Study of mechanical properties and oxidation resistance of reduced graphene oxide reinforced ceramic composites

The mechanical properties and oxidation resistance of reduced graphene oxide (rGO)/ceramic composites are investigated in this thesis, focusing on the effects of 1) sintering temperature and 2) rGO dimensions on rGO reinforcement performance in rGO/ceramic composites as well as 3) graphene oxide (GO) addition on the oxidation resistance of rGO/ceramic composites. The effect of ceramic sintering temperature on GO reduction and toughening effect of rGO was studied using the rGO/Al2O3 ceramic composites fabricated by spark plasma sintering (SPS) at 1350–1450 °C with 1 wt% GO. The hardness of the rGO/Al2O3 decreased and more composite chipping under indentation occurred in the samples with increasing sintering temperature. Meanwhile, GO was thermally reduced to a higher degree when sintered at a higher temperature. The higher degree of GO reduction resulted in a weaker interlayer shear strength for rGO. Upon indentation, sliding and lateral crack propagation occurred along the in-plane direction of the rGO sheets, which led to decreased hardness and chipped indentation. In addition, rGO/Al2O3 composite samples exhibited anisotropic hardness, indentation fracture toughness (Kc) and indentation damage on the perpendicular and parallel surfaces related to SPS pressure direction. The anisotropic mechanical behaviour was due to the alignment of rGO sheets caused by uniaxial pressure applied during SPS. The effects of GO dimensions on the microstructure and mechanical properties of rGO/Al2O3 composites were studied by comparing composite samples reinforced by two types of GO with different thicknesses and lateral sizes. The thin GO promoted grain size refinement and enhanced grain boundary pinning effects. Al2O3 composite reinforced by thin and large GO had higher hardness and Kc than composite reinforced by thick and small GO. The addition of thick GO leads to a reduction of hardness and generation of lateral cracks at the Al2O3 grain boundaries under indentation due to low rGO interlayer strength. A model calculation showed that the spacing between two parallel rGO platelets was one Al2O3 grain at rGO addition of 0.77 wt%. We concluded that the increased hardness and Kc are due to the fine grain size and thin rGO at Al2O3 grain boundaries. The oxidation behaviour of rGO/Al2O3 with various GO additions was investigated at 800–1200 °C for the first time. After oxidising at 800 and 1000 °C, the hardness of the composites with less than 3 vol% GO was comparable with those at room temperature. The rGO was completely oxidised in composites with more than 5 vol% GO above 800 °C due to the interconnected rGO network which provided oxygen diffusion paths. The excellent oxidation resistance of rGO/Al2O3 composite samples with less than 3 vol% GO below 1000 °C is due to the Al2O3 grain seal effect and low oxygen diffusion coefficient. The oxidation behaviours at 1200 °C were also studied. Due to the enhanced oxygen diffusion coefficient, significant oxidation was observed in composites with less than 3 vol% GO. The oxidation rate at 1200 oC increased by an order of magnitude when the added GO was increased from 0.5 to 3 vol%. The rGO/SiC composites with various GO additions were fabricated at 1750 °C by SPS to study the effect of GO addition on oxidation resistance of rGO/SiC composites at 1200 oC in static air. Results showed that the indentation Kc first increased from 3.89 to 6.09 MPa m1/2 as GO addition increasing from 0 to 1 wt%, then decreased to 1.85 MPa m1/2 with increasing GO to 7 wt%. In addition, thermal conductivity decreased with increasing GO addition due to the finer grain size and enhanced interfacial phonon scattering. The oxidation rate increased by one order of magnitude as GO addition varied from 0 (5.24× 10-17 m2/s) to 7 wt% (7.44× 10-16 m2/s). The lower oxidation resistance in graphene-containing samples was mainly due to finer grain sizes and rGO layers on the grain boundary, providing more oxygen influx paths. It was found that the addition of GO weakened the oxidation resistance of the rGO/SiC composites.