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-TiO2 composite coating, i.e. the inner, transition and outer layers. The inner layer has a thickness of similar to 3 mu m, mainly composed of Ni and P elements. The transition layer of the coating has a relatively high content of TiO2 with a thickness of similar to 500 nm and a columnar-structure. The thickness of the outer layer was similar to 7 mu m, with almost evenly distributed Ni, P and TiO2. The hardness and Young's modulus of the composite coating were greatly improved to similar to 10 and similar to 200 GPa, respectively, compared to similar to 6 and similar to 110 GPa of the traditional Ni-P coating.