Effect of water film on the plastic deformation of monocrystalline copper

Abstract

The mechanical behavior of nanoscale materials is strongly influenced by the working conditions. The corresponding research about plastic properties, however, is very limited especially in liquid environments. In this work, the effect of a water film on the plastic deformation of monocrystalline copper (Cu) is performed by a nanoindentation process using molecular dynamics simulations. The results indicate that the water film induces strong fluctuation of the load–indentation depth curves and increase of the load at the same indentation depth. The plastic deformation of monocrystalline Cu is obviously strengthened due to the water film facilitating dislocation propagation sufficiently into the inner Cu substrate. In addition, the mechanism of plasticity response to the nanoindentation process is discussed based on the interaction force. The results reveal that with the water film thickness increasing, the interaction force between the indenter and monocrystalline Cu exhibits obvious delay and its value is significantly smaller compared with that of the nanoindentation without a water film. The water molecules under and around the indenter transmit the force stemming from the indenter to the Cu substrate, which strengthens the dislocation propagation into the inner substrate but blocks the dislocation propagation to the surface, leading to a larger plastic deformation of monocrystalline Cu. In addition, the effect tendency of the water film on the nanoindentation property of the Cu substrate slows down rather than increases continuously once the thickness is large enough. Our findings can help us to understand more thoroughly the plastic deformation mechanism of nanomaterials under liquid or humid conditions.