Application Of Solid Lubricants Research Articles

High power pulsed magnetron sputtering tailored low temperature CrAlN + MoWS coatings for dry tribological contacts

Gears are used in a variety of applications, for example, in automotive and aerospace industries. In these applications, gears are exposed to harsh conditions such as high Hertzian pressures. In many cases, lubricants are used to achieve a reduction in the coefficient of friction and, thus, in the wear of the components. Nevertheless, in some applications, the use of a lubricant is difficult, for example, in vacuum environments. One promising approach is the application of solid lubricants by using triboactive elements. The present study shows the potential of a process combining high power pulsed magnetron sputtering (HPPMS) and direct current magnetron sputtering (dcMS) for the deposition of CrAlN + MoWS coatings. In HPPMS/dcMS processes, especially, the share of HPPMS leads to higher ionization of the coating plasma and, therefore, to higher coating thicknesses on the teeth of gears. One major challenge in coating processes is the adjustment of the deposition temperature, since steels used for gear applications are temperature sensitive. Therefore, starting from an established coating process with a high coating temperature, the coating process was adjusted by reducing the number of used cathodes. By reducing the count of used dcMS cathodes, the share of HPPMS cathodes within the coating process was increased. Furthermore, segments of gears were coated in order to determine the coating thickness at different distances from the head of the gear tooth. A sufficient coating thickness distribution on the flank of the gear tooth is crucial for friction reduction and wear protection when used in gearboxes. The reduction of used cathodes led to a decrease in the coating temperature from a maximum coating temperature of Tmax = 281 °C to Tmax = 214 and 197 °C. It was shown by nanoindentation measurements that the effect of soft annealing was eliminated by reducing the coating temperature. At the same time, coatings deposited with increased HPPMS fraction led to a reduced coefficient of friction as well as lower wear volumes for pin-on-disk measurements in a dry tribological contact with an uncoated 100Cr6 counterpart. Moreover, these processes exhibited a good coatability of the gear tooth with a relatively high coating thickness even near the gear tooth root. The present study shows the potential of increasing the share of HPPMS during the deposition of coatings for dry-running applications. By increasing the HPPMS content, the tribological behavior as well as the coating thickness distribution on gears can be significantly improved.

Tribologically induced nanostructural evolution of carbon materials: A new perspective

Carbon-based solid lubricants are excellent options to reduce friction and wear, especially with the carbon capability to adopt different allotropes forms. On the macroscale, these materials are sheared on the contact along with debris and contaminants to form tribolayers that govern the tribosystem performance. Using a recently developed advanced Raman analysis on the tribolayers, it was possible to quantify the contact-induced defects in the crystalline structure of a wide range of allotropes of carbon-based solid lubricants, from graphite and carbide-derived carbon particles to multi-layer graphene and carbon nanotubes. In addition, these materials were tested under various dry sliding conditions, with different geometries, topographies, and solid-lubricant application strategies. Regardless of the initial tribosystem conditions and allotrope level of atomic ordering, there is a remarkable trend of increasing the point and line defects density until a specific saturation limit in the same order of magnitude for all the materials tested.

Eco-friendly sustainable machining with MoS2-based solid lubricant

Machining is one of the basic and inevitable operations in the manufacturing industry. The machining performance is estimated based on various parameters like cutting forces, surface roughness, tool–chip interface temperature, and specific energy. In the traditional approach, cutting fluids are greatly utilized to dissipate the generated heat during machining, but their utilization causes a threat to nature and human being’s health. Therefore, there emerges a necessity to determine user-friendly, sustainable, and eco-friendly substitutes for traditional cutting fluids. The field of advanced tribology has proposed the usage of solid lubricants due to their inherent properties, such as excellent tribological performance, reduced machining zone temperature due to lowered friction, and enhanced tool life. Therefore, in the present study, pure MoS2 and composite MoS2–TiO2 solid lubricants have been employed in the milling process of AISI 52100 steel with HSS end mill cutter. The machining was carried out in different conditions such as uncoated tool without use of lubricant, uncoated tool with use of lubricant, coated tool without use of lubricant, and coated tool with use of lubricant that were employed to analyze their effect on machining performance. The experimental results showed substantial improvement regarding reduced cutting forces, reduced temperature at tool–chip interface, improved surface finish, and average tool wear with the application of solid lubricants. Among the various lubricating conditions, composite MoS2–TiO2-coated tool with composite MoS2–TiO2 lubricant exhibits excellent machining performance.

