Mechanism of Interaction and Degradation of Perfluoropolyethers with a DLC Coating in Thin-Film Magnetic Rigid Disks: A Critical Review
Czeslaw Kajdas¹ and Bharat Bhushan²
This paper reviews, discusses, and analyzes the literature on the interaction and degradation mechanisms of perfluoropolyether lubricants with carbon protective overcoats used for magnetic media. Emphasis is placed on the degradation process of perfluoropolyether lubricants under sliding conditions. Detailed comments on various degradation mechanisms are presented. Particular stress is put on Z-DOL lubricant degradation mechanisms. It is believed that the dominant degradation mechanism is the low-energy electron initiated degradation mechanism.
Table 1 Chemical Structure and Selected Properties of PFPE Lubricants
(x10³ kg/m³ at 20°C)
|Z-03||CF3—O—(CF2—CF2—O)m—(CF2—O)n—CF3||—CF3||3600||1.82||30 @ 20°C|
|Z-15||CF3—O—(CF2—CF2—O)m—(CF2—O)n—CF3||—CF3||9100||1.84||150 @ 20°C|
|Z-DOLa||HO—CH2—CF2—O—(CF2—CF2—O)m—(CF2—O)n—CF2—CH2—OH ; (m/n ~ 2/3)||—OH||2000||1.81||80 @ 20°C|
34 @ 40°C
|SAa||F—(CF2—CF2—CF2—O)m—CF2—CF2—CH2—OH||—OH||3600||1.87||75 @ 40°C|
|—CF3||6800||1.91||1600 @ 20°C|
|a — Functional or polar lubricants|
none < ester < piperonyl < hydroxyl
showing that the adsorption energy between functional groups and carbons increases with the increasing hydrophilic affinity of these groups. In contrast, the hydrophilic affinity of lubricants is also of importance for interaction of the environmental humidity with lubricant molecules. This relates to the hydrogen bonding of polar molecules in which there is a partial separation of charge to give positive and negative poles. It is of note at this point that hydrogen bonds may also be somehow separated from other examples of van der Waals forces because they are stronger than other intermolecular bonds. The strength of the hydrogen bond in water has been estimated to be between 10 and 12 kJ/mol (Bonder and Pardue, 1989). For comparison, the bond between planes of carbon atoms in a graphite structure is estimated to have a dissociation enthalpy of 17 kJ/mol.
Table 2 Heat of Adsorption for PFPE's with Various Functional Groups to Various Carbon Surfaces
|Adsorbent (Carbon)||Functional Group (PEPE)||Heat of Adsorption (mJ/m²)|
|a — Piperonyl is :|
film magnetic media. As phophazenes have totally different chemical structure from PFPE they will be discussed separately in another review paper. It is only to note at this point that pertluoropolyether triazene and monophosphatriazene have been considered as
|Fig. 1 Physical processes associated with surface deformation during boundary friction (Kajdas, 1997).|
lubricant and additive for space application (Jones and Snyder, 1980; Jones, 1995) and for gas turbine engine oils (Gschwender et al., 1992), respectively. The characteristic feature for these two compounds is the presence of the six member ring including apart from phosphorus atoms 3 nitrogen atoms or 3 nitrogen atoms and one phosphorus atom (monophosphazene). The ring of the X-1P molecule is composed of nitrogen and phosphorus atoms. Such structures adsorb well on various substrates. The most recent papers (Zhao and Bhushan, 1999; Zhao et al., 1999a; Kasai, 1999) describe not only X-1P's role as PFPE lubricant in disk files, but also discuss its lubrication mechanism.
|Fig. 2 Surface activation during friction (Kajdas, 1994).|
|Fig. 3 Diagram for the Z-DOL molecule adsorbed on the carbon surface at 20 °C (Yanagisawa, 1994; Bhushan and Zhao, 1999).|
Table 3 Functional Group to Total Carbon Ratio on Hydrogenated DLC Surfaces and Their ICN's
|Functional Group||Formula||% Total Carbon||ICN|
|Fig. 4 Simplified diagram of the chemical structure of a hydrogenated carbon overcoat surface.|
The ratio of oxygen-containing functional groups presented in Fig. 3 does not relate to their percentage of total carbon. Even considering inorganic character numbers (ICN), as discussed by Yanagisawa (1994), the most abundant functional groups would relate to the ether group (15.7% of total carbon with ICN = 20), and the carbonyl group (6.0% of total carbon and ICN = 65). The presence of other oxygenated groups (—COOR, —COOH, and —OH) is much less significant, although their ICN's are high, particularly for the carboxyl group (ICN =150). The inorganic character number shows the relative magnitude of association between function groups.
