Coatings tribology—contact mechanisms and surface design

The fundamentals of coating tribology are presented by using a generalised holistic approach to the friction and wear mechanisms of coated surfaces in dry sliding contacts. It is based on a classification of the tribological contact process into macromechanical, micromechanical, nanomechanical and tribochemical contact mechanisms, and material transfer. The important influence of thin tribo- and transfer layers formed during the sliding action is shown. Optimal surface design regarding both friction and wear can be achieved by new multi-layer techniques which can provide properties such as reduced stresses, improved adhesion to the substrate, more flexible coatings and harder and smoother surfaces. The differences between contact mechanisms in dry, water- and oil-lubricated contacts with coated surfaces is illustrated by experimental results from diamond-like coatings sliding against a steel and an alumina ball. The mechanisms of the formation of dry transfer layers, tribolayers and lubricated boundary and reaction films are discussed.     

Developments in tribological materials in the next millennium

The materials needed for tribological contact systems in the next millennium will depend on the trend and pace of technological development, and environmental policy. The latter will certainly demand lower pollution and energy consumption, so materials should preferably be light, and work in most friction systems without lubrication, but with low friction, and no wear. The following properties, or requirements, of future tribomaterials can be conceived of in such a scenario: it seems likely that micro‐mechanical systems will achieve great importance, but silicon must be either surface protected or replaced by an optimised material; light metal alloys must be conditioned by surface treatment or coating; coatings must be improved, e.g., by nanomaterials; ceramic materials sintered at low temperatures from nano‐powders will be ductile and machinable, but retain the other beneficial properties of ceramics; and ultra‐precise surface finishing of materials may lead to nearly friction‐free and wearproof tribocontacts.     

A new tool for industrial tribology filling the gap between macro‐ and nano‐ tribology

The aim of this paper is to illustrate the similarities and variations between friction and wear data acquired with two different types of laboratory tribological test equipment that not only operate in the gap between macro‐ and nano‐testing, but also provide test parameters that allow an overlap across the whole scale from macro‐ to nanoscale friction and wear tests. Comparative wear tests were conducted under identical test conditions in the overlap region between macro‐ and micro‐regions using an existing piece of laboratory fretting test equipment and a novel one. The experimental test data obtained with these two experimental set‐ups are discussed.     

The current status of tribological surface science

Use of the ultrahigh vacuum methods of surface science allows the preparation and study of highly characterized solid–solid interfaces for tribological investigations. One of the limiting factors in the progress of our understanding of tribology has been the inability to generate solid–solid interfaces with well-defined, reproducible, atomic scale structures and compositions. The preparation of single crystalline surfaces of metals and the characterization of adsorbed molecules on these surfaces has been one of the major accomplishments of the field of ultrahigh vacuum surface science. Combining surface science instrumentation with an ultrahigh vacuum tribometer has made it possible to bring to the study of tribology the same quality of experimental measurement as is available for many other surface phenomena. One phenomenon that has been explored in great detail is the effect of the coverage of adsorbed species on friction between metal surfaces. Other measurements have included the study of friction anisotropy; the effect on the friction between two single crystal surfaces of their relative crystallographic orientations across the sliding interface. Finally, recent results have correlated the friction between adsorbate covered surfaces with the desorption energy of the adsorbed molecules. These results and measurements anticipated for the future offer the opportunity to address truly fundamental questions in tribology and complement many of the other new tools recently developed for the study of tribological phenomena.     

Computer support in tribology - experiments and database

As in all other areas of research today, so in tribology, computers play an important role. The exact analysis of tribological systems needs finite element methods (FEM) for calculation. Analysis of worn surfaces by SEM, AUGER, ESCA, XRS and the interpretation and documentation of measurements is performed with computers. The new approach to the basis of tribology by the use of scanning tunnelling microscopy (STM) and lateral force microscopy (LFM) would not be possible without computers. This paper points out the opportune tasks for computers in tribological simulation tests, such as complex test cycles, long time runs, automatic sensor calibration, measurements of fast varying signals, control and correction of mutual influences, and mentions the databases TRIBO (Tribology Index) for literature and Tribocollect for tribological data.     

