A novel method of using continuous tribo-electrification variations for monitoring the tribological properties between pure metal films

The traditional method of using the continuous variation of the friction coefficient with sliding distance to monitor the tribological properties between the contacts of soft metal films is generally low in sensitivity. This paper proposed the novel method of using instead the continuous variation of tribo-electrification voltage. This method was investigated experimentally for the dry friction sliding of iron on copper coated with a thin film of tin and was shown to be much superior to the traditional method in terms of sensitivity and ease of implementation. Moreover, it was observed that the continuous variations of the friction coefficient with sliding distance was very unstable but remained positive, making it indiscriminative for monitoring the tribological properties between the hard metal films. The continuous variations of the tribo-electrification voltage, on the other hand, showed either positive or negative polarity depending on the metal pairs, which allowed the identification of the surface where friction had occurred as well as the sliding surface. Finally, two continuous models to represent the tribo-electrification mechanisms for iron sliding against copper coated with a thin film of tin or nickel at different normal loads were proposed.     

Tribological properties of selected PVD coatings when slid against ductile materials

In this paper, a physical vapour deposited (PVD) deposited TiB₂ coating is compared in dry sliding with commercial PVD titanium nitride (TiN), titanium aluminium nitride (TiAlN) and titanium carbonitirde (TiCN) as to frictional properties and tendency of counter material pick-up. The aim is to investigate if the superior behaviour of the TiB₂ coating experienced in severe sliding applications against aluminium alloys can be extended to other materials with a similarly poor tribological characteristics.A new tribological test for sliding contact has been used. The test configuration involves two crossed elongated cylindrical test specimens which are forced to slide axially against each other at a constant sliding speed and a gradually increasing normal load, while recording the friction. The evaluation is performed by correlating the friction history with the width, topography and composition of the sliding tracks as detected by optical and scanning electron microscopy.Coated cemented carbide (CC) test cylinders have been slid against cylinders of a Ti alloy (Ti–6Al–4V), an Al alloy (Al 7075) and Inconel 718. It was shown that the TiB₂ surface displayed superior friction and anti-sticking properties, when tested against the aluminium alloy. Against the Ti and Inconel alloys no major difference between the coatings could be found. Instead, it is concluded that the friction coefficient is determined by the plastic properties of the counter material since a complete transfer layer instantly builds up on the coating.It proved possible to estimate the friction force from the width of the sliding tracks, the Vickers hardness of the counter material and simple plastic considerations. This estimation also verifies the unexpectedly low friction of all coatings against the Ti alloy.     

Thick rutile layer on titanium for tribological applications

In the present work, efforts have been made to oxidise the titanium surface, followed by very slow cooling to produce a thick and adherent oxide layer. The response of titanium to oxidation at various temperatures and timings has been investigated, in terms of layer thickness, phase evolution, surface morphology, oxide layer–substrate adhesion, hardness and tribological characteristics. A variety of experimental and analytical techniques, including X-ray diffraction, field-emission scanning electron microscopy, scanning electron microscopy, micro-hardness and tribological testing, have been used to characterise the resultant thermal oxidised surfaces. The results showed that a thick oxide layer with rutile TiO₂ and oxygen diffused Ti structure can be produced, which exhibited excellent adhesion with the titanium substrate, low friction and superior wear resistance during pin-on-disc sliding tests and thus good potential for tribological applications.     

Preliminary Study of Nano- and Microscale TiO2 Additives on Tribological Behavior of Chemically Modified Rapeseed Oil

This study compared the tribological evaluation of chemically modified rapeseed oil as a potential biodegradable automotive lubricant with and without nano- and microscale titanium dioxide (TiO₂) particles, focusing on the influence of TiO₂ particles to improve the friction reduction and antiwear characteristics of chemically modified rapeseed oil. TiO₂ nano- and microscale particles of anatase phase and rutile phase were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Further, the analysis of chemically modified rapeseed oil with and without TiO₂ additives was carried out to determine its tribological behavior using a pin-on-disc tribometer. The experimental results showed that the addition of TiO₂ nanoparticles exhibited good friction reduction and antiwear properties compared with the addition of microscale TiO₂ and without TiO₂ additives to chemically modified rapeseed oil. Nanoscale TiO₂ is suitable as an antiwear additive in chemically modified rapeseed oil.     

