DLC solid lubricant coatings on ball bearings for space applications

The environment of space offers special challenges for the lubrication of components in sliding and rolling mechanisms. Hydrogenated diamond-like carbon (DLC) films are being studied as solid lubricant coatings to simultaneously fulfil specifications regarding wear resistance and low friction behaviour under ambient atmosphere and in vacuum.In this paper, the tribological behaviour of highly hydrogenated DLC coatings (50 at% hydrogen) is assessed. Coating composition was optimised on flat AISI 52100 steel substrates based on ball-on-disc tribotest results in air, vacuum and dry nitrogen environments. The developed DLC coatings can be tailored to yield ultra-low friction values in vacuum (μ=0.008). The average friction coefficient range obtained in humid air, dry nitrogen and vacuum for the range of applied loads were, respectively, 0.22 to 0.27, 0.02 to 0.03, and 0.007 to 0.013.New in this work is that optimised DLC coatings were applied to ball bearings for space applications. The torque and life tests of coated pairs of angular contact bearings in air revealed that relatively high bearing torques are generated which increase with time, but the amount of coating wear generated during in-air operation appears relatively light. In vacuum, low torques are generated after a prolonged running-in period. Low-torque life exceeds that observed for MoS₂ by a factor of about two. It is concluded that, in contrast to MoS₂ coated bearings, DLC-coated bearings for space applications might therefore be capable of undergoing in-air ground testing without too much disruption of the subsequent in-space performance.     

Low temperature investigations on frictional behaviour and wear resistance of solid lubricant coatings

The tribological characteristics of polymer-based solid lubricant coatings under frictional stressing in vacuum at 293, 120 or 77 K were studied. Vacuum-friction apparatus with cryogenic pumps and low-temperature tribometer designed at SR&DB of ILTP&E was used for sliding tests. It was found that the coefficient of friction is somewhat higher at low than at room temperatures. The effect of temperature decrease on the wear life of solid lubricant coatings is ambiguous and determined by the direction of changes in physical and mechanical characteristics of a solid lubricant coating under cooling and by the rate of the process of a binder tribodestruction.     

Evaluating and predicting durability of bonded solid lubricant coatings under fretting conditions

Fretting wear is a common failure at contact surfaces of tight assemblies in industries, such as those of transport, power transmission and nuclear power station. Using friction reduction coatings is one of the most effective methods to palliate fretting failure. However, in view of numerous available coatings, it is still a tough task to evaluate them and to select the optimum one for a given application. In this paper, based on the investigation of fretting behaviors of three bonded solid lubricant coatings, an initial maximal dissipated energy density (Ed_0 max ini) approach and a local Archard factor (K_A0) approach were suggested to evaluate and predict coating durability. The lifetime of each coating under different values of test parameters can be fitted by one master curve. The master curves of a coating may be used to predict the coating lifetime only by running a new test for few cycles under relevant test conditions. For a given test condition, the durability of coatings can be easily evaluated by comparing their K_A0 master curves. Ed_0 max ini master curves include comprehensive tribological performance, which is helpful for coating selection.     

Improvement of tribological performance of steel by solid lubricant shot-peening in dry rolling/sliding contact wear tests

The effect of shot-peening treatment with the particulate MoS₂ solid lubricant on the wear resistance of steel in the dry rolling/sliding contact wear tests was investigated. The duplex shot-peening treatment with ceramic balls and the particulate MoS₂ solid lubricant provided excellent wear resistance under a severe loading and sliding condition because the uniform and minute surface roughness given by shot-peening treatment with ceramic balls could keep shot-peened MoS₂ particles with a low friction coefficient on the sample surface. Furthermore, the sample surface was covered with shot-peened MoS₂ particles by a MoS₂ layer formed during the rolling/sliding contact wear test.     

The tribological role of surface atoms: ultra-thin carbon and silver layers on the Si(111)

A few monolayers of a carbon film on an Si(111) substrate have been studied for tribological characteristics, focusing on the tribological role of the surface atoms.A monolayer of H and Ag on Si were also tested with a macroscopic diamond slider to examine the effect of the surface atoms on the friction, and it was observed that the sliding systems show an extraordinarily low friction in ultra-high vacuum. We could draw a conclusion: the chemical characteristics of the surface layer strongly affect the friction and nano-scale structures on the surface drastically change the macroscopic slider friction in these systems.     

