Interface characterization and scratch resistance of plasma sprayed TiO2-CNTs nanocomposite coating

TiO2 and TiO2-CNTs nanostructured coatings were fabricated by plasma spray with agglomerated nanocrystalline powders. The microstructure and interface between TiO2 and CNT of the nanocomposite coating were observed extensively using scanning and transmission electron microscopy. The scratch resistance of the two coatings was compared under a constant load scratch test. Results showed that both nanostructured coatings consisted of fully melted (FM) regions, partially melted (PM) with solid-state sintered regions, and pores. The addition of CNTs can effectively inhibit the growth of TiO2 grains and has a fine-grain strengthening effect. Besides, the interface bonding includes good chemical bonding by locally forming carbothermic reduction product in the FM regions and relatively weak physical bonding by van der Waals forces in the PM regions. In the scratch test, the lateral friction force and material removal volume of TiO2-CNTs coating were reduced remarkably compared to those of un-reinforced coating. The corresponding strengthening mechanisms induced by CNTs are scratched-out CNTs rolling on the scratch track and CNTs bridging inside the nanocomposite coating.