Effect of methane on magnetron sputtering graphite target deposited films and tribological properties of a-C:H:Ti/a-C:H friction pairs

In this study, we prepared a-C:H films with different nanostructures at different methane flow rates. The effect of the methane flow rate on the tribological properties of 440 steel/a-C:H friction pairs and a-C:H:Ti/a-C:H friction pairs in an atmospheric environment was studied by a reciprocating friction machine. The results show that there is no relationship between the tribological properties of 440 steel/a-C:H friction pair and methane flow rate. The tribological performance of the a-C:H:Ti/a-C:H friction pair was greatly improved. In particular, in the friction pair of a-C:H:Ti/a-C:H with a methane flow rate of 20 sccm, superlubricity is shown, and the wear rate is only 4.04 × 10⁻⁹ mm³/Nm. After tribological experiments, Raman spectroscopy, XPS, and other characterization methods were used to study the relationship between the nanostructure and tribological properties of a-C:H:Ti films and a-C:H prepared with different methane flow rates. This study is great significance to the application of a-C:H:Ti/a-C:H friction pair in mechanical parts under atmospheric environment.     

Effects of UV irradiation on tribological properties of nano‐TiO2 thin films

One‐layer and two‐layer nano‐TiO₂ thin films were prepared on the surface of common glass by sol–gel processing. Water contact angle, surface morphology, tribological properties of the films before and after ultraviolet (UV) irradiation were investigated using DSA100 drop shape analyzer, scanning probe microscopy (SPM), SEM and universal micro‐materials tester (second generation) (UMT‐2MT) friction and wear tester, respectively. The stored films markedly resumed their hydrophilicity after UV irradiation. But UV irradiation worsened tribological properties of the films. After the film was irradiated by UV, the friction coefficient between the film and GCr15 steel ball increased about 10–50% and its wear life shortened about 20–90%. Abrasive wear, brittle break and adherence wear are the failure mechanisms of nano‐TiO₂ thin films. It was believed that UV irradiation increased surface energy of the film and then aggravated adherence wear of the film at initial stage of friction process leading to severe brittle fracture and abrasive wear. Copyright © 2009 John Wiley & Sons, Ltd.     

The antioxygen radiation properties of the nanocomposite film consisted of hydrogenated amorphous carbon (a-C:H) and MoS2

MoS₂/a-C:H multilayer film and MoS₂/a-C:H composite film exhibit excellent tribological properties in vacuum, which can be used as the potential space lubricant. The radiation-protective properties of these two films in atomic oxygen (AO) are evaluated. The influences of AO radiation on structure, morphology, and tribological properties of the films were investigated. The results show that AO radiation mainly causes oxidation and increases sp² C content in both of the films. Furthermore, the MoS₂ sublayer on the surface of the multilayer film is oxidized heavily, whereas both the MoS₂ and the a-C:H matrix on the surface were oxidized in the composite film. As a result of this, the multilayer film exhibits high friction coefficient and short sliding lifetime in vacuum after AO radiation. Compared with that, the composite film exhibits lower friction and longer sliding time more than 3600 seconds in vacuum, which illustrates it has a good AO radiation protection. This indicates that MoS₂/a-C:H composite film is more likely to be used as a potential space lubricant.     

Investigation of transfer film of PTFE/bronze composites on 2024Al surface

Polytetrafluoroethylene (PTFE) composites filled with 10–30% volume content of bronze powder were prepared through molding and sintering process. Transfer films of these composites were prepared on surface of 2024 Al bar through friction method under certain condition. Roughness, morphology, andelement of these transfer films were investigated using scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) methods. Tribological propertiesof these transfer films sliding against GCr15 steel ball were tested using a DFPM reciprocating tribometer. Worn surfaces were observed and analyzed using SEM and EDS methods. It was found that uniformity and continuity of the transfer films were obviously improved by the increase of bronze content of the composites. Transfer films with better uniformity and continuity holds longer wear life. Considerably lower friction coefficient and longer wear life of these transfer films indicate that the transfer films prepared in the experiment could effectively prevent direct contact of metal friction pair and thus protect them from heavy wear. SEM and EDS analyses of the worn surfaces indicate that adhesion wear and fatigue wear were main wear modes of the transfer film. Copyright © 2009 John Wiley & Sons, Ltd.     

