Achieving Ultra-Low Friction with Diamond/Metal Systems in Extreme Environments

In the search for achieving ultra-low friction for applications in extreme environments, we evaluate the interfacial processes of diamond/tungsten sliding contacts using an on-line macro-tribometer and a micro-tribometer in an ultra-high vacuum. The coefficient of friction for the tests with the on-line tribometer remained considerably low for unlubricated sliding of tungsten, which correlated well with the relatively low wear rates and low roughness on the wear track throughout the sliding. Ex situ analysis was performed by means of XPS and SEM-FIB in order to better understand the underlying mechanisms of low friction and low-wear sliding. The analysis did not reveal any evidence of tribofilm or transferfilm formation on the counterface, indicating the absence of significant bonding between the diamond and tungsten surfaces, which correlated well with the low-friction values. The minimal adhesive interaction and material transfer can possibly be explained by the low initial roughness values as well as high cohesive bonding energies of the two materials. The appearance of the wear track as well as the relatively higher roughness perpendicular to the sliding indicated that abrasion was the main wear mechanism. In order to elucidate the low friction of this tribocouple, we performed micro-tribological experiments in ultra-high vacuum conditions. The results show that the friction coefficient was reduced significantly in UHV. In addition, subsequently to baking the chamber, the coefficient of friction approached ultra-low values. Based on the results obtained in this study, the diamond/tungsten tribocouple seems promising for tribological interfaces in spacecraft systems, which can improve the durability of the components

Atomistic Insights Into Lubricated Tungsten/Diamond Sliding Contacts

The reinforcement of coatings with diamond particles results in superior tribological performance in automotive applications. In addition to improving the coating's bulk properties, sliding of diamond on metallic counter bodies contributes to improved tribological performance. Therefore, in order to design better diamond-reinforced coatings, it is imperative to understand the atomistic mechanisms at sliding metal/diamond interfaces. Here, we investigate the interfacial tribochemical mechanisms leading to low friction in lubricated tungsten/diamond sliding contacts by combining reactive atomistic simulations with on-line tribometry experiments linked to chemical analysis. Reactive classical molecular dynamics simulations reveal the dehydrogenation of hexadecane lubricant molecules between tungsten/diamond contacts by proton transfer from the hexadecane to octahedral sites of the tungsten surface. Subsequent chemisorption of the radicalized hexadecane on dangling C-bond sites of the diamond surface leads to the formation of low-density hydrocarbon films, which significantly lower frictional resistance in the tribo-contact. Quasi-static density functional theory calculations confirm the classical molecular dynamics results and reveal that radicalized hydrocarbon molecules can also bond via C–O bonds on a WO3 layer covering the tungsten counter surface. The on-line tribometry experiments confirm the reduction of friction under hexadecane lubrication, and ex situ chemical analysis by means of X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and electron energy loss spectroscopy (EELS) provides evidence of the formation of a carbon-rich tribofilm on the diamond and tungsten-oxide surfaces as predicted by the atomistic simulations

Atomistic insights into lubricated tungsten/diamond sliding contacts

The reinforcement of coatings with diamond particles results in superior tribological performance for automotive applications. In addition to improving the coating’s bulk properties, sliding of diamond on metallic counter bodies contributes to improved tribological performance. Therefore, in order to design better diamond reinforced coatings, it is imperative to understand the atomistic mechanisms at sliding metal/diamond interfaces. Here, we investigate the interfacial tribo-chemical mechanisms leading to low friction in lubricated tungsten/diamond sliding contacts by combining reactive atomistic simulations with on-line tribometry experiments linked to chemical analysis. Reactive classical molecular dynamics simulations reveal the dehydrogenation of hexadecane lubricant molecules between tungsten/diamond contacts by proton transfer from the hexadecane to octahedral sites of the tungsten surface. Subsequent chemisorption of the radicalized hexadecane on dangling C-bond sites of the diamond surface leads to the formation of low-density hydrocarbon films, which significantly lower frictional resistance in the tribo-contact. Quasi-static density functional theory calculations confirm the classical molecular dynamics results and reveal that radicalized hydrocarbon molecules can also bond via C-O bonds on a WO3 layer covering the tungsten counter surface. The on-line tribometry experiments confirm the reduction of friction under hexadecane lubrication and ex situ chemical analysis by means of XPS, AES and EELS provide evidence of the formation of a carbon-rich tribofilm on the diamond and tungsten-oxide surfaces as predicted by the atomistic simulations

