Graphene overcoats for ultra-high storage density magnetic media

Hard disk drives (HDDs) are used as secondary storage in digital electronic devices owing to low cost and large data storage capacity. Due to the exponentially increasing amount of data, there is a need to increase areal storage densities beyond ~1 Tb/in2. This requires the thickness of carbon overcoats (COCs) to be <2 nm. However, friction, wear, corrosion, and thermal stability are critical concerns below 2 nm, limiting current technology, and restricting COC integration with heat assisted magnetic recording technology (HAMR). Here we show that graphene-based overcoats can overcome all these limitations, and achieve two-fold reduction in friction and provide better corrosion and wear resistance than state-of-the-art COCs, while withstanding HAMR conditions. Thus, we expect that graphene overcoats may enable the development of 4–10 Tb/in2 areal density HDDs when employing suitable recording technologies, such as HAMR and HAMR+bit patterned media

compatible HDM in HAMR-like conditions, with 0.25 s total irradiation time. Reference 29 also reported degradation of 4 nm a-C:H-based commercial COC on a FePt-based HDM in HAMRlike conditions, with total heating time~0.1 ms, corresponding to a 5-year drive life or 157.68 × 10 6 s 29 (see Supplementary Note 1).
Better thermal stability was reported in filtered cathodic vacuum arc (FCVA)-based COCs under laser irradiation in HAMR-like conditions 29 and thermal annealing up to~940 K 32 , consistent with the good thermal stability, i.e., no change in sp 3 content up to 1100°C, found in tetrahedral a-C (ta-C) films 33 . However, FCVA-based COCs are not yet used as HDM overcoats due to the presence of macro-particles 6,34 .
Graphene is an emerging material for lubrication [35][36][37][38][39] , as well as oxidation 40 and corrosion protection [41][42][43] . Reference 37 reported that single-layer graphene (1LG) reduced the steel COF from 0.9 to 0.3 with a coating lifetime up to 6500 cycles, and decreased the wear rate by two orders of magnitude. Multilayer graphene (MLG) (3-4 layers) showed excellent tribological performance with a COF < 0.2, a decrease of wear rate by three orders of magnitude, and sliding lifetime up to 47000 cycles on steel 37 . Reference 35 reported that 1LG exhibits superlubricity 35 and reduced the COF by 2-3 times and wear rate by two orders of magnitude for Au-based electrical contacts 38 . 1LG decreases the oxidation and corrosion of various metals, such as Ni 40,41,43 , Co 41 , Fe 41 , Pt 41 , Cu 40,42,43 , Ag 42 . Reduced graphene oxide was also used as a barrier coating 44 . Reference 45 reported that suspended 1LG has good thermal stability up to 2600 K, with a thermal conductivity up~2000 W/mK 46 . In ref. 47 , we showed that it is possible to achieve high mobilities~30,000 cm 2 V/s at room temperature in wet transferred, polycrystalline 1LG. Thus, scalable processes, such as wet transfer, can be used for integration and packaging 47,48 . All these characteristics make 1LG promising as a protective overcoat for both existing and HAMRbased technologies.
Here we use 1-4 layers of chemical vapor deposition (CVD) grown graphene (1-4LG) transferred on Co-alloy (current technology) and FePt-based (HAMR technology) HDMs, and test friction, wear, corrosion, thermal stability, and lube compatibility. We demonstrate that 1LG shows better performances than current 2.7 nm COCs on Co-alloy-based HDM, and good tribological properties with a COF < 0.2 for >10,000 cycles. We achieve very low COF~0.15 for 1LG, indicating high wear resistance and corrosion protection, with corrosion current densities < 5 nA/cm 2 for 2-3LG. Thermal stability tests confirm that 1LG on FePt can withstand HAMR-like conditions, without degradation. Graphene's superior performance and its thinness can enable the development of ultra-high-density magnetic data storage technologies, based either on current technology, as well as on HAMR, or HAMR combined with bit patterned media (BPM), where the magnetic storage layer is patterned into an array of pillars, each representing a single bit 49 . The combination of 1LG + HAMR + BPM may increase AD > 10 Tb/in 2 .
