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Wear behavior of three different AA6061-0.5 wt% CNT composites fabricated using different dispersion techniques by stir-squeeze-casting have been studied against a steel ball counter body. The uniform CNT distribution in composites fabricated by dropping pellets of Al-powder and CNT (PD composite) yields the lowest coefficient of friction (COF) and specific wear rate (SWR) for both the composite material and the counter body. Both COF and SWR increase as the load increases from 5 N to 10 N, but drop as the load increases further to 15 N. Even at 150 °C, the presence of CNTs improved the tribological performance of AA6061. A detailed study of CNT morphologies on the worn surface and Raman spectroscopy indicate their beneficial role as a solid lubricant to reduce COF and material loss during wear. • Wear behavior of Al-CNT composite strongly affected by CNT dispersion in Al melt. • The COF and SWR of pellet drop composites (PD) were 68 % and 77 % lower than AA6061. • At 150 °C, PD composite is more wear resistance than AA6061 even at higher load. • Stability of tribolayer determines the wear behavior of the composites. • Raman Peak shift and I D /I G ratio indicate structural change of CNTs due to wear. [ABSTRACT FROM AUTHOR]

As a typical two-dimensional layered material, hexagonal boron nitride nanosheets (BNNSs) have high mechanical strength, good conductivity, and excellent lubrication performance, which have great potential as solid nano lubricant additives in tribology. When BNNS is used as a water-based or oil-based lubricant additive, its inherent chemical inertness and poor dispersion stability make it difficult to achieve optimal anti-friction and anti-wear performance. In this paper, a simple and efficient preparation method for obtaining hexagonal boron nitride nanosheets (C-HPC-BNNSs) with surface carbonization modification and excellent dispersion stability in water-based lubricants was proposed. Using hydroxypropyl cellulose (HPC) as a modifier, a wet ball milling process combined with high-temperature carbonization treatment effectively reduced the thickness of BNNSs while uniformly modifying N-doped carbon layers on the surface of the nanosheets. Benefit from the smaller layer thickness of C-HPC-BNNSs, they exhibit excellent dispersion stability in the water-glycol solution and can maintain no significant agglomeration behavior for up to 30 days, which is the basis for the excellent lubrication performance of lubricant systems. In addition, the N-doped carbon layer modified on the surface is beneficial for enhancing the hydrophilicity of BNNSs and synergistically enhances the tribological properties of the material with BNNSs. The test results of the four-ball friction tester show that compared with the pure water-glycol solution and the water-glycol system with added C-HPC-BNNSs, the friction coefficient (COF) and wear volume of the system with added C-HPC-BNNSs could be as low as 0.095 and 1.99 × 10−4 mm3, respectively. This work has positive significance for the preparation, surface modification, and tribological application expansion of two-dimensional layered material BNNSs, which is conducive to further understanding the relationship between. [Display omitted] • Surface carbonized boron nitride nanosheets were formed by wet ball milling process combined with high temperature carbonization. • C-HPC-BNNS has excellent dispersion stability in water-glycol solutions and will not aggregate for up to 30 days. • The COF and wear volume of the water-glycol system with C-HPC-BNNS were as low as 0.095 and 1.99 × 10-4 mm3 , respectively. [ABSTRACT FROM AUTHOR]

Wheel flange wear on curved tracks is a significant issue that poses a threat to the safety of rail vehicles and leads to increased maintenance costs for railway operators. This study aims to develop a numerical model to predict wheel flange wear on small radius curves. The model integrates an efficient wheel-rail conformal contact model with laboratory-tested wear coefficients. By using virtual penetration and influence coefficients for conformal geometries, the model accurately simulates the conformal contact between the wheel flange and the rail gauge corner. The validity of the wheel flange wear prediction model is confirmed through field tracking testing. Using the proposed model, the effects of track layout parameters and solid lubricants on wheel flange wear are investigated. This study provides theoretical guidance for the operation and maintenance of the wheel-rail system to reduce wear. • A fast calculation model of wheel-rail conformal contact is developed. • Wear coefficient for wheel materials under lubrication conditions is measured. • Wheel flange wear model considering conformal and lubricated contact is proposed. • Effect of track parameters and lubricants on wheel flange wear is analyzed. • The proposed model is validated through a railway field test. [ABSTRACT FROM AUTHOR]

In order to modify the wear resistance of Stellite 6 superalloy as wear resistant coating at room temperature-1000 °C, the different contents (1.0, 2.5 and 4.0 wt%) of ZrO 2 reinforced Stellite 6 matrix coatings were fabricated over the Inconel 718 nickel alloy substrate by laser cladding technology. The microstructure, hardness and high-temperature wear behavior of Stellite 6 matrix coatings with ZrO 2 was systematically studied. The sliding wear test were done using a ball-on-disk tribometer against Si 3 N 4 at room temperature-1000 °C. The results showed that the ZrO 2 showed obvious fine-grain strengthening and dispersion strengthening effect. The hardness of coatings reinforced by ZrO 2 were 470–540 Hv. A critical valve of ZrO 2 content was existed for the wear performance of Stellite 6 matrix coatings. The coating with 2.5 wt% ZrO 2 had the desirable wear resistance, and the wear rate was in the order of 10−5 mm3/N.m. This was attributed to the high microhardness and the formation of solid lubricants, as well as the friction film on the sliding surfaces. • ZrO 2 reinforced Stellite 6 coatings were fabricated by laser cladding. • The wear performance of coatings was investigated at RT-1000 °C. • There existed a threshold content of ZrO 2 for the tribological performance. • Wear resistance was due to the synergistic effect of high hardness and friction film. [ABSTRACT FROM AUTHOR]

MoS 2 –Sb 2 O 3 coating is commonly used as solid lubricant for titanium alloys, but the wear-resistance performance is still limited. Core-shell annealed nanodiamond (AND) particle is used as reinforcement for MoS 2 –Sb 2 O 3 coating on Ti6Al4V substrate treated by plasma electrolytic oxidation (PEO) and laser surface texturing (LST). The incorporation of AND particles into the MoS 2 –Sb 2 O 3 coating results in a 39 % decrease in friction and significantly reduces wear on the composite coating. Structural transformation of AND particles to amorphous carbon and graphitic structure promotes the densification of top-layer coating, enhancing lubrication and wear-resistance. Change of contact stress by PEO-LST promotes the formation of dense tribofilm and re-orientation of MoS 2. This study has implications for the development of solid lubrication coatings for aerospace applications through functional nanomaterials and targeted designed surfaces. • Core-shell annealed nanodiamond particle enhances wear-resistance. • Structural transformation of annealed nanodiamond promotes densification of tribofilm. • PEO-LST surface changes contact mechanics condition during friction. • Increased contact stress by PEO-LST enhances lubricious tribofilm formation. [ABSTRACT FROM AUTHOR]

Additive manufacturing of self-lubricating alloys plays a crucial role in the production of complex wear-resistant components and in expanding repair capabilities, especially for intricate wear parts with low tolerances (e.g. with cooling channels). Herein, we report a novel approach providing nickel-based alloy with an excellent tribological performance in dry sliding contacts. Laser-deposited self-lubricating nickel alloys, infused with anti-wear additives of molybdenum disulfide, nickel sulfide, copper sulfide, or bismuth sulfide, were subjected to dry sliding wear tests against an alumina ball counterbody at a temperature range of up to 800 °C. The self-lubricating alloys exhibited a significant decline in friction (43 %) and wear (45 %) at room temperature, 400 °C (friction 40 %, wear rate 55 %), and 600 °C (MoS 2 -based, friction 58 %, wear rate 75 %). The MoS 2 -based alloy coating demonstrated excellent performance characteristics up to 800 °C (friction coefficient ⁓0.25, wear rate 11 × 10−6 mm3 N−1 m−1) due to the formation of a 'glazed' tribolayer. Wear mapping allowed to identification of a critical condition for self-lubricating alloys where positive transitions in wear mechanisms led to a synergistic lubrication mode involving the formation of tribologically induced new lubricious phases such as silver molybdate or nickel-bismuth intermetallic. This work provides a comparative evaluation of the micromechanisms, surface transitions, and tribochemistry of solid lubricants at a wide temperature range and a variety of applications. • Comparative study of five solid lubricants i.e. Ag, Ag + MoS 2 , Ni 3 S 2 , CuS, or Bi 2 S 3. • Superior running-in behavior of self-lubricating alloys especially at 400, 600 °C. • Decline by 40–50 % in CoF and wear rate of self-lubricating alloys at RT and 400 °C. • Exceptional decline for Ag + MoS 2 in CoF/Wear by 58 %/75 % at 600 °C, and 70 %/33 % at 800 °C. • For Bi 2 S 3, a lubricious glazed layer rich in Nickel–Bismuth intermetallic sustains at 800 °C. [ABSTRACT FROM AUTHOR]