Characterisation of friction properties between a laminated carbon fibres reinforced polymer and a monocrystalline diamond under dry or lubricated conditions

The machining of carbon fibre reinforced polymer (CFRP) is a hot topic for the aircraft industry. Such materials are considered as difficult to cut materials due to their heterogeneity and presence of hard fibres. In this context, a lot of finite element models have been developed in order to understand their material removal mechanisms. Among the scientific issues faced by these works, the identification of friction coefficients between CFRP and cutting tool materials remains unanswered. So, this paper aims to characterize the friction properties between composite and cutting tool materials. For instance, the paper focuses on the context of a laminated CFRP machined with a monocrystalline diamond tool under dry or under lubricated conditions. The specific tribological conditions during machining of such heterogeneous materials are discussed in the paper, especially the configuration of the tribosystem (‘opened tribosystem’) and the orientation of laminates and fibres during sliding. The great lack of friction coefficient is mainly due to the absence of relevant tribometers simulating the tribological conditions occurring in cutting. This paper presents the development of a new tribometer designed to simulate conditions corresponding to machining of CFRP materials. It provides quantitative values of friction coefficient depending on several key parameters. A range of sliding velocities and contact pressures has been tested. The influence of layers orientation and cutting fluids has also been investigated. It has been shown that friction coefficients are very low (∼0.06) in dry regime. Friction coefficient is not sensitive to contact pressure nor to sliding velocity. Additionally this works has revealed that a cutting fluid leads to a significant decrease in friction coefficients (∼0.02), which corresponds to a friction less situation.     

Humidity effects on tribology of advanced carbon materials

In micro and nanotribology, most exciting materials will be advanced carbon materials. These include carbon nanotubes (CNT), fullerene C₆₀, onionlike carbon and diamondlike carbon. However, tribology study on these materials started in the late 1990s, limited information of the influence of environmental atmosphere on tribology being available. The effect of relative humidity (RH) on micro and nanotribology of advanced carbon materials as well as well-known lamellar materials such as highly oriented pyro-graphite, molybdenum disulfide and mica has been studied. At high RH, friction and adhesion became high. All the materials except vertically aligned (VA) CNT showed RH dependence of friction. The primary reason for none of RH dependence of VACNT is the super hydrophobic property.     

Study on rotational fretting wear of bonded MoS2 solid lubricant coating prepared on medium carbon steel

The rotational fretting wear behaviors of the bonded MoS₂ solid lubricant coating and its substrate steel were comparatively studied under varied angular displacement amplitudes, constant normal load, and rotational speed. Dynamic analysis in combination with microscopic examinations was performed through SEM, EDX, XPS, optical microscope, and surface profilometer. The experimental results showed MoS₂ changed the fretting running regimes of substrate. The friction coefficients of MoS₂ were lower than those of the substrate. For MoS₂, the damage in partial slip regime was very slight. The damage mechanism of the coating in slip regime was main abrasive wear, delamination, and tribo-oxidation.     

The influence of solid lubricant for improving tribological properties in turning process

Control of machining zone temperature is achieved by providing effective cooling and lubrication. Though cutting fluids are widely used to carry away the heat in metal cutting, they cannot be recommended in the light of ecological and economic manufacture. Hence, there arises a need to identify eco‐friendly and user‐friendly alternatives to conventional cutting fluids. The present work features a specific study of the application of molybdenum disulphide as solid lubricant for improving tribological properties in turning and to overcome the limitations that arise with the use of cutting fluids or while dry machining. An experimental setup developed has been used to maintain constant flow rate of solid lubricant powder continuously on to the workpiece and tool interface zone. Results are encouraging with overall improvement in machining properties in terms considered parameters as compared to wet and dry techniques, due to the reduction in friction at tool/work and tool/chip interface. Copyright © 2010 John Wiley & Sons, Ltd.     

