Study on Nanometer-Thick Room-Temperature Ionic Liquids (RTILs

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Study on Nanometer-Thick Room-Temperature Ionic Liquids (RTILs) for Application as the Media Lubricant in Heat-Assisted Magnetic Recording (HAMR) Xiao Gong,†,‡ Benjamin West,† Alex Taylor,† and Lei Li*,† †

Department of Chemical & Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China

‡

ABSTRACT: Nanometer-thick room-temperature ionic liquids (RTILs) have been studied for potential application as media lubricants for heat-assisted magnetic recording (HAMR) in the hard disk drive (HDD) industry. The RTILs containing fluorinated anions are readily solvable in Vertrel XF solvent and can be applied on the media by dipcoating, which is compatible to the current industry process. The thermal stability, topography and tribological performance of the RTILs nanofilms have been characterized by thermogravametric analysis (TGA), ellipsometry, atomic force microscopy (AFM) and nanotribometry. The experimental results showed that RTILs are thermally more stable than the state-of-the-art media lubricants in HDDs, i.e., perfluoropolyethers (PFPEs) such as diolic perfluoropoly(oxyethylene-ran-oxymethylene) commercially known as Zdol. The lubricant uniformity of most studied RTILs is better than that of Zdol, and no dewetting is observed even when the RTIL nanofilm is as thick as 10 nm. The friction coefficient of subnanometer-thick RTILs is lower than that of Zdol. The structure− property relationship and the possible structure design for the future improvement have been discussed.

1. INTRODUCTION With the fast growth of nanotechnology, the material design of nanometer-thick lubricant is becoming more and more critical because many nanoscale devices have contacting surfaces during operation. Lubrication at the interface is key to device reliability.1 A typical example is a hard disk drive (HDD), which is the state-of-the-art device for information storage. In a HDD, a magnetic head flies only ∼10 nm above the magnetic media while the magnetic media is spinning at ∼10 m/s.2−5 The possible head−media contact would result in head crashing and thus the failure of the HDD. To protect the head−media interface from the tribology failure, a nanometer-thick lubricant is applied on top of the magnetic media to provide the required lubrication.4,5 The state-of-the-art media lubricant is perfluoropolyether (PFPE), such as diolic perfluoropoly(oxyethyleneran-oxymethylene) commercially known as Zdol.2−5 To maintain high growth rate in the areal density of HDDs, heat-assisted-magnetic-recording (HAMR) has been proposed to deliver 1 terabite per square inch (Tbpsi) or above data density.2,3 In HAMR, the media surface will be heated to or above the Curie temperature (TC) by laser irradiation and the estimated TC could be above 400 °C,2,3 which makes the thermal stability of the media lubricant a serious concern. The state-of-the-art media lubricant, such as Zdol, starts evaporating around 250 °C and will not survive the HAMR heating.4 Therefore, novel lubricants with much higher thermal stability need to be developed. In addition to thermal stability, HAMR © XXXX American Chemical Society

lubricants must also have good uniformity and excellent lubricity as a nanometer-thick film. Because nanometer-thick lubricants are also required in other micro/nanoelectromechanical systems (MEMS/NEMS) such as the micromirror components of commercial digital light processing (DLP) equipment6 and the high thermal stability of the lubricant is generally preferred due to contact-induced temperature rise,4 HAMR lubricant development could have impacts far beyond the HDDs. Room temperature ionic liquids (RTILs), synthetic molten salts with the melting temperature below room temperature, have attracted a lot of interest in the past 2 decades due to their excellent physiochemical properties.7−11 Specifically, the high thermal stability and promising tribology properties12−20 render the RTIL a promising lubricant candidate for HAMR application. Ngo et al.12 reported that the intrinsic thermal decomposition temperature of some imidazolium-based RTILs could be above 400 °C and the vapor pressure of the RTILs is very low. This conclusion was confirmed more recently by others,13,14 indicating the excellent thermal stability of RTILs. Since 2001, many studies have shown that bulk RTILs have better lubrication and antiwear properties than conventional Received: February 29, 2016 Revised: April 8, 2016 Accepted: May 23, 2016