PTFE matrix composites reinforced with α-Al 2 O 3 fillers are considered a gold standard polymeric solid lubricant. α-Al 2 O 3 PTFE exhibits ultra-low wear rates (k ∼ 10−8 mm3/Nm) when sliding against steel in the presence of water vapor. Mechanochemical interactions involving the breaking of PTFE chains, and subsequent oxidation in the presence of water vapor helps promote growth of interfacial films that are strongly bound to the sliding interface. In the absence of moisture, wear rates of α-Al 2 O 3 PTFE can increase by as much as three orders of magnitude, underscoring its importance to ultra-low wear behavior. A similar increase in wear rates of α-Al 2 O 3 PTFE in humid environments has recently been observed when sliding against brass counter surfaces, though reasons for this increase remain unclear. These findings suggest that mechanisms of ultra-low wear not only rely on ambient humidity but are also strongly dependent on the counter surface chemistry. In the present work, we test the hypothesis that counter surface chemistry drives alumina PTFE wear by systematically investigating the wear of an α-Al 2 O 3 PTFE composite against brass and aluminum substrates within the context of the morphological and mechanochemical evolution of the sliding interface. Wear tests are conducted under reciprocating, dry sliding conditions. Results presented in this work offer insights on mechanochemical mechanisms that drive wear of α-Al 2 O 3 PTFE against brass and aluminum, and provide a framework for developing strategies for controlling wear. • Dry sliding wear of α-Al 2 O 3 PTFE increases when sliding against brass and aluminum, relative to steel by nearly three orders of magnitude. • Spectroscopic analysis suggests increase in wear is due to partial oxidation of PTFE, resulting in less strongly bound interfacial films. • Preconditioning of polymer pins is explored as a viable route for recovering ultra-low wear against non-ferrous substrates. [ABSTRACT FROM AUTHOR]

Investigations into effects of humidity and oxygen on a polymer-bonded MoS 2 -based dry film lubricant (DFL) coating, on grit-blasted Ti–6Al–4V using a cylinder-on-flat configuration, are reported. Sustained low friction and low wear occurred in both low-humidity air and low-humidity argon, producing significant extension of fretting wear lifetimes. A cracked-paved structure was observed in low-humidity air which was not observed in low-humidity argon. The Mo-DFL coatings were observed to be highly sensitive to humidity. As humidity increased, a notable rise of friction and wear ensued which reduced the system lifetime. Different friction evolution was observed in high-humidity air and high-humidity argon before failure, with a short-lived but established low friction phase in high-humidity air and a consistent high friction phase in high-humidity argon. Although oxidation of the MoS 2 to MoO 3 is a key process of degradation of DFLs of this type, this oxidation reaction does not operate in the absence of humidity. • Experimental fretting tests carried out on MoS2 loaded polymer DFL systems in a range of environmental conditions. • Fretting is very sensitive to RH, low RH fretting lifetimes were an order of magnitude larger than for atmospheric conditions. • Oxygen appears to contribute to the formation of a low friction phase within the evolution of friction. [ABSTRACT FROM AUTHOR]

In order to simulate the fade behaviour of friction between copper-based brake pads and brake disc during emergency braking of high-speed railway train, accelerated fade experiments were performed by executing cyclic emergency braking under high braking speeds and overload conditions. The fade behaviour and its relationship with variations in the microstructure and composition of the friction layer were investigated in detail. The results showed that continuous emergency braking resulted in the formation of fatigue cracks in the friction layer, and the high temperature led to the softening and flow of copper-rich phase existing in the surface layer. Those promoted the formation of friction layer composed of copper-rich phase and iron-rich phase alternately. The softening copper-rich phase was the primary cause of the fading friction coefficient, serving as a solid lubricant to reduce the clamping force on hard particles and in turn reduce the friction resistance. Moreover, the accumulated damage and rapid transfer of the friction layer on the friction surface weakened the brake pad tolerance and caused the friction coefficient to fade more rapidly. • The changing relationship of instantaneous friction coefficient and temperature during braking is exhibited. • The relation between the variation of friction layer and the friction coefficient is explained in detail. • The main factor that induces the fade behaviour of copper-base brake pads is elucidated. [ABSTRACT FROM AUTHOR]

Tools in cold forging processes experience high loads. In order to reduce the process loads, solid and liquid lubricants are used. The use of lubricants is disadvantageous due to ecological, economical, and legislative reasons. Novel approaches in cold forging aim at the use of ecologically benign lubricants or avoiding the usage of lubricants completely. This paper focuses on the substitution of lubricants in cold forging by surface textures on workpieces and self-lubricating coatings on tools. The potential of surface textures on workpieces and self-lubricating tool coatings to reduce friction and wear in dry forging was demonstrated in preliminary work using analogy experiments. After analogy tests under dry tribological boundary conditions have shown positive results in preliminary work, dry full forward extrusion tests were performed in this work. Firstly, the focus of the investigations was on feasibility. Secondly, the potentially best combination with regard to friction and wear reduction was selected. Thirdly, in addition to force profiles, the surface of cold forging dies was studied by means of SEM. At the same time, the processes were supported by a validated numerical FE process simulations with the simulation software Forge NxT 2.1. The dry full forward extrusion experiments were successful, because the specimens were extruded until the final point and could be ejected without failure. It was observed that the highest wear of the dies occurred not in the region with the highest tool load but in the region with relatively low loads. Furthermore, it was shown that no wear model suitable for cold forging exists so far. • The research article deals with the study of dry full forward extrusion. • In particular, investigations into the most influential signs of wear are new. • Imaging of wear was investigated using finite element method firstly in cold forging. [ABSTRACT FROM AUTHOR]

Coolant plays an important role in grinding, as it preserves the workpiece integrity by reducing the friction, as well as cooling the surface. In grinding, the coolant is generally applied at high flow rates, through the flooding technique, which can cause health and environmental damage and moreover high costs. An alternative is to use less coolant with the addition of solid lubricant particles, which improve the tribological behavior of the fluid, such as multilayer graphene that is a small solid lubricant with high thermal conductivity. The aim of this present study is to investigate the influence of a multilayer graphene concentration (0.05 wt% and 0.10 wt%) dispersed in semi-synthetic oil applied using the Minimum Quantity of Lubrication (MQL) technique on the grinding performance of Inconel 718 alloy under various machining conditions. Graphene-free oil, flooding and dry conditions were also tested for comparison. The output parameters evaluated were: surface roughness, microhardness, residual stresses, grinding power and SEM images. The cooling-lubrication fluid properties (viscosity and thermal conductivity) were also obtained. The results showed that grinding with the lowest graphene concentration in the milder condition provided the lowest roughness values and microhardness variation, required about 50% less grinding power, inhibited surface crack generation, led to smoother surfaces and resulted in compressive residual stress. [ABSTRACT FROM AUTHOR]

The work addresses a particular case of interaction between adhesive and abrasive wear in the electrical contact field, characterized by a selective adhesive transfer layer. The system studied is composed of a resistive element (NiCr thin layer) and a contactor (AgPd), forming an angular position sensor lubricated with a standard silicone oil/PTFE grease. The angular position/electrical resistance relationship is disturbed by wear of the NiCr surfaces, enhanced by the formation on the AgPd contactor of a third body enriched in Ni. From a practical point of view, it has been found that this detrimental Ni-selective transfer formation can be controlled by inserting tumble finishing with lamellar alumina powder as the last step of NiCr track manufacturing. Acting like a solid lubricant, alumina clusters of 1–5 µm in size adhere to NiCr surface giving rise to a delay or cancel of Ni adhesion on the contactor which lead to a significant improvement of electrical behavior of the position sensor. • We studied the wear mechanisms taking place in an electrical contact inside a sensor. • The resistance degradation of AgPd/NiCr contact is related to the wear process. • Selective transfer layer rich in nickel adhered on the AgPd surface is point out as the major factor limiting the electrical performances of the contact. • Tumble finishing treatment using Alumina particles was found to improve the electrical performances of the contact by stopping the growth of the transfer layer (3rd body). [ABSTRACT FROM AUTHOR]