Development of amorphous carbon coating with luminescent silica/CdSe/ZnS quantum dots underlayer for wear monitoring

Amorphous carbon (a-C) coating with luminescent wear-sensing underlayer is developed for achieving the tribological coating with wear monitoring capability. Wear monitoring of a-C coating by luminescent spectroscopy to determine the remaining thickness of the coating after the wear test is also demonstrated. The coating structure consists of a-C films deposited by R.F. magnetron sputtering method onto the luminescent layer, which is made from silica coating containing CdSe/ZnS quantum dots (Silica/QD). A thin Si-intermediate layer is added between a-C and silica/QD layer for adhesion improvement. The physical as well as tribological properties of the coatings are analysed. Furthermore, wear monitoring of a-C films is also demonstrated to determine the remaining coating thickness after the tribological test. The demonstration is carried out by firstly formulating a relation between luminescent intensity detected from the coating and coating thickness. Then the luminescent intensity is measured again from the wear track. The remaining thickness is finally determined by using the relationship between luminescent intensity and coating thickness. The fabricated coating exhibits a smooth surface with the average surface roughness of 1.35 nm and a friction coefficient of 0.1. The demonstration of wear monitoring shows that the remaining thickness of the coating after the tribological test determined by luminescent spectroscopy is compared well with the thickness measured by profilometry. This suggests that wear monitoring of a-C films by luminescent spectroscopy technique is feasible.     

Microstructural Characterization of Wear Particles Formed During Tribological Stressing of TiC and Ti(C,N) Coatings

An attempt was undertaken to obtain a better understanding of the tribological properties of two wear-resistant coatings on tool steel by structural and microchemical analysis of wear particles using a transmission electron microscope. Coatings were deposited by physical vapor deposition and plasma-assisted chemical vapor deposition techniques and tribological properties were derived from reciprocating sliding tests of the coatings against alumina balls. Three types of wear particles were identified by electron diffraction and energy dispersive X-ray spectroscopy: nanocrystalline rutile (TiO₂), nanocrystalline graphite and microcrystalline graphite. Low coefficients of friction, of the order of 0.2, were attributed to the formation of solid lubricant films of sub-stoichiometric TiO_2-x Magnéli phases and/or graphite.     

Effect of transfer film structure, composition and bonding on the tribological behavior of polyphenylene sulfide filled with nano particles of TiO2, ZnO, CuO and SiC

The tribological behavior of polyphenylene sulfide (PPS) filled with inorganic nano particles was studied. The fillers investigated were TiO₂, ZnO, CuO and SiC whose sizes varied from 30 to 50 nm. The polymer composites were compression molded with varying proportions of these fillers. Wear and friction tests were performed in a pin-on-disk configuration at a sliding speed of 1.0 m/s, nominal pressure of 0.65 MPa, and counterface roughness of 0.10 μm Ra. The polymer composite pins slid against hardened tool steel counterfaces. The transfer films of the composite materials formed on the counterfaces during sliding were studied by optical microscopy and X-ray photoelectron spectroscopy (XPS) and the adhesion between the transfer film and counterface was measured in terms of the peel strength. It was found that the wear rate of PPS decreased when TiO₂ and CuO were used as the fillers but increased with ZnO and SiC fillers. The optimum wear resistance was obtained with 2 vol.% CuO or TiO₂. These filled composites had the coefficients of friction lower than that of the unfilled PPS. The wear behavior of the composites is explained in terms of the topography of transfer film and adhesion of transfer film to the counterface as observed from peel strength studies. There is a good correlation observed between the transfer film-counterface bond strength and wear resistance.     