Friction and wear of polyphenylene sulfide composites filled with micro and nano CuO particles in water-lubricated sliding

The tribological behavior of polyphenylene sulfide (PPS) composites filled with micro and nano CuO particles in water-lubricated sliding condition were studied. Pin-on-disk sliding tests were performed against a steel counterface of surface roughness 0.09–0.11 μm. The lubrication regimes were established from friction data corresponding to various combinations of loads and sliding speeds. Later experiments were performed using the sliding speed of 0.5 m/s and contact pressure of 1.95 MPa, which corresponded to boundary lubrication regime. Micro CuO particles as the filler were effective in reducing the wear of PPS but nano CuO particles did not reduce wear. The steady state wear rate of PPS-30 vol.% micro CuO composite was about 10% of that of unfilled PPS and the coefficient of friction in this case was the lowest. The examination of the topography of worn pin surfaces of nano CuO-filled PPS by SEM revealed grooving features indicating three-body abrasion. The transfer films formed on the counterfaces during sliding were studied by optical microscopy and AFM. The wear behavior of the composites in water-lubricated sliding is explained using the characteristics of worn pin surfaces and transfer films on the counterface.     

Characterisation and nano‐friction behaviour of molecular deposition films

The structure and the nano‐friction behaviour of a new kind of ultrathin film, a molecular deposition (MD) film, on an Au substrate were studied. The MD film is formed by the electrostatic attraction between opposite charges of cationic and anionic compounds, and a multilayer film can be built through alternating deposition of bipolar cationic and anionic compounds. Monolayer, bilayer, trilayer, and tetralayer MD films on Au substrates were examined. MD films with an alkyl terminal group were also investigated. It was found that while the MD film on an Au substrate reduced the friction, its nano‐friction behaviour was unstable because of the active terminal group. However, if the MD film was formed with an alkyl terminal group, its nano‐friction behaviour became stable and its friction decreased markedly. Therefore, this film termination method could contribute to the nano‐tribological application of MD films.     

Adhesion of cleaned nanoscopic metal contacts

Nanoscopic metal contact in the context used in this paper means atomic force microscope (AFM) tips in contact with cleaned, flat, smooth surfaces in ultrahigh vacuum. Such idealised contact conditions are of more and more relevance for technical friction systems despite the fact that for the latter the loads and the velocities are usually higher, the materials used are impure (e. g. alloys) and contaminants or lubricants are present between the sliding surfaces. Sliding contacts in micro‐system technology, which is unequivocally a key technology of the future, will presumably be compared with such idealised conditions. Another important reason for the investigation of nanoscopic contacts is the strong interest in the understanding of the elementary processes and mechanisms causing friction. The investigation of friction systems with a reduced number of influencing parameters offers a chance to gain insight into elementary processes and to compare the results with theoretical models based on physical principles, such as molecular dynamics simulation.     

Microtribological behaviour of thin DLC films using different testing methods

To enhance the lifetime and reliability of microcomponents, thin microtribological films are applied to microparts. With reduction of the component size, investigation methods for tribological testing must be adapted. This paper studies the microtribological behaviour of thin diamond‐like carbon (DLC) films using different testing methods. To tie in with macroscopic results, to determine friction we used the well‐known pin‐on‐disc test with spherical surfaces of 10 mm diameter under a typical load of 3 N. For investigations of the behaviour under single asperity contact, Atomic Force Microscope (AFM) methods with applied loads of a few hundred micronewtons were used. Investigations on thin DLC films showed that the friction coefficient under single asperity contact is strongly dependent on the applied load and the resulting contact area. Especially for thin films (up to a few hundred nanometres) the friction coefficient is influenced by the substrate material. With decreasing substrate Young's modulus the friction coefficient also decreases. On the other hand, an increase in the abrasive wear resistance was observed using soft substrate materials. In this paper we show that the friction coefficient was also reduced by a simple surface structure. For investigations we used photolithography to create concentric circles in different substrates. This resulted in a behaviour like riding on rails for the pin‐on‐disc test. Depending on the tribological pairing the friction coefficient was reduced to more than 50% of the original value. Copyright © 2006 John Wiley & Sons, Ltd.     

