Tribological Properties of Hard Carbon Films on Zirconia Ceramics

In this study, the authors investigated the tribological properties of hard diamondlike carbon (DLC) films on magnesia-partially stabilized zirconia (MgO-PSZ) substrates over a wide range of bads, speeds, temperatures, and counterface materials. The films were 2 μm thick and produced by ion-beam deposition at room temperature. Tribological tests were conducted on a ball-on-disk machine with MgO-PSZ balls, in open air of 30 to 50% relative humidity under contact loads of 1 to 50 N, at sliding velocities of 0.1 to 6 m/s, and at temperatures of 400°C. Al₂O₃ and Si₃N₄ balls were also rubbed against the DLC-coaled MgO-PSZ disks, primarily to assess their friction and wear performance and to compare it with that of MgO-PSZ balls. A series of long-duration lifetime tests was run at speeds of 1, 2, and 6 m/s under a 5 N load to assess the durability of these DLC films. Results showed that the friction coefficients of MgO-PSZ balls sliding against MgO-PSZ disks were 0.5-0.8, and the average specific wear rates of MgO-PSZ balls ranged from 1 × 10^?5 to 5 × 10^?4 mm³/N·m, depending on sliding velocity, contact load, and ambient temperature. The friction coefficients of MgO-PSZ balls sliding against the DLC-coaled MgO-PSZ disks ranged from 0.03 to 0.1. The average specific wear rates of MgO-PSZ, balls were reduced by three to four orders of magnitude when rubbed against the DLC-coaled disks. These DLC films could last 1.5 to 4 million cycles, depending on sliding velocity. Scanning electron microscopy and micro-laser Raman spectroscopy were used to elucidate the microstructural and chemical nature of the DLC films and worn surfaces.     

Friction and wear of diamondlike carbon on fine-graindiamond

Friction and wear behavior of ion-beam-deposited diamondlikecarbon (DLC) films coated on chemical-vapor-deposited (CVD),fine-grain diamond coatings were examined in ultrahigh vacuum,dry nitrogen, and humid air environments. The DLC films wereproduced by the direct impact of an ion beam (composed of a 3 :17 mixture of Ar and CH₄) at ion energies of 1500 and700 eV. Sliding friction experiments were conducted withhemispherical CVD diamond pins sliding on four differentcarbon-base coating systems: DLC films on CVD diamond; DLC filmson silicon; as-deposited, fine-grain CVD diamond; andcarbon-ion-implanted, fine-grain CVD diamond on silicon. Resultsindicate that in ultrahigh vacuum theion-beam-deposited DLC films on fine-grain CVD diamond (similarto the ion-implanted CVD diamond) greatly decrease both thefriction and wear of fine-grain CVD diamond films and providesolid lubrication. In dry nitrogen and in humid air,ion-beam-deposited DLC films on fine-grain CVD diamond films alsohad a lowsteady-state coefficient of friction and a low wear rate. Thesetribological performance benefits, coupled with a wider range ofcoating thicknesses, led to longer endurance life and improvedwear resistance for the DLC deposited on fine-grain CVD diamondin comparison to the ion-implanted diamond films. Thus, DLCdeposited on fine-grain CVD diamond films can be an effectivewear-resistant, lubricating coating regardless of environment.     

Genesis of superlow friction and wear in diamondlike carbon films

Diamondlike carbon (DLC) films offer enormous possibilities for applications that require low friction and high wear resistance. The range of physical, chemical, mechanical, electrical, and optical properties offered by these films is also exceptional and can meet the increasingly multifunctional application needs of machine elements, microelectronics, and biological systems. Since the early 1990s, carbon has been used as a precursor in our laboratory for the design and synthesis of superhard and low-friction carbon films. As a result of systematic studies over the years, in 1997, we developed a new class of DLC films that provide friction and wear coefficients of 0.001–0.005 and 10⁻¹⁰–10⁻⁹ mm³/N m, respectively, in inert-gas or vacuum test environments. This paper will mainly concentrate on the tribology of these superlow-friction carbon films and provide an update on our understanding of the fundamental tribological mechanisms of such films. It will also expand on the effects of hydrogen within the films and gaseous species within the test environments on friction and wear.     

