Mobile PFPEs

All of the studies discussed above have shown that some silane SAMs are efficient in reducing the coefficient of friction, the work of adhesion, and stiction properties however, their wear resistance is not sufficient to provide high durability to the MEMS components [42]. One possible reason for the low wear durability of SAMs is the lack of a mobile portion in the lubricant. Hence, there is no replenishment in these layers as molecules are continuously removed from the contact area during the wear process. Moreover, the worn particles generated as a result of material removal act as a third body and further accelerate the wear of the film. Therefore, we proposed a lubrication concept of overcoating SAMs (bonded) with an ultrathin layer of per-fiuoropolyether (PFPE) (bound + mobile) to improve the wear durability of SAMs and hence that of the Si substrate (fig. 6.1) [43, 44]. The mobile PFPE is expected to lubricate and replenish the worn regions and hence enhance the wear durability. 

The concept of overcoating PFPE onto SAMs is similar to that of magnetic harddisk lubrication, where a combination of both bonded and mobile PFPEs is routinely used to better protect the hard-disk surface. For example, in the studies by Katano et al. [48], Chen et al. [49], and Sinha et al. [50], a combination of both bonded and mobile PFPEs on hard-disk surfaces has shown higher wear durability than the use of either bonded or mobile PFPEs alone. In a study by Choi et al. [51], PFPE overcoating onto SAMs-modifled hydrogenated amorphous carbon surface has shown higher wear durability than only a SAM-coated or PFPE-coated carbon surface. 

FIGURE 6.6 Molecular model of PFPE on (a) OTS SAM and (b) APTMS/GPTMS (refer to text for details). Thicker lines in (b) are used for strongly adsorbed, and the thinner lines for mobile PFPE molecules. (Reprinted from Satyanarayana, N., and Sinha, S. K. 2005. J. Phys. D Appl. Phys. 38 (18) 3512-22. With permission from Institnte of Physics Pnblishing.)... 

The mechanism for improved tribological properties in the case of PFPE-coated SAM surfaces may be summarized as an optimum combination of the chemical bonding between PFPE and SAM molecules and the presence of an optimum amount of mobile PFPE. 

As a crucial factor that dominates the behavior of lubricant flow, the mobility of PFPE molecules has been studied extensively in both experiments and simulations, through observing the spreading of the lubricant on solid substrates. Investigators, including Novotny [46], O Connor et al. [47], Min et al. [48], and Ma et al. [49], in collaboration with IBM scientists, carried out systemic experimental studies on spreading... 

The interaction between PFPEs and disk overcoat is another significant factor to affect the properties of lubricant films. PFPEs with functional endgroups (e.g., Zdol and Ztetraol) perform better than PFPEs with nonfunctional endgroup (e.g., Z03) for retention and evaporation at the expense of the surface mobility or replenishment ability. However, strong endgroup functionality can lead to the layering and instability (e.g., surface nonuniformity/dewetting) of PFPE films (Karis... 

The application of the cyclophosphazene N3P3(OC6H4F-4) (OC6H4CF3-3)e (n 2 code name X-IP) as a lubricant is well known. Comparative studies of the tribological properties of X-IP and ionic liquids of al-kylimidazolium tetrafluoroborates have shown a preference for the tetrafluo-roborates as lubricants. The interaction of X-IP with a carbon-coated head at the head-disk interface of hard disk magnetic storage systems in the presence of a perfluoropolyether (PFPE) lubricant, has been diseussed. Attention has been paid to the application of a novel lubricant (A20H) which consists of (234) as the major component and small quantities of (235). The new lubricant displays a lower mobility on carbon surfaces than hydroxyl-... 

The present concept of overcoating SAMs with PFPE to improve the wear durability of SAMs of different functional groups has not been tried previously from the view of MEMS lubrication. The concept of overcoating PFPE onto epoxy nanocomposite bilayers with the purpose of improving the wear durability has been proposed by Julthong-piput [45] however, the results are not available. A similar concept of overcoating a mobile hydrocarbon-based lubricant onto hydrocarbon layers chemically bound onto... 

As presented in fig. 6.4b, thermal treatment shows approximately 30% improvement in the wear durability of PFPE-coated APTMS SAM, whereas there is a reduction in wear durability in the case of OTS SAM, GPTMS SAM, and bare Si due to the thermal treatment after PFPE overcoating. From table 6.3, it is clear that there is a very minimal increase in the percentage of bonded PFPE after thermal treatment for Si/APTMS/PFPE, whereas there is an appreciable increase in bonded PFPE after thermal treatment for Si/GPTMS/PFPE and Si/PFPE. Alternatively, we can say that the thermal treatment after PFPE coating reduced the mobile fraction of PFPE on GPTMS SAM and Si. Therefore, the changes in the mobile and bonded... 

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