Nanoscale Boundary Lubrication of Diamond-like Carbon Coatings with Fluorinated Compounds*
Institute of Chemistry and Central Petroleum Laboratory, Warsaw University of Technology, Poland
Progress in the technology of magnetic media has brought about a remarkable increase in recording density. The most important factor determining the utility of magnetic disks is durability against head wear, and this durability is controlled by several factors. The present paper discusses the tribology of these media, particularly from the viewpoint of boundary lubrication. In that context there are two characteristic features of this lubrication regime: specific standard lubricants (fluoropolyethers such as Z-DOL and perfluoropolyetliers such as Z-15) and the newer application of these lubricants in the form of films only a few nanometers thick. Advanced phosphazene-type fluorinated compounds are of most interest at present, so these compounds are discussed in more detail. The emphasis is on X-1P lubricant used either alone or as an additive for fluoro- and perfluoropolyethers deposited on protective diamond-like carbon coatings.
boundary lubrication, fluorinated lubricants, diamond-like carbon, computer disks
* — Published in : J. Synthetic Lubricants 18-1, April 2001. (18) 18
INTRODUCTIONNew sophisticated techniques for measuring friction, wear, lubricant thickness, surface topography, and adhesion (all on a micro- to nanoscale) for imaging lubricant molecules, and the availability of superconductors for atomic scale simulations have led to the development of a new field of research called microtribology, nanotribology, molecular tribology, or atomic-scale tribology [1-3]. The converse of macrotribology, micro/nanotribology studies the wear behaviour of components at least one of which has extremely small mass. Since negligible wear occurs in these circumstances, tribological performance at this scale is mostly controlled by the lubrication of component surfaces. Under these specific nanoscale boundary lubrication conditions, lubricant films are very thin.
Chemistry and general tribological characteristicsPhosphazenes are ring or chain compounds consisting of alternating phosphorus and nitrogen atoms with two substituents attached to phosphorus. Owing to the presence of phosphorus and nitrogen, phosphazenes are inherently fire resistant. Cyclic phosphazenes are either liquids or low melting point crystalline solids. Their physical properties vary considerably with molecular weight and substituents. Phosphazenes are of interest in applications where fire resistance and thermal stability are important considerations. With the proper selection of substituents, thermally and hydrolytically stable ring and chain compounds, including fluids with low pour points and good thermal stability, have been synthesised . Phosphazenes have properties necessary for advanced lubricants and/or additives for high performance applications [7-8]. Recently, a number of substituted aryloxycyclotriphosphazenes were investigated with a view to meeting the lubrication requirements of the integrated high performance turbine engine technology initiative . Consideration of various properties and economic factors led to the identification of a derivative containing four meta-CF3, and two para-F substituents, as the leading fluid candidate.
X-1P boundary film formationRecently, the dynamics of the formation and loss of boundary films formed during the lubricated sliding of steel surfaces were investigated over a range of temperatures and applied loads : X-1P lubricant and mineral oil with and without zinc dialkyldithiophosphate (ZnDDP) additive were used in tests on a
SELECTED PHOSPHAZENE-TYPE COMPOUNDS AS ADVANCED TOPICAL LUBRICANT ADDITIVES AND/OR LUBRICANTS
General informationMechanical interactions between the head and the medium are minimised by lubrication of the magnetic medium. Magnetic storage devices used for information (audio, video, and data processing) storage and retrieval are tapes, floppy disks, and hard disk drives. Magnetic media fall into two categories : particulate media, where magnetic particles are dispersed in a polymeric matrix and coated on to a polymer substrate for flexible media (tapes and floppy disks), or on to a rigid substrate (typically aluminium, and more recently glass) for rigid disks; and thin-film media, where continuous films of magnetic materials are deposited on to the substrate by vacuum techniques.
X-1P additive and/or lubricantGeneral characteristics The X-1P product is a mixture of p-fluorophenoxy- and m-trifluoromethyl-phenoxy-substituted cyclic phosphazenes. The phosphazene ring in X-1P may exist in a puckered or planar form [25,26]. In the planar form, the phosphazene ring is sterically shielded by the six phenoxy groups, and it is unlikely that strong adhesive interaction occurs between partially fluorinated phenyl rings and the carbon coating. In the puckered form, a basal plane is defined by the nitrogen atoms and three phenoxy groups in the equatorial positions, and the three phenoxy groups in the axial positions are directed upward. A strong interaction is then possible between the exposed phosphazene ring and the carbon coating. Selected properties of X-1P and Z-DOL are presented in Table 1. Advanced analytical techniques have been used to investigate the degradation mechanism and oxidative stability of X-1P cyclophosphazene lubricant .
Trihological performance A cyclic phosphazene lubricant, X-1P, is considered as one of the most advanced lubricants for both rigid and flexible thinfilm magnetic media. It is a very special non-polymer compound, which can be used either as an additive for regular PFPE lubricant or just as a superb lubricant. Early information on the tribological properties of X-1P was published in 1992 . The first papers considering the unique tribological performance relevant to the disk-drive industry were published in 1994 and 1995 [15-16]. These papers generated significant interest in the application of this special non-polymeric chemical as a lubricant for advanced recording media. It was shown that X-1P by itself performs well on hard disks during CSS and stiction tests with a lubricant thickness of about 0.5 nm . Most of the early work testing X-1P involved its use as a pure lubricant coated by dipping or draining, resulting in a thickness of less than 1 nm. This work was published for both hot/wet environments  and ambient conditions . Laptop computing had increased the importance of lubricant performance in adverse environments, especially high humidity (80% RH) and somewhat elevated temperature (30°C). Contact start-stop (CSS) test results with X-1P show it to have a tribological performance similar to or better than Z-DOL, as a topical lubricant on carbon coated thin-film rigid disks under ambient conditions; however, X-1P significantly outperforms Z-DOL in CSS testing under hot/wet (30°C, 80% RH) conditions . Additionally, X-1P was found to be stable when heated in the presence of either water or a standard slider material Al2O3-TiC.