Electrodeposited composite coatings consisting of reinforcement, solid lubricant and tough metallic matrix usually exhibit excellent tribological properties, including high resistance to wear and corrosion, low coefficient of friction, and other desired properties. However, if the distribution of the reinforcement is not homogeneous due to particle agglomeration, especially for fine or nano-sized particles, the performance of the coatings could be largely deteriorated. In the work, Cu-Sn-PTFE- TiO 2 composite coatings were made using a pulse electroplating process. Homogeneous distribution of TiO 2 particles in the coatings was achieved using highly homogeneous dispersion of nano TiO 2 Sol in the plating solution rather than directly dispersing the TiO 2 nanoparticles in the solution. Such fabricated Cu-Sn-PTFE- TiO 2 coatings showed smooth surface, refined grains and high micro-hardness, leading to excellent tribological properties and corrosion resistance. However, too much TiO 2 Sol led to changes in the solution's pH value, accompanied with agglomeration of the TiO 2 Sol, lowering the quality of the coatings with decreased resistance to failure during sliding wear tests. This study demonstrates that application of nanoparticle sol in a proper concentration range is an effective new approach to achieve homogeneous distribution of reinforcing nanoparticles in electrodeposited composite coatings for high wear resistance, low friction and corrosion resistance. • Homogeneous dispersion of TiO 2 nanoparticle in TiO 2 Sol containing plating solution resulted in Cu-Sn-PTFE-TiO2 composite coatings with homogeneously distributed nanoparticles in a fine-grained matrix. • With an appropriate TiO 2 Sol concentration, the Cu-Sn-PTFE-TiO 2 coating shows refined crystalline grain and high hardness, leading to improved tribological properties. • Too much TiO 2 Sol however promoted agglomeration of the TiO 2 Sol, thus deteriorating the coating. • The Sol is a new and effective method of making homogeneous nano-composite coatings with higher mechanical strength and improved tribological properties. [ABSTRACT FROM AUTHOR]

Development of low friction surface materials matched with low friction lubricants is an important route to improving engine efficiency and reducing emissions. Diamond-like carbon (DLC) is a material of interest because it combines the durability of diamond with the low friction properties of graphite. Consequently, various forms of DLC have been studied as durable, low friction surface coatings. At the same time, friction modifiers (FMs) are often used in engine lubricants to produce low friction surface layers. Molybdenum-based friction modifiers are commonly used additives that mechanochemically decompose to cover surfaces with MoS 2 , a solid lubricant. FMs can also reduce friction on DLC surfaces. However, molybdenum can be harmful to DLC and cause rapid degradation and breakthrough of the DLC surface layer. We have found that the wear debit of molybdenum FMs trends with surface affinity of the additive. The adsorption isotherms of 4 Mo-based FMs on boron-doped diamond-like carbon (BDLC) were measured with a quartz crystal microbalance (QCM). The strongest adsorbate, Mo dithiophosphate (MoDTP), which also forms the thickest film, protects the BDLC surface enabling low friction while keeping wear low. Theoretical predictions based on molecular dynamics simulations combined with molecular-thermodynamic theory (MD-MTT) show that adsorption of MoDTP on BDLC deviates significantly from Langmuir behavior and free energy of adsorption for the first monolayer is significantly more negative than the multilayer adsorption free energy observed experimentally. The relationship between wear and adsorption behavior is consistent with experimental observations in the literature. These results also suggest that additive performance in a fully formulated lubricant is related to fundamental adsorption properties measured on the neat additive. • Molybdenum based friction modifiers reduce friction and accelerate wear on boron-doped diamond-like-carbon (BDLC). • Highly surface active (i.e. more negative ∆G ads) Mo friction modifiers cause less wear on BDLC. • Molecular dynamics-molecular-thermodynamic theory (MD-MTT) can predict adsorption isotherms. • Mo-dithiophosphate forms a thick multilayer adsorbed film on BDLC. [ABSTRACT FROM AUTHOR]

Owing to their HCP structure and inherent resistance to deformation, Cobalt-containing alloys are often used for low-friction and wear-resistant purposes. Here, the tribological performance of nanostructured Nickel–Cobalt coatings with 0–83 wt% Co was investigated. Three Co-rich coatings with similar microhardness and grain sizes showed different friction and wear behaviors that cannot be solely attributed to the HCP phase fraction. The coating with the highest Co content of 83 wt% achieved the lowest wear rate. However, the lowest surface friction was observed in Ni-76 wt% Co with a large amount of wear debris covering the contact surface of its counterpart. Nevertheless, these debris led to micro-cutting of the coating and a high wear rate. Detailed surface investigation of these coatings and their counterparts revealed a significant difference in material transfer, distribution of wear products, and wear mechanisms. Our analysis showed that the tribological performance of Ni–Co coatings can be dominated by surface-related features (such as surface oxide integrity, tribofilm, and wear debris) and lattice internal stress over the underlying phase fractions. [Display omitted] • Co-rich coatings with similar hardness and grain size differ in friction and wear. • Wear-resistant coating had high HCP fraction, low stress, and intact surface oxide. • Hybrid FCC-HCP coating with high internal stress showed largest wear debris. • Large and abundant debris led to surface micro-cutting and high wear rate. • Certain wear debris helped reduce direct metal contact and lowered friction. [ABSTRACT FROM AUTHOR]

Carbon fabric (CF)/polymer composites are potential lubricating materials, which could be used for plane bearing lining, sliding guide rails, and other moving parts of advanced equipment. But the low bonding strength at the interface between the carbon fibers and the polymer matrix is a great challenge to developing high-performance CF/polymer composites. In this study, the CF is coated with an active transition layer using high-adhesion polydopamine (PDA) as an ideal plate for the TiO 2 micro/nano hierarchical structure to construct CF-PDA-TiO 2. This hierarchical structure is made of micro-bundles, each composed of numerous nanorods, and decorated on CF surface densely and uniformly. The interfacial combination of CF-PDA-TiO 2 and phenolic resin (PF) shows excellent performance, and the interlaminar shear strength (ILSS) of CF-PDA-TiO 2 /PF is improved from 10.48 MPa to 37.22 MPa, attributed to the improved interfacial physicochemical interaction. Furthermore, the tribological performance of CF-PDA-TiO 2 /PF with size of Φ30 mm✕5 mm is characterized by sliding against 440C stainless steel ball with diameter of 8 mm using ball-on-disc sliding wear tester (MPX-3). The tribological results demonstrate that the friction coefficient and wear rate of CF-PDA-TiO 2 /PF are reduced by 37.91% and 77.6%, compared to CF/PF. Moreover, the CF-PDA-TiO 2 /PF maintains low friction coefficient and wear rate under high-load and high-speed sliding conditions. This is mainly because TiO 2 nanorod bundles exhibit micro/nano multi-scale interfacial enhancement, promoting stress distribution and micro-cracks deflection etc. Moreover, TiO 2 nanorod bundles could promote the formation of a uniform transfer film to decrease friction and wear. This work implies that interface construction of micro/nano hierarchical structure is a promising way to enhance the tribological performance of CF/polymer composites. Construction of TiO 2 micro/nano hierarchical structure on the surface of CF using PDA as an active transition layer to enhance the tribological performance of CF/PF composite. [Display omitted] • A novel TiO 2 micro/nano hierarchical structure was constructed on CF surface. • The interlaminar shear strength and tribological performance of CF/PF were enhanced significantly. • Multi-scale synergistic enhancement of TiO 2 micro/nano hierarchical structure was proposed. • A high-performance lubricating material of CF/PF was constructed. [ABSTRACT FROM AUTHOR]

