DLC Coatings Research Articles

Tribological properties of Ti-doped DLC coatings under ionic liquids lubricated conditions

In this paper, titanium doped diamond-like carbon (Ti-DLC) coatings were prepared onto AISI 52100 steel substrates using medium frequency magnetic sputtering process, and were analyzed using the Raman and transmission electron microscope (TEM). Two kinds of 1,3-dialkyl imidazolium ionic liquids (ILs) were synthesized and evaluated as lubricants for Ti-DLC/steel contacts at room temperature, and PFPE as comparison lubricant. The tribological properties of the ILs were investigated using a ball-on-disk type UMT reciprocating friction tester. The results indicated that the ILs have excellent friction-reducing properties, the friction coefficient kept at a relatively stable value of 0.07–0.06, which was reduced approximately by 47% compared with perfluoropolyether (PFPE). The worn surfaces of Ti-DLC coatings were observed and analyzed using a MICROXAM-3D non-contact surface profiler, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The Ti-DLC coatings using ionic liquids lubricating systems are considered as potential lubricating system in vacuum and space moving friction pairs.

Comparative Study of Friction and Wear Behavior of Diamond-Like Carbon Coating under the Lubrication of PAGs Containing Various Additives

The friction and wear properties of the DLC coatings were evaluated while being lubricated with pure PAG, PAG containing PN and ZDDP using reciprocating ball-on-disk sliding UMT tester, respectively. The morphologies of the worn surfaces of the DLC coatings were observed using a scanning electron microscope (SEM). The results indicated that the DLC coatings exhibited better tribological properties under the lubrication of PAG containing PN or ZDDP than that of pure PAG. In addition, PN and ZDDP as additives show different tribological properties. The former offers better anti-wear ability, the latter offers better friction-reducing properties.

A grey-fuzzy Taguchi approach for optimizing multi-objective properties of zirconium-containing diamond-like carbon coatings

Metallic, diamond-like carbon (Me-DLC) coatings are widely used in the mold industry due to their excellent wear resistance. In this paper, zirconium-containing DLC (Zr-DLC) coatings are prepared using an unbalanced magnetron sputtering process. Factors that affect the wear resistance of coatings consist of friction coefficients, hardness and anti-sticking properties. This paper aims to develop a method that optimizes the multi-objective properties of Zr-DLC coatings. For statistical purposes, the experimental parameters for the Zr-DLC coatings were put in place with a L18(3 5) orthogonal array. In order to optimize these properties, a method combining the grey, fuzzy and Taguchi approaches was established. While using the developed grey-fuzzy Taguchi method (GFTM), the experimental results from four properties can be integrated into a performance index. A comparison of the integrated performance index between the initial and optimal conditions shows that the magnitude increases from 0.46 to 0.81. The gain for each property by the GFTM from the initial condition is reported as 35% positive.

Modulation of residual stress in diamond like carbon films with incorporation of nanocrystalline gold

Residual stress modulation in the diamond-like carbon coatings with incorporation of gold nanoparticles was studied critically. The films were deposited on glass and Si (1 0 0) substrates by using capacitatively coupled plasma chemical vapor deposition. Stresses in the films were determined from the broadening of the optical absorption tail and were found to decrease from 2.3 GPa to 0.48 GPa with increasing gold content (2–7 at.% Au) in the DLC matrix. Gold incorporation also made the films harder than the corresponding DLC coatings. Modulation of stress with nanocrystalline gold content in the DLC matrix was related to the relative amount of sp 2/sp 3 content in the DLC films.

The wear and friction behaviour of engineering coatings in ambient air and dry nitrogen

This work reports on the structural and wear properties of a range of engineering coatings including TiN, TiAlN, CrAlN, MoS2/Ti and a number of different DLC coatings, deposited on tool steel substrates. The tribological properties of the coatings were characterised by sliding wear tests in different environments of humid air and in dry nitrogen. Microstructural assessment was performed using scanning electron microscopy and atomic force microscopy (AFM). DLC coatings produced the lowest friction coefficient in dry nitrogen and in humid air, demonstrating their versatility. The coefficient of friction can be attributed to the oxidation of MoS2 at the wear track to form MoOx that is known to cause an increase in the friction coefficient. In the case of DLC the main effect of changing atmosphere was both friction coefficient and local wear mode/nature of debris produced; in general moist air produced far lower friction coefficients and wear rates; attributed to surface absorbates, which in turn modify local ductility and friction at asperity contacts. Delamination occurred in an IBAD (ion beam assisted deposition)-produced DLC, which was subjected to further investigation by scratch testing. AFM was used to investigate in detail the wear track morphology in the IBAD DLC and a reference DLC. The highest friction coefficients were generated by the TiAlN and CrAlN coatings.