Research emphasises that X-1P is advantageous for use in pseudo-contact recording due to protection of the head . When working with very thin layers the lubricant serves as a boundary layer. It was also stressed that X-1P serves to protect the head from the catalytic decomposition of PFPE-type lubricants.
It was found that an X-1P film of 1 nm exhibits a low coefficient of kinetic friction ranging from an initial value of 0.18 to 0.36 at 2 x 105 disk revolutions. The coefficient of friction for Z-DOL, at the same thickness, is initially similar to X-1P but then increases rapidly to reach 0.7 at 7 x 10³ disk revolutions. The coefficient of friction of Z-DOL with X-1P as an additive is initially at the same value; however, it increases rapidly after 104 revolutions to reach 0.55 after 2 x 104 revolutions. Figure 2 shows that X-1P has the highest durability, followed by Z-DOL with 10 wt.% X-1P additive.
Action mechanism The experimental findings described above have been accounted for by a new mechanism approach with the concept of hydrogen bonding interaction and triboemission . The new mechanism was specially developed for the Al2O3-TiC surface sliding against a disk with Z-DOL lubrication at high environmental humidity. The mechanism, a speculative one, also takes into account the role of X-lP lubricant in tribological performance.
Tribological performance of X-IOO and XML-86 phosphazene additives Experimental investigations have been carried out on the tribological behaviour of these additives in Z-DOL, a commonly used PFPE-type hard-disk lubricant . CSS tests, stiction tests, and constant-speed drag tests were performed to evaluate the stiction and friction behaviour of the two new additives compared with X-1P additive performance in Z-DOL: the lubricant/additive weight ratio investigated was 95:5. The disks were lubricated by dip coating and the thickness of the lubricant layer was about 2 nm for all disks, measured with FTIR spectroscopy. Test results showed that the Z-DOL/X-100 mixture had better tribological performance than Z-DOL. Disks lubricated with Z-DOL/X-100 have a lower coefficient of stiction and friction than disks lubricated with Z-DOL alone, the variations with Z-DOL/X-100 are generally small and the increase in stiction is only a weak function of CSS cycles. Both X-100 and X-1P are effective as additives, in enhancing the tribological performance of Z-DOL.
Additive-lubricant interaction with the carbon coating The interaction mechanism of the X-1P additive in Z-DOL lubricant with disk and slider surfaces has been investigated . It was suggested that the X-1P molecules would lie underneath the Z-DOL molecules and next to the carbon coating. To understand the interaction mechanism better, Figure 12 compares the contact angles of similar lubricants and water on different types of solid surfaces. It can be seen that lubricant X-1P and water exhibit higher contact angles than Z-DOL and X-1P as an additive in Z-DOL. These contact angle measurements
Degradation mechanism of X-1P lubricantIn accordance with the PFPE lubricant catalytic degradation mechanism , X-1P efficacy has usually been attributed to the passivating action of the catalytically active slider material (Al2O3-TiC). This idea might be important if the catalytic degradation mechanism were predominant; but this is not the case. The most recent review of the PFPE lubricant degradation mechanism  suggests that catalytic degradation is not relevant because the kinetics are very slow at asperity temperatures. Based on a wide variety of reviewed experimental data it is reasonable to emphasise that anionic intermediates produced by low-energy electrons play an important part in both the electron-mediated degradation process of PFPE lubricants and the chemical bonding of PFPE lubricant films with DLC surfaces under sliding conditions. Therefore, the X-1P mechanism question seems to be open, and more detailed research is needed to understand this issue better.
CONCLUSIONSThis paper has reviewed and discussed selected literature concerning micro- and nanotribology of magnetic recording media, and particularly the boundary lubrication process. Specific standard PFPE lubricants along with advanced phosphazene-type fluorinated additives and/or extremely effective lubricants are considered in detail. Emphasis is put on the X-1P phosphazene compound used alone, and as an additive for Z-DOL, deposited on protective DLC coatings. Speculative action and degradation mechanisms related to the X-1P additive lubricant are discussed, mostly focusing on a new suggested mechanism of PFPE lubricant degradation in the presence of the X-1P additive. This mechanism has been specially developed for Al2O3-TiC surface sliding against the disk at high environmental humidity, with Z-DOL containing the X-1P phosphazene compounds. More research is required to understand the complexity of this X-1P lubricant/additive action/degradation mechanism better. The same is true for the newest phosphazene-type additive, X-100.
AcknowledgementsThe author wishes to express his sincere thanks to Professor Bharat Bhushan and Dr. Zheming Zhao from the Computer Microtribology and Contamination Laboratory, Ohio State University, Ohio, USA, for their help, fruitful discussions, and suggestions while writing this paper.