Biotribological properties of Ti/TiB2 multilayers in simulated body solution

Ti/TiB₂ multilayers with various modulation ratios were prepared by magnetron sputtering on biomedical titanium alloy Ti6Al4V. The tribological properties of the multilayers sliding against ultra-high molecular weight polyethylene under lubrication with Hank׳s solution were also investigated. The results demonstrated that the tribological properties strongly depended on the modulation ratios of multilayers. The coefficient of friction of multilayers with a modulation ratio of 1:5 was 0.1, a reduction by 28.6%; the wear volume loss of UHMWPE decreased by almost one order of magnitude compared to that of Ti6Al4V alloy, exhibiting excellent anti-friction and anti-wear properties. The oxidation wear of Ti6Al4V alloy could be restrained effectively and converted to abrasion wear and/or adhesive wear by the laminate structures in the multilayers, suggesting that this material may serve as a potential candidate for the surface modification of artificial joints.     

DLC薄膜的表面形貌及其摩擦学性能研究

以真空蒸发碳离子束辅助镀膜法制备了DLC薄膜，通过原子力显微镜(AFM)和扫描电子显微镜(SEM)观察了该薄膜的表面形貌，对该薄膜的表面形貌对其摩擦学行为的影响进行了研究。研究发现：用真空蒸发碳离子束辅助镀膜的方法制备的类金刚石薄膜表面光滑，颗粒均匀，粒度小，摩擦因数降低；DLC薄膜比弹簧钢片及Ti6Al4V球基体耐磨；DLC薄膜的摩擦学性能在摩擦过程中会进一步改善。     

Sliding friction and wear of Cu–graphite against 2024, AZ91D and Ti6Al4V at different speeds

Cu–graphite composite fabricated by powder metallurgy art is no longer novel material. However, it might be a versatile self-lubricating material sliding against different metals and alloys. In this connection, understanding towards its tribological behavior and wear mechanism is very important. Sliding tribological behaviors of Cu–graphite composite against different counterparts, specified as 2024 aluminium alloy, AZ91D magnesium alloy, and Ti6Al4V titanium alloy, were investigated over varied sliding speeds at room temperature in air. The friction and wear tests were conducted on a pin-on-disk tribo-meter. Tribological performance of Cu–graphite composite strongly depended on its counterpart materials. Cu–graphite composite could provide friction reduction in sliding against 2024 and Ti6Al4V. Cu–graphite composite was a good self-lubricating material in sliding against AZ91D at low speeds but not at 0.25 and 0.50 m/s. Wear mechanism of Cu–Gr composite was related to the transfer, counter-transfer, mechanical mixing and tribo-oxidation at tribo-interface. Sliding speed had influences on tribo-interface and thereby wear mechanism. Finally, the effects of naturally occurred oxide film and sliding speed were discussed.     

Tribological behaviors of composite molecular deposition films

Polyallylamine hydrochloride (PAH)/graphite oxides (GO) ultrathin film and the multilayer film of PAH incorporated with TiO₂ enwrapped by polyacrylic acid (PAA), namely PAH/PAA(TiO₂) composite film, were prepared by molecular deposition (MD) method in laboratory. Both of them were heated to change the film forming dynamic force from electrostatic force to covalent bond so as to increase the bonding strength of the films. The structure and nanotribological properties of the films were analyzed by atomic force microscope (AFM) and ultraviolet-visible (UV) spectroscopy. It was found that these films had a much smaller friction force than their substrates and the friction force was dependent on the morphology and/or hardness of the films.     

Tribological behavior of TiN films depositid by reactive magnetron sputtering under low pressure

The effect of deposition conditions on the tribological behavior of titanium nitride thin films produced by reactive magnetron sputtering has been studied. Dependences of the hardness, the width of the friction track, the friction coefficient, and the volume wear of the TiN films on the N₂ reactive gas flow rate have been obtained. Conditions of deposition under which the coatings with the best tribological characteristics are formed have been determined.     

