Deformation Mechanisms of NiP/Ni Composite Coatings on Ductile Substrates

Abstract

NiP/Ni composite coatings with different thicknesses were prepared on coarse-grained Ni substrates by electrodeposition. The tensile tests show that compared with the substrate, the toughness and strength of the samples with multilayer composite coatings are greatly improved. The uniform elongation is increased from 24% to 43%, and the yield strength is increased from 108 to 172 MPa. In the deformation process, the geometrically necessary dislocations accumulate, resulting in long-range back stress, leading to strain hardening, showing synergistic strength and ductility. The mechanical properties of composite coatings are strongly affected by the layer thickness. Molecular dynamics studies show that there is a more uniform distribution of the shear strain in thinner coatings, and the propagation of shear transformation zones (STZs) is restrained, preventing the formation of a large shear band. With the decrease of thickness, the deformation of the NiP layer changes from shear fracture to the coexistence of uniform deformation and shear deformation. The interface resistance of the multilayer structure increases the resistance of crack propagation and alleviates the effects of NiP layer cracking on substrate cracking. Multilayer amorphous/crystalline coatings therefore may increase the toughness of the Ni substrate.