Tribological Properties Of Diamond-like Carbon Coatings Research Articles

Surface Roughness Influence on Tribological Behavior of HiPIMS DLC Coatings

The application of diamond-like carbon (DLC) coatings in dry machining of difficult-to-machine materials has been gaining popularity due to high inertness, low coefficient of friction (COF), and high hardness of these coatings. Although the effect of surface roughness on the tribological properties of DLC coatings is of paramount importance, usually it is overlooked and coatings performance analysis was accomplished generally on highly polished substrates. The generation of polished surfaces is a time-consuming, labor-intensive process and, in most cases, not feasible for the industry due to its high cost. This article focuses on determining the effect of substrate (cemented carbide, WC-Co) surface roughness on the load-bearing capacity and tribological properties of DLC coatings deposited by High Power Impulse Magnetron Sputtering (HiPIMS) in Ne–Ar gas plasma. The DLC films were deposited onto WC-Co substrates with three different surface roughness profiles and their tribological performance were evaluated using a reciprocating tribotest. The high surface roughness resulted in increased wear rate due to high levels of asperities and increased potential for premature delamination of the coatings, while also causing severe damage to the counterbody due to inhibition of transfer film formation.

Mass-Produced Cu Nanoparticles as Lubricant Additives to Enhance the Tribological Properties of DLC Coatings

In this paper, Nano copper (Cu) particles with a core-shell structure and good spherical shape were prepared by an innovative method called mass-produced nanoparticles (MPNP). The prepared Cu nanoparticles have good dispersibility and are agglomeration-free in Pao oil. In particular, the effects of nano-Cu particles with different mass fractions on the tribological properties of the steel against diamond-like carbon (DLC) coating were studied systematically. The results showed that the nano-Cu particles with the mass fraction of 0.1 wt.% and the steel/DLC friction pairs had good synergistic lubrication. The friction mechanism of the metal nano-particles and carbon-based coatings mainly depends on the physical effects such as nano-bearing and nano-filling of the nano-particles, which has little correlation with the shear film formation of the metal nano-particles. Therein, the surface polish behaviors of the metal nano-particles and carbon-based coatings are the key to bringing the nano-bearing mechanism of nano-particles into full play. Therefore, the Cu nanoparticles prepared by MPNP show excellent tribological performance and possess broad prospects in the fields of lubricant additives.

Investigation of changes in the properties of diamond-like films under friction by the XPS method

The combination of unique physicochemical, mechanical and tribological properties of diamond-like coatings determines the prospects for their use in critical friction units, including those operating in a rarefied atmosphere and vacuum. The properties of diamond-like carbon (DLC) coatings depend on the contribution of the sp2 and sp3 fractions of the carbon hybrid atomic electron orbitals. Modern methods of determining the graphite and diamond proportion in coatings are time-consuming and insufficiently accurate. In addition, the determination of the sp3/sp2 ratio is often difficult due to the displacement of the energy position of the C1s electron line. In this paper, the change in the chemical state of carbon over the thickness of a diamond-like coating is studied by X-ray photoelectron spectroscopy. Analysis of the carbon line fine structure of the differential graphite spectra (sp2 bonds) and diamond (sp3 bonds) allowed us to establish the parameter δ, which determines the ratio of the graphite and diamond components in the DLC coating. Profiling with Ar+ ions of the diamondlike coating surface showed that with an increase in the etching time, the proportion of amorphized carbon increases, which means that the antifriction properties increase with the abrasion of the coating. The obtained regularities allow us to predict changes in the tribological properties of DLC coatings during operation. Ion profiling also allows to determine the thickness of coatings with high accuracy.

