Sol-gel enhanced Ni-P composite coatings

A novel technique, sol-gel enhanced Ni-P electroless composite coatings on Mg alloys, has been developed. A transparent TiO2 sol solution was added into conventional electroless-plated (EP) Ni-P solution at a controlled speed, leading to in situ synthesis of Ni-P-TiO₂ composite coating. The results indicated that the coatings produced in this way possess a much increased hardness of 1025 HV₂₀₀, compared to 710HV₂₀₀ of the composite coatings prepared with traditional methods. Tests also indicated that the wear resistance of the new coatings is significantly improved compared with the traditional composite coatings. The width of the wear tracks was reduced from 500 μm to 160 μm. Heat treatment at ~250̊C can further improve the hardness and wear resistance of the coatings. This is a new concept to form metal/nano-particle composite coatings. The traditional way to produce ceramic particle reinforced metal coating is to mix fine particles into the coating solution. The particles will be co-deposited with metal to form a composite coating. The traditional method, however, has the intrinsic disadvantage of particle agglomeration. The nano-particles tend to join together as clusters, and distribute non-uniformly in the metal matrix. In this new method, the sol solution containing the desirable ceramic components is directly introduced into the electroless coating solution. Ceramic particles and metal are deposited out together to form a metal-ceramic coating. There is not a step of particles mixing in the coating solution, therefore avoiding particle agglomeration and non-uniform distribution. Microstructure of the novel coatings is investigated by XRD and electron microscopy. It is believed that the superior hardness and wear resistance come from the reinforcement of the highly-dispersed nano-oxide particles.