Solid-state Lubricant Research Articles

Computational investigation of the mechanical and tribological responses of amorphous carbon nanoparticles

Nanoparticles are a class of materials that have seen increasing use as friction and wear reducers in tribological applications. Amorphous carbon (a-C) films have been the subject of significant scientific and industrial interest for use as solid-state lubricants. Here, we present classical molecular dynamics simulations to investigate the mechanical and tribological responses of a-C nanoparticles that are subjected to external forces between hydrogen-terminated diamond surfaces. Over the range of a-C nanoparticle diameters (2–5 nm) and hydrogenation (0%–50%) considered, the simulations predict a consistent mechanical response where each nanoparticle is highly elastic. The simulations predict that the transition from elastic to plastic response is directly related to an increase in the percentage of carbon-carbon crosslinking within the individual nanoparticles. Contrarily, the simulations also predict that the tribological response is noticeably impacted by changes in diameter and hydrogenation. This is because during friction, hydrogen passivates the unsaturated carbon atoms near the nanoparticle's surface, which prevents interfacial bond formation and allows the nanoparticle to roll within the interface. From these findings, it is demonstrated that a-C nanoparticles are able to provide good tribological performance only when sufficient chemical passivation of the nanoparticles is maintained.

Characterisation of friction reduction with tangential ultrasonic vibrations using a SDOF model

Active control of friction between sliding surfaces is of fundamental and practical interest in automotive applications. It has been shown that the friction force between sliding surfaces decreases when ultrasonic vibration is superimposed on the sliding motion. This principle can be applied to systems in which solid state lubrication or friction modulation is advantageous. The ultrasonic vibration may be applied longitudinally or normal to the direction of motion. A number of friction models have been considered in order to analyse this phenomenon. The degree of friction reduction has been shown to depend on the ratio of the sliding velocity to the vibration velocity. Since friction is a system response, it is necessary to include system dynamics in the analysis of ultrasonic lubrication. A nonlinear single-degree-offreedom-model is formulated and numerically approximated to quantify the effect on friction reduction of control force, intrinsic coefficient of friction, mass load, tangential contact stiffness at the sliding interface, and system stiffness. Model results are in close agreement with experimental measurements.

Study of urological devices coated with fullerene-like nanoparticles

Insertion of endoscopes and other medical devices into the human body are ubiquitous, especially among aged males. The applied force for the insertion/extraction of the device from the urethra must overcome endoscope-surface-human-tissue interactions. In daily practice a gel is applied on the endoscope surface, in order to facilitate its entry into the urethra, providing also for local anesthesia. In the present work, a new solid-state lubricant has been added to the gel, in order to reduce the metal-urethra interaction and alleviate the potential damage to the epithelial tissue. For that purpose, a urethra model was designed and fabricated, which allowed a quantitative assessment of the applied force for extraction of the endoscope from a soft polymer-based ring. It is shown that the addition of MoS2 nanoparticles with fullerene-like structure (IF-MoS2) and in particular rhenium-doped nanoparticles (Re:IF-MoS2) to Esracain gel applied on the metal-lead reduced the friction substantially. The Re:IF-MoS2 showed better results than the undoped fullerene-like nanoparticles and both performed better than the gel alone. The mechanism of friction reduction is attributed to fullerenes' ability to roll and act as a separator between the active parts of the model.

Copper Substitution and Noise Reduction in Brake Pads: Graphite Type Selection

Graphite is commonly used in brake pads. The use of graphite powder has the main goal of solid state lubrication and friction coefficient stabilization. In this article results on resin bonded brake pads with focus on noise performance and heat dissipation are presented. Experimental tests are based on model friction materials with a known formulation and a reduced number of components for a better identification of the role of the graphite type. Results clearly indicate that both noise performance and thermal conductivity are strongly affected by the type of graphite. Guidelines for the selection of graphite types for optimized friction materials are given.

