Partho S. Goohpattader

Probing the Role of an Atomically Thin SiNx Interlayer on the Structure of Ultrathin Carbon Films

Filtered cathodic vacuum arc (FCVA) processed carbon films are being considered as a promising protective media overcoat material for future hard disk drives (HDDs). However, at ultrathin film levels, FCVA-deposited carbon films show a dramatic change in their structure in terms of loss of sp3 bonding, density, wear resistance etc., compared to their bulk counterpart. We report for the first time how an atomically thin (0.4 nm) silicon nitride (SiNx) interlayer helps in maintaining/improving the sp3 carbon bonding, enhancing interfacial strength/bonding, improving oxidation/corrosion resistance, and strengthening the tribological properties of FCVA-deposited carbon films, even at ultrathin levels (1.2 nm). We propose the role of the SiNx interlayer in preventing the catalytic activity of Co and Pt in media, leading to enhanced sp3C bonding (relative enhancement ~40%). These findings are extremely important in view of the atomic level understanding of structural modification and the development of high density HDDs.

Activated drops: Self-excited oscillation, critical speeding and noisy transport

A small drop (∼10μl) of water exhibits critical speeding dynamics on an inclined super-hydrophobic pillared surface, in that it moves very slowly at first, but speeds up rapidly after a critical velocity is reached. During the mobile phase, some of the natural vibration modes of the drops are self-excited on a pillared surface, but not on a smooth hydrophobic surface. Additional experiments were carried out with glycerin and the solutions of water and glycerin that allowed their density and surface tension to be held more or less constant, while their viscosity could be varied. The terminal velocities of these drops following the critical speeding did not exhibit the expected decrease with increasing viscosity, but showed a highly non-linear behavior, exhibiting a maximum at an intermediate viscosity. Any of these drops moves steadily on a sub-critically inclined pillared substrate when it is subjected to a mechanical noise, the dynamics of which is remarkably similar to that obtained from another designed experiment in which the drops were made to cross a physical barrier assisted by an external noise. The dynamics of the low viscosity (1mPa s to 5.3mPa s) drops are amenable to a Kramers-like transition rate in the low friction limit, although the overall dynamics is found to be sub-Arrhenius. This work highlights the importance of the fluctuation of a drop that is either self-excited or that induced by an external noise in its motion on a surface.Graphical abstract

Enhanced Tribological, Corrosion, and Microstructural Properties of an Ultrathin (<2 nm) Silicon Nitride/Carbon Bilayer Overcoat for High Density Magnetic Storage

An ultrathin bilayer overcoat of silicon nitride and carbon (SiNx/C) providing low friction, high wear resistance, and high corrosion resistance is proposed for future generation hard disk media. The 16 Å thick SiNx/C overcoat consists of an atomically thin SiNx underlayer (4 Å) and a carbon layer (12 Å), fabricated by reactive magnetron sputtering and filtered cathodic vacuum arc (FCVA), respectively. When compared with monolithic overcoats of FCVA-deposited carbon (16 Å) and sputtered SiNx (16 Å), the SiNx/C bilayer overcoat demonstrated the best tribological performance with a coefficient of friction < 0.2. Despite showing marginally less electrochemical corrosion protection than monolithic SiNx, its ability to protect the magnetic media from corrosion/oxidation was better than that of an ∼27 Å thick commercial hard disk overcoat and 16 Å thick monolithic FCVA-deposited carbon. From X-ray photoelectron spectroscopy and Raman spectroscopy analyses, it was found that the introduction of the 4 Å SiNx underlayer facilitated higher sp(3) hybridization within the carbon layer by acting as a barrier and promoted the formation of strong bonds at the SiNx/C and the SiNx/media interfaces by acting as an adhesion layer. The higher sp(3) carbon content is expected to improve the thermal stability of the overcoat, which is extremely important for future hard disk drives employing heat assisted magnetic recording (HAMR).

