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.     

Tribological behaviours of PA/UHMWPE blend under dry and lubricating condition

Dry-sliding and lubricated friction and wear behaviours of polyamide (PA) and ultra-high molecular weight polyethylene (UHMWPE) blend were studied using a pin-on-disc method (polymer pin sliding against a stainless steel disc) at room environment. The tribological performance of PA and UHMWPE were also investigated for the purpose of comparison. The worn surfaces were examined using a scanning electron microscope (SEM) and optical microscope. It was observed that PA specimen demonstrated highest friction coefficient, UHMWPE the lowest in both dry-sliding and lubricated sliding test. The friction of PA could be sufficiently decreased by blending with UHMWPE. Statistical analysis suggested the relationship between the wear volume loss and the sliding distance could be expressed by a linear model for dry-sliding, while a logarithmic model was determined for lubricated sliding. The difference in wear modes between both sliding series suggested that there was change in the mode of material removal process. The lower wear rate in lubricated sliding was attributed to the elastohydrodynamic or partial elastohydrodynamic lubrication through the development of a continuous lubricant film between the polymer and the counterface, while the high wear rate of the specimens, in dry-sliding test, was mainly caused by fatigue process due to the repeated action of tearing and crack-propagation.     

Scuffing Performance of Amorphous Carbon During Dry-Sliding Contact

Scuffing is a major problem that limits the life and reliability of sliding tribo-components. When scuffing occurs, friction force rises sharply and is accompanied by an increase in noise and vibration; severe wear and plastic deformation also occur on the damaged surface. Attempts have been made over the years to combat scuffing by enhancing the surface properties of the machine elements, and by methods involving lubricant formulation and coating application.

In this study, the authors evaluated the scuffing performance of an amorphous, near-frictionless carbon (NFC) coating that provides super-low friction under dry sliding conditions. The test configuration used a ball-on-flat contact in reciprocating sliding. The coating was deposited on HI3 steel. An uncoated 52100 steel ball was tested against various coated flats in room air. Compared to uncoated surfaces, the carbon coating increased the scuffing resistance of the sliding surfaces by two orders of magnitude. Microscopic analysis shows that scuffing occurred on coaled surfaces only if the coating had been completely removed. It appears that depending on coating type, the authors observed that coating failure occurs before scuffing failure by one of two distinct mechanisms: the coating failed in a brittle manner and by spoiling, or by gradual wear.     

Tribological studies of epoxy and its composite coatings on steel in dry and lubricated sliding

Industrial lubricants are invariably used with additives (with high sulfur and phosphorous contents) for tribological performance enhancement. However, these additives are environmentally very harmful. Hence, there is an urgent need to find alternate solutions for enhancing the tribological performance of lubricants and components without the use of harmful additives. The objective of this work is to investigate the feasibility of using polymer composite coatings in enhancing the tribological properties of steel surfaces in dry and base oil lubricated conditions. Pure epoxy and its composite (with 10?wt-% of graphene or graphite powder) films were coated onto steel substrates and tested under dry and base oil lubricated conditions. Friction and wear experiments were conducted on a ball on cylinder tribometer between polymer/composite coated cylindrical steel surface (shaft) and an uncoated steel ball as the counterface. Tests were conducted at various normal loads and speeds. In dry condition at 3 N load and 0.63?m s^??1 sliding speed, the wear life of epoxy was increased by five times and coefficient of friction was nearly the same (0.18) on inclusion of graphene nanoparticle. In lubricated case, epoxy/graphene composite coating performed eight times and more than five times better than pure epoxy and epoxy/graphite respectively.     

Role of surface texture of harder surface on subsurface deformation

Plastic deformation and damage accumulation below the contact surfaces play an important role in sliding wear of ductile materials. In this study, metallographic techniques have been used to determine the extent of plastic deformation and strain localization events that occur at various depths beneath the worn surface in the subsurface zones of the ductile pins slid against harder plate with various surface textures. Results showed that the magnitude of plastic strain gradient and the depth of highly deformed zone correlate with coefficient of friction, which in turn depends on the surface texture of harder counterface, under both dry and lubricated conditions. It was also observed that the gradient of equivalent strain, as it approaches the worn surface, was higher under dry conditions when compared to that under lubricated conditions.     

