Nikhil Subhashchandra Tambe

Scale dependence of micro/nano-friction and adhesion of MEMS/NEMS materials, coatings and lubricants

Scale dependence of micro/nanotribological properties is studied for various materials, coatings and lubricants used in micro/nanoelectromechanical systems (MEMS/NEMS). The adhesive force and friction force dependence on rest time and sliding velocity and the effect of relative humidity and temperature on the scale dependence of these properties is studied. The scale dependence of the coefficient of friction is attributable to the sample surface roughness and the scan size. For larger scan sizes the sliding interface encounters larger asperities and so friction force is higher. The adhesive force is higher on the microscale although on the nanoscale surface forces such as electrostatic attraction that are generally negligible on the microscale can become dominant. The difference in the adhesive force on the micro- and nanoscale for different rest times, relative humidities and temperatures is due to the meniscus force dependence on the sample surface roughness. The velocity dependence of the friction force shows significant scale dependence due to the scale dependent roughness and the higher contact pressures that are encountered on the nanoscale.

Friction model for the velocity dependence of nanoscale friction.

The velocity dependence of nanoscale friction is studied for the first time over a wide range of velocities between 1 microm s(-1) and 10 mm s(-1) on large scan lengths of 2 and 25 microm. High sliding velocities are achieved by modifying an existing commercial atomic force microscope (AFM) setup with a custom calibrated nanopositioning piezo stage. The friction and adhesive force dependences on velocity are studied on four different sample surfaces, namely dry (unlubricated), hydrophilic Si(100); dry, partially hydrophobic diamond-like carbon (DLC); a partially hydrophobic self-assembled monolayer (SAM) of hexadecanethiol (HDT); and liquid perfluoropolyether lubricant, Z-15. The friction force values are seen to reverse beyond a certain critical velocity for all the sample surfaces studied. A comprehensive friction model is developed to explain the velocity dependence of nanoscale friction, taking into consideration the contributions of adhesion at the tip-sample interface, high impact velocity-related deformation at the contacting asperities and atomic scale stick-slip. A molecular spring model is used for explaining the velocity dependence of friction force for HDT.

Nanotribological characterization of self-assembled monolayers deposited on silicon and aluminium substrates

Silicon and aluminium are the substrates of choice for various micro/nanoelectromechanical systems (MEMS/NEMS) including digital micromirror devices (DMD®). For efficient and failure-proof operation of these devices, ultrathin lubricant films of self-assembled monolayers (SAMs) are increasingly being employed. In this study, we investigate friction, adhesion and wear properties of various SAMs. Surface properties such as contact angle, adhesive force, friction force and coefficient of friction are compared for SAMs with hydrocarbon and fluorocarbon backbone chains with different chemical structures, chain lengths and end groups. The influence of relative humidity, temperature and sliding velocity on the friction and adhesion behaviour is studied for various SAMs. Failure mechanisms of SAMs are investigated by wear tests and the potential mechanisms involved are discussed. These studies are expected to aid the design and selection of proper lubricants for MEMS/NEMS.

A new atomic force microscopy based technique for studying nanoscale friction at high sliding velocities

Tribological studies on the micro/nanoscale conducted using an atomic force microscope (AFM) have been limited to low sliding velocities (< 250 µm s−1) due to inherent instrument limitations. Studies of tribological properties of materials, coatings and lubricants that find applications in micro/nanoelectromechanical systems and magnetic head-media in magnetic storage devices that operate at high sliding velocities have thus been rendered inadequate. We have developed a new technique to study nanotribological properties at high sliding velocities (up to 10 mm s−1) by modifying the commercial AFM set-up. A custom calibrated nanopositioning piezo stage is used for mounting samples and scanning is achieved by providing a triangular input voltage pulse. A capacitive sensor feedback control system is employed to ensure a constant velocity profile during scanning. Friction data are obtained by processing the AFM laser photo-diode signals using a high sampling rate data acquisition card. The utility of the modified set-up for nanoscale friction studies at high sliding velocities is demonstrated using results obtained from various tests performed to study the effect of scan size, rest time, acceleration and velocity on the frictional force for single crystal silicon (100) with native oxide.

