Tribological characterisation of hard coatings with and without DLC top layer in fretting tests

The potential of coatings to protect components against wear and to reduce friction has led to a large variety of protective coatings. In order to check the success of coating modifications and to find solutions for different purposes, initial tests with laboratory tribometers are usually done to give information about the performance of a coating. Different Ti‐based coatings (TiN, Ti(C,N), and TiAlN) and NiP were tested in comparison to coatings with an additional diamond‐like carbon (DLC) top coating. Tests were done in laboratory air at room temperature with oscillating sliding (gross slip fretting) with a ball‐on‐disc arrangement against a ceramic ball (Al₂O₃). Special attention was paid to possible effects of moisture (relative humidity). The coefficient of friction was measured on line, and the volumetric wear at the disc was determined after the test from microscopic measurements of the wear scar and additional profiles. The friction and wear behaviour is quite different for the different coatings and depends more or less on the relative humidity. The DLC coating on top of the other coatings reduces friction and wear considerably. In normal and in moist air the coefficient of wear of the DLC top‐layer coating is significantly less than 10⁻⁶ mm³/Nm and the coefficient of friction is below 0.1. In dry air, however, there is a certain tendency to high wear and high friction. Copyright © 2006 John Wiley & Sons, Ltd.     

Behaviour of MoDTC in DLC/DLC and DLC/steel contacts

Diamond-like carbon (DLC) coatings are beginning to be used on machine components parts because of their excellent friction and wear resistance properties. It is hence important to be able to formulate lubricants able to work effectively with these coatings. This requires knowledge of how the various surface-reactive additives generally employed in lubricants behave with DLCs. This paper compares the behaviour of seven types of DLC, a-C, a-C:H, a-C:H:W, a-C:H:WC, Si-DLC, ta-C, ta-C:H, lubricated with molybdenum dialkyldithiocarbamate (MoDTC) solution. It is found that a-C and a-C:H:WC give lower boundary friction than the other types of DLC. MoDTC improves the wear resistance of DLC/DLC contacts but appears to greatly degrade the wear resistance properties of some DLCs in DLC/steel contacts, even though Mo-derived tribofilms form on all DLCs.     

Friction and wear behaviour of hard coatings in vibrating contacts

The friction and wear behaviour of thin hard coatings, such as TiN and the promising class of C-based coatings (a-C, a-C:H, and diamond for example), are compared under oscillating and reciprocating sliding conditions. The typical effects of test parameters, such as stroke, frequency, normal force, relative humidity and test duration, are described as a basis for the proper selection of test conditions or, conversely, for the selection of suitable coatings for particular practical applications. Friction and wear data from over 1000 vibrating tests using thin hard coatings against 100Cr6 and against Al₂O₃ have been compiled in a database. This allows easy manipulation and comparison of test results. Using selection criteria and filter procedures (e. g., lifetime of coatings, friction limits, and critical wear rate), suitable coating systems for different test conditions can be chosen from the database. The effects of test parameters on friction and wear behaviour and changes have anyway to be known for meaningful tribotesting, as well as for the selection of coatings.     

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.     

Tribological properties of tribofilms formed from ZDDP in DLC/DLC and DLC/steel contacts

Diamond-like carbon coatings (DLCs) are considered to hold great promise for improvement in friction and wear resistance of engine parts. It is hence interesting to know whether conventional engine oil additives such as ZDDP can form tribofilms and reduce friction and wear in DLC contacts as effectively as they do in steel on steel contacts. This paper compares the behaviour with ZDDP of six different DLC coatings. It is seen that ta-C gives lower boundary friction than the other types while a-C:H gives better wear prevention. A ZDDP-derived tribofilm forms on all DLCs but a pad-like structure is seen only on W-DLC in DLC/DLC tribopairs.     

