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