- Researchers achieve detailed high-speed imaging of the microparticle impact process.
- The study revealed that both the surface and the particle melt to some extent at high impact velocities.
The combination of high temperatures, pressures, and deformation rates that arise during an impact can trigger a wide range of materials responses, including nanocrystallization, solid-state splashing, crater formation, unusual phase transformations, and various chemical reactions.
Amongst these, the most common phenomenon is impact-induced erosion, which frequently occurs at space (as micrometeorites pummeling satellites), and on the ground (for instance, as coatings being sprayed on a metal).
Due to small length scales and short timescales involved in the process, the details of the phenomenon have not been clearly understood, until now.
Recently, MIT researchers conducted an experiment in which they imaged and analyzed the impact process of a microparticle to predict how it will react after the collision: whether the particle will knock material off the surface or will bounce away.
Single Particle Impact Experiments
For their experiment, the team chose a few tiny particles (approximately 10 micrometers in diameter), accelerated them up to 1067 meters per second, and collided them with a plan solid surface.
Impact of 10 μm particle (coming in from the left) moving at 1067 m/s | Courtesy of the researchers
They used two laser beams in this process: one for accelerating particles (by instantly evaporating a substrate surface that ejects particles at high-speeds) and other for illuminating the flying particles as they collided with the surface.
The collision process was recorded in real-time using a microparticle impact testbed capable of capturing videos at 100 million frames per second.
At micro scales, metallic particles alter their interaction with targets as the velocity of collision increases.
- For an extremely weak impact (less than 0.1 m/s), the particles rebound with their initial kinetic energy.
- For slightly higher impact velocities (less than 10 m/s), the particles bounce off with a fraction of their initial kinetic energy while plastically deforming the target and themselves.
- For higher impact (about 100 m/s), the particles make an adhesive bond with the target, building up coatings or bulk substances.
- At even higher impact velocities (more than 1000 m/s), a material loss can occur. It could damage things like satellites and space vehicles.
The scanning electron micrograph of impact area | Courtesy of researchers
Why Is This Important?
Studies done in the past have relied on post-mortem examination only. Thus, conditions necessary to trigger erosion and the detailed mechanism of impact-induced erosion weren’t well-understood. The high-speed imaging revealed that both the surface and the particle melt to some extent at high impact velocities.
Today, high-speed particle impacts are used for several purposes in various industries, such as cleaning surfaces, applying coatings, and cutting materials. However, we have been controlling these processes without knowing the underlying physics of the process.
As per industry standards, we have always thought that higher impact speeds lead to better outcomes. However, the study shows that this isn’t always the case. In fact, the quality and strength of the coating decline drastically at supersonic speeds.
The researchers used data obtained from these experiments to build a general model for predicting the response of microparticles moving at a given speed.
So far, the team has experimented with pure metals only. In the next study, they will include alloys and other materials as well. Also, they will analyze collisions at different angles rather than just the right-angle impacts tested so far.