Friction and Wear Behavior of Ultra-High Molecular Weight Polyethylene Sliding Against GCr15 Steel and Electroless Ni–P Alloy Coating Under the Lubrication of Seawater

The friction and wear behavior of ultra-high molecular weight polyethylene (UHMWPE) sliding against GCr15 steel and electroless Ni-P alloy coating under the lubrication of seawater was investigated and compared with that under dry sliding and lubrication of pure water and 3.5 wt.% NaCl solution, respectively. It was found that under the lubrication of aqueous medium, the friction and wear behavior of UHMWPE mainly depended on the corrosion of counterface and the lubricating effect of the medium. Because of serious corrosion of counterface by the medium, the wear rates of UHMWPE sliding against GCr15 under the lubrication of seawater and NaCl solution were much larger than that under other conditions, and such a kind of wear closely related to the corrosion of counterface can be reckoned as indirect corrosive wear. However, when sliding against corrosion-resistant Ni–P alloy under the lubrication of seawater, the lowest coefficient of friction and wear rate of UHMWPE were obtained, owing to superior lubricating effect of seawater. Moreover, periodic ripple patterns were observed on the worn surfaces of UHMWPE sliding against GCr15 under the lubrication of seawater and NaCl solution, which were ascribed to the intelligent reconstruction of surface microstructure of UHMWPE upon large plowing effect of the counterface asperities. Based on scanning electron microscopic (SEM) and three-dimensional (3D) profile analyses of the worn surfaces of UHMWPE, a stick–slip dynamic mechanism was proposed to illustrate the pattern abrasion of UHMWPE. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Influence of surface texture on boundary lubricated sliding contacts

The friction and wear behaviour of boundary lubricated sliding surfaces is influenced by the surface texture. By introducing controlled depressions and undulations in an otherwise flat surface, the tribological properties can be improved. Lubricant can then be supplied even inside the contact by the small reservoirs, resulting in a reduced friction and a prolonged lifetime of the tribological contact.In the present paper, well-defined surface textures were produced by lithography and anisotropic etching of silicon wafers. The wafers were subsequently PVD coated with thin wear resistant TiN or DLC coatings, retaining the substrate texture. The size and shape of the depressions were varied and evaluated in reciprocating sliding under dry and boundary lubricated conditions.     

Review of engineered tribological interfaces for improved boundary lubrication

Recent advances in smart surface engineering and coating technologies offer unique possibilities for better controlling friction and wear under boundary or marginally lubricated rolling, sliding or rotating contact conditions. Specifically, such coatings can be tailored to meet the increasingly multi-functional application needs of future engine systems by enabling them to operate in lower viscosity oils with reduced sulfur and phosphorous. Using these technologies, researchers have already pioneered the development of a variety of nano-composite and super-hard coatings providing longer tool life in demanding machining and manufacturing applications. The same technologies can also be used in the design and development of novel coating architectures providing lower friction and wear under boundary-lubricated sliding conditions. For example, such coatings can be tailored in a very special way that while one of the phases can favorably react with certain additives in engine oils to result in an ideal chemical boundary film; the other phases can provide super-hardness and hence resists wear and scuffing. Because of their very dense microstructure and high chemical inertness, these coatings can also provide superior protection against oxidation and corrosive attacks in aggressive environments. The use of solid lubricant coatings may also improve the tribological properties of sliding contact interfaces under boundary lubricated sliding conditions. When fluid and boundary films fails or is broken down, such coatings can carry the load and act as a back-up lubricant. Other smart surface technologies such as laser texturing and/or dimpling, laser-glazing and -shotpeening have also become very popular in recent years. In particular, laser texturing of control or coated surfaces have opened up new possibilities for further manipulation of the lubrication regimes in classical Stribeck diagrams. Controlling dimple size, shape, orientation, and density, researchers were able to modify both the width and the height of the boundary lubrication regimes and thus achieve lower friction and wear at sliding and rotating contact interfaces. Overall, smart surface engineering and coating technologies have matured over the years and they now become an integral part of advanced machining and manufacturing applications. They can also be used to meet the increasingly stringent and multi-functional application needs of demanding tribological applications. In this paper, selected examples of recently developed novel surface engineering and coating technologies are introduced, and the fundamental tribological mechanisms that control their friction and wear behavior under boundary lubrication regimes are presented.     

