Microstructure, wear and crack propagation evolution of electrodeposited nickel-nano diamond composite coatings: Molecular dynamic modeling and experimental study

Nickel electrodeposition coatings are prone to cracking during wear and indentation which limits their application in different industries. This paper aims to study the effect of adding nanodiamonds as one of the alternative methods to improve the toughness and crack propagation of nickel-nanodiamond composite coatings produced by electrodeposition. This study utilized both experimental analysis and molecular dynamics simulation to understand the role of nanodiamonds on crack propagation. The wear behavior and toughness of the coatings were measured using ball on disk in reciprocating condition and Rockwell B indentation test. Material characterization was carried out using scanning electron microscopy, Raman spectroscopy, and microhardness measurement. The crack propagation and indentation test were modeled and developed by molecular dynamics simulation in LAMMPS. The ratio of diamond to disordered sp3 atoms decreases during the wear test which results in blocking crack propagation. Moreover, graphitization was also observed which reduced the friction coefficient. The molecular dynamics simulation results show nanodiamonds not only block the crack propagation but also changes the shape of the crack. The formation of amorphous carbon when the crack reaches nanodiamonds and sp3 to sp2 phase transformation was found to be the main reason for blocking the crack extension mechanism. During the indentation test, both experimental and molecular dynamics simulation results verify that nanodiamonds undergo sp3 to sp2 phase transformation which leads to the formation of uniaxial stress and increases the coating toughness.