Enhancing AW/EP property of lubricant oil by adding nano Al/Sn particles

This paper presents results of experiments to enhance antiwear/extreme pressure (AW/EP) properties of a lubricant oil by adding metal nano particles. In this experiment, Al, Sn and Al + Sn nano-particles were selected as trial additives. The AW and EP properties were evaluated on Four-Ball test machine, while the feature and composition of the wear scar surface were investigated by scan electron microscope (SEM) and energy dispersion spectrum (EDS). The test results show that the AW and EP performance can be improved within a wide load range by adding Al + Sn nanoparticles. Analysis of the enhancement mechanism has also been conducted in this experiment and presented in this paper. It is found that nano-Sn particles can be deposited on the friction surface when the pressure was moderate and act as AW additive. It is also found that the nano-Al particles can be deposited under the condition of high load pressure and act as EP additive. Thus, the AW and EP properties of tested lubricant oil have been improved at the same time due to adding both Al and Sn.     

Synthesizing diamond film on Cu,Fe and Si substrate by in-liquid microwave plasma CVD

The purpose of this study is to synthesize diamond onto Si, Cu, and Fe (SUS632J2) substrates and to analyze the effect of carbon diffusion on their surfaces. Diamond was synthesized using the in-liquid microwave plasma chemical vapor deposition (IL-MPCVD) as a novel method for synthesizing diamond on various base materials. The IL-MPCVD method is superior one due to its efficiency in terms of cost, space and speed as compared to a conventional gas phase microwave plasma CVD (MPCVD). Microwaves of 2.45 GHz generated plasma in a solution which was comprised of methanol: ethanol (M:E = 97:3). Evaluation of deposited diamond films was done by a Scanning Electron Microscope (SEM) and Raman spectroscopy. Results shows that the IL-MPCVD method can form diamond films on Cu, Si and Fe substrates. The minimum time of film formation of Cu, Si and Fe are 2.5, 3.5 and 5 min, respectively. The material that forms carbide layers such as Si is a better substrate to form diamond film by the IL-MPCVD than other metal substrates such as Cu and Fe. Synthesizing diamond directly on the Fe substrate results in poor quality layers. The effect of carbon diffusion influences diamond film nucleation and diamond growth. In order to alleviate the carbon diffusion and improve the quality of the diamond film on the Fe substrate, Si has been sputtered on the Fe substrate as an interlayer. It is found that the diamond film can be formed on a Fe substrate using a Si interlayer and that heat treatment and thickening the interlayer improve its quality.     

Sliding wear response of zinc based alloy as affected by suspended solid lubricant particles in oil lubricant

Abstract

Wear behaviour of a zinc based alloy has been studied in partially lubricated condition. The test environment comprised a mixture of oil plus graphite/talc particles. The composition of the lubricant mixture was varied by changing the concentration of the solid lubricant particles suspended in the oil lubricant. Wear response of the alloy was noted to improve in terms of decreased wear rate, frictional heating and friction coefficient initially with the increasing concentration of the solid lubricant particles suspended in the oil lubricant. A critical content of the solid lubricant led to the best wear performance of the samples. This was followed by a reversal in the trend at concentrations of the solid lubricant particles in the lubricant mixture that were greater than the critical one. Wear behaviour of the alloy has been substantiated through the characteristics of wear surfaces, subsurface regions and debris particles.     

Characteristics of fluid flow and heat transfer in a fluidized heat exchanger with circulating solid particles

The commercial viability of heat exchanger is mainly dependent on its long-term fouling characteristic because the fouling increases the pressure loss and degrades the thermal performance of a heat exchanger. An experimental study was performed to investigate the characteristics of fluid flow and heat transfer in a fluidized bed heat exchanger with circulating various solid particles. The present work showed that the higher densities of particles had higher drag force coefficients, and the increases in heat transfer were in the order of sand, copper, steel, aluminum, and glass below Reynolds number of 5,000.     

Influence of humidity on the friction of diamond and diamond-like carbon materials

The friction of diamond and diamond-like carbon (DLC) materials was evaluated in reciprocating sliding wear testing under controlled relative humidity. The testing conditions were a displacement stroke of 100 μm, an oscillatory frequency of 8 Hz and a normal load of 2 N. The coefficient of friction of diamond and hydrogen-free DLC (a-C) coatings against a corundum sphere in the steady regime decreased with an increase in relative humidity. A water layer physisorbed at the interface between the mating surfaces played two major roles: acting as a lubricant and increasing the true area of contact. However, it was noticed that the friction coefficient of the hydrogenated DLC (a-C:H) coatings first increased and then decreased with increasing relative humidity in the steady state. There appeared to be a critical relative humidity for the a-C:H coatings, at which the steady-state friction reached the maximum value. The frictional behaviour of the a-C:H coatings also showed dependence on the wear test duration. The interaction between hydrogen and oxygen at the interface between the a-C:H coating and water layer was mainly responsible for such behaviour.     