Diffusive motion with nonlinear friction: apparently Brownian

We study the diffusive motion of a small object placed on a solid support using an inertial tribometer. With an external bias and a Gaussian noise, the object slides accompanied with a fluctuation of displacement that exhibits unique characteristics at different powers of the noise. While it exhibits a fluidlike motion at high powers, a stick-slip motion occurs at a low power. Below a critical power, no motion is observed. The signature of a nonlinear friction is evident in this type of stochastic motion both in the reduced mobility in comparison to that governed by a linear kinematic (Stokes-Einstein-like) friction and in the non-Gaussian probability distribution of the displacement fluctuation. As the power of the noise increases, the effect of the nonlinearity appears to play a lesser role, so that the displacement fluctuation becomes more Gaussian. When the distribution is exponential, it also exhibits an asymmetry with its skewness increasing with the applied bias. A new finding of this study is that the stochastic velocities of the object are so poorly correlated that its diffusivity is much lower than either the linear or the nonlinear friction cases studied by de Gennes [J. Stat. Phys. 119, 953 (2005)]. The mobilities at different powers of the noise together with the estimated variances of velocity fluctuations follow an Einstein-like relation.

Solvent-Vapor-Assisted Dewetting of Prepatterned Thin Polymer Films: Control of Morphology, Order, and Pattern Miniaturization

Ultrathin (<100 nm) unstable polymer films exposed to a solvent vapor dewet by the growth of surface instability, the wavelength (λ) of which depends on the film thickness (h(f)). While the dewetting of a flat polymer thin film results in random structures, we show that the dewetting of a prepatterned film results in myriad ordered mesoscale morphologies under specific conditions. Such a film undergoes rupture over the thinnest parts when the initial local thickness of these zones (h(rm)) is lower than a limiting thickness h(lim) ≈ 10 nm. Additionally, the width of the pattern grooves (l(s)) must be wider than λ(s) corresponding to a flat film having a thickness of h(rm) for pattern-directed dewetting to take place over surface-tension-induced flattening. We first present an experimentally obtained morphology phase diagram that captures the conditions where a transition from surface-tension-induced flattening to pattern-directed-rupture takes place. Subsequently, we show the versatility of this technique in achieving a variety of aligned mesopatterns starting from a prepatterned film with simple grating geometry. The morphology of the evolving patterns depends on several parameters such as the initial film thickness (h(f)), prepattern amplitude (h(st)), duration of solvent vapor exposure (SVE), and wettability of the stamp used for patterning. Periodic rupture of the film at regular intervals imposes directionality on the evolving patterns, resulting in isolated long threads/cylindrical ridges of polymers, which subsequently disintegrate into an aligned array of droplets due to Rayleigh-Plateau instability under specific conditions. Other patterns such as a double periodic array of droplets and an array of holes are also possible to obtain. The evolution can be interrupted at any intermediate stage by terminating the solvent vapor annealing, allowing the creation of pattern morphology on demand. The created patterns are significantly miniaturized in size as compared to features obtained from dewetting a flat film with the same hf.

Stochastic rolling of a rigid sphere in weak adhesive contact with a soft substrate

We study the rolling motion of a small solid sphere on a fibrillated rubber substrate in an external field in the presence of a Gaussian noise. From the nature of the drift and the evolution of the displacement fluctuation of the ball, it is evident that the rolling is controlled by a complex non-linear friction at a low velocity and a low noise strength (K), but by a linear kinematic friction at a high velocity and a high noise strength. This transition from a non-linear to a linear friction control of motion can be discerned from another experiment in which the ball is subjected to a periodic asymmetric vibration in conjunction with a random noise. Here, as opposed to that of a fixed external force, the rolling velocity decreases with the strength of the noise suggesting a progressive fluidization of the interface. A state (K) and rate (V) dependent friction model is able to explain both the evolution of the displacement fluctuation as well as the sigmoidal variation of the drift velocity with K. This research sets the stage for studying friction in a new way, in which it is submitted to a noise and then its dynamic response is studied using the tools of statistical mechanics. Although more works would be needed for a fuller realization of the above-stated goal, this approach has the potential to complement direct measurements of friction over several decades of velocities and other state variables. It is striking that the non-Gaussian displacement statistics as observed with the stochastic rolling is similar to that of a colloidal particle undergoing Brownian motion in contact with a soft microtubule.

Experimental investigation of the drift and diffusion of small objects on a surface subjected to a bias and an external white noise: roles of coulombic friction and hysteresis.

