The role of hydrogen on the friction mechanism of diamond-like carbon films

The structure, properties and tribological behavior of DLC films are dependent on the deposition process, the hydrogen concentration and chemical bondings in the films. The present paper reports selected tribological experiments on model DLC films with different hydrogen contents. The experiments were performed in ultrahigh vacuum or in an atmosphere of pure hydrogen or argon in order to elucidate various friction mechanisms. Two typical friction regimes are identified. High steady-state friction in UHV (friction coefficient of 0.6) is observed for the lowest hydrogenated and mostly sp²-bonded DLC film. Superlow steady-state friction (friction coefficient in the millirange) is observed both for the highest hydrogenated film in UHV, and for the lowest hydrogenated film in an atmosphere of hydrogen (10 hPa). The high steady-state friction in UHV, observed for the lowest hydrogenated film with a dominant sp² carbon hybridization, is associated with a –^* sub-band overlap responsible for an increased across-the-plane chemical bonding with a high shear strength similar to what is observed with unintercalated graphite in the same UHV conditions. Superlow friction is correlated with a hydrogen saturation across the shearing plane through weak van der Waals interactions between the polymer-like hydrocarbon top layers. This regime is observed during the steady-state period if the film contains enough hydrogen incorporated during deposition. If this condition is not satisfied (i.e., for the film with the lowest hydrogen content), the limited diffusion of hydrogen from the film network towards the sliding surfaces seems to be responsible for a superlow running-in period. The superlow friction level can be reached over longer time periods by suitable combinations of temperature and molecular hydrogen present in the surrounding atmosphere during friction.     

Ultralow friction of DLC in presence of glycerol mono-oleate (GNO)

This paper presents a unique tribological system that is able to produce no measurable wear of material combination and that reduces friction markedly in the ultralow regime under boundary lubrication. Ultralow friction (0.03) was obtained by sliding hydrogen-free Diamond-Like-Carbon ta-C against ta-C lubricated with Poly-alpha Olefin base oil containing Glycerol Mono-Oleate (GMO) additive. The origin of ultralow friction in these conditions has been investigated by surface analysis techniques. Results are in agreement with the formation of a OH-terminated carbon surface. This new surface chemistry might be formed by the tribochemical reaction of alcohol function groups with the friction-activated ta-C atoms. The origin of low friction could be due to the very low-energy interaction between OH-terminated surfaces.     

类金刚石膜在水环境下的摩擦学行为

测试了 CoCrMo合金表面沉积类金刚石薄膜与CoCrMo在水溶液润滑下的摩擦磨损行为。结果表明：摩擦副在不同浓度牛血清白蛋白溶液润滑下的平均摩擦因数均在0.10左右,CoCrMo合金销磨损量最小值为1.69×10-5 mm3;相同条件下,生理盐水溶液润滑的磨损量为1.38×10-5 mm3,且销表面有转移膜生成;牛血清白蛋白溶液润滑时,界面蛋白吸附层屏蔽了转移膜的形成。根据结果可知,转移膜的形成经历了磨屑附着、连续转移、局部脱落的过程。     

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.     

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。     

Influence of Deposition Positions on Fretting Behaviors of DLC Coating on Ti-6Al-4V

Abstract

The influence of diamond-like carbon (DLC) coating positions—coated flat, coated cylinder, and self-mated coated surface tribopairs—on the fretting behaviors of Ti-6Al-4V were investigated using a fretting wear test rig with a cylinder-on-flat contact. The results indicated that, for tests without coating (Ti-6Al-4V–Ti-6Al-4V contact), the friction (Q_max/P) was high (0.8–1.2), wear volumes were large (0.08–0.1?mm³) under a large displacement amplitude of ±40 µm and small (close to 0) under a small displacement amplitude of ±20 µm, and the wear debris was composed of Ti-6Al-4V flakes and oxidized particles. For tests with the DLC coating, under low load conditions, the DLC coating was not removed or was only partially removed, Q_max/P was low (≤0.2), and the wear volumes were small. Under high load conditions, the coating was entirely removed, Q_max/P was high (0.6–0.8), and the wear volumes were similar to those in tests without coating. The wear debris was composed of DLC particles, Ti-6Al-4V flakes, and oxidized particles. The DLC coating was damaged more severely when deposited on a flat surface than when deposited on a cylindrical surface. The DLC coating was damaged more severely when sliding against a DLC-coated countersurface than when sliding against the Ti-6Al-4V alloy.     

