Lowering friction coefficient under low loads by minimizing effects of adhesion force and viscous resistance

Conditions (normal load, sliding speed, ambient conditions, and material) to obtain the lower friction coefficient were studied by measuring the friction and pull-off forces between a metal pin (copper or gold) and a plate (steel or single crystal silicon). First, a pin was rubbed against a plate under a normal load between −12 and 870 μN at a sliding speed between 0.012 and 9.6 μm/s. The friction force was measured during reciprocating sliding motion. The pull-off force was measured before and after each friction force measurement. All the force measurements were taken in high vacuum at 10⁻⁵ Pa, dry argon at 1 atm, and ambient humid air of 38 and 60% relative humidity. Then, the friction coefficient was calculated by dividing friction force by the sum of normal load and pull-off force. In high vacuum, when a copper pin was rubbed against either a silicon or steel plate, the friction coefficient decreased to less than 0.05 with decreasing sliding speed. The effect of sliding speed on the friction coefficient suggests that under a low normal load the viscous resistance of liquid contributed to the friction force. When a gold pin was rubbed against a silicon plate, the friction coefficient was not affected by sliding speed.     

Optimum film thickness of thin metallic coatings on silicon substrates for low load sliding applications

The frictional behaviour of thin metallic films on silicon substrates sliding against 52100 steel balls is presented. The motivation of this work is to identify an optimum film thickness that will result in low friction under relatively low loads for various metallic films. Dry sliding friction experiments on silicon substrates with soft metallic coatings (silver, copper, tin and zinc) of various thickness (1–2000 nm) were conducted using a reciprocating pin-on-flat type apparatus under a controlled environment. A thermal vapour deposition technique was used to produce pure and smooth coatings. The morphology of the films was examined using an atomic force microscope, a non-contact optical profilometer and a scanning electron microscope. Following the sliding tests, the sliding tracks were examined by various surface characterization techniques and tools. The results indicate that the frictional characteristics of silicon are improved by coating the surface with a thin metallic film, and furthermore, an optimum film thickness can be identified for silver, copper and zinc coatings. In most cases ploughing marks could be found on the film which suggests that plastic deformation of the film is the dominant mode by which frictional energy dissipation occurred. Based on this observation, the frictional behaviour of thin metallic coatings under low loads is discussed and friction coefficients are correlated with an energy based friction model.     

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.     

Water Lubrication of Silicon Nitride in Sliding

Several studies have shown that the coefficient of friction of self-mated silicon nitride in water decreases from an initially high value to about 0.002 after a certain run-in period. Since the worn surfaces become extremely smooth, the low friction is attributed to the initiation of hydrodynamic lubrication by a thin water film at the interface. The possibility of mixed lubrication, i.e., hydrodynamic lubrication by water and boundary lubrication due to the presence of colloidal silica on the wearing surfaces, has also been proposed. The purpose of our study is to investigate the influence of load, speed, and surface roughness on the duration of the run-in period. The results confirmed that a low coefficient of friction is obtained following a run-in period when a wear scar of sufficient size is developed to reduce the contact stress. The run-in period, during which the coefficient of friction is fairly high, was shorter for smoother surfaces and at higher loads and speeds. The low friction behavior was found to be unstable and occasional high friction spikes were observed. The surfaces of the wear tracks and wear scars contained a series of striations parallel to the sliding direction and exhibiting plastic deformation, delamination and fracture. The striations that appeared to be associated with the high friction spikes, could form as a result surface film breakdown. Although these results are consistent with the proposed mechanisms of hydrodynamic lubrication or mixed lubrication, it is proposed that the low friction behavior may be also related to fundamental interactions between two hard and elastically deforming surfaces covered with hydrogen-terminated oxide films.     