We study the stochastic motion of a small solid block or a small water drop on a flat solid support in the presence of an external noise and a bias. The bias is caused either by inclining the plane of the support, as is the case with the solid block, or by creating a gradient of wettability, as is the case with a water drop. Both the solid block and the water drop exhibit drifted Brownian-like motion. There are, however, differences between the motion described here and that of a classical drifted Brownian motion, in that the Coulombic friction (for solid on solid) or wetting hysteresis (for water drops on a solid) accounts for a significant resistance to motion in addition to the kinematic friction. Although the displacement distribution here is non-Gaussian, the variance of the distribution increases with time, indicating that the overall motion follows simple diffusion. The diffusivity and the mobility of the solid object are considerably lower than the values expected when the diffusion is governed by only kinematic friction. The experimental diffusivity increases with the power of the noise with an exponent of 1.61, which is close to that (1.74) of an analysis based on the Langevin equation when the Coulombic friction is taken into account in addition to the kinematic friction. The ratio of diffusivity and mobility increases slightly sublinearly with the power of the noise with an exponent of about 0.8. The experimentally observed relaxation time of the process is, however, considerably smaller than the Langevin relaxation time. When the experimental ratio of diffusivity and mobility is taken into account in the distribution function of the displacement, the later quantity becomes amenable to an analysis that is similar to the conventional fluctuation relations.

Random motion with interfacial contact: Driven diffusion vis-à-vis mechanical activation

Rolling of a small sphere on a patterned support of an elastomer is governed by a non-linear friction. No motion occurs when the external field is weaker than the frictional resistance. However, with the intervention of an external noise, a viscous friction like behavior emerges; thus the sphere rolls with a uniform drift velocity that is proportional to the applied field. At a very low noise strength, the sphere exhibits a stick-slip behavior with motion occurring always along the bias. With the increase in the noise strength, the sphere exhibits a diffusive drift accompanied with forward and backward displacements. During this stage of driven diffusive motion, the ratio of the integrated probabilities of the negative-to-positive work fluctuations decreases monotonically with the time of observation, from which a temperature like intensive parameter can be estimated. This parameter conforms to Einstein’s ratio of diffusivity and mobility that increases almost linearly, even though the diffusivity increases super-linearly, with the strength of the noise. A new barrier crossing experiment is introduced that can be performed either with a hard (e.g. a steel ball) or with a soft (e.g. a water drop) sphere in contact with a periodically undulated substrate. The frequency of barrier crossing follows a transition state equation allowing a direct estimation of the effective temperature. These experiments as well as certain numerical simulations suggest that the effective temperature of a system controlled by a non-linear friction may not have a unique value.

Comparison of Corrosion and Tribological Properties of Ultrathin (<2 nm) Carbon Films on Hard-Disk Media by DC Sputtering and FCVA Processes

Ultrathin carbon overcoats (COCs) are of great interest for future hard-disk media. In view of future requirements, the key research focus lies in investigating/maintaining tribological and oxidation resistance properties of different types of COCs with thicknesses <;2nm. We report on the tribological and oxidation resistance properties of sputtering and filtered cathodic vacuum arc (FCVA) processed ultrathin COCs of thicknesses in the range of ~1.2-1.6nm. FCVA-deposited 1.6nm COC exhibits excellent properties in terms of lower coefficient of friction (COF, 0.2-0.25), higher wear resistance and greater oxidation resistance. In contrast, sputtered 1.2 and 1.5nm COCs show higher COF (~0.4-0.5), poor wear and oxidation resistance properties. The tribological and oxidation resistance performance of 1.2 and 1.6nm thick FCVA-processed COCs are found to be comparable with, or even better than, significantly thicker ~2.7nm commercial COC, demonstrating the effectiveness of the FCVA processed ultrathin protective magnetic media overcoats.

Noise-activated dissociation of soft elastic contacts

Adhesive forces are capable of deforming a soft elastic object when it comes in contact with a flat rigid substrate. The contact is in stable equilibrium if the total energy of the system arising from the elastic and surface forces exhibits a minimum at a zero or at a slightly negative load. However, as the system is continually unloaded, the energy barrier decreases and it eventually disappears, thus leading to a ballistic separation of the contact. While this type of contact splitting has received wide recognition, what has not been much appreciated with these types of soft adhesion problems is that rupture of a contact can also occur at any finite sub critical load in the presence of a noise. The soft contact problems are unique in that the noise can be a-thermal, whereas the metastable and stable states of the thermodynamic potential can arise from the competition of the elastic and the inter-facial energies of the system. Analysis based on activated rate theory and simulations based on stochastic dynamics show that the contact rupture dynamics is amenable to a force and noise induced escape of a particle from a potential well that is generic to various types of colloidal and macromolecular processes. These ideas are useful in understanding the results of a recent experiment involving the noise activated rolling dynamics of a rigid sphere on a surface, where it is pinned by soft micro-fibrils.