The effect of atomic oxygen flux and impact energy on the damage of spacecraft metals
Introduction
As missions continue to move to deeper space there is a pressing need to better understand the performance of the materials that will take us there. One of the biggest threats to materials in space is high-energy atomic impacts. For example, in the low Earth orbit (LEO) atomic oxygen (AO) impacts represent a significant source of erosion and performance degradation (Banks et al., 1989). These impacts can reach energies of 4.5 eV and, depending on the material, can lead to material loss and reduced performance (Zhang and Minton, 2001). Significant in-situ testing and simulations have been performed to better understand the effect of AO oxygen collisions in the LEO (Banks et al., 1989). The results of these studies have allowed scientists to better understand how a material performs and which materials are most suitable for LEO exposure. However, while the LEO has received fairly exhaustive research, as these materials move even deeper into space they can be subjected to significantly higher energy collisions with both cosmic dust and the interstellar medium. For example, the Cassini-Huygens Cosmic Dust Analyzer has detected nanoscale dust particles at speeds higher than 200 km/s (Srama et al., 2004). While impacts with these dust particles would likely be catastrophic, in addition to nanoscale dust, impacts with high-speed interplanetary/interstellar gases may also hinder performance (Parker, 1953). For example, Hoang et al. (2017) considered the interaction of relativistic spacecrafts with the interstellar medium. The study attempted to quantify the potential effects of collisions with both interstellar gas and dust using theoretical predictions. Findings indicated that atomistic gas bombardment could result in macroscale surface damages for a quartz material traversing a gas column with a proton number density of 2 × 1018 cm−2 at 0.2 the speed of light. As a result, while many spacecraft materials may be assumed to be non-reactive with AO and thus suitable for the LEO, there may exist an energy barrier that could be overcome with a harsh enough environment. In other words, suitability in the LEO does not guarantee performance with higher energy collisions. However, unlike the LEO environment, materials cannot be easily tested in-situ and simulating the conditions in a laboratory environment is both costly and extremely complex.
In addition to spacecraft impacts, sputtering on planetary and lunar surfaces is a critical process to better understanding atmospheric formation and resource utilization. For example, Farrell et al. have considered the role high-energy solar radiation plays on the sputtering and generation of new compounds from lunar regolith (Farrell et al., 2015). As another example, there is significant research interest for in-situ sampling of high-energy (7–18 km/s) gas molecules and grains from the Enceladus plume. These plumes are rich in water and complex organics, making Enceladus one of the only bodies in the solar system besides the Earth that contains several of the basic requirements for life (Postberg et al., 2018). However, previous fly-bys have suggested that readings from the Ion and Neutral Mass Spectrometer (INMS) on board the Cassini spacecraft may be influenced by high-energy interactions with the ejected plume (Walker et al., 2015). Research has shown that ejected ice grains can lead to the vaporization of titanium within the INMS, potentially explaining these unexpected readings. Therefore, further research is needed to better understand the rate of vaporization and how this can be reduced to obtain better confidence in sampling results.
Overall, there is a clear need for more feasible methods of simulating and understanding the performance of materials in extremely harsh space environments where high-energy impacts can occur via several different processes. One alternative to traditional testing methods is the use of the molecular dynamics (MD) technique to simulate the atomistic impacts on various substrates. Depending on the force field used, these simulations are capable of effectively modelling the bond breaking, temperature evolution, and damage of the substrate as impacts and chemical reactions occur. Therefore, these simulations offer the unique ability to view the evolution of the substrate during impact, as opposed to simply tracking its surface erosion after the impacts have occurred. Recently, MD has been used to simulate the performance of both polymers and metals in the LEO orbit. Rahnamoun and Van Duin (2014) used the ReaxFF force field to study the effect of AO on Kapton, POSS polyamide, and amorphous silica. During impact the simulations tracked the mass loss and temperature growth. Further, the predicted erosion coefficient agreed closely with previously reported experimental values. Similar to this work, Zeng et al. (2015) used MD to study the disintegration of PVDF and FP-POSS, two common spacecraft polymers. Results showed that the FP-POSS performed better under AO impact and was immune until a certain number of impacts. Building on this work, Morrissey et al. (2019) used MD to simulate the erosion yield of silver and gold; two commonly used spacecraft metals. Predicted erosion yields closely matched test data and results indicated that substrate temperature evolution was critical to subsequent erosion. While these studies clearly show the promising potential of using MD to model the effect of high-energy impacts, findings were limited to energies found in the LEO. Therefore, these studies did not attempt to understand the effect of impact energy on the substrate. Moreover, findings typically only report the erosion yield and often do not consider the effect collisions may have on the remaining substrate. With increasing number of impacts is the non-eroded substrate damaged? How does this damage evolve with increasing impact energy? Unlike traditional testing which occurs on the macroscale, MD simulations occur on the atomistic scale and should be able to answer these important questions. Further, understanding the effects of impact energy on damage is crucial to material selection for future deep space missions.
As can be seen from the above review, future deep space missions are likely to encounter higher energy atomistic collisions. Therefore, there is a pressing need to understand the performance of materials subjected to these environments. Materials that are considered ‘immune’ to AO erosion in the LEO will need to be tested again with these higher impact collisions. Previous research on AO impact simulations has shown strong accuracy with field and laboratory erosion rates (Morrissey et al., 2019, Rahnamoun and Van Duin, 2014, Zeng et al., 2015). While the erosion rate is an important value to consider, it does not provide an insight into the state of the substrate. As a result, the purpose of this study was to use molecular dynamics to track the damage evolution of various metal substrates during AO impact as a function of impact energy and dosage. Unlike previous simulations which focused solely on erosion yields from LEO conditions, this study tracked the microstructure damage by computing the number of defect atoms using a Wigner-Seitz defect analysis. Substrate damage was then modeled as both a function of impact dose, slab temperature, and impact energy. This allows for a more detailed discussion on the effect of high-energy impacts on the microstructure of space metals.
Section snippets
Molecular dynamics and force field selection
MD simulates the movement of atoms and molecules within a body. Simulations begin with the initial positions and velocities of all atoms within the system. An interatomic potential is then used to define forces between interacting particles and thus calculate the atomic accelerations. Finally, Newton’s equations of motion are then used to predict the next set of positions and velocities incremented by a prescribed timestep. AO simulations require a force field that can describe the bond
Verification of forcefields
Due to the harsh environment and high energies being considered one of challenges is a lack of experimental data for comparison. Unlike LEO simulations where experimental and field data are readily available, there is limited data for high-energy AO collisions on metals as would be potentially seen in deep space. Therefore, care must be taken to first validate the force fields prior to use. Previous studies have shown that the force fields used in the current study were able to accurately
Conclusions
In conclusion, this study used the ReaxFF force field to determine the effect of impact energy on the damage of silver and aluminum due to atomic oxygen. While previous research has used these methods to simulate the LEO environment, research is needed on the performance of these materials when subjected to the higher energy collisions possible in deep space. Simply assuming a material is suitable based on its LEO performance is not sufficient. Moreover, as opposed to focusing on erosion rate,
Acknowledgments
The author’s would like to acknowledge the Natural Sciences and Engineering Research Council (NSERC - PGS-D) for this financial support towards this project.
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