13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Here we suggest a mechanochemical mechanism to understand the possible degradation routines for grafting groups on silicon surfaces. In tribo-conditions, the dangling bonds may not only form at top silicon atoms but also form by the rupture of grafted carbon groups. Consider the quasi-static vertical stretching of the end carbon atom on hexanol grafted at Si(111) surface in Figure 9. Using the DFT calculation by the Si7H16 model, the chain will break at the end C-C bond, releasing the CH3· radical. This character gives an explanation of the “tribochemical polymerization” [23] of short CxHy compounds found by TOF-SIMS in pentanol lubricating experiments, including surface-MEMS devices [24] and macro- ball(SiO2)-on-disc(Si) experiments [23]. Figure 10 shows the rupture details of different grafted alcohols. Before a critical strain of approximately the same, the Si-O-R chain is stretched uniformly. And at the critical strain (about 0.2), the chain “breaks” at the end C-C bond, where the energy increase is about 0.86~1.25 times of forming a Si··Si pair. However, different chains have different trends to release carbon radicals. For hexanol, there is a jump at the break point and the CH3 group is almost fully released to form radical. For ethanol, the jump is not significant and the CH3 group still has some quantum connection with the main chain when the strain is more than 0.2. The situation of propanol is between ethanol and hexanol. But for fluoroalcohols, the CHxF3-x group can not get rid of the influence of main chain as fatty alcohols since the Si-O-R chain relaxes in a continuum manner. Therefore, the fluoroalcohols is much more stable than fatty alcohols when they are grafted on silicon surfaces and the latter can degrade by local mechanochemical effect. The longer the alcohol, the easier to break. This degradation may be harmful for lubrication and fluoroalcohols are better lubricant than fatty alcohols. Figure 9. The quasi-static stretching of grafted hexanol on Si7H16 cluster. (a) Initial state, (b) just before C-C break, (c) just after C-C break.
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