Characterization and tribological investigation of self-assembled lanthanum-based thin films on glass substrates

Rare-earth (RE) (lanthanum-based) thin films were prepared on hydroxylated glass substrates by a self-assembling process from specially formulated solution. Atomic force microscope (AFM) and X-ray photoelectron spectrometry (XPS) and scanning electron microscope (SEM) are used to characterize the thin films. The tribological properties of the as-prepared thin films sliding against a steel ball were evaluated on a friction and wear tester. The tribological experiment shows that the friction coefficient of glass substrate reduced from 0.85 to 0.13 after the formation of RE self-assembled film (SAM) on its surface. And the RE self-assembled film has longer wear life (2880 sliding pass). It is demonstrated that RE self-assembled film exhibited good wear resistant property. The superior friction reduction and wear life of RE films are attributed to good adhesion of the film to the substrate and special characteristic of the RE elements.     

Mechanical and Tribological Properties of Diamond-Like Carbon Films Deposited by Electron Cyclotron Resonance Microwave Plasma Chemical Vapor Deposition

Mechanical and tribological properties of diamond-like carbon (DLC) films deposited by electron cyclotron resonance microwave plasma chemical vapor deposition were analyzed by nanoindentation, nanoscratch and ball-on-disk sliding tests. As the results, hardness and residual stress which depended on the substrate bias voltage had combined effects on the scratch resistance of the films. In sliding friction tests, the transferred layer on the surface of the counterpart accounts for the decrease of friction coefficient with increasing sliding distance. Atomic force microscopic images of the DLC films and the counterpart Si₃N₄ ball surfaces indicate that the sliding friction process could be treated as a periodical scratching process with many indenters.     

Tribological Dependence of the High-Performance Ferrous-Based Coating on Different Coating Counterparts in Engine Oil

A ferrous-based coating with significant chromium was fabricated on aluminum alloy substrate using a plasma spray technique. The tribological performance of the as-fabricated ferrous-based coating sliding against different coatings including Cr, CrN, TiN, and diamond-like carbon (DLC) in an engine oil environment were comparatively studied. Results showed that the high hardness of the sprayed ferrous-based coating was achieved due to the dispersion strengthening effect of Cr₇C₃ phase embedded in the austenite matrix. The ferrous-based coating exhibited low friction coefficients when coupled with these four coating counterparts, which could be attributed to the boundary lubricating effect of engine oil. However, both friction and wear of the ferrous-based coating were different when sliding against these different coating counterparts, which might be closely related to the surface roughness, self-lubricating effect, and mechanical properties of the coupled coatings. Ferrous-based coating sliding against CrN and DLC coatings exhibited good tribological performance in engine oil. The best coating counterpart for the ferrous-based coating in an engine was DLC coating.     

Effect of the functional groups in ionic liquid molecules on the friction and wear behavior of aluminum alloy in lubricated aluminum-on-steel contact

Four imidazolium-based room temperature ionic liquids containing phosphonyl functional groups, i.e. 1-(3′-O,O-diethylphosphonyl-n-propyl)-3-alkylimidazolium tetrafluoroborates and hexafluorophosphates, were synthesized. The physical properties of the resulting synthetic products were evaluated, and their tribological behaviors as the lubricants for an aluminum-on-steel sliding system were evaluated on an oscillating friction and wear tester, with the emphasis being placed on the effect of the O,O-diethylphosphonyl groups in the ionic liquid molecules on the tribological behaviors. Thus the friction and tests were conducted at a frequency of 25 Hz, a sliding amplitude of 1 mm, and for a duration of 30 min. The worn aluminum surface was analyzed by means of scanning electron microscopy, energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy. It was found that the synthesized ionic liquids had better friction-reducing and anti-wear ability for the aluminum-on-steel system than their nonfunctionalized courterparts (1-ethyl-3-hexylimidazolium tetrafluoroborate, coded as L206, and 1-propyl-3-octylimidazolium hexafluorophosphate, coded as LP308). Especially, they had much better load-carrying capacity than L206 and LP308. The tribological behaviors of the synthetic lubricants were dependent on both the anions and the side-substituted alkyl chains attached to the imidazolium cations. Moreover, physical adsorption and complicated tribochemical reactions were involved during the sliding process of the Al-on-steel system under the lubrication of the synthetic functionalized ionic liquids, which led to the generation of physically adsorbing and chemically reacting films composed of five-member-ring complex compounds, metal fluorides, nitrogen oxide, and FePO₄ on the rubbed Al surface. Those physically adsorbing and chemically reacting films contributed to effectively decrease the friction and wear of the aluminum sliding against steel.     

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.     