Friction and wear behaviour of hard and superhard coatings at cryogenic temperatures

The work presents data on friction and wear behaviour of pin-on-disc pairs with superhard diamond-like carbon (DLC) coatings and hard coatings of zirconium nitride (ZrN) and titanium nitride (TiN) in liquid nitrogen with loads of 2.5 and 10 N and sliding speed of 0.06 m/s. It is shown that at cryogenic temperatures the friction coefficients of pairs of two types of DLC coatings obtained by vacuum-arc deposition of filtered high-speed carbon plasma fluxes depend to a great deal on the mechanical properties of the coatings defined by predominant sp² or sp³ hybridization of valence electrons. A friction coefficient of 0.76 was observed for friction pairs of superhard (90 GPa) DLC coatings having properties similar to those of diamond. For “softer” DLC coatings of 40 GPa and properties similar to those of graphite the friction coefficient shows lower values (0.24–0.48) dependent on normal load and counterbody material. The DLC coatings obtained by the filtered arc technology exhibit good wear resistance and have strong adhesion to the substrate under friction in liquid nitrogen. With a normal load of 10 N under cryogenic temperature a low wear rate (of the order of 7.2×10⁻⁴ nm/cycle) was found for superhard DLC coatings. The friction coefficient of pairs with hard ZrN and superhard DLC coatings on steel discs was revealed to be linearly dependent on the counterbody material hardness between 20 and 100 GPa. The hardness of the pin was varied by means of depositing TiN or DLC coatings and also by using high-hardness compounds (boron nitride and synthetic diamond). Proceeding this way can be promising since it offers the possibility of creating low-temperature junctions of required friction properties.     

Dry Sliding of Nitrided NiTiHf

The unlubricated friction and wear properties of the superelastic material NiTiHf, treated by either gas nitriding or plasma nitriding, have been investigated. Pin-on-disk testing of the studied materials was performed at sliding speeds from 0.01 to 1 m/s at normal loads of 1, 5, or 10 N. For all of the studied friction pairs (NiTiHf pins vs. nitrided disks) over the given parameters, the steady-state coefficients of friction varied from 0.7 to 1.6, and those of the control (NiTiHf on NiTiHf) varied from 0.5 to 1.2. Pin wear factors ranged from approximately 10^?6 against the NiTiHf and plasma-nitrided disks to approximately 10^?4 for the gas-nitrided disks. The plasma-nitrided disks provided wear protection in several cases and tended to wear by adhesion. The gas-nitrided treatment generated the most pin wear but essentially no disk wear except under the most severe of the studied conditions (1 N load and 1 m/s sliding speed). The results of this study are expected to provide guidance for design of aerospace components such as gears and fasteners.     

Tribological behaviour of MW PACVD diamond coatings in vibrating contacts

Six diamond coatings were produced by microwave plasma-assisted chemical vapour deposition (MW PACVD) on polished SiC substrates. Process gas pressure and process gas composition were varied systematically, resulting in different coating morphologies, roughnesses and grain sizes. Friction and wear were investigated for unlubricated vibrating contact conditions in air at room temperature. Tribological tests on diamond coatings were performed in a ball-on-disc configuration, with steel balls (100Cr6) and alumina balls acting as counterbodies. For comparison of the tribological performance of the diamond coatings, standard tests were performed with a stroke of 200 μn, a load of 10 N, and a frequency of 20 Hz. The effect of relative humidity (RH) was investigated by testing in dry (3% RH), in normal (50% RH), and in moist air (100% RH), respectively To investigate running-in effects and wear propagation, the test duration (10⁴, 10⁵, and 1.2·10⁶ sliding cycles) was varied. For steel/diamond pairings, high wear at the steel ball and high friction were measured, influenced by roughness and grain size of the diamond coating and by relative humidity. For Al₂O₃ against diamond, a pronounced running-in of friction and wear was found. High friction and high wear at the beginning of each test are followed by a stationary phase with extremely low friction coefficients (⩽ 0.05) and with wear rates below the limit of resolution. This running-in depends on grain size and roughness of the coating, on relative humidity, as well as on the operational parameters. Surface analysis using SEM, EDX, LRS and AFM was conducted, to investigate tribologically induced surface changes.     

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.     

Wear–Corrosion Resistance of DLC/CoCrMo System for Medical Implants with Different Surface Finishing

The field of medical implants in the human body is a growing area with diverse tribological aspects. This application field has its own specific characteristics, dominated by stringent quality requirements due to the human suffering and sometimes life-threatening consequences of a surface failing to fulfil its required function. Combined wear–corrosion tests could provide more complete information about the implant behaviour in the aggressive body environment than separate wear and corrosion testing. Combined wear–corrosion experiments were performed using a reciprocating ball-on-plate apparatus equipped with an electrochemical cell. Untreated CoCrMo alloy samples as well as diamond-like carbon (DLC) coated samples were used as plate. The DLC coatings were tested with three different surface finishes: as-deposited, polished with diamond and brushed. All DLC coated samples with and without mechanical finishing had lower corrosion activity under wear–corrosion conditions and also smaller wear tracks when compared with the CoCrMo alloy. The current density for the coated alloy was about two orders of magnitude lower on average (10^?5 vs. 10^?3 A cm^?2) and had a final coefficient of friction of only 50% of the uncoated metal (0.15 vs. 0.3). The brushed DLC showed the lowest current density and its behaviour was better than polished DLC and DLC as-deposited up to a potential of +0.93 V.     