Data-Driven Model for Estimation of Friction Coefficient Via Informatics Methods

As technologies progress, the development of new mechanical systems demands the rapid determination of friction coefficients of materials. Data mining and materials informatics methods are used here to generate a predictive model that enables efficient high-throughput screening of ceramic materials, some of which are candi- date high-temperature, solid-state lubricants. Through the combination of principal component analysis and recursive partitioning using a small dataset comprised of intrinsic material properties, we develop a decision tree-based model comprised of if-then rules which estimates the friction coefficients of a wide range of materials. This data- driven model has a high degree of accuracy with an R 2 value of 0.8904 and provides a range of possible friction coefficients that accounts for the possible variability of a material's actual friction coefficient.

Friction and Wear of Pyrophyllite on the Atomic Scale

Pyrophyllite, Al2Si4O10(OH)2, a layered aluminosilicate material, has several properties which suggest that it might exhibit desirable frictional properties for solid-state lubrication. These include a lamellar structure and a low-energy basal plane. Here, we utilize atomic force microscopy to investigate the tribological performance of mineralogical pyrophyllite samples and compare the results to those of other minerals and solid lubricants. The results indicate that pyrophyllite exhibits low friction forces and shear stresses, on the same order of other commonly used solid lubricant materials, and a high resistance to deformation within a single layer. These fundamental findings highlight the potential use of pyrophyllite as a possible tribological material and alternative solid lubricant.

Fate of methanol molecule sandwiched between hydrogen-terminated diamond-like carbon films by tribochemical reactions: tight-binding quantum chemical molecular dynamics study

Recently, much attention has been given to diamond-like carbon (DLC) as a solid-state lubricant, because it exhibits high resistance to wear, low friction and low abrasion. Experimentally it is reported that gas environments are very important for improving the tribological characteristics of DLC films. Recently one of the authors in the present paper, J.-M. Martin, experimentally observed that the low friction of DLC films is realized under alcohol environments. In the present paper, we aim to clarify the low-friction mechanism of the DLC films under methanol environments by using our tight-binding quantum chemical molecular dynamics method. We constructed the simulation model in which one methanol molecule is sandwiched between two hydrogen-terminated DLC films. Then, we performed sliding simulations of the DLC films. We observed the chemical reaction of the methanol molecule under sliding conditions. The methanol molecule decomposed and then OH-termination of the DLC was realized and the CH3 species was incorporated into the DLC film. We already reported that the OH-terminated DLC film is very effective to achieve good low-friction properties under high pressure conditions, compared to H-terminated DLC films. Here, we suggest that methanol environments are very effective to realize the OH-termination of DLC films which leads to the good low-friction properties.

Tribochemical Reaction Dynamics Simulation of Hydrogen on a Diamond-like Carbon Surface Based on Tight-Binding Quantum Chemical Molecular Dynamics

Diamond-like carbon (DLC) has recently attracted much attention as a solid-state lubricant, because of its resistance to wear, low friction, and low abrasion. Several factors, such as the hydrogen atoms in DLC and transfer film formation are important for improving the tribological characteristics of DLC. In this paper, we discuss the low-friction mechanism of DLC by using our tight-binding quantum chemical molecular dynamics method. The method employs a DLC film sliding simulation in order to explore the effect of hydrogen atoms on the carbon-based transfer film. The formation of C–C bonds between DLC films increases friction, while surface hydrogen atoms suppress C–C bond formation, which results in the low-friction state. Moreover, the steric effect of hydrogen molecule generation was found to remove the load from the substrate, inhibiting C–C bond formation. In addition, we determined that surface hydrogen atoms play a key role in the cleavage of C–C bonds formed during sliding of DLC films.

How to pick a winner when choosing a PM graphite

In discussions of alloying additives for improving PM manufactured parts the role of carbon is often overlooked. However the role of carbon is crucial in two key respects. First, carbon assists steel matrix hardening by diffusion into iron particles. Second, it decreases the co-efficient of friction by solid state lubrication. Swiss/Italian research has looked at how graphite type and particle size distribution can affect flowability of the mix, dimensional change after sintering and mechanical properties of PM parts. So how do manufacturers select graphite powder when considering their process metallurgy?

