Effect of annealing temperature on the structural and mechanical properties of coatings prepared by electrophoretic deposition of TiO2 nanoparticles

In the present study, TiO2 nanoparticles coatings were prepared on 316L stainless steel substrate by electrophoretic deposition (EPD) process. The effect of a thermal treatment on the obtained coatings was studied. Indeed, after electrophoretic deposition, a thermal treatment is required to evaporate the solvent from the coating and to improve its cohesion and its adhesion to the substrate. Three temperatures were selected: 650 °C, 750 °C and 850 °C. The TiO2 coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDXS) and atomic force microscopy (AFM). Results of electron microscopy and atomic force microscopy observations showed that a smooth and uniform porous layer was provided with particle sizes of about 20-100 nm. XRD measurement indicated that TiO2 coating was single crystalline with a pure rutile structure after heating at 850 °C during 2 h. The mechanical properties of the TiO2 coatings were investigated by means of nano-indentation and scratch tests. It was observed that the increase of annealing temperature improves the hardness, the Young's modulus as well as the coatings adhesion to the substrate. Moreover, a new scratch test mode called 'wear mode' was performed to evaluate the wear resistance of the coatings. It was noticed that when increasing the annealing temperatures, the friction coefficient and the wear resistance of the coated samples were improved. Therefore, the best tribological properties were obtained at 850 °C regarding to the self-lubrication role induced by the rutile phase.