Unravelling friction anisotropy by atomic force microscopy

Area of Science:

• Tribology and Nanomechanics
• Surface Science and Materials Engineering

Background:

• Friction anisotropy significantly impacts natural phenomena and technological applications, yet its nano- and microscale mechanisms remain poorly understood.
• Bridging the gap between atomistic and macroscale friction models requires a deeper comprehension of intermediate length-scale phenomena.
• Nano-/micro-electromechanical systems (NEMS/MEMS) and other advanced technologies necessitate precise control over friction behavior.

Purpose of the Study:

• To review the contributions of Atomic Force Microscopy (AFM)-based techniques to understanding friction anisotropy at the nano- and microscale.
• To synthesize 30 years of research, identifying common friction anisotropy mechanisms across various material systems and conditions.
• To provide insights for refining theoretical friction models and advancing technological applications.

Main Methods:

• Utilizing Atomic Force Microscopy (AFM) for high-resolution investigation of friction forces at the nano- and microscale.
• Analyzing AFM findings on friction anisotropy in atomically flat crystals, quasicrystals, 2D materials, and organic materials.
• Exploring friction anisotropy influenced by solid and fluid adsorbates, including polymers and liquid lubricants.

Main Results:

• AFM has elucidated friction anisotropy arising from surface topography in various crystalline and organic materials.
• The presence of adsorbates, such as polymers and lubricants, significantly influences friction anisotropy mechanisms.
• Common underlying mechanisms of friction anisotropy have been identified across diverse material systems.

Conclusions:

• AFM is an indispensable tool for studying nano- and microscale friction anisotropy due to its high resolution and sensitivity.
• A comprehensive understanding of friction anisotropy mechanisms is crucial for developing advanced materials and technologies.
• This review consolidates knowledge to guide future research in tribology and materials science.