Separating anisotropic and isotropic friction between atomic force microscope tips and atomically flat surfaces

Layered materials are the most important class of solid lubricants. Friction on their surfaces has complex origins. Most experimental methods so far only give total friction force and cannot separate contributions from different origins. Here, we report a method to separate anisotropic and isotropic friction forces on atomically flat surfaces such as ${\mathrm{MoS}}_{2}$, graphite, h-BN, and mica by combining a two-dimensional friction force microscope technology and a two-dimensional friction model. We found that the friction force of most atomically flat surfaces is anisotropic, the total force on the tip misaligns with the scan direction, and the friction anisotropy vanishes under low sliding velocity. Our two-dimensional friction model explains experimental observations. It reveals the existence of elemental hopping combinations and the isotropic component in total friction. The misalignment angle can be used to calculate the ratio of anisotropic and isotropic friction components and the ratio of resistance forces from different lattice directions. The separation of anisotropic and isotropic friction forces will offer an avenue for studying the properties of individual friction components, which can boost the study of friction mechanisms in the future and benefit the application of solid lubricants.

High-Temperature Solid Lubricants and Self-Lubricating Composites: A Critical Review

Solid lubricants are described as solid materials of intentionally introduced or in situ formed on contact surfaces in relative motion for the purpose of lowering friction and wear and providing protection from damage. Solid lubricants and advanced self-lubricating materials are widely used in modern industries, especially in aerospace, aviation, automotive, metallurgy, materials forming, and machining industries, and have attracted great interest in lubrication applications under very severe circumstances such as elevated temperatures, heavy loads, ultrahigh vacuum, extreme radiation, strong oxidation, and chemical reactivity environments. Many efforts have been made to develop self-lubricating composites by a variety of material preparation techniques, which include powder metallurgy, physical/chemical vapor depositions, thermal spraying, electrodeposition, laser cladding, and additive manufacturing. Although several reviews on the development of high-temperature solid lubricants have been published, most of them only focus on a type of material, a specific process, or application. In this paper, a comprehensive review is provided to present the state-of-the-art progress in solid lubricants, self-lubricating composites/coatings, and their effective functions that can be used over a wide variety of environmental conditions, especially at elevated temperatures. The solid lubricants considered include representative soft metals, layered structure materials (e.g., graphite, hexagonal boron nitride, transition metallic dichalcogenides, MAX phase), chemically stable fluorides, binary or ternary metallic oxides, especially alkaline earth chromates, and sulfates, and synergistic effects from these solid lubricants. This paper also provides new insights into design considerations of environmental adaptive solid lubrication, and the challenges and potential breakthroughs are further highlighted for high-temperature solid lubrication applications.

Nanofriction Properties of Mono- and Double-Layer Ti3C2Tx MXenes.

Unique structure and ability to control the surface termination groups of MXenes make these materials extremely promising for solid lubrication applications. Due to the challenging delamination process, the tribological properties of two-dimensional MXenes particles have been mostly investigated as additive components in the solvents working in the macrosystem, while the understanding of the nanotribological properties of mono- and few-layer MXenes is still limited. Here, we investigate the nanotribological properties of mono- and double-layer Ti3C2Tx MXenes deposited by the Langmuir-Schaefer technique on SiO2/Si substrates. The friction of all of the samples demonstrated superior lubrication properties with respect to SiO2 substrate, while the friction force of the monolayers was found to be slightly higher compared to double- and three-layer flakes, which demonstrated similar friction. The coefficient of friction was estimated to be 0.087 ± 0.002 and 0.082 ± 0.003 for mono- and double-layer flakes, respectively. The viscous regime was suggested as the dominant friction mechanism at high scanning velocities, while the meniscus forces affected by contamination of the MXenes surface were proposed to control the friction at low sliding velocities.