Tribological behaviors of DLC films in sulfuric acid and sodium hydroxide solutions

Tribological behaviors of three typical kinds of diamond-like carbon (DLC) films (a-C, a-C:Cr, and a-C:H) in sulfuric acid and sodium hydroxide solutions were investigated. The a-C film showed the lowest stable coefficients of friction (COF) in both sulfuric acid and sodium hydroxide solutions but the worst wear resistance in sulfuric acid solution. The a-C:H film showed the highest COF in sulfuric acid solution and the best wear resistance in both sulfuric acid and sodium hydroxide solutions. The a-C:Cr film exhibited superior comprehensive tribological performance in sulfuric acid solution, while in sodium hydroxide solution, high COF and very poor wear resistance was observed. What is more, friction and wear mechanism was revealed by investigating the friction-induced material evolutions on the sliding surface.     

Pencil sketch graphene films as solid lubricant on steel surface: Observation of transition to grapehene/amorphous carbon

During recent years, graphene as a solid lubrication material have been thoroughly studied under nano or micro scales, but rarely reported at industrial conditions. In present work, graphene films as solid lubricant were prepared on the surface of 201 stainless steel substrates by pencil sketch. And then the friction tests from 5 to 65 N were carried out via a homemade tribo-tester and used GCr15 balls (ø = 5 mm) as friction pairs. Not surprisingly, graphene films cannot bear the loads beyond 5 N, but interestingly, via gradually increasing the loads, graphene films show prominent load performance and steady state of friction coefficients at about 0.12 while the loads varied from 5 to 65 N. Compared with bare steel, the coefficient of graphene films reduced by about 80%, and the wear volume reduced to 1/28 when variable load (from 5 N to 30 N) were applied. Raman spectra shown that the structure of graphene had been changing into diamond-like carbon films with graphene distributed inside, which was confirmed by HRTEM that graphenes were coming with amorphous carbon. Considering the roughness of steel wafers (170 nm), one can speculate that, with graphene films' protection, the steel has no abrasion but plastic deformation instead. It is concluded that the shearing force induced the film densification via sp² to sp³ changing that enforced cross-linking. This cross-linking carbon matrix was responsible for high load bearing and the graphene exfoliated into graphene under shearing force contribute to low steady-state friction. Benefiting from sketch, one can get a lubrication film on any substrates with complex topography, our results shed light on the growth of graphene films for industrial use.     

Tribological properties of multilayer nanostructure TiO2 thin film doped by SiO2

Multilayer of TiO₂ and TiO₂:SiO₂ thin films were grown on a glass substrate by sol?Cgel processes, followed by high temperature treatment at 500?°C. The fine grained TiO₂ films controlled by SiO₂ dopant showed very good wear resistance and endurance life. Energy dispersive X-ray spectroscopy was used to indicate the elements in the films. X-ray diffraction analyses indicated that TiO₂ and TiO₂:SiO₂ film contain only anatase phase. The morphologies of the original and worn surfaces of the samples were analyzed by means of scanning tunneling microscope and scanning electron microscopy. The tribological properties of TiO₂ and TiO₂:SiO₂ thin films sliding against AISI52100 steel pin were evaluated on a pin on disk friction and wear tester. The results showed that 25-layer TiO₂:SiO₂ films are superior in reducing friction and resisting wear compared with the glass substrate.     

