Langmuir 1998, 14, 4929-4934
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Contact Angle Measurements of Lubricated Silicon Wafers and Hydrogenated Carbon Overcoats Junhua Wu and C. Mathew Mate* IBM Research Division, Almaden Research Center, San Jose, California 95120 Received February 17, 1998. In Final Form: June 4, 1998 We have investigated the dependence of contact angles on the thickness of perfluoropolyether lubricants on silicon wafers and on disks overcoated with amorphous hydrogenated carbon (CHx). Similar lubricants and carbon-overcoated disks are found in many of today’s disk drives. From the contact angles, we observe that the work of adhesion for alkanes on these lubricated surfaces can be separated into a term that describes the direct interaction with the lubricant material and into a term that describes how the interaction with the lubricant material and into a term that describes how the interaction of the alkane with the substrate surface is mediated by the intervening lubricant layer: ∆W ) ao′ + bo′e-L/Lo, where L is the lubricant layer thickness, Lo is the screening length of the substrate interaction through the lubricant layer, and ao′ and bo′ are parameters that describe the strength of the interactions. The contact angles for alkanes are further analyzed using the Zisman method to determine the critical surface tension of these surfaces. Water contact angle measurements reveal that the surfaces become more hydrophobic with increasing thickness of the perfluorinated lubricants. Moreover, the contact angles decrease in time via a nonexponential process and relax at a slower rate for samples with thicker lubricant films. In general, we observe that adding a perfluoropolyether lubricant lowers the surface energy by screening out the interaction with the underlying substrate. The screening distance depends on the type of a substrate, lubricant material, annealing condition, and type of liquid in the contact angle droplet, but is always in the range of 2-10 Å for alkane droplets and 20-47 Å for water droplets.
Introduction The surface energy of a material is one of the more striking manifestations of the forces that act between atoms and molecules.1,2 For a layer of molecules absorbed on a solid substrate, the surface energy arises not only from the forces within the molecular layer but also from the interactions between the molecules and the underlying substrate. These interactions determine the properties that make molecular layers on substrates valuable in many technological applications: lithography, paints, colloids, and lubrication, among others. A common technique to investigate the surface energies and underlying interactions is to measure the contact angles of droplets of test liquids deposited on surfaces covered with molecular films.3 In this paper, we explore what can be learned from contact angle measurements on how perfluoropolyether lubricants interact with two types of surfacessbare silicon wafers and disks overcoated with an amorphous hydrogenated carbon (CHx). Contact angle measurements are frequently used in the magnetic storage industry to gain insights into the chemical nature of surfaces inside disk drives.4-8 One can use this technique to monitor manufacturing processes for finishing the surface of components; that is, if the (1) Adamson, A. W. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons, Inc.: New York, 1997. (2) Israelachvili, J. Intermolecular & Surface Forces, 2th ed.; Academic Press: London, 1991. (3) Contact Angle: Wettability and Adhesion; ACS Advances in Chemistry 43; American Chemical Society: Washington, DC, 1964. (4) Bhushan, B. Tribology and Mechanics of Magnetic Storage Devices, 2nd ed.; Springer: New York, 1996. (5) Lee, J. K.; Smallen, M.; Engvero, J.; Lee, H. J.; Chao, A. IEEE Trans. Magn. 1993, 29, 276. (6) Ruhe, J.; Blackman, G.; Novotny, V. J.; Clarke, T.; Street, G. B.; Kaun, S. J. Appl. Polym. Sci. 1994, 53, 825. (7) Perry, S. S.; Mate, C. M.; White, R. L.; Somorja, G. A. IEEE Trans. Magn. 1996, 32, 115. (8) Tyndall, G. W.; Leezenberg, P. B.; Waltman, R. J.; Castenada, J. Tribol. Lett. 1998, 4, 103.
