Carbon nanotube toughened hydroxyapatite by spark plasma sintering: Microstructural evolution and multiscale tribological properties

Carbon nanotube (CNT) reinforced hydroxyapatite (HA) composite synthesized using spark plasma sintering is investigated in this study. Quantitative microstructural analysis suggests that CNTs play a role in grain boundary pinning and are responsible for the improved densification and retention of nanostructure throughout the thickness of the sintered pellet. HA crystal forms coherent interface with the CNT, resulting in a strong interfacial bond. The uniform distribution of 4 wt.% CNTs in the HA matrix, good interfacial bonding and fine HA grain size help to improve the fracture toughness by 92% and elastic modulus by 25% as compared to the HA matrix without CNT. Toughening mechanisms have been explained in terms of interfacial shear strength and pull-out energy of CNT from the HA matrix. CNT plays a major role in improving the wear resistance of HA matrix at both macro- and nano-scale. It is concluded that graphene layer removal from the CNT surface occurs during macro-wear, but not for nano-wear. Thus, the coefficient of friction (CoF) in HA-CNT decreases in macro-wear due to lubrication available through delaminated graphene layers.