Investigation of transfer phenomenon by X-ray photoelectron spectroscopy and tribological properties of polymers sliding against polymers

Tribological properties of polymeric materials were investigated with various polymer-polymer combinations by means of a pin-on-disk wear testing apparatus. The specific wear rate was related to the cohesive energy density of the polymeric materials, but hat of a given polymer slider was dependent on the mated polymeric materials, and a high wear rate was observed in the sliding against a polymer counterface with higher cohesive energy than that of the slider. By means of X-ray photoelectron spectroscopy it was confirmed that the polymer transfer did occur on the mated polymer, and the degree of covering of the track with transfer films could be estimated. The covering ratio with transfer films was dependent on the polymer-polymer combinations. Poly(tetrafluoroethylene) (PTFE) transfer film on various polymers was very effective in reducing friction irrespective of the covering with the transfer films. With the other polymers, high density poly(ethylene) (HDPE), poly(propylene) (PP), and polyacetal (PAc), the transfer film was less effective in reducing friction than PTFE transfers and friction in the steady state dependent on the polymer-polymer combinations.     

Small amplitude reciprocating wear performance of diamond-like carbon films: dependence of film composition and counterface material

Small amplitude (50 μm) reciprocating wear of hydrogen-containing diamond-like carbon (DLC) films of different compositions has been examined against silicon nitride and polymethyl-methacrylate (PMMA) counter-surfaces, and compared with the performance of an uncoated steel substrate. Three films were studied: a DLC film of conventional composition, a fluorine-containing DLC film (F-DLC), and silicon-containing DLC film. The films were deposited on steel substrates from plasmas of organic precursor gases using the Plasma Immersion Ion Implantation and Deposition (PIIID) process, which allows for the non-line-of-sight deposition of films with tailored compositions. The amplitude of the resistive frictional force during the reciprocating wear experiments was monitored in situ, and the magnitude of film damage due to wear was evaluated using optical microscopy, optical profilometry, and atomic force microscopy. Wear debris was analyzed using scanning electron microscopy and energy dispersive spectroscopy. In terms of friction, the DLC and silicon-containing DLC films performed exceptionally well, showing friction coefficients less than 0.1 for both PMMA and silicon nitride counter-surfaces. DLC and silicon-containing DLC films also showed significant reductions in transfer of PMMA compared with the uncoated steel. The softer F-DLC film performed similarly well against PMMA, but against silicon nitride, friction displayed nearly periodic variations indicative of cyclic adhesion and release of worn film material during the wear process. The results demonstrate that the PIIID films achieve the well-known advantageous performance of other DLC films, and furthermore that the film performance can be significantly affected by the addition of dopants. In addition to the well-established reduction of friction and wear that DLC films generally provide, we show here that another property, low adhesiveness with PMMA, is another significant benefit in the use of DLC films.     

Wear-Resistant PTFE/SiO2 Nanoparticle Composite Films

Polytetrafluoroethylene (PTFE) is a polymer that is well known for its exceptional tribological properties and, as such, it is commonly used to reduce the coefficient of friction between surfaces. In recent years it has also been established that by incorporating nanoparticle fillers in PTFE, it is possible to extend the polymer's life by reducing its wear rate. Although much study has been placed on bulk PTFE, very little study has been focused on thin films. This article demonstrates that SiO ₂ nanoparticles can be used as a filler to significantly reduce the wear of PTFE thin films while also maintaining a low coefficient of friction. The wear resistance and coefficient of friction of PTFE/SiO ₂ composite films on stainless steel substrates were tested using a linear reciprocating tribometer and compared to pure PTFE films and bare stainless steel to evaluate the benefit of incorporating the SiO ₂ filler in the film. The composite films showed a significant improvement in wear resistance when compared to pure PTFE films. The coefficient of friction for the composite film remained low and stable during a 50 g normal load friction test for a duration of approximately 300 cycles, whereas that of PTFE showed an increasing trend at onset. In addition, of 1.7 and 3.3 wt% SiO ₂ concentrations in solution, 3.3 wt% SiO ₂ showed better performance, with a much higher wear resistance than that of 1.7% SiO ₂ after being subjected to a 1,000-cycle abrasive wear test.     