Friction and wear behaviour of low-friction coatings in conventional and alternative fuels

Today low-friction PVD coatings are used regularly in combustion engines to reduce wear and energy loss due to friction. Three coatings based on transition-metal dichalcogenides and three DLC coatings were tested with respect to tribological behaviour in non-conformal sliding contact, in five conventional and alternative fuels and fuel blending components. The friction and wear proved to vary substantially between the different tested systems. The DLC coatings exhibited extremely good wear properties, but also higher friction. Contrastingly the TMD coatings showed promising friction results, but in their present forms they do not offer sufficient wear resistance in the tested severe contact situation.

Finite Element Method for Predicting the Cohesive Strength of DLC Film on 316L Stainless Steel by Four Point Bend Test and Validation with Experimental Results

The mechanical performance of DLC coatings on 316L stainless steel deposited by a saddle field fast atom beam source has been evaluated using the four point bend (FPB) test. Two different deposition parameters, pressure and current were varied when depositing the films. Load-displacement measurements were carried out during the bend test to determine the load corresponding to crack initiation. This load designated as the cohesive strength of the coating which is also called the cracking resistance of coating and provides a measure of the strength of the coating. The cohesive strength of the coating was calculated based on elementary beam theory. Scanning Electron Microscopy (SEM) was used to determine the location of the crack. Finite element analysis was used to predict the stress distribution across the coating thickness. The experimental work on FPB tests has been used to support the numerical (finite element) model for the determination and prediction of film cohesive strength. It was observed that at lower deposition current, the cohesive strength increases with increased deposition pressure whereas, for higher deposition current, these values do not increase with increasing deposition pressure. The model takes into account the film’s Young’s modulus, thickness and deposition pressure and current, and has shown that it is capable of predicting film cohesive strength when combined with a theoretical formulation for brittle fracture. It has been observed that the maximum stress develops at the outer surface of the film and propagates through the film-substrate interface. This result has only been validated for films with higher Young’s modulus compared to that of the substrate material.

Influence of different surface modification technologies on friction of conformal tribopair in mixed and boundary lubrication regimes

► Friction coefficient in mixed lubrication regime can be reduced by effective running-in or by polish the surfaces before test. ► Effective running-in can be achieved by using hard DLC coatings on journal or soft phosphate coating on bearing. ► Running-in gives lower friction in mixed regime compared to polished surfaces before test. Moving machine assemblies are generally designed to operate in full film lubrication regime to ensure high efficiency and durability of components. However, it is not always possible to ensure this owing to changes in operating conditions such as load, speed, and temperature. The overall frictional losses in machines are dependent on the operating lubrication regimes (boundary, mixed or full-film). The present work is thus aimed at investigating the role of different surface modification technologies on friction of a sliding bearing/roller tribopair both in boundary and mixed lubrication regimes. A special test rig comprising of two bearings was built for the experimental studies. Tribological tests were conducted in a wide speed range to enable studies in boundary and mixed lubrication regimes. The influence of application of different surface modification technologies on both the sliding bearing and the roller surfaces on friction has been studied. The rollers used in these studies were provided with five different coatings (hard DLCs and a soft self-lubricating coating). Additionally, two uncoated rollers having different surface roughness were also studied. Uncoated bearings were used in all tribopairs except two. These two bearings were coated with DLC and phosphate coatings respectively and uncoated rollers were the mating counterparts. Friction measurements were made on the new as well as the previously run-in surfaces. It was found that the rollers with self-lubricating coating resulted in lowest boundary friction closely followed by the rollers with the hardest DLC coatings. The DLC coating applied on to the bearing showed lower boundary friction after running-in. Mixed friction has been found to be mainly dependent on the surface topography characteristics of both the original and the run-in surfaces of bearings and rollers. The harder DLC coatings and the phosphated bearing showed the lowest mixed friction due to an efficient running-in of the bearing surface.