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DOI: 10.1021/acs.iecr.6b00822 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research lubricating oils.15 Compared to bulk performance, there have been much less reports14,19−22 on the tribology performance of nanometer-thick RTILs to date. Palacio et al.16 reported that the coefficient of friction (COF) of two imidazolium-based RTILs on silica substrate is low and only slightly higher than that of the PFPE control. Zhao et al.17 studied three nanometer-thick tetrafluoroborate (BF4) RTILs on silicon wafers and found that the COF of RTILs are highly dependent on the chemical structure and, in general, the COF of RTILs is higher than that of Zdol. Werzer et al.18 conducted colloid probe atomic force microscopy study on silica−ethylammonium nitrate (EAN)−mica interface and concluded that the RTIL acts as an intrinsic boundary lubricant due to their capacity to be adsorbed strongly to the solid surfaces and form lamellar structures. Because the nanometer-thick media lubricant was used in HDDs, the thermal stability of the nanometer-thick RTILs is more relevant to HAMR condition. However, all previous thermal studies on RTILs were on bulk materials. Meanwhile, because the media lubricant in a HDD is on top of a carbon overcoat (COC) layer, it is practically critical to understand the friction and uniformity of nanometerthick RTILs on the COC substrate. To address the above-mentioned issues, in the current research, we have experimentally evaluated the thermal stability, the lubricant uniformity and the friction of four nanometerthick RTILs with Zdol as control. The experimental results showed that RTILs are thermally more stable than Zdol. On the basis of a zero-order kinetic model, the best RTIL is estimated to be able to survive ∼107−108 HAMR writes, about 4 times as much as Zdol, when the heating temperature is from 300 to 500 °C. The RTILs containing fluorinated anions are readily solvable in 2,3-dihydrodecafluoropentane and can be applied on the media by dip-coating, which is compatible to the current industry process. The lubricant uniformity of nanometer-thick RTILs is excellent and no dewetting is observed even when the RTIL nanofilm is as thick as 10 nm. The friction coefficient of RTILs is also comparable to Zdol.

trifluorophosphate (HMIM FAP) (high purity grade, >99%; mp, −14 °C; viscosity, 116 cP) and ethyldimethyl-(2methoxyethyl)ammonium tris(pentafluoroethyl)trifluorophosphate (MOEDEA FAP) (high purity grade, >99%; mp, 99%; mp, 3 °C; viscosity, 93 cP), 1-ethyl-3methylimidazolium tris(pentafluoroethyl)trifluorophosphate (EMIM FAP) (high purity grade, >99%; mp, −1 °C; viscosity, 75 cP), 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)-

Figure 1. Chemical structure of FAP RTILs and PFPE Zdol. B

DOI: 10.1021/acs.iecr.6b00822 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research correlation between the very thin RTIL films, we fixed their optical constants using a Cauchy dispersion equation: 0.00250 n(λ) = 1.45 + λ2

angle was automatically calculated by the vendor-supplied software. Each static water contact angle (WCA) and hexadecane contact angle (HCA) measurement were repeated three times and the results were shown as “mean ± standard deviation”. The surface energy was determined based on WCA and HCA using the Fowkes model.28

Here n is the refractive index and λ is the wavelength in micrometers. 2.3. Characterizations. Thermal Stability. To evaluate the thermal stability of the bulk materials, the thermogravometric analysis (TGA) was conducted using a SEIKO-220 TG system. Typical sample sizes were around 20 mg. The sample was heated up from room temperature to 600 °C at a scan rate of 10 °C/min in the ambient air. The weight percentage of the remaining sample was plotted against the heating temperature. All the samples were contained in Platinum pans. To evaluate the thermal stability of the nanofilms, the sample was heated at the desired temperature for 1 s by a hot plate. The temperature of the sample was determined by a thermal pyrometer, with a maximum inherent error of ±1.5 °C, as shown in Figure 2.

