Quasicontinuum Simulation Of Geometric Effect On Onset Plasticity Of Nano-scale Asperities On FCC Metal Surface

Most metal surfaces have certain degree of surface roughness,which perform as a large number of nano-scale asperities and pits in the microstructure.To understand mechanical behaviors of these nano-scale asperities are great helpful for the mechanism of friction,wear,contact deformation of materials.Statistical models were usually used to evaluate surface morphology effect on mechanical property.It is lacking a sysmetric research on mechanical reponose of single asperity.In this study,multiscale quasicontinuum(QC)method is used to explore geometric effect on onset plasticity of nano-scale asperities on FCC metallic(Al,Cu,Au,Ag,Ni,Pt)thin films during nanoindentation on the(001)surface and the effects of geometrical factors of nano-scale single asperity and the spatial factors of multi asperities on onset plasticity of the thin film.The results show that for the single asperity film,the initial indentation yield stress of the film is significantly reduced compared with that of the flat surface.For different FCC metallic asperity,under the condition of restricting the formation of full dislocation at nano-scale,the deformation mechanism of different metallic asperity at yielding can be divided into two types:the single partial dslocation formation(Cu,Ag,Ni)and deformation twinning formation(Al,Pt,Au).Further studies reveal that it intends to form a single partial dislocation morphology,when satisfies with ?homo barier/?heter barrier 1.5 and ?USF/?SF>2.9 of the material properites;otherwise,it inclines to form a deformation twinning,where ?USF is the unstable stacking fault energy,?SF is the stable stacking fault energy,?UT is the unstable twinning energy,?homo barrier is the homogeneous nucleation barrier(=?USF-?SF)and ?heter barrier is the heterogeneous nucleation barrier(=?UT-?SF).The Al and Cu asperity is selected as the two represented mateirals for further studys.It is shown that for a rectangular asperity,the aspect ratio(width/height)affects indentation yield stress of the initial plasticity of both films without much change.But the further response of the dislocations is obviously different,especially when the width/height(AR=D/H)is over(?).For the Cu asperity film,the stress increases significantly with applied load and the stress drop corresponds to the partial dislocation formed at different locations.While,the stress responses without much change for the Al asperity film and the stress drop corresponds to the appearance of new partial dislocation which is arranged side by side with the existing dislocation,forming the deformation twinning gradually.For a trapezoidal asperity,the initial indentation yield stress decreases with the base angle(a)gradually,especially when the a is over 54.7°.This may be due to the trapezoidal side surface index from(001),(110)to(111)gradually,passing 54.7° corresponds to the(111)side surface.The different surface energy can lead to different stress level required for dislocation nucleation.Meanwhile,the interaction energy between the substrate and the asperity has also be changed with the base angle(a).Both energy changes lead to the variety of initial indentation yield stress.The simulation is also found the effect of the base angle on initial indentation yield stress is more significant for more anisotropic Cu.The effects of the height difference(dh),width(D)and interspacing(I)of the asperities on the onset plasticity of a multiple-asperity surface are studied.Compared with the same height asperities,the existence of dh significantly reduces the indentation modulus and the initial indentation yield stress.With the increasing dh,the subsequent indentation yield stress will rise significantly.With the increase of D,the indentation modulus and indentation yield stress increase.With the increase of I,the change of the indentation modulus and initial indentation yield stress are not evident,while the subsequent indentation yield stress varies significantly.Using polyfitting technique,the intergal effect of the(dh,D)on the total indentation yield stress has been revealed.These simulation results are useful to evaluate mechanical properties of the real surface at macroscale and also to provide guidelines for designing such micro-electromechanical systems and nano-scale functional devices.