Room-Temperature Chemical Synthesis of Shape-Controlled Indium Nanoparticles

Shape-controlled metal nanoparticles are of interest because of their wide range of properties that are useful for applications that include optics, electronics, magnetism, and catalysis. Indium metal is an attractive target for nanoparticle synthesis because it is superconducting, plasmonically active, and is a component in low-melting solders and solid-state lubricants. Indium nanoparticles are typically synthesized using harsh physical or chemical techniques, and rigorous shape control is difficult. Here we present a simple and robust kinetically controlled process for synthesizing shape-controlled indium nanoparticles. By controlling the rate of dropwise addition of a solution of NaBH4 in tetraethylene glycol to an alcoholic solution of InCl3 and poly(vinyl pyrrolidone), indium nanoparticles are formed with shapes that include high aspect ratio nanowires and uniform octahedra and truncated octahedra. The zero-dimensional indium nanoparticles exhibit an SPR band centered around 400 nm, and all morphologies are superconducting (Tc = 3.4 K) with higher critical fields than bulk indium.

Bulk vs Nanoscale WS2: Finite Size Effects and Solid-State Lubrication

To investigate phonon confinement in nanoscale metal dichalcogenides, we measured the low-temperature specific heat of layered and nanoparticle WS2. Below 9 K, the specific heat of the nanoparticles deviates from that of the bulk counterpart. Further, it deviates from the usual T 3 dependence below 4 K due to finite size effects that eliminate long wavelength acoustic phonons and interparticle-motion entropy. This separation of nanoscale effects from T 3 dependence can be modeled by assuming that the phonon density of states is flexible, changing with size and shape. We invoke relationships between the low-temperature T 3 phonon term, Young's modulus, and friction coefficient to assess the difference in the tribological properties. On the basis of this analysis, we conclude that the improved lubrication properties of the nanoparticles are extrinsic.

Synthesis of silicon carbide nanorods from mixture of polymer and sol–gel silica

Silicon carbide possesses many unique properties like superior strength, corrosion and oxidation resistance at elevated temperature [1]. Silicon carbide in the fibrous form is employed for the development of ceramicmatrix and metalmatrix composites (CMCs and MMCs) [2, 3]. Recently there is growing interest in the synthesis of nanofibres, which could be used for the development of nanocomposites, solid-state lubricants, catalysts and components of magnetic devices [4]. Nano silicon carbides are synthesized as nanorods, nanowires, nanotubes and nanofibres [5–8]. Silicon carbide nanomaterials which can be used as reinforcing material are generally prepared by chemical vapor deposition (CVD) [9], arc-discharge [10], polymer blend technique [11] and by reaction between carbon nanotubes and silicon monoxide [12]. The authors had also employed and reported polymer blend technique for the development of pitch based carbon fibres [13, 14]. It was seen by the authors during their efforts to develop carbon fibres from pitches modified with polymers that polyphenylene oxide–polystyrene (PPO–PS) blend does not give fibres on spinning and gives rod like materials [13]. Based on the results of the above studies, attempts have been made in the present investigation to synthesize SiC nanorods by blending sol–gel derived silica with PPO–PS polymer. Commercially available PPO–PS was used as carbon source and methyltriethoxysilane [(MTEOS) Lancaster, 98%] and tetraethoxysilane [(TEOS) E’Merck, 98%] are used as silica source. PPO–PS was carbonized at 1000 C in inert atmosphere to determine carbon residue. PPO–PS was dissolved in dichloromethane and was mixed with MTEOS sol, which was prepared by hydrolyzing a mixture of MTEOS, water and dichloromethane. The mixture was stirred well for about 2 h. The sol-containing polymer was then allowed to gel at room temperature and dried at 60 C to obtain polymer incorporated with sol–gel derived silica, which is the silicon carbide nanorods precursor. The same experiment was repeated with TEOS sol. The SiC precursor prepared as above was carbonized at 1000 C and analyzed for carbon and silica contents. The carbonized product was further heated at 1400 C under argon atmosphere to get a mixture of SiC, carbon and silica. The pyrolysed products (Heat treatment temperature = 1400 C) were oxidised in air at 800 C to remove the free carbon. The carbonized, pyrolysed and oxidized products of PPO–PS + MTEOS and PPO– PS + TEOS were characterized by scanning electron microscopy (SEM, LEO-440). The oxidized sample derived from PPO–PS + MTEOS derived silica was also characterized by transmission electron microscopy (TEM, Jeol JEM 2000 CX). The FTIR spectra of the precursor (PPO–PS + MTEOS derived SiO2) and the pyrolysed sample (heat treatment temperature = 1400 C) were recorded by KBr pellet method using Perkin Elmer FTIR 2000. X-ray diffraction V. Raman (&) G. Bhatia P. R. Sengupta Carbon Technology Unit, Division of Engineering Materials, National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India e-mail: vasantha@mail.nplindia.ernet.in