Investigations on the friction force anisotropy of the silicon lattice

State of the art thin film technologies allow silicon, the material of choice in semiconductor applications, to be used for micro-electro-mechanical systems (MEMS) type microactuators. To investigate the suitability of silicon for these applications, friction force tests for a silicon-silicon interface were performed. For microactuators using friction bearings there is a great need for a general understanding of friction and wear phenomena. Since silicon wafers in general exhibit a single crystalline structure, the investigations included activities regarding the influence of the single crystal silicon’s orientation. The test result shows a periodic change in the coefficient of friction depending on the slider’s rotational position. For instance, a single crystal silicon disk with a {1 1 1} surface crystal orientation exhibits six recurring maxima of the friction force per rotation when tested against a specimen with the same crystal orientation. The contact between wafer and specimen results in a coefficient of friction μ reaching its maximal value of 0.5 every 60° of rotation. To find the root cause for this repetitive behavior, the sliding directions for maximal friction values were compared to the wafer’s respective crystal orientation. For a {1 1 1} silicon wafer, the atoms at the surface are arranged in equilateral triangles. The angle of 60° between the atoms in these triangles corresponds with the periodicity of the friction force. It therefore may be concluded that the coefficient of friction follows the crystal structure. Depending on the lattice orientation, the friction force varies by more than 50%. This information is crucial for designing a micro-slide bearing as well as choosing a combination of lattice orientations that yield minimal friction.     

Tribological Behavior of TiC/a-C:H-Coated and Uncoated Steels Sliding Against Phenol–Formaldehyde Composite Reinforced with PTFE and Glass Fibers

Tribological experiments on phenol–formaldehyde composite reinforced with polytetrafluoroethylene (PTFE) and glass fibers were performed against 100Cr6 steel and TiC/a-C:H thin film-coated 100Cr6 steel. In both cases, the coefficient of friction increases with increasing sliding distance until a steady-state value is reached. Although the steady-state values of the coefficient of friction are very close and ultralow, the wear rate of the PTFE composite liner at a long sliding distance (1,000 m) is reduced when the steel ball is coated with the TiC/a-C:H coating. This behavior is mainly attributed to the smoother surface after long sliding and the improved wear resistance of TiC/a-C:H coating. PTFE transfer films are evident on the surfaces of the hard counterparts. The average thickness of the transfer film on TiC/a-C:H-coated surfaces is about 3.8 nm. On the surface of uncoated steel ball, a continuous but non-uniform transfer film of around 13.9 nm average thickness was found.     

Sliding velocity dependency of the friction coefficient of Si-containing diamond-like carbon film under oil lubricated condition

In this study we investigated the sliding velocity dependency of the coefficient of friction for a Si-containing diamond-like carbon (DLC-Si) film in an automatic transmission fluid (ATF) under a wide range of contact pressures. The DLC-Si film and a nitrided steel with a surface roughness, Rz_JIS, of around 3.0 μm were used as disk specimens. A high-carbon chromium steel (JIS-SUJ2) bearing ball was used as a ball specimen. Friction tests were conducted using a ball-on-disk friction apparatus under a wide range of sliding velocites (0.1-2.0 m/s) and contact pressures (P_max: 0.42-3.61 GPa) in ATF. The friction coefficients for the nitrided steel had a tendency to decrease with an increase in sliding veloicity under all the contact pressure conditions; however, the friction coefficients for the DLC-Si film were stable with respect to sliding velocities under all the contatct pressures. These results indicate that the DLC-Si film suppresses the stick-slip motion during sliding againt steel in ATF, which is a desired frictional characteristic for the electromagnetic clutch disks used under lubrication. Furthermore, the DLC-Si film showed a higher wear resistance and lower aggression on the steel ball specimen than the nitrided steel. There were less hydrodynamic effects on the friction coefficient for the DLC-Si film possibly due to maintenance of the initial surface roughness and its poorer wettability with the fluid. X-ray photoelectron spectroscopy (XPS) analysis of the sliding surfaces revealed that the adsorption film derived from the succinimide on the sliding surfaces of the DLC-Si film and the mating steel ball also contributed to the sufficient and less sliding-velocity-dependant friction coefficients.     