contact angle on a test sample is out of specification then the manufacturing process is adjusted until the component surface has the desired contact angle. The question naturally arises as to what is a desirable contact angle for these surfaces and what surface properties influence the contact angle. For example, disk manufacturers generally try to achieve disk surfaces with the highest possible water contact angles, as this is believed to indicate strong aversion to water adsorption. Water aversion is a desirable property in that it helps in humid environments to reduce corrosion of the disk media and formation of water meniscuses at the recording head-disk interface that can lead to high adhesion forces. Another desirable consequence of high contact angles for water and other test liquids is the implied low surface energy, which slows down the adsorption of contaminants from the environment and lowers friction and adhesive forces between two contacting surfaces, such as when a recording head is dragged over a disk.9,10 Obviously, a better understanding of the physical and chemical factors that influence contact angles would be of great value to the storage industry, as well as other industries that make use of this common technique. Experimental Details The disks used here have a structure similar to those found in today’s disk drives:11 95 mm in diameter Al-Mg substrate with a Ni-P coating onto which a Cr underlayer, cobalt-alloy magnetic layer, and a 140-Å-thick amorphous hydrogenated carbon (CHx) overcoat are sputter deposited. In the data zone, where the contact angle measurements are performed, the surface is very smooth with an rms typically of 7 Å, as determined by (9) Mate, C. M. Data Storage 1997, (July/Aug), 45. (10) Mate, C. M.; Homola, A. M. In Micro/Nanotribology and its Applications; Bhushan, B., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997. (11) Johnson, K. E.; Mate, C. M.; Merz, J. A.; White, R. L.; Wu, A. W. IBM J. Res. Dev. 1996, 40, 511.
S0743-7463(98)00189-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/30/1998
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Figure 1. Schematic of a contact angle droplet on a substrate topped by a thin-layer lubricant film. atomic force microscopy (AFM). The carbon overcoats have high degree of hydrogen incorporated, 30-35 at. %. The lubricants were deposited onto the disk surfaces after the sputter deposition of the carbon overcoat without any further processing or cleaning. Silicon (100) wafers were purchased from 3M, cleaned in a UVozone chamber for 10 min prior to lubricant deposition, and had typically a 15-Å thick-SiOx surface layer as determined by ellipsometry. One of two types of perfluoropolyether (PFPE) lubricants used was Fomblin Z-Dol [HOCH2CF2(OCH2CF2)m(OCF2)nOCF2CH2OH] supplied by Ausimont USA, which was fractionated by supercritical CO2 extraction by Phasex Corp. to have a nearly monodispersed molecular weight distribution (Mw/Mn < 1.1) with Mw ) 1600 and 4700. The other perfluoropolyether lubricant studied was Demnum-SA [HOCH2CF2(OCF2CF2CF2)nF] provided by Diakin Industries with Mw ) 4000. Both the lubricants are linear chain polymers with a chain diameter of about 6 Å. Consequently, 6 Å is the minimum lubricant thickness required to obtain complete coverage of molecules lying flat on the surface, which can be thought of as monolayer coverage. Since the packing of the linear polymers may be irregular and the polymers may not lie completely flat on the substrate, a thickness greater than 6 Å may needed to obtain a complete monolayer coverage. The concept of coverage in the presence of the liquid droplet will be discussed again during our discussion of screening lengths. The lubricants were deposited using the dip coating technique.12 A substrate was dipped first into a tank containing the lubricant dissolved in a volatile solvent (PF5060 from Dupont). Then, the substrate was withdrawn from the tank, pulling up a thin film of solution with it and leaving a uniform lubricant film on the substrate surface after the solvent evaporates. The thickness of the resulting lubricant film can be controlled by adjusting either the withdrawal speed from the solution or the concentration of the lubricant in the solution. The average lubricant thickness was determined by ellipsometry and grazingangle Fourier transform infrared spectroscopy. The accuracy of these techniques has been recently verified in our laboratory by using X-ray reflectometry and electron spectroscopy for chemical analysis.13 The effect of heat treatment on lubricated silicon wafers was investigated by annealing some samples on a hot plate at 100 °C for 1 h in air to promote possible reactions between the hydroxyl end groups of the lubricant molecules with the reactive sites on the silicon oxide surface. Contact angle measurements were performed at room temperature (24 °C) by releasing a small liquid droplet from a syringe onto the freshly lubricated surfaces and measuring the advancing contact angle when the speed of the contact line is near zero. Figure 1 illustrates the contact angle of the droplet on the lubricant film.