3. RESULTS AND DISCUSSION 3.1. Thermal Stability. The TGA results of four bulk RTILs, with Zdol as control, are shown in Figure 3. For Zdol,

Figure 3. TGA results of RTILs and Zdol (10 °C/min heating rate in air). Figure 2. Experimental setup for determining thermal stability of RTIL nanofilms.

the weight loss starts at around 150 °C. At around 320 °C, the weight loss reaches 50%. At 440 °C, there is 100% weigh loss. All the RTILs studied here showed improved thermal stability. For BMIM FAP, the weight loss does not start until around 280 °C, which is around 130 °C higher than that of Zdol. By relating the chemical structure to the thermal stability, the TGA results suggest that the aromatic imidazolium ring and the length of the aliphatic substitute on the imidazolium ring are two critical factors. Ionic liquids are composed of large and unsymmetrical cations and anions.11 On the one hand, this unique structure dampens the electrostatic interactions between cations and anions and renders RTILs liquid at room temperature. 11 On the other hand, the cation−anion interaction is still strong enough to result in the low volatility. The TGA results showed that the weight loss of MOEDEA FAP occurs at significantly lower temperature than those of BMIM FAP and EMIM FAP. As shown in Figure 1, this can be attributed to the fact that MOEDEA does not have an aromatic imidazolium ring, which reduces the packing efficiency of the caions/anions in bulk and thus decreases the cation−anion attractive interaction. Similarly, the weight loss of HMIM FAP also occurs at significantly lower temeprature than those of BMIM FAP and EMIM FAP. This can be explained by the long aliphatic substitute on the imidazolium ring, which makes the compact packing difficult and reduces the electrostatic interaction between cations and anions. Another crucial factor impacting the weigh-loss is the chemical decomposition. It has been reported that increasing the alkyl chain length of the Nsubstitute of the imidazolium RTILs reduces the thermal stability, which can be attributed to the increased stability of both the carbocation and carbon radicals.29,30 According to this, HMIM FAP has lower thermal stability than BMIM FAP and EMIM FAP. The fact that the cation of MOEDEA is noncyclic

Afterward, the change in nanofilm thickness as determined by ellipsometry, was recorded as the percentage of the remaining thickness with respect to different heating temperatures. Each nanofilm thickness measured was repeated three times and the average value was reported, which was shown as “mean ± standard deviation”. Nanofilm Uniformity on the COC. The uniformity of RTIL nanofilms on the COC substrate was characterized by tappingmode AFM using a Veeco Dimension V SPM. A MikroMasch NSC15/Aluminum-backside probe (resonance frequency, 325 kHz; force constant, 46 N/m) was used for all tests. Friction. Friction experiments were performed using a CSM Instruments Nano Tribometer (NTR2) placed on a Kinetic Systems antivibration platform. The tribometer uses a dual beam cantilever and high-resolution capacitive sensors that allow for accurate microscale measurements. All tests were conducted with a 2 mm diameter stainless steel sphere as the counterface, which was cleaned with isopropyl alcohol between tests. The stainless steel sphere has a modulus of 200 GPa, AFM roughness (Ra) of 50 nm and a Poisson’s ratio of 0.28.26,27 All tests were conducted at 21.0−24.0 °C and relative humidity ranging from 30 to 50%. The counterface was contacted with the sample at a predetermined normal load and the sample reciprocated for a predetermined number of cycles at 0.20 cm/s. The friction test on all the samples were repeated three times and the reported results were shown as “mean ± standard deviation”. Surface Energy. Deionized water and hexadecane were used to test contact angle, which was conducted with a VCA optima XE contact angle system at room temperature. Each droplet with 2 μL was introduced on the sample surface. Static contact C

DOI: 10.1021/acs.iecr.6b00822 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Number of Laser Writesa

results in the lower thermal stability, which is also consistent with previous studies.29,30 As shown in Figure 4, the thermal stability of all RTILs (∼1 nm film on silicon wafer) is also better than that of PFPE Zdol.

a

T (°C)

BMIM

Zdol

300 400 500

92 40 21

35 10 4

In million.