High-speed tribology of PFPEs with different functional groups and molecular weights coated on DLC

In this article, we study the friction and wear durability of perfluoropolyether (PFPE) with different functional groups and molecular weights (MW) for a range of disk rotational speeds (500–7200 rpm or 1.2–17.33 m/s). A 4 mm diameter silicon nitride ball under a normal load of 4 g was employed as slider against PFPE lubricated diamond like carbon (DLC) film on magnetic hard disk. The coefficient of friction increases with increasing speeds, to certain extent, but it decreases for the higher speeds. At very high speeds, the fluctuations in the coefficient of friction of low MW PFPEs were larger than those of high MW PFPEs. The optical microscope image of the ball after sliding showed that evaporation might have occurred more easily in low MW than in high MW when sliding speed was increased due to the frictional heat generated at the interface. The wear lives of Z-lube (carboxyl group at both ends) and Z-dol are significantly higher than AS1 (alkoxy silano group at both ends) at low speed (1.2 m/s). In comparison to low MW PFPEs, high MW PFPEs show better wear durability at higher rotational speeds.     

Effect of counterparts on the friction and wear behavior of polyelectrolyte multilayers

Polyelectrolyte multilayers (PEMs) were prepared on Si substrates by alternative deposition of poly(sodium 4-styrenesulfonate) (PSS) as polyanion and poly (diallyldimethylammonium chloride) (PDDA) as polycation. The PEM film was characterized by means of ultraviolet-visible light absorption spectrometry and atomic force microscopy. The friction and wear behavior of the polymer film sliding against brass, 440C stainless steel, Si₃N₄ and WC balls was evaluated on a microtribometer. It was found that the multilayer film was uniform and compact, and it registered a lowered friction coefficient and extended antiwear life while sliding against soft counterparts, in particular, a brass ball. This could be because the polymeric transfer film had an enhanced adhesion on the soft metallic counterpart in the presence of inter-transferred metallic debris. Contrary to the above, the PEM film had a higher friction coefficient and shorter antiwear life while sliding against Si₃N₄ and WC balls, possibly owing to a higher shearing stress in the presence of stiff and hardly deformable hard counterparts. In other words, the polymeric transfer film on the hard couterparts, if any, would be easily scaled off, leading to decreased antiwear life. Moreover, the differences in the friction and wear behavior of the PEM film sliding against different counterparts were closely related to the differences in the chemical and crystallographic structure of the counterparts (ceramics Si₃N₄ and WC, and metals brass and stainless steel).     

Tribological Performance of UHMWPE and PFPE Coated Films on Aluminium Surface

A thin layer of Ultra High Molecular Weight Polyethylene (UHMWPE) or UHMWPE + PFPE is coated onto cylindrical aluminium (Al) pin (4.6 mm diametre) surface with the aim of providing wear resistant coating on this soft and tribologically poor metal. The coefficient of friction and wear life of the coated samples are investigated on a pin-on-disk tribometre under different normal loads (394–622 g) and two sliding speeds (0.1 and 0.31 m/s) against uncoated Al disk as the counterface. Both coatings provide coefficient of friction values in the range of 0.02–0.2 as compared to 0.4–1.0 for uncoated Al. There is tremendous improvement in the wear life of the pin, with UHMWPE + PFPE film giving wear life approximately twice to thrice higher than that with only UHMWPE film. A thin polymer film is transferred to the disk surface during sliding providing very long-term wear life (continuous low coefficient of friction) despite visual removal of the film from the pin surface. The present films will have applications in gears and bearings as solid or boundary lubricants for automotive and aerospace component.     

Molecular Orientation, Crystallinity, and Topographical Changes in Sliding and their Frictional Effects for UHMWPE Film

This paper presents a study on the frictional anisotropy of semi-crystalline UHMWPE polymer film deposited on DLC-overcoated Si substrate. For UHMWPE film slid against a silicon nitride ball, there is a remarkable difference in the coefficient of friction between the forward and reverse directions after the slider has been initially slid against the film for certain number of cycles. The changes in the friction are greatly influenced by the initial number of sliding cycles. This frictional behavior is explained in terms of crystallinity change and molecular orientational effects on UHMWPE and micro-topographical effects due to the initial sliding. Nanoscratch test is conducted to understand the friction of the polymer film in the sliding track and the data are compared with the macroscale friction data. The results show that the friction in the reverse of the initial sliding direction is high in comparison to that in the forward direction and this behavior mainly depends upon the number of initial sliding cycles. The initial sliding cycles affect the crystallinity and molecular orientation of the film, as well as the film topography. This combined effect on the polymer film results in an anisotropic frictional behavior of the film.     

