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The emergence of nanotechnology and surface engineering techniques provides new opportunities for designing self-lubricant coatings with enhanced properties. In recent years, green coating technologies have played a vital role in environmental preservation. This article mainly reviews five typical types of self-lubricant coatings including MoN coatings, VN coatings, WN coatings and TMN (Transition Metal Nitride) soft-metal coatings, and DLC (Diamond-like Carbon) with lubricant agents deposited by PVD (Physical Vapor Deposition) for the demanding tribological applications, which is the latest research into the green lubricant coatings. Furthermore, it is of great significance for designing the green self-lubricant coatings to adapt the demanding tribological applications to meet the industrial requirements. [ABSTRACT FROM AUTHOR]

Abrasive wear is a wear mechanism that results in a loss of material from the interaction of a surface with hard particles. This type of wear is frequently found in the surface of machining tools. Microscale abrasion equipment is often used to characterize the resistance to abrasive wear of a surface. The different parameters able to control micro-abrasion wear tests, such as ball rotation, sliding distance between ball and surface sample, abrasive slurry concentration, normal load acting on the sample, and abrasive flow rate over the sample, have been widely studied. The combination of different variables, including sliding distance, concentration of abrasive particles, their hardness, and size of abrasive particles, promotes the transition between two-body, three-body, or mixed abrasive wear modes. However, the influence of the ball surface on the dragging of abrasive particles, which is reflected in the wear modes, is still poorly studied. One of the variables possible to control and less studied is the influence of the ball surface texture on the dragging of abrasive particles in micro-abrasion wear tests. This work intends to correlate the effect of different testing times (500, 1000, and 1500 cycles) and different concentrations of 3 μm Al2O3 abrasive slurry (25, 35, and 45 g/100 mL) on the micro abrasion resistance of a TiN thin coating film, using balls of AISI 52100 steel whose texture and roughness were prepared by 60 s chemical etching. The rotation speed of each test was 80 rpm, applying a normal load of 2 N. Subsequently, the craters were carefully analyzed using SEM to evaluate the transition of the wear mode as a function of the applied load, the abrasive particle concentration, and the sliding distance. The textured ball tracks were observed via SEM to assess the particle dynamics. The results showed that, contrarily to what is reported in the literature regarding wear modes where rolling is promoted with increasing abrasive concentration, in this work grooving took place instead. This is a result of the rough balls use in the experiments which, due to the embedment of abrasive particles in the ball grooves, promotes the abrasion mechanism. The higher the abrasive concentration, the higher the grooving mechanism, since more particles are available to scratch the surface. [ABSTRACT FROM AUTHOR]

Diamond-like-carbon "DLC" coatings can be deposited in many different ways, giving a large range of material properties suitable for many different types of applications. Hydrogen content significantly influences the mechanical properties and the tribological behavior of DLC coatings, but its determination requires techniques that are not available in many research centers. Thus, it is important to find alternative indirect techniques, such as Raman spectroscopy or nanoindentation (hardness measurements), which can give comparative and indicative values of the H contents in the coatings, particularly when depositions with a reactive gas flow are being studied. In this work, "DLC" composite coatings with varying H content were deposited via Physical Vapor Deposition (PVD) magnetron sputtering in a reactive atmosphere (Ar + CH4). An Ion Beam Analysis was used to determine the elemental depth profile across the coating thickness (giving both average C:W:H ratios and film density when combined with profilometer measurements of film thickness). The hardness was evaluated with nanoindentation, and a decrease from 16 to 6 GPa (and a decrease in the film density by a factor of two) with an increasing CH4 flow was observed. Then, the hardness and Raman results were correlated with the H content in the coatings, showing that these indirect methods can be used to find if there are variations in the H content with the increase in the CH4 flow. Finally, the adhesion and tribological performance of the coatings were evaluated. No significant differences were found in the adhesion as a function of the H content. The tribological properties presented a slight improving trend with the increase in the H content with a decrease in the wear rate and friction. [ABSTRACT FROM AUTHOR]

