PFPEs

Kajdas, C., and Bhushan, B., Mechanism of Interaction and Degradation of PFPEs with a DLC Coating in Thin-Film Magnetic Rigid Disks A Critical Review," /. Info. Storage Proc. Syst.,Vol. 1,1999, pp. 303-320. 

However, the assumption of molecule orientation normal to the surface is not convincing enough for this author, and it does not consist well with the results of the molecular d5mamics simulations for the alkane confined between solid walls. An example in Fig. 3 shows that the chain molecules near the wall are found mostly lying parallel, instead of normal, to the wall [6]. This means that the attractions between lubricant molecules and solid wall may readily exceed the inter-molecule forces that are supposed to hold the molecules in the normal direction. Results in Fig. 3 were obtained from simulations for liquid alkane with nonpolar molecules, but similar phenomenon was observed in computer simulations for the functional lubricant PFPE (per-fluoropolyether) adsorbed on a solid substrate [7], confirming that molecules near a solid wall lie parallel to the surface. 

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. 

The PFPE liquid used in the experiment has a typical backbone group -CF2 as shown in the diagram of PFPE. 

A thermogravimetric apparatus (TGA) was used to verify the weight loss of the chemical materials of different samples which are PFPE (Sample 1), PFPE+AI2O3 (Sample 2), PFPE+X-IP (Sample 3), and PFPE+AI2O3+X-IP (Sample 4) at the temperature of 220 °C, respectively. The weight of PFPE of Sample 1 was 500 mg for Sample 2, PFPE was 500 mg plus 200 mg of AI2O3 for Sample 3, PFPE was 500 mg and X-1P was 100 mg for Sample 4, the weights of... 

PFPE, AI2O3, and X-IP were 500 mg, 200 mg, and 100 mg, respectively. The materials in Samples 2 to 4 were sufficiently mixed to ensure that the liquid PFPE and X-IP completely wet the alumina powders. These specimens were put in a closed space hlled with inert nitrogen. The flow rate of the nitrogen gas was kept at 20 milliliters per minute. The environmental temperature of the system was set at 220 ° C and the duration time was 250 minutes per sample for each individual operation procedure. PFPE used in the experiment was Z-dol and the alumina was in ultra-fine powders with chemical analytic grade purity. 

As shown in Fig. 12, the weight loss of PFPE for Sample 1 was very small during the whole 250 minutes reaction process, except for an apparent loss in the first 20 minutes because of evaporation. For Sample 3, the weight loss was similar to that of Sample 1, so that the reaction between PFPE and X-IP was very weak. However, for Sample 2, PFPE + AI2O3, the reaction rate was expected to increase greatly with time because the weight loss grew very fast with... 

X-IP is useful to reduce the reaction between PFPE and AI2O3. In this test, a usual dipping method was used to prepare X-IP films on the surfaces of the head. The X-IP solution was diluted by 1 -methoxynonafluorobutane. The thickness of the X-1P film was controlled by adjusting the solution concentration and the time of the samples being immersed... 

Fig. 12—TGA results for specimens at 220°C 1-PFPE, 2-PFPE + AI2O3, 3-PFPE+X1-P, 4-PFPE+AI2O3+XI-P.

Surfaces of heads were checked after the CSS tests by an optical inspection. For the heads with X-IP hlms about 1.3 nm thick, both clear surfaces of the head and disk in the landing zone were obtained after the CSS test as shown in Figs. 18(a) and 18(h). Nevertheless, there were some liquid droplets of PFPE on the surfaces of heads with X-1P films at 2.53 nm thick as shown in Figs. 18(c)and 18(d). It also can be seen that there were footprints on the surfaces of disks where the head had parked for 24 h as shown in Figs. 18(e) and 18(f). It indicates that the thick X-IP him will result in the dewetting of PFPE. 

The results from TGA and FTIR tests indicate that PFPE will be more stable when X-IP is in the presence of the system... 

At the present, perfluoropolyether or PFPE, a random copolymer with a linear principal chain structure, has been widely used in HDD as the lubricant. Its chemical structure can be described by A-[(0CF2CF2)p-(0CF2)g]-0-A (p/q s2/3), with the average molecular weight ranging from 2,000 to 4,000 g/mol. Here, the symbol -X denotes the end-bead (eb), corresponding to -CF3 (nonfunctional) in PFPE... 

Mate and Novotny [42] studied the conformation of 0.5-13 nm thick Z-15 on a clean Si (100) surface by means of AFM and XPS. They found that the height for PFPE molecules to extend above a solid surface was no more than 1.5-2.5 nm, which was considerably less than the diameter of gyration of the lubricant molecules ranging between 3.2-7.3 nm. The measured height corresponds to a few molecular diameters of linear polymer chains whose cross-sectional diameter is estimated as 0.6-0.7 nm. The experimental results imply that molecules on a solid surface have an extended, flat conformation. Furthermore, they brought forward a model, as shown in Fig. 28, which illustrates two... 

Spreading of PFPE films on amorphous carbon surfaces... 

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... 

Fig. 29—Sketch for the structure of functional PFPE molecules [45], where Rg is the radius of gyration of PFPE molecules and a is the diameter of PFPE molecules.

The experiments further reveal that the spreading of functional PFPE proceeds in a layering fashion, which is apparently different from that of nonfunctional PFPE. It is reported [49] that in spreading of Z-dol multiple layers develop from the liquid front, but each layer exhibits a different thickness. The thickness of the first layer is close to the diameter of gyration of the polymer in bulk and the second layer is nearly twice as thick as the first layer. No such layering was observed in the case of Z-15. Instead, the liquid front evolves smoothly with time, leading to a gradual and diffusive profile. 

Fig. 31—Thickness profiles at time t = 100,000, 300,000, and 500,000 MD steps, where X is in the direction of PFPE spreading, Z is in the direction of PFPE falling down (a) nonfunctional PFPE (b) functional PFPE [51].

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