Results and Discussion Dependence of Contact Angles and Work of Adhesion on Lubricant Thickness. Figure 2a shows the contact angles for a series of alkanes on CHxovercoated disks lubricated with different thicknesses of Z-Dol-4700. For each alkane, the contact angle increases with increasing lubricant thickness before reaching a plateau at thicknesses greater than 20 Å. As expected from Young’s equation (eq 2 below), θdecane < θdodecane < θhexadecane at the same lubricant thickness. (12) Gao, C.; Lee, Y. C.; Chao, J.; Russak, M. IEEE Trans. Magn. 1995, 31, 2982. (13) Toney, M. F.; Mate, C. M.; Pocker, D. IEEE Trans. Magn., in press.
Figure 2. (a) Contact angle measurements and (b) cos θ or reduced work of adhesion vs lubricant thickness for Z-Dol-4700lubricated CHx-overcoated disks. Solid lines in (b) show the fit of the data to eq 6, cos θ ) ∆W/γl - 1 ) ao + boe-L/Lo.
We discuss the interaction between the test liquids and the lubricated substrates using the concept of the work of adhesion between a liquid-solid interface as defined by the Dupre´ equation
∆W ) γs + γl - γls
(1)
where γs is the surface energy of the lubricated sample, γl the surface tension of the test liquid, and γls the interfacial surface energy between the liquid and the sample. Making use of Young’s equation
cos θ ) (γs - γls)/γl
(2)
we obtain the Young-Dupre´ equation
∆W ) γl(1 + cos θ)
(3)
In light of this expression, we may use a liquid of known surface tension to determine the work of adhesion of the liquid by measuring the relevant contact angle. It is convenient to express the work of adhesion in a reduced, dimensionless form, which is simply the cosine of the contact angle
cos θ ) ∆W/γl - 1
(4)
Figure 2b shows the reduced work of adhesion calculated from the contact angles of Figure 2a, which shows a rapid decrease as the lubricant thickness increases to 10 Å. The interaction of the alkane droplet with the lubricated surface is considered to split into a direct contact term
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Table 1. Comparison of Parameters in Eq 6 Demnum-SA-4000 on silicon wafers annealed
Z-Dol-4700 on CHx disks aged in air fresh
Z-Dol-1600 on CHx disks fresh
Z-Dol-4700 on silicon wafers
average
0.58 0.48 0.40
0.60 0.49 0.42
0.57 ( 0.02 0.48 ( 0.01 0.41 ( 0.01
0.40 0.51 0.50
decane dodecane hexadecane
0.56 0.46 0.41
0.56 0.49 0.42
(a) Parameter ao 0.58 0.54 0.49 0.47 0.42 0.40
decane dodecane hexadecane
0.60 0.79 0.94
0.61 0.69 0.74
0.31 0.34 0.30
(b) Parameter bo 0.35 0.38 0.38
0.25 0.31 0.36
decane dodecane hexadecane
5.7 4.6 2.8
4.4 3.4 2.5
6.0 5.0 4.5
(c) Parameter Lo (Å) 6.7 6.0 5.0
6.2 5.7 5.0
with the lubricant and a term that represents the interaction of the alkane with the substrate mediated by the intervening lubricant film (refer to Figure 1). We assume that the first term is constant and the second has an exponential decay dependence on lubricant thickness, i.e.,
∆W ) ao′ + bo′e-L/Lo
(5)
where L is the thickness of the lubricant layer, Lo is the screening length of the substrate interaction through the lubricant layer, and ao′ and bo′ are parameters that describe the strength of the interactions. Therefore, the reduced work of adhesion can be described by
cos θ ) ∆W/γl - 1 ) ao + boe-L/Lo
(6)
where ao, bo, and Lo are parameters to be further discussed below. The solid lines in Figure 2b show that the experimental values are well fitted by the exponential decay expression, eq 6. (A power law decay was also tried, but did not fit the data as well as eq 6.) Table 1 lists the values of the parameters ao, bo, and Lo obtained by fitting eq 6 to the cosines of the contact angles measured for six different types of lubricants, substrates, and annealing conditions. Since all the data sets are well fitted by eq 6, Table 1 completely summarizes our experimental results of the contact angles with alkanes. Examining the tables, we can reach the following generalizations: (ao) For the same test