Surface‐capped molybdenum sulphide nanoparticles — a novel type of lubricant additive

Compounds containing molybdenum sulphide are widely employed as friction‐modifying and antioxidant additives in lubricants. However, the synthesis of these types of compound can often be complex and expensive. Tribologically active lubricant additives can be prepared as inorganic nanosized surface‐capped particles, the surfaces being coated with compounds that interact and/or react with the inorganic material. This paper reports on the synthesis and characterisation of surface‐capped molybdenum trisulphide nanoparticles. The synthesis was carried out using a reverse micellar system. The size and composition of the nanoparticles were determined, and their tribological properties as lubricant additives were compared with those of commercially available additives. It was found that the surface‐capped molybdenum sulphide nanoparticles show good friction‐modifying and anti‐scuffing behaviour.     

Surface forces, surface chemistry and tribology

Scanning probe methods have been applied to the investigation of tribological phenomena on the nanometre and nanonewton scale. The systems studied have included parallel investigation of identical tribosystems on the macro and nano scales, where the inherent differences in the AFM/LFM and flat-on-disk experiments have been compared; oxide-covered surfaces in contact under electrolytes, where the adhesion hysteresis and frictional behaviour was shown to be strongly dependent on the solution pH; and polymer surfaces, where advantage can be taken of variations in the interactions between the scanning tip and different polymers, to perform chemically sensitive, high-resolution surface imaging of polymer blends.     

摩擦学设计主要是摩擦学系统的设计

在一般意义下的讨论了摩擦学设计,涉及 摩擦学设计的目标和特征。强调摩擦学设计不能仅仅被理解为理解为摩擦副的设计,更能被理解为摩擦,磨损或润滑的设计。介绍了国内在转子－轴承－密－齿轮系统、推力轴承系统、缸套－活塞－活塞环系统、主动磁轴承系统和摩擦学机敏材料系统设计方面所做的一些工作。国家自然科学基金委员会对这些 强有力的经费支持。     

离子渗硫模具钢的摩擦磨损机理研究

对模具钢5CrNiMo进行低温离子渗硫处理，表现得到了厚约10μm的渗硫层。在MM－200型摩擦磨损试验机上进行了渗硫层干摩擦条件下的摩擦学试验。利用原子力显微镜（AFM）和扫描电镜（SEM）观察了渗硫层的表面、截面与磨痕形貌；利用X射线衍射仪（XRD）分析了渗硫层的相结构；利用扫描俄歇微探针（SAM）分析了表面元素沿深度的分布。渗硫层表面多孔且不平，由纳米尺度的微粒随机叠嵌而成。渗硫层的纳米结构有利于改善其减摩耐磨性能。摩擦磨损结果表明，模具钢渗硫层的摩擦学性能明显优于未渗表面。     

Benzimidazolyl phosphates as anti‐wear additives in poly(ethylene glycol) for steel/steel contacts

Four novel benzimidazolyl phosphates (BPs) were synthesized and evaluated as anti‐wear additives in poly(ethylene glycol) for steel/steel contacts. The friction experiments were carried out on an Optimol SRV‐I oscillating reciprocating friction and wear tester (SRV) both at room temperature and high temperature. The worn surfaces of the steel discs were analysed by JSM‐5600LV scanning electron microscope and PHI‐5702 multifunctional X‐ray photoelectron spectrometer. It was indicated that poly(ethylene glycol) with 2 wt% BP additives had better friction‐reducing and anti‐wear properties than the commercial lubricant additive, tricresyl phosphate. Excellent tribological performance of BPs could be ascribed to the formation of high quality boundary films that consisted of the ordered adsorption films and tribo‐chemical reaction films. Copyright © 2013 John Wiley & Sons, Ltd.     

