Sol-enhanced triple-layered Ni-P-TiO₂ composite coatings

A novel coating process, TiO₂ sol enhanced Ni-P electroless composite coatings on carbon steel, is presented in this paper. Transparent TiO₂ sol was added into the electroless plating Ni-P solution at a controlled rate, leading to in situ synthesis of a triple-layered Ni-P- TiO ₂ composite coating, i.e. the inner, transition and outer layers. The inner layer has a thickness of ∼3 μm, mainly composed of Ni and P elements. The transition layer of the coating has a relatively high content of TiO₂ with a thickness of ∼500 nm and a columnar-structure. The thickness of the outer layer was ∼7 μm, with almost evenly distributed Ni, P and TiO₂. The hardness and Young's modulus of the composite coating were greatly improved to ∼ 10 and ∼200 GPa, respectively, compared to ∼6 and ∼110 GPa of the traditional Ni-P coating.