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The high temperature grain growth behaviour of Y 2 O 3 partially stabilised ZrO 2 and ZrO 2 -CNT (0.5 and 2 vol%) composites was investigated in a series of isothermal experiments carried out between 1500 and 1800 °C. The grain growth activation energy was found to be 417 ± 20 kJ mol − 1 for the ZrO 2 , which increased with the addition of CNTs to 509 ± 41 and 497 ± 17 kJ mol − 1 for the 0.5 and 2 vol% composites respectively. A sintering exponent of 2 was observed for the ZrO 2 , corresponding to normal grain growth, whilst the nanocomposites showed a CNT drag dominated sintering mechanism, with exponents of 2.7 and 3.2 for the 0.5 and 2 vol% composites respectively. This work confirms and quantifies the direct influence of CNTs on the grain growth mechanism and activation energy of ZrO 2 -CNT composites. [ABSTRACT FROM AUTHOR]

The effects of hot-pressing (HP) and spark plasma sintering (SPS) methods on the grain size, microstructural features, and mechanical behaviour of graphene nanoplatelet/carbon nanotubes (GNTs) reinforced Al 2 O 3 nanocomposites were comprehensively studied. Different graphene nanoplatelet to carbon nanotube ratios were selected as the overall reinforcement content of composites prepared using HP and SPS. Highly densified samples (>98%) were obtained at 1650 °C under 40 MPa in Ar atmosphere, with dwell times of 1 h and 10 min for HP and SPS respectively. Both types of sample showed a mixture of inter- and transgranular fracture behaviour. A 50% grain size reduction was observed for samples prepared by HP compared to SPS samples. Both types of samples achieved a high flexural strength and fracture toughness of >400 MPa and 5.5 MPa m 1/2 , whilst SPS samples peaked at relatively lower GNT contents than those for the HP samples. Based on analyses of the morphology, grain sizes and fracture mode, similar toughening mechanisms for both types of sample were observed, involving the complex characteristics of the combined GNT fillers. [ABSTRACT FROM AUTHOR]

Abstract: Graphene was prepared using liquid phase exfoliation and dispersed in an alumina matrix using an ultrasonication and powder processing route. Al2O3–graphene composites with up to 5vol% content were densified (>99%) using SPS. The fracture toughness of the material increased by 40% with the addition of only 0.8vol% graphene. However for higher graphene contents the improvement in fracture toughness was limited. Graphene changed the mechanism of crack propagation for the alumina matrix from inter-granular to trans-granular. The formation of an inter-connecting graphene network promoted easy fracture for concentration ⩾2vol%. Elastic modulus remained nearly constant for up to 2vol% and decreased significantly for 5vol% due to the formation of the inter-connecting graphene network. Fracture toughness measured with the indentation and chevron notch methods were consistent up to 2vol% and at 5vol% the percolating network of graphene resulted in easy crack propagation with significant discrepancy between the results for the two methods. [Copyright &y& Elsevier]

The processing conditions for preparing well dispersed silica–graphene nanoplatelets and silica–graphene oxide nanoplatelets (GONP) composites were optimized using powder and colloidal processing routes. Fully dense silica–GONP composites with up to 2.5 vol% loading were consolidated using spark plasma sintering. The GONP aligned perpendicularly to the applied pressure during sintering. The fracture toughness of the composites increased linearly with increasing concentration of GONP and reached a value of ∼0.9 MPa m1/2 for 2.5 vol% loading. Various toughening mechanisms including GONP necking, GONP pull-out, crack bridging, crack deflection and crack branching were observed. GONP decreased the hardness and brittleness index (BI) of the composites by ∼30 and ∼50% respectively. The decrease in BI makes silica–GONP composites machinable compared to pure silica. When compared to silica–Carbon nanotube composites, silica–GONP composites show better process-ability and enhanced mechanical properties. [ABSTRACT FROM AUTHOR]