Ni 3 Al-based composite containing Ag/Cu modified h BN nanosheets (Cu- h BN)/Ag-Cu- h BN have been fabricated by vacuum hot press sintering and their tribological performance has been evaluated at RT, 200, 400, 600 and 800 °C at a constant load of 10 N and a sliding speed of 0.2 m/s using a ball-on-disc rotary tribometer against a Si 3 N 4 ball. The main aim of the study is to examine the occurrence of a synergetic action between Ag and Cu- h BN in attaining low friction and anti-wear properties from room temperature to 800 °C. The results indicated a significant reduction in the coefficient of friction by the addition of a combination of Ag and Cu- h BN in comparison to either Ag or Cu- h BN. Ni 3 Al-Ag-Cu- h BN attained the lowest coefficient of friction (∼0.19) and wear rate (1.85 x 10-5 mm3/Nm) at 800 °C, which has been ascribed to the supportive lubrication provided by Ag and Cu- h BN. The investigation also revealed that Ag provided lubrication at temperatures below 500 °C, whereas Cu- h BN and other lubricious oxides like Ag 2 MoO 4 , Ag 2 Mo 2 O 7 , MoO 3 , CuO and NiMoO 4 provided low friction and anti-wear properties beyond 500 °C. Self-lubricating Ni 3 Al-Ag-Cu- h BN composites offer themselves as a potential candidate for high-temperature applications where the components work under relative sliding motion. [Display omitted] • Ni 3 Al-Ag/Cu- h BN/Ag-(Cu- h BN) composites were fabricated by hot press sintering. • The tribological performance from RT to 800 °C was investigated. • Ni 3 Al-Ag-(Cu- h BN) composite showed the lowest coefficient of friction and wear rate. • Ag and Cu- h BN provided low friction and anti-wear property from RT to 800 °C. [ABSTRACT FROM AUTHOR]

The comprehensive utilization of biomass resources is to be prepared into functional nanomaterial. This paper prepared carbon quantum dots (CQDs) using citric acid. A new CQDs/polyimide composites (CQDs/PI) coating fabricated on the surface of steel disks, has been widely used in bearing sleeve material or coatings in marine equipment parts and aerospace industries. Tribological properties of coating materials were investigated at different loads and reciprocating speeds of 10 mm/s under dry sliding conditions using a UMT-2 tribometer and compared to those of commercial MoS 2 particles in PI composites. Scanning electron microscopy accompanied by energy spectroscopy (SEM/EDS) and 3D laser scanning microscopy were used to characterize the morphologies of the abrasion marks to clarify the tribological mechanisms. Results show that different levels of CQDs can improve composite coatings' anti-wear and friction reduction properties. The tribological mechanisms of CQDs were attributed to the enhancement of the hardness of the polyimide coatings by the CQDs and the formation of a transfer film on the surface of the ceramic balls during the friction process reducing the direct contact at the friction sub-interface.1 • Polyimide coatings including CQDs and MoS 2 particles were successfully prepared. • CQDs and MoS 2 particles can improve the friction and wear properties of PI. • Tribological behavior of CQDs/PI is better than that of MoS 2 /PI. • Molybdenum disulfide can cause premature failure of composite materials. • Formation of transfer films resulted in the low friction coefficient and wear rate. [ABSTRACT FROM AUTHOR]

The third body formed in a contact between HS25 cobalt-based superalloy versus ceramic under fretting wear (small reciprocating displacements) was investigated. This tribomaterial, commonly called “glaze layer”, was created from nanosized, compacted and sintered wear debris and adheres on both rubbed parts. The glaze layer was investigated both from tribological and rheological points of view. In terms of tribology, the glaze layer was found stable above 450 °C, providing low friction and very low wear rate in the interface. To study the mechanical properties of the third body, micropillars have been FIB-machined within the glaze layer and compressed between 25 °C and 500 °C. Low temperature testing showed a brittle and hard behavior for the glaze layer which was confirmed by nanoindentation. By contrast, glaze layer at 500 °C evidenced a perfect ductile response with high strain rate during fretting and no brittleness. Made of 10–20 nm nanocrystals embedded in a ceramic-metallic amorphous matrix, the glaze layer is put forward in the brittle to ductile transition: the amorphous matrix may act as a metallic glass. This allows the correlation between glaze layer ability to plastically accommodate the strain without fracturing processes, thus without being damaged through debris generation. The ejected debris flow is stopped and the wear rate becomes negligible. [ABSTRACT FROM AUTHOR]

Leaded tin bronze alloys are widely used in heavy machinery bearings operating in boundary and mixed lubrication regions due to the excellent dry lubrication properties of lead. However, restrictions on the use of lead have created an increasing demand for lead-free or low-lead bearing materials. In the present study, suitability of a novel bismuth bronze bimetal material for possible substitution of leaded tin bronze was studied with a special thrust bearing test device, which simulates the contact conditions in the main thrust bearing of mineral crushers. The oil-lubricated test bearings have a flat-on-flat type contact with oil grooves and a constant eccentric motion against a case hardened steel counter plate under a periodically increased axial pressure. The test was continued until a sudden rise in friction, which indicates bearing failure and risk of an imminent seizure. The bismuth bronze showed a load capacity of the same level with the reference material, continuously cast CuSn10Pb10. Characterization by electron microscopy showed that the dry-lubricating bismuth precipitations had a fine grain size and an even distribution, which explains the good load carrying capacity. It was concluded that the bismuth bronze has potential for substituting the leaded tin bronzes in the studied operating conditions. [ABSTRACT FROM AUTHOR]

This paper is aimed to characterize the new NiCrAlMo-12.5 wt. % Ag- X wt. % CaF 2 /BaF 2 (X = 5 or 10) solid-lubricating composites, which have high strength and self-lubricity simultaneously from room temperature to 800 °C and thus would be a potential candidate for aerospace bearing applications. Additionally, clarification of the influence of fluoride content on the tribological responses of this material is another object. Sliding friction and wear performances of these composites as bearing materials are evaluated using a pin-on-rotating-flat disc configuration against an Inconel718 alloy pin. Also, the surface tribo-chemical reactions and mechanical properties of the composites are investigated and related to their tribological behaviors. Reducing the fluoride eutectic content leads to significant improvement on the wear resistance of the composites above 600 °C, which is strongly dependent on the surface tribo-chemical reactions and the interaction with the counterpart, not on the mechanical properties. The wear rate of the Inconel718 counterpart over the entire testing temperature range is as low as 10 -7 mm 3 /(Nm). [ABSTRACT FROM AUTHOR]

Poly aryl ether ketone (PAEK) is known as a high performance polymer superior to PEEK (poly ether ether ketone) in many aspects such as temperature stability, tribo-performance etc. Recently few efforts are put to enhance the tribo-potential of PAEK by developing composites containing short fibers of carbon and glass and most favored lubricants such as graphite, PTFE (poly tetrafluoroethylene), hexa boron nitride (hBN) etc. The wear and friction got significantly reduced to the extent that the composites showed very high potential as bearing materials for harsh operating conditions. Mica and boric acid (BA) are known as very good solid lubricants (SLs) with layer lattice structures. Potassium titanate (KT) powder also has been very popular ingredient of friction materials responsible for increasing wear resistance (W R ) and reducing friction fluctuations. It is, however, never explored in bearing materials for enhancing tribo-potential. Keeping this in view, a series of composites was prepared by injection molding with identical parent composition but differing in type of solid lubricants (hBN, KT, BA and Mica). The tribo-performance was evaluated in adhesive wear mode on tribometer under severe operating conditions (700–900 N, velocity 1.63–3.5 m/s). Composites exhibited very low specific wear rates (K 0 ) (2–8×10 –16 m 3 /Nm) and very low coefficient of friction (µ) (0.04–0.08), which decreased with load and velocity. KT proved to be the most performing filler, while Mica was least effective followed by BA. Worn surface analysis by SEM (Scanning electron microscopy) and EDAX (energy dispersive x-ray) helped to understand wear mechanisms. [ABSTRACT FROM AUTHOR]

Solid lubricants (SLs) are known to enhance the tribo-performance of oils significantly. Current work focusses on issue of exploration of combination of SLs in different amounts along with dispersant (Polyisobutylene succinimide −1 wt% const.) for possible synergism. Hexagonal boron nitride (hBN) and polytetrafluoroethylene (PTFE) are known as anti-friction, Anti-wear additives. PTFE is also known as an effective extreme-pressure (EP) additive. A series of oils based on Group III (Gr III) as base oil containing PIBSI (1%) along with increasing amount of hBN (0–4%) and simultaneous decrease in PTFE (4–0%) were prepared. The tribo-performance of oils was evaluated on 4 ball tester for EP (weld load) and anti-wear performance. Formulations were characterized for physical properties density, viscosity, flash point. Also the stability of oils was observed using Dynamic Light Scattering studies. Results revealed the inclusion of these two SLs showed better tribo-performance (43% in Weld Load) than with presence of PTFE in oil. The characterization of wear and a possible wear mechanism was given by observing the worn surface under a scanning electron microscope and energy dispersive X-ray analysis. [ABSTRACT FROM AUTHOR]