Effect of nitrogen and ammonia on transfer and deposition of hydrocarbon radicals in different regions of hollow-cathode glow discharge

The effect of addition of nitrogen or ammonia in an amount equal to the flow of methane entering as a 7: 1 H₂/CH₄ mixture into a hollow-cathode dc glow flow discharge on the rate of deposition/erosion of amorphous hydrocarbon (a-C:H) films at 300 K has been studied. The introduction N₂ or NH₃ into the mixture facilitates the transition from deposition to erosion of a-C(N):H films in the hollow cathode, but has a little effect on the growth rate of a-C(N):H films in the positive column and in the afterglow of the discharge. It has been suggested that the changes in the a-C:H film deposition/erosion rate are due to the formation of hydrogen cyanide, mainly, on the hollow-cathode surface.     

Effects of bias voltage on structure and properties of TiAl‐doped a‐C:H films prepared by magnetron sputtering

Hydrogenated TiAl‐doped a‐C:H films were deposited on Si substrates by middle frequency magnetron sputtering TiAl target in argon and methane gas mixture atmosphere. Effects of substrate bias voltage on structure and properties of the films, such as the surface morphology, hardness, chemical nature and bond types, were investigated by means of atomic force microscopy (AFM), XPS, Raman spectroscopy and nanoindentation. The friction and wear behaviors of the deposited films were characterized on an UMT‐2MT tribometer. SEM was utilized to analyze the wear scar on steel balls and debris after sliding on the deposited films under dry friction conditions. The results demonstrated that the film deposited at ? 100 V exhibited low friction coefficient which is attributed to the easier formation of graphitized transfer layer. Copyright © 2010 John Wiley & Sons, Ltd.     

The effect of interface electrostatic interaction based on acid–base theory on friction behavior

Fluorine‐containing amorphous carbon films [fluoring‐containing diamond‐like carbon (F‐DLC)] were fabricated on Si wafer by direct current plasma enhanced chemical vapor deposition (dc‐PECVD) technique using CF₄ and Ar as gas sources, confirmed by XPS and Raman analyses. The friction tests were carried out on a rotating ball‐on‐disk apparatus in high vacuum atmosphere (≤5.0 × 10^?4 Pa) at the load of 0.5 N selecting glass (mainly containing silicon–oxygen tetrahedron structure) and Al₂O₃ with the same hardness and surface roughness as the counterpart balls. The results indicate that glass/F‐DLC results in lower friction coefficient of 0.14 than that of the Al₂O₃/F‐DLC (0.20). At the same time, no wear was occurred, and the transfer layer was not formed on the counterpart ball for glass/F‐DLC, while the wear of Al₂O₃/F‐DLC is slightly larger than that of glass/F‐DLC. However, just like the glass ball, there is no formation of transfer layer on the Al₂O₃ ball surface. Furthermore, the chemical state of fluorine in the film after friction, which mainly existed in the form of the C–CF and C–F bonds, did not change compared with the F‐DLC film, while the fluorine content has changed significantly. As a result, it is assumed that interface electrostatic interaction based on acid–base theory plays an extremely important role in the process of friction. Copyright © 2017 John Wiley & Sons, Ltd.     

Tribological behaviour of nanostructured Ti-C:H coatings for biomedical applications