The details and nomenclature of the COCs are in Table 1. The structural properties of the FCVA and sputter-deposited COCs can be found in refs. 50,51 . HAMR-compatible FePt-based HDMs are shown in Fig. 1f. See Supplementary Notes 2 and 3 for further details.
Graphene growth and transfer. Graphene is grown by chemical vapor deposition (CVD) and placed on the HMD by wet transfer 52,53 , as described in the "Methods" section. The quality and uniformity of the samples are assessed by Raman spectroscopy 54 . Unpolarized Raman spectra are recorded at 514.5, as well as at 785 nm, close to the 850 nm used for HAMR writing tests 25,55 , with a Renishaw InVia spectrometer equipped with a Leica DM LM microscope and a ×100 objective with a numerical aperture 0.85. A 514.5 nm Raman spectrum of 1LG on Cu before the transfer is shown in Fig. 2a. The photoluminescence (PL) background due to the Cu foil is removed using baseline subtraction 56 . The D to G intensity ratio I(D)/I(G) is <0.1, indicating a defect concentration n d < 2.4 × 10 10 cm −2 54,57-59 (Fig. 2b). The 2D peak position can be fitted with a single Lorentzian with Pos(2D)~2705 cm −1 and full-width-half-maximum, Fig. 1 Hard disk media structures. a Schematic cross-section of a hard disk drive with magnetic medium, disk overcoat, lubricant, fly height, head overcoat, head. The head-medium spacing and fly height/clearance are indicated by arrows. The slider containing the read/write head and the head overcoat are shown as well. For 1 Tb/in 2 , the head-media spacing is~8.9-6.5 nm 6 , based on the sum of overcoat thickness (2.5-2 nm) 6 , lubricant thickness (1.2-1 nm) 6 , touch down height (2-1 nm) 6 , fly clearance (1.2-1 nm) 6 and head overcoat thickness (2-1.5 nm) 6 . b Full stack of bare CoCrPt:Oxide-based hard disk media comprising a glass substrate, seed bottom underlayer, and antiferromagnetic layer (A-FM) between two soft-magnetic under layers (SUL), followed by two intermediate layers and a co-based magnetic recording layer. c Bare medium (BM). d Magnetic medium with~2.7 nm commercial COC. e Magnetic medium with~2.7 nm commercial COC and~1 nm commercial lube. f Full stack of bare FePt-based hard disk medium (HDM) containing glass substrate, a layer of CrRu followed by layers of MgO and FePt. NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-22687-y ARTICLE FePt/MgO/CrRu/glass samples grown by magnetron sputtering. CrRu and MgO seed layers are grown at 400°C and 1.5 mTorr; FePt is grown at 600°C and 3.5 mTorr as discussed in ref. 22 .
Current commercial HDD technology uses a~2.5-3 nm COC deposited by plasma-based processes 12,63 . The thickness and roughness of 1-4LG are measured by Atomic Force Microscopy (AFM), as discussed in Supplementary Note 5. This shows that the thickness of 4LG on HDM is~2.1 nm ( Supplementary Fig. 3), well below that of the current commercial COC. Graphene is transferred without such an energetic process as that used for commercial COC deposition. Thus, it does not adversely affect the magnetic performance. Even considering that COCs slightly affect the HDM magnetic properties, the influence of graphene will be much less than commercial COCs deposited by energetic processes, where energetic atoms/ions (neutrals also) hit and participate in deposition onto the HDM. Reference 64 reported that 1LG grown by CVD on Co (and Ni) does not affect the magnetic properties of these materials. In order to confirm this in our case, we perform polar magneto-optical Kerr effect measurements on BM, BM covered with~2-3 nm COC, and BM covered with 1-4LG. 1-4LG coating results in a small change in the hysteresis loop. The coercivity changes from 510 mT (BM) tõ 545 mT (1-4LG and COC). The remanent magnetization is unchanged at~97-98% of the saturation magnetization when compared to BM. The coercivity and the remanent magnetization of 1-4LG-coated BM are similar to BM+COC, indicating that graphene coating does not have any detrimental effect on the media, see Supplementary Note 6 for details.