Ionic Liquid Lubrication Effects on Ceramics in a Water Environment

Ionic liquids were studied to determine their effectiveness as boundary lubricant additives for water. The chemical and tribochemical reactions that govern their behavior were probed to understand lubrication mechanisms. Under water lubricated conditions, silicon nitride ceramics are characterized by a running-in period of high friction, during which time the surface is modified causing a dramatic decrease in friction and wear. Two mechanisms have been proposed to explain the friction and wear behavior. Si₃N₄ sliding against itself may result in tribochemical reactions that form a hydrated silicon oxide layer on the surface of the sliding contact. This film has been suggested to mediate friction and wear. Others have suggested that tribo-dissolution of SiO₂ results in an ultra smooth surface and after a running-in period of high wear, the lubrication mode becomes hydrodynamic. The goal of this study was to examine the effects that ionic liquids have on the friction and wear properties of Si₃N₄, in particular their effects on the running-in period. Tribological properties were evaluated using pin-on-disk and reciprocating tribometers. The tribological conditions of the tests were selected to produce mixed/hydrodynamic lubrication. The relative lubrication mode between mixed and hydrodynamic was controlled by the initial surface roughness. Solutions containing 2 wt% ionic liquids were produced for testing purposes. Chemical analysis of the sliding surfaces was accomplished with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The test specimens were 1 in diameter Si₃N₄ disks sliding against 1/4 in Si₃N₄ balls. The addition of ionic liquids to water resulted in dramatically reduced running-in periods for silicon nitride from thousands to the hundreds of cycles. Proposed mechanisms include the formation of BF_x and PF_x films on the surface and creation of an electric double layer of ionic liquid.     

Nano/Microtribological Properties of Ultrathin Functionalized Imidazolium Wear-Resistant Ionic Liquid Films on Single Crystal Silicon

Ionic liquids (ILs) are considered as a new kind of lubricant for micro/nanoelectromechanical system (M/NEMS) due to their excellent thermal and electrical conductivity. However, so far, only few reports have investigated the tribological behavior of molecular thin films of various ILs. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick film on a substrate is a critical step for their application in MEMS/NEMS devices. To this end, four kinds of ionic liquid carrying methyl, hydroxyl, nitrile, and carboxyl group were synthesized and these molecular thin films were prepared on single crystal silicon wafer by dip-coating method. Film thickness was determined by ellipsometric method. The chemical composition and morphology were characterized by the means of multi-technique X-ray photoelectron spectrometric analysis, and atomic force microscopic (AFM) analysis, respectively. The nano- and microtribological properties of the ionic liquid films were investigated. The morphologies of wear tracks of IL films were examined using a 3D non-contact interferometric microscope. The influence of temperature on friction and adhesion behavior at nanoscale, and the effect of sliding frequency and load on friction coefficient, load bearing capacity, and anti-wear durability at microscale were studied. Corresponding tribological mechanisms of IL films were investigated by AFM and ball-on-plane microtribotester. Friction reduction, adhesion resistance, and durability of IL films were dependent on their cation chemical structures, wettability, and ambient environment.     

Tribochemical interactions of Si-doped DLC film against steel in sliding contact

This study concerns the effects of tribochemical interactions at the interface of Si-DLC (silicon-doped diamond-like carbon) film and steel ball in sliding contact on tribological properties of the film. The Si-DLC film was over-coated on pure DLC coating by radio frequency plasma-assisted chemical vapor deposition (r.f. PACVD) with different Si concentration. Friction tests against steel ball using a reciprocating type tribotester were performed in ambient environment. X-Ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) were used to study the chemical characteristics and elemental composition of the films and mating balls after tests. Results showed a darkgray film consisting of carbon, oxygen and silicon on the worn steel ball surface with different thickness. On the contrary, such film was not observed on the surface of the ball slid against pure DLC coating. The oxidation of Si-DLC surface and steel ball was also found at particular regions of contact area. This demonstrates that tribochemical interactions occurred at the contact area of Si-DLC and steel ball during sliding to form a tribofilm (so called transfer film) on the ball specimen. While the pure DLC coating exhibited high coefficient of friction (∼0.06), the Si-DLC film showed a significant lower coefficient of friction (∼0.022) with the presence of tribofilm on mating ball surface. However, the Si-DLC film possesses a very high wear rate in comparison with the pure DLC. It was found that the tribochemical interactions strongly affected tribological properties of the Si-DLC film in sliding against steel.     

Polymer Jet Printing of SU-8 Micro-Dot Patterns on Si Surface: Optimization of Tribological Properties

Tribological properties of optimized SU-8 patterns (micro-dots with varying pitch) on Si (silicon) were evaluated using a ball-on-disk tribometer. Sliding tests on the patterns were conducted against a 2-mm diameter Si₃N₄ ball at varying normal loads and sliding velocities. It was observed that the pitch of the SU-8 pattern on Si substrate had a significant effect on the initial coefficient of friction and wear durability. Initial coefficient of friction studies have concluded that the SU-8 polymeric micro-dots improved the tribological properties by sharing the normal force and reducing the contact area. For the wear durability test, ultra-thin layer of perfluoropolyether was over-coated onto SU-8 micro-dot specimens, and the optimized pitch specimens have shown wear durability of more than 100,000 cycles at a normal load of 350 mN.     

