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Why Material Defects Are a Good Thing

| Author/ Editor: Florian Aigner* / Alexander Stark

Material wear has been described for many years with simple, empirical laws. The University of Applied Sciences of Vienna is looking at the underlying causes. And discovered: A metal without any faults would be unusable.

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If materials were flawless, flying would be a dangerous thing.
If materials were flawless, flying would be a dangerous thing.
(Source: gemeinfrei / Pixabay)

It doesn't matter if it's a cogwheel, rolling bearing or hip joint: Wear cannot be avoided. Wherever materials touch, where pressure and friction occur, material failure occurs at some point. Small cracks can grow under certain conditions, perhaps a tooth will break out of a gear wheel and needs to be replaced. These processes have been analyzed for decades by means of tribology, the science of friction and wear. “There are many existing empirical values that still help us to predict material wear in a meaningful way,” says Prof. Carsten Gachot from the Institute of Design Sciences and Product Development at Vienna University of Technology. But these empirical rules are not enough. “We need to understand what happens during friction and wear on a microscopic level. Only then can we say which metals or alloys will last as long as possible for a certain purpose.”

There are Defects Everywhere in the Material

Gachot and his colleague from the tribology research group, Dr. Stefan Eder, can use computer simulations to explain important details of wear effects. The research also brought an unexpected result to light. Not every mistake is undesirable. Many irregularities, which are called material defects, are even indispensable and improve the properties of the material enormously. “When a metal is depicted on an atomic scale in textbooks, you usually see perfectly regular atoms, one next to the other,” says Gachot. But reality looks completely different. “There are tiny irregularities everywhere. Sometimes an atom is missing or whole atomic levels shift against each other. When the surface is loaded, the grain structure of the material becomes finer, depending on the load and the initial microstructure, and thus tolerates the external load in a better way."

For a long time, it was only possible to test materials on a macroscopic level - for example by attempting to measure the characteristic relationship between load and deformation: Small loads deform the workpiece slightly, a load twice as large deforms it about twice as much. At higher loads, this correlation is no longer completely linear and at some point, the deformation persists, even if the loading force is removed. When the load direction is reversed, the material “remembers” its original microstructure somehow and restores this state. “We can now calculate such processes on the computer and find completely new microscopic explanations for macroscopic effects that have long been known,” says Gachot. “We can do this with the help of molecular dynamics simulations at the atomic level that can be performed with modern supercomputers.”

Material Defects Make Planes Safe

To eliminate errors in the material completely is not what Gachot has in mind - on the contrary. “Many important properties of materials only come about as a result of these supposed defects,” explains Gachot. “When we get on an airplane, we want the hull not to be brittle and fragile, but flexible and elastic.” This is only made possible by microscopic irregularities on an atomic scale: If defects are built into the metal, these defects can migrate, the material can deform and thus react to external pressure without immediately failing. A metal without any faults would be unusable.

Even if two materials rub against each other, material defects play an important role. Gachot's team investigates on the computer which effects occur - and the results fit excellently to experimental findings. “Experiments remain indispensable, but we have more scope on the computer,” says Gachot. "We can freely choose the properties of the material - it would often not be possible to produce a material sample with exactly these specific properties. This allows the tribology team to specify exactly which material parameters cause certain wear effects. Depending on the type of load, temperature or other variables, different metal alloys may be the best solution. “No one can try all the variants in the experiment," says Gachot. “But with computer support, a whole new, exciting chapter in tribology has begun.”

This article was first published by MM MaschinenMarkt.

Original by Simone Käfer / Translation by Alexander Stark

* Dr. Florian Aigner is a staff member of the Vienna University of Applied Sciences in 1040 Vienna (Austria), Tel. (00 43 1) 5 88 01-4 10 27

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