Radial Nanofretting of Single Crystal Copper and Silicon under High Load

As a new tribological movement mode,nanofretting refers to a cyclic movement of contact interfaces with the relative displacement amplitude in nanometer scale.Since it widely exists in microelctromechanical systems(MEMS),nanofretting damage may become a key tribological problem besides microwear and adhesion.Using a nanoindenter,the radial nanofretting behaviors of single crystal copper and Si(110) were studied under various peak indentation forces.It was found that the residual deformation depth decreased quickly with the increase in the number of nanofretting cycles.The indentation curves of copper and silicon exhibited a hysteresis loop especially in the initial nanofretting cycles,which indicated an energy dissipative process in nanofretting.The energy dissipation in nanofretting may be attributed to the friction between diamond indenter and sample,the plastic deformation of sample,as well as the stress-induced phase transition process in nanofretting.The energy dissipation was the highest in the first cycle and then decreased dramatically to a constant after 20 cycles.Both the contact stiffness and elastic modulus increased sharply in the first several cycles and attained constants thereafter.Even the projected area of the indents in copper and silicon showed an increase with the increase in the number of nanofretting cycles,the radial nanofretting damage exhibited their unique behaviors.The radial nanofretting damage in copper was mainly identified as the pileup of the wrinkles around indents.Whereas,the radial nanofretting damage in silicon was characterized as the initiation and propagation of the cracks on the edges of plastic zone.