Macro- and nanotribological properties of organosilane monolayers prepared by a chemical vapor adsorption method on silicon substrates

Organosilane monolayers are novel ultrathin films used to control the physicochemical properties, such as friction and wear, of solid surfaces. In this study, the authors prepared alkylsilane and fluoroalkylsilane monolayers with a series of chain lengths by a chemical vapor adsorption method. The monolayers tribological properties were investigated by lateral force microscope (LFM) and friction tester. LFM nanoscale measurements of tribological properties showed that alkylsilane monolayer gave lower lateral force than the Si substrate surface. The lateral force decreased as the length of the alkyl chain increased. On the macroscale, friction test revealed that the organosilane monolayers gave lower dynamic friction coefficients than the Si substrate surface in air at room temperature. The longer the alkyl chain, the greater the wear resistance of the organosilane monolayers. Friction experiments using tetradecane as a lubricant showed better tribological properties than were obtained in air. Furthermore, microscopically line-patterned two-component organosilane monolayers were prepared and their macroscopic friction behavior was investigated. Even though the height difference between the two-components was less than 1 nm, friction force anisotropy between the parallel and perpendicular directions against the line pattern was observed.     

Effects of SEBS-g-MA on tribological behaviour of nylon 66/organoclay nanocomposites

The tribological behaviour of nylon 66, nylon 66/organoclay nanocomposites and nylon 66/(SEBS-g-MA+organoclay) nanocomposites was studied by means of a pin-on-disk apparatus. The morphologies of the transfer films and the worn surfaces of specimens were observed with an optical microscope and a scanning electron microscopy (SEM). Wear of all of the specimens increases with increasing load; meanwhile the coefficient of friction almost linearly decreases. Wear of nylon 66/organoclay is larger than that of nylon 66. Adding SEBS-g-MA to the nylon 66/organoclay improves the wear resistance. The coefficient of friction of nylon 66/organoclay is slightly less than that of nylon 66 at various loads. The coefficient of friction of nylon 66/(SEBS-g-MA+organoclay) is the lowest at every load. If the transfer film is thin, uniform and continuous, the wear loss and the coefficient of friction are low.     

A method for the tribological testing of thin,hard coatings

A new method has been developed for tribological testing of thin, hard antiwear coatings, using a ball‐on‐disc tribosystem, under conditions of dry sliding. In this, an Al₂O₃ ball is pressed against a coated steel disc. Wear debris is removed from the contact zone by a stream of dry argon in this novel method. This improves the stability of the tribological properties and the repeatability of the test results. All test conditions are precisely defined, in particular: the type of motion, air relative humidity, ambient temperature, sliding speed, load, tribosystem spatial configuration, substrate material, substrate hardness and roughness, and coating thickness. The method developed has been used to test various physical vapour deposition coatings (deposited by the vacuum arc method), i. e., single‐layer TiN, Ti(C,N), CrN, and Cr(C,N), and multilayer Cr(C,N)/CrN/Cr and Cr(C,N)/(CrN+Cr₂N)/CrN/Cr. It is shown that CrN coatings exhibit the best antiwear properties, and Ti(C,N) the worst. Friction coefficients for CrN and Cr(C,N) coatings are much lower than for the more commonly used TiN. Multilayer coatings have better antiwear properties than single‐layer ones.     

Fretting Wear Study on Micro-Arc Oxidation TiO<Subscript>2</Subscript> Coating on TC4 Titanium Alloys in Simulated Body Fluid

Tribological properties of TiO₂ coatings synthesized by micro-arc oxidation (MAO) on the surface of TC4 titanium alloys were investigated at the fretting contact against 440C stainless steel in simulated body fluid (SBF). Fretting experiments were carried out by ball-on-flat contact at various loads for 1 h, with an amplitude of 100 μm and a frequency of 5 Hz. Results show that MAO TiO₂ coatings presented good tribological properties with lower friction coefficient in SBF. Less wear volume was observed for MAO TiO₂ coatings compared with that for TC4 alloy. At lower load, the wear mechanism of MAO TiO₂ coatings was dominated to abrasive wear. With an increase of normal load, however, fretting corrosion increased due to chemical reactions with SBF, and therefore, fretting fatigue coexisting with abrasive wear became the predominant mode.     

