Tribological Behavior Of Diamond-like Carbon Films Research Articles

Effects of sp3 bond ratio and modulation ratios on nanotribology behaviors of diamond-like carbon coatings investigated using atomistic nanoscratching simulations.

Diamond-like carbon (DLC) films are amorphous solids in which carbon atoms are linked by hybridized sp3, sp2, and sp bonds. The mechanical and tribological properties of DLC films can be adjusted by tuning the sp2/sp3 bond ratio. These films are typically used as protective coatings for components such as dies, automotive engines, mechanical seals, hard disk drives, biological engineering devices, and micro/nano-electromechanical systems. Further exploration of the mechanical and tribological behavior of DLC films is important for enhancing functional design for the above applications. The simulation results show that single-layer DLC with a higher sp3 ratio has better resistance to indentation. Single-layer DLC with a lower initial sp3 ratio has lower friction and a shorter repeated cycle in the friction force curve due to an increase in the graphitization of the friction interface. Single-layer DLC with a higher sp3 ratio has a higher coefficient of friction because compared with the normal force, the friction force is much more sensitive to an increase in the sp3 ratio. Molecular dynamics simulations based on the Tersoff potential were performed using the open-source code LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator).

Tribological behaviors of DLC films with hierarchical surface textures under water lubrication: A molecular dynamic simulation

Tribological behaviors of diamond-like carbon (DLC) films with different levels of hierarchical surface textures with lubricant water molecules are investigated through molecular dynamics simulation. The friction forces stabilize at a small value for small normal loads, due to the complete separation between DLC films by water molecules, while friction forces with large normal loads show complicated changes under the cooperation of interfacial evolution and water behaviors. Under large normal loads, friction force increases firstly due to the direct contact of surface textures which are subsequently worn and graphitized, resulting in the temporary stabilization of friction force at a large value. With their further wearing, the amount of interfacial carbon clusters decreases and water molecules distribute evenly at interface, which leads to the gradual decrease and final stabilization of friction force. During the sliding, the water molecules show a restoration in the structure and amount of hydrogen bonds, thus making these molecules play different roles in various stages, i.e., these molecules demonstrate a better diffusion during the friction rise and an enhanced separating effect for DLC films during the friction stabilization. Furthermore, the same amount of water molecules in the one-level hierarchical (L1) model has a larger bearing capacity than that in the two-level hierarchical (L2) model. When the normal load exceeds the bearing capacity of water, the friction force for model L2 is more stable and smaller than that for model L1 after running-in periods due to flattened interfaces and evenly distributed water molecules.

Tribological behaviors of DLC films in sulfuric acid and sodium hydroxide solutions

Tribological behaviors of three typical kinds of diamond‐like carbon (DLC) films (a‐C, a‐C:Cr, and a‐C:H) in sulfuric acid and sodium hydroxide solutions were investigated. The a‐C film showed the lowest stable coefficients of friction (COF) in both sulfuric acid and sodium hydroxide solutions but the worst wear resistance in sulfuric acid solution. The a‐C:H film showed the highest COF in sulfuric acid solution and the best wear resistance in both sulfuric acid and sodium hydroxide solutions. The a‐C:Cr film exhibited superior comprehensive tribological performance in sulfuric acid solution, while in sodium hydroxide solution, high COF and very poor wear resistance was observed. What is more, friction and wear mechanism was revealed by investigating the friction‐induced material evolutions on the sliding surface.

Probing carbon-based composite coatings toward high vacuum lubrication application

Abstract For challenges in minimizing friction and wear of spatial mechanical systems, synergetic lubrication coatings were prepared by spinning liquid lubricants and hybrid greases on the diamond-like carbon (DLC) films, and were evaluated whether they could achieve a long-term safe and reliable operation under high vacuum. Under high vacuum conditions, liquid lubricants significantly reduce the friction and wear of DLC films. Hybrid greases not only show excellent lubrication performance, but also greatly enhance the tribological behavior of DLC films, especially the grease with optimal proportion. Such excellent tribological performance of DLC-based composite coatings at low applied loads depends on the synergy of DLC and fluid film, in reverse the tribo-chemical reaction film under harsh working conditions.