liquid, ao remains nearly constant for the six different experiments, i.e., ao ) 0.57 ( 0.02 for decane; ao ) 0.48 ( 0.01 for dodecane; and ao ) 0.41 ( 0.01 for hexadecane. We interpret the phenomena in that the surface behaves like that of the bulk perfluoropolyethers for very thick lubricant films. Since the bulk perfluoropolyethers (Z-Dol and Demnum-SA) have very similar surface tensions,14 we expect an alkane droplet to have similar interactions with the different types of thick perfluoropolyether films used here. In addition, ao decreases with increasing surface tension of the alkane test liquid, reflecting the decreasing affinity between the surface and the test liquid. (bo) This parameter reflects the difference, with and without a lubricant, that interaction between the test alkane and the substrate contributes to the reduced work of adhesion. It generally increases with increasing alkane surface tension. Demnum-SA has a stronger adhesion interaction than Z-Dol. Decreasing the molecular weight (14) Ausimont and Diakin data sheets.
10.5 8.0 6.2
of Z-Dol from 4700 to 1600 seems to reduce the interaction strength slightly. (Lo) This parameter measures the spatial range of the interaction between the substrate and alkanes across the lubricant layer. For Z-Dol-4700 on CHx-overcoated disks, the spatial range is shorter than for the same lubricant on silicon wafers. Molecular weight and annealing have little effect on the spatial range. Demnum-SA on silicon wafers has about twice the screening power (half the screening distance) of Z-Dol-4700 on silicon wafers. Since alkanes interact with surfaces predominately via dispersive interactions, Lo represents how well the lubricant layer screens out these interactions between the substrates and alkanes. Lo can be influenced by several factors. One factor is how well the dielectric properties of the lubricant layers are at screening electromagnetic interactions. Since the refractive indexes and dielectric constants of bulk Z-Dol and Demnum-SA are similar, dielectric screening alone is not an adequate explanation of the variation of Lo observed from type of sample to another in Table 1c. Another factor should be how well the lubricant molecules cover the surface in the presence of the alkane liquids. In Table 1c, Lo ranges from 2.8 to 10.5 Å, comparable to the polymer lubricant chain diameter of 6 Å. In this thickness range, the packing of the lubricant molecules may deviate from the simple continuous, uniform layer structure illustrated in Figure 1, since discrete molecules lying flat on the surface cannot give rise to a complete coverage until the lubricant thickness is greater than 6 Å. Also, if the alkanes start to displace the lubricant molecules so that they no longer lie flat on the surface or become dissolved into the droplet, then it should take a thicker lubricant film for the contact angle to reach the plateau value. Consequently, the shorter Lo observed for the Demnum-SA-lubricated surfaces may be evidence that, when these molecules are at the substrate-alkane interface, they achieve a more uniform coverage at thinner lubricant thicknesses. Zisman Analysis. A widely used method for analyzing contact angle data is the empirical formulation developed by Zisman and co-workers.1,3 They proposed that the cosine of the contact angle could be described by
cos θ ) az + bzγl ) 1 - β(γl - γc)
(7)
for a homologous series of liquids such as alkanes. Zisman called γc the critical surface tension as it characterizes the surface tension of a liquid that just wets the sample surface. Figure 3 shows the plot of the cos θ vs the alkane surface tension for one of our samples. For the Zisman plot, we have used cos θ determined by eq 6 with the fitting
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Figure 3. Zisman plot showing the linear regression of cos θ vs γl of different alkanes for the same surface with 3.8-Å-thick Fomblin Z-Dol-4700-lubricated layer.
