Effect Of Solid Lubricants Research Articles

Development and characterization of TiAlN (Ag, Cu) nanocomposite coatings deposited by DC magnetron sputtering for tribological applications

AISI 420 stainless steel is currently used in the manufacture of surgical and dental instrumentation due to its hardenability, acceptable biocompatibility and resistance to corrosion. However its resistance to wear is relatively low and therefore multiple strategies of surface modification are used to reduce the wear volume and to confer other specific properties on this steel. In this work, TiAlN coatings doped with five different content proportions of Ag and Cu nanoparticles (11 at.% to 20 at.%) were deposited onto 420 steel by means of DC magnetron sputtering, using two targets of Ti/Al and Ag/Cu (both 50/50 at.%), which were placed facing each other at 180° and spaced 42 cm apart. The microstructure, and chemical and phase composition, were analyzed by X-ray diffraction, scanning and transmission electron microscopy and energy dispersive X-ray spectroscopy, while roughness was determined using atomic force microscopy. Hardness and Young's modulus were evaluated by nanoindentation measurements. Finally, the tribological properties were assessed using the ball on disc test method. The composite coatings initially showed a continuous decrease in hardness and a significant reduction in the coefficient of friction and wear volume with the increase in Ag–Cu content, due to the solid lubrication effect of the metallic nanoparticles. However, above 17 at.% of Ag–Cu, the coefficient of friction and wear volume of the coating increased, due to the low hardness of the coating, which lead to the wear process being mainly cohesive type between coating and substrate, and adhesive type between coating and ball. The lowest wear volume, of 7.7 × 10−5 mm3, was exhibited by the coating deposited with 17 at.% Ag–Cu. This was much lower than that of uncoated steel which was 5.4 × 10−4 mm3. This new nanostructured coating could be considered for potential application in manufacturing processes where protection of tools against wear is required, due to its adequate balance between hardness and wear resistance.

Mussel-Inspired Graphene Film with Enhanced Durability as a Macroscale Solid Lubricant.

Graphene has exhibited massive potential as a macroscale solid lubricant, but its durability is limited due to the weak adhesion between graphene sheets and the substrate. Here, inspired by mussel adhesive protein (MAP), effective reinforcement of the graphene-substrate interaction to attain remarkable enhancement on the durability of the graphene film is presented. The mussel-inspired graphene (mGr) film exhibits a coefficient of friction stabilizing at 0.16 up to 490000 sliding cycles in the friction testing against the silicon nitride ball; in the identical sliding condition, comparatively, the graphene (Gr) film without MAP only lasts 4300 sliding cycles. The analysis of Raman and ATR-FTIR demonstrates that, on the one hand, the MAP film firmly adsorbs onto the substrate via forming metal-catechol coordination bonds with metal atoms; on the other hand, it establishes strong interactions with graphene sheets by hydrogen bonding as well as the π-π overlap. As an interlayer, MAP retains graphene sheets within the contact interface in the form of a compact tribo-layer, which results in an over 2 orders of magnitude enhancement of durability for the mGr film. This strategy of improving the graphene-substrate adhesion via MAP offers an avenue for the development of effective and reliable graphene-based solid lubricants for engineering applications.

On the Solid Lubricity of Electrophoretically Deposited Carbon Nanohorn Coatings

In this study, dahlia-type carbon nanohorns (CNH) have been deposited onto a stainless steel substrate by using electrophoretic deposition. Secondly, the lubrication properties of the carbon nanohorn coating have been researched by tribometry and compared to an uncoated reference. Wear track analysis has been conducted to identify the underlying tribo-mechanisms. Additionally, Raman spectroscopy was employed to study the structural changes of the CNH during dispersion and tribological testing. Furthermore, energy dispersive X-ray spectroscopy (EDX) was used in order to investigate the chemical composition of the wear tracks’ surface. This work has shown that CNH coatings have the ability to maintain effective solid lubrication on a polished stainless steel surface. A temporary friction reduction of 83% was achieved compared to the uncoated reference. Moreover, the lubricity was active for significant periods of time due to the formation of a Mg(OH)2 layer which provides a certain degree of substrate adhesion as it holds the CNH in the wear track. Once this holding layer wanes, the CNH are gradually removed from wear track resulting in an increase of the coefficient of friction. The complete removal of CNH from the wear track as well as considerable oxide formation was confirmed by EDX. Moreover, the amount of defects in the CNHs’ structure increases by being exposed to tribological strain. Adhesion has been identified as the dominant wear mechanism.

