PSI - Issue 23

Ivo Dlouhy et al. / Procedia Structural Integrity 23 (2019) 431–438

436

Ivo Dlouhý et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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fillers without affecting their performance, coming from specific structure, including their shape. Studies carried out up to now have shown that nanotubes, and nanosheets in particular, often produce localised porosity partly because of forming agglomerates in case of higher fractions and partly also because of being localised on powder boundaries. The common problem with mixture preparation of matrix precursor and nano-filler is a prevention of particle agglomeration, so the mixing techniques of ceramic matrix and nanotubes/nanosheets powders requires high energy to overcome the high surface energy of graphene and BNNS causes their agglomeration. On the other side there are positive effects of nanotubes and nanosheets on sintering, mainly in polycrystalline matrices. Among others, this is seen in grain growth reduction by grain wrapping. However, the nanofillers also inhibits densification as driving forces for densification and grain growth are interrelated. In addition, the flat morphology of nanosheets is more capable to survive high pressure processing that the tubular structure of nanotubes. This makes the nanotubes more susceptible of buckling and fracture during processing. The almost 2D nanosheets structure brings a higher specific surface enabling better to control interaction with the matrix.

Fig. 3: Effect of nano- filler content on Young’s modulus (rigidity) and Vickers hardness It follows from Fig. 3 , that for both the nanotubes and nanosheets, the rigidity in terms of Young’s modulus and hardness are enhanced only for low contents of the reinforcing phase. In such a case the crack deflection model published by Kotoul M. (2003) incorporating Young’s modulus increase appears to be valid only for nanofiller contents up to 2 %. D ecrease of the both Young’s modulus and hardness can be observed once exceeding this value. Comparing the hardness for composites formed by BS matrix reinforced by nanotubes and nanosheets (Fig. 4) it is evident that the role of the nanosheets is more effective. For polycrystalline matrices, attempts to apply higher volume fractions of nanosheets result in either a decrease in toughness or low K IC improvements that can be achieved comparing to much lower nanosheets contents. Partly this is because the nanosheet cannot enter inside the grains in polycrystalline ceramics and higher contents yields more than one nanosheet occurrence at the grain boundaries. In overall , the toughening effects of nanotubes and nanosheets can be seen in two groups of micromechanisms, the first one belongs to common micromechanisms like crack deflection and branching, crack bridging etc. Nanosheets, both GNSs as well as BNNSs, have energy dissipating mechanisms that are unlike the conventional toughening mechanisms of whisker and fibre reinforcements. It is a “ matrix independent ” intrinsic energy dissipating mechanisms which consist of nanosheet kinking and interlayer sheet sliding. Effectiveness of fibre type pull-out reinforcements, such as graphen and BNNSs, is heavily dependent of the interfacial stress transfer between the nanosheet and the matrix, e.g. Zhang L. (2014). Exceptional strength and elastic modulus of nanosheets, both graphen and boron nitride, maximise the resistance to crack propagation by bridging mechanism. There is however additional energy dissipation due to the weak van der Waals bonding between individual layers. Once the bonding of nanosheet to matrix is stronger than the bonding strength between nanosheet layers the interlayer shear fracture can occur as observed in fracture surfaces (Fig. 4). Embedded (between two grains) GNPs provide toughening mostly by crack bridging, however, the confinement by