铝掺杂类石墨薄膜的构筑及其摩擦学性能研究

利用磁控溅射技术在硅片表面上制备铝掺杂类石墨(Graphite-like carbon,GLC)薄膜,采用原子力显微镜、场发射扫描电镜、X射线光电子能谱仪、纳米压痕仪等考察铝掺杂对薄膜表面形貌、结构及其机械性能的影响规律;利用旋转的球一盘微摩擦试验机考察薄膜的摩擦行为.结果表明:铝的引入使GLC薄膜表面更加平整、致密;GLC薄膜的硬度和弹性模量随着掺铝量的增加而增加;低载低速时,薄膜摩擦因数随着铝含量的增加而增加,高载高速时,摩擦因数随铝掺入量的增加明显降低且更稳定.     

Surface Modification of Polyether Ether Ketone (PEEK) with a Thin Coating of UHMWPE for Better Tribological Properties

The effect of normal load and sliding speed on the tribological properties of a thin film of ultra-high-molecular-weight polyethylene (UHMWPE) coated onto a polyether ether ketone (PEEK) substrate sliding against a stainless steel ball in dry conditions are investigated. Wear tests are carried out with a ball-on-disc configuration to evaluate the tribological properties of the plasma-treated PEEK samples coated with UHMWPE film at different normal loads (5, 7, and 9 N) and linear speeds (0.1, 0.2, and 0.5 m/s). The coated samples exhibited a very low coefficient of friction of ～0.09 compared to that of uncoated PEEK samples, which showed a coefficient of friction of ～0.3.     

Preparation of functional ionic liquids and tribological investigation of their ultra-thin films

Novel ionic liquid materials based on 1-alkyl-4-[5-(alkylsulfanyl)-1,3,4-oxadiazol-2-yl] pyridinium tetrafluoroborate derivatives were synthesized. Ultra-thin ionic liquid films were prepared on silicon wafers by means of spin-coating and their tribological properties were investigated. The wear life increases when the chain length decreases. Under low load, they show good tribological properties which are closely related to the interaction between ionic liquid and substrate surface, especially the film of 1-ethyl-4-[5-(ethylsulfanyl)-1,3,4-oxadiazol-2-yl] pyridinium tetrafluoroborate, it has low friction coefficient and long wear life. So, this kind of ionic liquid can be potentially applied as ultra-thin lubricating coating.     

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.     

Comparison of the effects of ionic liquid and anti‐wear additives on the tribological properties of Cr―Mo―N coatings

Cr―Mo―N coating was prepared by magnetron sputtering, and its crystallinity and phase structure were analysed by X‐ray diffraction. The tribological properties of the coating separately lubricated with L‐P106 ionic liquid was compared with poly‐alpha‐olefin (PAO) and PAO‐containing MoDTP and ZnDTP additives with a ball‐on‐disc reciprocal friction and wear tester. It was found that the tribological properties of as‐prepared Cr―Mo―N coating vary with varying lubricant systems. Namely, the results indicated that the L‐P106 has better friction‐reducing and wear resistance properties than that of MoDTC and ZnDTP. The analyses indicated that Cr―Mo―N coatings and lubrication films can be considered as a solid–liquid duplex lubricating systems. Copyright © 2014 John Wiley & Sons, Ltd.     

The tribological properties of YBa2Cu3O7 films in ambient environment

In the present study, high-T_c superconducting thin YBa₂Cu₃O₇ films and polysilicon films were prepared to investigate the initial sliding friction properties using a ball-on-flat tribometer when samples were moved against a sapphire ball or a steel ball in ambient environment. The surface topography was measured with atomic force microscope (AFM). After five times testing, the experimental results indicate that the friction coefficient of YBa₂Cu₃O₇ films is lower than that of polysilicon films when sliding against a sapphire ball and almost the same when sliding against a steel ball. In particular, the initial friction of YBa₂Cu₃O₇ films is more stable when sliding against a sapphire ball. However, the initial friction of polysilicon films fluctuates during a cycle period when sliding against a sapphire ball. They are both stable when sliding against a steel ball. Although, the surface profile of the YBa₂Cu₃O₇ film is rough and can be seen to be rougher than the polysilicon film, but the friction coefficient of the YBa₂Cu₃O₇ film is lower than that of polysilicon film. Also, although the topography of YBa₂Cu₃O₇ films changes during friction, the friction coefficients are stable. This clearly shows that the initial sliding friction of YBa₂Cu₃O₇ films under microfriction is stable. The observation signifies YBCO film is a good film to prevent stick–slip motion in ambient environment. The wear properties of YBa₂Cu₃O₇ films suggest that the superconducting outgrowths (CuO) are loose and they can be easily removed.     