Annealing treatment of amorphous silicon generated by single point diamond turning

Mechanical material removal during ultraprecision machining of semiconductor crystals normally induces surface damage. In this article, Raman micro-spectroscopy has been used to probe structural alteration as well as residual stresses in the machined surface generated by single point diamond turning. The damage found is characterized by an amorphous phase in the outmost surface layer. In addition, the results of in-situ re-crystallization annealing of micromachined silicon monitored by micro-Raman spectroscopy are reported for the first time. It is also shown that the annealing heat treatment influenced surface roughness, whereby R_max was equal to 24.3 nm and 47.5 nm for the non-treated and annealed surfaces, respectively.     

简述润滑、冷却用油劣化后的再生处理

论述润滑、冷却用油劣化后再生处理的经济实用性及劣化油再生处理中所面临的技术、设备要求,介绍一项较经济实用的劣化油再生处理技术,可以对企业的节能降耗和环境保护工作起到积极作用。     

High-temperature tribological properties of strontium sulfate films formed on zirconia-alumina, alumina and silicon nitride substrates

In our present study, Al₂O₃, (ZrO₂–3 mol% Y₂O₃)–39.6 mass% Al₂O₃ and Si₃N₄ substrates coated with SrSO₄ and SrSO₄–10 mass% Ag films were prepared, and the friction and wear properties of these specimens were investigated using a reciprocating ball-on-disk tribometer in the temperature range from room temperature to 1073 K in air. It was clarified that (ZrO₂–3 mol% Y₂O₃)–39.6 mass% Al₂O₃ substrates coated with chemically precipitated SrSO₄ particles and the substrates coated with SrSO₄–10 mass% Ag films prepared by mechanically grinding and annealing at 1073 K for 3.6 ks exhibited low friction coefficients and low wear rates at all the testing temperatures. In addition, the average friction coefficients of Si₃N₄ substrates were reduced above 673 K by coating with chemically precipitated SrSO₄ particles.     

The effect solid particle lubricant contamination on the dynamic behavior of compliant journal bearings

The proposed work concerns a theoretical and numerical investigation of the effect of solid particle contamination of lubricant oils on the static and dynamic characteristics of a finite length compliant journal bearing operating under isothermal conditions with laminar flow. In the present investigation, we use simple models based on the Einstein's mixture theory, which is characterized by the presence of suspended rigid particles in a fluid. Using the classical assumptions of lubrication, a Reynolds equation is derived and solved numerically by the finite difference method. The displacement field at the fluid film bearing liner interface due to pressure forces is determined using the elastic thin layer model. The results obtained show that the presence of suspended rigid particles in the lubricating oil (solid contamination) has significant effects on the hydrodynamic performance characteristics such as the pressure field, friction force, flow rate, elastic surface deformation as well as stability maps of the rotor‐bearing system (critical mass and whirl frequency) especially at high volumetric concentration.     

金刚石刀具性能及其应用研究

描述了金刚石的物理特性,对金刚石刀具的分类及其性能行了介绍,包括天然金刚石、聚晶金刚石、聚晶金刚石复合片、化学气相沉积金刚石涂层刀具.分析和对比了不同类型金刚石刀具的应用场合,为企业在加工难加工材料时选用超硬金刚石材料刀具时提供参考.     

Monitoring of diamond disk wear in stone cutting by means of force or acceleration sensors

The application of sensor systems is becoming more commonplace in improving productivity, automation, and reliability. The sensors employed in such systems possess signal and information ability for enhancing the monitoring and control of machining processes. Although measuring force and acceleration signals have been commonly used for the monitoring of metal machining processes, their application to stone cutting has not been well developed, which is perhaps due to the complexity of the interaction between the stone and the diamond disk. In order to enhance knowledge in this area of applications, a multi-sensor system was developed and installed for the monitoring of stone cutting by diamond mill. The signals acquired and analysed by the system include force and acceleration under different machining conditions. The measured signal data was used to perform time-domain analysis. The results indicate the feasibility of using the RMS features of force or acceleration signals along z-axis for the monitoring of disk wear.