DLC-based solid–liquid synergetic lubricating coatings for improving tribological behavior of boundary lubricated surfaces under high vacuum condition

► Five DLC-based solid–liquid lubricating dual-layer coatings were successfully fabricated. ► Solid–liquid lubricating coatings of MACs, IL, Zdol exhibited excellent tribological properties. ► Solid–liquid lubricating coatings of PAO and silicon oil showed the poor tribological behaviors. ► Such difference depends on nature of liquid lubricant, and synergy lubricating mechanism with DLC. In the present study, five kinds of liquid lubricants, including MACs, IL, Zdol, PAO and silicon oil, were homogeneously spun on DLC coatings and consequently the DLC-based solid–liquid lubricating dual-layer coatings were successfully fabricated. The tribological behaviors of the DLC/steel combinations and steel/steel combinations lubricated with above five liquid lubricants were comparatively investigated using a high-vacuum tribometer simulated for space environments. The SEM, 3D surface profiler, Raman spectrum and EDS were employed to analyze the worn surfaces of the friction pairs. The analysis results showed that three kinds of solid–liquid lubricating coatings including MACs, IL, Zdol exhibited excellent tribological properties, in which DLC/steel combinations lubricated with these three liquid lubricants exhibited smoother running-in and much lower steady-state friction than steel/steel combinations, and the wear rates of DLC/steel combinations were in the range of 1–3 orders of magnitude lower than that of steel/steel combinations. But for PAO and silicon oil, they demonstrated the poorest tribological behaviors with high friction coefficients and wear rates. Such significant improvement in tribological performance of DLC-based solid–liquid lubricating dual-layer coatings under high vacuum can be attributed to the synergy lubrication mechanism by the combination of the solid lubrication effect of DLC film and the boundary lubrication of the liquid lubricants. Moreover, the differences among different liquid lubricants in tribological properties of DLC-based solid–liquid lubricating dual-layer coatings under high vacuum were possible due to the differences in viscosity and components of the five kinds of liquid lubricants.

Enhanced DLC wear performance by the presence of lubricant additives

Lubricant additives play significant role for reducing friction and wear of mechanical elements. The additives presented in 5W30 oil were developed for metal surfaces. However, they have been used in engine pieces covered with DLC coatings because they also offer the potential to reduce friction losses and wear in automotive applications. The friction and wear tests were carried out by using a UMT-CETR ball-on-disk tribometer in rotational mode under 5W30 synthetic oil at 100 °C. The X-ray photoelectron spectroscopy (XPS) showed the presence of Mo and S in the wear tracks. These elements are from decomposition of ZDDP and MoDTC additives producing MoS2 in DLC surface, which offers enhanced durability by low wear rate.

Comparative Study of Friction and Wear Behavior of Diamond-Like Carbon Coating under Various Lubricants

The friction and wear properties of AISI 52100 steel and DLC coatings were evaluated while being lubricated with silicone oil, PAO and PAG lubricants by using a reciprocating ball-on-disk sliding UMT tester. The morphologies of original surface and worn surfaces for the DLC and Ti doped DLC coatings were observed by using a scanning electron microscope. The results show that the DLC coatings have better tribological properties than AISI 52100 steel under silicone oil, PAO and PAG lubrication conditions. In addition, the DLC coatings have much better wear resistance than the AISI 52100 steel.

Tribological properties of diamond-like carbon coatings prepared by anode layer source and magnetron sputtering

In this work we studied the tribological properties of DLC coatings prepared by two deposition techniques. The emphasis was given on double layer Cr/DLC coatings deposited by a closed drift ion beam technique (anode layer source, ALS) with C2H2 and N2 carrier gases. For comparison, the same types of substrates were coated by unbalanced magnetron sputtering in the CemeCon CC800/9 deposition system. Pin-on-disk experiments showed that the DLC coatings possess excellent wear resistance (wear rate down to 12μm3/Nm) and also low values of coefficient of friction (down to 0.055). The presence of a carbon transfer layer, which is mainly responsible for good tribological properties, was observed on the wear scars of ball surfaces by optical microscopy. In addition, we measured the Vickers microhardness (1000–3100HV), performed the scratch test (LC in the range 40–100N) and Rockwell indentation test to measure adhesion. Coating surface has been analyzed by atomic force microscopy (AFM) and by profilometry.

