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

Diamond-like carbon (DLC) is a metastable form of amorphous carbon with significant sp3 bonding. DLC is a semiconductor with a high mechanical hardness, chemical inertness, and optical transparency. This review will describe the deposition methods, deposition mechanisms, characterisation methods, electronic structure, gap states, defects, doping, luminescence, field emission, mechanical properties and some applications of DLCs. The films have widespread applications as protective coatings in areas, such as magnetic storage disks, optical windows and micro-electromechanical devices (MEMs).

/pdf/diamond-like-amorphous-carbon-3pczopr0z7.pdf

Diamond-like amorphous carbon

During the past two decades, diamond-like carbon (DLC) films have attracted an overwhelming interest from both industry and the research community. These films offer a wide range of exceptional physical, mechanical, biomedical and tribological properties that make them scientifically very fascinating and commercially essential for numerous industrial applications. Mechanically, certain DLC films are extremely hard (as hard as 90 GPa) and resilient, while tribologically they provide some of the lowest known friction and wear coefficients. Their optical and electrical properties are also extraordinary and can be tailored to meet the specific requirements of a given application. Because of their excellent chemical inertness, these films are resistant to corrosive and/or oxidative attacks in acidic and saline media. The combination of such a wide range of outstanding properties in one material is rather uncommon, so DLC can be very useful in meeting the multifunctional application needs of advanced mechanical systems. In fact, these films are now used in numerous industrial applications, including razor blades, magnetic hard discs, critical engine parts, mechanical face seals, scratch-resistant glasses, invasive and implantable medical devices and microelectromechanical systems. DLC films are primarily made of carbon atoms that are extracted or derived from carbon-containing sources, such as solid carbon targets and liquid and gaseous forms of hydrocarbons and fullerenes. Depending on the type of carbon source being used during the film deposition, the type of bonds (i.e. sp 1 ,s p 2 ,s p 3 ) that hold carbon atoms together in DLC may vary a great deal and can affect their mechanical, electrical, optical and tribological properties. Recent systematic studies of DLC films have confirmed that the presence or absence of certain elemental species, such as hydrogen, nitrogen, sulfur, silicon, tungsten, titanium and fluorine, in their microstructure can also play significant roles in their properties. The main goal of this review paper is to highlight the most recent developments in the synthesis, characterization and application of DLC films. We will also discuss the progress made in understanding the fundamental mechanisms that control their very unique friction and wear behaviours. Novel design concepts and the principles of superlubricity in DLC films are also presented. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0022-3727/39/18/R01/pdf

Tribology of diamond-like carbon films: recent progress and future prospects

Ceramics have some of the highest strength- and stiffness-to-weight ratios of any material but are suboptimal for use as structural materials because of their brittleness and sensitivity to flaws. We demonstrate the creation of structural metamaterials composed of nanoscale ceramics that are simultaneously ultralight, strong, and energy-absorbing and can recover their original shape after compressions in excess of 50% strain. Hollow-tube alumina nanolattices were fabricated using two-photon lithography, atomic layer deposition, and oxygen plasma etching. Structures were made with wall thicknesses of 5 to 60 nanometers and densities of 6.3 to 258 kilograms per cubic meter. Compression experiments revealed that optimizing the wall thickness-to-radius ratio of the tubes can suppress brittle fracture in the constituent solid in favor of elastic shell buckling, resulting in ductile-like deformation and recoverability.

Strong, lightweight, and recoverable three-dimensional ceramic nanolattices

A few years after the invention of the scanning tunneling microscope (STM), the atomic force microscope (AFM) was developed Instead of measuring tunneling current, a new physical quantity could be investigated with atomic-scale resolution: the force between a small tip and a chosen sample surface This paper reviews progress and recent results obtained with AFM and other closely related techniques in the field of nanotribology, and attempts to point out many of the unresolved questions that remain The goal of this paper is to demonstrate that AFM is capable of producing atomic-scale knowledge As such, the authors will focus upon some of the contributions of the AFM to nanotribology They will almost exclusively discuss results that shed light on the actual atomic and molecular processes taking place, as opposed to the more applied investigations of microscale properties which are also carried out with AFM They will accompany this discussion by mentioning related theoretical efforts and simulations, although their main emphasis will be upon experimental results and the techniques used to obtain them, as well as suggested future directions In many ways, AFM techniques for quantitative, fundamental nanotribology are only in a nascent stage; certain key issues such as force calibration,more » tip characterization, and the effects of the experimental environment, are not fully resolved or standardized The authors thus begin with a critical discussion of the relevant technical aspects with using AFM for nanotribology 289 refs« less

/pdf/scratching-the-surface-fundamental-investigations-of-4kjq5s243s.pdf

Scratching the Surface: Fundamental Investigations of Tribology with Atomic Force Microscopy.

We study the deposition of tetrahedral amorphous carbon (ta-C) films from molecular dynamics simulations based on a machine-learned interatomic potential trained from density-functional theory data. For the first time, the high sp^{3} fractions in excess of 85% observed experimentally are reproduced by means of computational simulation, and the deposition energy dependence of the film's characteristics is also accurately described. High confidence in the potential and direct access to the atomic interactions allow us to infer the microscopic growth mechanism in this material. While the widespread view is that ta-C grows by "subplantation," we show that the so-called "peening" model is actually the dominant mechanism responsible for the high sp^{3} content. We show that pressure waves lead to bond rearrangement away from the impact site of the incident ion, and high sp^{3} fractions arise from a delicate balance of transitions between three- and fourfold coordinated carbon atoms. These results open the door for a microscopic understanding of carbon nanostructure formation with an unprecedented level of predictive power.

