Satish Achanta

Nanotribology of MEMS/NEMS

Micro-/Nano- electromechanical systems (MEMS/NEMS) are future devices that have a spectrum of applications ranging from rocket technology to biological sciences. Although MEMS devices are known for over two decades, very few categories of them are used in commercially applications due to their poor reliability. Tribological phenomena like stiction, friction, and wear are major issues affecting the reliability of contact MEMS/NEMS devices and micromotors, microgears, nanosliders, etc., are some examples in which reliability is greatly hampered by such dissipation processes. In recent years, lot of research was dedicated for improving the reliability of MEMS/NEMS through lab scale tribological studies e.g., nanotribological studies. Similar tribological studies were earlier carried out on components like magnetic storage devices, electrical connectors, etc., and were successfully tackled. Present chapter is an overview of the research done over the years to understand the phenomena like stiction, friction and wear in such devices. This chapter addresses the variety of tribological problems associated with MEMS/NEMS, tribological characterization through lab scale and in-situ techniques, and various solutions that are used for improving the reliability of such devices with respect to tribological problems.

Effect of Al and Cr addition on tribological behaviour of HVOF and APS nanostructured WC–Co coatings

Abstract Nanostructured WC–Co based coatings were investigated. The main focus was given to the effect of Al and Cr addition on their tribological behaviour. These coatings were successfully deposited from engineered nanosized powders using high velocity oxy fuel (HVOF) and atmospheric plasma spraying (APS). Porosity level was <3%. Crystal sizes of around 20–30 nm determined by TEM in such coatings, confirm the retention of a nanosize after thermal spraying. The nanostructured coatings were tested for their tribological characteristics and compared to industrial micrometre sized WC–Co coatings and common wear resistant engineering materials. It was found that decarburisation of the coating constituents is a critical issue and has a large impact on the tribological behaviour of the coatings. Proper selection of spraying technique, spraying parameters and distribution of phases are shown to be key factors for achieving nanostructured coatings with high wear resistance.

Investigation of Friction in the Meso Normal Force Range on DLC and TiN Coatings

In recent years “low load” tribology has received more attention due to the emergence of special devices like MEMS, and new materials such as bioimplants, polymers, and textured surfaces. The tribological characterization of MEMS materials is challenging, because the devices operate at nominal contact pressures of only a few MPa and low wear rates of nm/h. The evaluation of materials with conventional high load tribological tools or with AFM type techniques is not appropriate because the contact pressures or contact geometries are not suitable. Furthermore, even in many conventional applications, the contact pressures and wear rates are more moderate than the ones achieved in conventional tribometers. In this work, the friction and wear behavior of the commonly used industrial coatings titanium nitride (TiN) and diamond-like carbon (DLC) are investigated with normal forces in between Newton (conventional tribology) and nanoNewton (nanotribology). The investigation of friction at such low contact pressures was achieved using a high precision microtribometer MUST® operating in a linearly reciprocating ball-on-flat configuration; a ball-on-disk configuration is also possible. The equipment works on the principle of deflection measurements of a flexible cantilever beam with light optical sensors bearing a force resolution of 0.1 μN. As the applied normal force reduces from mNs to μNs, the effect of surface roughness becomes strikingly apparent in the recorded tangential force. The friction of TiN coating appears to be less sensitive to surface roughness variations and chemical changes of the countermaterial than DLC. The behavior of DLC at low contact pressures is different from the well known low friction behavior of DLC coatings at high contact pressures. The importance of the appropriate tribological tool for evaluating materials at low contact pressures has been illustrated through this study.

Nanocoatings for tribological applications

Abstract: An overview is given of the possible applications of nanostructured coatings for mitigating friction and wear. Different aspects of nanostructured coatings like the available deposition methods, the structure-to-property relationships, and the suitable tribological characterization techniques are reviewed. Finally, the challenges linked to the further implementation of such novel nanostructured coatings into industrial practice are critically discussed.

Tribological behaviour of mineral and rapeseed oils containing iron particles

Purpose – The purpose of this paper is to investigate the tribological behaviour of commercially available SAE 10 mineral and rapeseed oils containing Fe particles synthesized directly in the oil phase. Design/methodology/approach – Sub-micron Fe particles (50-340 nm) were synthesized by wet chemical reduction reaction of FeSO4 by sodium borohydride in the rapeseed and mineral oils in the presence of surfactant: block copolymer (ENB 90R4) or oxyethylated alcohol (OS-20). A four-ball wear tribometer was used to investigate the tribological properties of mineral and rapeseed oil: coefficient of friction (COF), wear scar diameter and wear loss. Viscosity measurements of oil solutions and determination of synthesized Fe particles size were performed as well. Findings – The presence of Fe particles (0.1 weight per cent) in the rapeseed and mineral oils caused the little change in the COF but resulted in marked improvement of anti-wear property. The oils containing Fe particles with slightly higher viscosity ar...

Nanotribology on individual phases of duplex steel: combining roughness, material effects, and friction

In this study, lateral force microscopy (LFM) technique was used to investigate local friction and wear behaviour on individual phases of dual phase steel. Important factors influencing friction interpretation at nanoscale are investigated. A nanoprobe made of silicon nitride (20 nm tip radius) was used for this investigation. The difference in phases is clearly apparent when the surface is smooth but with a slight increase in surface roughness, the frictional difference between the phases got masked. A clear direct dependence of friction force on normal force was observed at nanoscale as predicted by Derjaguins friction model. This model appeared to be valid irrespective of the surface roughness modifications on different phases of the material. The tip wear phenomenon was detected through adhesion force measurements before and after the test. Even at nanoscales, the wear resistance was found to be directly dependent on the hardness of the phases.

Investigation of Friction Over a Wide Range of Normal Forces

Friction at different force, length, and time scales is of great interest in tribology. The mechanical, chemical, and physical (atomic) interactions, each operating at their own time length and force scale, makes friction complex. This work is an attempt to improve the understanding of friction at normal forces ranging from nN up to N. This investigation was carried out under reciprocating ball-on-flat sliding conditions on engineering surfaces like diamond-like carbon (DLC) and dual phase steel. The test equipments used for this investigation are an atomic force microscope, a microtribometer, and a macrofretting tester. It was observed that for a hard/hard tribocouple like DLC/Si3 N4 , the variation in the coefficient of friction is negligible whereas the variation is large when the tribocouple is hard / soft like in dual phase steel / Si3 N4 . By changing the surface roughness of the material, the dependence of friction on normal force could be altered or manipulated.Copyright