A new insight into promotion action of Co2+ in Ni-diamond composite electrodeposition

•Addition of Co2+ in Watt’s bath remarkably increased the content of particles in deposits for Ni-diamond system.•The cathodic zero potential has a significant effect on the entry of particles into coatings.•Addition of Co2+ can not only increase Faraday resistance but also reduce reduction rate of metallic cations.•Physical model of diamond particles deposition state before and after the addition of Co2+ was established.