|Fig. 5 Aromatic layer plane with functional side groups (Janzen, 1982).|
Hydrogen incorporated in the amorphous carbon changes its chemical and physical properties. For example, electrical resistance and hardness of the carbon film vary with the amount of hydrogen. A hydrogenated carbon surface chemistry differs significantly from a nonhydrogenated one (Wang et al., 1996). The same is true for the triboemission process (Nakayama, Bou-Said, et al., 1997). However, it is not due to dangling bonds. Yanagisawa(1994) discussed the question why dangling bonds are not saturated by hydrogen, referring to papers by Wada et al. (1980). The authors of the two latter papers emphasized that dangling bonds in carbon are relatively stable. Dangling bonds are not easy to bond to hydrogen because double bonds in sp² carbon would be saturated first with hydrogen (Cho et al., 1990). According to Jansen et al. (1985), even if the carbon is hydrogenated, the density of unpaired
|Fig. 7 Concentration of carbonyl and hydroxyl end groups of a-C:H films (Wang et al., 1996).|
Fig. 8 Relation between charge intensity and hydrogen content in carbon film (Nakayama, Bou-Said, et al., 1997).
Fig. 9 Friction coefficient during the sliding of a diamond stylus against hydrogenated carbon film, as function of hydrogen content in carbon film
The emission intensities of the negatively charged particles and positively charged particles are low in that region of hydrogen content. Figure 8 also manifests that the charge intensity of negative particles is greater than that of positive particles, particularly in the low hydrogen content region. In the region of hydrogen contents between 15 at. % and 37 at. %, the emission intensity of the charged particles increased very rapidly, accompanied by the photon emission. It is of note at this point that the friction coefficient also increased from a low to a higher value in the region of hydrogen content from 15 to 27 at. %. The relationship of the friction coefficient values and hydrogen content in the carbon films tested is presented in Fig. 9. From these results, Nakayama, Bou-Said, et al. (1997) concluded that a microplasma state is formed at the frictional contacts of diamonds sliding on hydrogenated carbon films.
|• Dangling bond; — Covalent bond; « Hydrogen bond interaction; (x) Functional group|
Interestingly, such degradation was not observed with the Krytox lubricant. The complete degradation of Fomblin Z was ascribed to the presence of acetal groups (—O—CF2—O—) in the polymeric chain, as suggested earlier by Jones et al. (1983).
|Fig. 11. Thermal degradation process of PFPE lubricants (temp. >350 °C); the free radicals can react with PFPE molecules and oxygen molecules, recombine with other free radicals, or decompose.|
→ R1—CF3 + FC(O)—R2
where R1 is the left part of Z-DOL molecule from the above-shown acetal sector, and R2 is the right portion of Z-DOL molecule.
|Fig. 12. Interaction between a Lewis acid site on AlF3 and an acetal sector of Z-lube (Kasai et al., 1991).|
4.4. Electron-Induced Degradation MechanismIt is well known that a wide variety of materials, after mechanical treatment, become electron emitters. The term "exoelectron emission" relates to low-energy electrons and originated from the investigation of the emission in freshly formed metals, which were accounted for as a consequence of an exothermal transformation process of the surface (Kramer, 1950). Figure 1 demonstrates that exoelectron emission occurs when a material's surface is disturbed by plastic deformation, abrasion, fatigue cracking, or phase transformation. The energy of exoelectrons is considered to be very low, of the order of 1-3 eV (Chaikin, 1967; Dickinson et al., 1997). Wei et al. (1998) reported that the energy of triboelectrons ranges form 0.5 to 80 eV, depending on the materials and test conditions. However, there is no reference given either to who measured the energy of these electrons or to the apparatus or technique used. There is also no information related to the mentioned materials and test conditions. Grunberg (1958) reviewed the exoelectron emission phenomena. Kajdas (1994) discussed the importance of low-energy electrons for the initiation of tribochemical reactions, proceeding according to the NIRAM approach. The principal thesis of the model is that lubricant molecules, for example, alcohols (Kajdas, 1987), form anions that are then chemisorbed on the positively charged areas of rubbing surfaces. Vurens et al. (1992) examined the bonding of the thin PFPE films by low-energy electrons and UV. They provided evidence that the bonding takes place through an interaction of the perfluoropolyether molecule, with a low-energy photoelectron emitted by excitation of the substrate by the UV photons, and demonstrated that for a Demnum S200 film on a diamondlike carbon substrate bombarded with low-energy electrons, the most abundant fragment ions comprise the following negative species: F¯, C3F5O2¯, and C3F7O¯. Therefore, the electron mediated degradation mechanism of perfluoropolyether lubricants seems to be of particular importance.
