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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they were synthesised by the injection catalytic chemical vapour deposition (CCVD) method, as described by Singh C. (2003). The composite samples were prepared by colloidal hetero-coagulation followed by spark plasma sintering; the details have been described by Cho J. (2011). Two different BNNTs types, cylindrical and bamboo like, commercially available, were incorporated into BS matrix. The BNNTs were produced by self-propagating high temperature synthesis and CVD annealing process, Wang J. (2011). It was necessary to purify and functionalise after receiving them. In order to get a homogeneous powder mixture the initial BS powder (particle size d 50 was 183 nm) was ball milled with 0.25 and/or 5 wt.% the BNNTs in ethanol. The slurry was dried and sintered using SPS facility at 775 °C and under a pressure for 7 min. Disc shaped samples in diameter of 20 mm were obtained and further investigated by Tatarko P. (2014). Graphen was prepared using liquid phase exfoliation and dispersed in alumina (particle size d 50 was 200 nm) using an ultrasonication, as described by Porwal H. (2013b). Alumina/graphene composites with up to 5 vol.% content were densified using SPS (HP D25/1, FCT Germany) at 1350 °C with a dwell time 5 min under 50 MPa pressure , the details are available in the same paper of Porwal H. (2013b). The BNNSs were prepared using bulk h-BN powder applying a liquid phase exfoliation technique, Saggar R. (2013). For the preparation of nanocomposites containing 1, 2.5 and 5 wt.% of BNNSs, dry BNNSs were suspended in ethanol and ultrasonicated for 2 h. Borosilicate glass powder (particle size d 50 was 183 nm) was then added to the ultrasonicated BNNSs and ball milled. The consolidation was carried out under the same conditions as in case of alumina/graphen composites, for details check paper of Saggar R. (2013). For fracture toughness determin ation rectangular bar samples (typically 2×3×25 mm) were used with chevron notches. The samples were tested in 3-point bending with crosshead speed of 0.01 mm/min and 1 kN load cell (Instron 8862 machine, USA). At least five samples were tested for each condition. Load vs deflection traces were recorded and the fracture toughness, K IC , was calculated from the maximum load, max , and the corresponding minimum value of geometrical compliance function, m∗ in , using the equation IC = max m∗ in / 1/2 . The chevron notch depth, 0 , was measured from SEM micrographs of broken specimens. Details and advantages of the method for fiber reinforced CMCs crack resistance evaluation have been described earlier by Dlouhy I. (2001). Fracture toughness was measured also by indentation technique. Crack length produced by Vickers indentations usually with a load of 9.8 kN for 10 s (Zwick/Roell ZHU/Z2.5, Germany) was measured. Calculation of the indentation toughness was carried out using the equation suggested by Anstis G.R. (1981) IC = 0.016 ( ) 1/2 3/2 , in which E is the Young’s modulus, H is measured hardness, P is indentation load and c is crack length at indents corners. At least 10 indents were measured for each sample in randomly chosen and well separated locations. The K IC determination using both the chevron notch and indentation tests showed that both techniques are capable to provide consistent evidence of the toughening role of nanofiller. However, it was also proven that the K IC values measured by the indentation were overestimated, in our first experiments, confirming the literature reports about the difficulty of providing accurate absolute K IC measurement when applying the indentation technique. The elastic modulus was measured using the impulse excitation resonance method using GrindoSonic Mk5i (J. W. Lemmens N.V., Belgium) on polished rectangular bar shaped samples. A piezoelectric vibration detector was used to detect the vibrations produced in the beam by tapping them with a light mechanical impulse. A mean value of 15 recorded measurement frequencies for each sample was considered, the averaged values of the frequencies were used to calculat e the Young’s modulus applying Genemod software. In order to examine the toughening mechanisms, SEM observation (Tescan LYRA 3 XMU, Czech Republic) of the fracture surfaces and the indentation crack paths was carried out. 3. Results Ceramic and glass matrix composites were prepared after careful precursors functionalisation and optimisation of the sintering technology, full details have been published in our papers by Cho (2011), Porwal H. (2013b), Tatarko