Effect of counterface on the tribology of UHMWPE in the presence of proteins

Artificial joints in orthopedics occupy a principal position owing to the increase in number of cases suffering from arthritis and associated diseases in addition to impairment caused by accidents. In this work, one of the most commonly used joint material, i.e. ultrahigh molecular weight polyethylene (UHMWPE), was tested against the duplex stainless steels instead of the conventional 316 L stainless steel. The UHMWPE was found to exhibit the lowest friction coefficient and wear rates when lubricated with water followed by globulin and glucose. The friction coefficient in the presence of egg albumen was higher along with high wear rates recorded. Post-test evaluation of surface roughness and wear scar/track analysis was performed to identify the wear mechanisms. Worn surfaces were analyzed using a differential scanning calorimeter for changes in crystallinity with sliding. The specimens tested under lubricated conditions with glucose, egg albumen and globulin indicated the presence of reaction products on the worn surface. Adhesive and corrosive wear mechanisms were the predominant modes of wear identified on the polymer samples. The wear tracks indicated that the proteins did react with the counterface material forming a thin deposit on them. Low temperature nitriding of the duplex stainless steel counterfaces were performed and the UHMWPE specimens were tested under similar conditions against the nitrided surfaces. Low temperature nitriding of the counterface did result in improved tribological behavior of UHMWPE and the corrosive effects were minimal.     

Friction and Wear Properties of Micro Textured DLC Coated Surfaces in Boundary Lubricated Sliding

In the present study, the friction and wear properties of boundary lubricated textured surfaces were investigated. The capability to feed lubricant into the interface of a sliding contact and to isolate wear particles was related to the shape, size and orientation of the texture patterns. Well-defined surface textures of square depressions or parallel grooves of different widths and distributions were produced by lithography and anisotropic etching of silicon wafers. Subsequently the wafers were PVD coated with thin, wear resistant DLC coatings, retaining the substrate texture. The surfaces were evaluated in reciprocating sliding against a ball-bearing-steel ball under starved or amply lubricated boundary lubrication conditions.     

Nanocomposite UHMWPE–CNT Polymer Coatings for Boundary Lubrication on Aluminium Substrates

The boundary lubrication regime plays a very important role in determining the life span of any of the two mating parts under liquid-lubricated conditions. It is during the start\stop cycles when insufficient fluid is available to fully separate the surfaces in relative motion and thus unusual wear takes place; a case of boundary lubrication. The aim of this work is to study the feasibility of using polymer coatings as boundary lubricants. This study investigates the friction and wear properties of ultra-high molecular weight polyethylene (UHMWPE) films coated on aluminium substrates under dry and base oil (without any additives)-lubricated conditions. In order to increase the load bearing capacity of the UHMWPE coatings, 0.1 wt% of single-walled carbon nanotubes are added. Experiments are carried out on a custom-built tribometer simulating a line contact between a polymer-coated cylindrical Al surface (shaft) and a flat uncoated Al plate as the counterface. The experimental parameters such as the normal load and the sliding speed are selected to simulate the boundary and mixed lubrication regimes for comparison purposes. Specific wear rates of the polymer films and bare Al surface under lubricated conditions are also calculated. Stribeck curves have been generated to evaluate the effectiveness of the pristine UHMWPE and the nanocomposite coatings in the various regimes of lubrication, especially the boundary lubrication regime. It is observed that the selected polymer coatings are effective in protecting the metallic surfaces without causing any observable oil contamination with wear debris.     