/pdf/growth-mechanism-and-origin-of-high-sp-3-content-in-fkem7zlhx1.pdf

Growth Mechanism and Origin of High sp^{3} Content in Tetrahedral Amorphous Carbon.

Material deformations and the influence of coating thickness and elastic modulus were analysed by three-dimensional finite element method (FEM) modelling on microlevel, by stress, strain, and displacement computer simulations and by experimental studies with a scratch tester. The studied tribological contact was a diamond ball sliding with increasing load on a thin titanium nitride (TiN) coating on a flat steel substrate. The ball was modelled as rigid, the coating was linearly elastic, and the steel substrate was elastic–plastic, taking into account strain hardening effects. It was shown that a thin TiN ceramic coating on a steel substrate has only a very slight effect on friction and on the plastic deformations (i.e., the groove formation) in the surface, but changes considerably the stress pattern at the surface. The stress simulations showed how a thicker hard coating on a soft substrate has a better load-carrying capacity that a thinner one. Higher tensile stresses at the coating/substrate interface increase the risk for interface cracks and delamination of the thicker coating. A stiffer hard coating on a soft substrate has a better load-carrying capacity than a more elastic one. The stiffer coating will accommodate higher tensile stresses with the same indentation depth compared to a more elastic one. The results show that much more attention should be given to optimizing the elastic properties of the coating than previously has been done. In many cases, it can be much more effective to improve the wear resistance of the coated surface by focusing on the elastic modulus of the coating than changing the coating thickness.

Tribological contact analysis of a rigid ball sliding on a hard coated surface. Part II: Material deformations influence of coating thickness and young's modulus

Amorphous hydrogenated carbon (a-C:H) films were deposited by the r.f. plasma technique on stainless steel substrates. Pin-on-disc experiments were carried out over a wide range of normal loads (5–40 N) and sliding velocities (0.1–3.0 m s−1) in order to study the friction and wear performance of the coating against steel and alumina. The friction coefficient of a-C:H films against both steel and alumina pins decreased with increasing load and sliding velocity. On the pin wear surface, tribolayer formation was detected. The wear of the alumina pins increased with increasing load and sliding velocity when they contacted the coating. However, the thick tribolayer formed on the wear surface of the steel pin protected it from excessive wear when high loads and sliding velocities were applied. The wear surfaces were analysed by secondary ion mass spectroscopy and Auger electron spectroscopy. The analyses revealed that the thick tribolayer formed on the pin wear surface mainly consisted of the oxides of the pin material. However, evidence of carbon was found in the sliding deposit formed in front of the contact area of the pin and also in some cases on the pin wear surface. Carbon played an important role in the low friction behaviour although the amount of carbon was low. It is assumed that a thin tribolayer with low shear strength, consisting of carbon species, is formed on the disc wear surface. The coating wear increased when the normal load was increased. Some transfer of pin material was observed on the coating wear surface.

Effect of tribofilm formation on the tribological performance of hydrogenated carbon coatings

Low friction coatings are being developed for a variety of reasons. Practical conditions in which machines and machine elements have to operate are ever expanding and more often involve features which preclude liquid or grease lubrication. An example of such conditions very common in everyday life is the food-processing industry, where any kind of lubrication is impossible owing to the risk of contamination of a product. Some other conditions include hostile environments such as high temperature or vacuum. High temperatures are common, for example, in the chemical industry and power generation, whereas vacuum is often associated with space technology. A new motivation for development of low friction coatings is a general trend to reduce lubrication for reasons of environmental compliance. All these together challenge materials technology and surface engineering especially. Accordingly, activity in the area of the development of coating materials, sophisticated deposition methods as well as post treatments of coatings has been vigorous in recent years.

In this paper we first review typical conditions in which low friction coatings are sought as well as the demands for properties which coatings should fulfil in order to function in an appropriate way. This is followed by a more detailed presentation of three different coatings for three different conditions. Amorphous hard carbon films deposited by arc discharge deposition are hydrogen free, have high hardness and are best characterized as an amorphous diamond. The films have been shown to possess a friction coefficient below 0.2 or even below 0.1 in dry-sliding conditions associated with good wear resistance. Potential conditions in which these films could be used are dry sliding at room temperature and in a normal atmosphere with adequate humidity. The second coating family which will be presented is based on molybdenum disulphide (MoS2). These are typical coatings for use in a vacuum. Many attempts have been made to overcome the many limitations of these coatings with post-treatments. In this presentation, ion beam and laser processing of MoS2 coatings with or without alloying are dealt with. The third set of conditions includes high temperatures. This is one of the most difficult areas of low friction coatings and no final solutions for practical purposes have been developed.

Jari Koskinen

Papers

Growth Mechanism and Origin of High sp^{3} Content in Tetrahedral Amorphous Carbon.

Tribological contact analysis of a rigid ball sliding on a hard coated surface. Part II: Material deformations influence of coating thickness and young's modulus

Effect of tribofilm formation on the tribological performance of hydrogenated carbon coatings

Present progress in the development of low friction coatings

Graphene oxide in water lubrication on diamond-like carbon vs. stainless steel high-load contacts