Radial nanofretting behavior of four typical structural materials in MEMS under high loads

Nanofretting refers to a cyclic movement of contact surfaces with the relative displacement amplitude in nanometer scale, where the contact area and normal load are usually much smaller than those in fretting. To understand the radial nanofretting behaviors under high loads, the nanofretting tests of a Berkovich diamond tip against typical structural materials in MEMS (polycrystalline copper, monocrystal silicon, and nickel titanium (NiTi) shape memory alloys) were performed with a nanoindenter. The experimental results indicate that: (i) the residual deformation depth in a nanofretting cycle quickly decreases to zero with the increase in the number of cycles; (ii) both the contact stiffness and the projected area of the indents on four materials attain to constants after the initial increase; (iii) the force vs. displacement curve exhibits a hysteresis loop due to the energy dissipation in nanofretting cycle. The energy can be dissipated in nanofretting by different ways corresponding to various materials, such as plastic deformation, interface friction, as well as the stress-induced phase transition. Under high loads, the main nanofretting damage of materials is plastic deformation, which may be almost finished during the first several cycles. Besides plastic deformation, the typical nanofretting damage in copper is identified as the pileup of the wrinkles on the edge of indents. In silicon, the damage is characterized as the initiation and propagation of the cracks on the edge of plastic zone of indents. Due to the stress-induced reversible phase transition, the superelastic NiTi could exhibit a good damping property under the complex loading condition, and therefore shows the excellent ability against nanofretting damage in four materials.