Tribological Properties of DLC Coatings in Helium

The aim of this research work was to investigate tribological properties of low-friction DLC coatings when operating in helium atmosphere. Two commercial DLC coatings (a-C:H and Me-C:H) were included in the investigation and compared to reference PTFE-based coatings, normally used on components operating in helium. Coatings were deposited on hardened 100Cr6 bearing steel discs and tested against uncoated steel balls in low-load pin-on-disc contact configuration. Investigation was focused on the effect of substrate roughness (R _a ?=?0.05?C0.2???m) and contact conditions, including contact pressure (150?C350?MPa) and sliding speed (0.2?C0.4?m/s) on the coefficient of friction of DLC coatings operating in helium. Results of this investigation show that for low-load sliding contact DLC coatings provide low friction in helium atmosphere, similar to soft PTFE-based coatings. At the same time DLC coatings investigated were found to substantially reduce wear of the coated surface. However, while the wear of the coated part has been more or less eliminated, application of DLC coating prolongs running-in and increases wear of the steel counter-part. Furthermore, also in helium atmosphere tribolgical behaviour of DLC coatings showed dependence on the coating type and contact conditions.     

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.     

Ultra-High Tribological Performance of Magnetron Sputtered a-C:H Films in Sand-Dust Environment

The hydrogenated amorphous carbon (a-C:H) films were prepared on AISI 440C steel substrates using a RF magnetron sputtering graphite target in the CH₄ and Ar mixture atmosphere. The friction and wear behavior of a-C:H films were comparatively investigated by pin-on-disc tester under dry sliding and simulated sand-dust wear conditions. In addition, the effects of applied load, amount of sand and sand particle sizes on the tribological performance of a-C:H films were systemically studied. Results show that a-C:H films exhibited ultra-high tribological performance with low friction coefficient and ultra-low wear rate under sand-dust environments. It is very interesting to observe that the friction coefficient of a-C:H film under sand-dust conditions was relatively lower when compared with dry sliding condition, and the wear rate under sand-dust conditions kept at the same order of magnitude (×10⁻¹⁹ m³/N m) with the increase of applied load and particle size as a comparison with the dry sliding condition. Based on the formation of “ridge” layer (composite transfer layer), a transfer layer-hardening composite model was established to explain the anti-wear mechanisms and friction-reducing capacity of a-C:H solid lubrication films under sand-dust conditions.     

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.     

Structure and water-lubricated tribological properties of Cr/a-C coatings with different Cr contents

The Cr containing amorphous carbon coatings (Cr/a-C) with varying Cr content were deposited using unbalanced magnetron sputtering. The results revealed that the chromium carbide nano-clusters were formed when the Cr content exceeded 4.9 at%. The critical load increased while the hardness decreased after the Cr element incorporation. Although the low Cr containing Cr/a-C coatings (≤4.9 at%) exhibited similar friction coefficient with a-C coatings, but the initial friction coefficient, running-in distance and wear rate of SUS440C balls all decreased. However, the Cr/a-C coatings with high Cr content (11.98–14.09 at%) would worsen the tribological properties because chromium carbides acted as abrasive wear particles during tribotests.     

Influence of Silicon on the Wear Properties of Amorphous Carbon Under Dry and Lubricated Conditions

Diamond-like carbon (DLC) coatings have drawn much attention as potential surface coatings for engineering contacts because of their excellent friction and wear resistance properties. However, much less is known about their friction and wear mechanisms, especially at higher contact pressures. In this study, two amorphous carbon DLCs, with (a-C:Si) and without Si (a-C), have been investigated to understand the influence of Si on friction and wear under dry, base oil- and fully formulated oil-lubricated conditions. Si does not affect friction but significantly affects wear. a-C:Si shows lower wear than a-C but imparts higher wear on the steel counterpart. The steel counterpart that forms a hybrid tribolayer (transferred carbon from DLC + additive-derived tribofilm products) exhibits superior wear resistance properties.     