高速钢表面镀覆DLC膜的结构及其摩擦特性研究

以高纯石墨作靶材、N2/Ar为工作气体,采用非平衡磁控溅射技术在高速钢上制备了光滑、致密、均匀的掺氮DLC膜。用XPS、Raman光谱仪表征了DLC膜的结构,并在球-盘摩擦试验机上研究了其摩擦特性。结果表明:工作气体中的N2比例对DLC膜结构及其摩擦特性影响较大,随着N2比例的增加薄膜中sp3的比例减少,而sp2和碳氮键的比例增加;随着N2比例的增加,薄膜摩擦因数先减后增,这与薄膜中sp3比例下降和sp2比例增加而导致薄膜的硬度以及内应力变化有关。就本试验而言,工作气体中N2比例不应高于20%。     

The respective role of oxygen and water vapor on the tribology of hydrogenated diamond-like carbon coatings

The tribological behavior of diamond-like carbon coatings (DLC) strongly depends on the chemical nature of the test environment. The present study proposes to explore the influence of water vapor and oxygen on the friction behavior of a hydrogenated DLC coating exhibiting ultralow friction in ultrahigh vacuum (friction coefficient below 0.01). Using a UHV tribometer, reciprocating pin-on-flat friction tests were performed in progressively increasing or decreasing partial pressures of pure oxygen and pure water vapor. The maximum gaseous pressures of oxygen and water vapor were 60 hPa and 25 hPa (1 hPa = 100 Pa), respectively, the second value corresponding to a relative humidity (RH) of 100% at room temperature. It was found that, for the pressure range explored, oxygen does not change the ultralow friction behavior of DLC observed in UHV. Conversely, water vapor drastically changes the friction coefficient at pressures above 0.5 hPa (RH = 2%), from about 0.01 to more than 0.1. Electron energy loss spectroscopy and in situ Auger electron spectroscopy have been performed to elucidate the friction mechanisms responsible for the tribological behaviors observed with the two different gaseous environments. In all cases no significant oxidation has been observed either inside the wear scars or in the wear debris particles. Ultralow friction is systematically associated with a homogeneous carbon-based transfer film. The higher friction observed at partial pressure of water vapor higher than 0.5 hPa, is associated with a thinner transfer film. Consequently friction seems to be controlled by the transfer film whose kinetics of formation strongly depends on the partial pressure of water vapor. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Combination of DLC coatings and EP additives for improved tribological behaviour of boundary lubricated surfaces

The aim of the present investigation was to obtain some further understanding of the mechanism responsible for low-friction behaviour of W-containing DLC coatings (W-DLC) when lubricated with EP additivated oil. Boundary lubricated wear and friction tests were performed under reciprocating sliding motion using a high frequency test rig and a contact pressure of 1.5 GPa. Additionally, some of the tests were performed in a load-scanning reciprocating test rig, with the contact pressure being in the range from 2.4 to 5.6 GPa. The influence of concentration of a sulphur-based EP additive on the friction behaviour was investigated.This investigation showed that W-DLC coatings greatly improve the tribological properties of boundary-lubricated surfaces, especially when pairing coated and uncoated steel surfaces. The improved tribological behaviour was found to be governed by the gradual formation of a WS₂ type tribofilm on the steel counter-face or on revealed steel substrate. The friction level depends on the additive concentration.     

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.     

Experimental investigation of friction in entrapped elastohydrodynamic contacts

Start-up friction is a performance-limiting aspect of hydraulic motor operation. This study was conducted to gain a better understanding of the roles played by contact pressure, speed, and oil type to start-up friction behavior for contacts containing trapped pockets of highly pressurized oil, also known as elastohydrodynamic lubrication (EHL) entrapments. An apparatus was built to measure the start up friction response for ball-on-plane sliding contact with simultaneous observation of the contact region by optical interferometry. Baseline trials for all cases were conducted in the absence of any entrapment and then repeated after forming an entrapment. An impact, activated by solenoids, was used to create a small separation whereby oil would fill the gap and then become trapped as the load rapidly brought the surfaces back into contact.In all cases, entrapment substantially decreased the start-up friction. Additionally, the short-lived entrapments provide the greatest reduction in start-up friction. Therefore, the method of entrapment that may be implemented with least delay before the initiation of sliding has the greatest potential.     

类金刚石碳基薄膜在1-乙基-3-甲基咪唑四氟硼酸盐中的摩擦学性能

利用射频等离子体增强化学气相沉积技术,以甲烷为气源,在单晶硅(P(001))衬底上制备类金刚石碳基薄膜(DLC);利用高速往复摩擦磨损试验机分别测试DLC薄膜/Al2O3球摩擦副在大气环境下和1-乙基-3-甲基咪唑四氟硼酸盐离子液润滑下的摩擦磨损性能;利用光学显微镜,X射线光电子能谱和三维轮廓仪分别对磨痕、磨痕表面元素和磨损率进行考察。实验结果表明:DLC薄膜在离子液润滑时,在低载荷下减摩作用明显,但在较高载荷下摩擦因数较无离子液润滑时高,且不随载荷增加而变化,推测是离子液形成了边界润滑膜;XPS分析表明这层边界润滑膜可能是由离子液物理吸附在摩擦接触面上形成的,并且对DLC薄膜有很强的抗磨作用。     

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.     