滑动速度对油润滑表面胶合磨损的影响

在油润滑条件下,钢对钢摩擦副的胶合摩损不仅取决于润滑油膜是否破裂,而且取决于在摩擦表面上化学反应膜的形成情况。本文研究了在油润滑条件下滑动速度对钢摩擦副胶合的影响。在低滑动速度下摩擦表面易于形成反应膜,油膜破裂后并不直接发生胶合。胶合发生在高温、高摩擦系数的恶劣条件下。在高滑动速度下油膜破裂后很容易发生胶合,发生胶合前的表面温度和摩擦系数都比较低。     

磨料粒度对表面微织构纯钛干摩擦性能的影响

为了提高钛及钛合金钻具在超深钻探、深海钻探和外太空钻探工程中的减摩抗磨性能。利用激光表面加工技术在工业纯钛（TA2）表面制备了不同参数的点阵微织构。采用MS-T3000摩擦磨损试验机测试了微织构钛合金在不同粒度模拟月壤作用下的摩擦学性能。利用扫描电子显微镜和能谱分析仪分析磨痕形貌及元素含量。研究结果表明：当磨料粒度小于微织构点阵的直径时,磨料压入微织构点阵里,磨料具有滚动和滑动两种运动方式。当粒度大于微织构点阵的直径时,磨料不能完全压入微织构点阵里,磨料对微织构TA2表面产生了滑动犁削作用。由于两种磨料磨损的作用机理不同,同等条件下,小粒度的磨料作用下的微织构TA2的摩擦因数和磨损率较大粒度磨粒作用下的最大减少量分别为50%和53%。考虑磨料粒度与微结构的匹配性,可以大大降低摩擦减少磨损。     

Friction,Wear, and Scuffing Characteristics of Marine Engine Lubricants with Nanodiamond Particles

Many kinds of additives are generally added to engine lubricants to improve performance. These chemical additives are harmful to both humans and the environment. For this reason, the research trend in the lubricant industry is to reduce the use of chemical additives in engine oils. Carbon materials like nanodiamonds are candidates among many physical additives. Nanodiamond particles are round, very hard, chemically stable, and highly heat conductible. In this research, nanodiamond particles were uniformly dispersed in marine engine lubricants. A matrix synthesis method was used for dispersion with various concentrations. Friction and wear tests were performed to measure the friction and wear amounts, and scuffing tests were performed. The friction coefficients were decreased with the addition of nanodiamond particles. Due to their octagonal and almost spherical shape, the particles could act as rolling contact elements between two lubricated sliding surfaces. In addition, it was found that there was a proper concentration of nanodiamond to minimize the wear amounts, which was 0.3 wt%. From the scanning electron microscopy (SEM) analysis many agglomerated particles were found on the sliding surfaces with a high concentration of particles over 0.3%. The excessive amount of nanodiamonds acted as abrasive debris and ploughed the contact surfaces. Finally, as the concentration of nanodiamonds increased, the scuffing life increased due to a reduction in friction, and the rate of temperature increase was reduced due to the high heat conductivity of nanodiamonds.     

Dry sliding friction and casing wear behavior of PCD reinforced WC matrix composites

The friction and casing wear properties of PCD reinforced WC matrix composites were investigated using a cylinder-on-ring wear-testing machine against N80 casing steel counterface under dry sliding conditions. The results indicate that the friction and casing wear rate of PCD reinforced WC matrix composites are the lowest among the materials. As the applied load and sliding speed steadily increase, the friction coefficients of PCD reinforced WC matrix composites decrease. In addition, the casing wear rates increase with increasing load, but decline with sliding velocity. The dominant wear mechanism of the PCD composite is the micro-cutting wear, accompanied by adhesive wear.     

Effects of anti-wear additives on friction and wear properties of Cr2O3 coating

The effects of some anti-wear additives on the friction and wear behaviour of plasma-sprayed Cr₂O₃ coating were investigated using a block-on-ring tester at ambient conditions. The results show that zinc dialkyldithiophosphate (ZDDP), tricesyl phosphate (TCP) and tributyl phosphate (TBP) significantly reduce the wear of Cr₂O₃ coating lubricated by paraffin oil. Additive concentrations as well as sliding time have great influence on the wear. The friction coefficient varies slightly with test conditions. The analysis by XPS of worn surfaces indicates that the wear resistance of these additives is due to the formation of tribochemical reaction films by reacting with Cr₂O₃ coatings.     