Tribochemical Behaviors of Onion-like Carbon Films as High-Performance Solid Lubricants with Variable Interfacial Nanostructures.

Onion-like carbon (OLC), spherical nanoparticles consisting of carbon shells, is capable of providing exceptional lubrication effects. Nevertheless, the underlying mechanism, especially the tribo-induced evolution of interfacial nanostructures and their correlation with the friction states, is not clear. In this work, OLC films with a thickness of ∼1 μm were synthesized by electrophoretic deposition on the mirror-polished stainless steel. The lubricity was evaluated by tailoring the sliding aspects including applied normal load, contact time, and counterface materials. It is found that the friction reduction level is highly dependent on the material transfer and transformation of the OLC surface and the physicochemical nature of the as-formed tribolayer in the contact areas. The subsurface of the OLC film always undergoes a deep amorphization transformation upon sliding. It is interesting to note that the tribolayer formed on the bare steel ball is mainly composed of highly ordered graphene-like nanoflakes derived from the sliding-induced degradation of OLC nanospheres. In comparison, the nanospherical carbon structure can be retained in the topmost subsurface of the tribolayer formed on the ceramic Si3N4 ball. Such a nanosphere-/amorphization-coupled interface is capable of providing a robust lubrication state under high contact stresses. The findings identify a new lubrication mechanism for the spherical carbon nanostructure, rendering them effective solid lubricants.

Natural articular cartilage: A smart biointerface

In healthy joints, the cartilage surface coated with phospholipid multi-bilayers is governed by the lamellar-repulsive hydration mechanism. The phospholipid bilayers of biological surface are investigated by friction testing, while surface degradation is controlled by wettability. This paper proposes the presence of cartilage surfaces that are negatively charged results in highly efficient lubrication in natural joints. At pH �7.4, the mechanism of lamellar-repulsion between contacting cartilage surfaces is supported by charged macromolecules of synovial fluid and phospholipid lamellar phases. It was proven experimentally that phospholipid (PLs) bilayers with lamellar structure can act as an effective solid lubricant in friction and wear under biological test conditions

Understanding the Unique Role of Phospholipids in the Lubrication of Natural Joints: An Interfacial Tension Study

Some solid lubricants are characterized by a layered structure with weak (van der Waals) inter-interlayer forces which allow for easy, low-strength shearing. Solid lubricants in natural lubrication are characterized by phospholipid bilayers in the articular joints and phospholipid lamellar phases in synovial fluid. The influence of the acid–base properties of the phospholipid bilayer on the wettability and properties of the surface have been explained by studying the interfacial tension of spherical lipid bilayers based on a model membrane. In this paper, we show that the phospholipid multi-bilayer can act as an effective solid lubricant in every aspect, ranging from a ‘corrosion inhibitor’ in the stomach to a load-bearing lubricant in bovine joints. We present evidence of the outstanding performance of phospholipids and argue that this is due to their chemical inertness and hydrophilic–hydrophobic structure, which makes them amphoteric and provides them with the ability to form lamellar structures that can facilitate functional sliding. Moreover, the friction coefficient can significantly change for a given phospholipid bilayer so it leads to a lamellar-repulsive mechanism under highly charged conditions. After this, it is quickly transformed to result in stable low-friction conditions.

The effect of nanoparticle-added solid lubricant on the formability of sheet metal

In sheet metal forming processes, formation without fracture is related to sheet metal formability. Parameters such as tool geometry, blank holder force, and lubricant are changed in order to push the formability of material to its maximum capacity. Lubricants play an important role in sheet metal forming processes. Depending on the type of lubricant, the formability of the sheet metal can be increased, the surface quality of the product can be improved, and the tool life can be extended. In this study, nanoparticles were added to solid lubricants, and the effect of nanolubricants on sheet metal formability was investigated with Erichsen test. Test specimens were observed by SEM, and the nanoparticle effect was investigated by EDX mapping. In addition, the optimum nanoparticle concentration was determined, and the effects of the nanoparticle on solid lubricants were compared with other lubricants which were used in sheet metal forming. As a result of the study, it was observed that the nanoparticle additive increased the formability by 13%. Results showed that the formability of the sheet metal increased up to certain nanoparticle concentrations, but it decreased above this limit.