Compatibility of DLC coatings with formulated oils

In the present study, the tribological performance and compatibility of hydrogenated amorphous carbon coating (a-C:H) and metal-doped diamond-like carbon (DLC) coating (Me-C:H) with formulated oils under the boundary lubrication regime was investigated. The investigation employed ball-on-flat contact geometry in reciprocating sliding motion and six formulated oils (manual gearbox oil, automatic gearbox oil, hydraulic oil, compressor oil, and normal and high performance motor oil), with pure poly-alpha-olefin (PAO) oil used as a reference. In addition, DLC coatings behavior in diesel and gasoline fuel was evaluated.Compared with the uncoated steel surfaces a-C:H coatings give improved wear resistance in base PAO as well as in fully formulated oils and fuels. On the other hand, W-doped DLC coatings show the lowest steady-state friction under boundary lubrication, especially when using oils with high additive contents.     

Gradient DLC-Based Nanocomposite Coatings as a Solution to Improve Tribological Performance of Aluminum Alloy

The low hardness and poor tribological performance of aluminum alloy as moving component greatly restricts their wide applications in automotive fields. In this letter, an attempt to deposit gradient Ti/TiN/Si/(TiC/a-C:H) multi-layer on aluminum alloy is thus effectively performed by a combined arc ion plating and magnetron sputtering process based on the concept of involving coatings with a functionally graded interface. Multi-layered structure within DLC-based coatings has shown to significantly improve the load-bearing capacity, anti-wear and self-lubricating ability of Al alloys. The friction coefficient of gradient DLC-based coatings decreased to 0.18 under dry sliding condition while kept at 0.05 under the oil-lubricated conditions. The wear rate of gradient DLC multilayers was lower by two and even three orders of magnitude when compared with Al alloys both under dry wear and oil-lubricated conditions. Such gradient DLC-based coatings with good adhesion strength, high hardness, and excellent tribological performance are considered as potential protective surfaces of Al alloys for engine parts.     

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.     

碳基薄膜水润滑性能的研究进展

评述了碳基薄膜如类金刚石薄膜(DLC)和非晶氮化碳(a-CNx)薄膜水润滑的研究现状和进展。分析了第2元素加入和摩擦副材料对碳基薄膜在水中摩擦磨损特性的影响,探讨了碳基薄膜在水中的磨损机制。指出:氢化或氮化碳基薄膜的磨损率与摩擦副材料的水合反应有关,若摩擦副材料易于摩擦水合反应,碳基薄膜的磨损率很低;3种DLC薄膜在水中的磨损率与DLC的种类和对磨钢球材料无关,都在10-8mm3/(N.m)的数量级上变动;a-CNx/Si基非氧化物陶瓷摩擦副显示很低的摩擦因数和低的磨损率;在相同条件下,a-CNx薄膜比a-C薄膜更能显示优异的水润滑性能。     

Effect of polar groups in lubricants on sliding speed dependent friction coefficients of DLC coatings

The speed dependent friction coefficients of two types of DLC coatings, a-C:H and ta-C, were evaluated when lubricated with 1-hexadecene, which did not contain any functional group, and with oleic acid and oleyl alcohol that did. The friction coefficient measured for ta-C at a low sliding speed of 0·01 mm s^?1 with the 1-hexadecene lubricant that did not contain any functional group was 0·22, which was higher than the value of 0·11 seen for a-C:H. The friction coefficients measured for a-C:H and ta-C at a high sliding speed of 50 mm s^?1 with 1-hexadecene were 0·10 and 0·06 respectively. The friction coefficients measured with oleic acid and oleyl alcohol were 0·02 for a-C:H and less than 0·001 for ta-C. The results showed that the friction coefficient of ta-C was more strongly influenced by the functional group in the lubricants than that of a-C:H. It is assumed that this difference between the two coatings can be attributed to a difference in the capability to form a tribochemical reacted film under boundary lubrication. Under mixed lubrication, differences in lubricity also affected this friction coefficient difference, in addition to the properties of the tribochemical reacted film.     

