PSI - Issue 23

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

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

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P. (2014) and Saggar R. (2013). In order to follow the quantitative effects the composites containing different contents of the nano-filler randomly distributed in the matrix were prepared.

3.1. Carbon nanotubes in silica glass matrix

A model CNTs/glass composite system based on a dense and almost entirely amorphous silica glass matrix was used to investigate the extent of nanotube toughening in brittle materials. The particular advantage of the glass matrix is the absence of confounding microstructural features that could affect K IC , such as porosity or grain boundaries. The relative density of the sintered samples was found to exceed 98 % in all cases independently of CNTs content (from 0 to 15 wt.%). SEM observations of fracture surfaces confirmed absence of porosity and that the random nanotube distribution reached in the green body was maintained following the high temperature consolidation process. Fracture toughness of the composite, as measured by both methods, chevron notch technique and indentation toughness, increases linearly to 100-120 % relative to silica monoliths, up to unusually high CNT loadings (15 wt.%). Toughening mechanisms associated with CNT crack bridging, crack deflection, and CNT pull-out, respectively were identified, although the last appeared to dominate. Simplified quantitative estimate by Cho (2011) suggested that the conventional fibre pull-out mechanism is indeed sufficient to explain the level of toughening observed experimentally in these CNT/glass composites. For the CVD grown CNTs, the small diameter is associated with a reduction in pull-out toughening, but a broadly constant debonding contribution compared to conventional carbon fibre system. However other mechanisms may become significant in the future, for nanotubes with distinctive characteristics, such as small diameter, high strength, Cho (2011).

3.2. BN nanotubes in borosilicate glass matrix

Boron nitride nanotubes with two different inner morphologies, i.e. hollow cylindrical and bamboo-like, were used to reinforce a borosilicate glass (BS) matrix. The obtained results have shown that the boron nitride nanotubes contributed directly to the reinforcement of the amorphous BS matrix.

Fig. 1: Typical toughening mechanisms in composites reinforced by nanotubes a) BN nanotubes in silica glass - crack bridging; b) BNNT in silica glass – crack deflection/branching; c) fracture surface – telescopic pull-out (sword-in-sheet). Young’s modulus and the hardness of the 5 wt.% BNNTs composites decreased by 7 % (68.0 and 5.8 GPa respectively), when compared to the pure amorphous BS glass (71.0 GPa and 6.24 GPa, respectively). This is related to the fact that the relative density of the BS glass composites decreases slightly with increasing BNNT content. The absolute values of K IC as well as the relative increment in the fracture toughness measured by both the indentation as well as the chevron notched beam techniques are very similar, reaching about 30 % for the 5 wt.% BNNTs composites (1.10 MPam 1/2 ) when compared to the pure borosilicate glass (0.85 MPam 1/2 ). Significant BNNTs pull-out, bridging, stretching, as well as a crack deflection around the BN nanotubes were observed (Fig. 1 a)b)) as the apparent toughening mechanisms in the reinforced glass matrix composites. The shorter pull-out of the bamboo like BNNTs was produced by a combination of their initial length and easier disconnection in the inner nanotube joints. The hollow inner structure of the cylindrical BNNTs allowed inter/wall sliding to be active, while the outer walls of the nanotubes were strongly bound to the matrix. The pull-out contribution was