工艺条件对类金刚石薄膜在不同介质环境下摩擦学性能的影响研究

考察了基底负偏压对类金刚石薄膜(DLC)在无水和有水环境下摩擦性能的影响。利用电子回旋共振等离子体化学气相方法沉积制备DLC薄膜,利用激光拉曼(Raman)、原子力显微镜(AFM)和纳米硬度计表征了其结构特征,用UMT型多功能摩擦磨损实验机考察了其摩擦性能,并用光学显微镜分析了磨痕特征。结果表明:随着基底偏压的增加,表面粗糙度减小;在无水条件下,基底偏压较低的DLC薄膜摩擦因数较高,并存在一定的波动性,基底偏压较高时,摩擦因数较低。在有水条件下,基底偏压对摩擦因数影响不大。总体来说,加水后薄膜磨损较为严重。     

The tribological behaviour of glass filled polytetrafluoroethylene

Polymers and polymer composites are steadily gaining ground over metals in the field of engineering applications in tribology. Laboratory wear tests were carried out under ambient temperatures with no lubricant as well as in distilled water at an average sliding velocity of 0.2 m/s and contact pressures of 2.6–6.4 MPa. Three forms of glass viz. glass fibres, glass beads and glass flakes, each with a content of 25% weight were used in this study. Both hollow and solid glass beads were used. The sliding wear of the different glass filled PTFE composites was dependent on their ability to form transfer films on the counterface. The glass bead filled PTFE showed comparatively thicker films and higher wear rates than other forms of glass filled grades. The glass fibres and solid glass beads showed the lowest wear whilst hollow beads showed the highest under both low and high pressures due to crumbling and crushing of the beads during the sliding process. The glass flake filled PTFE showed relatively high but stable wear results up to 4.5 MPa above which the wear rate increased dramatically. A marginal increase in wear was achieved by using high aspect ratio glass fibres to the PTFE matrix. No correlation between the size of glass reinforcement and wear rate was established. The addition of a lamellar solid lubricant to the glass fibres reduced both the wear and friction of PTFE. The study of the transfer film growth by means of an optical microscope revealed that it was due to the mechanical interlocking of the polymer fragments into the metal asperity valleys. The compositional changes in the transfer film were studied by XPS which, among other things, showed presence of metal fluoride on the metal counterface.     

Lubrication of carbon fibre-reinforced polymers part I—Water and aqueous solutions

An examination has been made of the friction and wear properties of carbon fibre-reinforced polymers sliding against metals in water, sea water and other aqueous solutions. The conditions of sliding were chosen to minimize hydrodynamic effects. All the fluids inhibit the formation of transfer films of carbon/polymer debris on the counterface and the wear rates are generally greater than those obtained in dry conditions. The topography and type of counterface play a major role in the wear process and corrosion by the fluid is also important. Type II fibre composites generally exhibit lower wear than those containing Type I fibre, and compare favourably with existing materials. Additions of small amounts of finely divided abrasive to carbon fibre composites are very effective in reducing wear in water. Wear rates in sea water and other aqueous solutions on some counterfaces can be significantly lower than in pure water.     

Carbon Nanotube Reinforced Polyimide Thin-film for High Wear Durability

In this paper, the influence of single walled carbon nano tubes (SWCNTs) addition on the tribological properties of the polyimide (PI) films on silicon substrate was studied. PI films, with and without SWCNTs, were spin coated onto the Si surface. Coefficient of friction and wear durability were characterized using a ball-on-disk tribometer by employing a 4 mm diameter Si₃N₄ ball sliding against the film, at a contact pressure of ∼370 MPa, and a sliding velocity of 0.042 ms⁻¹. Water contact angle, AFM topography, and nano-indentation tests were conducted to study the physical and mechanical properties of the films. SWCNTs marginally increased the water contact angle of PI film. The addition of SWCNTs to PI has increased the hardness and elastic modulus of pristine PI films by 60–70%. The coefficient of friction of PI films increased slightly (∼20%) after the addition of SWCNTs, whereas, there was at least two-fold increase in the wear life of the film based on the film failure condition of coefficient of friction higher than 0.3. However, the film did not show any sign of wear even after 100,000 cycles of rotation indicating its robustness. This increase in the wear durability due to the addition of the SWCNTs is believed to be because of the improvement in the load-bearing capacity of the composite film and sliding induced microstructural changes of the composite film.     

Measurement of Friction Surface and Wear Rate between a Carbon Graphite Brush and a Copper Ring

A pin-on-disc surface friction tester designed by the authors was used to measure the surface friction temperature of a carbon graphite brush sliding against a copper ring under the condition of no flow by electric current. Microelectromechanical systems (MEMS) fabrication techniques including sputtering, lithography, and etching processes were utilized to fabricate metal film temperature sensors on the surface of the carbon graphite brush. The true temperature measurement system of the friction surface of the carbon graphite brush was then established. A USB-DAQ system and LabVIEW software were adopted for the data acquisition/management of analog signals. In this research, the relationship between friction temperature and wear rate was investigated using experiment data. Optical and scanning electron microscope (SEM) images of the friction surface of the carbon graphite brush were taken and analyzed to study the effect of temperature on the friction surface. From this experiment, it was found that the debris of a mixture of brush and copper particles softens under high temperature, adheres to the brush surface, and forms flattened areas. These smooth, flattened areas in turn lower the friction coefficient.     

