PSI - Issue 23

Ivo Dlouhy et al. / Procedia Structural Integrity 23 (2019) 431–438 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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recognised up to 400 nm for both types of the BNNTs, but became less significant when the pull out length was larger due to the lack of energy dissipation during the sword-in-sheet mechanism. Extensive crack bridging by BNNT was found in the present investigation across the maximum crack openings of about 300 nm for the radial indentation cracks. It was observed that the entangled BNNTs are stretched while their ends in the crack opening are embedded strongly in the borosilicate glass matrix (Fig. 1c).

3.3. Alumina nanocomposites reinforced by graphene

There was an improvement of about 40 % in the fracture toughness of the composites prepared with the addition of only 0.8 vol.% graphene (CNSs) as measured by indentation and 25 % and by chevron notch method. Graphen was found to be anchored in between the grains of alumina. Different toughening mechanism were observed including graphene pull-out, crack bridging, crack deflection and crack branching. A transgranular mechanics of crack propagation became more dominant with increasing graphene content. Graphen toughens the composites by making the crack paths more tortuous. The change in the mechanism of crack propagation is new compared to fibre reinforced ceramic composite. The fracture toughness and elastic modulus of the nano-composites decrease for composites with more than 2 vol.% of graphene. This is attributed to an increase in number of sites with inter connecting graphen nanosheets. Until the 2 vol. % the graphen appears to an effective reinforcing agent for alumina ceramics however. 3.4. BN nanosheets in borosilicate glass matrix BNNSs with concentrations of 1 to 5 wt.% used as reinforcement in borosilicate glass matrix shown relative density higher than 98 for all the composites investigated. The Young’s modulus of the BNNSs/BS composite samples did not change significantly comparing to BS glass. Nonetheless, the fracture toughness and flexural strength of the BS/BNNSs (5 wt.%) composite simultaneously increase by about 45 % from 0.76 MPam 1/2 and 82.2 for BS to 1.10 MPam 1/2 and 118.8 MPa for BS/BNNS (5 wt.%) composites due to several toughening mechanism including crack deflection, crack bridging and pull-out (Fig. 2). It was also observed that there was a good interfacial bonding between BS glass and BNNSs, leading to higher strength values for the composites.

1µm

1µm

500 nm

Fig. 2. Typical toughening micromechanisms as observed in composites reinforced by nanosheets (BNNSs in borosilicate glass matrix) a) crack deflection, crack kinking and crack bridging, b) crack branching, both in ceramographic section; c) nanosheet pull-out in fracture surface.

4. Discussion

The key issue of this investigation has been to evaluate, in what extent the incorporation of nanotubes and nanosheets effectively contribute to the toughening of ceramic and/or glass matrix not losing at the same time strength, hardness and other mechanical properties. The findings obtained can be seen in two levels: the first one being associated with composites processing, the other set of findings is seen in field of fracture micromechanisms that contributes to toughening under active assistance of nanotubes and/or nanosheets. CMCs reinforced by nanotubes and nanosheets are in almost all cases produced from powder precursors and hence the initial powder mixture preparation appears to be a critical step in ensuring homogenous dispersion of the nano