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December 7, 2018

What happens when materials take tiny hits?

Impact-erosion

Above, the moment of impact as a 10 micrometer particle impacts a metal surface.

From Extreme Tech:

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MIT studies micro-impacts at 100 million frames per second

Engineers know that tiny, super-fast objects can cause damage to spacecraft, but it's been difficult to understand exactly how the damage happens because the moment of impact is incredibly brief.

A new study from MIT seeks to reveal the processes at work that produce microscopic craters and holes in materials.

The hope is that by understanding how the impacts work, we might be able to more durable materials.

Accidental space impacts aren't the only place these mechanisms come into play.

There are also industrial applications on Earth like applying coatings, strengthening metallic surfaces, and cutting materials.

A better understanding of micro-impacts could also make these processes more efficient.

Observing such impacts was not easy, though.

For the experiments, the MIT team used tin particles about 10 micrometers in diameter accelerated to 1 kilometer per second.

They used a laser system to launch the projectile that instantly evaporates a surface material and ejects the particles, ensuring consistent timing.

That's important because the high-speed camera pointed at the test surface (also tin) needed specific lighting conditions.

At the appointed time, a second laser illuminated the particle allowing the camera to follow the impact at up to 100 million frames per second.

In previous studies of micro-impacts, researchers had to rely entirely on "post-mortem" analysis of the impact damage.

Watching it unfold in real-time and comparing that to the final product revealed several important factors.

At speeds above a certain threshold, the team discovered a pivotal period of melting when the particle hits the surface.

That plays a crucial role in eroding the material.

Using the high-speed camera data, the team developed a model that can predict how a particle will interact with the surface.

It might bounce away, stick, or knock material loose and leave a crater that weakens the surface.

This is important especially in industrial applications because the conventional wisdom has long been that higher velocities are more effective.

We now know that is not always the case.

The research so far has focused on pure metals, but most industrial and space applications rely on alloys.

Expanding the test to more materials is next on the agenda.

Likewise, the researchers plan to fire particles at surfaces from varying angles — these initial tests were straight-down impacts only.

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Below, the abstract of the paper, which appeared November 29 in Nature.

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Melt-driven erosion in microparticle impact

Impact-induced erosion is the ablation of matter caused by being physically struck by another object. While this phenomenon is known, it is empirically challenging to study mechanistically because of the short timescales and small length scales involved. Here, we resolve supersonic impact erosion in situ with micrometer- and nanosecond-level spatiotemporal resolution. We show, in real time, how metallic microparticles (~10-μm) cross from the regimes of rebound and bonding to the more extreme regime that involves erosion. We find that erosion in normal impact of ductile metallic materials is melt-driven, and establish a mechanistic framework to predict the erosion velocity.

December 7, 2018 at 12:01 PM | Permalink


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