The low adhesion of amorphous carbon (a-C) films on ferrous alloys has restricted massive industrial application since their development, restricting application to mechanical and electromechanical components. Although different mechanisms have been raised to describe the a-C film adhesion, the chemical affinity and the role of oxygen at the interfaces are the key issue. Nevertheless, a quantitative approach considering the oxygen adsorption and the adhesion strength/base pressure relationship is still not proposed, which would be of special interest in industry. In this study, we analyze the influence of the base pressure on the adhesion of amorphous carbon (a-C) films onto a ferrous alloy intermediated by a nanometric silicon interlayer. The different base pressure deposition resulted in different adhesion of the films. By means of structural and chemical techniques, the oxygen content at the interfaces was quantified and correlated with the base pressure before thin film deposition. We propose a quantitative physicochemical model that correlates the a-C film adhesion with oxygen located at the interfaces, which is indirect evidence of its previous presence in the deposition chamber, as a fraction of the residual gases from the base pressure. As adhesion depends on oxygen content, we used the Langmuir isothermal law to evaluate this dependence with good agreement. This model may be of potential interest in plasma surface engineering and process automatization not only for carbon-related materials deposited on metals, mainly in its potential use as a solid lubricant. [ABSTRACT FROM AUTHOR]

W–S–N self-lubricant coatings were deposited by d.c. reactive magnetron sputtering at two total discharge pressures of 0.6 and 1.2 Pa. The nitrogen content and S/W ratio varied from 0 to 30 at.%, and 0.6 to 1.6, respectively. The effect of deposition conditions on chemical composition, structure and mechanical properties was studied; coatings with the most promising properties have been selected for tribological testing. Coatings were nanostructured with nanograins of tungsten disulfide; the size of which decreased with increasing nitrogen content. Hardness increased with N content up to 7 GPa for coating deposited with a discharge pressure of 1.2 Pa; coatings deposited at lower pressure exhibited hardness around 9 GPa. Friction and wear behavior in humid air and dry nitrogen was evaluated using a pin-on-disk tribometer. The wear tracks and the ball wear scars were analyzed by Raman spectroscopy and scanning electron microscopy. Although the friction coefficient in dry nitrogen was significantly lower than in humid air, the wear rate was similar for both environments. Detailed wear track surface analysis showed that the tribolayer formed on the surface contained mostly tungsten disulfide and tungsten oxide. [ABSTRACT FROM AUTHOR]

ABSTRACT Carbon-based coatings with different W contents were deposited by direct current magnetron sputtering in reactive and non-reactive atmospheres. All deposited coatings have compact morphologies with amorphous (tungsten-free) or nanocrystalline structures (tungsten-doped). The latter one was indicated by very broad peaks in X-ray Diffraction spectra in the position of tungsten carbide suggesting W-carbide nanoparticles embedded in an amorphous carbon matrix. The hardness increased from 10 to 15 GPa with increasing W content. The coatings were tribological tested at dry and lubricated conditions with increasing temperature in a coating/steel configuration. In dry sliding, the friction coefficient increases with the increase of the temperature reaching values higher than 1.0. The friction is significantly lower in lubricated contact using three different oils: poly-alpha-olefin, paraffin and olive oil. The olive oil shows promising lubricating properties at the temperature lower than 70°C; however, at higher temperature, the coatings were quickly worn through. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Magnetron sputtered Ag:TiN thin films, with Ag contents ranging from 0 to ~50 at. %, were subjected to a set of annealing treatments at four different temperatures (from 200 to 500 °C), aiming at studying the effect of Ag addition on their morphological and structural features, as well as on their electrical and mechanical properties. The increase of the annealing temperature revealed that significant morphological modifications were induced, mainly in the samples with higher Ag contents (represented by the sample with an Ag content of 36.3 at. %), which revealed extensive Ag segregation to the surface. The increase of the annealing temperature up to 500 °C, besides confirming the presence of metallic Ag in the coatings, also resulted in some major structural changes, promoting an increase in the coating's crystallinity and an extensive Ag grain growth phenomenon. Nevertheless, the resistivity values of all annealed samples were found to be only slightly higher when comparing to the as-deposited ones, which indicated that beyond the significant changes in the samples morphology (e.g., Ag segregation), there seemed to be some important coating resistance in terms of the electrical response. Furthermore, the annealed samples also exhibited lower reduced modulus up to 400 °C when compared with the unannealed ones, consistent with some increase of the samples' elasticity. [ABSTRACT FROM AUTHOR]

