Radial Nanofretting Of Copper Silicon And Niti Shape Memory Alloys

As a new tribological movement mode, nanofretting refers to a cyclic movement of contact interfaces with the relative displacement amplitude in nanometer scale. It will damage the surface inactivate some micro apparatus in MEMS, thus reduce the service life of MEMS. Radial nanofretting is the primary running mode of nanofretting, and it widely exists in structure beam, films, hinge, micro-bearing, micro-spring, and other main components of microelctromechanical systems (MEMS), may become a key tribological problem besides microwear and adhesion.Based on nanohardness, this thesis studies the operational mechanism of radial nanofretting of four typical materials of MEMS-copper, silicon, nickel titanium shape memory alloy under different cycle indexes and loads, tests and analyzes the surface topography of samples by using scanning electro microscope (SEM), nanohardness, atomic force microscope (AFM), and other equipments; analyzes its nanofretting damage and operational mechanism. And studies the damage to four different typical materials in MEMS under the same load and with different cycle indexes in detail, and emphases on the influence to the running of radial nanofretting and damage by stress-induced phase transition in silicon and NiTi. Main conclusions are drawn as follows:1. In the radial nanofretting under high load by Berkovich tip, the residual deformation depth of the four different typical materials in nanofretting cycle quickly decreases to zero with the increase in the number of cycles. The indentation curves of MEMS materials exhibited a hysteresis loop especially in the initial nanofretting cycles, which indicated an energy dissipative process in nanofretting. The energy dissipation was the highest in the first cycle and then decreased dramatically to a constant after 20 cycles. The contact stiffness and elastic modulus show an increase sharply in the first several cycles and attained constants thereafter.2. In the radial nanofretting under high load by Spherical tip, SE NiTi experienced elastic deformation of austenite, phase transition from austenite to martensite under loading , the deformations recoved after unloading, which causes SE NiTi to exhibit superelastic effect. SME NiTi experienced elastic deformation of austenite, phase transition from austenite to martensite in the first cycle, but SME NiTi and SE NiTi have some differences, during unloading SME NiTi experienced reversible phase transition from martensite to austenite, shows the well-known shape memory effect. Through analyzing, we found that the Load-Displacement curve and uniaxial tension Stress-Strain curve of the two materials are consistent with each other.3. Form a serial experiment of radial nanofretting with two different tips, the damages of four kinds of materials have their own characteristics. 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. The projected area of the indents in copper, silicon, SE NiTi and SME NiTi showed an increase with the increase in the number of nanofretting cycles and attained constants after 20 cycles. In the four kinds of materials, SE NiTi is the one which has the greate stability to resist Nanofretting damage and exhibit a good damping property under the complex loading condition in radial nanofretting.