Surface softening in metal-ceramic sliding contacts: An experimental and numerical investigation

This study investigates the tribolayer properties at the interface of ceramic/metal (i.e., WC/W) sliding contacts using various experimental approaches and classical atomistic simulations. Experimentally, nanoindentation and micropillar compression tests, as well as adhesion mapping by means of atomic force microscopy, are used to evaluate the strength of tungsten?carbon tribolayers. To capture the influence of environmental conditions, a detailed chemical and structural analysis is performed on the worn surfaces by means of XPS mapping and depth profiling along with transmission electron microscopy of the debris particles. Experimentally, the results indicate a decrease in hardness and modulus of the worn surface compared to the unworn one. Atomistic simulations of nanoindentation on deformed and undeformed specimens are used to probe the strength of the WC tribolayer and despite the fact that the simulations do not include oxygen, the simulations correlate well with the experiments on deformed and undeformed surfaces, where the difference in behavior is attributed to the bonding and structural differences of amorphous and crystalline W-C. Adhesion mapping indicates a decrease in surface adhesion, which based on chemical analysis is attributed to surface passivation

Micro-tribological performance of metal-doped MoS2 coatings

The mechanical and tribological properties of pure MoS2, pure Au, Au-MoS2 and Ti-MoS2 coatings were evaluated and examined at a microscopic scale. The metal doped MoS2 coatings had varying metal content, 5-10at% for Ti and 10-90% for Au. Reciprocating sliding wear tests were performed with a range of initial Hertzian contact pressures from 0.41 to 3.5 GPa and in air at two humidity levels (i.e. "low" being 3-5%RH and "high" being 30-40%RH). Titanium and gold were chosen for this study as metal additives due to their positive influence on the mechanical properties of the coating. The friction and wear behavior at the micro-scale were directly compared to tribological properties at the macro-scale, which were performed using an in situ tribometer. Reciprocating micro- and macro- wear tests were performed with spherical diamond tip (with 10 and 50 µm radii) and a sapphire tip (with a radius of 3.175 mm), respectively. The range of initial Hertzian contact pressures for macro-scale (i.e. between 0.41GPa and 1.2GPa) overlapped with that for micro-scale. However, the initial Hertzian contact diameters (2*a) were very different (i.e. 0.8-2.3 µm for micro-scale and 60-180 µm for macro-scale). It was observed that the small addition of Ti or Au to MoS2 improved the microtribological properties (i.e. lower friction and less wear) compared to pure MoS2 coatings. The improved microtribological properties with metal additions were attributed to an increase in the mechanical properties, decrease in adhesion, and a decrease in the interfacial shear strength. In terms of the different length scales, lower steady state friction was observed for macrotribology compared to microtribology. The higher friction at the micro-scale was explained by the greater adhesion effects and additional velocity accommodation modes (e.g. microplowing or plowing). The microplowing or plowing at the microscopic scale was attributed to the tip roughness and the inability to sustain a stable transfer film throughout the tests at high humidity. In addition, using in situ and ex situ techniques, three different stages for solid lubrication were identified based on differences in contact area, tip shapes, and environmental conditions. The first stage has been previously observed with macrotribology on MoS2 coatings at low humidity levels. The second stage, on the other hand, was observed for micro-tribology where the contact size is significantly smaller compared to stage one. The main wear