Amorphous hydrogenated carbon films for tribological applications I. Development of moisture insensitive films having reduced compressive stress

Although earlier investigations on the tribological behaviour of amcrphous hydrogenated carbon (AHC) films in sliding contact with steel showed encouraging results, four open issues were identified. They were: (a) dependence of friction and wear on humidity (i.e., the friction coefficient and the wear increased with humidity), (b) limitations on film thickness (i.e., films greater than 2 μm thick delaminated due to large compressive stress), (c) deposition of films on substrates other than silicon and (d) lubricant compatibility (i.e., formation of lubricant-derived antiwear films on AHC film surfaces). Steps were taken to address some of these open issues by incorporating silicon in AHC films. Friction and wear tests were conducted on AHC films containing various amounts of silicon. Incorporation of silicon in AHC films rendered the friction coefficients and the wear of a steel counterface insensitive to moisture. Silicon incorporation in AHC films also significantly reduced compressive stress. This allowed deposition of 10 μm thick films. These effects were achieved without any compromise with the friction coefficient and the film wear if the amount of silicon in the film was kept within a certain concentration range. In addition, silicon-containing AHC films were thermally more stable than silicon-free films. Experiments conducted with two lubricants resulted in significantly lower wear of the silicon-free AHC films than that obtained for unlubricated sliding. Similar friction coefficients were obtained for AHC film/steel and steel/steel combinations in lubricated sliding.     

Reducing Friction Force of Si Material by Means of Atomic Layer-Deposited ZnO Films

With excellent lubricating property, zinc oxide (ZnO) films are promising candidates to act as protective coatings in Si-based microelectromechanical system devices for the purpose of decreasing friction forces of silicon (Si) material. In this paper, the nanotribological behavior of ZnO films prepared by atomic layer deposition on a Si (100) substrate is investigated by an atomic force microscope. The ZnO films have various thicknesses ranging from 10.0 to 182.1 nm. With the increase of film thickness, the root-mean-square roughness of the films increases, while the ratio of hardness to Young’s modulus (H/E) decreases. Due to their large surface roughness, the thick ZnO films are low in adhesion force. The friction force of the ZnO films is smaller than that of the Si (100) substrate and is greatly influenced by their adhesion force and mechanical property. In a low-load condition, the friction force is dominated by the adhesion force, and thus, the friction force of the ZnO films decreases as film thickness increases. While in a high-load condition, the friction force is dominated by plowing. Films with higher H/E possess smaller friction force, and thus, the friction force increases with the decreasing film thickness.     

Frictional behaviour of Pb‐Sn thin‐film lubrication

The frictional properties of lead‐tin thin films (thickness of 0.05–0.19 μm) with two types of copper interlayer were investigated. The thin film and the interlayer were formed on a silicon wafer surface by vacuum deposition. Friction tests were carried out using a ball‐on‐disc apparatus in a vacuum chamber. The thin copper interlayer reduced the friction coefficient and prolonged the film life. The effect of load on the friction coefficient is explained by an equation derived using the Hertzian contact area between a sphere and a plate. The thicker copper interlayer did not reduce the friction coefficient but markedly extended the life of the film. In this case, the dependence of the friction coefficient on the load is explained by an equation derived using the Hertzian contact area between a sphere with surface roughness of second order and a plate.     

Measurements of tribological properties of poly(pyrrole) thin film bearings

We report the results of a recent study on the tribological properties of electropolymerised thin films at light loads and low speeds. Poly(pyrrole) films incorporating different counter-ions have been electrochemically deposited onto gold electrodes on the plano-convex glass substrates and studied extensively. The measuring apparatus has been greatly improved from that reported earlier and now provides simultaneous monitoring of frictional force and wear. High precision capacitive gauging is employed to provide high resolutions of frictional force of better than 100 μN and height variation (wear) of 2 nm. A large number of specimens of poly(pyrrole) grown from five different counter-ions were prepared and their performances evaluated. The film morphology of each type of film was examined by atomic force microscopy (AFM) for control of the variability of film formation. Results are presented for the friction coefficients and wear rates observed for the films typically at a load of 2 N and a sliding speed of 5 mm s⁻¹. The effects of normal loading force and sliding speed on the friction coefficient are also discussed with a load range of 0.2–5 N and a sliding speed up to 30 mm s⁻¹.     