4.5. Mechanical DegradationThe application of an adequate shear stress results in the ordering of the PFPE chain and a corresponding
decrease in friction (Hirz et al., 1992). Under operating conditions, the removal of PFPE lubricant molecules from sliding and flying tracks, comprises its displacement and loss. The removal rate depends on film thickness, chemical structure, and molecular weight. Microscale friction and properties of molecularly thin liquid films most recently were discussed in detail by Bhushan (1999b). In flying and sliding, the lubricant removal rate from monolayer films is significantly slower than from multilayer films (Novotny and Baldwinson, 1991). The lubricant loss in sliding can be due to polymer scission (Buchholz and Wilson, 1986). Direct mechanical scission at shear rates of 108-1010 s -1; is possible (Novotny and Baldwinson, 1991; Bhushan, 1999b); however, other factors can contribute to the lubricant degradation as well. Mechanical scission of PFPE bonds leads to the formation of free radicals, similarly as in the thermal degradation process. Figure 16 provides a schematic presentation of the PFPE degradation process caused by high shear stresses.
5. DEGRADATION OF PFPE LUBRICANTS UNDER SLIDING CONDITIONS
5.1. General InformationUnder disk-drive operation conditions, the head slider slides on the disk surface during start and stop operations. CSS operations lead to elastic and plastic deformation of asperities and some wear. The generated energy impacts the lubricant molecules, which are both physically and chemically adsorbed. The adsorbed and chemically bonded PFPE molecules undergo important changes caused by physical and chemical processes. Physical changes include the mechanical displacement of the molecules and their possible desorption or evaporation. Chemical changes are much more complex and can proceed according to different mechanisms, as discussed in Section 4. The common denominator of all these specific mechanisms is the lubricant degradation process leading to its loss, mostly in the form of gaseous species generated at the sliding contacts. Currently, for detection of gaseous species generated by sliding in a high vacuum chamber, the most widely used measurement technique relates to quadrupole mass spectrometry. Data concerning the detection of gaseous wear species from lubricated thin-film disks have been presented by Vurens et al. (1992), Strom et al. (1993), Novotny et al. (1994), Bhushan and Ruan (1994), Bhushan and Cheng (1997), and Li and Bhushan (1997).
5.2. Discussion on PFPE Lubricant Degradation MechanismsNovotny et al. (1994) applied high-resolution mass spectrometry to monitor in situ gaseous wear products generated during the sliding of lubricated carbon-carbon interfaces in a vacuum. Lubricant loss, alteration, and removal as precursors of the wear of solids were determined by scanning microelipsometry. The amount of altered lubricant that was left on the disk and slider surfaces was quantified by X-ray photelectron spectroscopy (XPS), and the chemical structure of altered lubricant was evaluated by time-of-flight mass spectrometry. It was believed that the dominating process, before the wear of solids occurs, is the tribochemical scission of fluorocarbon polymers. In other words, the processes occurring at slider-disk interfaces take place before a microscopic wear of solids, excluding local microscopic wear at asperities. Novotny et al. (1994) also compared the mass spectra obtained during sliding experiments with those obtained in thermal desorption and found that the spectrum during sliding at 0.5 m/s resembled the thermal spectrum at 300 °C rather than that at 90 °C. Thus, they speculated that the loss mechanism of lubricants could be of two types: either transient interfacial temperatures were high enough to evaporate lubricant from the surface, or polymer scission produced fragments small enough to be gaseous. It is of note at this point, however, that according to the work by Bair et al. (1991) and Bhushan (1992), local increases in temperature at contact spots are less than 100 °C for typical test conditions. From the mass spectrum of wear fragments and thermal and electron decomposition products of PFPE lubricants, Vurens et al. (1992) concluded that the degradation of PFPE lubricants in sliding is an electron mediated process, since most of the product fragments generated by low-energy electrons were comparable with that in sliding experiments.
6. CONCLUDING REMARKSThis review paper is mostly focused on interaction and degradation mechanisms of PFPE lubricants with DLC protective overcoats for magnetic media. Major stress is put on the degradation process of these lubricants under sliding conditions. A detailed discussion of various degradation mechanisms of Z-DOL lubricant is provided, which includes thermal decomposition, catalytic degradation, electron-mediated degradation, and mechanical degradation processes. Comparing various degradation mechanisms, we suggest that the catalytic degradation mechanism proposed by other authors is not relevant, because kinetics is slow at asperity temperatures. Based on a wide variety of reviewed or discussed experimental data, it is reasonable to emphasize that anionic intermediates (negative ions and/or negative-ion-radical species) produced by low-energy electrons play an important part in both
ACKNOWLEDGMENTThe authors acknowledge Dr. Zheming Zhao for his help in figure preparation and for fruitful discussions. Financial support for this research was provided by the industrial membership of the Computer Microtribology and Contamination Laboratory.