Wear of Silicon Carbide in Sliding Contact with Lubricated Thin-Film Rigid Disk

Silicon carbide (SiC) is a potential ceramic material for recording heads, yet its tribological performance against lubricated thin-film rigid disks is not fully known. Square pins with a 100 mm radius spherical surface were made from hot pressed SiC, chemical vapor deposited (CVD) SiC, and Al₂O₃TiC, and tested with lubricated thin-film disks. The pin-on-disk tests showed that the region of contact on the spherical surface of the SiC and CVD-SiC pins wears away to form a circular wear plateau with smears in and around the plateau. The wear plateau is formed rapidly in the first 1000 drag revolutions and then very gradually grows in size with further revolutions. Analysis of the smears showed that a large fraction of the smears contained SiO₂ which had been oxidized from SiC due to high temperatures generated at the pin surface in contact with the disk. In contrast, tests with Al₂O₃. TiC pins did not show any formation of a wear plateau on the pins.     

Abrasive Wear of DLC/PVD Multilayer Coatings: AFM Studies

The tribological behaviour of multilayered coatings deposited on plain carbon steel was investigated by microscale abrasion tests (MSATs). The multilayered coatings consisted of an outer diamond‐like carbon (DLC) layer, a physical vapour deposition (PVD) nitride‐based interlayer, and an inner electroless Ni‐P layer. PVD TiN‐ and Ti(C,N)‐coated samples with and without the DLC outer layer were studied in order to evaluate the influence of each layer on the tribological behaviour of the multilayer‐coated system. The MSATs were carried out using a device based on ball‐cratering geometry: a hard steel sphere was rotated against the coated specimen in the presence of an aqueous suspension of SiC particles. The wear coefficients of the multilayers were calculated from the diameter of the wear craters. The morphology of the wear scars produced by the MSATs was studied by atomic force microscopy (AFM). The wear damage was described by measuring the r.m.s. roughness (S_q) on the sides of the wear craters. Roughness values were related to the wear coefficients (k_c) for the different multilayers on the basis of mathematical elaboration typical of the ‘design of experiment’ (DOE) statistical technique. The presence of the DLC outer layer reduced the roughness of the crater sides and significantly increased the wear resistance of the multilayer only in the case of the PVD TiN sublayer.     

Friction and wear properties of CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/SiC in water lubrication

Amorphous carbon nitride coatings (a-CN_x) were deposited on SiC disk by ion beam assisted deposition (IBAD). The tribological behavior of a-CN_x coating sliding against SiC ball in water was investigated and compared with that of SiC/SiC system at room temperature. The influences of testing conditions on friction coefficient and specific wear rate of both kinds of tribopairs were studied. The worn surfaces on disks were observed by scanning electron microscope (SEM). The results indicate that the running-in period of a-CN_x/SiC was shorter than that of SiC/SiC system in water. At a sliding velocity of 120 mm/s, the mean steady-state friction coefficients of SiC/SiC (0.096) was higher than that of a-CN_x/SiC (0.05), while at 160 mm/s, lower friction coefficient (0.01) was obtained for SiC/SiC in water. With an increment of normal load, the mean steady-state friction coefficients after running-in first decreased, reaching a minimum value, and then increased. For self-mated SiC, the specific wear rate of SiC ball was a little higher than that of SiC disk, while for a-CN_x/SiC, the specific wear rate of SiC ball were 10 times smaller than that of a-CN_x coating. Furthermore, the specific wear rate of SiC ball sliding against a-CN_x coating was reduced by a factor up to 100~1000 in comparison to that against SiC in water. The wear mechanism of SiC/SiC system in water is related to micro-fracture of ceramic and instability of tribochemical reaction layer. Conversely, wear mechanism for a-CN_x/SiC is related to formation and transfer of easy-shear friction layer.     

