DLC-coated Surfaces Research Articles

Quantitative analysis of nano-wear on DLC coatings by AFM

Quantitative analysis of surface modifications due to wear tests of a diamond-like carbon (DLC)-coated cylindrical steel test sample was carried out by atomic force microscopy and confocal microscopy. In order to get a reliable metrics for the quantitative assessment of surface modifications due to nano-wear, different surface texture parameters like arithmetic average roughness Sa, surface kurtosis Sku, surface skewness Ssk, and core roughness depth Sk were analysed. The results show that, after application of a Gaussian filter to reduce the influence of the underlying longer-range surface topography variations, the surface skewness parameter Ssk was the most sensitive and reliable parameter to indicate the very small wear modifications of the DLC-coated surface. Further studies were carried out to investigate the influence of pixel sampling distance, scanning area and scanning tip dimensions.

The Influence of DLC Coating on EHL Friction Coefficient

High hardness, high elastic modulus, low friction characteristics, high wear and corrosion resistance, chemical inertness, and thermal stability are factors that make diamond-like carbon (DLC) coatings the subject of many studies. For the same reasons they also seem suitable for use in, amongst others, machine components and cutting tools. While most studies in the literature focus on the influence of coatings on wear and friction in boundary lubrication and pure sliding contacts, few studies can be found concerning rolling and sliding elastohydrodynamic lubrication (EHL) friction, especially in the mixed and full film regime. In this article tests are carried out in a Wedeven Associates Machine tribotester where an uncoated ball and disc pair is compared to the case of coated ball against uncoated disc, coated disc against uncoated ball, and coated disc against coated ball. The tests are conducted at two different temperatures and over a broad range of slide-to-roll ratios and entrainment speeds. The results are presented as friction maps as introduced in previous work (Björling et al. in J Eng Tribol 225(7):671, 2011). Furthermore a numerical simulation model is developed to investigate if there is a possibility that the hard, thin DLC coating is affecting the friction coefficient in an EHL contact due to thermal effects caused by the different thermal properties of the coating compared to the substrate. The experimental results show a reduction in friction coefficient in the full film regime when DLC-coated surfaces are used. The biggest reduction is found when both surfaces are coated, followed by the case when either ball or disc is coated. The thermal simulation model shows a substantial increase of the lubricant film temperature compared to uncoated surfaces when both surfaces are coated with DLC. The reduction in friction coefficient when coating either only the ball or the disc are almost the same, lower than when coating both the surfaces but still higher than the uncoated case. The findings above indicate that it is reasonable to conclude that thermal effects are a likely cause for the decrease in coefficient of friction when operating under full film conditions, and in the mixed lubrication regime when DLC-coated surfaces are used.

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.

Friction, Wear, and Surface Film Formation Characteristics of Diamond-Like Carbon Thin Coating in Valvetrain Application

The friction and wear characteristics of thin diamond-like carbon (DLC) coatings have been investigated extensively in recent years mostly in laboratory bench tests. These coatings are known to provide significant friction reduction in the absence of lubricants. In the presence of lubricants, the friction benefits of these coatings are not clearly demonstrated. The current investigation is focused on exploring the friction reduction potential of a DLC coating obtained from a supplier in laboratory bench tests and in a motored valve train test. The DLC coating was deposited on the bucket tappet. In laboratory bench tests, results showed significant friction reduction in the absence of any lubricant but not in the presence of engine oil. In motored valve train tests a significant reduction in friction torque was observed when compared against a slightly rougher uncoated bucket, but no reduction was observed when compared against uncoated bucket tappet with comparable surface finish. Under boundary lubrication conditions, no lubricant-derived surface films were present on the DLC-coated surface. However, under mixed lubrication conditions, evidence of patchy antiwear surface films could be observed on DLC-coated buckets. The antiwear film appears to be primarily composed of calcium phosphate.

Surface properties and blood compatibility of commercially available diamond‐like carbon coatings for cardiovascular devices

The aim of this study was to determine the relationships between the surface properties and blood compatibility of in-use diamond-like carbon (DLC) coatings for cardiovascular components. Commercially available DLC films were characterized with respect to surface topography and wettability, protein adsorption from human plasma, and platelets adhesion/activation. Fibrinogen (Fng) and human serum albumin (HSA) adsorbed onto the sample surfaces were in particular quantified as two of the main proteins involved in blood compatibility. A low tendency of platelets to spread and form aggregates onto the DLC-coated surfaces has been described and related to a low Fng-to-HSA adsorption ratio. This study provides evidence that the rapid and tenacious binding of albumin molecules to DLC materials tends to passivate the surfaces and to inhibit Fng adsorption, thus imparting thromboresistance to the carbon coatings by rendering the surfaces less adhesive and activating for platelets. Albumin preferential adsorption was ascribed to high chemical heterogeneity of the DLC sample surfaces. The DLC films tested present a favorable behavior as regards blood compatibility with respect to platelet thrombus formation by reason of their surface properties.

