average value is reported. Fracture toughness was estimated by direct crack measurement (DCM) using the Anstis equation (Eq. 21.1) [24] 21 Graphene Reinforced Silicon Carbide Nanocomposites: Processing and Properties 167
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