Layer-dependent frictional properties of Ti3C2Tx MXene nanosheets

MXenes are an emerging class of two-dimensional lamellar materials with exceptional architectural variety, tunable chemical compositions, and chemical inertness. Owing to their unique structure, with a highly tunable interlayer distance and binding properties, MXenes can potentially exhibit excellent nanoscale solid lubrication properties. Herein, we report the layer dependence of the frictional characteristics of atomically thin Ti3C2Tx MXene nanosheets. The frictional properties of the isolated Ti3C2Tx nanosheets deposited on a clean Si surface were investigated using friction force microscopy. The friction decreased with an increase in the number of layers, starting from monolayer Ti3C2Tx. The layer dependence of friction was attributed to the reduced elastic compliance with an increase in the number of Ti3C2Tx layers. A partially suspended Ti3C2Tx monolayer exhibited a higher degree of friction than the substrate-supported Ti3C2Tx monolayer, indicating the role of elastic compliance in the friction mechanism of Ti3C2Tx nanosheets. The reduced elastic compliance of Ti3C2Tx nanosheets decreases the localized out-of-plane puckering-type deformation surrounding the sliding indenter, thereby reducing the resistance to frictional sliding. The results indicate that tuning their out-of-plane mechanical properties by varying the chemical architecture, chemical composition, surface terminations, and interlayer intercalations can render MXenes as potential candidates for versatile nanoscale solid lubrication applications.

In Situ‐Formed Ultralow Wear Tribofilms Induced by Poly(Acrylic Acid)‐co‐MXene‐Modified Polymer‐Like Carbon Films

Herein, two types of tribocouple pairs based on polymer‐like carbon (PLC) films are designed through the in situ‐formed MXene coating. The in situ‐formed MXene coatings are synthesized via the in situ organic–inorganic chemical composite technique‐induced anionic polymers with different molecular weights (≈450 000 and ≈4 000 000, respectively). Next, the tribotesting condition is controlled in the ambient environment under different loads by bare and PLC‐coated bearing steel balls as counterbodies. During the whole sliding process, a lubricity state (friction coefficient is about 0.21 and wear rate is about 1.5 × 10−6 mm3 N−1 m−1) is realized on the self‐mated PLC system. Meanwhile, the contact surfaces are covered by graphitic 2D Ti3C2. Compared with the high‐molecular‐weight modifier system, most poly(acrylic acid)‐co‐MXenes grafted by the low‐molecular‐weight poly(acrylic acid) form far more sp 2‐C phases. Moreover, active groups are potentially involved in forming a tribofilm, which enhances the overall lubricity of MXenes during sliding. This verifies the key to achieving low friction and ultralow wear based on the formation of the nanostructured sliding interface upon MXenes and provides an expanded application of solid lubricants.

Application of solid lubricant for enhanced frictional efficiency of deep drawing process

Manufacturing processes are usually energy intensive, contributing to the global carbon dioxide emissions. Deep Drawing is one of the most common types of sheet metal forming processes with great potential for energy efficiency improvement. In this paper, the optimised combination of molybdenum disulphide (MoS2) and graphite is proposed as a solid lubricant to reduce the punching force and energy consumption of deep drawing process. Different mixtures of MoS2 and graphite are prepared and their tribological performance are measured using experimental tests on tribometer. In order to investigate the friction reduction rate in deep drawing process, finite element simulation of the drawing process is performed. Results show that friction reduction using proposed combination of lubricants has significant effect on punching force and would provide greater process efficiency.

Phase behaviour of (Ti:Mo) S2 binary alloys arising from electron-lattice coupling

While 2D materials attract considerable interests for their exotic electronic and mechanical properties, their phase behaviour is still largely not understood. This work focuses on (Mo:Ti) S2 binary alloys which have captured the interest of the tribology community for their good performance in solid lubrication applications and whose chemistry and crystallography is still debated. Using electronic structures calculations and statistical mechanics we predict a phase-separating behaviour for the system and trace its origin to the energetics of the d-band manifold due to crystal field splitting. Our predicted solubility limits as a function of temperature are in accordance with experimental data and demonstrate the utility of this protocol in understanding and designing TMD alloys.

Effective Application of Solid Lubricants in Spacecraft Mechanisms

Solid lubricants, antiwear coatings, and self-lubricating composites are used in applications on spacecraft where oils and greases cannot be used because of the need to avoid lubricant volatility/migration, and where the application requires significant temperature variation, accelerated testing, higher electrical conductivity, or operation in boundary conditions. The purpose of this review is to provide spacecraft designers with tools that can aid in the effective use of solid-based tribological materials, both to increase their usage, and to reduce anomalies. The various tribological material formulations are described, including how their materials, physical, and chemical properties affect their performance. Included are typical solid lubricants like PTFE and bonded or sputter-deposited MoS2, as well as low shear metal coatings, hard coatings, and composite materials (including bulk composites and nanocomposite coatings). Guidance is given on how to develop mechanisms that meet performance requirements, but also how to optimize robustness, so that success is achieved even under unforeseen circumstances. Examples of successful applications are given, as well as how to avoid potential pitfalls, and what the future of solid tribological materials may hold.

