Synthesis of Ni-P-TiO₂ coatings with advanced mechanical and catalytic properties

There was processed the technique of composite Ni-P-TiO₂ coatings producing by chemical deposition method via adding of TiO₂ sol into the chemical deposition solution of the following composition, g/l: 6H₂O - 25; NaH₂PO₂H₂O - 20; CH₃COONa3H₂O -15, NH₂CH₂COOH - 20. In addition this solution was stabilized by thiourea in an amount of 1 mg/l. The deposition duration was 2 hours. The phosphorous content in the coatings was 5.8-6.5 wt. %. The obtained coatings were characterized by a uniform distribution of the dispersed phase and higher microhardness, which increased from 5.72 ± 0.09 GPa (for the original Ni-P coating) up to 6.14 ± 0.17 GPa (after incorporation of TiO₂ particle). Annealing leads to an increase in the microhardness of coatings. The highest values of microhardness of coatings (10.63 ± 0.43 GPa) were achieved after annealing at the temperature of 400C. Adding of the following surfactants: sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulfonate, hydroxypropyl methyl cellulose into the electroless nickel plating solution reduces internal stresses, which prevents the formation of microcracks in the coatings which are the cause of their rapid corrosion failure. Addition of SDS reduces the Tafel coefficients for composite Ni-P-TiO2 coating both in acidic (0.5 M H₂SO₄) and in alkaline media (1M KOH). The rate of hydrogen evolution reaction increases ~ 4 times under acidic conditions. Furthermore, the catalytic activity of composite Ni-P-TiO2 coatings nearly doubles in the alkaline medium as compared with Ni-P coatings. Influence of the SDS additive can be explained by increasing of share of the active surface of Ni-P-TiO₂ coating achieved by reducing the size of TiO₂ particles included, owing to which coating surface become more active. Using of the SDS additive reduces roughness of Ni-P-TiO₂ coatings, which may be attributable to the inclusion of smaller particles of TiO₂, because the SDS additive prevents their aggregation in electroless nickel plating solution.

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