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A novel building block material for the generation of non-brittle ceramic composites consisting of micron-sized alumina platelets homogeneously coated with multi-wall carbon nanotubes (MWCNTs) bound to their surface is described. The MWCNT phase is grown in situ from immobilised metal catalyst particles using chemical vapour deposition techniques. In-depth Raman and scanning electron microscope studies revealed that this approach solves the typical issue of MWCNT-agglomeration in ceramic matrices and paves the way to excellent control over MWCNT purity and concentration within the resulting composite material. Moreover, we show that the preparation of the catalyst is the most important factor for the generation of uniformly distributed MWCNTs of high-quality on these platelets. With these MWCNT-coated alumina building blocks, we have manufactured nacre-like biomimetic composites using spark plasma sintering. The resulting composites are electrically conductive and three-point bending tests show a transition from brittle/catastrophic failure to graceful failure, holding great promise towards multifunctional, tough and strong lightweight ceramic composite manufacturing. Image 1 [ABSTRACT FROM AUTHOR]

Graphene Nano-Sheets (GNS) with different lateral sizes (193, 373 and 1070 nm) were produced using liquid phase exfoliation and controlled centrifugation. Alumina-GNS (0.8 vol%) composites with different size GNS were consolidated using spark plasma sintering. The fracture toughness, hardness and elastic modulus of the composites decreased with increasing GNS size. Microstructural investigation of the composites suggested that their mechanical properties were directly related to: (1) average number of GNS present per unit volume in composite; and (2) relative size of GNS to alumina grains. Small size GNS resulted in a 35% improvement of fracture toughness of alumina-GNS composite. The mechanism of crack propagation changed as the size of the GNS increased. The smaller size GNS produced various toughening mechanisms, while the larger size GNS, with similar size to the alumina grains, resulted in grain boundary sliding. The composite with larger size GNS was machinable using ordinary tungsten carbide drill bits. [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]

Graphene nano-platelets (GNP) were synthesised using a liquid phase exfoliation method and then used to produce well dispersed silica-GNP composites. The tribological properties of the silica-GNP composites with different vol% GNP loading were investigated. The wear behaviour at room temperature was studied using a ball on disc technique with alumina and borosilicate (BS) balls as the tribological counterpart. In the case of alumina balls, GNP above a critical concentration decreased the coefficient of friction of the composite by ~20% while increasing the wear resistance of the composite by ~5.5 times. In the case of using BS balls there was an improvement of ~8.5 times in the wear resistance while the coefficient of friction did not change. In both cases a percolating network of GNP above a critical concentration provided a lubricating effect to the silica matrix, resulting in a significant improvement in the tribological properties of the nano-composites. The extent of damage to GNP during tribology testing was quantified using Raman spectroscopy. [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]

Abstract: Highly transparent pure alumina (65.4%) with an average grain size of 200nm was fabricated using high-pressure (>400MPa) spark plasma sintering. For the first time, highly transparent alumina ceramics were produced using low cost carbon fiber composite for the pressing punches. The high pressure device was also used to fabricate fully dense nanostructured alpha alumina with average grain size of 107nm. The developed low cost high pressure sintering apparatus might be applied to produce a wide range of pore free nanostructured materials. [Copyright &y& Elsevier]