The development of a mechanically stable, functionally graded Ti-doped a-C:H interface layer in combination with a functional a-C:H coating requires a reduction of the brittle phases which induce generally problems in the transitions from Ti to TiC/a-C:H. The core objective of this study was to develop an optimum interlayer between the substrate and the functional top layer for biomedical applications, namely for tooth implants. Since the interlayer may be exposed to the sliding process, in the case of local failure of the top layer it has to fulfil the same criteria: biocompatibility, high wear resistance and low friction.The functional Ti-C:H layers with thickness in the range 2.5–3.5 μm were deposited by a magnetron sputtering/PECVD hybrid process by sputtering a Ti-target in a C₂H₂ + Ar atmosphere in dc discharge regime. The sets of coating samples were prepared by varying the C and H concentrations controlled by the C₂H₂ flow during the deposition process. The tribological properties were evaluated on a pin-on-disc tribometer at room temperature (RT) and at 100 °C using 440C balls with a diameter of 6 mm. The tests at 100 °C were performed to investigate the effect of the sterilization temperature on the tribological properties and the coating lifetime as well. The tribological performance was examined with respect to the friction coefficient, the wear rates of the coating and the counter-parts and the analysis of the wear debris. The Ti/C ratio decreased almost linearly from 4.5 to 0.1 with increasing C₂H₂ flow; the hydrogen content showed a minimum of 5 at.% at C₂H₂ flow of 30 sccm, while for lower flows it was about 10 at.%. The coatings could be divided into three groups based on the C₂H₂ flow: (i) 10–15 sccm, exhibiting severe abrasive damage during the sliding tests, (ii) 20–45 sccm, showing the highest hardness and friction values, and (iii) 52–60 sccm, with moderate hardness and minimal values of the friction coefficient and the wear rate.     

Micropatterning of Sol-Gel Derived Thin Films Using Hydrophobic-Hydrophilic Patterned Surface

Formation process of convexly shaped oxide micropatterns using hydrophobic-hydrophilic patterned surface has been examined, and this technique was applied to several oxide thin films such as SnO₂, ZrO₂, TiO₂ and Al₂O₃. Hydrophobic-hydrophilic patterned surfaces were prepared on glass substrates by selective UV irradiation through a photomask on double-layered films of a very thin TiO₂ gel film as the underlayer and a hydrolyzed fluoroalkyltrimethoxysilane layer as the top layer. Precursor solutions were then spin-coated on the hydrophobic-hydrophilic patterns, and the coated substrates were dried at room temperature. The micropatterns of oxides were very difficult to be formed on the hydrophobic-hydrophilic patterned surfaces from metal-alkoxides as a precursor solution, but convexly shaped micropatterns were formed on the hydrophilic regions of the pattern when metal chlorides or oxychlorides were used as starting materials. This patterning technique potentially has a wide variety of applications such as fabrication of micro-optical components and finely patterned transparent electrodes.     

Friction and wear properties of combined surface modified carbon fabric reinforced phenolic composites

Carbon fabric (CF) was surface treated with silane-coupling agent modification, HNO₃ oxidation, combined surface treatment, respectively. The friction and wear properties of the carbon fabric reinforced phenolic composites (CFP), sliding against GCr15 steel rings, were investigated on an M-2000 model ring-on-block test rig. Experimental results revealed that combined surface treatment largely reduced the friction and wear of the CFP composites. Scanning electron microscope (SEM) investigation of the worn surfaces of the CFP composites showed that combined surface modified CFP composite had the strongest interfacial adhesion and the smoothest worn surface under given load and sliding rate. SEM and X-ray photoelectron spectroscopy (XPS) study of carbon fiber surface showed that the fiber surface became rougher and the oxygen concentration increased greatly after combined surface treatment, which improved the adhesion between the fiber and the phenolic resin matrix and hence to improve the friction-reduction and anti-wear properties of the CFP composite.     

The structure and tribological properties of aluminum/carbon nanocomposite thin films synthesized by reactive magnetron sputtering

Hydrogenated nanocomposite aluminum/carbon thin films (Al/a‐C:H) were fabricated on stainless steel and silicon wafer substrates via unbalanced reactive magnetron sputtering from an Al target in CH₄/Ar plasma. The composition and structure of Al/a‐C:H films were investigated by high‐resolution transmission electron microscope (HRTEM), XPS and micro‐Raman spectroscopy. Nanoindenter, interferometer and ball‐on‐disc tribometer were carried out to evaluate the hardness, internal stress and tribological properties of Al/a‐C:H films. HRTEM observations confirmed that the metallic Al nanocrystallites were uniformly dispersed in the amorphous carbon matrix. XPS and Raman analyses indicated that the sp² content increased with the increase of Al content in the films. Nanoindenter and interferometer tests exhibited that the uniform incorporation of Al nanocrystallites can diminish drastically the magnitude of internal stress with maintaining the higher hardness of as‐deposited films. Especially, the ball‐on‐disc tribometer measurements revealed that the nanocomposite film with 2.3 at.% Al content exhibited relatively better wear resistance and self‐lubrication performance with a friction coefficient of 0.06 and wear rate of 3.1 × 10^?16 m³/ N·m under ambient air, which can be attributed to the relatively higher hardness, the formation of continuous graphitized transfer film on counterface and the reduced reaction of oxygen with carbon. Copyright © 2010 John Wiley & Sons, Ltd.     