Molecular dynamics simulations of atomic-scale tribology between amorphous DLC and Si-DLC films

Silicon-doped diamond-like carbon (Si-DLC) films possess the potential to improve wear performance of DLC films in humid atmospheres and at higher temperatures. But many experimental results of Si-DLC films show that their structures and tribological properties changed greatly with silicon content. Therefore, molecular dynamics (MD) simulations were used to study the sliding friction process between DLC and Si-DLC films on un-lubricated boundary condition. The results show that a part of sp² bonding of the Si-DLC films is converted into sp³ bonding with the addition of silicon atoms, and the sp³/sp² ratio increases with the increase in silicon content. A transfer film between the DLC and Si-DLC films is formed and the friction force changes with the silicon content. Moreover, the simulations have showed that the silicon addition promotes the bonding of interfilms being formed.     

Friction of diamond-like carbon films in different atmospheres

Diamond-like carbon (DLC) films constitute a class of new materials with a wide range of compositions, properties, and performance. In particular, the tribological properties of these films are rather intriguing and can be strongly influenced by the test conditions and environment. In this paper, a series of model experiments are performed in high vacuum and with various added gases to elucidate the influence of different test environments on the tribological behavior of three DLC films. Specifically, the behavior of a hydrogen-free film produced by a cathodic arc process and two highly hydrogenated films produced by plasma-enhanced chemical-vapor deposition were studied. Flats and balls used in these experiments were coated with DLC and tested in a pin-on-disc machine under a load of 1 N and at constant rotational frequency. With a low background pressure, in the 10⁻⁶ Pa range, the highly hydrogenated films exhibited a friction coefficient of less than 0.01, whereas the hydrogen-free film gave a friction coefficient of approximately 0.6. Adding oxygen or hydrogen to the experimental environment changed the friction to some extent. However, admission of water vapor into the test chamber caused large changes: the friction coefficient decreased drastically for the hydrogen-free DLC film, whereas it increased slightly for one of the highly hydrogenated films. These results indicate that water molecules play a prominent role in the frictional behavior of DLC films—most notably for hydrogen-free films but also for highly hydrogenated films.     

The wear characteristics of ultrahigh molecular weight polyethylene against a high density alumina ceramic under wet (distilled water) and dry conditions

In recent years there has been growing interest in the use of high density alumina ceramic material for the femoral ball in association with ultrahigh molecular weight polyethylene (UHMWPE) for the acetabular component in total replacement hip joints.The wear characteristics of UHMWPE pins sliding against a high density alumina ceramic disc in the presence of distilled water in a tri-pin-on-disc machine have been revealed in very long-term experiments reported in this paper. A total sliding distance in excess of 6000 km was achieved and very low mean wear coefficients of the order of 10^?8 mm³ N^?1 m^?1 were recorded.Experiments were also carried out over a shorter sliding distance under dry conditions and the average wear coefficient of 2 × 10^?7mm³N^?1m^?1 was consistent with earlier findings. In these dry tests, comet-like streaks of polyethylene were transferred to the ceramic counterface, but no such transfer was noted during the wet tests. When distilled water was added to the test chamber after a considerable period of dry sliding, the wear coefficient rapidly decreased to about 10^?8 mm³ N^?1 m^?1 and the streaky transfer film disappeared from the ceramic counterface.The possibility of hydrodynamic action between the wear face on the pins and the counterface was investigated by reversing the direction of sliding. Surface topography changes on both the pins and the discs and friction and bulk temperatures of the pins were recorded throughout the tests.It is concluded that the excellent dry wear coefficients of UHMWPE sliding on alumina ceramic counterfaces are about twenty times greater than those experienced by the same materials in the presence of distilled water. The tribological advantage of the ceramic with respect to stainless steel having a similar surface roughness has been confirmed in dry sliding involving UHMWPE, but further work is required to determine whether or not the same advantage can be achieved under wet conditions.     

The effects of iron particles on the friction and wear mechanisms of ceramics in ethanol

For the combinations of an Si₃N₄ pin and five kinds of ceramic disk (SiC, Si₃N₄, Al₂O₃, ZrO₂, TiC), a friction and wear test was carried out in ethanol and in ethanol containing iron particles (1 wt.%, average diameter d = 200 nm, D = 12 μm under cohered condition) under a load in the range 5.88–11.50 N, at a sliding velocity of 0.138–0.196 m s⁻¹. A topographical analysis was also performed on the microasperities of the wear surfaces to estimate the behavior of the iron particles, and the degree of surface damage. As a result, the following facts were found. (1) The addition of iron particles in ethanol decreased both the wear rates of SiC and TiC disks and the mating pins, and also decreased the wear rate of the Al₂O₃ disk but increased that of the mating pin. The addition increased the wear rates of both ZrO₂ and Si₃N₄ disks and the mating pins. (2) The average coefficients of friction with the addition of iron particles were greater than those without iron particles. (3) The wear rates of pin and disk depended on the topographies of wear surfaces and the wear index Γ.     