Bulk versus nanoscale WS2: Finite size effects and solid state lubrication

Metal dichalcogenide nanostructures have recently attracted attention due to their unique closed-cage structures, hierarchy of length scales, and outstanding solid state lubrication behavior. To understand the finite size effects and tribological properties in these nanoscale materials, we measured the low temperature specific heat of layered and nanoparticle WS2. Below 9 K, the specific heat of the nanoparticles deviates from that of the bulk. Further, the thermal response of the nested nanoparticles deviates from the usual T3 dependence below 4 K because of both finite size effects and inter-particle-motion entropy. This separation of nanoscale effects from T3 dependence can be modeled by assuming that the phonon density of states is flexible, changing with size and shape of the nanoparticle. We also invoke relationships between the low temperature T3 phonon term, Young’s modulus, and friction coefficient to assess the difference in the tribological properties. Based on this analysis, we conclude that the improved lubrication properties of the nanoparticles are extrinsic in origin.

Solid-state flow, mechanical alloying, and melt-related phenomena for [0 0 1] single-crystal W ballistic rod penetrators interacting with steel targets

Single-crystal [0 0 1] clad with Inconel 718 and unclad W penetrators in steel targets have been examined by optical metallography and electron microscopy to provide a comprehensive microstructural overview of the penetration process, including solid-state penetrator flow and erosion, target flow, and solid-state mixing of the target and penetrator. Selective etching techniques as well as energy-dispersive X-ray mapping, revealed unambiguous evidence of target (steel) and penetrator mechanical mixing. These mixtures, which included some isolated melt regimes, differ in composition for clad and unclad samples, and were observed to intercalate within the material being eroded by DRX-assisted flow. The cladding material appears to influence the solid-state flow by functioning as the principal flow interaction regime between the target and penetrator. This suggests that by selecting some optimum cladding material, which might function as a solid-state lubricant at the interface regime, the penetration performance of the rod could improve.

Dynamics of bulk versus nanoscaleWS2: Local strain and charging effects

We measured the infrared vibrational properties of bulk and nanoparticle WS$_2$ in order to investigate the structure-property relations in these novel materials. In addition to the symmetry-breaking effects of local strain, nanoparticle curvature modifies the local charging environment of the bulk material. Performing a charge analysis on the \emph{xy}-polarized E$_{1u}$ vibrational mode, we find an approximate 1.5:1 intralayer charge difference between the layered 2H material and inorganic fullerene-like (IF) nanoparticles. This effective charge difference may impact the solid-state lubrication properties of nanoscale metal dichalcogenides.