Microstructural study and residual stress measurement of a hot rolling work roll material during isothermal oxidation

Compact oxide scale at a high speed steel (HSS) work roll surface during hot rolling process can act as an effective solid lubricant and a protective layer for better product surface quality. In this study, considering practical hot rolling process time, microstructure and surface roughness of three oxide scales on a HSS work roll have been systematically characterized with a help of scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). It has been found that growth of the oxide scale follows a parabolic rate law and the dominant oxide phase is hematite. In addition, surface roughness increases dramatically when the oxidation time is less than 30 min and then changes slightly, when the HSS specimens were isothermally oxidized at 600 °C in a dry air. Residual stress evolutions with oxidation time have also been examined in details using X-ray diffraction (XRD) method. Measurement of microhardness at the HSS specimen surface revealed that development of defects and decrease of compressive residual stress in the oxide layer lead to a slight decrease in the microhardness.

Nickel Aluminum Matrix Solid-Lubricating Composite Lubricated by Silver and Silver Vanadate Formed by Tribochemistry at Elevated Temperature

The synergistic effect of solid lubricants plays a significant role in wide-temperature-range lubrication, where the combination of lubricious oxide and Ag is the promising solid lubricants. In this paper, the friction and wear performances of Ni3Al with the addition of Ag and V2O5 solid-lubricating composites were evaluated from room temperature to 1000 °C. It was found that Ni3Al matrix composite with the addition of V2O5 has high friction coefficient of 0.3–0.7, while Ni3Al matrix composite with simultaneous addition of Ag and 2 wt % V2O5 has a relatively low friction coefficient of 0.25–0.4 between room temperature and 1000 °C and wear rate with the magnitude of 10−5 mm3/N m at high temperatures. The results revealed that nickel aluminum matrix solid-lubricating composite lubricated by silver and in situ formed silver vanadate at elevated temperature achieves a wide-temperature-range lubrication, which is attributed to the synergistic action of silver and silver vanadate formed at high temperatures.

Influence of WS2/SnS2 on the tribological performance of copper-free brake pads

PurposeThe purpose of this study is to investigate the influence of solid lubricants (tungsten disulfide [WS2]/ Tin disulfide [SnS2]) on the tribological performance of brake pads.Design/methodology/approachIn this study, the brake pads were developed by varying the solid lubricants (WS2/SnS2) without varying the other ingredients. The brake pads were developed as per the industrial procedure. Thermal stability was found for varying ingredients and developed pads. The physical, mechanical and thermal properties of the developed brake pads were analyzed as per the industrial standards. The tribological properties were analyzed using the Chase test. The worn surface analysis was done using scanning electron microscopy, elemental mapping and three-dimensional profile analysis.FindingsThe experimental results indicate that the WS2-based brake pads possess good physical, chemical and mechanical properties with stable friction and less wear rate due to its good lubrication film formation and thermal stability natures of WS2.Originality/valueThis paper explains the effect of solid lubricants in brake pads for enhancing the tribological performance by the shearing of crystal structure, thermal stability and tribo film properties of the lubricants.

Type and concentration effects of particulate solid lubricants on the microstructure, friction, and wear of electrodeposited Ni composite coatings

Nickel–MoS2 composite coatings were obtained by electrodeposition from a nickel electrolyte containing suspended MoS2 particles. The coating composition, morphology, crystalline structure, microhardness, and frictional behavior were studied as a function of MoS2 concentration. The results obtained in this study revealed that the codeposited lubricant particles strongly influenced the composite nickel coating properties. It was found that increasing codeposited MoS2 decreases the average grain size of nickel crystallites and leads to the formation of clusters which, in turn, lead to rough coatings with a high and variable thickness. The results of tribological response indicated that the reduction of friction coefficient and the improvement of wear resistance were performed until an optimal value of MoS2 concentration, which provided the best condition that promoted the tribo-layer stability and maintained the matrix integrity. A comparison of tribological and micromechanical properties between the coating containing the optimal fraction of MoS2 particles and the coating containing nearly the same fraction of graphite particles has been undertaken. Unlike the case of the addition of graphite particles, the microhardness of composite coating has been enhanced with the incorporation of MoS2 particles. However, the incorporation of graphite particles in the coating induced more effective lubrication and wear resistance.

