Self-Healing Materials and Damage From Shock Induced Nanobubble Collapse: Reactive Molecular Dynamics Simulations
Priya Vishishta
University of Southern California / USA
Bonding and Structure of Ceramic-Ceramic Interfaces
Interfaces in multicomponent systems are critical in determining their material behavior. High-resolution TEM identified rich interfacial phases of thickness ~ 10 Å, which have distinct atomistic structures that do not exist in bulk phases. An archetypal ceramic/ceramic interface is formed between Al2O3 and SiC. For better understanding and design of Al2O3/SiC interfaces, the central question is: What are the nature of interfacial bonding at the atomistic level and its consequence on interphase structures?
Our quantum molecular dynamics (QMD) simulations of a-Al2O3 (0001)/3C-SiC (111) interfaces revealed profound effects of thermal annealing for producing strong interfaces consisting solely of cation-anion bonds. Bond purification of the interfacial bonds is observed (i.e. elimination of cation-cation and anion-anion bonds) due to thermal annealing, using the Mulliken bond-overlap population (BOP) analysis. A Si-terminated SiC surface and Al2O3 form a stronger interface (Si-interface) with a Si-O bond density of 12.2 nm-2, whereas the C-interface has an Al-C bond density of 9.46 nm-2. The interfacial bond strengthening is accompanied by the formation of an Al2O3 interphase with a thickness of 2-8 Å.
Damage on Silica Surface from Shock Induced Nanobubble Collapse
Despite a great deal of research on cavitation bubbles, several important questions about the mechano-chemistry of bubble collapse near a solid surface remain unanswered. In particular, an atomistic understanding is lacking. We have addressed these questions using billion-atom reactive molecular dynamics (RMD) simulations. These are the largest RMD simulations done on 163,840 processors IBM BlueGene/P at Argonne Leadership Computing Facility. The simulations revealed more chemical activity in water nanojets and vortex rings formed by collapsing nanobubbles, in cavitation pits on silica surface and around secondary shock waves. We found that the structural damage is intimately related to the chemical processes initiated by water nanojets, and can be mitigated by incorporating suitable gas in the nanobubble. We examine chemical and mechanical damages due to shock-induced collapse of a nanobubble near a silica slab. When the shock front hits the nanobubble, water molecules at the periphery of the bubble rush in to form a focused nanojet in the direction of the shock-wave propagation. We observe an increase in the number of H3O+ ions when the nanojet hits the distal side of the bubble. We also performed two sets of simulations in which the nanobubble contained an inert gas. The simulated systems contained 109 and 108 atoms and the initial bubble radii in these systems were again 97.6 nm and 40 nm, respectively.
Acknowledgement: Research supported by DOE-BES Grant Number DE-FG02-04ER46130.
References:
1. “Bonding and Structure of Ceramic-Ceramic Interfaces”, Kohei Shimamura,Fuyuki Shimojo,1,2 Rajiv K. Kalia,1 Aiichiro Nakano,1 and Priya Vashishta, Physical Review Letters 111, 066103 (2013).
2. “Nanobubble Collapse on a Silica Surface in Water: Billion-Atom Reactive Molecular Dynamics Simulations”, Adarsh Shekhar, Ken-ichi Nomura, Rajiv K. Kalia, Aiichiro Nakano, and Priya Vashishta, Physical Review Letters, 111, 184503 (2013).