Laser irradiation stability. HAMR is not possible without laser heating. References 22,25,65 reported that FePt does not degrade upon laser heating, with lubricant and COC being the main concerns when it comes to laser irradiation. Consistent with refs. 25,55 , here we use an IR laser to examine the 1-4LG irradiation stability. To test whether 1LG can withstand HAMR conditions we consider 1LG transferred on L1 0 -FePt-based HDM 25,27,55 and on Si/SiO 2 . Reference 55 achieved T c at 1.3 mW/μm 2 by optimizing aperture optics. Reference 25 suggested using a laser power density~10 7 W/cm 2 and~0.35 × 10 7 W/cm 2 for FePt-based HDM. We perform Raman measurements at 785 nm, the available Raman wavelength closest to that used in HAMR (~830 nm 25,55 ), with a spot size~1.24 μm, as determined by the razor blade technique, see Supplementary Note 7 for details. We vary the power density from~1.3mW/μm 2 (0.013 × 10 7 W/cm 2 ) to~31.5 mW/μm 2 (0.315 × 10 7 W/cm 2 ) in order to examine the laser irradiation-driven evolution of the SLG Raman spectrum under HAMR power densities.
We first consider Si/SiO 2 /1LG. We record spectra at different power densities for~4 min to achieve a good signal-to-noise ratio. Figure 2c shows that 1LG on Si/SiO 2 has no D peak even for the highest power density. Pos(G) downshifts from~1588 to 1583 cm −1 , indicating an increase in T~312.5 K with respect to room temperature, by taking −0.016 cm −1 /K as a shift of Pos(G) with T 66 . Figure 2d plots the data for 1LG on FePt HDM, with I(G) normalized to that at 1.3 mW/μm 2 . In this case, the FePt substrate is rotated at 4100 rpm on a circular track with a diameter~4 mm to simulate rotating HDD conditions, with a 20 min acquisition time, much larger than the total laser irradiation time expected for a 5-years life of HAMR-based HDDs 29 . No D peak is seen for all power densities, thus confirming the stability of 1LG. The Raman data show that no visible changes occur during the irradiation test measurements.
Friction and wear. The HDM comprises magnetic/metallic layers, including Co-alloys-based magnetic storage layers 14,20 , with a high COF~0.6-0.8 11,15 and wear 11,15 . Therefore, they can experience mechanical damage whenever intermittent contact occurs with the head 11,15 , and are susceptible to corrosion 8,11,14 and oxidation 11,15 , leading to HDD degradation or damage. The cur-rent~2.7 nm commercial COCs have a high COF~0.3-0.5 11,15 and wear in a ball-on-disk tribological environment 11,15 , which can result in damage, hence durability concerns, see Supplementary Note 1 for more details.
Reference 11 used FCVA to deposit~1.5-2 nm carbon films for protection of Co-alloy-based HDM and reported low COF~0.25, wear, and corrosion. Here, we use 1-4LG grown by CVD for Coalloy-based and HAMR HDDs.
Since in HDDs the HDM spins during operation, COF and wear need to be examined in a setup mimicking HDD operation. AFM and other tip-based tools are used to measure COF and wear 39 . In contrast to setups with rotating geometry, tip-based tools measure the COF based on the movement of the tip in the lateral direction 39 . Ball-on-disk measurements (rotation-based geometry) have a similar assembly as HDDs, with samples rotating while the counterface is in contact with the surface 11,35 , thus measuring COF. We perform ball-on-disk tests using a nano-tribometer (CSM Instruments) in a cleanroom, to have a controlled environment with T = 23 ± 1°C and a relative humidity~55 ± 5%. A sapphire ball (Al 2 O 3 ) of diameter~2.0 ± 0.1 mm and surface roughness~5.0 ± 0.1 nm is used as the counterface because the hard disk head is made of an Al 2 O 3based composite 6 . During the test, a normal load~20 mN and a rotational speed~100 rpm are used, corresponding to a linear speed~1.05 cm/s for 10,000 cycles.