Durability studies of micro/nanoelectromechanical systems materials, coatings and lubricants at high sliding velocities (up to 10mm∕s) using a modified atomic force microscope

Most micro/nanoelectromechanical (MEMS/NEMS) devices and components operate at very high sliding velocities (of the order of tens of mm/s to few m/s). Micro/nanoscale tribology and mechanics of these devices is crucial for evaluating reliability and failure issues. Atomic force microscopy (AFM) studies to investigate potential materials/coatings for these devices have been rendered inadequate due to inherent limitations on the highest sliding velocities achievable with commercial AFMs. We have developed a technique to study nanotribological properties at high sliding velocities (up to 10mm∕s) by modifying the commercial AFM setup with a customized closed loop piezo stage for mounting samples. Durability of materials, silicon, poly(methylmethacrylate) (PMMA) and poly(dimethlysiloxane) (PDMS), diamond-like carbon (DLC) coating and lubricants such as self-assembled monolayer of hexadecanethiol (HDT) and perfluropolyethers Z-15 and Z-DOL used in MEMS/NEMS applications, is studied at various normal loads and s...

Identifying materials with low friction and adhesion for nanotechnology applications

The advent of micro/nanostructures and the subsequent miniaturization of moving components for various nanotechnology applications have ascribed paramount importance to tribology and mechanics on the nanoscale. Materials with low friction and adhesion are desirable for avoiding premature failures. We present the coefficient of friction and adhesion dependence on the Young’s modulus over a range of sliding velocities for an array of materials. A contour map is developed to identify tribologically suitable materials. This approach provides a fundamental insight into the mechanical property dependence of friction and adhesion and simplifies the material selection process for nanotechnology applications.

Nanoscale friction mapping

Contrary to classical friction laws postulated by Amontons and Coulomb centuries ago, nanoscale friction force is found to be strongly dependent on the normal load and sliding velocity. Many materials, coatings, and lubricants that have wide applications in micro/nanoelectromechanical systems show reversals in friction behavior corresponding to transitions between different friction mechanisms. We have developed a contour map to provide a fundamental insight into the normal load and sliding velocity dependence of friction force and thus help identify and classify dominant friction mechanisms.

Nanoscale friction and wear maps

Friction and wear are part and parcel of all walks of life, and for interfaces that are in close or near contact, tribology and mechanics are supremely important. They can critically influence the efficient functioning of devices and components. Nanoscale friction force follows a complex nonlinear dependence on multiple, often interdependent, interfacial and material properties. Various studies indicate that nanoscale devices may behave in ways that cannot be predicted from their larger counterparts. Nanoscale friction and wear mapping can help identify some ‘sweet spots’ that would give ultralow friction and near-zero wear. Mapping nanoscale friction and wear as a function of operating conditions and interface properties is a valuable tool and has the potential to impact the very way in which we design and select materials for nanotechnology applications.

Sliding contact energy measurement using a calibrated acoustic emission transducer

Acoustic emission (AE) transducers are commonly used to monitor the intensity of head-disk contact. However, analytical methods used to obtain electrical energy of the AE output signal and mechanical energy dissipated in typical head-disk contacts are still, to a large extent, not well understood. In the present study we introduce a method to obtain the electrical energy of the AE output voltage signal. In calibration experiments this electrical energy is shown to be directly proportional to the mechanical energy released from the mechanical contact. We have developed analytical models to calculate the energy released from the sliding contact. Some experimental measurements and their correction is also shown.

Role of particulate contamination on friction and wear and durability of load/unload and padded picosliders

The role of particulate contamination on friction and wear at the head-disk interface (HDI) for load/unload (L/UL) picosliders and contact-start-stop padded picosliders is studied. L/UL picosliders performed better than padded picosliders in the presence of particulate contamination and in a clean environment. Disk wear was higher for harder and smaller sized particles. Disk wear decreases with an increase in disk speed in the presence of smaller particles as these can escape more easily through the HDI at higher speeds. Track following causes more and earlier disk damage as compared to sweeping. In the case of padded picosliders, HDI contamination causes wear of pads eventually leading to partial or complete removal of pads.