Research On Strengthening Micromechanisms And Tribological Behaviour Of Cu/Ni Multilayers With Molecular Dynamic Simulations

Multilayers are being increasingly studied for the substantial strength and hardness enhancements which are not simply some combination of the properties of the bulk constituents. The current research on multilayers mainly focuses on experimental tests of the properties and theoretical analysis of their micromechanisms using continuum analysis, both of which have limitations. In this paper, the nanoindentation and dry friction models of Cu/Ni multilayers have been established using Molecular Dynamic (MD) simulations to investigate the stress distribution, dislocation nucleation and moving during nanoindentation and friction, analyze the effects of interface structures on multilayer properties, obtain the micromechanisms of strengthening enhancement and establish the relationship between the microstructures and the tribological properties of Cu/Ni multilayers. The innovative work and relative results in this paper are concluded as follows: 1.This paper established the simulation model of nanoindentation by means of Molecular Dynamic Simulations, defined the loading method and the load of the indenter. In this model, the indenter is rigid, and the load is defined as the sum of all the forces interacting with atoms within the cutoff distance from the indenter surface. The simulations show that the material at atomic scale has viscoelasticity, which will affect the measurement of hardness and elastic modulus. The solution is to drift thoroughly before unloading. Due to the elastic anisotropy of crystal, the position of the maximum shear stress is displaced from the dislocation nucleation position, which is also off the loading axis and occurs nearer the surface than predicted by continuum analysis of cylindrical indentation in isotropic media. For FCC metals, during indentation, the dislocation loops first nucleate and then glide along the {111} plane. The variation of indenter-radius size will affect the stress distribution under the indenter, and the viscoelasticity at atomic scale makes the simulation results (for example, the indent depth, the load and the maximum shear stress when dislocation nucleates) dependent on the loading speed. 2.The misfit dislocation network structure at the Cu/Ni interface differs according to the crystallographic orientations of the film relative to the substrate. Misfit dislocation network at (111)Cu||(111)Ni interface is a triangle with dislocation lines parallel to [1 10], [1 01] and [ 011]directions, respectively. While at (001)Cu||(001)Ni interface, a square network of edge-type dislocations accommodates the misfit. These misfit dislocations form a stable spatial network in Cu/Ni multilayer structure, baffle the glide dislocations, deform and consume some energy with external loading, and contribute to the strength enhancement of Cu/Ni multilayers. But the contribution is dependent on the wavelength of Cu/Ni multilayers. When the wavelength is more than the critical value λc, the stress distribution of misfit dislocation network does not change too much according to the layer thickness; When the wavelength is less than λc, the baffling of glide dislocations by misfit dislocation network is weakened rapidly with the decreasing of the wavelength. The results show that, considering the effect of misfit dislocation network in Cu/Ni multilayer structures, the critical wavelength λc is 12nm. 3.The alternating stress field, which includes the image force due to a modulus difference across an interface and the stress field of misfit dislocation network due to the lattice mismatch, plays an important role in strength enhancement of Cu/Ni multilayers because of its resistance to glide dislocations. The magnitude is dependent on the wavelength. The results show that the alternating stress field together with the interface roughness and diffusion of atoms across an interface makes the shear stress peak when the wavelength is approximately 9 nm, which is more than the 1.9nm predicted by theoretical analysis. 4.Due to the confinement of interfaces, the glide dislocations are trapped in individual Cu layers, and are in positions of equilibrium. This kind of distribution can enhance the strength and ductility of Cu/Ni multilayers, and enhance its wear resistance ability. 5.The mechanism of stick-slip phenomenon varies at different scale. At the atomic scale, the regular arrangement of atoms on the sliding surfaces makes a large and a small "sawtooth"displayed in the friction force curve. The magnitudes of sawtooth depend on the load, sliding speed, and the lattice difference across the sliding surface. 6.The micromechanisms of friction and wear on a Ni thin film are interpreted, and the effects of misfit dislocations, the load and sliding speed on the friction force are analyzed. During friction on the Ni film, the misfit dislocation network will deform and consume a certain amount of energy, prevent the glide dislocations from moving into the substrate, and finally cause laminar wear debris. For a single asperity sliding contact on Ni films, the friction force increases with the increasing load. But the curve between the friction force and load has a horizontal segment, which can be interpreted as the effect of misfit dislocation network, and hence the coefficient is decreased. Due to viscoelasticity at micro scale, the friction force is dependent on velocity, and the relationship is different with different loads. 7 . Cu/Ni multilayers are prepared on Cu substrate using a single-bath electrodeposition method, their microstructures and mechanical properties are studied using SEM, AFM and nanoindentation equipment. The results show that the hardness of Cu/Ni multilayers peaks when the wavelength is 46 nm, and the plot of hardness vs. wavelength agrees with the theoretical analysis. This thesis has been supported by the National Science Foundation of China (Approval No. 50071014) and the Ministry of Communication of China (Approval No. 200232522504)...