Since in HDDs the contact occurs occasionally 6 , the 10,000 cycles in our setup are much higher than the HDD operational lifetime. After each test, the wear track and ball images are captured using an optical microscope. To check repeatability, tests are performed 2-7 times. When two surfaces are in contact, and at least one of the surfaces starts to slide with respect to the other, friction and wear occur 67 . As a result, a wear track is formed. Figure 3 plots representative friction curves for BM, CMC, CMCL, and BML (Fig. 3a) coated with 1-4LG (Fig. 3b), and BM coated by FCVA (Fig. 3c). The average COFs are in Fig. 4, including HDM coated by DC-sputtering. BM has the highest COF~0.8, with substantial wear, as confirmed by optical images of balls and wear tracks in Figs. 5 and 6. The COF of~2.7 nm CMC reaches~0.4 at 10,000 cycles, but with strong fluctuations betweeñ 0.2 and 0.6 at 1500 up to~3500 cycles (Fig. 3a), and negligibly improves wear with respect to BM (Fig. 3a). The transfer of 1-4LG reduces COF < 0.2 for all samples, and gives non-fluctuating, smooth friction curves. The COF for 1-4LG-coated samples (without lube) is~4 times lower than BM and~2 times lower than CMC, despite a reduction of thicknesses~7 times (for 1LG) to 2 times (for 4LG), using the theoretical 1LG thickness, with respect to CMC. Figures 5b, d and 6b, d reveal that the wear track width of 1-4LG-coated samples is~2-4 times lower and debris transferred to the ball are smaller than BM and CMC, indicating higher wear resistance. All FCVA COCs with thicknesses from~0.3 to~1.8 nm show~2-5 times higher COF than 1-4LG-coated samples, with and without lube, apart from 1LG without lube. Pulsed DC sputtered COCs have~2-3 times higher COF than 1-4LG-coated samples.
To further analyze the wear tracks, Auger Electron Spectroscopy (AES) imaging is performed using a JEOL JAMP Auger Microprobe, see Supplementary Note 4 and Supplementary Fig. 1 for details. Before AES, scanning electron microscope images are taken to select the AES locations. The AES images inside and outside the wear tracks show the carbon-containing sites, and indicate that the amount of carbon on the wear track increases with increasing number of graphene layers, N. The Co and Cr intensities inside the wear tracks are higher for 1LG, and decrease with increasing N, due to the increase in C and the <1-3 nm sampling depth of AES 68 . The O signal in the wear track appears due to ambient oxygen as the samples are exposed to air before AES, with some contribution from the HDM oxide.
After ball-on-disk tests, the wear tracks are analyzed by recording Raman spectra across the wear track at different positions. Reference 58 introduced a three-stage model of amorphization. Stage 1: graphene to nanocrystalline graphene. Stage 2: nanocrystalline graphene to low sp 3 amorphous carbon. Stage 3: low sp 3 to high sp 3 amorphous carbon. For all spectra in Figs. 5 and 6, as the D peak increases, D′ and D+D′ appear, whereas the I(2D) weakens when approaching the wear track, indicating an increase in disorder according to stage 1. The broad peak between 500 and 1000 cm −1 is due to the glass substrate 69 . At the center of the wear track, all second-order Raman features (i.e., 2D, 2D′, and D+D′) merge, while I(D) decreases and D,G become broader, indicating an increase of disorder 58 . 2-4LG is less damaged than 1LG, as for the spectra in   Effect of lubricant. Current commercial HDDs use a layer of perfluoropolyether (PFPE) lubricant (lube) 11 on top of COCs to further reduce friction and wear, and minimize surface energy. Given the lubricating and corrosion protection properties of 1LG itself, lube may not be needed for 1LG. However, exploring the compatibility of 1LG with lube is useful from the fundamental viewpoint.
