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This thesis focuses on the design concepts, implementation and preliminary evaluation of a novel micro-tribometer measuring-head test-rig intended to contribute to microtribometery (the measurement of small-scale friction, wear, etc.) which is widely recognised as a resource tool and source of important data design in many fields of advanced technology. Commercially-available instruments clearly address major needs in this respect, but there remains room to question whether their frequency response and other operational ranges suffice for all current applications. It is designed specifically to address questions such as positional stability under low-amplitude oscillatory testing and increased sliding speed and contact repetition rates on small samples. It features a sensing-head that can be used with various means of driving a sample against it. Force actuators are included both to compensate for lateral misalignment in the normal direction and to hold position (while measuring friction) in the lateral direction. Consequently, a single lightweight steel cantilever sensing beam of reasonably high stiffness can be used, helping both resonance and robustness. This design allows several modes of operation from a full null-servo, through running with a “dither” signal, to passive spring sensing similar to commercial instruments. Its purposes are therefore to reveal new information about micro-friction in new materials (especially thin film polymer contacts in MEMS), to discover whether current methods approaches their practical limits and to act as a demonstrator for new generation of high-performance micro-tribometers. As a test bed for measurement and control strategies, the first prototype has been aimed at typical forces in the mN range, sub-50 nm displacement resolution and responses up to around 100 Hz. Basic calibration shows performance acceptably close to the design ideals. Its potential is here illustrated by very promising results from demonstration experiments: the system is shown to clearly track surface topography and to reveal instances of a known frictional behaviour that contravenes Amontons’ laws. All micro-tribological tests followed the same procedure and were performed in a temperature and humidity controlled metrology laboratory, normally at 20 ± 1˚C and 40 ± 5% relative humidity. |
