Microstructure and Properties of Duplex Ni-P-TiO2/Ni-P Nanocomposite Coatings

Duplex Ni-P-TiO2/Ni-P coatings were electrodeposited onto brass substrates. High phosphorus Ni-P coatings were plated as the inner layer to improve the corrosion resistance, while low phosphorus sol-enhanced Ni-P-TiO2 coatings were deposited on the top to strengthen the mechanical property. The microstructure, morphology, mechanical property and corrosion resistance of coatings were investigated systematically. The results show that duplex coatings exhibit both excellent mechanical property and good corrosion resistance. The hardness was further improved from similar to 545 HV50 of duplex Ni-P/Ni-P coating to similar to 622 HV50 of duplex Ni-P-12.5mL/L TiO2/Ni-P coating, while the hardness of single Ni-P coating was only similar to 387 HV50 . The corrosion resistance of duplex coatings improved significantly by adding a suitable amount of TiO2 sol. Duplex Ni-P-12.5mL/L TiO2/Ni-P coating presented the highest corrosion resistance. However, adding excessive quantities of sol (more than 12.5mL/L) caused nanoparticle agglomeration and created a porous structure in the outer layer, deteriorating the properties of coatings.