Friction measurements on lube-coated samples (Fig. 3) show that in certain cases the COF is higher with L, than without. Taking the error bars into account, a significant difference in COF is only observed for 1LG and 1LGL. 1LG has wetting transparency 73 , i.e., it is thin enough that its introduction does not affect the wettability of an underlying substrate. Without overcoat, the L containing HDM, as in BML, shows very high COF~0.55 and wear, due to the metal (medium) L-induced catalysis 74 . 1LG is not enough to avoid the interaction of L with medium, hence higher COF~0.45 and wear is observed in 1LGL. Without L, 1LG shows very low COF~0.15 and wear. A minor difference in case of 2LG versus 2LGL can be seen. However, for 3LG versus 3LGL and 4LG versus 4LGL the difference in average COF is marginal. L on 1LG results in higher and inconsistent COF and wear, as some measurements show lower COF~0.15, some higher~0.78, with COF increasing after few thousands cycles. The COF of 1LG, on an average, is~0.45, i.e.,~3 times its non-lubricated counterpart. Thus, COFs of 2-4LG are more or less similar to those of the non-lubricated counterparts, suggesting that 2-4LG without L are lubricious enough, and that L does not improve lubricity. The D and G peaks in the Raman measurements in Figs. 5 and 6 on 1-4LG confirm the presence of carbon on the wear tracks, and transfer of debris to the balls, similar to the non-lubricated counterparts.
Corrosion. Co-alloys have a great propensity to corrode, mainly due to Co oxidation 75 . This results in the loss of magnetic properties 75 , hence this is one of the major concerns for the longterm functionality and durability of HDDs 14,51,75 . To examine the corrosion protection efficiency of 1-4LG and compare their performance with state-of-the-art COCs, the corrosion of different uncoated-and coated-HDM, exposed to an electrolyte solu-tion~0.1 M NaCl similar to that used in refs. 75 on a~0.24 cm 2 area, is investigated using an electrochemical corrosion method 51,75 . The measurements are performed with a 3-electrode setup with a Pt wire as counter electrode, Ag/AgCl as reference electrode and HDM as working electrode, to which the potential is applied 51 . Each test consists of anodic and cathodic sweeps, where the potential is varied and the corresponding current measured 51 . Every sweep is conducted at different locations, with at least 3 sets of 6 sweeps on each sample. The so-called Tafel's analysis 75 is done by plotting anodic and cathodic curves on a semi-logarithmic scale of potential versus log current. The linear part of the logarithmic anodic and cathodic currents are  75 , and the intercept of these lines gives the corrosion current 75 , and the corrosion current density J corr when divided by the contact area.
The corrosion protection efficiency (CPE) defines how efficient the COC is in protecting against HDM corrosion. This is defined as 51 : where J 0 corr is the BM corrosion current density and J corr that of coated HDM.
When a metal is exposed to a corrosive solution, it releases ions that leave behind electrons, which can be observed in an anodic reaction as 75 : where M represents the metal and n the number of electrons released by it. For Co-alloy-based systems, the metal dissolution, which decreases the anode conductivity 75 , can be written as 75 : As the corrosion reaction involves the transfer of electrons and ions between metal and solution 75 , the corrosion rate varies with corrosion current 75 , hence J corr varies inversely with corrosion resistance, i.e., the COC ability to reduce the HDM corrosion 51,75 .