Superlubricity behaviors of Si3N4/DLC Films under PAO oil with nano boron nitride additive lubrication

The tribological properties of Si₃N₄ ball sliding against diamond‐like carbon (DLC) films were investigated using a ball‐on‐disc tribometer under dry friction and oil lubrications, respectively. The influence of nano boron nitride particle as lubricant additive in poly‐α‐olefin (PAO) oil on the tribological properties of Si₃N₄/DLC films was evaluated. The microstructure of DLC films was measured by Raman spectroscopy and X‐ray photoelectron spectroscopy. The experimental results show coefficient of friction (COF) of Si₃N₄/DLC films was as low as 0.035 due to the formation of graphite‐like transfer films under dry friction condition. It also indicates that the tribological properties of Si₃N₄/DLC films were influenced significantly by the viscosity of oil and the content of nano boron nitride particle in PAO oil. COF increases with the viscosity of PAO oil increasing. Si₃N₄/DLC films exhibit the superlubricity behaviors (μ=0.001 and nonmeasurable wear) under PAO 6 oil with 1.0 wt% nano boron nitride particle lubrication, indicating that the improved boundary lubrication behaviors have indeed been responsible for the significantly reduced friction. Nano boron nitride additive is used as solid lubricant‐like nano scale ball bearing to the pointlike contact and a soft phase bond with the weak van der Waals interaction force on the contact surface to improve the lubrication behaviors of Si₃N₄/DLC films. The potential usefulness of nano boron nitride as lubricant additive in PAO oil for Si₃N₄/DLC films has been demonstrated under oil lubrication conditions. The present work will extend the wide application of nano particle additive and introduce a new approach to superlubricity under boundary lubrication in future technological areas. Copyright © 2013 John Wiley & Sons, Ltd.     

Characteristics of Ti–Nb,Ti–Zr and Ti–Al containing hydrogenated carbon nitride films

Nanocomposite Me–C–N:H coatings (Me is TiNb, TiZr or TiAl), with relatively high non-metal/metal ratios, were prepared by cathodic arc method using TiNb, TiZr and TiAl alloy cathodes in a CH₄ + N₂ atmosphere. For comparison purposes, a-C–N:H films were also produced through evaporating a graphite cathode in a similar atmosphere. The films were characterized in terms of elemental and phase compositions, chemical bonds, texture, hardness, adhesion and friction behavior by GDOES, XPS, Raman spectroscopy and XRD techniques, surface profilometry, hardness and scratch adhesion measurements, and tribological tests. The nanocomposite films consisted of a mixture of crystalline metal carbonitride and amorphous carbon nitride. The non-metal/metal ratio in the films composition was found to range between 1.8 and 1.9. For the metal containing nanocomposites, grain size in the range 7–23 nm, depending on the metal nature, were determined. As compared with the a-C–N:H, the Me–C–N:H films exhibited a much higher hardness (up to about 39 GPa for Ti–Zr–C–N:H) and a better adhesion strength, while the coefficients of friction were somewhat higher (0.2–0.3 for Me–C–N:H and 0.1 for a-C–N:H).     