The production of self-lubricating composites containing second phase particles is one of the most promising choices for controlling friction and wear in energy efficient modern systems. To gain a better understanding of the wear behaviour of such materials, Fe-Si-C matrix composites containing solid lubricants added during the mixing step were studied in this work. The samples were produced using powder metallurgy route, with total contents of 5, 7.5 and 10% in terms of volume of hexagonal boron nitride (hBN) and graphite mixtures as the solid lubricants. The composite's tribological properties were evaluated under reciprocating sliding conditions and their mechanical properties were tested using tensile tests. Additionally, after interruptions at different stages of the reciprocating tests, the wear scars were characterised by Raman spectroscopy and scanning electronic microscopy, to evaluate the evolution of the wear with test time. Higher total solid lubricant contents greatly increased the scuffing resistance of the composites, but decreased the mechanical properties. Furthermore, increasing the hBN content reduced both properties. Among the composites studied, the samples containing 1% vol hBN and 9% vol graphite showed the best combination of mechanical and tribological properties. [ABSTRACT FROM AUTHOR]

Laser alloying with boron and solid lubricants (CaF 2 or BaF 2 ) was used in order to produce the self–lubricating layers on 100CrMnSi6–4 bearing steel. The surface of the base material was coated with a paste consisting of alloying material blended with a diluted polyvinyl alcohol solution. Then, this surface was re-melted by the laser beam using TRUMPF TLF 2600 Turbo CO 2 laser. The produced layer consisted of the two zones: the re–melted zone (MZ) and the heat affected zone (HAZ). The re–melted zone was composed of eutectic mixture (iron borides with martensite) and CaF 2 or BaF 2 particles, locating close to the surface. In the HAZ, the martensite, retained austenite, pearlite and sorbite were observed. The layers were free of cracks or gas pores. The microhardness of the re–melted zone was equal to about 800 HV and was lower than that-characteristic of the laser-alloyed layer only with boron. During the wear resistance test, the tribofilm was created on the worn surface. This tribofilm consisted of the calcium fluoride or barium fluoride, which was smeared on the surface. This way, the solid lubricant reduced the mass loss of mating parts and improved their tribological properties. [ABSTRACT FROM AUTHOR]

In order to meet the increasing demands of friction reduction and wear resistance in heavy-duty diesel engines, the refined design of surface topography of cylinder liner and piston ring (CLPR) has been a major concern. Based on electrolytic micro-texturing and electrochemical deposition methods, micro-dimples filled with solid lubricant have been separately applied to the CLPR surfaces. This double-sided textured design of CLPR exhibited significantly lower friction coefficient and wear depth than the non-dimpled CLPR when the lubricating oil containing zinc dialkyldithiophosphates (ZDDP) was used under different strengthening loads and temperatures. Solid lubricants were released from the double-sided textures, forming mutually transferable solid lubricant films on the sliding surface. Although various products containing molybdenum oxides, molybdenum sulfides, phosphate of different chain lengths, zinc compounds, iron sulfide, etc. Appeared in the tribochemical reactions of ZDDP and MoS 2 with increasing temperature and load, but the lubricating effect was still maintained. These results can provide some insight into the design of CLPR surfaces in order to achieve a high quality tribofilm under heavy load conditions. • Double-sided microtextures filled with solid lubricants are prepared. • Two channels of fluid and solid lubrication are provided at the tribopair interface. • Multiple protection against asperity contacts with double-sided textured surface. [ABSTRACT FROM AUTHOR]

Solid lubricants, like molybdenum disulfide (MoS 2) are often used in space applications, and subject to prolonged periods where components sit stationary before use (i.e., during storage, transport or in between duty cycles). When sliding is resumed after dormancy, the friction behavior of MoS 2 can vary due to a variety of factors such as accrued adsorbates and oxidation. This phenomenon, referred to as the dwell or stop-time effect, was first investigated over 50 years ago, and is characterized by an increase in the coefficient of friction and a prolonged run-in back to steady-state friction. After nearly five decades, the fundamental driving mechanism for the dwell effect is still not well understood. In this work, the dwell-effect for MoS 2 coatings is studied through intermittent sliding experiments with dwell times ranging from 30 s to 48 h (172,800 s) at pressures from 7 × 10−9 to 2 × 10−1 torr. Vacuum pressure was varied to investigate the role of surface contaminants (i.e., water). Results suggest that the change in the coefficient of friction is driven by interactions of water with the sliding interface. The role of microstructure on the dwell effect was investigated using nanocrystalline sputter-deposited and highly oriented spray-deposited MoS 2 coatings. Results show that the shear-modified surfaces of sputter-deposited coatings have a ∼2 × smaller dwell effect than spray-deposited surfaces due to the reduction of potential edge-sites that can interact with contaminants. Additionally, intermittent sliding experiments after vacuum annealing show that the contribution of latent water can be minimized by driving intercalated water from the coating. • The dwell-time effect was investigated for MoS2 coatings with different microstructures. • Frictional changes are dominated by bulk and surface contaminants. • Annealing minimizes the dwell-effect by removing bulk water effects. • Sliding-induced surface modifications impact frictional changes with time. [ABSTRACT FROM AUTHOR]

Improving the energy efficiency of mechanical systems and reducing the usage of lubricant oils and related additives are challenging tasks of the 21st century. Consequently, the application of tribopolymers increases due to their easy processing (injection moulding) and lower density (mass reduction). Among them are the poly(ether-ether-ketone) (PEEK) and the poly(aryl-ether-ketone) (PAEK) with outstanding mechanical properties, and poly(tetrafluorethylene) (PTFE), a solid lubricant. This work produced powder-like PAEK + PTFE blends (2%–30% PTFE) by low and high-energy grinding processes; then, the specimens were shaped by hot pressing. The morphology and composition of the post-grinding particles were analysed by white light interferometry (WLI), scanning electron microscopy (SEM) and X-ray dispersive energy spectroscopy (EDS). The fractography and thermal analysis of the specimens were performed via SEM-EDS and differential scanning calorimetry (DSC). For tribological evaluation, reciprocating (2Hz, 10 mm) cylinder-on-flat tests were conducted to determine the lubricious regime duration, the friction coefficient (COF) during the steady-state regime, and wear rates, where the worn volumes (specimen and counterbody) were measured by WLI. High-energy grinding leads to smaller and more plastically deformed particles. In addition, the incorporation of PTFE into PAEK is more homogeneous with a higher crystalline degree. The self-lubricating effect is manifested from 5% of PTFE, and the best results are reached for 15% of solid lubricant, with an average wear rate of 9.4 × 10−16 mᶟ/Nm and COF of 0.08, against 5.3 × 10−14 mᶟ/nm and 0.32 for pure PAEK, respectively. • The two mixing methods used to produce the blend (PAEK + PTFE) resulted in completely different blends structural morphology. • But high energy method induces the formation of polymers with higher crystallinity degrees. • Higher tribological performance was reached from 5% of PTFE concentration. [ABSTRACT FROM AUTHOR]

The processing of Polyamide-12 (PA12) by Laser Sintering is one of the most well-established Additive Manufacturing (AM) processes for producing functional components for end-use applications. However, its further adoption within industry remains hindered by an incomplete understanding of resultant part quality and the impact this has on component wear. The scope of this research was to investigate the dry sliding behaviour of Laser Sintered Polyamide-12, as well as evaluate whether the inclusion of solid lubricant fillers effect the friction and wear properties of parts produced. Polytetrafluoroethylene (PTFE), Graphite and Molybdenum disulphide (MoS 2) were added to Polyamide-12 powder in 1 : 100 mass ratios, respectively, to create three different polymeric composites. Mechanical blending and Laser Sintering then ensued; the latter was performed using identical processing parameters throughout. Tribological performance was evaluated by ball-on-flat uni-directional wear testing. Tensile testing was also carried out to help elucidate what wear mechanisms were active during sliding, as well as identify whether solid lubricant inclusion impacted mechanical performance. Results showed that in all instances solid lubricant inclusion significantly influenced the friction and wear properties of resultant composites, without compromising their mechanical performance when compared with neat-PA12. More specifically, it was demonstrated that the individual additions of PTFE and MoS 2 could reduce the coefficient of friction and specific wear rate of Laser Sintered PA12 components by as much as 50% and 78%, respectively. • Polymeric composite powders comprising of Polytetrafluoroethylene (PTFE), Graphite and Molybdenum disulphide (MoS 2) individually blended with Polyamide-12 (PA12) in mass ratios of 1 : 100 (solid lubricant : PA12) were processable by Laser Sintering. • In this study, the inclusion of solid lubricants within Polyamide-12 powder prior to Laser Sintering did not compromise the mechanical performance of the resultant composites. • The individual addition of PTFE and MoS 2 was demonstrated to reduce the coefficient of friction and specific wear rate of Laser Sintered PA12 components by as much as 50% and 78%, respectively. [ABSTRACT FROM AUTHOR]

