Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Refractory high entropy alloys (RHEAs) have been proven to have excellent mechanical properties with a potential use as protective thin films. However, the combination of high hardness with low friction and wear is a major challenge in the design of RHEA films. In this study, we show that NbMoWTa/Ag self-lubricating multilayered films give a remarkable reduction in friction and at same time maintain high hardness. Interestingly, it is found that the bcc superlattice dominates in both NbMoWTa and Ag layers and the interfaces become coherent when the individual layer thickness h is reduced below 10 nm. The film properties are then strongly dependent on h ranging from 100 to 2.5 nm, and the most promising properties are obtained when the interface structure transforms from incoherent to coherent one. Especially, the multilayer with h = 2.5 nm exhibits superior tribological performance over monolithic NbMoWTa due to the significant coherent strengthening along with the self-lubricating ability in the multilayer. This tailored phase transition and coherent structure between the matrix and lubrication phases can also provide an optimal wear rate-coefficient of friction combination, which is higher than most of the Ag-containing self-lubricating films. The current work might open a new route toward the development of innovative self-lubricating RHEA films with excellent tribological properties.

Design and Characterization of Self-Lubricating Refractory High Entropy Alloy-Based Multilayered Films.

Effects of structure relaxation and surface oxidation on nanoscopic wear behaviors of metallic glass

Abstract The excellent properties of metallic glass (MG) films make them perfect candidates for the use in miniature systems and tools. However, their high coefficients of friction (COFs) and poor wear resistance considerably limit their long-term performance in nanoscale contact. We report the fabrication of a MG/graphene multilayer by the repeated deposition of Cu 50 Zr 50 MG with alternating layers of graphene. The microstructure of the multilayer was characterized by the transmission electron microscopy (TEM). Its mechanical and nanotribological properties were studied by nanoindentation and nanoscratch tests, respectively. A molecular dynamics (MD) simulation revealed that the addition of graphene endowed the MG with superelastic recovery, which reduced friction during nanoscratching. In comparison with the monolithic MG film, the multilayer exhibited improved wear resistance and a low COF in repeated nanowear tests owing to the enhanced mechanical properties and lubricating effect caused by the graphene layer. This work is expected to motivate the design of other novel MG films with excellent nanowear properties for engineering applications. 

/pdf/design-and-characterization-of-metallic-glass-graphene-2vi8fvrr.pdf

Wan Wang

Papers

Atomistic insights into the deformation mechanism of a CoCrNi medium entropy alloy under nanoindentation

Molecular dynamics simulation of the tribological performance of amorphous/amorphous nano-laminates

Molecular dynamics simulation of deformation mechanism of CoCrNi medium entropy alloy during nanoscratching

Atomistic understanding of incipient plasticity in BCC refractory high entropy alloys

Effect of a water film on the material removal behavior of Invar during chemical mechanical polishing