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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the ceramic grains inhibited other intrinsic energy dissipating mechanisms such as kinking and sliding of GNPs. The GNPs between grains may also facilitate the sliding of grains as sheets of graphene become shared.

1 µm

500 nm

Fig. 4: BS glass matrix containing BN nanosheets, interlayer shear sliding in nanosheets a) and b) as observed in fracture surface; c) schematics of the interlayer shear fracture. It follows from the Fig 5 that toughness enhancement appears to be more effective in polycrystalline matrices comparing to glass matrices. This is associated with the softer glass matrix comparing to alumina and zirconia and also with the absence of grain boundaries in glass matrices. The toughness enhancement is thus partly associated with composite sintering when the nanosheets (mainly) limit the grain growth at increased sintering temperature. Toughness enhancement appears to be more effective in case of nanosheets comparing to nanotubes (Fig. 5). This may be associated with higher effective contact area to the matrix. Taking into account similar finding for hardness/Y oung’s modulus, composites containing up 2 wt. % of nanosheets possess both, increase of the rigidity as well as fracture toughness enhancement.

Fig. 5. Quantitative comparison of toughening mechanisms (a) effect of nanotubes content left (b) nanosheets content effect right.

5. Conclusions The study has shown that effect of nanotubes and nanosheets incorporation into brittle ceramic matrix can be seen in two levels in principle: (i) their effects at stage of microstructure formation, in particular in polycrystalline ceramic matrices and (ii) toughening mechanisms affecting the crack propagation. (i) In the first case, decrease of defectiveness of the microstructure has been observed enabling to reach high density of the composite. The main effect however must be seen in fine grain structure formation in which the nanosheets in particular are effectively acting against grain growth at high temperatures. (ii) In the second case, synergy of several toughening mechanisms active during crack initiation/propagation and the following toughening effects have been observed: crack bonding and branching, crack bridging and pull-out, namely