Friction and Wear Properties of Micro Textured DLC Coated Surfaces in Boundary Lubricated Sliding

In the present study, the friction and wear properties of boundary lubricated textured surfaces were investigated. The capability to feed lubricant into the interface of a sliding contact and to isolate wear particles was related to the shape, size and orientation of the texture patterns. Well-defined surface textures of square depressions or parallel grooves of different widths and distributions were produced by lithography and anisotropic etching of silicon wafers. Subsequently the wafers were PVD coated with thin, wear resistant DLC coatings, retaining the substrate texture. The surfaces were evaluated in reciprocating sliding against a ball-bearing-steel ball under starved or amply lubricated boundary lubrication conditions.

Influence of EP additive concentration on the tribological behaviour of DLC-coated steel surfaces

The influence of the concentration of a conventional extreme-pressure (EP) additive on the wear and friction behaviour of DLC-coated steel surfaces was investigated. Special emphasis was put on exploring a DLC/steel combination operating under boundary lubricated conditions and on determination of how the DLC against steel situation differs from conventional steel against steel contacts in boundary lubricated sliding. Tests were performed in a load-scanning reciprocating test rig, which allowed a wide spectrum of loads to be tested in a single test. The sliding speed was set to 0.1 m/s and the contact pressure was in the range from 2.4 to 5.6 GPa. This investigation indicated that the life of boundary lubricated DLC-coated surfaces involves several stages. After running-in, the DLC coating, although unable to form a tribofilm on top of the coating, takes an active part in forming a low friction tribofilm on the uncoated steel counterpart or on the exposed steel substrate, which eventually leads to the final wear out of the coating. Further, the concentration of additive has a significant influence on the tribological behaviour of DLC-coated steel surfaces. Contrary to the common rule for steel/steel contacts, reduction of the EP additive concentration may lead to improved tribological behaviour of DLC-coated surfaces.

Diamond-Like Carbon Films for Silicon Passivation in Microelectromechanical Devices

ABSTRACTThe surface properties of diamond-like carbon (DLC) films deposited by a vacuum arc technique on smooth silicon wafers are presented with specific emphasis given to stiction reduction in microelectromechanical systems (MEMS). The low deposition temperatures afforded by the vacuum arc technique should allow for easy integration of the DLC films into the current fabrication process of typical surface micromachines by means of a standard lift-off processing technique. Using X-ray photoelectron spectroscopy (XPS), contact angle analysis, and atomic force microscopy (AFM), the surface chemistry, microroughness, hydrophobicity, and adhesion forces of DLC-coated Si(100) surfaces were measured and correlated to the measured water contact angles. DLC films were found to be extremely smooth and possess a water contact angle of 87°, which roughly corresponds to a surface energy of 22 mJ/m2. It is shown that the pull-off forces measured by AFM correlate well with the predicted capillary forces. Pull-off forces are reduced on DLC surfaces by about a factor of five compared to 10 nN pull-off forces measured on the RCA-cleaned silicon surfaces. In the absence of meniscus forces, the overall adhesion force is expected to decrease by over an order of magnitude to the van der Waals attractive force present between two DLC-coated surfaces- To further improve the surface properties of DLC, films were exposed to a fluorine plasma which increased the contact angle to 99° and lowered the pull-off force by approximately 20% over that obtained with as-deposited DLC. The significance of these results is discussed with respect to stiction reduction in micromachines.

Nanoscale/Multilayer Gradient Materials for Application in the Electromagnetic Gun Systems

AbstractAnalysis of fired rails from electromagnetic railguns indicates severe surface damage occurs due to high current arcing and tribological mismatch. We have explored the behavior of several nanoscale multilayered materials as possible routes to improve the thermomechanical properties of the rail and armature materials . Structures investigated include (i) Ti-Co alloy on Ta-Cu alloy on dlc (diamond-like carbon) on stainless steel; (ii) Ti-Co alloy on Ta-Cu alloy on dlc on Cu, (iii) Ti-Co alloy on Ta-Cu on Cu; and (iv) Ti-Co on Ta-Cu alloy on Al. The alloys were all 50:50 at% and film thicknesses were in the range 400-1000 Å. The films were formed using a repetitively pulsed vacuum arc plasma deposition method with substrate biasing- and IBAD-like techniques. The surfaces were characterized by scanning electron microscopy, transmission electron microscopy, Rutherford backscattering spectroscopy, optical microscopy, microhardness measurements, arc erosion resistance and scratch resistance tests. Preliminary results show improvement in the microhardness, arc erosion resistance and scratch resistance, most especially for the dlc-coated surfaces. This kind of multilayered approach to the fabrication of electromagnetic railgun and armature surfaces could be important for future advanced Electromagnetic EM Gun systems.