High temperature solid lubricating materials show important demands in applications for moving machine assemblies under extreme operating conditions. In this study, the tribological behavior of a nickel-alloy based self-lubricating composite (82.5%nickel-alloy–12.5%Ag–5%BaF 2 /CaF 2 ) against Si 3 N 4 ball was evaluated under various temperatures and sliding speeds. Results showed that the friction coefficient and wear rate of the composite were greatly dependent on testing temperature and sliding speed. The composite exhibited favorable lubricity in the wide temperature range from 25 °C to 800 °C. The friction and wear mechanisms were explored by observing and analyzing the worn surfaces. [ABSTRACT FROM AUTHOR]

Poly-ether-ether-ketone (PEEK) is a high-performance, temperature-resistance polymer that is finding an increasing range of applications. In order to even enhance PEEK׳s mechanical and tribological properties particles of different compositions, shapes and sizes are added into its matrix. PEEK has already been combined with many different particles; however, very rarely MoS 2 and WS 2 – two state-of-the-art solid-lubricants – were added into the PEEK matrix. Furthermore, a comprehensive tribological study combining MoS 2 and WS 2 particles of different sizes and concentrations has not yet been reported. In this investigation we looked at the effect of micro- and nanosized MoS 2 and WS 2 particles in PEEK on the dry-sliding tribological behaviour against stainless steel (100Cr6). A non-conventional technique, i.e., the sintering of dry-pressed compacts, was used to prepare the PEEK composites. The results show that all the particles, irrespective of their composition and size, reduce the friction (up to 30%); however, the nanoscale particles require a higher concentration to form an effective low-friction tribofilm. The formation of a tribofilm is necessary to reduce the wear of all the composites (up to 51%); this is strongly promoted by the addition of nano- or microparticles of both the MoS 2 and WS 2 materials. In addition, the hardness, which is greatly increased by the addition of all the particles, significantly improves the wear behaviour. The results of XPS analyses showed that the oxidation occurs during tribological sliding, which reduces particles beneficial wear behaviour effects. [ABSTRACT FROM AUTHOR]

Fluoropolymers have unique mechanical, chemical, and tribological properties (low friction coefficients) but their use as solid lubricants is inhibited by high wear rates (1–5×10 − 4 mm 3 /Nm). The addition of certain types of α-alumina has been shown to reduce the wear rate of PTFE by over three orders of magnitude, but due to its extremely high molecular weight PTFE cannot be screw injection molded. However, PFA, a perfluorinated copolymer of tetrafluoroethylene (TFE) and a perfluorinated alkylvinyl ether (PAVE), can be. Teflon® PFA 340 samples with various weight fractions of α-alumina (0%, 5%, 7.5%, 10%) were injection molded, and samples from each mold were wear tested against stainless steel ( P =6.3 MPa, v =50.8 mm/s). Experiments showed that the friction behavior of the PFA 340-α alumina composite was very close to that of both unfilled PFA 340 and PTFE-α alumina composites. The wear rate of unfilled PFA 340 was 1.4×10 −4 mm 3 /Nm, and dropped to 4.0×10 − 8 mm 3 /Nm for the PFA-α alumina composites. Just as in the case of PTFE-α alumina composites, these PFA composites generated brown-colored tribofilms on both the polymer and metal surfaces, which were indicative of tribochemical changes. ATR-IR and FTIR spectra of each surface showed evidence for the generation of perfluorinated carboxylate salts and waters of hydration. This spectral similarity between PTFE and PFA 340 samples shows that the same tribological mechanism found in PTFE-α alumina composites is responsible for ultralow wear in PFA-α alumina composites as well. [ABSTRACT FROM AUTHOR]

Al 2 O 3 /Mo self-lubricating structural ceramics with laminated-structure are potential candidates for high-temperature applications because of their excellent self-lubricating and mechanical performances. This study aims at revealing the mechanisms of how a three-dimensional lubricating layer affects the tribological properties of Al 2 O 3 /Mo laminated composites at temperatures from 25 to 800 °C. A three-dimensional lubricating layer was formed by considering texture pattern as storage dimples and coating solid lubricants (SLs) on the textured surface of Al 2 O 3 /Mo laminated composite. The friction properties and wear mechanisms at temperature from 25 to 800 °C in a continuous heating process were studied. It is found that the synergy effect of micro-textures and SLs influence the tribological properties of material. Moreover, the tribochemical reaction of SLs stored in the micro-dimples at high temperature improves the lubricating ability of materials, thus realizing continuous lubrication within a wide temperature range. We demonstrated that the friction coefficient of textured surface that coated with MoS 2 /CaF 2 –BaF 2 SLs was kept lower than 0.50 when subjected to dry sliding wear against Al 2 O 3 ceramic pin at 25–800 °C, and it was even lower than 0.15 and 0.35 at 25–200 °C and 800 °C, respectively. This work is an extension of studies that were previously published in Wear journal. [ABSTRACT FROM AUTHOR]

Graphite, a solid lubricant having good thermal conductivity (TC) and lubricity is used as a friction modifier in friction material (FM) formulations. Since graphite can be from different origins (synthetic, natural, treated etc.) and particles can be of various sizes, shapes and varying in crystallinity and purity, a careful selection is required for achieving desired performance properties of the FMs. Thermo-graphite (TG), as claimed to be more thermally conducting along with number of beneficial properties by the manufacturer was used to develop multi-ingredient FM. Two FMs differing in only type of graphite (TG and natural graphite-NG) were developed and evaluated for various performance properties. Tribo-performance evaluation of both the FMs on inertia dynamometer (schedule JASO C406) led to the conclusion that TG based FM proved superior to NG based one in almost all the properties barring recovery performance. [ABSTRACT FROM AUTHOR]

This study explores the feasibility of employing MAX phase composite as impregnated solid lubricants and/or as bulk material for advanced high temperature tribological applications. The development of microstructure of Ti-based MAX phase composites Ti 3 SiC 2 –TiC and Ti 3 AlC 2 –Ti 2 AlC fabricated by spark plasma sintering and their dry sliding tribological properties against bearing steel, Si 3 N 4 , and Al 2 O 3 counterfaces were investigated using a ball on disc tribometer at 400 °C. An orientation relationship between the Ti 3 SiC 2 matrix and TiC particle was established wherein Ti 3 SiC 2 /TiC interphase is a coherent boundary from which Ti 3 SiC 2 grows epitaxially from TiC. Against steel, the MAX phase composites – in stark contrast to Si 3 N 4 and Al 2 O 3 – exhibited a negative wear behaviour due to the excessive wear and subsequent sintering of the transferred Fe particles at the sliding interface. Intrinsic lubrication mechanisms involving tribo-oxidation, tribochemical reaction, and mechanochemical reaction played a vital role in the friction and wear properties. The friction and wear properties of the Ti 3 AlC 2 –Ti 2 AlC MAX phase composite was superior to the Ti 3 SiC 2 –TiC composite owing to better oxidation kinetics governing the oxide growth and their retention. Evidently, the wear of the MAX phase is expected to proceed upon extensive deformation incorporating a range of microscale micromechanisms. • Ti 3 SiC 2 /TiC interface is a coherent interface with low interfacial energy. • Tribo-oxidation enhances tribosintering • Oxidation kinetics plays a role in tribofilm spallation. • The role of counterface material was studied. • Wear of MAX phase proceeds by deformation. [ABSTRACT FROM AUTHOR]

