under a normal load condition. A calibrated strain gauge was instrumented to measure the friction force (in the sliding direction) between the head and the disk interface. The frictional force at an interface is written as, \( F = \mu L + F _ { A } \), where F is the total friction at the interface,\( \mu \) the non-dimensional coefficient of friction, L the applied normal load, and \( F _ { A } \) the adhesion component of friction.40–42 Figure 4 shows the delta friction force as a function of normal load when applied -1 \( V \) and +1 V on the head overcoat. Here, we have subtracted the friction contribution at low load (=1.25 mN) from all other load conditions to measure delta friction force. Dashed line is the linear fit with slope being the coefficient of friction. For positive voltage cycle on head, the friction coefficient is 0.4 in comparison to 0.2 for negative voltage cycle on the head. This further demonstrates the importance of the electrochemical activity on the carbon overcoat during sliding is determining the friction properties and long term durability of the overcoat.

Figure 4. Frictional properties under an applied voltage: Shows the delta friction force as a function of normal load under a repetitive cycle of applied +1 V and -1 V on head. Dashed line is the linear fit with slope being the friction coefficient.

# Chemical marking of contact location

As a practical application of surface passivation, it was used to chemically mark the disk overcoat surface. Intentional contacts at two distinct tracks (radius 21 \( ^ { \prime \prime } \) m and 23 \( ^ { \circ } \) m on disk) were made for 3 seconds under a normal load of 3.75 mN at the head disk interface. The head is held at -1 V with respect to the disk. Figure 5b shows the decay in the interfacial current with the same head at two distinct tracks. Once the head passivates the first track on disk, the initial current on the new track is similar to the initial current of the previously passivated track. This further shows that for negative head voltages, the surface passivation dominated the electrochemical activity on the disk leaving the head in pristine condition. We used the same head to scan probe the electrical conductivity of the complete disk. Figure 5c shows the interfacial current as probed by the same head at different tracks. The electrical current on the passivated track is found to be significantly lower than the untreated area of the disk surface. We believe that this surface passivation of the carbon overcoat can have significant applications for high speed lithography. It is worth mentioning that recently AFM has been used to perform similar surface passivation of graphitic surfaces but AFM operates at typically six orders of magnitude slower sliding speed.43 Chemical analysis of the oxide formed on carbon overcoat is still missing and requires more work.

# Discussion

In summary, we have outlined a quantitative analysis of voltage assisted nanoscale electrochemical wear on carbon overcoat at high sliding speed interfaces. At high sliding speeds, in-situ measurements were performed of the interfacial current and the associated wear amount due to the electrochemical process. In addition, the effect of electrochemical activity on the interface is further quantified by measuring the friction force and the friction coefficient. It is found that the voltage assisted electrochemical activity greatly influences the interfacial wear and frictional properties. Positive voltage applied to the head leads to high

Figure 5. Chemical marking of contact location: (a) Cartoon of disk depicting the intentional contact at two location, Radius =21 mm and 23 mm. (b) Marking: dots represent the passivation of carbon overcoat at two locations. (c) Detection: shows the interfacial current as probed by the same head at different tracks.

wear on the head overcoat but no head overcoat wear was observed for negative applied voltage. As a useful application, we exploited the electrochemical passivation to mark the head-disk contact regions on the disk. The contact regions can be clearly identified by the associated conductivity variations of the surface. We believe that the observed voltage assisted asymmetric nanoscale wear will lead to additional experimental and simulation work, and will help to understand precisely the chemical origin of the involved process.

Our results are expected to have strong impact on fundamentally improving the carbon overcoat for various applications. The effect of interfacial voltage on the sliding interface is expected to be of great importance for understanding and improving the wear properties in nanoscale devices.

# Methods

# Sample preparation

Disk: the rotating disk is a commercial 2.5” CoCrPt:oxide based hard disk media fabricated onto a glass substrate. Outermost thin film layer of hard disk media consist of 3 nm amorphous nitrogenated carbon overcoat coated with a molecular layer of perfluoropolyether polymer lubricant (∼ 1 nm thick).

Head: the head is a commercially available with read and write elements fabricated on a ceramic substrate. Similar to the disk, the head is also coated with a carbon overcoat with 1.4 nm diamond like carbon on top of a 0.3 nm Silicon based adhesive layer. The head surface is carefully etched (known as air bearing surface (ABS)) such that while flying on top of the disk an air lift force is generated that keeps it afloat in the nanometer distance over the disk.

# Contact detection between the head and disk

Contact between the head and disk is monitored using a piezo-electric based acoustic emission (AE) sensor of the type PICO -200-750 \( - r \mapsto \cdot \) z. It detects elastic propagating waves generated during the head-disk contact events.34 Figure 6 shows a typical contact detection between the head and the disk. Vertical clearance between the head and disk is set using the embedded micro-heater inside the head. For protruding head making a contact with disk, AE signal increases sharply compare to non-contact condition.

# Interfacial current and friction measurements

Electrical measurement: In all interfacial measurement, the head-disk interface is first voltage biased then the contact is made using the micro-heater. The voltage bias is done using a HP 3314A source and the corresponding interfacial current is measured using Agilent 4155C. Interfacial current measurement are done under a positive normal load condition.

Friction measurement: Contact friction force is measured using a calibrated strain gauge mounted at the end of the suspension. The strain gauge signal is measured and amplified using a Vishay 2311 signal conditioning amplifier.