Friction and Wear of Various DLC Films in Water and Air Environments

Three types of diamond-like carbon (DLC) films, pure DLC, F-containing DLC, and a Si-containing DLC film, were deposited on a WC–Co substrate by a plasma-enhanced CVD technique. Friction and wear properties were determined using a ball-on-plate type reciprocating friction tester in water, comparing the water results to those in ambient air. The friction coefficient of DLC and F–DLC films in water was considerably lower than that in air. With Si–DLC, the friction was almost the same level in both water and air, and was less than 0.1. The specific wear rate of films in water was much smaller than that in air and varied around the low level of 10^–8 mm³/Nm in water, The mating ball wear was also less than 10^–8 mm³/Nm. With DLC and F–DLC films, the transferred amount of material on the friction surface of a mating ball was larger in a water environment than that in air. With a Si–DLC film, the difference in the transferred amount when exposed to either the water or air environment was negligible.     

载流下MoS2/Ag纳米复合薄膜摩擦学性能

MoS₂基纳米复合薄膜具有良好的摩擦学性能,但较差的导电性能限制了其在载流条件下作为润滑材料的应用。为提高MoS₂基纳米复合薄膜的导电性能,采用非平衡磁控溅射系统沉积2种不同Ag含量的MoS₂/Ag纳米复合薄膜,并在不同的电流条件下研究MoS₂/Ag纳米复合薄膜与GCr15钢球对摩时的摩擦学性能。结果表明：在载流下2种MoS₂/Ag纳米复合薄膜表现出相似的摩擦性能,而低掺杂MoS₂/Ag薄膜具有更佳的耐磨性能,这归因于低掺杂MoS₂/Ag薄膜具有较好的力学性能;无载流时,MoS₂/Ag纳米复合薄膜在摩擦过程中生成的氧化物颗粒增加了磨损、降低了润滑性,磨损机制主要为磨粒磨损;电流小于0.5 A时,电流促进了转移膜形成,使得摩擦因数降低,但磨损率增加,磨损机制主要为黏着磨损;当电流大于0.5 A时,由于电弧烧蚀加速了薄膜的磨损,磨损机制主要为磨粒磨损、黏着磨损和电弧腐蚀磨损。     

Friction and Wear Behaviors of Several Polymers Sliding Against GCr15 and 316 Steel Under the Lubrication of Sea Water

Ocean tribology, a new research field of tribology, is currently being established and developed. The tribological behaviors of polyether ether ketone (PEEK), poly(phenyl p-hydroxybenzoate) (PHBA), polyimide (PI), and perfluoroethylene propylene copolymer (FEP) sliding against GCr15 and 316 steel rings under the lubrication of sea water were studied and compared with that under the lubrication of pure water. The results show that the friction and wear behaviors of a polymer under the lubrication of aqueous medium are not only related to the properties of polymer itself, but also to the corrosive effect and lubricating effect of the medium. When a polymer slid against GCr15 steel under sea water lubrication, the friction coefficient and wear rate of polymer were much larger than that under pure water lubrication because of indirect corrosive wear. However, when sliding against corrosion-resistant 316 steel, polymers PEEK, FEP, and PI exhibited lower coefficients of friction and wear rates under sea water lubrication, this was attributed to better lubricating effect of sea water as a result of the deposition of CaCO₃ and Mg(OH)₂ on the counterface. On the contrary, the friction coefficient and wear rate of PHBA sliding against 316 steel under sea water lubrication were larger than that under pure water lubrication, which may be related to the properties of PHBA itself.     

Dependence of the friction process on the molecular structure and architecture of thin polymer films

The frictional properties of a homologous series of poly(n-alkyl methacrylates) (PnAMA) and a series of poly(methyl methacrylate) (PMMA) films, cast from a variety of solvents, are characterized. The choice of polymer film was driven by the consideration of the possible mechanisms for the accommodation of a macroscopically applied shear stress by molecular entities. Two possible mechanisms are proposed: (i) the relative flexibility of the polymer backbone chain. For this purpose the PnAMAs have been chosen. By varying the length of the substituent chain, the relative molecular freedom around the backbone chain is altered. These molecular differences are sensed in the frictional properties at the macroscopic level, and (ii) the molecular organization is also proposed to be a factor in determining the friction response of a particular polymer film. For this purpose, the frictional properties of PMMA films cast from different solvents are investigated. There is observed to be a strong influence of the molecular organization on the frictional properties of the solvent cast PMMA films. The molecular probe employed to characterize the molecular environment is vibrational spectroscopy. Conformationally sensitive vibrational modes are used to determine the relative flexibility of the backbone chain and the organization of the chain network.     