mechanism is still adhesion, but there is also some micro-plowing. The final stage was observed for humid sliding in microtribology, where no transfer films were observed and therefore the main wear mechanism was plowing.Les propriétés mécaniques et tribologiques de revêtements de MoS2 pur, d'Au pur, de Au-MoS2 et de Ti-MoS2 ont été évaluées et examinées à l'échelle microscopique. Les revêtements nanocomposites étudiés contenaient 5-10 % at. de Ti et 10-90 % at. d'Au. Des tests d'usure par glissement alternatif ont été mis en œuvre, l'échelle de pression Hertzienne de contact initiale variant de 0.41 à 3.5 GPa, dans une atmosphère d'air avec deux niveaux d'humidité contrôlée (le niveau le moins élevé se situant entre 3 et 5 % HR et le plus élevé entre 30 et 40 % HR). Pour cette étude, le titane et l'or ont été choisis comme additifs métalliques pour leur influence positive sur les propriétés mécaniques des revêtements. Les comportements de friction et d'usure des revêtements à l'échelle microscopique ont été directement comparés à leurs propriétés tribologiques à l'échelle macroscopique, dont les tests étaient effectués à l'aide d'un tribomètre in situ. Des tests sclérométriques alternatifs ont été réalisés aux échelles microscopiques et macroscopiques avec des pointes de diamant sphérique (10 et 50 µm de rayon) et une pointe de saphir (ayant un rayon de 3.175 mm). La gamme de pression Hertzienne de contact utilisée à l'échelle microscopique (entre 0.41 GPa et 1.2 GPa) était très proche de celle utilisée à l'échelle macroscopique. Cependant, le diamètre de contact Hertzien initial (2*a) était très différent, soit 0.8 – 2.3 µm à l'échelle microscopique et 60 – 180 µm à l'échelle macroscopique. Les résultats montrent que l'ajout de faibles quantités de Ti ou d'Au au MoS2 améliore les propriétés micro-tribologiques (comportements à la friction et à l'usure atténués) en comparaison avec des revêtements de MoS2 pur. L'amélioration des propriétés micro-tribologiques due à l'addition de métaux a été attribuée au renforcement des propriétés mécaniques, une adhésion plus faible et une baisse des contraintes de cisaillement interfaciales. Si l'on compare des tests micro- et macro-tribologiques effectués sur des étendues de longueur variées, ces derniers étaient caractérisés par une friction en régime permanent moins élevée. Le comportement de friction plus accentué dans le cas des tests réalisés à l'échelle microscopique s'explique sur la base d'effets d'adhésion plus importants et des modes additionnels de compensation de vitesse (labourage ou micro-labourage). Les tendances au labourage ou micro-labourage observées à l'échelle microscopique ont été attribuées à la rugosité de la pointe de diamant et à la difficulté de maintenir une couche de film de transfert en place lors de tests effectués dans des conditions d'humidité élevée. L'utilisation de techniques in situ et ex situ a également permis de déterminer trois stades de lubrification solide, en se basant sur des différences observées à la zone de contact, dues aux formes des différentes pointes et aux conditions environnementales appliquées. Le premier stade, avait été identifié auparavant, lors de tests de macro-tribologie sur des revêtements de MoS2, à un niveau d'humidité faible. Par contre, le deuxième stade n'a été observé que lors de tests de micro-tribologie où la taille de la zone de contact était bien plus petite que dans le cas du premier stade. A ce stade, le mécanisme d'usure est principalement relié au comportement d'adhésion du revêtement, avec une influence possible de l'effet de micro-labourage. Le stade final de lubrification a été observé lors de tests de micro-tribologie réalisés dans des conditions d'humidité élevée et caractérisés par l'absence du film de transfert. De cette observation, il a été déduit que le principal mécanisme d'usure du film à ce stade de lubrification correspondait au labourage

