Grand Challenge: Formation of microscopic and macroscopic craters on gold and silica surfaces

12.03.2010

In their Grand Challenge project, Juha Samela and Kai Nordlund (University of Helsinki) investigated what happens when rapidly moving gold atom clusters hit solid gold surfaces, and argon atom clusters hit silica (silicon dioxide) surfaces. This research involved extensive use of CSC supercomputing resources.

The research was conducted by simulating the dynamics of atomic collisions to determine what level of atomic interaction is needed to induce structural changes that involve as many as millions of atoms, giving rise to, for example, crater formation and matter transitions. When small or large clusters of atoms collide, the shape of the resultant craters is similar but the generation mechanism is quite different.

In macroscopic collisions (i.e. involving large atom clusters) the colliding atom cluster and the substrate immediately "under" it are first compressed into a very small volume. In the next phase, this densification expands explosively, forming a semispherical crater. Since the collision energy is stored in the densification, the crater size is directly proportional to the amount of energy, which, in turn, is proportional to the size of the atom cluster.

Atoms of small atom clusters are pushed between the atoms of the target substrate, and no particular densification is formed. The kinetic energy of the impact is divided between the atoms of the target substrate, and the atoms keep colliding with each other. At the impact area, the substrate becomes either liquid or gas and is displaced from the target substrate leaving a crater behind. Hence, when small or large clusters of atoms collide, the shape of the resultant craters is similar but the generation mechanism is quite different. This is seen in experiments by the different scaling of crater dimensions depending on whether it is the size or speed of the colliding atom clusters that is the variable.

The colliding argon and gold atom clusters contained from about 1000 to more than 100,000 atoms. They hit the surface orthogonally at approximately 20 km/s, which is roughly the speed of micrometeoroids in space. Impact regions must be sufficiently large to prevent shock waves created due to the collision from being immediately echoed from the crater rims and hence, interfering with the simulation results. The target substrate regions comprised more than 100 million atoms over an area of 15x15 nanometers. The size requirement is the very reason why simulations require so much computing capacity.

In the present research, the size of the colliding atom cluster was gradually increased to determine the size class at which the colliding mechanism changes from microscopic to macroscopic particle collision. In the case of gold, the transition to macroscopic collision behavior occurs when the colliding cluster contains approximately 10,000 atoms.

When a sufficiently large argon cluster collides with silicon oxide substrate, simulations show a compressed zone in front of the cluster. However, the atom concentration in silica is lower than in gold, and argon atoms are lighter than gold atoms. Therefore, the densification is not as dense as in gold collisions and more of its energy is dissipated away from the substrate than into the substrate. Consequently, the crater size is almost macroscopically scaled, but some microscopic scaling is still observable when the colliding atom clusters contain more than 100,000 atoms. Hence, the transition into macroscopic collision behavior is more gradual than with gold.

The studies also clarified, whether a powerful impact causes crack propagation in the normally fragile silica. No cracks were observed in simulations. This may be due to the fact that the amorphous material models describing the interactions cannot produce real-world crack propagation.

Computational Grand Challenge projects are aimed at high-impact scientific research that requires computational or data resources exceeding CSC's standard project quotas or level of services. CSC allocates a support group for each GC project, and the GC calls are arranged twice a year.

Publications

K. Nordlund, T.T. Järvi, K. Meinander1 and J. Samela: Cluster ion-solid interactions from meV to MeV energies. Appl. Phys. A, 91, 561-566 (2008)

J. Samela and K. Nordlund: Atomistic Simulation of the Transition from Atomistic to Macroscopic Cratering. Phys. Rev. Lett. 101, 027601 (2008)

J. Samela and K. Nordlund; Transition from Atomistic to Macroscopic Cluster Stopping in Au. Nucl. Instr. Methd. B, 267 (2009) 2980-2986

J. Samela and K. Nordlund: Classical molecular dynamics simulations of hypervelocity nanoparticle impacts on amorphous silica. Phys. Rev. B volume 81, issue 5 (2010)

More information:

Tomi Kutilainen: Atom-by-atom formation of a crater. CSC News 1/2010.

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