一种新型摩擦材料在水润滑下的摩擦学性能研究

采用环块式摩擦磨损实验研究了一种新型摩擦材料在水润滑状态下不同载荷与转速对试样摩擦学性能的影响,并对比干摩擦条件下的摩擦学性能变化,借助磨损表面形貌观察分析其磨损机理。实验结果表明:水润滑条件下,摩擦系数随着载荷的增大而减小,随着转速的提高先增加后减小;磨损率随着载荷与转速的提高都减小。相同载荷与转速下,干摩擦时磨损机理以磨粒磨损和黏着磨损为主,而水润滑条件下水形成边界润滑,磨损机理以磨粒磨损和轻微的黏着磨损为主;水润滑条件下摩擦系数和磨损率均低于干摩擦,主要是由于水起到了润滑和冷却的作用,阻止了转移膜的形成,并在材料表面形成水膜起到了边界润滑的作用。     

Tribological Properties and Thermal Stability of Various Types of Polyimide Films

Thermal exposure experiments at 315° and 350°C were conducted on seven different types of polyimide film to determine which was the most thermally stable and adherent. The polyimides were ranked according to the rate of which they lost weight and how well they adhered to the metallic substrate. Friction and wear experiments were conducted at 25°C (room temperature) on films bonded to 440C HT stainless steel. Friction, film wear rates, wear mechanists, and transfer films of the seven films were investigated and compared. The polyimides were found to fall into two groups as far as friction and wear properties were concerned. Group I had lower friction but an order of magnitude higher film wear rate than did group II. The wear mechanism was predominately adhesive, but the size of the wear particles was larger for group I polyimides.     

硼酸酯与聚醚复配水溶液的摩擦学性能研究

水的润滑性能较差,因此需要选取性能良好的水溶性添加剂对其摩擦性能进行改善。研究选取椰油酰胺聚氧乙烯醚(CPOE)和三乙醇胺硼酸酯(BN)作为添加剂,分别对CPOE和CPOE-BN水溶液的摩擦学性能进行研究。使用NGY-6纳米级膜厚测量仪测试两种溶液的成膜性能,结果表明两种溶液在低浓度下均具有良好的成膜能力。利用四球试验机对两种溶液的减摩抗磨性能、稳定性能和极压性能进行了研究,结果表明两种溶液在低浓度下均具有良好且稳定的减摩抗磨性能,BN的加入可以大大提高CPOE水溶液的减摩性能和高载荷下的抗磨性能。利用XPS技术对CPOE-BN磨痕表面的化学元素进行了分析,结果表明在磨痕的表面生成了氮化硼、硼化铁等化合物,起到了良好的减摩抗磨效果。研究表明,CPOE和BN具有良好的协同润滑效果,是一种具有良好工业应用前景的绿色水基润滑添加剂。     

Tribochemical effects on the friction and wear behaviors of a-C:H and a-C films in different environment

Friction and wear behaviors of hydrogenated amorphous carbon (a-C:H) and hydrogen-free amorphous carbon (a-C) films sliding against Si₃N₄ balls were investigated in different testing environments. The result showed that two films with extreme chemical disparity (one hydrogenated, and the other hydrogen free) showed distinct different friction and wear behaviors, and the friction and wear behaviors of the both films were strongly dependent on the environment. For a-C:H films, much low friction coefficient and wear rate were obtain in dry N₂. In the water and/or oxygen containing environments, the friction coefficient and wear rate of a-C:H films were obviously increased. On the contrary, a-C films only provided low friction coefficient and wear rate in the presence of water and/or oxygen in the test chamber. In dry N₂, the highest friction coefficient and wear rate were observed for a-C films. By investigating the worn surfaces of the films using XPS, it was proposed that the environment dependence of the friction and wear behaviors of the films was closely related with the friction-induced chemical interactions between the films and water and/or oxygen molecules. The specific roles of hydrogen, water and oxygen molecules and their tribochemical effects on the friction and wear mechanism of the films are discussed.     