Wear behaviour of hydrogenated DLC in a pin-on-disc model test under lubrication with different diesel fuel types

The wear behaviour of hydrogenated diamond like-carbon (DLC) coating in DLC/steel tribological contact in a pin-on-disc model test under lubrication with two diesel fuels is presented in this work. The first diesel fuel was standard EN590 that contained ester-based antiwear additives. In contrast to EN590, the second diesel fuel, called GDK650, did not contain antiwear additives. It was experimentally observed that the antiwear additives are detrimental to the DLC. The effects of load, speed and temperature on the DLC and steel counterbody wear were investigated. Steel counterbody wear volume was found to be not affected by pressure, temperature, speed and lubricant, whereas the DLC-coating revealed correlation between the parameters and wear rate. Regarding the results of the tribological tests under both diesel lubrications, new mathematical wear laws were developed.     

Adhesion and friction properties of hydrogenated amorphous carbon films measured by atomic force microscopy

Atomic force microscopy has been used to measure adhesion and friction forces at the interface between an oxidized metal probe tip and amorphous carbon films of varying hydrogen contents (12.3–39.0 atomic percent hydrogen). The interface of an oxide surface and a hard carbon coating models the unlubricated head-disk interface of current hard disk products. Adhesion forces normalized by the radius of curvature of the contacting tip range from 1.09 to 8.53 N/m. Coefficients of friction values, measured as the slope of the friction versus load plot, range from 0.33 to 0.87. A trend of increasing adhesion forces and coefficients of friction is observed for increasing hydrogen content in the films. We attribute the increase in adhesion and friction to increases in the surface free energy of the carbon films with the incorporation of hydrogen.     

TiNi表面磁控溅射DLC薄膜的纳米压痕与摩擦性能

采用室温磁控溅射技术在TiNi合金表面制备出DLC/SiC(类金刚石/碳化硅)双层薄膜(SiC为中间层),采用拉曼光谱仪、纳米压痕仪和球-盘式摩擦磨损仪研究DLC薄膜的结构、纳米压痕和摩擦性能.结果表明:制备的DLC/SiC薄膜石墨含量高、纳米硬度(5.493 GPa)低、弹性模量(62.2447 GPa)低.在以氮化硅球(半径为2mm)为对摩件,4.9N载荷、室温、Kokubo人体模拟体液润滑下,该DLC/SiC薄膜具有低且稳定的摩擦因数,其平均值约为0.094.     

ECR结合中频磁控溅射制备掺Cu类金刚石膜工艺及性能研究

为改善DLC膜的内应力及导热问题,采用ECR微波等离子体化学气相沉积及中频磁控溅射的方法制备掺Cu类金刚石膜,研究溅射电流对薄膜中Cu含量、薄膜表面形貌、结构及机械性能的影响.结果表明:改变溅射电流能有效地控制类金刚石膜中金属含量,拉曼光谱显示,制备的薄膜为典型的类金刚石薄膜结构;Cu的掺入使得类金刚石膜的硬度和耐磨损性能下降,但在一定溅射电流下可得到薄膜结构及机械性能均较好的掺Cu类金刚石膜.     

类陨石坑表面类金刚石薄膜的制备及摩擦特性

为了提高DLC（Diamond-like Carbon）类金刚石薄膜与SAE1060碳素钢基材的结合强度,以延长发动机活塞环的使用寿命,研制了一种带有复合阳极的RF-DCCVD双电源化学气相沉积设备。利用锯齿结构的辅助阳极产生尖端放电,制备了具有微米类陨石坑非连续结构的DLC薄膜,并利用Ball-on-Disk摩擦评价试验机评价了薄膜的摩擦特性。着重研究了极间距S -T对薄膜表面类陨石坑密度的影响;最后利用拉曼光谱仪分析了薄膜结构和成分。结果表明：在同样的电压下,类陨石坑的密度随着电极间距的增加而减小,最佳电极间距S -T为40~60 mm,此时不仅具有比较适中的类陨石坑密度,对DLC薄膜的摩擦特性影响不大,而且具有较强的界面结合强度。当S-T为50 mm,施加载荷为3 N时,薄膜的破坏寿命达到了130万循环,比光滑表面的薄膜延长了30万循环。得到的结果显示微米类陨石坑非连续结构能够有效地释放膜内的残余压缩应力,延长SAE1060碳素钢基材上沉积类金刚石薄膜的使用寿命。     

A predictive analytical friction model from basic theories of interfaces, contacts and dislocations

Friction between crystalline bodies is described in a model that unifies elements of dislocation drag, contact mechanics, and interface theory. An analytic expression for the friction force between solids suggests that dislocation drag accounts for many of the observed phenomena related to solid–solid sliding. Included in this approach are strong arguments for agreement with friction dependence on temperature, velocity, orientation, and more general materials selection effects. It is shown that calculations of friction coefficients for sliding contacts are in good agreement with available experimental values reported from ultrahigh vacuum experiments. Extensions of this model include solutions for common types of dislocation barriers or defects. The effects of third-body solid lubricants, superplasticity, superconductivity, the Aubry transition, and supersonic dislocation motion are all discussed in the framework of dislocation-mediated friction.