Tribochemical effects on the friction and wear behaviors of a-C:H and a-C films in different environment

Friction and wear behaviors of hydrogenated amorphous carbon (a-C:H) and hydrogen-free amorphous carbon (a-C) films sliding against Si₃N₄ balls were investigated in different testing environments. The result showed that two films with extreme chemical disparity (one hydrogenated, and the other hydrogen free) showed distinct different friction and wear behaviors, and the friction and wear behaviors of the both films were strongly dependent on the environment. For a-C:H films, much low friction coefficient and wear rate were obtain in dry N₂. In the water and/or oxygen containing environments, the friction coefficient and wear rate of a-C:H films were obviously increased. On the contrary, a-C films only provided low friction coefficient and wear rate in the presence of water and/or oxygen in the test chamber. In dry N₂, the highest friction coefficient and wear rate were observed for a-C films. By investigating the worn surfaces of the films using XPS, it was proposed that the environment dependence of the friction and wear behaviors of the films was closely related with the friction-induced chemical interactions between the films and water and/or oxygen molecules. The specific roles of hydrogen, water and oxygen molecules and their tribochemical effects on the friction and wear mechanism of the films are discussed.     

Fatty Acid Adsorption on Several DLC Coatings Studied by Neutron Reflectometry

The application of diamond-like carbon (DLC) coatings on the contacts of mechanical systems is becoming widespread thanks to their excellent tribological properties. Numerous studies of DLC coatings have been reported over the past decade and, as a result, the understanding of their lubrication has improved. The tribological properties of boundary-lubricated contacts are drastically affected by adsorbed layers; however, due to the variety of lubricant additives and coating structures, no general adsorption mechanisms for DLC coatings have been put forward until now. This has, unfortunately, hindered improvements in their lubrication performance. Many of the essential physical properties of the adsorbed layers also remain undefined. In this work, we used neutron reflectometry to determine the thickness and the density of the adsorbed layers of fatty acid molecules on coatings of a-C, a-C:H, a-C:H:F and a-C:H:Si. The results showed that a 0.9-nm-thick layer adsorbed onto the a-C and a-C:H coatings. In contrast, both doped coatings, i.e. the a-C:H:F and a-C:H:Si, showed a poorer adsorption ability towards the fatty acid molecules than the a-C and a-C:H. Continuous adsorption layers were not detected on the a-C:H:F and a-C:H:Si; however, the possibility of adsorption in lower quantities cannot be ruled out.     

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.     

An overview on the tribological behavior of diamond-like carbon in technical and medical applications

Diamond-like carbon (DLC) coatings are a class of materials and, depending on the deposition conditions and the tribological system, different outstanding tribological properties can be obtained. The most important subclasses of DLC are hydrogenated amorphous carbon (a-C:H) and hydrogen free tetrahedral amorphous carbon (ta-C). When DLC slides against a metal, the formation of the so-called transfer layer on the metallic counterpart can protect the metal from further wear. Adhesion and cohesion of this transfer layer is a critical issue and is also influenced by the environmental atmosphere including the relative humidity. When DLC is running against a hard and chemically inert counterface, such as sapphire, the formation of a transfer layer is not observed but low wear is still obtained. This is attributed to the lubricating properties of the graphitic wear residues. Due to its unique combination of properties, DLC is already used in different industrial applications such as magnetic storage media, diesel injection pumps, sliding bearings, car valve rockers, gears, tappets of racing motorcycles, laser barcode scanner windows in supermarkets, VCR head drums, textile industry parts, motor cycle forks, razor blades, etc. In medical applications, i.e. the coating of load bearing joints that slide against ultra high molecular weight polyethylene (UHMWPE), the different in vitro experiments apparently showed contradicting results, mainly due to the different experimental setups and especially the different liquids used as lubricants. However, when DLC slides against DLC in medical applications low wear rates could be demonstrated in different in vitro tests.