During braking, high temperatures can be generated on the contact surfaces and inside the brake pads. Solid lubricants, i.e., metal sulphides, play an important role in brake performance, but they can react, oxidise or decompose with exposure to temperature, modifying their tribological properties. This paper investigates the chemical reactions between different metal powders (Cu, Fe, Sn and Zn) and Sb 2 S 3 that take place in a low-metallic brake pad during braking in conditions were high temperature is generated. The reaction products were characterised by SEM–EDS and the glancing XRD technique. Moreover, thermal treatments (in air and argon) of binary mixtures, containing the same weight ratio of these metal powders and Sb 2 S 3 contained in the brake pad, were carried out and analysed via DSC–TGA and XRD techniques. The paper finds that the reaction features and reaction products depend on the temperature generated during braking, the specific combination of metal/lubricant and the atmosphere conditions (oxidant or inert). In general, the reaction products are a combination of an intermetallic (metallic antimonide) and a metal sulphide (from the metal powder), which can be found both on the surface and inside the brake pad. [ABSTRACT FROM AUTHOR]

To improve the friction and wear properties of Al 2 O 3 /TiC ceramic tool material, laser surface textures combined with burnished MoS 2 solid lubricants, burnished WS 2 solid lubricants and magnetron sputtered WS 2 /Zr coatings by physical vapor deposition (PVD) methods were fabricated on its surface. The friction and wear tests were performed by dry sliding friction tests against AISI 1045 hardened steel balls using a ball-on-disk tribometer. The effect of the surface textures combined with muti-solid lubricant coatings on the tribological behavior of Al 2 O 3 /TiC ceramic was investigated. Results show that a combination of laser surface textures and solid lubricants can effectively improve the tribological properties of Al 2 O 3 /TiC ceramic surface and protect the counter-face of the ball. The burnished WS 2 solid lubricants combined with surface textures are more effective in reducing the friction and wear compared with burnished MoS 2 solid lubricants. The WS 2 /Zr coatings deposited on the textured sample show the best efficiency in improving the tribological properties compared with the textured samples with burnished MoS 2 and WS 2 solid lubricants. However, the ball sliding against the textured sample with burnished WS 2 lubricants exhibits the smallest wear rate. In addition, the possible mechanisms of laser surface textures combined with different solid lubricant types were discussed. [ABSTRACT FROM AUTHOR]

Polybenzimidazole (PBI) is a specialty polymer with highest thermal stability. However, its potential to develop composites has not yet been explored because of problems associated with processing. To overcome this problem, blend of Polyetherketone (PEK) and PBI was developed for the first time, and its mechanical and tribological properties were investigated. Further, a systematic series of composites containing solid lubricants (SLs), short fibers of glass and carbon was developed and its physical, mechanical, thermal and tribological properties were investigated. The composites with either SLs or fibers did not show good tribo-potential. However, when these were used together, performance was significantly better. When compared to the commercial composite based on PBI–PEEK blend and with similar composition, the new composites demonstrated excellent tribo-potential with very low friction coefficient ( µ )≈0.05 and specific wear rate ( K 0 )≈1×10 −16 m 3 /N m along with very high PV safe-limit value≈63 MPa m/s. [ABSTRACT FROM AUTHOR]

Tribological and mechanical properties of α-olefin-copolymer, impact-modified Polyamide 66, containing pristine and plasma treated Polytetrafluoroethylene (PTFE) micro-powders as solid-lubricants have been investigated. The PTFE powders were subjected to low pressure 2.45 GHz microwave plasma treatment with H 2 and NH 3 as the process gases to aid their dispersion in PA66. Formation of polar surface groups in conjunction with significant defluorination was observed for both H 2 (F/C atomic ratio 1.30) and NH 3 (F/C atomic ratio 1.13) plasma treated powders. The H 2 PTFE-impact modified PA66 composites exhibited a 25% increase in their impact energy absorption capabilities (25.0 J) than their pristine counterparts (20.0 J), along with a significant reduction of the specific wear rate at higher pv -values. At the pv -value of 6 MPa m/s, the specific wear rates for composites containing hydrogen plasma treated PTFE and nitrogen plasma treated PTFE were reduced by 33% (0.7×10 −6 mm 3 /N m) and 50% (0.6×10 −6 mm 3 /N m), respectively, as compared to pristine PTFE–α-olefin PA66 composites (1.1×10 −6 mm 3 /N m); while showing similar coefficient of friction values. The use of plasma functionalized PTFE powders thus provides a facile route for the production of impact modified PA66 compounds with significantly lower coefficient of friction and higher wear resistance for applications like bearings and sliding elements where impact strength as well as good tribological properties are required. [ABSTRACT FROM AUTHOR]

The tribological performance of metal phosphate composite coatings for sliding parts at 400 °C was experimentally evaluated. In this work, an aluminum–magnesium–chromium phosphate composite binder was fabricated and the binder was mixed with WS 2 , MoS 2 , graphite, and Sb 2 O 3 as lubricating fillers. The friction coefficients and wear lives of the coated specimens of various formulations in sliding contact against S45C disc at 400 °C in air were tested and compared using a reciprocating friction test rig. Results showed that the average friction coefficients of composite coatings were improved by about 40% when Sb 2 O 3 is replaced by Sn in the same amount of material formulation. It was observed that the addition of Sn resulted in a lower friction coefficient presumably by the melt lubrication effect. [ABSTRACT FROM AUTHOR]

This paper proposes a method to improve the tribological properties of Al 2 O 3 /Mo laminated composites. Eight types of micro-textures with different patterns and area densities were produced on the surfaces of Al 2 O 3 /Mo laminated composites. The friction properties and wear mechanisms at room temperature and 800 °C were studied. Results show that the method is effective in achieving composition-lubrication on the surface of laminated ceramics by laser texturing. We demonstrated that the friction coefficient can be reduced to 0.5 and 0.22 at room temperature and 800 °C, which were 0.39 times and 0.55 times lower than those of untextured Al 2 O 3 /Mo laminated composites, respectively. The contribution of surface texturing to the tribological properties of the laminated composites was mainly related to the increase of the frictional contact pressure, trapping wear debris in the patterns and promoting the formation of lubricating film at 800 °C. [ABSTRACT FROM AUTHOR]

The process of machining of hard materials such as titanium and its alloys, has an important role in the development of new machining techniques, such as cutting tools and coatings, with the aim to increase the machinability of these materials. Due to the poor thermal properties of the titanium, the cutting tool suffers accelerated wear which affects the tool life. This paper describes a study of the use of solid lubricant during the machining of Ti-6Al-4V in a turning operation. TiAlN (PVD) coated cemented carbide tool (class S15) was used. Dry tests, tests with lubricant and tests with lubricant with 20% by weight of solid lubricants: graphite mesh 325 (average particle size of 40 μm), graphite mesh 625 (20 μm) and molybdenum disulphide (MoS 2 , 6 μm) were made. The output parameters are: tool life, surface roughness, cutting force and interfacial temperature (tool-work thermocouple). Tool flank wear, a major problem in machining of titanium alloys, was significantly reduced due to the action of the solid lubricants. Experimental results showed the superiority of molybdenum disulphide in relation to graphite in the turning process: reducing the tool wear, prolonging the tool life and improving the surface roughness. The solid lubricant may be a viable alternative to dry and wet turning. Thus, it may be inferred that proper application of solid lubricants in machining of Ti-6Al-4V alloy results in reduction in tool wear, which can lead to enhancement of productivity. [ABSTRACT FROM AUTHOR]

A new processing route to promote the in situ generation of solid lubricant particles has been presented recently. This route takes an advantage of a powder injection moulding process combined with a plasma-assisted debinding and sintering process (PADS). Graphite nodules were generated in situ in the volume of sintered steels during sintering simply by the dissociation of silicon carbide (SiC) particles, which were mixed into the metallic matrix powders before injection. In this work, we present and discuss the effect of the sintering temperature (1100, 1150 and 1200 °C) and SiC content (0–5 wt%) on the tribological behaviour (friction coefficients and wear rates of specimens and counter bodies) of these new materials. The tribological behaviour was analysed using linear reciprocating sliding tests (constant load of 7 N, 60 min duration). It was shown that the presence of graphite nodules significantly reduced the friction coefficients and wear rates, whereas the sintering temperature hardly affected these parameters. The results were compared with those caused by other forms of graphite (nodules in nodular cast iron and powder graphite) and were discussed in terms of the crystalline structure of the analysed graphite using micro-Raman spectroscopy. Chemical analyses of the wear scars using scanning electron microscopy (SEM–EDX) and Auger electron spectroscopy (AES) showed a tribolayer that was composed predominantly of carbon and oxygen. This tribolayer is removed and restored during sliding and is continuously replenished with graphite. Analyses of the wear scars showed traces of plastic deformation on both samples and counter-bodies and the predominance of abrasion as the main wear mechanism. [ABSTRACT FROM AUTHOR]