Rapid selection of environmentally friendly layered alkaline-earth metal phosphates as solid lubricants using crystallographic data

Lubricating technologies are essential for saving energy and identifying effective, environmentally friendly layered materials that meet the demands of solid lubricants is an important direction. Herein, we probed the relationships between load-carrying capacity and crystal structure. The results showed that increasing the sliding resistance of interlayers using corrugated layers increased the load-carrying capacity of layered solid lubricants. This finding expands on the traditional lubrication mechanism of layered materials. Following the rules, rapid selection of layered potassium magnesium and calcium phosphates (K-LMP and K-LCP) as effective solid lubricants was achieved using crystallographic data and strict filtering criteria. In order to prepare materials suitable for lubrication, the synthesis of K-LMP and K-LCP was optimised. These materials could be utilised in the food, textile or marine machinery industries in the future. The developed method could successfully guide the synthesis of application-oriented materials.

3D hierarchical structure of MoS2@G-CNT combined with post-film annealing for enhanced lithium-ion storage

We present a three-dimensional hierarchical structure of a ternary nanocomposite consisting of MoS2, graphite, and carbon nanotubes, as a new anode material for lithium-ion batteries. The 3D structure (3D MoS2@G-CNT), prepared by a simple and scalable high-energy ball milling process, contains 2D heterogeneous clusters between MoS2 and graphite, which are effectively interconnected by 1D carbon nanotubes. During the formation of 2D heterogeneous clusters, the surface area of MoS2 is significantly increased due to exfoliation driven by the solid lubrication effect from mechanical milling. The morphology and structural properties of 3D MoS2@G-CNT are analyzed by various techniques including XRD, Raman, SEM, TEM, and BET. At the optimal ratio of ingredients, 3D MoS2@G-CNT shows excellent electrochemical performance in terms of specific capacity, long-term cycle life, and rate capability. Specifically, at the best experimental conditions, 3D MoS2@G-CNT delivers ∼1600mAhg−1 at 0.2Ag−1 after 100 cycles and ∼1200mAhg−1 at 0.5Ag−1 after 450 cycles. To achieve these, we also introduce a post-film annealing step which induced rearrangement of the binder, thus improving the adhesion between the electrode material and current collector. Enhanced charge transport and mechanical integrity after this treatment are confirmed by FTIR, ex-situ SEM, and photographs.

Influence of Temperature on Friction and Wear Characteristics of Polyaryletherketones and Their Composites Under Reciprocating Sliding Condition

The tribological behavior of different polyaryletherketones (PAEKs) and their composites was studied under reciprocating sliding conditions at room temperature as well as at elevated temperature (120 °C). The effect of glass, carbon fiber and solid lubricants (PTFE and graphite) filled in PAEK matrix was investigated at different normal loads. It was found that friction and wear characteristics are strongly influenced by the type of PAEK matrix and the material composition. The temperature has a more pronounced influence on the coefficient of friction and specific wear rates of PAEKs and their composites than the normal load. Friction and wear mechanisms are discussed, using scanning electron micrographs of the worn surfaces. A relationship between the average coefficient of friction and specific wear rates of PAEKs and their composites is also reported.

Role of solid lubricant (MoS2 and graphite) variations on characteristics of brake lining composite

Solid lubricants have an essential role in brake lining composite. They can reduce wear that occurs either on the brake lining composite or the contact surface. In this study, the effect of solid lubricants (MoS2 and graphite) on brake lining composite at various sliding speeds and contact pressure has been investigated. Three brake lining composites were made with the same formulation except for the addition of the solid lubricants, all of which have 15% graphite, 12% graphite + 3% MoS2, and 9% graphite + 6% MoS2. The coefficient of friction test with various contact pressure and sliding speed was conducted by using a pin on disc tribometer. The result showed that coefficient of friction and specific wear was strongly influenced by the solid lubricant. Compared with graphite, MoS2 had a better lubrication performance. An increase in MoS2 content decreased the coefficient of friction either on contact pressure or sliding speed variation. MoS2 also reduced the specific wear as well as the temperature rise in the disc.

Influence of Surface Design on the Solid Lubricity of Carbon Nanotubes-Coated Steel Surfaces

Topographically designed surfaces are able to store solid lubricants, preventing their removal out of the tribological contact and thus significantly prolonging the lubrication lifetime of a surface. The present study provides a systematic evaluation of the influence of surface structure design on the solid lubrication effect of multi-walled carbon nanotubes (MWCNT) coated steel surfaces. For this purpose, direct laser writing using a femtosecond pulsed laser system is deployed to create surface structures, which are subsequently coated with MWCNT by electrophoretic deposition. The structural depth or aspect ratio of the structures and thus the lubricant storage volume of the solid lubricant is varied. The frictional behavior of the surfaces is recorded using a ball-on-disk tribometer and the surfaces are thoroughly characterized by complementary characterization techniques. Efficient lubrication is achieved for all MWCNT-coated surfaces. However, and in contrast to what would be expected, it is shown that deeper structures with larger lubricant storage volume do not lead to an extended lubrication lifetime and behave almost equally to the coated unstructured surfaces. This can be attributed, among other things, to differences in the final surface roughness of the structures and the slope steepness of the structures, which prevent efficient lubricant supply into the contact.