Friction and Wear Performance of Boron Doped,Undoped Microcrystalline and Fine Grained Composite Diamond Films

Chemical vapor deposition(CVD) diamond films have attracted more attentions due to their excellent mechanical properties. Whereas as-fabricated traditional diamond films in the previous studies don’t have enough adhesion or surface smoothness, which seriously impact their friction and wear performance, and thus limit their applications under extremely harsh conditions. A boron doped, undoped microcrystalline and fine grained composite diamond(BD-UM-FGCD) film is fabricated by a three-step method adopting hot filament CVD(HFCVD) method in the present study, presenting outstanding comprehensive performance, including the good adhesion between the substrate and the underlying boron doped diamond(BDD) layer, the extremely high hardness of the middle undoped microcrystalline diamond(UMCD) layer, as well as the low surface roughness and favorable polished convenience of the surface fine grained diamond(FGD) layer. The friction and wear behavior of this composite film sliding against low-carbon steel and silicon nitride balls are studied on a ball-on-plate rotational friction tester. Besides, its wear rate is further evaluated under a severer condition using an inner-hole polishing apparatus, with low-carbon steel wire as the counterpart. The test results show that the BD-UM-FGCD film performs very small friction coefficient and great friction behavior owing to its high surface smoothness, and meanwhile it also has excellent wear resistance because of the relatively high hardness of the surface FGD film and the extremely high hardness of the middle UMCD film. Moreover, under the industrial conditions for producing low-carbon steel wires, this composite film can sufficiently prolong the working lifetime of the drawing dies and improve their application effects. This research develops a novel composite diamond films owning great comprehensive properties, which have great potentials as protecting coatings on working surfaces of the wear-resistant and anti-frictional components.     

Effect of Molecular Structure on Friction and Wear of Polymer Thin Films Deposited on Si Surface

In this study, the influence of the molecular structure (linear or with bulky side groups) of polymer films covalently attached to Si surface on tribological properties is investigated. Two polymers, PE (polyethylene) and PS (polystyrene), are selected where PE has simple linear molecular structure whereas PS has linear molecular structure but contains bulky benzene groups located at the sides of the linear chain. PE and PS molecules, both with reactive maleic anhydride groups, are chemisorbed onto Si via an intermediate APTMS SAM (3-aminopropyltrimethoxysilane self-assembled monolayer). Water contact angle measurements, AFM (atomic force microscopy), ellipsometry, and XPS (X-ray photoelectron spectroscopy) are used to identify and characterize the polymer films. Tribological properties are studied using a microtribometer where a 4 mm diameter Si₃N₄ ball is used as the counterface. Among the two polymer films investigated, Si/APTMS/PE has shown very low coefficient of friction (0.08) and high wear life (∼4,400 cycles) than those of Si/APTMS/PS. Surprisingly, Si/APTMS/PS did not show any improvement in tribological properties when compared to that of bare Si. The present study proves that the polymer with linear molecular structure without the bulky side groups show good tribological properties even when it is coated as a thin film and hence such polymers can be used as thin-films for reducing friction and wear of substrates such as Si or other materials.     

Investigation of the Tribological Behaviors of Polymer Molecular Deposition Films by Tribometer Based on Interferometer

The polymer molecular deposition films including polyelectrolyte molecular deposition (PEMD) film and nanoparticles composite molecular deposition (NPs/MD) film have been prepared using the molecular deposition method and the in situ synthesize method. The polymer molecular deposition films were characterized by atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The tribological behaviors of the substrate and polymer molecular deposition films were investigated by a tribometer based on interferometer. It is found that the NPs/MD film has a lower friction force and a better anti-wear property than the PEMD film under the dry friction. The poly alpha olefin (PAO2) and water films confined between samples and steel ball surfaces have been investigated using thin film interferometry. The friction force of substrate was lower than the polymer molecular deposition films under PAO2 lubrication. The friction forces alteration of PEMD film and NPs/MD film were similar and consistent, and lower than that for substrate under water lubrication.     

Changes in the mechanical properties of oil used in a gasoline engine

This study is concerned with the changes in and deterioration of the mechanical properties of oil used in a gasoline engine. The properties analysed were friction and antiwear performance, wear debris, load-carrying ability and the formation of surface films. It was found that the oil run in an engine deteriorated so as to increase the wear and friction and decrease the load-carrying ability as the running distance of oil was increased. The main cause of deterioration was related to the ability to form a protective film in the contact zone. When the film was produced by additives (sulphur), this could properly protect the surface in the contact zone undepleted from wear and friction. But as the oil deteriorated, it could not form such a film and so its protective ability on sliding surfaces diminished.