Improvement in friction and wear characteristics using CaF2 as a solid lubricant at different conditions

During machining, adequate cooling and lubrication is much needed to reduce friction, wear, and temperature by efficient heat dissipation resulting in improved surface quality and longer tool life. The various alternative approach developed to provide lubrication and cooling includes minimum quantity lubrication, cryogenic cooling, and machining with the use of solid lubricants. The efficiency of solid lubricant particles greatly depends on the particle size, the wt% of solid lubricant, and base fluid used to prepare a lubricant mixture. Therefore, an attempt has been made in the present investigation, to evaluate the tribological properties of CaF2 with different grain sizes and weight ratios in a base oil. Results demonstrated the anti-wear and friction ability of CaF2 as a solid lubricant by improving the tribological properties of sliding surfaces. An increase in friction coefficient is observed with larger grain size (30 and 50 μm) with 10 and 20 wt% of CaF2 as compared with 10 μm particle size. Results also showed the ability of solid lubricant particles with smaller particle sizes (10 μm) to maintain a thin film of lubrication. As a comparison, CaF2 mixed with a base oil performed equally well compared to MQL under different loading conditions. The reduction in friction coefficient and wear is observed with solid lubricant application due to its excellent lubricity even at higher load and pressure. The analysis shows the presence of wear on disc surface in the form of visible grooves and wear debris in dry and MQL condition whereas lubricating effects of CaF2 is dominant.

MoS2 solid lubricant application in turning of AISI D6 hardened steel with PCBN tools

This research investigates the effect of the application of MoS2 mixed with oil in the hard turning of AISI D6 cold work tool steel with PCBN tools. A 33 statistical factorial planning was used to run the tests, variables: cutting speed, feed rate; lubri-cooling conditions: dry cutting, pure oil applied by MQF (minimum quantity of fluid), oil mixed with solid lubricant (MoS2) applied by MQF. The main tool wear mechanisms were abrasion, attrition (or adhesion), and diffusion. The EDS of the PCBN confirmed the presence of MoS2, indicating that solid lubricant particles took place in the lubrication process at the cutting region. The results suggest that use of solid lubricant in hard turning may be a viable alternative to tackling the challenge of machining difficult-to-cut materials.

The Tribological Behavior of As-Sprayed Graphene Oxide–Tungsten Disulfide Hybrid Coatings

The application of solid lubricants in the metal forming of aluminum alloys is imperative due to their potential for the reduction of both friction and aluminum adhesion. This study focuses on examining the potential application of hybrid graphene oxide–tungsten disulfide (GO/WS2) coatings in the forming of aluminum alloys. A series of experiments with different GO/WS2 concentrations (between 1:1 and 12:1) was conducted to investigate the effects of increasing GO concentration on the tribological performance of the coatings. The coefficient of friction (COF) was evaluated using pin-on-disc experiments. The durability of the coatings (the sliding distance prior to the initiation of coating failure) was also recorded. Subsequent surface characterization was performed from within the sliding tracks induced on the coated Al-Mg discs and the mechanisms responsible for friction and wear are discussed. The GO/WS2 hybrid coatings provided low COF values (within the range of 0.18−0.27) and mitigated against aluminum adhesion before failure. An optimum GO/WS2 ratio (6:1) was noted as providing the optimum combination of low friction and improved durability. The concentration of GO in the hybrid coatings influenced the friction performance and durability of coatings, which was related to the formation of tribolayers on the aluminum surface and wear-induced transfer layers on the counterface surface. The formation of the tribolayers was attributed to the mixture of fragmented WS2 platelets, GO flakes, and carbon binder comprising the tribolayer.

Performance assessment of CaF2 solid lubricant assisted minimum quantity lubrication in turning

Abstract Machining is one of the most widely used manufacturing processes and conventional cutting fluids have been used in large quantity to tackle the effects of heat generated during metal cutting. Looking at the effects of the cutting fluids on the environment as well as an increase in total production cost, the performance of CaF2 solid lubricant assisted minimum quantity lubrication in turning is studied. The performance of solid lubricant is judged by measuring surface roughness, chip-tool interface temperature and tool flank wear in different machining conditions. The effect of different concentration of solid lubricant is analyzed under various cutting conditions. A comparison of the results is provided with those obtained in dry, wet and minimum quantity lubrication (MQL) machining. The results indicate the improvement in process performance with the application of solid lubricant assisted lubrication. The use of the solid lubricant with MQL can be considered as a low-cost environment-friendly alternative.