3.2. Lubricant Uniformity. In HDDs, because the flying head is only ∼10 nm above the magnetic media,2−5 the media surface must be extremely flat to avoid any possible head-media contact. As a result, the uniformity of the media lubricant is critical. The AFM results, as shown in Figure 5, showed that the roughness of the BMIM FAP/COC (Ra = 0.145 nm) is similar to that of Zdol/COC (Ra = 0.116 nm) with similar lubricant thickness. Indeed, for BMIM FAP on COC, no dewetting was observed when the lubricant thickness is 4 nm as shown in Figure 5. Meanwhile, 4 nm Zdol/COC shows clear dewetting, which is in line with a previous report32 that dewetting occurs to Zdol when the lubricant film is above one-monolayer thick, i.e., ∼2 nm. The results indicate that BMIM FAP has better uniformity than Zdol. To uncover fully the molecular origin of the good uniformity of RTIL nanofilms, AFM images were taken for ∼10 nm thick RTILs/COC. As shown in Figure 6, similar to that of BMIM FAP/COC, AFM of ∼10 nm HMIM FAP/COC and EMIM FAP/COC also show very smooth feature, indicating excellent lubricant uniformity. Interestingly, AFM of 11 nm MOEDEA/ COC clearly shows dewetting, indicating worse lubricant uniformity than BMIM FAP, HMIM FAP and EMIM FAP. Because BMIM, HMIM and EMIM contain an imidazolium ring and MOEDEA does not, the difference in the lubricant uniformity can be attributed to π−π stacking between sp2 carbon and the imidazolium ring based on our recent studies.19 In the positively charged imidazolium ring, the charge is delocalized (π+).33 When the imidazolium ring is deposited on COC surface that has ∼70% sp2 carbon, π−π+ interaction could occur at the interface. The π−π+ interaction originates from both electrostatic and nonelectrostatic interaction and the exact nature is still a matter of investigation.33 However, previous studies19,33,34 did show that, if the π−π+ interaction occurs between sp2 π electron and the imidazolium ring, the imidazolium ring takes a parallel orientation to the solid substrate, e.g., π−π+ stacking, because the imidazolium ring is a delocalized π system. As a result, RTIL cations form a highly ordered “lamella-like” structure on the solid surface. In other words, the solid surface serves as a template to orient the imidazolium ring in an ordered parallel geometry, which does not exist in the bulk liquid. This parallel geometry initiates the extensive layering of cations and anions and the Coulombic force between anions and cations of RTILs could further promote the growth of the “lamella-like” structure. As a result, all the RTILs containing imidazolium rings show excellent uniformity and the only RTIL that does not contain imdazolium ring, i.e., MOEDEA FAP, exhibits worse uniformity. 3.3. Friction and Surface Energy. The coefficient of friction (COF) of RTILs/COC, with Zdol/COC as control, has been measured by a CSM nanotribometer. As shown in Figure 7a, the COFs of ∼1 nm RTIL/COC are slightly higher, but still comparable to that of Zdol. For ∼1 nm RTILs on COC, the COF is between 0.15 and 0.19 at three different normal loads: 3, 5 and 10 mN. Then the RTIL/COC and

Figure 4. Thermal stability of IL thin films (top, experimental results; bottom, kinetic models).

For Zdol, the 50% weight loss of 1 nm film occurs at around 240 °C and the complete weight loss occurs at 350 °C. For the best RTIL, BMIM FAP, the 50% weight loss of 1 nm film occurs at around 290 °C and the complete weight loss does not occur even at 360 °C. It is worth noting that the thermal stability of the nanofilms is significantly lower than that of the bulk for both RTILs and Zdol. This result suggests that the evaporation contributes to the weight loss of the nanofilms because the surface/area ratio is much higher for thin films. However, the change in decomposition mechanism due to nanoconfinement effect cannot be excluded and further research is required to clarify the mechanism. The experimental data shown in Figure 4 has been analyzed by a zero-order kinetic model,31 which is also illustrated in Figure 4. On the basis of the curve fitting results using the zero-order kinetic model, the apparent activation energy (E) of the weight loss process was determined and thus the lifetime (number of HAMR laser writes) of various RTILs and PFPE Zdol under different HAMR heating temperatures was also estimated. Here we assume that (1) 1 ns time duration for each HAMR laser write and (2) 50% of the lubricant loss is defined as “failure”. As shown in Table 1, from 300 to 500 °C, the lifetime of all RTILs are longer than that of Zdol. The lifetime of BMIM FAP, the most thermally stable RTIL, is around 4 times that of Zdol. At 400 °C, BMIM FAP can survive ∼40 × 107 laser writes whereas Zdol can only survive ∼10 × 107 laser writes. Therefore, regarding the thermal stability, RTILs are very promising as a HAMR lubricant candidate. D

DOI: 10.1021/acs.iecr.6b00822 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. AFM image of RTIL/COC and Zdol/COC.