Indirect monitoring of the dynamics at the contact interfaces of a pair of abrading titanium specimens

The novel method of using continuous tribo-electrification variations to monitor the dynamic tribological properties between metal films has been applied successfully [Y.P. Chang, A novel method of using continuous tribo-electrification variations for monitoring the tribological properties between the pure metal films, Wear, 262 (2007) 411–423]. The method was shown to produce clear and strong signals, superior to monitoring continuous friction coefficient variations. However, the above method was only shown to be suitable for the tested material pairs that were studied. In this paper, the method was improved and applied to monitoring the dynamic tribological properties between titanium oxide (TiO₂) films in the friction process. The experiment was conducted on a purposed-designed friction tester with a suitable measuring system. In order to investigate the tribological property of titanium (Ti) sliding against Ti with TiO₂ films in detail, the continuous variations of electrical contact resistance and friction coefficient were measured for monitoring the onset of film rupture between the TiO₂ films and the chemical reactions between the interfaces. Wear loss was measured by an accuracy balance and scan electron microscopy was used to observe the microstructures and material transfer. The experiments demonstrated that the novel method of using electrical contact resistance variations has great potential for monitoring the dynamic tribological properties and the chemical reactions of titanium specimens.     

Crown-Type Ionic Liquids as Lubricants for Steel-on-Steel System

A series of novel imidazolium-based crown-type phosphate ionic liquid lubricants have been designed and synthesized. Because the anions of ionic liquids are organic phosphate, crown-type ionic liquids would not corrode steel. The crown-type ionic liquids exhibit better tribological properties than conventional lubricants, which were evaluated by a ball-on-flat type Optimol-SRV oscillating friction and wear tester. The chemical compositions of the boundary films generated on the steel worn surfaces were analyzed with the use of a scanning electron microscope and X-ray photoelectron spectrometer. The results indicate that crown-type ionic liquids would generate complex lubricating films, composing organometallic polymer, polyphosphate, FePO₄, Fe₃O₄, etc., lubricated under different loads to exhibit the friction reduction and anti-wear abilities.     

Effect of nitrogen atoms desorption on the friction of the CNx coating against Si3N4 ball in nitrogen gas

Phase transition of CNx coatings by sliding against a Si₃N₄ ball has been studied by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) to understand this super-low friction phenomena in N₂. A pin-on-disk type tribometer was constructed to determine the tribological properties of this coating when sliding against a Si₃N₄ ball in N₂. The analytical results by AES and XPS showed that the nitrogen atoms desorbed from the top layers of the coating, and that the layers changed to a graphite-like structure without nitrogen during a friction coefficient decrease to lower than 0.01. The structural transition of CNx is discussed in this paper.     

Ionic liquid lubrication of electrodeposited nickel–Si3N4 composite coatings

Nano-Si₃N₄ particles were electrodeposited with nickel on copper substrate from a Ni bath. The friction and wear properties of the Ni/Si₃N₄ composite coating were evaluated while being lubricated with several various oils using a ball-on-disk sliding tester. The morphologies of the worn surfaces were observed using a scanning electron microscope. The chemical states of several typical elements on the worn surfaces were examined by means of X-ray photoelectron spectroscopy (XPS). Results indicated that the electrodeposited Ni/Si₃N₄ composition coating had excellent tribological properties while being lubricated with the ionic liquid. This was partly attributed to the high hardness of the electrodeposited nickel composite coating containing nano-sized Si₃N₄ and the tribochemical reaction between the lubricant and the sliding surface.     

Tribological evaluation of α, -diimidazoliumalkylene hexafluorophosphate ionic liquid and benzotriazole as additive

Ionic liquids of α, -diimidazoliumalkylene hexafluorophosphate were synthesized. The tribological properties of the synthetic ionic liquid and the ionic liquid contained additive for contacts of steel/steel were investigated by Optimol SRV oscillating friction and wear tester under ambient conditions. The synthetic ionic liquid presented low friction coefficients and small wear volumes, especially under higher temperatures. The ionic liquid doped with benzotriazole (BTA) showed excellent anti-wear ability. The worn surfaces and chemical nature of the boundary films generated on the metal surfaces were analyzed by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). SEM results showed slight abrasion on the worn surfaces and XPS results indicated the formation of FeF₂, FeF₃, Fe₃O₄, and FePO₄ by the tribochemical reactions of ionic liquid with iron during the sliding process.