Multilayer DLC coatings via alternating bias during magnetron sputtering

To combat the high residual stress problem in monolayer diamond-like carbon coatings, this paper fabricated multilayer diamond-like carbon coatings with alternate soft and hard layers via alternating bias during magnetron sputtering. The surface, cross sectional morphology, bonding structures and mechanical properties are investigated. The atomic force microscopy images indicate low bias results in rougher surface with large graphite clusters and voids suggesting low coating density. The multilayered coatings demonstrate relatively smooth surface stemming from higher bias. The cross sectional images from field emission scanning electron microscopy indicate coating thickness decreases as substrate bias increases and confirm that higher bias results in denser coating. Delamination is observed in monolayer coatings due to high residual stress. The trend of sp 3/sp 2 fraction estimated by X-ray photoelectron spectroscopy is consistent with that of I D/I G ratios from Raman spectra, indicating the change of bonding structure with change of substrate bias. Hardness of multilayer diamond-like carbon coating is comparable to the coatings deposited at low constant bias but the adhesion strength and toughness are significantly improved. Alternately biased sputtering deposition provides an alternative when combination of hardness, toughness and adhesion strength is needed in an all diamond-like carbon coating.

The Effect of Maximum Normal Impact Load, Absorbed Energy, and Contact Impulse, on the Impact Crater Volume/Depth of DLC Coating

In this work, the influence of maximum normal impact load, absorbed energy, and contact impulse, on the impact crater volume/depth of a hydrogen-free diamond-like carbon coating (commonly known as DLC) has been studied. The tungsten high speed steel (SKH2) specimen discs were coated with DLC using the Physical Vapour Deposition (PVD) method. The 90o impact test was performed using a self-developed impact tester, where the DLC coated disc was impacted by a chromium molybdenum steel (SCM420) pin, at 400 impact cycles, under lubricated conditions. The results show that the most crucial factor, affecting the impact crater volume/depth of DLC coating under impact, is the maximum normal impact load.

Bias voltage effect on the structure and property of the (Ti:Cu)-DLC films fabricated by cathodic arc plasma

Titanium copper–diamond-like carbon (Ti:Cu)-DLC films were deposited onto silicon via cathodic arc evaporation process using titanium (Ti) and copper (Cu) target arc sources to provide Ti and Cu in the metal containing diamond-like carbon (Me-DLC) film systen. Acetylene reactive gases served as the carbon sources, which were activated at 180 °C at 20 mTorr. Varying substrate bias voltages from − 80 to − 250 V were used in order to provide the (Ti:Cu)-DLC structure. The structure, interface, and mechanical properties of the produced film were analyzed by transmission electron microscope (TEM), IR Fourier transform (FTIR) spectra, and Rockwell C hardness. The process parameters were compared by studying the various mechanical properties of the films, such as microhardness and adhesion. The results showed that the Ti-containing a-C:H/Cu coatings exhibited an amorphous layer of Ti-DLC layer as well as a nanocrystalline layer of Cu multilayer structure. The mechanical properties of the coatings, as determined by Rockwell C indents testing and the ball-on-disc test, varied with the the applied negative DC bias voltage. These (Ti:Cu)-DLC coatings are promising materials for soft substrate protective coatings.

Electrochemical behavior of diamond-like-carbon coatings deposited on AlTiC (Al 2O 3 + TiC) ceramic composite substrate in HCl solution

The electrochemical characterization/corrosion behavior of diamond-like carbon thin films is worthwhile to study and needed in the field (as there has been limited comprehensive evaluation of this across all types of DLC in the literature). In this paper, newly developed tetrahedral amorphous carbon (ta-C) and hydrogenated amorphous carbon (a-C:H) films, prepared by filtered cathodic arc deposition (FCVA) and plasma enhanced chemical vapor deposition (PECVD) respectively were deposited over AlTiC (Al 2O 3 + TiC) ceramic composite substrate. Electrochemical impedance spectroscopy (EIS) and polarization measurements have been used to evaluate the coating performance in 2 M HCl solution. This ceramic substrate is used widely for the hard disk drives and read and write heads in computer. The memory of the hard disks can be increased by improving the surface quality and decreasing the pinholes. The DLC coatings were modified under different preparation conditions by changing the nitrogen-doping ratios as an attempt for improving the surface distribution and minimizing the surface coating defects.

