Effect of progressive wear on the friction characteristics of nickel LIGA processed rotors

In microsystems engineering the understanding of tribological behaviour on the microscale is one of the key factors in the design of devices with moving parts such as micromotors. This study has investigated the friction coefficients of LIGA processed nickel rotor samples on an alumina substrate and how the measured friction was affected by wearing the nickel rotors. The nickel rotors are typical of those that would be used in electrostatic micromotors. The rotors were loaded only by their own mass, resulting in normal forces of as low as 10^-7 N. High coefficients of friction were measured for the smallest rotor samples. A relationship was found linking the mass of the rotor samples and the measured coefficient of friction, which showed, that for very small rotors and light loads, the friction coefficients can be very high. This is obviously a major concern for the miniaturisation of devices, if they are to work efficiently (or at all). The mass dependence relationship was investigated for increasing wear of the rotor samples and found to be less apparent with wear. It is suggested that this is a direct result of the increase in true contact area as wear progresses, resulting in a decreased sensitivity to the load.