Tribological mechanism of diamond-like carbon films induced by Ti/Al co-doping

Co-doping two metals into diamond-like carbon (DLC) films exhibits a desirable combination of low residual stress and hard hardness for further application, but the insight into tribological mechanism induced by the co-doped metals is still not fully clarified yet. In this work, we fabricated the Ti/Al co-doped DLC films (Ti/Al-DLC) with various metal concentrations using the hybrid ion beam system, and the tribological properties of films were systematically investigated. Results revealed that the co-doped Ti/Al metals played an important role in the tribological behaviors of DLC films; the film deposited at 2.5 A (Ti10.06at.%Al4.78at.%) exhibited the lowest friction coefficient of about 0.05 and wear rate of 1.56 × 10−16 m3 N−1 m−1. This attributed to the formation of thick transfer layer in the friction interface, which could be described as a dual or hierarchy nanostructure constructed of cross-linking amorphous carbon networks and hard phase (mainly TiC and Al2O3) structures.

Tribological behavior of DLC films and duplex ceramic coatings under different sliding conditions

Diamond-like carbon (DLC) films are effectively utilized to improve the tribological properties of materials due to their low friction coefficient, good wear resistance and high hardness. In recent years, different pre-treatments such as plasma assisted diffusional processes have been used to increase long-term durability of these coatings. In the present work, DLC films were deposited on untreated and plasma nitrided AISI 4140 low-alloy steel samples using physical vapor deposition method (PVD). The effect of nitride layers on the structural and mechanical properties of DLC films was examined by XRD, SEM, microhardness tester and reciprocating tribotester. The wear tests were performed under dry, saltwater and lubricant environments. The microhardness results indicated that surface hardness considerably increased after duplex treatment and this caused to increase plastic deformation resistance of the material. The reciprocating test results showed that DLC films deposited on untreated and plasma nitrided samples reduced the coefficient of friction because of the formation of transfer film between sliding/mating surfaces. Also, wear rates improved significantly after duplex treatment in consequence of high plastic deformation resistance of duplex treated samples. The highest wear resistance was obtained in lubricated condition while the lowest wear resistance was for the samples tested in dry conditions.

Tribological behavior of DLC films in space and automotive oil under boundary lubrication

The extremely hardness, high elastic modulus, excellent wear and corrosion resistance, high thermal and chemical stability, and the low-friction nature of the diamond-like carbon (DLC) coatings open further possibilities in improving tribological performance and reliability of different machine components. However, despite of the low friction coefficients normally observed for DLCcoated surfaces under dry sliding conditions, only a few DLC-coated tribological components are likely to be operated completely without a lubricant. In this paper, it was investigated the tribological behavior of DLC films in space and automotive oil under boundary lubrication. DLC films was grown over 316L stainless steel disks using plasma enhanced chemical vapor deposition. The tribological performance of the DLC film under Fomblin and 5W30 oil was investigated. Transferred materials on the DLC surface was examined using Raman Spectroscopy and contact angle. The combined effect of the transferred layer and the water content on oils might contribute to the effective improvement of the frictional and wear rate behaviors, which depends on the chemical structure and end groups in the lubricating oil.

Improved Tribological Behavior of DLC Films Under Water Lubrication by Surface Texturing

Textured diamond-like carbon (DLC) films with the pattern of parallel grooves were developed by depositing DLC on textured stainless substrates in a PVD system. The texturing effects on tribological performance of DLC in water-lubricated condition were investigated. Results show that introducing specific patterns into DLC film not only retains the low friction coefficients, but also dramatically extends coating lifetime through affecting the coating delamination behavior and graphitization process during friction. Besides the adherence difference induced by surface texturing which could influence the delamination, another possible mechanism, “buffer stripes”, which is characteristic of the lateral soft/hard periodical structure, was proposed by us based on the Micro-Raman spectroscopy and nanoindentation analysis. Additionally, a much lower graphitization for textured DLC during friction may also be responsible for the improved wear resistance.

Tribological Behavior of DLC Films by RF-PECVD at the Elevated Temperature

The tribological behaviors of Diamond-like carbon (DLC) films, prepared by the radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) method, were studied in the room temperature and the elevated temperature. The ball-on-disk tests with DLC films on steel specimens were conducted at a sliding speed of 60 rpm, a load of 10 N, and surrounding temperatures of 25°C and 75°C. The results show that the coefficients of friction and the amounts of wear of DLC films were decreased at higher temperature. After tests the wear tracks of hydrogenated DLC film were analyzed by Raman spectroscopy.