parameters in Table 1. This procedure smoothes out the scatter in the experimental data. The line through the points intersects at the cos θ ) 1 axis to give γc. Figure 4 shows the plots of γc vs lubricant thickness for all types of lubricants and samples used in our experiments. With the exception of the Z-Dol-1600 data, the values for γc can be satisfactorily fitted by an equation similar to that for work of adhesion,
γc ) al + ble-L/Ll
(8)
where al, bl, and Ll are fitting parameters, collected in Table 2. Comparing the critical surface tensions of Z-Dol-4700 and Demnum-SA on silicon wafers, we find Demnum-SA to be much more effective at screening the interactions from the silicon substrates and that thermally bonding Demnum-SA enhances this screening. Comparison of the Zisman and Fowkes Methods. The Fowkes method1,3 starts from the following premises: (1) for a hydrocarbon liquid sitting on a surface, only dispersive or van der Waals interactions are important across the interface; and (2)
γsl ) γs + γl - 2(γds γdl )1/2
(9)
where γdl and γds represent the components of the surface tension or energy due to dispersive interactions for liquid and solid, respectively. This leads to the Girifalco-GoodFowkes-Young equation for the cosine of the contact angle
cos θ ) -1 + 2(γds γdl )1/2/γl
(10)
For alkanes γdl ) γl, so the Girifalco-Good-FowkesYoung equation becomes
cos θ ) -1 + 2(γds /γl)1/2
(11)
To apply the Fowkes analysis method to alkane test liquids, one makes a forced linear regression of cos θ vs γl-1/2 through cos θ ) -1 at γl-1/2 ) 0. The value of γl at which cos θ ) +1 is similar to the Zisman critical surface tension γc and provides the value of the dispersive component of the sample’s surface energy γds . Figure 5
Figure 4. Critical surface tensions vs lubricant thickness: (a) for Demnum SA-4000 and Z-Dol-4700 as lubricants; (b) for lubricant Z-Dol-1600. The solid lines in (a) show the fit of the data to eq 8, γc ) al + ble-L/Ll. Table 2. Parameters for the Critical Dispersive Surface Tension γc in the Zisman Method, Fitted to Eq 8 Demnum-SA-4000 on silicon wafers annealed al bl Ll (Å)
11.9 14.1 12.1
10.6 16.0 9.2
Z-Dol-4700 on CHx disks aged in air fresh 13.9 7.7 9.5
11.3 8.8 12.5
Z-Dol-4700 on silicon wafers 14.8 8.8 18.1
shows an example of our contact angle data plotted using the Fowkes method. Unfortunately, the fit to the experimental data is poor in that a line made to go through cos θ ) -1 at γl-1/2 ) 0 does not fit well through the three points from the three alkane test liquids. The apparent reason for the poorer fit of the Fowkes method compared to the Zisman method is that the Fowkes method has only one adjustable parameter (γds ), while the Zisman method applies two adjustable parameters (β and γc). Also, in Figure 5 the three data point lie relatively far from the origin, further making the application of the Fowkes method to determine dispersive surface energies highly problematic for these surfaces. The reader is referred to the paper by Tyndall et al.8 for a further discussion of the application of the Fowkes method to determine the dispersive and polar contributions to the surface energy of lubricated disk surfaces. Water Contact Angles with Lubricated Surfaces. With a large dipole moment and a strong tendency for hydrogen bonding, water molecules interact quite differently with surfaces compared to alkanes, which interact with surfaces mainly through dispersive interactions. Usually, we say that a surface is hydrophilic if it has a low water contact angle and hydrophobic if it has a high water contact angle. Figure 6a shows the cosine of the water
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Figure 6. Cosine of water contact angles as function of lubricant thickness: (a) Fomblin Z-Dol-lubricated substrates and (b) Demnum SA-lubricated substrates. Solid lines show the fit of the data to eq 12, cos θwater ) aw + bw/((L/Lw)3 + 1). Figure 5. Determination of dispersive surface energy by the Fowkes method (solid line) for the same surface of Figure 3. The dashed line is the Zisman fitting.