Increasing the tribological performances of Ti–6Al–4V alloy by forming a thin nanoporous TiO2 layer and hydroxyapatite electrodeposition under lubricated conditions

In this study, comparative investigation of (i) untreated Ti–6Al–4V alloy, (ii) nanoporous thin TiO₂ layer formed by controlled anodic oxidation and (iii) electrodeposited hydroxyapatite coatings into porous oxide layer was carried out for evaluation of sliding-wear performances in a bio-simulated environment. Wear mechanisms, wear volumes and friction coefficients of the three types of surfaces under lubricated conditions in a bio-simulated solution were recorded and analyzed. The results presented herein show that, under the investigated tribocorrosion conditions (under reciprocating sliding), both surface treatments applied have improved the wear resistance and friction coefficients as compared to the untreated Ti–6Al–4V alloy surface.     

Nanotribological properties of novel lubricants for magnetic tapes

Two classes of novel lubricants, perfluoropolyethers (PFPE) and ionic liquids (ILs), were deposited on metal film magnetic tapes. The adhesive force and coefficient of friction of lubricated and unlubricated tapes were investigated at the nanoscale with an atomic force microscope (AFM) as a function of various humidity and temperature conditions. Microscale tests with a ball-on-flat tribometer were also performed in order to study the length-scale effects on friction. Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated by monitoring the friction force, surface potential and contact resistance with the AFM. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) experiments were conducted to determine the chemical species that affect intermolecular bonding and as an aid in interpreting how the lubricant film tribological properties vary with the environmental conditions. Z-TETRAOL, one of the PFPEs, was found to exhibit the lowest adhesion and friction among the lubricant films studied. The ionic liquid 1,1′-(pentane-1,5-diyl)bis(3-hydroxyethyl-1H-imidazolium-1-yl) di[bis(trifluoromethanesulfonyl)imide)] exhibited comparable nanotribological properties with the PFPEs. This is attributed to the presence of hydroxyl groups at its chain ends, which can hydrogen bond with the surface similar to PFPEs.     

Wear resistant multilayer nanocomposite WC1−x/C coating on Ti–6Al–4V titanium alloy

A significant improvement of tribological properties on Ti–6Al–4V has been achieved by developed in this study multilayer treatment method for the titanium alloys. This treatment consists of an intermediate 2 μm thick TiC_xN_y layer which has been deposited by the reactive arc evaporation onto a diffusion hardened material with interstitial O or N atoms by glow discharge plasma in the atmosphere of Ar+O₂ or Ar+N₂. Subsequently, an external 0.3 μm thin nanocomposite carbon-based WC_1−x/C coating has been deposited by a reactive magnetron sputtering of graphite and tungsten targets. The morphology, microstructure, chemical and phase compositions of the substrate material after treatment and coating deposition have been investigated with use of AFM, SEM, EDX, XRD, 3D profilometry and followed by tribological investigation of wear and friction analysis. An increase of hardness in the diffusion treated near-surface zone of the Ti–6Al–4V substrate has been achieved. In addition, a good adhesion between the intermediate gradient TiC_xN_y coating and the Ti–6Al–4V substrate as well as with the external nanocomposite coating has been obtained. Significant increase in wear resistance of up to 94% when compared to uncoated Ti–6Al–4V was reported. The proposed multilayer system deposited on the Ti–6Al–4V substrate is a promising method to significantly increase wear resistance of titanium alloys.     

Sliding Wear of Ni–17.5Si–29.3Cr Alloy under Water Lubrication

The tribological properties of Ni–17.5Si–29.3Cr alloy against Si₃N₄ under water lubrication conditions were studied on a ball-on-disc reciprocating 1tribotester. The effects of load and sliding speed on tribological properties of the alloy were investigated. The worn surfaces of the alloy were examined with SEM, TEM and an X-ray photoelectron spectroscope (XPS). It was found that the tribological properties of the alloy were closely dependent on the sliding conditions. Wear rate with the load of the alloy increased slightly at low and moderate load and increased dramatically at high load. Wear rate with the sliding speed of the alloy increased slightly at low and moderate sliding speed and increased dramatically at high sliding speed, which showed the same trend as that with the load. The friction coefficient increased with the load (especially at high load), and decreased with sliding speed at low sliding speed and increased significantly at high sliding speed. Wear mechanism of the alloy was mainly microploughing and delamination at low and moderate load and transformed to microfracture and delamination at high load.