Wear and friction are crucial issues in many industrial sectors: What happens when one surface slides across another? What changes are expected to happen in the material? What does this mean for the durability and safety of machines?
What happens at the atomic level cannot be observed directly. However, an additional scientific tool is now available for this purpose: for the first time, complex computer simulations have become so powerful that wear and friction of real materials can be simulated on an atomic scale.
The tribology team at Vienna University of Technology (TU Wien), led by professor Carsten Gachot, has now proven that this new research field now delivers reliable results in a current publication in the scientific journal ACS Applied Materials & Interfaces. The behavior of surfaces consisting of copper and nickel was simulated with high-performance computers. The results correspond amazingly well with images from electron microscopy—but they also provide valuable additional information.
Friction Changes Tiny Grains
To the naked eye, it does not look particularly spectacular when two surfaces slide across each other. But on the microscopic level, highly complicated processes take place: "Metals, as they are used in technology, have a special microstructure," explains Dr. Stefan Eder, first author of the current publication. "They consist of small grains with a diameter of the order of micrometers or even less."
When one metal slides over the other under high shear stress, the grains of the two materials come into intense contact with each other: they can be rotated, deformed, or shifted and they can be broken up into smaller grains or grow due to increased temperature or mechanical force. All these processes, which take place on a microscopic scale, ultimately determine the behavior of the material on a large scale—and thus they also determine the service life of a machine, the amount of energy lost in a motor due to friction, or how well a brake works, in which the highest possible friction force is desired.
Computer Simulation and Experiment
"The result of these microscopic processes can then be examined in an electron microscope," says Eder. "You can see how the grain structure of the surface has changed. However, it has not yet been possible to study the time evolution of these processes and explain exactly what causes which effects at which point in time."
This gap is now being closed by large molecular dynamics simulations developed by the tribology team at TU Wien in cooperation with the Excellence Centre of Tribology (AC²T) in Wiener Neustadt and Imperial College in London. Atom by atom, the surfaces are simulated on the computer. The larger the simulated chunk of material and the longer the simulated time period, the more computer power is needed.
"We simulate sections with a side length of up to 85 nanometers, over a period of several nanoseconds," says Eder. That doesn't sound like much, but it's remarkable—even the Vienna Scientific Cluster 4, Austria's largest supercomputer, may sometimes be busy with such tasks for months at a time.
The team investigated the wear of alloys of copper and nickel—doing so using different mixing ratios of the two metals and different mechanical loads.
"Our computer simulations revealed exactly the variety of processes, microstructural changes and wear effects that are already known from experiments," says Eder. "We can use our simulations to produce images that correspond exactly to the images from the electron microscope. However, our method has a decisive advantage—we can then analyze the process in detail on the computer. We know which atom changed its place at what point in time, and what exactly happened to which grain in which phase of the process."
Understanding Wear—Optimizing Industrial Processes
The new methods are already met with great interest from industry.
"For years, there has been an ongoing discussion that tribology could benefit from reliable computer simulations. Now we have reached a stage where the quality of the simulations and the available computing power are so great that we could use them to answer exciting questions that would otherwise not be accessible," says Gachot. In the future, they also want to analyze, understand, and improve industrial processes on the atomic level.