Solid lubricants for composite materials machining

Abstract The paper describes the relevance of application of solid lubricants for edge-cutting and abrasive machining of blanks from hard-to-process materials, including composite structural materials. Solid lubricants are used in cases where the use of cutting fluids is limited. Solid lubricants make it possible to control machining processes. The paper suggests the use of two formulations of solid lubricants, stearine-based Composition No. 1 and molybdenum disulfide (MoS2) based Composition No. 2, for which comparative experimental data are given characterizing their application in tests of drilling of two samples made of corrosion-resistant steel 14Cr17Ni2 (analogue of AISI 431) and carbon fiber reinforced plastic (CFRP) KMU-4e. The test results proved high efficiency of solid lubricants expressed in the increased durability of cutting tools.

Fretting wear behavior of duplex PEO/chameleon coating on Al alloy

Plasma electrolytic oxidation (PEO) is an attractive technology for improving resistance to wear, heat and corrosion of aluminum alloys. PEO results in a hard, well-adhered alumina ceramic coating with a morphology which is graded from a dense region near the substrate interface to a porous outer region. Such properties mean that PEO can be an ideal underlying layer for the application of solid lubricants which can be entrapped in outer pores and provide reservoirs for the tribological contact lubrication. This study investigates the fretting wear behavior and adaptive mechanisms for a PEO-produced alumina surface of about 11–12GPa hardness with a top layer of an MoS2/Sb2O3/C chameleon solid lubricating coating, the composition of which was designed to self-adapt in variable humidity environments for friction and wear reduction. Coupons of AA 6082 alloy were coated by the PEO process and then were over-coated by a burnishing process with a MoS2/Sb2O3/C chameleon coating to prepare a duplex coating combination. The coated surfaces were investigated using nanoindenation, Raman spectroscopy and scanning electroscopy and were then subjected to fretting wear tests against steel and alumina balls with variable amplitude (0–100μm) and loads (10–100N) in both humid air and in dry nitrogen environment conditions. The tests demonstrated low friction coefficients, considerable reduction in critical amplitude for the stick-slip transition, and self-adaptive tribological behavior in cycled environment tests. Friction coefficients of the order of 0.10–0.15 in humid air and 0.06–0.09 in dry nitrogen were recorded and linked with the surface self-adjustment from graphite to MoS2 lubrication, respectively, which was confirmed by Raman spectroscopy. The studies demonstrate the effectiveness of the PEO/chameleon duplex coating system for the friction reduction of and fretting wear in the gross-slip regime, as well as significantly reducing the critical amplitude of stick-slip transition for fatigue wear mitigation.

Research of lamellar lubricants mechanical-and-physical properties and structure

in the article ascertaining the applicability of lamellar solid lubricants is considered for the purpose of wear resistance increase of metal surface elements by their contact interaction in different operational conditions. Forms of binding in the structure of solid lubricants are described through the example of molybdenum disulfide MoS2, as well as change properties at standard and high (350-480°C) exposure temperature. After solid lubricant application the additional treatment is made on the work surface of flanged wheels in the wheel set of a locomotive using plastic deformation for getting of multilevel micro relief resulting in working service increase of the wheel set in comparison with control set from 1.6 to 3 times.

Applying solid lubricant to the grinding-wheel surface

The application of solid lubricant to the cutting surface of grinding wheels in the course of machining is considered. This technique improves the efficiency and quality of grinding. Two methods of lubricant supply are considered, and an appropriate attachment design is proposed.

An assessment of environmentally conscious lubrication techniques in grinding: use of minimum quantity lubrication, solid lubricants and cryogenic cooling

The quality of work piece produced by the grinding operation greatly depends upon the heat generated due to friction and high specific energy of surface. The heat generated due to friction not only affects the integrity of work piece surface but also limits the applicability of the work piece material being ground. To overcome the effects of high heat generation in grinding, cooling and lubrication is generally provided. In this study, an effort is made to review the status on various newer, efficient and environment conscious lubrication techniques in grinding operation. It includes the method of minimising quantity of lubricants, application of solid lubricants and cryogenic cooling such techniques can reduce the cost and environmental hazards in grinding process and also leads towards the cleaner production. The performance and limitations of these methods of lubrication are highlighted here. The review also suggests some of the possible avenues of future research in cutting fluid application for grinding process.