contact angles for Fomblin Z-Dol-lubricated surfaces, and Figure 6b for Demnum-SA-lubricated surfaces. In all the cases, we find that the cosine of the water contact angle monotonically decreases with increasing lubricant thickness, indicating that the addition of perfluorinated lubricant makes the surfaces more hydrophobic, i.e., increases its aversion to water. The lines in Figure 6 show fits to the experimental data using the following empirical expression
cos θwater ) aw + bw/((L/Lw)3 + 1)
(12)
The parameters aw, bw, and Lw relate, respectively, to the water interaction with an infinitely thick lubricant film, the difference in water interaction between the lubricated and unlubricated surface, and the screening length of the interaction. The results of the fitting parameters are tabulated in Table 3. From Figure 6 and Table 3, we may conclude the following: (1) Unlike alkane droplets, the water droplets have different contact angles for very thick lubricant films. In other words, the water still interacts with underlying substrates either through the thick film or, more likely, by displacing a significant fraction of the lubricant molecules in the layer. The greater difficulty of screening out the substrate interactions with water compared to alkanes is also apparent in the values of screening distance Lw, which are in some cases almost 1 order magnitude higher than those for Lo in Table 1c. (2) The bare silicon wafers are much more hydrophilic than the bare CHx-overcoated disks, probably due to a larger number of polar surface species in the silicon oxide surface layer, generated during ultraviolet ozone cleaning.
Table 3. Parameters Derived in Terms of Eq 12 for the Reduced Work of Adhesion with Water Demnum-SA- Z-Dol-4700 on CHx disks 4000 on Z-Dol-1600 Z-Dol-4700 silicon wafers aged on CHx on silicon annealed in air fresh disks fresh wafers aw 0.89 bw 0.098 Lw (Å) 37.4
0.47 0.37 16.6
0.019 -0.13 0.51 0.54 46.8 42.8
-0.032 0.59 23.4
0.97 0.025 19.7
(3) Adding the lubricants Fomblin Z-Dol or DemnumSA to CHx surfaces makes them much less hydrophilic. However, the water contact angles remain very small on the silicon wafers even with the added lubricant. This indicates that the water is still able to interact strongly with the silicon surfaces, most likely by displacing the adsorbed lubricant molecules. By thermally annealing the Demnum-SA, one is able to react the hydroxyl end group with the polar species on the silicon surface, anchoring the lubricant chains to the surface, making them difficult for the water droplets to displace. Figure 6b shows that the thermally annealed Demnum-SA is much more hydrophobic than the unannealed surface. Time Dependence of Contact Angle. The interaction of the lubricant molecules with substrate surfaces is, of course, a kinetic process. First, the molecules must dissipate extra vibrational and rotational energies, as well as adopt energetically favorable configurations. Then, the functional end groups will interact and possibly bond covalently with reactive sites on the substrate surfaces. Similar time-dependent processes can also go on at the interface between the liquid molecules in the contact angle droplet and the lubricated surface. Since all these processes bring the system to the lowest energy state, the surface energy of the lubricated surface and the interfacial energy between the test liquid and the lubricated surface should both decrease over time.
