Nanoscale Wear in Extreme tribology applications

02.07.2024

Introduction

Extreme tribology is a branch of tribology that focuses on the behavior of materials under extreme conditions, such as elevated temperatures, extreme pressures, or high sliding speeds. This field investigates friction, wear, and lubrication phenomena in harsh environments and is crucial for industries like aerospace, automotive, and energy. The connection between nanoscale and extreme tribology is due to many extreme tribological phenomena occurring at the nanoscale. Nanoscale wear is strongly influenced by the scale effect. As the scale decreases, localized stresses and strain concentrations at contact points can lead to increased wear rates. Additionally, nanoscale asperities and surface features can promote abrasive wear and generate wear particles. The specific details and quantitative aspects of the scale effect may vary depending on the materials, surface conditions, and experimental setups. However, these general trends underscore the importance of considering the scale effect when understanding and predicting tribological behavior at the nanoscale.

Need for Nanoscale Understanding

When materials and interfaces are scaled down to the nanoscale, their behavior undergoes significant changes, giving rise to unique tribological properties and challenges. In most solid-solid interfaces of technological importance, contact occurs at numerous small irregularities or asperities. At the nanoscale, surface roughness becomes more significant due to the heightened influence of individual asperities on overall contact behavior. Smaller surface irregularities play a dominant role in determining friction, wear, and contact characteristics as the scale decreases. Contact parameters such as contact area, pressure distribution, and contact stiffness are also affected by the scale effect. Reduced contact area leads to localized pressure distribution, resulting in increased stresses and strain concentrations at contact points. Contact stiffness tends to increase due to decreased contact area and enhanced surface interactions. Additionally, friction behavior at the nanoscale is significantly influenced by the scale effect, with adhesion and surface forces playing a more prominent role, leading to higher friction coefficients. The increased influence of surface roughness and contact parameters can result in complex friction behavior, including stick-slip motion and nanoscale wear. The scale effect also impacts thermal distribution at the interface between contacting surfaces, with reduced contact area leading to concentrated heat generation and elevated interface temperatures. This localized heating can influence contact mechanics, material properties, and wear behavior, potentially altering overall tribological performance.

Figure-1 Increased wear depth with increasing force [2]

Models developed to study the nanoscale wear

Various computational models have been developed to study nano-wear and wear at the nanoscale, providing valuable insights into fundamental wear processes and mechanisms. Molecular dynamics (MD) simulations have become popular for investigating atomic-scale wear and material deformation, offering insights into nano-wear phenomena such as surface interactions and material transfer. Coarse-grained models provide a compromise between atomistic and continuum models, allowing for efficient simulation of wear processes over longer time and length scales. Archard’s classic wear model has been extended to include size effects at the nanoscale, considering parameters like asperity size and surface roughness to explain variations in wear behavior. Adhesion-based models focus on adhesive wear phenomena, incorporating adhesive forces and contact mechanics to predict wear behavior driven by adhesion. Finite element analysis (FEA) and continuum-based models simulate material deformation and stresses under sliding conditions, predicting wear rates and identifying critical wear regions. Multiscale models integrate atomistic simulations with continuum-based approaches to bridge the gap between microscopic and macroscopic wear behavior, enabling a comprehensive understanding of wear phenomena across different scales. These computational models, coupled with advancements in computational power, contribute significantly to our understanding of nano-wear and aid in predicting wear behavior, optimizing materials, and designing wear-resistant surfaces and coatings in various industries.

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Figure-2 The schematic representation illustrates the atom-by-atom attrition wear process, driven by the reduction in the atomic bond energy barrier caused by frictional shear stress. This phenomenon can be understood using Arrhenius kinetics, which describes how the rate of a reaction increases with temperature due to the greater energy available for atomic rearrangement [3]

Conclusion

Nano-scale wear is a critical phenomenon impacting the performance, lifespan, and durability of various devices and systems. Scanning probe microscopy techniques, such as atomic force microscopy (AFM) and friction force microscopy (FFM), are pivotal in studying surface characteristics and wear-related phenomena at the nanoscale. Advanced microscopy techniques offer valuable insights into the mechanisms and dynamics of nano-scale wear. Factors like lubrication strategies, stress levels, sliding velocity, and atomic-scale reactions significantly influence nano-wear behavior. Controlling and minimizing nano-scale wear have important implications for improving device performance, extending component lifespan, enhancing durability, increasing energy efficiency, achieving cost savings, and promoting environmental sustainability. The application of nano-tribology principles and measurement methods can bridge the gap between macro and nano-tribology studies, facilitating a better understanding of tribological mechanisms across different scales.

Reference

[1] Sadeghi, B., Cavaliere, P., Shabani, A., Pruncu, C.I. and Lamberti, L., 2024. Nano-scale wear: A critical review on its measuring methods and parameters affecting nano-tribology. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 238(2), pp.125-155.

[2] Bhushan, B., Israelachvili, J.N. and Landman, U., 1995. Nanotribology: friction, wear and lubrication at the atomic scale. Nature, 374(6523), pp.607-616.

[3] Kim, H.J., Yoo, S.S. and Kim, D.E., 2012. Nano-scale wear: a review. International Journal of Precision Engineering and Manufacturing, 13, pp.1709-1718.

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[4] Braun, O.M. and Naumovets, A.G., 2006. Nanotribology: Microscopic mechanisms of friction. Surface Science Reports, 60(6-7), pp.79-158.

I am currently working as a Postgraduate Researcher at the University of Leeds, where I am actively involved in research activities. Prior to this, I successfully completed my master's degree through the renowned Erasmus Mundus joint program, specializing in Tribology and Bachelor's degree in Mechanical Engineering from VTU in Belgaum, India. Further I handle the social media pages for Tribonet and I have my youtube channel Tribo Geek.

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