The BM shows the highest J corr , indicating the greatest propensity to corrode (Fig. 8). Introduction of 1-4LG reduces HDM corrosion. J corr decreases with N, from~5.3 nA/cm 2 for 2LG to~4.4 nA/cm 2 for 4LG, Fig. 8, and CPE increases. The COC defects and pinholes are the active corrosion sites 75 , and their electrical conductivity could further add galvanic corrosion 75 . The COC should be defect-and pinhole-free, smooth, and with excellent barrier properties to minimize HDM oxidation and corrosion. Reference 43 showed that 1LG can be used as a corrosion barrier for metallic surfaces, and suggested that this should be uniform and defect-free to achieve corrosion protection. In our case, 1LG reduces J corr by~2.5 times with respect to BM, but it is twice that of BM with~2.7 nm COC. For 2LG, J corr is~3.8 times smaller than BM, and marginally higher than COC. This is remarkable as the 2LG thickness~0.7 nm, is~4 times lower than~2.7-nm-thick COC. Beyond 2LG, J corr and CPE remain similar, adding marginal anti-corrosion improvement (Fig. 8). The higher corrosion protection in 2-4LG with respect to 1LG is mainly due to the fact that the increase in N improves the HDM coverage, leading to the reduction of active corrosion sites. 2-4LG display corrosion protection similar to COC, attributed to better barrier properties. Figure 8 indicates that, for a similar thickness, graphene-based overcoats have lower J corr than amorphous COCs, indicating greater protection. Figure 8 shows that the corrosion resistance performance of 1LG is~2.5 times better than BM, but~2 times higher than COCs. By adding graphene layers the corrosion resistance is improved. The difference in corrosion current density between COC and 2-4LG is 30-10%. Thus, the corrosion resistance of 2-4LG approaches that of COCs. Taking into account that the theoretical thickness of 1LG is~8-9 times lower than state of the art COCs, and that a COC having the same thickness as 1LG would provide no protection and be full of holes 19 , 1LG comfortably beats any COC of the same thickness (i.e., same storage density).

Discussion
Friction is defined as resistance to sliding 36,76 . At the macroscopic level, Amonton's law 76 states that the frictional force between two bodies varies proportionally to the normal force. Hence: where F [mN] is the frictional force and W [mN] is the normal force. This does not take into account the area of contact at the microscopic level 76 . Reference 76 suggested that the contact between two bodies contains several smaller contacts, called asperities, with the sum of the areas of these asperities being lower than the apparent macroscopic area 76 . Thus, from ref. 76 , F in micro-tribology can be expressed as: where τ is the shear strength, i.e., the stress required to shear the contacting interfaces and enable sliding, expressed as shear force/ area and ∑A asp is the sum of the areas of the asperities, also called real contact area 76 . Hence, friction also depends on ∑A asp . Reference 76 showed that the stresses over regions of contact reach the elastic limit of the material and cause plastic deformation. Thus, the average pressure (p) in a contact region is governed by the normal force, and is given by p = W/A 77 . The zone of deformation increases with increase in load, and p on the region of contact tends to a stable value, which eventually causes the deformation to become entirely plastic. The process continues until the area of contact becomes sufficient to support the load W. Consequently, the normal force can be expressed as: Thus, Eqs. (4)-(6), give: A co-alloy is mechanically softer 78 than Al 2 O 3 79 . When a metallic co-alloy surface slides against Al 2 O 3 , it causes the HDM surface to undergo plastic deformation 76 , generating wear and high COF 0.8, Figs. 3 and 4, consistent with the friction results for metallic surfaces in ref. 76 . This could be due to the formation of an adhesive contact at the HDM-ball interface, due to the high surface energy of Co-alloy-based HDM~42.8 mN/m 80 , and the presence of contaminants at the interface that may enhance the interaction between the two bodies 81 . When 1LG is placed on HDM, the COF decreases~4 times and negligibly changes with N.