Effects of sliding velocity and normal load on friction and wear characteristics of multi-layered diamond-like carbon (DLC) coating prepared by reactive sputtering

Friction and wear tests were performed to investigate effects of sliding velocity and normal load on tribological characteristics of a multi-layered diamond-like carbon (DLC) coating for machine elements. The DLC coatings which consist of sequentially deposited gradient Cr/CrN, W-doped DLC (a-C:H:W) and DLC (a-C:H) layers were formed on carburized SCM 415 Cr–Mo steel disks using a reactive sputtering system. The tests against AISI 52100 steel balls were performed under various sliding velocities (0.0625, 0.125, 0.25, 0.5, 1 and 2 m/s) and normal loads (6.1, 20.7 and 49.0 N) in ambient air (relative humidity=26±2%, temperature=18±2 °C). Each test was conducted for 20 km sliding distance without lubricating oil. The results show that friction coefficients decrease with the increase in sliding velocity and normal load. Wear rates of both surfaces decrease with the increase in normal load. The increase in sliding velocity leads initially to the increase in wear rates up to the maximum value. Then, they decrease, as the sliding velocity increases above specific value that corresponds to the maximum wear rate. Through surface observation and analysis, it is confirmed that formation of transfer layers and graphitized degree of wear surfaces of DLC coatings mainly affect its tribological characteristics.     

Effects of fullerene C60 on the friction and wear characteristics of ceramics in ethanol

For the combinations of a pin of Si₃N₄ and five kinds of disk, the friction and wear test was carried out in ethanol, and in ethanol containing C₆₀ particles (1 wt%). A topographical analysis was also performed on the micro-asperities of the wear surfaces to estimate the behavior of C₆₀ particles, and the degree of surface damage. As a results, the following facts were found. (1) The addition of C₆₀ particles in ethanol decreased the wear rates of such ceramic disks as Al₂O₃, SiC and TiC and of the mating Si₃N₄ pins. (2) The addition of C₆₀ particles decreased the mean coefficient of friction for SiC, Si₃N₄, and TiC disks. (3) The wear rates of pin and disk depended on the topographies of worn surfaces such as the mean depth of micro-grooves, the mean tip radius of micro-asperities and so on.     

Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 2. The analysis of wear mechanism

This paper describes the tribological performance of diamond-like carbon (DLC) coatings deposited on AISI 440C steel substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process. A variety of analytic techniques were used to characterize the coatings, such as Raman spectroscopy, atomic force microscopy (AFM) and nano-indentation. The sliding wear and friction experiments were carried out by the conventional ball-on-disk tribometry against 100Cr6 steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). All the wear tests were conducted under dry sliding condition in ambient air for a total rotation cycle of 1 × 10⁵ (sliding distance ∼2.2 km). Surfaces of the coatings and the steel balls were examined before and after the sliding wear tests. The DLC coatings that had been tested all showed relatively low values of friction coefficient, in the range of 0.1-0.2 at a steady-state stage, and low specific wear rates (on the order of 10⁻⁸ mm³/Nm). It was found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Plastic deformation became more evident on the coating surface during the sliding wear test at higher contact stresses. The friction-induced transformation of the coating surface into a graphite-like phase was revealed by micro-Raman analysis, and the flash temperature of the contact asperities was estimated. It was suggested that the structural transformation taking place within the wear tracks was mainly due to the formation of compact wear debris layer rather than the frictional heating effect. On the other hand, an adherent transfer layer (tribolayer) was formed on the counterface, which was closely related to the steady-state friction during sliding and the wear mechanisms. Fundamental knowledge combined with the present tribological study led to the conclusion that adhesive wear along with abrasion was probably the dominant wear mechanism for the DLC/steel sliding systems. Additionally, fatigue processes might also be involved in the wear of the coatings.     

类金刚石薄膜水润滑摩擦学特性研究进展

综述类金刚石薄膜水润滑摩擦学特性的研究进展,评述薄膜在水环境中的摩擦磨损特性,分析薄膜种类、元素掺杂、对摩材料以及微结构对DLC薄膜水润滑摩擦学特性的影响,并阐述DLC薄膜在水中的摩擦磨损机制。指出:DLC薄膜水润滑摩擦学特性受薄膜制备参数和摩擦试验环境影响,通过与微结构的耦合可以进一步改善类金刚石薄膜的摩擦学特性。同时还展望了类金刚石薄膜水润滑摩擦学未来研究方向。     

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。