基于神经网络泛函数的摩擦学系统转化模型研究

建立了摩擦学特性影响规律的径向基神经网络模型，可以较准确地计算和预测不同摩擦学系统条件下摩擦系数与各影响参数之间的关系；进一步基于神经网络建立了不同系统之间的泛函数关系，基本解决了摩擦学系统的条件转化，为摩擦学设计、程序化计算和分析提供了一种方便有效的工具。     

Dry sliding tribology of dioctyldithiophosphoric acid deposited on steel at 150 °C

Dioctyldithiophosphoric acid was deposited on 7 nm (RMS) rough steel surfaces at 150 °C to create a densely packed (area-95%) deposit of glassy phosphate islands of maximum 500 nm height. This standard substrate was subjected to dry lubricated ball-on-disc tribology using a 2 mm diameter steel ball in the 25–500 mN normal load range at a sliding velocity of 1 mm/s. The experiments recorded friction coefficient of the order of 0.18 but the life of the film as it wears out with sliding distance was found to decline with increasing load. The life–load characteristics that we record in these experiments, we argue, can only be explained if there is a load induced growth of the deposited film during sliding. We record this load induced growth to yield phosphate islands which are significantly smaller but harder than those grown by thermal activation alone.     

The tribology of plastics — an overview

With the widening use of plastics, it has become an urgent task to study their behaviour in friction systems. The present paper reviews the applications of plastics during the past decades and their function in friction systems. The paper analyses the basic differences between metals and plastics in structure and properties, and points out the limitations, or inapplicability, of using metal tribology to solve problems in plastic friction systems when even different kinds of high-molecular materials have different shapes and applications. The paper also looks at the variety of plastic surfaces, and briefly presents the mechanism, theory, and calculation of the fiction, wear, and lubrication of plastics; theories of plastic wear are summarised. Also presented are the technical functions, simple application techniques, and economic benefits of plastic friction systems.     

轮胎摩擦学的研究与发展

介绍了轮胎摩擦学的基本知识,并对其发展现状作了综合、归纳、分析与评述,强调应建立轮胎的弹性响应模型、准确的摩擦系数模型和热模型。指出轮胎和道路间的润滑机理及其影响因素,优化轮胎结构和材料的机敏化,改进摩擦学设计和模拟试验等,都应作为今后轮胎摩擦学系统的研究重点。     

Aerospace mechanisms and tribology technology: Case study

The success of many tribological products used or tribological systems operated in vacuum depends on adequate control of adhesion between two or more materials. Adhesion (e.g., in adhesive bonding) is the mechanical force or strength required to separate the surfaces in contact. This foreword is concerned with adhesion, which has greatly contributed, and should continue to contribute, to tribological problems, such as high friction, solid-state welding, scuffing or scoring, high wear, and a short lubricant life, in vacuum.     

Mechanical properties of tribologically modified nanolayers

Wear in modern mechanical systems is characterised by ultra‐low wear rates in the range of a few nanometres per hour. The surfaces of the materials involved, as well as their volumes, experience modifications as regards morphology, crystalline arrangement, and chemical bonds. The depth of the tribological interactions under given conditions extends to a micrometre or less. To probe these thin modified zones, synchrotron‐based X‐ray diffraction and nano‐indentation were applied. The specimens were taken from tribometer experiments employing the radionuclide technique to measure ultra‐low wear rates. To obtain different degrees of tribological interaction, various loads and sliding velocities were applied. The specimens were subdivided according to the total power dissipated during the tribometer experiment.