At the nanoscale, the friction of 1LG on Si/SiO 2 against Si tips, measured at a low load~1 nN and scan speed~1-10 μm/s, was explained based on out-of-plane deformation in front of a scanning probe tip, the so-called puckering effect 39,82 . This enhances the contact area, hence friction. The out-of-plane deformation is suppressed with increasing N 82 , resulting in reduced COF. This is not applicable to ball-on-disk tribological tests of 1-4LG, due to the significantly larger dimension (ball radius~2.0 ± 0.1 mm) of the counterface, much higher load~20 mN and higher speed 100 rpm or 1.05 cm/s, which do not precisely differentiate the frictional characteristics of 1-4LG. Thus, all coated media show similar friction (Fig. 3b). Since the layers are coupled by van-der-Waals forces, they shear easily, as their interfacial shear strength  is low, with a shear force per unit area α~12.8 ⋅ 10 18 N/m 3 83 and a resulting shear modulus C 44~4 .3 GPa 83 . The ease of shearing at sliding contacts results in smoother frictional curves (and marginally lower COF) in 2-4LG with respect to 1LG. The improved wear resistance for 1-4LG-coated HDM is attributed to reduced friction, wear, and excellent mechanical properties of 1LG: breaking strength~42 N/m 84 , Young's Modulus~1 TPa 84 , flexibility (1LG can be stretched up to~20% without breaking 84 ).
When compared to BM, the COF reduction of 1-4LG-coated HDM could be attributed to reduced adhesive interaction (lower surface energy) 36,85 , as well as incommensurability of the lattice planes sliding against each other at the tribological interface 35 , occurring when the hills of a surface with lattice spacing a do not match the valleys of another surface of lattice spacing b, such that b/a is an irrational number 35 . This is consistent with what suggested in ref. 35 for 3-4LG on Si/SiO 2 sliding against an a-C:Hcoated steel ball, and for 1-3LG on a steel sliding against a steel ball 35,37 in macroscale ball-on-disk conditions, e.g., with a normal force in the z direction N z~0 .5-3 N, and speeds~0.6-25 cm/s. The AES on the wear tracks and Raman measurements across the wear tracks, after ball-on-disk tribological tests, in Figs. 5 and 6 show that a C signal is still present in all 1-4LG samples, even though disorder-induced peaks appear. Reference 86 also found C on wear tracks after microscale friction and wear tests of graphene on SiC. The counterface also reveals transferred debris, consisting of disordered carbon and underlying substrate atoms. This implies that, when a 20 mN load is applied and the sample starts to rotate at~2.1 cm/s, 1LG could be turned into patches, due to the large contact load. Reference 86 reported transformation of continuous layers of 1-2LG into 1-2LG patches during microscale tribology, despite using lower load~0.1-1 mN and speeds~30-50 μm/s compared to us. This led to a distribution of 1LG patches along the wear track, and transfer of 1LG-containing debris on the counterface. This also happens for 2-4LG-coated HDM. Raman measurements in Figs. 5 and 6 show disordered carbon on the wear tracks, with disorder lower than for 1LG, as revealed by I(D)/I(G) (Fig. 7). This implies that 1LG acts as lubricant when in contact with another 1LG, and is not completely removed during tribological tests. AES also reveals a progressive increase in C and C-containing sites on the wear tracks with increasing number of layers. The debris transferred to the ball also contain C. Therefore, the formation of disordered carbon debris on both surfaces facilitates smooth sliding, and contributes to maintaining the low COF. The marginally lower friction in 2-4LG as compared to 1LG can be linked to I(D)/I(G) (Fig. 7a), with lower disorder corresponding to lower friction, although this does not apply for 1-4LGL. Figure 9 is the proposed mechanism for friction reduction for 1LG and 3LG-coated HDM, where C debris are on both ball and wear track. To validate these assumptions, we perform molecular dynamics (MD) simulations of a Co|Pt-sapphire system with and without 1 and 4LG, as discussed in Supplementary Note 8.
The COF is estimated as the ratio of sliding and normal force, block averaged every 25 ps. Figure 10a-e compares the computed COF and surface disorder of Co|Pt, 1LG, and 4LG. The surface disorder represents the friction-induced damage/wear of the HDM, and its value is derived considering 0% surface disorder before friction measurements. The amount of disorder in HDM is quantified using the centrosymmetry parameter, a measure of the local lattice disorder around an atom. This is 0 for a perfect lattice, whereas when a point defect exists, i.e., when the symmetry is broken, it assumes a larger positive value, see Supplementary Note 8 for details. The simulations indicate that BM develops substrate disorder up to~14.5% within 350 ps (Fig. 10b, e).