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.     

The Influence of DLC Coating on EHL Friction Coefficient

High hardness, high elastic modulus, low friction characteristics, high wear and corrosion resistance, chemical inertness, and thermal stability are factors that make diamond-like carbon (DLC) coatings the subject of many studies. For the same reasons they also seem suitable for use in, amongst others, machine components and cutting tools. While most studies in the literature focus on the influence of coatings on wear and friction in boundary lubrication and pure sliding contacts, few studies can be found concerning rolling and sliding elastohydrodynamic lubrication (EHL) friction, especially in the mixed and full film regime. In this article tests are carried out in a Wedeven Associates Machine tribotester where an uncoated ball and disc pair is compared to the case of coated ball against uncoated disc, coated disc against uncoated ball, and coated disc against coated ball. The tests are conducted at two different temperatures and over a broad range of slide-to-roll ratios and entrainment speeds. The results are presented as friction maps as introduced in previous work (Bj?rling et al. in J Eng Tribol 225(7):671, 2011). Furthermore a numerical simulation model is developed to investigate if there is a possibility that the hard, thin DLC coating is affecting the friction coefficient in an EHL contact due to thermal effects caused by the different thermal properties of the coating compared to the substrate. The experimental results show a reduction in friction coefficient in the full film regime when DLC-coated surfaces are used. The biggest reduction is found when both surfaces are coated, followed by the case when either ball or disc is coated. The thermal simulation model shows a substantial increase of the lubricant film temperature compared to uncoated surfaces when both surfaces are coated with DLC. The reduction in friction coefficient when coating either only the ball or the disc are almost the same, lower than when coating both the surfaces but still higher than the uncoated case. The findings above indicate that it is reasonable to conclude that thermal effects are a likely cause for the decrease in coefficient of friction when operating under full film conditions, and in the mixed lubrication regime when DLC-coated surfaces are used.     

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.     

Si3N4 as a biomaterial and its tribo‐characterization under water lubrication

Hip implant wear is recognised as the main cause of hip implant failure therefore has been widely investigated both experimentally and clinically, demonstrating the coexistence of abrasive, adhesive, fatigue and corrosive wear. Many clinical in vivo and bulk material wear rate data from published literature have been presented for non‐oxide ceramic implants. Several studies have shown that the coefficient of friction of self‐mated silicon nitride in water decreases from an initially high value to about 0.002 after a certain run‐in period. Since the worn surfaces become extremely smooth, the low friction is attributed to the initiation of hydrodynamic lubrication by a thin water film at the interface. The possibility of mixed lubrication, i.e. hydrodynamic lubrication by water and boundary lubrication due to the presence of colloidal silica on the wearing surfaces, has also been proposed. Influence of load, speed and surface roughness on the duration of the run‐in period of silicon nitride under water lubrication was investigated in this study. The results confirmed that a low coefficient of friction is obtained following a run‐in period when a wear scar of sufficient size is developed to reduce the contact stress. The run‐in period, during which the coefficient of friction is fairly high, is shorter for smoother surfaces and at higher loads and speeds. The striations that appeared to be associated with the high‐friction spikes can be formed as a result of surface film breakdown. Although the results are consistent with the proposed mechanisms of hydrodynamic lubrication or mixed lubrication, it is proposed that the low‐friction behaviour may also be related to fundamental interactions between two hard and elastically deforming surfaces covered with hydrogen‐terminated oxide films. Copyright © 2016 John Wiley & Sons, Ltd.     

