13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- will be worn out and the wear rate is limited by the reaction rate, which depends on the activation energy Ea of silicon and adsorbed small molecules: ] ( d ) / exp[ 0 0 E E k T d K f dt dh h b a a − − ⋅ = = , (3) where da is the silicon atom diameter and K is a probability multiplier to express the uniform assignment of multi-contact wear. dE is a modifier for sharp contact between fresh asperities. Since the initial shape can be worn out quickly in bulk-fabricated devices, dE is omitted to prevent redundant discussion [13]. Therefore, the transient wear rate is a time-independent constant, which induces h h t⋅ = 0 or t ∝ρ . Therefore, using the relationship of C ∝ Δ ρ , two modes of ΔC should be observed at the beginning of wear. Figure 3 shows the shuttle vibration degradation in nitrogen and ethanol vapor conditions. The initial degradation can be fitted by the C t Δ ∝ or 2 C t Δ ∝ quite well, consistent with the analysis of wear mechanisms from surface morphologies. Figure 3. The typical vibration degradation of the driving shuttle in nitrogen and ethanol vapor environments. 2 C d t⋅ = Δ (red line) is used to fit the data for ethanol vapor condition. More kinds of gas environments have been tested on our tribotesters. In addition to dry nitrogen, O2/N2 mixture, n-hexane vapor and HFE 7100 (CF3(CF2)3OCH3) vapor also exhibit adhesive type. According to the density functional theory (DFT) calculation, n-hexane and HFE 7100 have very limited reactivity with silicon materials. Oxygen is easy to react with silicon in tribo-conditions [14]. However, lacking of inert groups, oxygen can not saturate dangling bonds to avoid atom bridges between couple surfaces. Besides ethanol, typical corrosive wear can be found in short chain fatty alcohols, such as methanol and hexanol, as well as propylene oxide. More complex wear processes are observed in water and fluoroalcohol vapor conditions, where a short linear vibration degradation can usually be found before the time-square degradation. Using the DFT calculation, fluoroalcohols show a weaker reactivity than fatty alcohols (Sec. 4). Hence they can not passivate initial sharp asperities as effectively as fatty alcohols to avoid adhesive wear. The reason for the boundary lubrication behavior of water is not very clear. A possible explanation is that silicon surfaces passivated by –OH groups still has possibilities to form atom bridges like Si-O-Si to cause adhesive wear when the local stress is high enough. Using the time scaling of t ∝ρ and t ∝ρ , the worn area increases in a manner of t 3/4 and t3/2 for adhesive and corrosive wear, respectively. For conventional macro structures, corrosive wear is usually more harmful since it has a superlinear growth. However, for MEMS devices, the permitted
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