High wear is also observed for BM in the experiments in Fig. 3a. An additional 250 ps where the forces on atoms are also collected are used to obtain the COF (Supplementary Fig. 5). The COF averaged over these 250 ps is~0.82 ± 0.1. 1LG reduces the average simulated COF to~0.18 ± 0.02. Figure 10c shows that 1LG maintains its structural integrity with fraction of disorder~1% on average, as a consequence of the low COF between the blocks, indicating higher resistance to wear as compared to BM. 4LG further improves the tribological behavior. The COF drops slightly to~0.14 ± 0.0.02 and substrate disorder is reduced to~0.1%, much lower than in BM, suggesting that the BM surface remains mostly unaffected. Simulations suggest partial transfer of graphene patches to the sapphire ball. The Raman analysis of wear tracks in Figs. 5 and 6 shows tribo-induced disorder in 1-4LG transfer of C to the ball. Thus, the patches containing debris on both surfaces are responsible for the lower COF in 1-4LG-coated HDM, maintaining higher wear resistance than BM.
Reference 87 showed that potential corrugations on sliding surfaces can impact friction force and anisotropy of MLG. It reported that higher friction forces result from larger corrugations of the potential energy and depend on N. This is consistent with our experiments in Figs. 3 and 4, and simulations in Fig. 10.
In summary, we analyzed 1-4LG-coated media and found that they can overcome the tribological and corrosion issues of current Co-alloy-based HDM, with laser irradiation stability on FePtbased HDM for HAMR. The overall performance of graphenecoated media exceeds that of thicker commercial COC, as well as other amorphous carbons of comparable/higher thicknesses prepared by FCVA and sputtering. Given the tribological, corrosion, and thermal stability characteristics coupled with an AFM thick-ness~4 times (for 1LG) to~1.3 times (for 4LG) lower than stateof-the-art 2.7-nm COCs, we expect 1-4LG-based overcoats to meet the requirements for 4-10 Tb/in 2 areal density HDDs, when employing suitable recording technologies, such as HAMR and HAMR+BPM. Graphene-coated media have better corrosion and wear resistance with a COF < 0.2. A single layer of graphene is enough to reduce corrosion~2.5 times and can withstand HAMR conditions. Our results imply that >2LG-based coatings could be used as a tribological interface for various other materials/devices, such as micro-and nano-electromechanical systems.

Methods
Graphene growth and transfer. 1LG is grown by CVD on a 35 μm Cu foil. The substrate is loaded into a hot wall tube furnace evacuated to~1 mTorr. The Cu foil is annealed in hydrogen at 1000°C for 30 min. This reduces the copper oxide surface and increases the Cu grain size. The growth process starts when 5 sccm CH 4 is added to H 2 . After 30 min the substrate is cooled down for several hours in vacuum (~1 mTorr) to room temperature and unloaded.
We use a polymethyl methacrylate (PMMA)-based wet transfer 52,53 . First,~500 nm PMMA is spin-coated on the sample. The PMMA/1LG/Cu stack is then placed in an aqueous solution of ammonium persulfate to etch Cu 52 . When Cu is fully etched, the graphene/PMMA-stack is placed into a de-ionized (DI) water bath to rinse any acid residuals and subsequently fished out using the HDM substrate. After drying for one day at room temperature, the sample is placed in acetone to remove PMMA, leaving 1LG on the HDM. By repeating the steps described above, several 1LGs are transferred to create an MLG stack. The same procedure is used to place 1LG and MLG onto FePt.

Data availability
All relevant data are available from the authors upon reasonable requests. Unique identifiers such as DOI and hyperlinks for any other data sets are also available from the authors upon reasonable request. Source data are provided with this paper.