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Figure 7. Cosine of contact angle time dependence of a hexadecane droplet on lubricated silicon wafers. Lines are fitted to the nonexponential eq 13, cos θ ) cos θo + Ee-(t/τ)β. Table 4. Parameters for the Relaxation of Cosine of Hexadecane Contact Angle on Z-Dol-4700-Lubricated Silicon Wafers lube thickness
cos θo E τ (min) β
0.63 nm
1.22 nm
2.47 nm
1.22 nm (aging 23 h in air)
0.98 -0.43 34.4 0.53
0.91 -0.47 290 0.45
0.58 -0.19 314 0.50
1.00 -0.56 990 0.43
All our contact angle measurements discussed so far were done within a few minutes of the droplet being placed on the surface, so only represent the quasi-equilibrium case. Now we look at the temporal behavior in contact angle measurements. Figure 7 shows the time dependence of the contact angle cosine for the least volatile hexadecane droplet on silicon wafers lubricated with different thicknesses of Z-Dol-4700. The time behavior is nonlinear and may be empirically described by the universal nonexponential function, the Kohlrausch-Williams-Watts (KWW) empirical law,15
cos θ ) cos θo + Ee-(t/τ)β
(13)
where θo is the equilibrium contact angle, τ the pseudorelaxation time, E the strength of the relaxation reflecting the deviation from equilibrium, and β the descriptor of the nonlinearity of the relaxation process. The fitting to eq 13, shown by the solid lines in Figure 7, generates the parameters tabulated in Table 4. From the figure and the table, we see that hexadecane droplets on thicker lubricant films tend to relax more slowly (larger τ). Besides the factors discussed above, an additional, likely cause for the slow decrease in the contact angle is a slow displacement of the adsorbed Z-Dol molecules by the hexadecane liquid. Conclusions We have investigated the dependence of contact angles on the thickness of perfluoropolyether lubricants on silicon wafers and on CHx-overcoated disks. For each type of (15) Williams, G.; Watts, D. C. Trans. Faraday Soc. 1970, 66, 80.
substrate and lubricant, the cosine of the contact angle (cos θ) for a series of alkanes is determined and related to the work of adhesion by the Young-Dupre´ equation ∆W ) γl(1 + cos θ). We find that work of adhesion for alkanes on these lubricated surfaces can be separated into a term that describes the direct interaction with the lubricant material and a term that describes how the interaction with the substrate is mediated by the intervening lubricant layer: ∆W ) ao′ + bo′e-L/Lo, where L is the lubricant layer thickness and Lo the screening length of the substrate interaction through the lubricant layer. The screening length Lo is very short (2-10 Å) and comparable to the 6-Å chain diameter of the lubricant polymer. So within a few layers of lubricant molecules, the alkanes are effectively interacting with a bulklike lubricant film. The data of cos θ for alkanes on these lubricated surfaces are further analyzed using the Zisman method to determine the critical surface tensions γc of the surfaces. γc is found to decrease exponentially with increasing lubricant thickness before reaching plateau values at a thickness of ∼20 Å, with the exception of Z-Dol-1600-lubricated surfaces, where γc does not follow the exponential decay law. Water contact angle measurements reveal to what degree the lubricated surfaces are hydrophobic or hydrophilic. We find that the surfaces become more hydrophobic with increasing thickness of the perfluorinated lubricants, but even with very thick lubricant films, the water molecules still interact through the lubricants with the underlying substrates. The contact angle of the least volatile alkane, hexadecane, was studied as a function of time. We observe that the contact angle slowly decreases in time via a nonexponential process and that the contact angle relaxes at a slower rate for samples with thicker lubricant films. Contact angle measurements are frequently used in the disk drive industry to monitor the chemical nature of component surfaces and to gain insight into the surface chemistry of these components. For example, disk manufacturers would like generally to produce disk surfaces with the lowest possible surface energies to minimize the adsorption of contaminants and to help reduce friction forces as the recording head drags across the disk surface.5 From the Young equation or the Girifalco-Good-Fowkes-Young equation, high contact angles imply low surface energies. We observe from our work that adding a perfluoropolyether lubricant lowers the surface energy by screening out the interaction with the underlying substrate. The screening length is found to depend on the substrate type, lubricant material, annealing conditions, and type of liquid in the contact angle droplet. The value of the screening length for alkanes is very short, 2-10 Å, and much longer for water, 20-47 Å. Since typically lubricated with 5-20 Å of perfluoropolyether lubricant, magnetic recording disks in disk drives should be reasonably well screened against the adsorption of alkanes, particularly in the upper part of this thickness range. However, disks with 5-20 Å of lubricant are only slightly screened against the adsorption of water, which may lead to problems for drives operating in high humidity environments. Acknowledgment. The authors thank Drs. J. Lyerla, G. Tyndall, and X. Wu for valuable discussions. LA980189G