A Comprehensive Numerical Study on Friction Reduction and Wear Resistance by Surface Coating on Cam/Tappet Pairs under Different Conditions

As a vital component in the valve train of internal combustion engines (ICEs), the cam/tappet pair undergoes high mechanical and thermal loads and usually works in a mixed and boundary lubrication regime. This leads to considerable friction loss and severe surface wear. Currently, the applications of diamond-like carbon (DLC) coatings for automotive components are becoming a promising strategy to reduce the friction and lower the wear. However, the practical performance of the coating is related to many factors, including friction coefficient, thermal properties, load conditions, and surface topography. In order to investigate these factors and successively improve the fuel efficiency and durability of the cam/tappet pair, a comprehensive multi-physics analytical model considering the mechanical, thermal and tribological properties of DLC coatings is established in this paper. Simulations are carried out for the coated as well as the uncoated cam/tappet conjunctions with different roughness at various ambient temperatures. The results show that both the fluid and asperity contact friction for the coated cam/tappet conjunction are significantly reduced due to their favourable characteristics. As a result, the friction loss of the coated cam/tappet pair is noticeably lower by almost 40% than that of the uncoated, despite a slightly higher asperity contact. In addition, the wear resistance of DLC coatings is also impressive, although the wear condition becomes progressively more severe with the increasing ambient temperature. Moreover, the roughness has complex effects on the friction and wear under different conditions.

Tight-Binding Quantum Chemical Molecular Dynamics Study on the Friction and Wear Processes of Diamond-Like Carbon Coatings: Effect of Tensile Stress.

Diamond-like carbon (DLC) coatings have attracted much attention as an excellent solid lubricant due to their low-friction properties. However, wear is still a problem for the durability of DLC coatings. Tensile stress on the surface of DLC coatings has an important effect on the wear behavior during friction. To improve the tribological properties of DLC coatings, we investigate the friction process and wear mechanism under various tensile stresses by using our tight-binding quantum chemical molecular dynamics method. We observe the formation of C-C bonds between two DLC substrates under high tensile stress during friction, leading to a high friction coefficient. Furthermore, under high tensile stress, C-C bond dissociation in the DLC substrates is observed during friction, indicating the atomic-level wear. These dissociations of C-C bonds are caused by the transfer of surface hydrogen atoms during friction. This work provides atomic-scale insights into the friction process and the wear mechanism of DLC coatings during friction under tensile stress.

MoDTC lubrication of DLC-involving contacts. Impact of MoDTC degradation

Recently Diamond-Like Carbon (DLC) coatings have attracted considerable attention due to their low friction, high hardness, good wear and corrosion resistance, and high thermal and chemical stability. Although considerable research has been conducted on the effect of molybdenum-based additives, e.g., molybdenum dithiocarbamate (MoDTC), on the boundary lubrication of DLC coatings, the wear mechanisms leading to the coating removal are not fully understood to date. Moreover, the impact of the degradation of the MoDTC-containing base oil on the tribological properties of DLC coatings has not been investigated. This study could help to elucidate the mechanisms by which the coating is worn out in presence of MoDTC-containing base oil.In this work, the friction and wear performances of DLC coatings with different hydrogen contents and doping elements have been investigated in the presence of MoDTC-containing base oil. To enable the correlation between these changes and the modification of the contact surface chemistry, tribofilms generated during friction were investigated by SEM combined with Energy Dispersive X-ray Spectrometry (EDX) and X-ray Photoelectron Spectroscopy (XPS). A new mechanism explaining the severe wear experienced by DLC coatings when lubricated with a MoDTC-containing base oil is proposed.

Comparison of Alcohol and Fatty Acid Adsorption on Hydrogenated DLC Coatings Studied by AFM and Tribological Tests

Adsorption on hydrogenated diamond-like carbon (DLC) coatings with respect to boundary lubrication has been studied only very rarely, even though such adsorption has proved to be one of the main boundary-lubricating mechanisms for metal contacts. For this reason, we used atomic force microscopy (AFM) and tribotests to study the ability of hexadecanol and hexadecanoic acid to adsorb onto DLC and affect the tribological properties of DLC coatings, where steel was used as a reference. We have shown that alcohols and fatty acids can adsorb onto the DLC under static conditions. However, under dynamic tribocontact conditions, although alcohol and fatty acid molecules help to decrease the wear of the coatings, they are inefficient in reducing the friction. In all the experiments the fatty acid proved to have superior adsorption abilities compared to the corresponding alcohol. Based on our results and the existing literature, tentative adsorption mechanisms that include an environmental species effect, a temperature effect and a tribochemical effect are discussed for DLC surfaces.