纳米SiO2对火焰喷涂尼龙1010涂层干摩擦磨损性能的影响

为了探讨纳米SiO2(n-SiO2)对火焰喷涂尼龙(PA)1010涂层干摩擦磨损性能的影响,采用MRH-3型环-块摩擦磨损试验机对不同n-SiO2含量的尼龙1010涂层的干摩擦磨损性能进行了测试;并利用扫描电子显微镜(SEM)对复合涂层的磨损表面进行观察,以探讨n-S iO2对火焰喷涂尼龙1010涂层摩擦磨损性能的影响机制。结果表明:n-SiO2的加入能明显提高尼龙涂层的耐磨性,降低摩擦因数,疲劳磨损、粘附磨损及犁切现象明显减轻;当n-SiO2含量为1.5%(质量分数)时,复合涂层摩擦磨损性能最佳,试验条件下磨损量降低近4倍,摩擦因数降低23%,跑合期降低44%,复合涂层与GCr15钢环对磨时的磨损机制主要为疲劳磨损和轻微的粘附磨损。     

The effects of contact configuration and coating morphology on the tribological behaviour of tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings under boundary lubrication

ABSTRACT

Tribological studies were carried out with tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings, varying in thickness and roughness, using two different contact configurations lubricated with seven types of hydraulic oils. Tribopair of cast iron and ta-C coated steel were tested in both non-conformal and conformal, unidirectional sliding contacts. The friction and wear results were mainly affected by the thickness of the coating in the non-conformal contact and the surface roughness of the coating in the conformal contact. Tests done with mineral base oil containing rust inhibitor in the non-conformal contact and with Polyalphaolefins and synthetic ester base oils in the conformal contact resulted in the lowest friction while that with mineral base oil containing zinc resulted in high friction and counterface wear. The results highlight the interdependence of contact configuration, lubricant chemistry, coating’s surface morphology and coating’s thickness in determining the tribological behaviour of ta-C coatings under boundary lubrication.     

Sliding wear behaviour of aluminum syntactic foam: A comparison with Al-10 wt% SiC composites

The sliding wear behaviour of cenosphere-filled aluminum syntactic foam (ASF) has been studied in comparison with that of 10 wt% SiC particle reinforced aluminum matrix composite (AMC) at a load of 3 kg and varying sliding speeds under dry and lubricated conditions using a pin-on disc test apparatus. The tribological responses such as the wear rate, the coefficient of friction and the frictional heating were investigated. The wear surfaces and subsurfaces were studied for understanding the wear mechanism. It was noted that the coefficient of friction, the wear rate, and the temperature rise for ASF are less than that for AMC in both dry and lubricated conditions. The craters (vis-à-vis exposed cenospheres) play an important role in the wear mechanism for ASF.     

接触应力和相对滑动速度对金属表面自修复膜生成的影响及机制

采用羟基硅酸镁粉体作为润滑油添加剂,在MMU-5G材料端面摩擦磨损试验机上,研究了不同接触应力和相对滑动速度对45^#钢/45^#钢摩擦副磨损表面自修复膜生成的影响及其机制,借助SEM及EDS测试分析摩擦副的表面形貌及表面成分组成。结果表明,接触应力和相对滑动速度对羟基硅酸镁粉体添加剂在磨损表面形成自修复膜影响显著。在接触应力为1.53,3.06,4.59,7.64MPa和在相对滑动速度0.416m/s的工况条件下,试样磨损表面有自修复膜生成。接触应力为3.06MPa和相对滑动速度为0.416m/s时,易于在短时间内达到磨损-自修复动态平衡,自修复效果最为理想。自修复膜的生成过程包含磨粒磨损和摩擦化学反应2个阶段。自修复膜的生成使得试样摩擦磨损表面平整光滑,可以有效降低金属磨损。