13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- bulk-fabricated side-wall micro tribotester is designed to test the wear life of silicon in different vapor environments. In this paper, we first introduce the tribotester and the experimental details. In Section 3, adhesive and corrosive wear are distinguished by the motion mode of the driving shuttle. In Section 4, we analysis the corrosive wear of fatty and fluoro- alcohols, pointing out that the solvolysis can not give a full explanation of tribochemical reactions of fatty alcohols on silicon surfaces. A mechanochemical mechanism is proposed to understand the stability of grafting groups in the presence of friction, helping to explain the stability of fluorocarbon chains on silicon surfaces. 2. Experimental details Every 50-µm-thick Si-MEMS tribotester consists of a driving shuttle and a pair of loading frames (Fig. 1). The on-chip buckle structure can push the loading frame over a certain displacement to maintain a deflection of the restriction beam to realize 2.7µN or 5.2µN normal loads. For each tribotester, a loading frame and its corresponding restriction beam serves as the reference pair working in the ethanol vapor environment, and the opposite pair is used to study the effects of other environments, such as dry nitrogen or fluoroalcohol vapors. The friction pair to be tested is blown by dry nitrogen or the bubbled vapor stream with saturation lower than 20%. This design can be as far as possible to cancel the interference of device fluctuation in fabrication [8] and give more reasonable judgments of anti-wear effect of candidate lubricants. Figure 1. The bulk-fabricated micro- tribotester and the optical path to monitor the vibration of the driving shuttle, top view (up) and side view (down). To drive the friction pair, the tribotester is stuck to a home-made one dimensional piezoelectric platform to force the shuttle vibrating near its nature frequency. The typical oscillation amplitude of the shuttle is about 4.5µm at 3850Hz. This method can offer a driving force no more than 10µN. And the on-chip buckle loading method avoids the necessity of finding the zero-point in a trial-and-error way. Thus this tribotester can be sensitive enough to observe the early stage of wear from its real starting point in MEMS devices. The vibration of the shuttle can reflect the friction/wear process. A HeNe 633nm laser focuses onto the edge of the shuttle to reflect a speckle filed. The light intensity of the carefully chose point in this speckle field denotes the position of the shuttle. Since the air resistance is measured only 1/20 of the average friction force, the vibration degradation ΔC (ΔC=C0-C, C0 is the vibration amplitude under empty load) reflects the real-time friction force ρ, which has a relationship of C ∝ Δ ρ deduced from the vibration equation under suitable simplifications [9].
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