Stribeck Curve Under Friction of Copper Samples in the Steady Friction State

The friction and wear of a pure copper block (99.98 wt% Cu) against a hardened steel disc were studied. The effect of sliding velocity and load on the friction coefficient and wear rate of Cu samples during steady tests was studied. Elasto-hydrodynamic (EHL), mixed (ML) and boundary lubrication (BL) regions were analyzed using the Stribeck curve. The lubrication number of Schipper, Z, was used in the analysis of the Stribeck curve. The transitions from one lubrication region to another are discussed. The mixed EHL region is characterized by stable low values of the friction coefficient, wear rate and temperature. Straight asperity contact is the dominant mechanism under friction of Cu–steel pair in the BL region. High-friction coefficients and wear rates, thin lubricant films and large wear grooves indicate straight asperity contact between rubbed surfaces in the BL region. Although the dominant mechanisms in the mixed EHL and BL regions are different in principle, a steady friction state is preserved in both cases. It is expected that the steady friction state in the BL and mixed EHL regions is associated with deformation and fracture of surface layers but these process occur at different scale levels. It was shown that under friction of Cu–steel pair, two types of ML regions are observed. The first is the stable steady friction of mixed EHL with low values of the friction coefficient and wear rate. The second type of the ML region is the region of unstable friction and wear when a decrease of lubricant film leads to a change of external (roughness, temperature, friction and wear) and internal (strain and stress) parameters. It was found out that a transition to the unstable ML region occurs within a narrow range of Z parameter under definite values of the load and sliding velocity.     

Nanofriction Mechanisms Derived from the Dependence of Friction on Load and Sliding Velocity from Air to UHV on Hydrophilic Silicon

This paper examines friction as a function of the sliding velocity and applied normal load from air to UHV in a scanning force microscope (SFM) experiment in which a sharp silicon tip slides against a flat Si(100) sample. Under ambient conditions, both surfaces are covered by a native oxide, which is hydrophilic. During pump-down in the vacuum chamber housing the SFM, the behavior of friction as a function of the applied normal load and the sliding velocity undergoes a change. By analyzing these changes it is possible to identify three distinct friction regimes with corresponding contact properties: (a) friction dominated by the additional normal forces induced by capillarity due to the presence of thick water films, (b) higher drag force from ordering effects present in thin water layers and (c) low friction due to direct solid–solid contact for the sample with the counterbody. Depending on environmental conditions and the applied normal load, all three mechanisms may be present at one time. Their individual contributions can be identified by investigating the dependence of friction on the applied normal load as well as on the sliding velocity in different pressure regimes, thus providing information about nanoscale friction mechanisms.     

Effect of molybdenum disulphide upon the friction and wear in ceramic–steel pair

Friction and wear tests between a stationary block and a rotating ring under lubrication with molybdenum disulphide (MoS₂) were carried out at room temperature at a sliding distance of 500 m. Silicon nitride and cemented carbide blocks were pressed against a bearing steel ring, silicon nitride-bearing steel and cemented carbide-bearing steel pairs, by a load of 1600 N. The effect of molybdenum disulphide upon the coefficient of friction and the wear of the steel ring was discussed for both pairs in comparison with mineral oil lubricants. Molybdenum disulphide was more effective in reducing the coefficient of friction and the wear of the ring than the oil lubricants. Various mechanical pretreatment for forming MoS₂ film on the ring surface prior to the sliding tests were also considered. The mechanical pretreatment enabled the sliding test with the low friction coefficient even without lubrication over the sliding distance of 500 m. In general, the coefficient of friction and wear loss of the steel ring were smaller in the silicon nitride-bearing steel pair than in the cemented carbide-bearing steel pair.     

摩擦偶件对PDDA/PSS分子沉积膜摩擦磨损行为的影响

利用分子沉积技术在单晶硅基底上制备了PDDA/PSS分子沉积膜。采用UV-vis吸收光谱对沉积过程进行了跟踪检测,用原子力显微镜观察了分子沉积膜的表面形貌,考察了摩擦偶件材料对PDDA/PSS分子沉积膜摩擦学行为的影响,并探讨了其磨损机制。实验结果表明,薄膜与较硬的偶件材料对摩时,剪切应力较大,薄膜很容易被磨穿,抗磨寿命极短;在相同实验条件下,薄膜与Cu球对摩时,薄膜的耐磨寿命最长,不锈钢球次之,与Si3N4球和WC球对摩时,薄膜的耐磨寿命较短。     

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.