Surface topography evolution of TiO2/SnO2 coatings during thermal oxidation of Ti/Sn composite coatings

Abstract Mechanical coating followed by thermal oxidation in the atmosphere was used to prepare TiO 2 /SnO 2 composite coatings. The coatings were characterized by X-ray diffraction (XRD), scanning electronic spectroscopy (SEM), energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and so on. Results show that continuous rutile TiO 2 coatings decorated with discrete SnO 2 particles were formed during thermal oxidation at the temperature range of 873–1073 K. Through the temperature range, a small quantity of SnO was formed. However, tiny amount of Ti 2 O 3 was formed only when oxidation temperature was 1073 K. The surface topography evolution of rutile TiO 2 crystals during the thermal oxidation and relevant mechanism was also examined. Equiaxed, rod, columnar and needle TiO 2 crystals with nanoscale were formed at temperatures from 873–1073 K. The topography evolution indicates that the diffusion of oxygen species into the formed rutile TiO 2 layer was predominant at 873 K. It favored the growth of equiaxed crystals in the tangent plane rather than radial direction of Al 2 O 3 ball substrate at the early stage of the thermal oxidation. However, the diffusion of Ti cations became prevailing compared with oxygen species at 1073 K. The reaction of Ti and oxygen species occurred at the external surface of the formed TiO 2 layer where rutile TiO 2 needles were formed. During thermal oxidation at 973 K, both the Ti cations and oxygen species were involved in the diffusion process and clusters of paired/parallel rutile TiO 2 nanorods were formed. The addition of metallic Sn remarkably affected the surface topography and the grain size of rutile TiO 2 crystals through regulating the diffusion of Ti cations and changing the local oxygen concentration surrounding Ti coatings. A proper additive amount refined the grains of rutile TiO 2 .