defined as the lubricant thickness after washing and is shown in Figure 7b. For all the RTILs, the bonded thickness is about 0.3−0.4 nm, which is significantly lower than that of Zdol, which is 0.54 nm. This can be attributed to the fact that there is no H-bonding at the RTIL/COC interface because RTILs do not have hydroxyl end groups as Zdol does.21 Interestingly, after solvent washing, the COF does not change significantly for BMIM FAP, HMIM FAP and EMIM FAP. Meanwhile, the COF of the Zdol is significantly higher than that of the unwashed Zdol sample. Moreover, The COF of washed Zdol is significantly higher than that of three above-mentioned washed RTILs even if the thickness of washed Zdol is higher than that of washed RTILs. The results indicate that the RTILs cover the COC better at lower thickness, which is highly desirable for media lubricant because this will potentially reduce the lubricant thickness and increase the data density of HDDs.2−5 This effect can also be attributed to the π−π+ stacking between sp2 graphitic carbon and the imidazolium ring and the resulting layering structure of RTIL molecules,19 which causes better coverage and lower friction. This hypothesis is further supported by the fact that the only RTIL, MOEDEA FAP, which shows an increase of COF after solvent washing, does not contain imidazolium ring. Instead, MOEDEA only has

Figure 6. AFM image of RTIL/COC.

Zdol/COC samples were washed with a good solvent, i.e., Vetrel XF.21 The bonded thickness, which characterizes the attractive interaction between the COC and the lubricant, is

Figure 7. COF of RTIL and PFPE on COC. E

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Industrial & Engineering Chemistry Research aliphatic molecular moiety and the π−π+ stacking is not possible. As a result, the coverage of very thin film is poor and the COF increases after solvent washing. Finally, we compared surface energy of BMIM FAP/COC with that of Zdol/COC. As show in Table 2. The surface

(5) Mate, C. M. Molecular tribology of disk drives. Tribol. Lett. 1998, 4, 119−123. (6) Henck, S. A. Lubrication of digital micromirrordevicesTM. Tribol. Lett. 1997, 3, 239−247. (7) Hayes, R.; Warr, G. G.; Atkin, R. Structure and Nanostructure in Ionic Liquids. Chem. Rev. 2015, 115, 6357−6426. (8) Gong, X.; Kozbial, A.; Li, L. What causes extended layering of ionic liquids on the mica surface? Chem. Sci. 2015, 6, 3478−3482. (9) Gebbie, M. A.; Valtiner, M.; Banquy, X.; Fox, E. T.; Henderson, W. A.; Israelachvili, J. N. Ionic liquids behave as dilute electrolyte solutions. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 9674−9679. (10) Perkin, S. Ionic liquids in confined geometries. Phys. Chem. Chem. Phys. 2012, 14, 5052−5062. (11) Hayes, R.; Warr, G. G.; Atkin, R. At the interface: solvation and designing ionic liquids. Phys. Chem. Chem. Phys. 2010, 12, 1709−1723. (12) Ngo, H. L.; LeCompte, K.; Hargens, L.; McEwen, A. B. Thermal properties of imidazolium ionic liquids. Thermochim. Acta 2000, 357, 97−102. (13) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N. V. K.; Brennecke, J. F. Thermophysical properties of imidazolium-based ionic liquids. J. Chem. Eng. Data 2004, 49, 954−964. (14) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation. Green Chem. 2001, 3, 156−164. (15) Zhou, F.; Liang, Y.; Liu, W. Ionic liquid lubricants: designed chemistry for engineering applications. Chem. Soc. Rev. 2009, 38, 2590−2599. (16) Palacio, M.; Bhushan, B. Ultrathin Wear-Resistant Ionic Liquid Films for Novel MEMS/NEMS Applications. Adv. Mater. 2008, 20, 1194−1198. (17) Zhao, W.; Mo, Y.; Pu, J.; Bai, M. Effect of cation on micro/ nano-tribological properties of ultra-thin ionic liquid films. Tribol. Int. 2009, 42, 828−835. (18) Werzer, O.; Cranston, E. D.; Warr, G. G.; Atkin, R.; Rutland, M. W. Ionic liquid nanotribology: mica−silica interactions in ethylammonium nitrate. Phys. Chem. Chem. Phys. 2012, 14, 5147−5152. (19) Gong, X.; Kozbial, A.; Rose, F.; Li, L. Effect of π−π+ Stacking on the Layering of Ionic Liquids Confined to an Amorphous Carbon Surface. ACS Appl. Mater. Interfaces 2015, 7, 7078−7081. (20) Rose, F.; Wang, N.; Smith, R.; Xiao, Q.-F.; Inaba, H.; Matsumura, T.; Saito, Y.; Matsumoto, H.; Dai, Q.; Marchon, B.; et al. Complete characterization by Raman spectroscopy of the structural properties of thin hydrogenated diamond-like carbon films exposed to rapid thermal annealing. J. Appl. Phys. 2014, 116, 123516. (21) Wang, Y.; Sun, J.; Li, L. What is the role of the interfacial interaction in the slow relaxation of nanometer-thick polymer melts on a solid surface? Langmuir 2012, 28, 6151−6156. (22) Gong, X.; Frankert, S.; Wang, Y.; Li, L. Thickness-dependent molecular arrangement and topography of ultrathin ionic liquid films on a silica surface. Chem. Commun. 2013, 49, 7803−7805. (23) Merzlikine, A. G.; Li, L.; Jones, P. M.; Hsia, Y.-T. Lubricant layer formation during the dip-coating process: influence of adsorption and viscous flow mechanisms. Tribol. Lett. 2005, 18, 279−286. (24) Prunici, P.; Hess, P. Ellipsometric in situ measurement of oxidation kinetics and thickness of (C2 - C20) alkylsilyl (sub)monolayers. J. Appl. Phys. 2008, 103, 024312−7. (25) Johs, B.; Hale, J. S. Dielectric function representation by Bsplines. Phys. Status Solidi A 2008, 205, 715−719. (26) Zhou, B.; Li, Y.; Randall, N. X.; Li, L. A study of the frictional properties of senofilcon-A contact lenses. Journal of The Mechanical Behavior of Biomedical Materials 2011, 4, 1336−1342. (27) Kozbial, A.; Li, Z.; Iasella, S.; Taylor, A. T.; Morganstein, B.; Wang, Y.; Sun, J.; Zhou, B.; Randall, N. X.; Liu, H.; et al. Lubricating graphene with a nanometer-thick perfluoropolyether. Thin Solid Films 2013, 549, 299−305. (28) Chen, H.; Li, L.; Merzlikine, A. G.; Hsia, Y.-T.; Jhon, M. S. Surface energy and adhesion of perfluoropolyether nanofilms on

