Preparation of carbon nanotube reinforced alumina composites and examination of their fracture mechanisms

Two kinds of the alumina composites made with chemical-vapor-deposition (CVD)-grown MWCNTs and thermal annealed version of this MWCNTs type that have highly crystalline layers and higher flexural rigidity were prepared to investigate the effects of such properties of the MWCNTs on the mechanical and electrical properties of the composites. SEM observations on the fracture surface demonstrated that no severe phase segregation was observed for the composites prepared by using the thermal annealed MWCNTs, whereas the CVD-grown MWCNTs revealed an inhomogeneous structure. The experimental results have shown that lower percolation threshold and higher electrical conductivity were observed for the thermal annealed MWCNT/alumina composites, presumably owing to the good dispersibility of the MWCNTs in the matrix. However no apparent change in the mechanical properties have been found in the two types of MWCNTs. TEM observation suggested that both types of MWCNTs, rather than pulling out from the alumina matrix, broke in the "sword-in-sheath" and "clean break" manner during crack opening in the composite. The present study implies that the use of MWCNTs having higher flexural rigidity leads to better MWCNTs dispersibility in the composites, and it should be critically important to prevent the rupture in MWCNTs.