Method for testing sliding frictional response of lubricious thin films used in plastic medical syringes

Lubricious thin films are used in plastic medical syringes in order to reduce the frictional forces between the syringe barrel and the rubber plunger. Polydimethylsiloxane (PDMS) liquid films are the current accepted technology for reducing the friction forces in plastic medical syringes. However, major issues with these PDMS films exist, including interactions of the film with the stored injectable drugs and variations in the frictional response as the syringes are aged over time. A new silicon based, lubricious octamethylcyclotetrasiloxane (L-OMCTS) thin film solid lubricant has been developed as a replacement for PDMS that provides acceptable and stable frictional responses without interacting with injectable drugs. A novel test method has been developed that can be used to successfully characterise the sliding frictional response of the L-OMCTS thin films at the syringe barrel and plunger interface. This test method will be used to provide future insight into how the frictional response of the L-OMCTS thin films is affected by various system parameters. This paper will mainly discuss the design of this new test method and provide some preliminary frictional response data.     

Evaluation of the friction of WC/DLC solid lubricating films in vacuum

The accuracy of nanopositioning is to a large extent limited by the friction-caused errors, particularly in vacuum environments. An investigation of the friction behaviour of sp²-bonds dominating diamond like carbon (DLC) coatings and WC_1−x/DLC, WC(N)/DLC multilayer coatings, which are considered to be used in nanopositioning in vacuum, have been performed by a vacuum microtribometer. By using an atomically smooth Si sphere as a counterface, the reciprocating sliding friction was measured at a normal load <5 mN, and running speed at a 1–100 μm/s in ambient air and in ultra high vacuum (UHV) at 10⁻⁷ Pa, and correlated with microstructures and properties of the coatings. When tested in UHV, the coefficient of friction (COF) for pure DLC coatings (thickness: 700 nm) changes significantly between 0.2 and 0.4. Once the thickness of DLC layers is limited to 5 nm by formation of multilayer coatings, the COF in UHV decreases by nearly one order to 0.02–0.05. We suggest that the deformation of DLC films and the transfer films determines COF. Thick DLC coatings can induce more plastic deformation and consumes more energy in sliding resulting in a high COF. Thickening of the transfer film in running leads to a continuous decrease of COF since the deformation of the transfer films turns easier. The low COF of multilayer coatings is mainly due to their confinement of the thickness of DLC films. A consistent velocity-strengthening frictional behaviour of both WC_1−x/DLC and WC(N)/DLC coatings in UHV indicates that the transfer films acting as a thin layer of granular material. Further study of the friction behaviour with the presence of such granular materials might be interesting for the further development of tribological coatings for vacuum applications.     

Complex approach to investigate mechanical and tribological characteristics of microcontacting objects for MEMS

Abstract

A complex approach based on atomic force microscopy (AFM) is developed to establish influence of nanoscale layer thickness on its elastic, adhesive and frictional properties of polymeric coatings for microelectromechanical systems. Thermoheating element was applied to perform AFM measurements with thermal effects in the temperature range from 20 to 120°C. Friction coefficients at high velocities of sliding and dependences of friction coefficient on the temperature of heated films at low velocities of sliding are defined. This study concludes that the Young’s modulus of ultrathin polymeric films on silicon substrate is reduced when thickness or temperature is increased.     

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.     

Tribological and mechanical properties of carbon nitride thin coating prepared by ion-beam-assisted deposition

Carbon nitride thin films may become good competitors for diamond-like carbon, due to their high hardness, high wear resistance, and low friction coefficient. At present, there are only a few studies of the effect of CN _x coating hardness and internal stress on its tribological properties, such as coating life and frictional behaviour. This work deals with tribological and mechanical properties of a carbon nitride coating prepared by ion-beam-assisted deposition (IBAD). Friction coefficients in the range of 0.10–0.12 were observed for the best CN _x coatings sliding against silicon nitride under ambient conditions. A nonlinear correlation between coating life and its internal stress and hardness was found.     

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.     

Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 1. The effect of processing parameters and operating conditions

Diamond-like carbon (DLC) coatings were prepared on AISI 440C steel substrates at room temperature by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process in C₂H₂/Ar plasma. Using the designed Ti/TiN/TiCN/TiC interfacial transition layers, relatively thick DLC coatings (1-2 μm) were successfully prepared on the steel substrates. The friction and wear performance of the DLC coatings was evaluated by ball-on-disk tribometry using a steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). By optimizing the deposition parameters such as negative bias voltage, DLC coatings with hardness up to 30 GPa and friction coefficients lower than 0.15 against the 100Cr6 steel ball could be obtained. The friction coefficient was maintained for 100,000 cycles (∼2.2 km) of dry sliding in ambient environments. In addition, the specific wear rates of the coatings were found to be extremely low (∼10⁻⁸ mm³/Nm); at the same time, the ball wear rates were one order of magnitude lower. The influences of the processing parameters and the sliding conditions were determined, and the frictional behavior of the coatings was discussed. It has been found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Therefore, it is feasible to prepare hard and highly adherent DLC coatings with low friction coefficient and low wear rate on engineering steel substrates by the ECR-CVD process. The excellent tribological performance of DLC coatings enables their industrial applications as wear-resistant solid lubricants on sliding parts.     

Application of ellipsometry to the characterization of reaction films formed by ZnDTPs on sliding steel surfaces

Spectroscopic ellipsometry (SE) was used to determine the thickness of zinc dialkyldithiophosphate (ZnDTP) reaction films formed on steel surfaces. Simultaneous measurements of friction coefficient and electrical contact resistance (ECR) were carried out using a cylinder-on-disk tribometer to form ZnDTP reaction films under a sliding condition, with monitoring of both the change of friction and the formation of ZnDTP reaction films during sliding. The film thickness was obtained by regression fitting of the data generated by the dispersion model to the experimental data. The results indicated that the thickness of the reaction film influenced the friction behavior of ZnDTP.     

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.     

Dry lubrication with soft metals: The tribological behaviour of a thin film of cadmium rubbing on carbon steel

Since cadmium has many properties which make it suitable for use as a solid lubricant, a thorough study of its behaviour on a modified pin-on-ring testing device with conformal mating surfaces was made. The subsurface temperature, friction force and wear depth were recorded simultaneously. Pins of fully annealed carbon steel C40 were used. The rings were successively coated by electroplating with nickel, copper and cadmium films to obtain good adhesion to the hard substrate and to minimize diffusion into the hard mating surface. Three velocities, 1, 0.5 and 0.1 m s^?1, and two loads, 20 and 100 N, were selected to provide a range of test conditions. Sections of the tested specimens were observed using both an optical microscope and a scanning electron microscope. The persistence of cadmium between the rubbing surfaces at various sliding distances was investigated using X-ray analysis. The behaviour of successive nickel and copper films and nickel films was investigated to confirm the good lubrication and wear properties of cadmium especially under conditions of heavy working.     

Generalized effects in confined fluids: New friction map for boundary lubrication

Recent advances in measuring the rheology and tribology of thin liquid films between shearing surfaces have enabled previously-inaccessible parameters to be measured accurately during frictional sliding. These include the real area of contact, the local asperity load and pressure, and the sheared film thickness. The results show striking non-continuum, non-bulk like effects when the thicknesses of sheared films approach molecular dimensions as occurs under most tribological conditions. Based on these new results, we assess the validity of current presentations of friction processes, such as the Stribeck curve, and propose new constitutive relations and a dynamic friction map, including an alternative Stribeck type curve representation, which are also formulated in terms of more accessible parameters.     

Microscopic real time observation of failure, wear and deformation on coating/subsurface

The deposition of surface coatings on industrial component is widely used for reducing wear and friction in the tribological application. Tribological properties of various kinds of coatings have been systematically analyzed. This paper mainly deals with the investigation of failure mechanism of the coating/subsurface through the microscopic observation in real time through the repetition of sliding friction. The results of observations suggest that the mechanical failure is characterized by the intrinsic properties of coating and substrate materials, e.g. the relation between the thickness of coating and the depth of the plastically deformed region in the substrate. In the present study the microscopic real time observation is performed to investigate the failure of coating/subsurface by use of ‘frictional surface microscope system’, which is assembled by combining a metallographic microscope and a frictional device operated in the field of view of the microscope.     