Table 2. Surface Energy of BMIM FAP/COC and Zdol/ COC coating materials (∼1 nm)

WCA (deg)

HCA (deg)

surface energy (mJ/m2)

BMIM FAP/COC Zdol/COC

52.2 ± 0.8 68.5 ± 1.1

10.5 ± 0.7 55.8 ± 0.9

50.2 ± 0.7 35.2 ± 1.0

energy of the BMIM FAP/COC is about 50.2 mJ/m2, which is larger than that of Zdol/COC and can be attributed to the fact that RTIL is not a perflurinated material whereas PFPE is.

4. CONCLUSIONS RTILs are much more thermally stable than Zdol, which is critical for HAMR application. Our experimental results on nanofilms indicated that RTILs can survive ∼4 × 107 HAMR writes when the heating temperature is 400 °C. Our research also showed that fluorinated RTILs are solvable in Vertrel XF, which makes it easy to apply RTILs on the media surface by dip-coating, the state-of-the-art lubricating technology in the HDD industry. The experimental results showed that the lube uniformity of RTILs on COC is better than that of Zdol. For Zdol, dewetting occurs when the lubricant is ∼2 nm thick. For most RTILs, no detwetting has been observed even when the lube is as thick as 10 nm. The improved uniformity has been attributed to π−π+ stacking between sp2 carbon in COC and the imidazolium ring in RTILs. The coefficient of the friction of thinner RTILs is also lower than Zdol. RTILs are “designer material” because it could have up to 1 million different chemical structures. Our current work suggested that RTILs, especially with the imidazolium cation and perfluorinated anion, have the potential as the media lubricant for HAMR. However, further structure optimization is needed.

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AUTHOR INFORMATION

Corresponding Author

*L. Li. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Advanced Storage Technology Consortium (ASTC) for the financial support. We thank Dr. Franck Rose from HGST for providing the COC samples.

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REFERENCES

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