Microstructure and wear performance of Al5083/CeO2/SiC mono and hybrid surface composites fabricated by friction stir processing

Abstract Friction stir processing (FSP) was utilized to produce surface composites by incorporating nano-sized cerium oxide (CeO2) and silicon carbide (SiC) particles individually and in combined form into the Al5083 alloy matrix. The study signified the role of these reinforcements on microstructure and wear behavior of the resultant surface composite layers. The wear characteristics of the resultant mono and hybrid surface composite layers were investigated using a pin-on-disc wear tester at room temperature. The microstructural observations of FSPed regions and the worn out surfaces were performed by optical and scanning electron microscopy. Considerable grain refinement and uniform distribution of reinforcement particles were achieved inside the nugget zone. All the composite samples showed higher hardness and wear resistance compared to the base metal. Among the composite samples, the hybrid composite (Al5083/CeO2/SiC) revealed the highest wear resistance and the lowest friction coefficient, whereas the Al5083/SiC composite exhibited the highest hardness, i.e., 1.5 times as hard as that of the Al5083 base metal. The enhancement in wear behavior of the hybrid composites was attributed to the solid lubrication effect provided by CeO2 particles. The predominant wear mechanism was identified as severe adhesive in non-composite samples, which changed to abrasive wear and delamination in the presence of reinforcing particles.

Dry friction and wear of self-lubricating carbon-nanotube-containing surfaces

The unfavorable environmental conditions of certain tribological systems, such as operation at high temperatures or under vacuum, set the need to replace liquid with solid lubricants. Multi-Wall-Carbon Nanotubes (MWCNTs) have been emphasized as a very effective solid lubricant. The particles have been used to create self-lubricating materials by acting as reinforcement phase in composites or as solid-lubricant coating that works in conjunction with textured surfaces to prevent the removal of particles from the contact. However, both approaches are restricted to some extent. In the case of composites, the solid lubricant concentration is limited so as not to influence the mechanical stability of the final component. For coated surface structures, the textured surfaces can degrade during the experiment. The present study focuses on the combination of these approaches in order to create enhanced self-lubricating surfaces with MWCNTs as the solid lubricant. A custom-made ring-on-block tribometer is used to study the behavior of laser textured MWCNT-coated and MWCNT-reinforced nickel matrix composites under the conditions of unidirectional sliding in conformal contact. It is shown that the combination of both approaches allows for a maximum 4-fold reduction in friction and a 115-fold reduction in wear rate compared to the reference. Additionally, the lubrication mechanism of the MWCNTs is investigated in more detail and a structural degradation model of the mechanically stressed MWCNTs is proposed. Our results highlight the integrated solution as a suitable approach for self-lubricating surfaces subjected to unidirectional sliding.

Friction and wear properties of Al-20Si-5Fe-2Ni-Graphite solid-lubricating composite at elevated temperatures

Abstract Tribological behavior of the Al-20Si-5Fe-2Ni alloy and its composite containing 30 wt % copper-coated graphite were investigated at 25–500 °C. The result indicated that the composite showed a significantly decreased wear rate at all test temperatures as compared to the alloy. Owing to the combined effect of solid lubrication and dispersion strengthening provided by the graphite particles and in situ formed Al2Cu intermetallic, the composite can avoid severe adhesive wear up to 500 °C. At 25 °C, the predominant wear mechanism of the composite is abrasive wear, and it transforms to abrasive wear and delamination wear at elevated temperatures.

Solid lubrication in fluid film lubrication

AbstractThis work aims to study the effect of solid lubrication combined with a liquid lubricant in a cooperative lubrication regime. The solid lubricant consists of sintered composites, which contain solid lubricant dispersed in a metallic matrix. The durability of the lubricious regime (μ < 0.2) was studied. The combined action significantly increased the durability of the lubricious regime (3 orders of magnitude). Additionally, the use of 3D triboscopy made it possible, using an original approach, to verify a significant increase in the friction coefficient at the dead centres of the strokes. This difference between the friction coefficients was discussed in terms of the debris accumulation and the change in the lubrication regime. A preponderant role was assigned to the accumulation of debris.