Lubricant films PFPEs

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... 

Fig. 28—Effect of van der Waals force on loading capacities of simple inclined and two-rail sliders, d is the thickness of PFPE lubrication film on disk, (a) simple inclined slider (b) two-rail slider.

Partially fluorinated X-IP has been used for a number of years as an additive in the inert lubricant PFPE film on the surface of a magnetic hard disk to enhance start/stop durability of PFPE lubricants [29,30]. Recently it has been used as a vapor lubricated film on the surface of the disks [31 ]. In order to avoid the PFPE being catalyzed to decomposition by the slider material AI2O3 (refer to Section 3.4), XI -P was also examined as a protective film on the surface of the magnetic heads [25,32]. The results of CSS tests indicate that the thermal stability of the lubricant was greatly improved in the presence of X-1P, and the thickness of X-1P film on the slider surface has an important influence on HDD lubrication properties. 

Molecularly thin lubricant film is an important application of nanoscale confined polymeric fluids, and is the focus of this chapter. Ultrathin lubricant films are necessary in high-density data storage to increase the reliability and performance of hard-disk drive (HDD) systems [2-4]. Spinoff and intermittent contact between the slider (or head) and the lubricated disk [ultrathin perfluoropolyether (PFPE) films are applied to the disk s carbon-overcoated surface, as shown in Fig. 1.1] cause loss and reflow of the lubricant film. The relevant HDD technology is summarized briefly in the end-of-chapter Appendix Section A.I, which provides an overview of how certain information technology devices are controlled by nanoscale chemistry. 

Although qualitatively similar to the experimental SME results, our previous simulation results had difficulties with particle evaporation [160]. Figure 1.34 illustrates particle evaporation. To overcome evaporation, the model parameters have been adjusted. 3D visualization techniques were also used to monitor the issue, and the full-blown 3D capabilities [157] allow for a detailed, nanostmctural analysis of the PFPE lubricant films. 

To represent the molecular structure with reasonable accuracy as well as to reduce computational time, the coarse-grained, bead-spring model [Fig. 1.28(b)] was employed to approximate a PFPE molecule. This simplifies the detailed atomistic information while preserving the essence of the molecular internal structure [167]. The off-lattice MC technique with the bead-spring model was used to examine nanoscale PFPE lubricant film structures and stability with internal degrees of freedom [168],... 

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 ... 

Example 11.4. McGuiggan et al. [492] measured the friction on mica surfaces coated with thin films of either perfluoropolyether (PFPE) or polydimethylsiloxane (PDMS) using three different methods The surface forces apparatus (radius of curvature of the contacting bodies R 1 cm) friction force microscopy with a sharp AFM tip (R 20 nm) and friction force microscopy with a colloidal probe (R 15 nm). In the surface force apparatus, friction coefficients of the two materials differed by a factor of 100 whereas for the AFM silicon nitride tip, the friction coefficient for both materials was the same. When the colloidal probe technique was used, the friction coefficients differed by a factor of 4. This can be explained by the fact that, in friction force experiments, the contact pressures are much higher. This leads to a complete penetration of the AFM tip through the lubrication layer, rendering the lubricants ineffective. In the case of the colloidal probe the contact pressure is reduced and the lubrication layer cannot be displaced completely. 

Considerable interest still exists in the application of fluorine-containing cyclophosphazenes in lubricant technology. Recent advances in the use of N3P3(OC6H4F-4) (OC6H4CF3-3)6-n (n 2 code name X-IP) as lubricant either by itself or as an additive to perfluoropolyethers (PFPE) have been reviewed. Addition of X-IP to PFPE films reduces the critical dewetting thickness on amorphous nitrogenated carbon compared to that of neat PFPE. The influence of X-IP on the stabilization of the PFPE lubricant for the slider/disk interface in hard disk drives has been studied. Micro-phase separation of X-IP... 

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 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. 

---