The Influence of the Temperature Increase on the Tribological Behavior of DLC Films by RF-PECVD

DLC (Diamond Like Carbon) films show very desirable surface interactions with high hardness, low friction coefficient, and good wear-resistance properties. The friction behavior of hydrogenated DLC film is dependent on tribological environment, especially surrounding temperature. In this work, the tribological behaviors of DLC (Diamond-like carbon) films, prepared by the radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) method, were studied in elevated temperatures. The ball-on-disk tests with DLC films on steel specimens were conducted at a sliding speed of 60 rpm, a load of 10 N, and surrounding various temperatures of 25, 40, 55 and 75. The results show considerable dependency of DLC tribological parameters on temperature. The friction coefficient decreased as the surrounding temperature increased. After tests the wear tracks of hydrogenated DLC film were analyzed by optical microscope, scanning electron spectroscopy (SEM) and Raman spectroscopy. The surface roughness and 3-D images of wear track were also obtained by an atomic force microscope (AFM).

Tribological properties of segment-structured diamond-like carbon films

Diamond-like carbon (DLC) has been widely recognized as a tribo-coating material acting like a high wear-resistant solid lubricant with a low friction coefficient. The tribological behavior of DLC films depends on the environmental conditions, the nature of the coating, and mechanical properties of the substrate. Particularly, when the DLC film is distorted with the deformation of a soft substrate, the fragile DLC film is considerably damaged. In this study, DLC films have been deposited by RF-plasma chemical vapor deposition (CVD) with methane gas on soft-material substrates, such as rubber and aluminum, and friction and wear tests have been conducted using a ball-on-disk test. In the case of rubber substrates, DLC films show a relatively low friction coefficient from 0.2 to 0.3, despite extensive substrate deformation. Results of observation of the surface of the rubber substrate suggest that the DLC film forms a “segment structure” automatically, and a “segment structure coating” was applied to an aluminum substrate. Ball-on-disk test results revealed that wear resistance of the segment-structured DLC film was approximately twofold higher than that of a conventional continuous DLC coating.

1030 軟質材料上へのダイヤモンド状炭素膜コーティングとその摩擦摩耗特性

Diamond-like carbon (DLC) films have been widely recognized as an excellent wear resistant coatings with a low friction coefficient against many kinds of counterparts. The tribological behavior of DLC films depend on not only the quality of the film but also the environmental conditions such as the substrate materials and the pressure applied. Particularly when the DLC films are distorted with the deformation of the soft substrate, the conventional DLC film should be damaged easily. In this study DLC films have been deposited by RF-plasma CVD from methane on the soft material substrates, such as a rubber and aluminum, and the friction and wear tests were conducted using a ball-on-flat configuration under some conditions. In the case of rubber substrate, DLC films show low friction coefficient from 0.2 to 0.3,even if large strain of ε=0.5 was applied to the substrate. A new coating method was proposed in the case of DLC coatings of Al substrate, which employs the segment structure of DLC film. It was found that the wear resistance of the Al substrate was significantly improved in comparison with the conventional DLC coatings.

Tribological behavior of sliding diamond-like carbon films under various environments

Tribological behaviors of amorphous diamond-like carbon (DLC) films were experimentally evaluated under various environments using a steel ball-on-disk wear-rig at dry sliding surfaces. The DLC films were prepared on Si wafer by r.f. PACVD method using benzene (C 6H 6). Every test was performed under the normal load 4.9 N and sliding velocity of 0.05 m/s, and the coefficients of friction of DLC films were measured with the contact cycle. When the test was performed under a vacuum, the coefficient of friction of DLC films showed a significant fluctuation with the contact cycles, remarkably accompanied with roll-shaped polymeric wear debris. Another test using thermally-baked DLC films showed that the friction was lower and more stable. Under a humidity controlled air, friction of DLC films was also stable. Tests with dry O 2 and N 2 gas were extended to find any other environmental factors which could affect the tribological behavior of DLC films. It was found that tribological behavior of DLC films was dependent on the formation of friction layers which were mainly affected by the environment. In the final discussion of this work, effects of environment on the tribological behavior of DLC films were combined and discussed in terms of tribo-chemistry of DLC films.