Ultra Low Friction of DLC Coating with Lubricant

The objective of this study was to find a trigger to make clear a mechanism of the ultra low friction by evaluating the friction property of DLC-DLC combination under lubrication with the simple fluid. The Pin-on-disc reciprocating and rotating sliding tests were conducted to evaluate the friction property. The super low friction property of pure sliding with the ta-C(T) pair coated by the filtered arc deposition process under oleic acid lubrication was found at the mixed lubrication condition. It was thought that the low share strength tribofilm composed of water and acid seemed to be formed on ta-C sliding interface. Additionally, the smooth sliding surface formed on ta-C(T) was seemed to be required to keep this tribofilm. Then, the super low friction was thought to be obtained by this superlubrication condition. Although the accurate and direct experimental data must be required to make clear this super low friction mechanism, the advanced effect obtained by the simple material combination is expected to be applied on the large industrial fields in near future.

The properties of fluorine-containing diamond-like carbon films prepared by pulsed DC plasma-activated chemical vapour deposition

Diamond-like carbon films containing up to 23.1 at. % of fluorine (F-DLC), were deposited onto silicon substrates by low-frequency, pulsed DC, plasma-activated, chemical vapour deposition (PACVD). The influence of fluorine on plasma current density, deposition rate, composition, bonding structure, surface energy, hardness, stress and biocompatibility was investigated and correlated with the fluorine content. X-ray photoelectron spectroscopy (XPS) analysis revealed the presence C–C, C–CF and C–F for F-DLC films with a low fluorine concentration (1.5–12.1 at. %), however for films with a higher fluorine content (23.0 at. %) an additional peak due to CF 2 bonding was detected. The addition of fluorine into the DLC film resulted in lower stress and hardness values. The reduction in these values was attributed to the substitution of strong C=C by weaker C–F bonds which induces a decrease in hardness. Ion scattering spectrometery (ISS) measurements revealed the presence of fluorine atoms in the outmost layer of the F-DLC films and there was no evidence of surface oxygen contamination. The water contact angle was found to increase with increasing fluorine content and has been attributed to the change of the bonding nature in the films, in particularly increasing CF and CF 2 bonds . Biocompatibility tests performed using MG-63 osteoblast-like cell cultures indicated homogeneous and optimal tissue integration for both the DLC and the F-DLC surfaces. This pulsed-PACVD technique has been shown to produce biocompatible DLC and F-DLC coatings with a potential for large area applications.

Tribological Behaviors of Different Diamond-Like Carbon Coatings on Nitrided Mild Steel Lubricated With Benzotriazole-Containing Borate Esters

Three synthesized benzotriazole-containing borate esters were separately added into poly-alpha-olefin (PAO) as additives, using molybdenum dithiocarbamate (MoDTC) as the comparison. The friction and wear behavior of Ti-DLC and Ti/Al-DLC coating on nitrided AISI-1045 steel sliding against AISI 52100 steel under the lubrication of PAO containing various additives was evaluated using a reciprocating ball-on-disk friction and wear tester. The morphology and chemical feature of the worn surfaces of the DLC coatings were observed and analyzed using a three dimensional (3D) surface profiler, a scanning electron microscope (SEM), and an X-ray photoelectron spectroscope (XPS). Results show that the three kinds of benzotriazole-containing borate esters as additives in PAO had much better tribological properties than MoDTC; the wear resistance of Ti/Al-DLC coating was better than Ti-DLC coating.

Space Tribometers: Design for Exposed Experiments on Orbit

Eight pin-on-disk tribometers have been made for testing materials in space on board the International Space Station. They will be exposed directly to the low earth orbit (LEO) environment on board the ''Materials on the International Space Station Experiments'' platform where they will experience extreme conditions including atomic oxygen, ultrahigh vacuum, radiation (including UV radiation), and thermal ranges from -40 to 60 C. In order to survive launch and LEO, these tribometers were designed to be extremely compact, rugged, and reliable. Pin-on-disk tribology experiments are now being per- formed with a 13.2 mm/s sliding velocity (14 RPM at 9 mm wear track radius) and a 1 N normal load with hemispherical pin of 1.5875 mm radius. Materials tested include MoS2/Sb2O3/Au, MoS2/Sb2O3/C, YSZ/Au/MoS2/ DLC, and SiO-doped DLC coatings, and bulk samples of polytetrafluoroethylene (PTFE) alumina nanocomposites and gold.