Bonded PFPEs

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... 

A thick is deposited on top. This is then covered with a molecularly thin film of lubricant to minimize wear during start-stop contacts and to passivate the disc surface against contamination and corrosion. High-molecular-weight perfluoropolyalkylether (PFPE) polymers are widely used for this purpose. In order to improve surface bonding, the PFPEs are modified with specific functional end groups. All these molecules have similar backbone structures, namely ... 

Figure 17 A sketch of the rigid units of an oligomeric PFPE molecule (a) the flexible bonds with freely jointed beads and springs for coarse-grained bead-spring model and (b) SRS model with polarity (red arrow).

Additionally, interesting investigations dealing with thermal stability of PFPE, as well as the effects of humidity and hydrocarbon contamination, have been examined in other studies [7,94-97,116]. Other topics of interest for PFPEs include chemical analyses [117-119], bonding mechanism of endgroups [84,120], molecular degradation [117-119,121-123], physical properties [105,124], and adsorption [125,126]. 

LJl) and van der Waals (LJ2) potentials were used for nonpolar bead-bead and bead-wall interactions, respectively. For polar interactions, exponential potential functions (EXP 1,2) were added to both bead-bead and bead-wall cases. For the bonding potential between adjacent beads in the chain, a finitely extensible nonlinear elastic (FENE) model was used. For example, PFPE Zdol... 

We assumed that all polymer chains in the system have the same number of beads, that is, that they are monodisperse. The number of beads Np in each polymer chain is chosen as Np =6, 10, or 16. PFPEs have rigid fluorocarbon backbone units connected via ether bonds, which give flexibility to the chain while keeping PFPEs stable in a liquid state at room temperature. The beads in our model reproduce the rigid units and are connected only via their... 

Recently [58], polyesters have been prepared from dimethylterephtalate, ethylene glycol and the diester CH302C—RF—O —RF—C02CH3. The copolycondensation by transesterification was studied and the authors investigated the morphology, the thermal and surface properties of these obtained polyesters. We can notice that the PET/copolymer (PET —PFPE) blend thus obtained contains up to 35% in weight of PFPE but 7% only are bond with the prepared PET. 

Figure 3.1-5 Trace (a) is the FTIR spectrum of D2O in scCOa microemulsions (156 bar and 32 °C). The very broad v(OD) band in the region 2700-2500 cm demonstrates the presence of bulk D2O. Trace (b) is the spectrum of a saturated solution of monomeric D2O dissolved in SCCO2 (156 bar and 32 °C) in the absence of PFPE surfactant. Note that the broad band of D-bonded water is absent. Only the antisymmetric and symmetric vibrations of free D2O at 2761 and 2653 cm can be seen. These same bands are also discernible in (a). The band marked indicates the presence of a small quantity of HDO, formed by proton exchange with the NH4 counterion of the surfactant. Adapted from M. J. Clarke, K. L. Harrison, K. P. Johnston, S. M. Howdle, J. Am. Chem. Soc. 1997, 119, 6399.

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. 

The purpose of XPS characterization was to identify whether or not the target film had been properly deposited, its chanical state and chanical interactions between SAM molecules and PFPE molecules, etc. For example, the amount of PFPE bonded can be obtained in the case of PFPE-overcoated SAMs with and without thermal treatment. Figure 6.3a shows the wide-scan spectra of the SAMs-modified and unmodified Si surfaces, and fig. 6.3b compares the wide-scan spectra of PFPE (as lubricated) onto SAM surfaces and Si. The wide-scan spectra of the three different SAMs qualitatively confirm the successful formation of respective SAMs on Si. For example, APTMS-modified Si shows a strong Nls peak, which must have resulted from the amine groups of APTMS molecules. The presence of the FIs peak on all PFPE-coated surfaces supports the presence of PFPE. 

The amount of PFPE remaining after rinsing of thermally treated PFPE was 65% on APTMS SAM, 80% on GPTMS SAM, 15% on OTS SAM, and 67% on bare Si (table 6.3). It is clear that the thermal treatment improves the extent of bonding between PFPE and reactive SAM molecules and Si. There was a slight increase in the... 

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. 

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