Scratch resistance and tribological properties of DLC coatings under dry and lubrication conditions

Scratch resistance and tribological properties of hydrogenated and hydrogen free diamond like carbon (DLC) coatings were investigated in ambient atmosphere under unlubricated and lubricated conditions using layers of oleic (C18H34O2) and linoleic acid (C18H32O2). Scratch resistance property improved in hydrogenated DLC while using linoelic and oleic acids compared to unlubricated condition. Coefficient of friction (CoF) was found to decrease under ambient dry conditions, while under lubrication condition, it was found to increase with normal load. At low load of 1N, hydrogenated and hydrogen-free coatings show a super low CoF of 0.001 and high wear resistance with oleic acid.

Tribological properties of DLC coatings and comparison with test results: Development of a database

Due to their good wear resistance and low friction coefficient diamond-like carbon (DLC) coatings have been studied intensively for many years and a great deal of research has been carried out in the field of tribology. To facilitate an overview and comparison of the research results, a database containing information on the mechanical and tribological properties of coatings and base materials as well as the test conditions has been designed. The database is constructed in such a way that new data can be added easily. Practicability of the database has been checked on pin-on-disc contact tests using different contact conditions. The main purpose in building the database has been to examine the scientific and the practical value of the numerous research results on DLC coatings and to find out to what extent these data are comparable with one another. The analysis of the database has showed that it enables a fast search and comparison between different research results.

Influences of optical emission settings on wear performance of metal-doped diamond-like carbon films deposited by unbalanced magnetron sputtering

We deposited amorphous diamond-like carbon (DLC) films using an unbalanced magnetron sputtering method on M2 tool steels. The deposition process was controlled using a closed-loop optical emission monitor (OEM), which regulated the flow of reactive gases of N 2 and C 2H 2 via a fast-responding piezo valve. By tuning the OEM settings for N 2 and C 2H 2, we were able to deposit a compound DLC coating consisting of Ti, TiN, TiCN, TiC and a-C:H/Ti in sequence. Excellent mechanical and wear performance was achieved. Microstructure and tribological properties of DLC coatings were characterized using transmission electron microscopy, electron probe microanalysis, Raman spectroscopy, Vickers, and wear tests. The Raman intensity ratio I D/ I G of the characteristic G and D bands decreases and the G line position moves toward 1550 cm −1 with the decrease of OEM settings, corresponding to a higher sp 3 content and higher microhardness. Wear tests demonstrate that the average friction coefficient of DLC films reduces from 0.33 to 0.14, and wear life increases from 900 to 24 000 m with the decrease of OEM settings. The Ti content, corresponding to enhanced wear properties, in DLC increases with OEM settings. Finally, we discovered that the phase transformation from TiC to DLC is strongly influenced by OEM settings, and is demarcated at an OEM setting of 20%.

A comparative study of state-of-the-art diamond-like carbon films

Diamond-like carbon (DLC) coatings represent a group of very similar but still very different materials. Their characteristics range from graphite-like to diamond-like to polymer-like, depending on the deposition techniques. The sensitivity of DLC coating properties to processing conditions can be considered a problem, but on the other hand it creates opportunities for adaptation to specific applications. In this study the mechanical and tribological properties of various state-of-the art DLC coatings (from six suppliers) were investigated to generate a basis of data for comparison that should facilitate the selection of DLC coatings for particular applications. Hardness and modulus of elasticity were measured by depth-sensing indentation. Scratch testing was used to determine the adhesive strength of the coatings. The friction and wear tests were conducted using a ball-on-flat and flat-on-flat configuration under unidirectional and reciprocating motion, respectively. The results of this work demonstrate that the mechanical and tribological properties of DLC coatings vary strongly. Each coating type has specific advantages and the complete range of properties should be studied carefully to select the appropriate coating in a particular application. A more general guideline for DLC selection is discussed.