Stainless steels (SS) are the most-used alloys for manufacturing fixed orthodontic appliances due to their attractive set of mechanical properties, biocompatibility, and high corrosion resistance. Nevertheless, during regular orthodontic treatments–taking at least around 2 years–the intraoral environment inevitably degrades these bioalloys, releasing metallic ions into the oral cavity. In the first part of this in vitro study, the corrosion resistance of commercial SS appliances (brackets, tubes, and bands) was evaluated in Fusayama-Meyer artificial saliva at pH values of 2.3 and 6.8 over the course of 30 days. As expected, the results corroborated that salivary pH highly influences corrosion behaviour. Released Ni, Cr, and Fe were within dietary intake values. In the second part, a novel approach for oral corrosion prevention based on the chemical inertness of DLC materials is presented. SS surfaces were functionalized with biocompatible a-C:H-sputtered coatings and submitted to the same experimental conditions. The anticorrosion ability of this system was demonstrated, preventing the pitting corrosion that occurred on the SS substrates. Despite the galvanic coupling effect due to the presence of the Cr-based interlayer, this study enhanced the potential use of the reactive sputter-deposited a-C:H coatings in orthodontics. [ABSTRACT FROM AUTHOR]

Hydrogenated amorphous carbon (a-C:H) coatings are attractive materials for protecting metallic surfaces in extreme biological environments like the human oral cavity, due to the unusual combination of mechanical properties, superior bioinertness, and relative easier and cheaper production. In this work, two a-C:H coatings were deposited on AISI 316L substrates by reactive magnetron sputtering with two CH4 flows to assess if this outstanding system could extend its application range to orthodontics. A 30-day immersion test in Fusayama-Meyer artificial saliva was conducted to mimic an extreme acidic intraoral pH. Extracts were quantified and used to perform in vitro assays with mono- and co-cultures of macrophages and fibroblast to assess cell viability, while mechanical and structural behaviors were studied by nanoindentation and visible Raman. The empirically estimated H contents of ~28 and 40 at.% matched the hard and soft a-C:H coating regimes of 18 and 7 GPa, respectively. After immersion, no important structural/mechanical modifications occurred, regardless of the H content, without corrosion signs, delamination, or coating detachment. However, the adhesion-promoting Cr-based interlayer seems to reduce corrosion resistance via galvanic coupling. The highest biocompatibility was found for a-C:H coatings with the lowest H content. This study indicates that sputtered a-C:H are promising surface materials in orthodontics. [ABSTRACT FROM AUTHOR]

Carbon-based coatings are used in many applications, particularly in sliding contacts to reduce friction and wear. To improve the tribological properties, these coatings are usually alloyed with metals; W is one of the most used since it helps improve the tribological performance at high temperatures. In this work, we compared the tribological performance of Diamond-Like Carbon alloyed with tungsten (DLC-W) films deposited by direct current magnetron sputtering (DCMS) with films deposited in a hybrid configuration DCMS + high power impulse magnetron sputtering (HiPIMS). The DLC-W coatings were produced with approximately the same W content. One hydrogenated film was deposited with the hybrid configuration for comparison purposes. Microstructure, structure, mechanical properties, and tribological behaviour were used to compare the coatings. All the films displayed a low-order structure of tungsten carbide embedded in an amorphous carbon matrix. The use of the hybrid HiPIMS/DCMS results in coatings with more compact morphologies due to the high ionization fraction of the species produced on the W target (W and Ar ionized species), which primarily will oppose the shadowing effect as the ions will reach the substrate at angles close to 90°. HiPIMS non-hydrogenated film is the more tribological, performing either at room or high temperature (150 °C) due to the much more compact morphology, which avoids the detachment of hard W-C particles, which are responsible for more efficiently scratching the film surface. Experiments revealed that wear behaviour in all the films is governed by the contact of the tribolayer formed on the counterpart composed of W–C, C and W–O against the surface of the film. [ABSTRACT FROM AUTHOR]

Carbon-alloyed transition metal dichalcogenide (TMD) coatings have great potential for providing a good tribological response in diverse operating environments. There are different ways to synthesize these coatings by magnetron sputtering, with no clear indication of the best possible route for potential upscaling. In this study, tungsten-sulfur-carbon (W-S-C) coatings were deposited by radio frequency (RF) magnetron sputtering via four different methods. All coatings were sub-stoichiometric in terms of the S/W ratio, with the bombardment of the growing film with backscattered Ar neutrals being the main mechanism governing the S/W ratio. The crystallinity of the films was dependent on the C and S contents. X-ray photoelectron spectroscopy (XPS) revealed W-S and W-C bonding in all coatings. Raman spectroscopy showed the presence of an a-C phase with predominant sp2 bonding. The hardness of the coatings may be related to the C content and the S/W ratio. A friction coefficient of 0.06–0.08 was achieved during sliding in ambient air by the coatings deposited in non-reactive mode with optimal C contents. The results indicate that sputtering in non-reactive mode should be the method of choice for synthesis of these coatings. [ABSTRACT FROM AUTHOR]