Multiscale Modeling And Simulation Of Heat Transfer At Matierial Interface

The interfaces of dissimilar materials suffer high stress gradients due to the presence of thermal and stiffness mismatches of bonded materials. Interfaces of dissimilar materials are prone to crack initiations,leading to delaminations.Reliable strength predictions for regions involving an interface cannot be made unless the critical values of fracture parameters are available.Heat transfer across interfaces is a critical consideration in a wide variety of scientific and engineering applications.When the system becomes extremely small,the atomistic effects have to be taken into account correctly and continuum method such as FEM is not capable of capturing all the information.At the nanoscopic length scale the molecular dynamics(MD) method takes correctly into account these effects,when the potential is chosen correctly.In this paper,atomic simulations of interfacial thermal conductance under the effect of nanocracks are proposed.The simulations results show the propagation mechanisms of cracks and the corresponding change of temperature distribution in the heat transfer process.The main works include:1.To construct a non-ideal interface,high pressure and high temperature conditions are applied to the system.Because temperature and pressure play important roles in interface forming.The simulation results showed that the thickness of interfacial region increases with the temperature increases.To effect the tensile deformation,the displacements of three layers of boundary atoms are controlled by time steps.Basing on the stress- strain curves,after reaching the maximum stress,the stress drops precipitously and plastic flow occurs.For the case of the constructed copper-aluminum interface,the curve is flatter than that of the ideal-contact case.The tensile strength of the constructed interface can reach 82%of that for the ideal-contact interface,demonstrating the effectiveness of diffusion-bonding process in creating bonds between the two materials.2.Model verification by experiments:firstly,to make the sample by depositing two metal file on the glass plate,with thickness of 300-500nm for per film.After applying thermal cycle conditions to the sample to mimic thermal loading,use scratch test and nano-indention test to get the results of interface critical force and materials properties. By the result of scratch test,the Al-W interface is stronger than Cr-W interface,and the effects of thermal cycling loading on critical force is less for Al-W interface,which is according with the MD results of interface fracture energy.And from nano-scratch test,MD results also show similar trend on Young's module by comparing with the experimental results.It means MD simulation can be used to analysis the interface properties qualitatively.3.The physical properties and failure mechanism on interface of dissimilar materials under thermal flux conditions.Before a steady state is reached, the temperature distributions near the interface and there is a jump in temperature.The temperature near the interface increases at first and then decreases severely,and the highest temperature lies in the interfacial region of Al block.It means high temperature region formed near the interface before the system reach steady state,and the large temperature difference will generate stress and cracks eventually.It is concluded that the characteristic of atoms' moving along the interface is caused by dissimilarity of material.And this conclusion is proved by a comparison study on Cu-Al interface and Cu-Cu interface.4.The nano-level cracks were added to the simulation materials by removing some atoms from the perfect lattice structure,and three types of cracks are studied.The propagation of cracks can be observed:at 2000 time steps,the movement of the atoms tend to enlarge the size of the original cracks;at 4000 time steps,some dislocations were emitted near the original cracks and a crack generated at the interface;at 8000 time steps,failure by coalescence of the crack at interface and original cracks,the delamination is generated;and at 20000 time steps,the delamination continue to expand.According to the results of temperature distribution,the propagation of crack will lead to formation of high temperature area along the direction of crack propagating,which is consisted with our results.According to the results of thermal flux for different crack models,the thermal flux increase with the crack length increase before 4000 time steps,and the crack close to the heat source has larger effect on the thermal flux.5.A multi-scale frame is proposed for interface heat transfer by coupling MD model and finite elements model concurrently.From the temperature distribution and thermal resistance results,the multi-scale model gives a similar trend and reasonable value on thermal resistance comparing with MD model,while with less computing time because of reduction of freedom by combining FE model.The multi-scale frame proposed here is meaningful for future multi-scale modeling of interfacial heat transfer.