Friction: Engineering’s frenemy and persistent puzzle
Scientists map advances in friction science and identify barriers to reliable prediction and control
image: The review evaluates physics-based friction models, highlights their strengths and limitations, and identifies key challenges to achieving predictive friction control.
Credit: Friction, Tsinghua University Press
Friction can be both friend and foe to engineers, and understanding it has never been an easy task. A review by Chinese Academy of Sciences researchers showed why controlling friction still relies heavily on trial and error, and what scientists must overcome to make friction more predictable.
The results were published in Friction on Oct. 28, 2025.
Friction is the force that resists motion when two surfaces slide against one another. Friction is everywhere, from tiny sensors inside smartphones to train brakes and industrial machinery. Yet despite centuries of study, scientists still struggle to predict exactly how surfaces will behave when they rub together.
“Friction is not caused by a single factor; instead, it arises from multiple coupled mechanisms and factors acting simultaneously,” said Wenhao He, associate professor from the Lanzhou Institute of Chemical Physics at the Chinese Academy of Sciences and lead author of the study.
“The key challenge is to quantify the contribution of each mechanism and factor under real working conditions.”
Many factors shape friction, including environmental conditions such as temperature and humidity, operating conditions like load and speed, and material properties and surface structure. Also, friction results from several physical processes occurring together, including deformation, chemical reactions, and energy transfer at the atomic scale.
The researchers systematically reviewed existing physics-based friction models designed to explain how friction behaves under different conditions. Rather than focusing on specific materials or isolated applications, the study evaluated models that capture the common physical principles governing friction across different scales.
Their findings highlighted the extent to which friction models have dramatically expanded scientific understanding over the centuries, from Leonardo da Vinci to modern times.
“Friction models have advanced our understanding to unprecedented levels and now serve as the theoretical bedrock for friction control,” He said.
Researchers can now logically explain many familiar friction behaviors, including how friction changes with contact area, applied load, sliding speed, the number of layers in two-dimensional materials, and the size of microscopic contact junctions. These advances have assisted engineers to better design systems ranging from micro- and nano-electromechanical devices to large moving machinery.
Yet major gaps remain.
“A pressing hurdle is the precise prediction of atomic-scale contact areas within macroscopic systems,” He said.
Although friction observed in everyday machines occurs at the macroscopic level, it ultimately originates from tiny contact regions between atoms. Existing contact theories can describe interactions across different length scales, but none can accurately define atomic-scale contact areas in real-world systems because atomic interactions are inherently complex.
Another major challenge is understanding more fully how friction dissipates energy.
“Current theoretical models allow us to calculate the potential energy landscape during friction, but key parameters that determine how much energy is irreversibly dissipated still have to be fitted to experiments,” He said. “If we could predict them directly from fundamental physics, truly predictive models of wear-free friction could become possible.”
Solving these problems could transform friction science from a largely empirical field into a predictive one.
By identifying both the strengths and shortcomings of current friction models, the review maps the progress of friction science and illustrates the major barriers preventing reliable friction control.
“We hope our findings help bridge the current knowledge gaps so that friction control can become more predictive, precise and effective,” He said.
Funding
- Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB 0470203),
- National Natural Science Foundation of China (No. 12302128),
- West Light Foundation of the Chinese Academy of Sciences (xbzg-zdsys-202305),
- CAS Project for Young Scientists in Basic Research (YSBR-023),
- LICP Cooperation Foundation for Young Scholars (HZJJ23-4).
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