Scaling Effects on Materials Tribology: From Macro to Micro Scale

The tribological study of materials inherently involves the interaction of surface asperities at the micro to nanoscopic length scales. This is the case for large scale engineering applications with sliding contacts, where the real area of contact is made up of small contacting asperities that make up only a fraction of the apparent area of contact. This is why researchers have sought to create idealized experiments of single asperity contacts in the field of nanotribology. At the same time, small scale engineering structures known as micro- and nano-electromechanical systems (MEMS and NEMS) have been developed, where the apparent area of contact approaches the length scale of the asperities, meaning the real area of contact for these devices may be only a few asperities. This is essentially the field of microtribology, where the contact size and/or forces involved have pushed the nature of the interaction between two surfaces towards the regime where the scale of the interaction approaches that of the natural length scale of the features on the surface. This paper provides a review of microtribology with the purpose to understand how tribological processes are different at the smaller length scales compared to macrotribology. Studies of the interfacial phenomena at the macroscopic length scales (e.g., using in situ tribometry) will be discussed and correlated with new findings and methodologies at the micro-length scale

Tribological Evaluation of Lead-Free MoS₂-Based Solid Film Lubricants as Environmentally Friendly Replacements for Aerospace Applications

Solid lubricants, such as MoS2 have been widely used in the aerospace industry with the primary purpose of reducing the friction and wear of tribological interfaces. MoS2 based solid film lubricants are generally doped with other compounds, which can help overcome some of their limitations related to environmental conditions. For instance, compounds like Sb2O3 and Pb have been traditionally used to improve the endurance life of these lubricants. However, with the recent zest in transferring to eco-friendly lubricants, there is a strong push to eliminate Pb based compounds. The main purpose of this work is to better understand the influence of Pb based compounds on the tribological behavior of MoS2 based solid film lubricants as well as to critically evaluate the performance of Pb free lubrication strategies. More specifically, the baseline ‘non-green’ lubricant was doped with Pb compound and Sb2O3 and the Pb compound in the ‘Green’ alternative lubricant was replaced by more Sb2O3. The wear test was done using a ball-on-disk tribometer for specific loads and for 5000 cycles. Ex-situ analysis was conducted using Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM), and micro-Raman to capture the interfacial processes of these lubricants at different loads. Overall, the non-green lubricant performed better in terms of the tribological behavior (i.e., lower friction and wear), which was attributed to the formation of a dense MoS2-based tribo-/transfer-film with the basal planes oriented in the parallel direction to the sliding. The finding on the interfacial phenomena provided critical insights into the development of novel green alternatives that may have the ability to replace Pb based compounds in the future for a sustainable environment

Tribological Performance of High-Entropy Coatings (HECs): A Review

Surface coatings that operate effectively at elevated temperatures provide compatibility with critical service conditions as well as improved tribological performance of the components. High-entropy coatings (HECs), including metallic, ceramics, and composites, have gained attention all over the world and developed rapidly over the past 18 years, due to their excellent mechanical and tribological properties. High-entropy alloys (HEAs) are defined as alloys containing five or more principal elements in equal or close to equal atomic percentage. Owing to the high configurational entropy compared to conventional alloys, HEAs are usually composed of a simple solid solution phase, such as the BCC and FCC phases, instead of complex, brittle intermetallic phases. Several researchers have investigated the mechanical, oxidation, corrosion and wear properties of high-entropy oxides, carbides, borides, and silicates using various coating and testing techniques. More recently, the friction and wear characteristics of high-entropy coatings (HECs) have gained interest within various industrial sectors, mainly due to their favourable mechanical and tribological properties at high temperatures. In this review article, the authors identified the research studies and developments in high-entropy coatings (HECs) fabricated on various substrate materials using different synthesis methods. In addition, the current understanding of the HECs characteristics is critically reviewed, including the fabrication routes of targets/feedstock, synthesis methods utilized in various research studies, microstructural and tribological behaviour from room temperature to high temperatures

Binary and ternary lubricious oxides for high temperature tribological applications: A review

Oxides and oxide-based coatings have been widely used as solid lubricants in demanding operating conditions to achieve low friction and wear due to their higher thermal and chemical stability. However, their tribological performance is highly dependent on the test temperatures and the surrounding environment. This article provides a comprehensive review of low-friction oxides and oxide-based coatings in relation to the influence of operating temperature on their tribological performance and their potential use as solid lubricants. Special emphasis is placed on the tribological behavior of binary and ternary oxides developed over the last few decades. Furthermore, this review summarizes the high temperature tribology, mechanisms and interfacial processes of the oxides leading to low friction coefficient and wear in high temperature applications

Surface softening in metal-ceramic sliding contacts: An experimental and numerical investigation

This study investigates the tribolayer properties at the interface of ceramic/metal (i.e., WC/W) sliding contacts using various experimental approaches and classical atomistic simulations. Experimentally, nanoindentation and micropillar compression tests, as well as adhesion mapping by means of atomic force microscopy, are used to evaluate the strength of tungsten?carbon tribolayers. To capture the influence of environmental conditions, a detailed chemical and structural analysis is performed on the worn surfaces by means of XPS mapping and depth profiling along with transmission electron microscopy of the debris particles. Experimentally, the results indicate a decrease in hardness and modulus of the worn surface compared to the unworn one. Atomistic simulations of nanoindentation on deformed and undeformed specimens are used to probe the strength of the WC tribolayer and despite the fact that the simulations do not include oxygen, the simulations correlate well with the experiments on deformed and undeformed surfaces, where the difference in behavior is attributed to the bonding and structural differences of amorphous and crystalline W-C. Adhesion mapping indicates a decrease in surface adhesion, which based on chemical analysis is attributed to surface passivation