Friction and Wear Characteristics of C/Si Bi-layer Coatings Deposited on Silicon Substrate by DC Magnetron Sputtering

The tribological behavior of carbon/silicon bi-layer coatings deposited on a silicon substrate by DC magnetron sputtering was assessed and compared to that of amorphous carbon and silicon coatings. The motivation was to develop a wear resistant coating for silicon using thin layers of amorphous carbon and silicon. Wear tests were conducted by sliding a stainless steel ball against the coating specimens under applied normal loads in the range of 20?~?50?mN. Results showed that the wear rate of the bi-layer coating was strongly dependent on the ratio of thickness between the carbon and silicon layers. The wear rate of the bi-layer coating with 25?nm thick carbon and 102?nm thick silicon layers was about 48 and 20 times lower than that of the single-layer amorphous carbon and amorphous silicon coating, respectively. In addition, the steady-state friction coefficient of the bi-layer coating could be decreased to 0.09 by optimizing the thickness of the layer. Finally, a model for the wear reduction mechanism of the carbon/silicon bi-layer coating was proposed.     

Microstructure and wear behaviour of Ni-based surface coating on copper substrate

The Ni-based surface coatings were prepared by a vacuum infiltration casting technique on copper substrate. The surface coatings were fabricated through copper melt penetrating into thin preforms whose thickness could change. By optimizing the processing parameters, compact surface coatings were achievable as confirmed through SEM observation. The surface coating was mainly composed of solid solution of Ni, solid solution of Cu and CrB. The macro-hardness of the coating was about HRC 58, and the micro-hardness of the coating shows a gradient distribution. The average micro-hardness of the coating was about HV450. Wear behaviour was investigated by using block-on-ring dry sliding linear contact at several loads (50 N–300 N) and two different sliding speeds (0.424 m/s and 0.848 m/s). Wear rate and friction coefficient were estimated using a method founded upon the PV factor theory. The surface oxidation predominated as the principle wear mechanism at low load. Meanwhile, adhesion and oxidation mechanism were observed when the coatings were tested at higher load more than 200 N. Friction coefficient decreased with increasing load and sliding speed.     

Study on Starved Lubrication Performance of a Cycloid Drive

The lubrication performance of cycloid drives affects the dynamic characteristics, mechanical efficiency, and contact fatigue behavior. Starved lubrication performance of a cycloid gear drive is studied using a numerical thermo-starved elastohydrodynamic lubrication model. The parameter of the inlet oil film thickness is chosen to represent the starvation degree. Effects of the inlet film thickness on the central film thickness, friction coefficient, frictional power loss, starting position of the effective film thickness, and lubrication efficiency are investigated. The optimum inlet film thickness is defined and is calculated under different rolling–sliding ratios, speeds, and loads. Finally, the optimum inlet film thickness during the meshing process of a cycloid drive is calculated.     

Effects of film thickness and contact load on nanotribological properties of sputtered amorphous carbon thin films

The nanotribological properties of amorphous carbon (a-C) films of thickness in the range of 5-85 nm sputtered on Si(1 0 0) substrates were investigated with a surface force microscope (SFM), using a Berkovich diamond tip of nominal radius of curvature approximately equal to 200 nm and contact (normal) loads between 10 and 1200 μN. The dependence of the friction and wear behaviors of the a-C films on normal load and film thickness was studied in terms of nanomechanical properties, images of scratched surfaces, and numerical results obtained from a previous analytical friction model. The increase of the contact load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it either remained constant or decreased slightly. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and plowing mechanisms contributed to the friction behavior in the intermediate- and high-load ranges. Thinner (thicker) a-C films yielded higher (lower) friction coefficients for normal loads less than 50 μN (low-load range) and lower (higher) friction coefficients for normal loads greater than 150 μN (high-load range). Elastic and plastic deformation, microcracking, and delamination of the a-C films occurred, depending on the contact load and film thickness ranges. The reduced load-carrying capacity, relatively low effective hardness (strength) obtained with thinner films, and dominant friction and wear mechanisms at each load range illustrate the film thickness and contact load dependence of the nanotribological properties of the sputtered a-C films.