PSI - Issue 23

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

432

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

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1. Introduction

Carbon nanotubes (CNTs) have been shown possessing excellent potential as reinforcements in a wide range of composite systems, among others thanks to their exceptional intrinsic mechanical and other functional properties. After a number of CNTs composites exploring polymer matrices, the CNTs have been also used as reinforcing fillers in ceramic and glass matrices in the last decade as summarised e.g. by Nieto A. (2016). Whilst CNTs have been sometimes taken as a ‘new generation’ of carbon fibre, the mechanical effects of a thousand -fold diameter decrease to a length scale that is closer to several lattice parameter dimensions than conventional fibres, is not yet fully clear. According to Ye F. (2006) the reinforcement of barium aluminosilicate glass by 10 vol.% CNTs produced 143 % increase in fracture toughness in the composite. However, CNTs face also a major challenge of agglomeration and entanglement making it difficult to realise their actual potential, which they possess once deagglomerated in the matrices as described e.g. by Mukhopadhyay M. (2010). Analogous toughening mechanisms can be expected from principally new type of nano-reinforcement. As shown by Bertolla (2019) it could be based on deconstruction of microfibrillated cellulose into rod-like cellulose nanocrystals (CNCs) in concentrated low modulus sodium silicate solutions. They have considerably high length (800 – 1200 nm), being stiffer than Kevlar and stronger than steel. Graphene nanosheets (GNSs) and graphen oxide nanosheets (GONSs) have also attracted attention thanks to their unique mechanical, thermal and electrical properties, e.g. Porwal H. (2013a,c). Graphene appears to be ideal reinforcing filler enabling to modify simultaneously mechanical properties and fracture resistance of ceramics/glasses. There are three crucial problems in preparation of these composites however: (i) preparation of good quality graphen, (ii) homogeneous graphene dispersion in ceramic matrix and (iii) retention of the graphitic structure during high temperature processing and/or composite operation. Although Fan Y.C. (2012) reported reduction of graphen oxides to graphen during SPS sintering as an effective method of graphen formation, commercially available graphen nanoplatelets further exfoliated in solvents using ultrasonication and/or ball milling have found a number of successful applications in ceramic/glass matrices, as shown by Kun P. (2012). Similar effects in affecting the fracture resistance as for graphen have been recognised when applying boron nitride (BN) nanotubes (BNNTs) and BN nanosheets (BNNSs). They also suppose unique contribution to microstructure formation in BNNTs/glass/ceramics matrix systems. As shown e.g. by Golberg D. (2007) and Verma V. (2007) BNNTs exhibit comparable stiffness, elastic modulus and yield strength to CNTs due to their prevailing structure. They also possess a high tensile strength of more than 30 MPa like in CNTs. Individual BNNTs have been reported by Wei X. (2010) to withstand higher tensile loads than CNTs. They are additionally advantageous over CNTs as they can withstand higher temperatures up to 950 °C without oxidising and hence are considered chemically inert. Boron nitride nanosheets (BNNSs) though having similar properties to BNNTs, have an extra advantage as reinforcements thanks to geometrical benefits, similar to those of graphene having over CNTs, e.g. Porwal H. (2013a). Owing to the two-dimensional geometry, BNNSs provide higher specific surface area for the matrix to interact in comparison to BNNTs. As shown by Saggar R. (2015) thanks to this geometry, they have lower tendency to entangle, and hence less complicated methods are required for processing and dispersion of BNNSs. A hydro-pressure processing as suggested by Taveri G. (2018) appears to be promising low temperature sintering procedure that could enable specific nanocomposites formation. The aim of the contribution is to provide a view inside the current state of the art on the factual effect of nanoscale fillers incorporated into ceramic/glass matrix on fracture resistance of the composite. Special attention is focused on micromechanisms really active during crack opening under different reinforcements in different matrices. The investigated materials included model ceramic/glass matrix composites based on silica and borosilicate glass (BS) matrices, and, in addition, common ceramic polycrystalline matrices like alumina and zirconia. The nano fillers, namely carbon and boron nitride nanotubes, graphen and boron nitride nanosheets have been explored in different volume fractions in order to investigate their quantitative effect on toughening mechanisms. A model composite system based on a dense and almost entirely amorphous silica glass matrix was used to investigate the extent of nanotubes toughening in brittle matrices. Commercial multi-walled CNTs have been used; 2. Materials and experimental methods