Tribological performance of fluoroalkylsilane modification of sol–gel TiO<Subscript>2</Subscript> coating

This paper explores the possibility of making coatings with super friction-reducing and wear protection properties by using both sol–gel and self-assembling techniques. The thin film of TiO₂ was firstly prepared on glass substrates via a sol–gel method, followed by sintering at 480°C. The self-assembled monolayer of Fluoroalkylsilane (FAS) were then prepared on TiO₂ thin film to obtain TiO₂–FAS dual-layer film. The contact angle measurement and X-ray photoelectron spectroscopy were used to determine the wetting behavior and chemical structure of films, respectively. The friction behavior of films sliding against a steel ball was examined on a macro friction and wear tester. It is shown that FAS is strongly adsorbed on sol–gel derived TiO₂ thin film, making it strongly hydrophobic. Good friction-reducing and wear protection behavior is observed for the glass substrate after duplex surface-modification with sol–gel TiO₂ and top layer of FAS.     

Photoredox Catalysis Using Heterogenized Iridium Complexes**

Heterogenized photoredox catalysts provide a path for sustainable chemical synthesis using highly tunable, reusable constructs. Here, heterogenized iridium complexes as photoredox catalysts were assembled via covalent attachment to metal oxide surfaces (ITO, ZrO₂, Al₂O₃) in thin film or nanopowder constructs. The goal was to understand which materials provided the most promising constructs for catalysis. To do this, reductive dehalogenation of bromoacetophenone to acetophenone was studied as a test reaction for system optimization. All catalyst constructs produced acetophenone with high conversions and yields with the fastest reactions complete in fifteen minutes using Al₂O₃ supports. The nanopowder catalysts resulted in faster and more efficient catalysis, while the thin film catalysts were more robust and easily reused. Importantly, the thin film constructs show promise for future photoelectrochemical and electrochemical photoredox setups. Finally, all catalysts were reusable 2–3 times, performing at least 1000 turnovers (Al₂O₃), demonstrating that heterogenized catalysts are a sustainable catalyst alternative.     

Sliding friction and wear behaviors of plasma sprayed aluminum–bronze coating in artificial seawater

The friction and wear behaviors of plasma sprayed aluminum–bronze (CuAl) coating sliding against silicon nitride (Si₃N₄) in artificial seawater were investigated and compared with those in pure water and dry sliding. The morphologies of the worn surfaces were analyzed by three‐dimensional non‐contact surface mapping and scanning electron microscopy. Moreover, chemical states of the tribochemical products of CuAl/Si₃N₄ in seawater were characterized by X‐ray photoelectron spectroscopy. Results show that the plasma sprayed CuAl coating possessed a specific wear rate (in order of 10^?7 mm³/Nm) in seawater more than 600 times smaller than that in dry sliding due to the great alleviation in abrasion wear and splats delamination. Besides, the CuAl/Si₃N₄ had a friction coefficient of 0.06 in seawater, significantly lower and more stable than those in pure water and dry sliding. The tribochemical products of CuAl/Si₃N₄ in seawater, which were proved to be silica, alumina, and their hydrates, transformed into a loosened wear‐debris layer under the coagulation effect of the seawater and dominated the excellent lubrication state in artificial seawater. Copyright © 2014 John Wiley & Sons, Ltd.     

Lower friction and higher wear resistance of fluorine‐incorporated amorphous carbon films

Plasma‐enhanced chemical vapor deposition was employed to fabricate hydrogenated amorphous carbon (a‐C:H) films and fluorine‐doped hydrogenated amorphous (a‐C:H:F) carbon films. For comparison purpose, the a‐C:H films were treated with CF₄ plasma. The bonding structure and tribological behavior of the films were investigated. The results indicate that the F presented mainly in the forms of C–F₃, C–F and C–F₂ groups in both the a‐C:H:F film and the surface CF₄ plasma processed hydrogenated amorphous carbon (F‐P‐a‐C:H) films. Moreover, the a‐C:H:F films, because of the transformation of sp³ to sp², possess a lower friction coefficient than that of the F‐P‐a‐C:H films. Copyright © 2013 John Wiley & Sons, Ltd.