Effects of processing upon the properties of soft magnetic composites for low loss applications

We have investigated the effects of processing variables and methods on the density, structure and magnetic properties of soft magnetic composite (SMC) materials primarily for use in low loss applications. Two commercially available SMC materials from Hoganas AB, Sweden were used throughout the study. The materials differed only in terms of the solid state lubricant incorporated in the powder. This was sufficient to affect the density of pressed samples. Both monotonic and cyclic pressing regimes were used to compact the materials, with pressures sufficiently large to control the bulk mechanical properties of the samples produced (Hardness typically 1.6 GPa). Samples were heat-treated to both relieve any stresses imposed during pressing, and to control the final composition through oxidation or non-oxidation processes. Magnetic properties were observed using a vibrating sample magnetometer and Barkhausen noise analyser and electrical properties determined using a micro-ohmmeter. Both magnetic and electrical properties were observed to be strongly dependent on sample density and heat treatment.

Novel tribological properties of anodic oxide coating of aluminum impregnated with iodine compound

Several iodine compounds are well known as solid-state lubricants in the halogen family, however, satisfactory results of their use in the manufacturing industry have not been reported. Subsequently the anodic oxide coating of aluminum impregnated with iodine compound in the concrete inclusion complex with iodine was prepared successfully by an electrochemical method and tribological properties of obtained anodic oxide coating were investigated [Proceedings of INTERFINISH 2000 15th World Congress and Exhibition Congresshaus Garmish-Parten—Kirchen, (2000)]. Iodine compound was found in micro-pores as iodine (I) of 0.1 mass% and formed iodophor of an amorphous structure. The tribological properties of obtained anodic oxide coating were estimated by friction and wear testing. The yielded coefficient of dynamic friction of a sample with iodine compound was less than 0.1. Obtained anodic oxide coating impregnated with PTFE, namely TUFRAM. The coefficients of dynamic friction of obtained anodic coating ADC12 aluminum die casting or A6061 aluminum alloy have proved to be lower in comparison with thin-film Sn on anodic oxide coating by electroplating. From these results, the impregnation of iodine compounds in the micro-pores of anodic oxide coating has proved to lower the coefficient of dynamic friction. We think this phenomenon originates from the orthorhombic system and lamellar structures of the iodine compound used this time, which are well known as solid lubricants.

Hollow nanoparticles of WS2 as potential solid-state lubricants

Solid lubricants fill a special niche in reducing wear in situations where the use of liquid lubricants is either impractical or inadequate, such as in vacuum, space technology or automotive transport. Metal dichalcogenides MX2 (where M is, for instance, Mo or W and X is S or Se) are widely used as solid lubricants. These materials are characterized by a layered structure with weak (van der Waals) inter-layer forces that allow easy, low-strength shearing1,2. Within the past few years, hollow nanoparticles (HNs) of MX2 with structures similar to those of nested carbon fullerenes and nanotubes have been synthesized3,4. Here we show that these materials can act as effective solid lubricants: HN-WS2 outperforms the solid lubricants 2H-MoS2 and 2H-WS2 in every respect (friction, wear and lifetime of the lubricant) under varied test conditions. We attribute the outstanding performance of HN-WS2 to its chemical inertness and the hollow cage structure, which imparts elasticity and allows the particles to roll rather than to slide.

高温で低摩擦係数を示すグラファイト系固体潤滑剤

高温で低摩擦係数を示すグラファイト系固体潤滑剤

Ion beam analysis of steel surfaces modified by nitrogen ion implantation

Conventional ERDA in combination with RBS and the 14N(d, α) 12C nuclear reaction has been applied to understand the wear behaviour of 210Cr46 tool steel after nitrogen implantation. However, the presence of the neighbouring elements 12C and 16O within the 14N-implanted layers results in both strong limitations for the conventional ERDA and the necessity to registrate them analytically. The ERDA with Bragg ionization chamber proved useful allowing depth profile measurements with element separation in a single run. The solid-state lubrication caused by a nonmetallic carbon covering of the steel surface was deduced to increase the wear resistance drastically.