High frequency reciprocating sliding wear behavior and mechanisms were investigated at 25, 150 and 400 °C for solid lubricant oxide coated carbon–carbon composites (CCC). Atomic layer deposition (ALD) was employed to infiltrate a trilayer nanolaminate coating of ZnO/Al 2 O 3 /ZrO 2 into the porous CCC. The ALD coatings exhibited excellent conformality and pore-filling down to ~55 μm into the CCC, which cannot be achieved by other coating techniques. Compared to uncoated CCC, ALD infiltration resulted in a 64% improvement in the wear rate to 1.5×10 −6 mm 3 /N·m at 25 °C, while a 31% improvement was determined during 150 °C sliding. Conversely, higher temperature sliding at 400 °C resulted in increasing wear rates by an order of magnitude due to combined oxidation of the steel counterface and exposed CCC. Cross-sectional focused ion beam-transmission electron microscopy (FIB-TEM) studies of the worn surfaces were performed to elucidate the solid lubrication mechanisms. It was determined that a highly deformed composite surface tribolayer of nanolaminate coating and amorphous C was responsible for improved sliding wear compared to the uncoated CCC that just exhibited an amorphous C tribolayer. Inside the deformed composite tribolayer, a high density of near surface ZnO (0002) basal plane sliding/shear-induced stacking faults was observed in comparison to unworn ALD nanolaminates. Activating subsurface basal stacking faults in ALD ZnO promotes intrafilm shear/slip and hence improves wear resistance. The slip of partial dislocations results from a dislocation glide process along the ZnO (0002) basal planes. Thus, ALD ZnO/Al 2 O 3 /ZrO 2 nanolaminate coatings are potential candidates for providing wear reduction from low to intermediate temperatures in moving mechanical assembles, such as CCC bushings that experience fretting-type wear. [ABSTRACT FROM AUTHOR]

The wear and friction behavior of materials depends on the structure of the surface layers that can develop during sliding contact. The evolving dynamic recrystallization process of the friction layer of Ni 3 Al matrix composites (NMCs) at different contact loads was characterized. A pin-on-disk contact geometry was used. Silicon nitride ceramic was chosen as the pin material because of its inertness. A wear-induced tribo-layer composed of a grain growth layer and a nanocrystalline (NC) structured layer was formed. The soft wear-induced (0.6 GPa) layer, supported by the harder NC structure layer (7.4 GPa), is proposed to be beneficial both to achieving a low friction coefficient and a high wear resistance for the NMCs. [ABSTRACT FROM AUTHOR]

Tribological and mechanical properties of Polyethersulfone (PESU) composites, containing pristine and plasma modified polytetrafluoroethylene (PTFE) micro-powders as solid-lubricants have been investigated. Low pressure 2.45 GHz microwave plasma functionalisation of PTFE powders was carried out using H 2 and NH 3 as process gases to introduce functional polar groups on the PTFE surfaces to enhance their wettability and mixing with PESU. As compared to pristine PTFE (F/C atomic ratio 1.86), X-ray photoelectron spectroscopy analysis confirmed the significant deflourination for both the NH 3 (F/C atomic ratio 1.13) and H 2 (F/C atomic ratio 1.30) plasma treated samples along with the attachment of polar surface moieties. An increase in the interaction between the plasma functionalised PTFE powders with PESU matrix was confirmed based on an increase in the glass transition temperature of the PESU–PTFE composites. The plasma treated PTFE–PESU composites exhibited nearly 75% higher force absorption capabilities (3.3 kN) than their pristine PTFE–PESU counterparts (0.96 kN). Moreover, the plasma treated PTFE–PESU composites exhibited a wear rate (3.42±0.51×10 −06 mm 3 /Nm) which was nearly half of that of pristine PTFE–PESU composites (5.75±0.80×10 −06 mm 3 /Nm). Thus, low-pressure microwave plasma modification offers an efficient route for surface functionalisation of solid lubricants, like PTFE, for enhanced dispersion in high-performance polymers. [ABSTRACT FROM AUTHOR]

A fully instrumented ring-on-ring galling test has been developed based on the ASTM G196 test principle. Torque records allow computation of the coefficient of friction and other tribological parameters to capture wear behavior of the interface. Contact electrical resistance is also recorded helping to detect galling onset in the contact. A solid lubricant anti-galling solution in OCTG connections involving a coated versus steel interface has been studied. Tests have been stopped at different sliding distances to perform surface analysis and characterize wear mode and severity. Wear scars have been expertized using optical, SEM, EDS, surface and μ-hardness profile analysis such that the degradation scenario of the coating and the steel surface can be drawn up. Moreover, an analysis in terms of friction coefficient can be developed describing the evolution from pure lubricated interface to fully galled interface. • Development of an ASTM G196 like macro ring-on-ring test system. • Investigation of the onset galling phenomena of a solid lubricant interface. • Couple analysis of the coefficient of friction, acoustic emission and electrical contact resistance. • The sigmoid evolution of COF is monitored by the galling area extension. • The incipient galling process is related to very low 0.12 friction coefficient. [ABSTRACT FROM AUTHOR]

In the present work, the friction and galling-prevention properties of a lubricating grease with the addition of Bi 2 S 3 particles have been investigated. To achieve this, commercial lithium greases containing Bi 2 S 3 , hexagonal boron nitride, and graphite were employed to lubricate steel–aluminum contacts under different tribological configurations, namely reciprocating sliding tests with varying normal loads and sliding speeds and load scanner tests at 25, 200 and 300 °C. The addition of Bi 2 S 3 enhanced the tribological response of the grease in reciprocating tests and provided protection against galling up to temperatures of 200 °C. However, its performance severely deteriorated at 300 °C. Thus, Bi 2 S 3 showed promising properties as an additive for lubricating greases operating under a wide range of normal loads, sliding speeds, and moderate temperatures. [Display omitted] • Bi 2 S 3 provides a stable COF in reciprocating tests with different loads and speeds. • The wear rate of the Bi 2 S 3 grease-lubricated pairs is lower than graphite and hBN. • Bi 2 S 3 provides excellent galling protection up to temperatures of 200 ° C. • Bi 2 S 3 shows promising properties for being used in Al forming operations. [ABSTRACT FROM AUTHOR]

The use of Inorganic Fullerene-like (IF) nanoparticles in lubricants has proved extremely effective to reduce friction and wear under severe boundary lubrication conditions. It has furthermore been suggested that the synthesis of smaller nanoparticles containing many structural defects would benefit friction and wear reduction, as they would penetrate and exfoliate more easily in the contact, leading to the quick formation of homogeneous tribofilms. In this study, four different types of IF-MoS 2 were synthesized so as to be able to differentiate the influence of both the size and the morphology of the nanoparticles on their tribological behavior. Pure-sliding, reciprocating tribological testing of these four types of nanoparticles revealed their excellent friction-reducing properties in severe boundary lubrication, with splash lubrication taking place for a high number of cycles. High wear reduction was also obtained and confirmed using optical profilometry. Although the nanoparticle structure was found to have an influence on their effectiveness in time, all the nanoparticles tested – regardless of size or crystallinity – were found to achieve the same performances as long as proper oil recirculation took place, ensuring a continuous feeding of the contact in nanoparticles. The formation of MoS 2 tribofilms on the wear surfaces was confirmed using XPS analyses and observed on FIB cross sections, and their nature was discussed in the light of the associated tribological results. As the size and morphology of the IF-MoS 2 did not affect their performance in the range studied, their friction reducing properties were compared to those of bulk h-MoS 2 tested in the same conditions. The benefits of using spherical nanoparticles such as IF-MoS 2 was clearly shown. [ABSTRACT FROM AUTHOR]

Bonded MoS 2 solid lubricant coatings are extensively used in tribology to reduce friction coefficient and wear rate. This coating strategy is particularly appreciated in aeronautical applications to limit fretting wear damage. A major question, however, concerns prediction of endurance for such palliatives ( Nc : µ > µ c ). Focusing on a MoS 2 -bonded solid lubricant coating fretted against a Ti–6Al–4V counter-body, an extensive fretting wear analysis coupling a large spectrum of contact pressures, sliding amplitudes, contact sizes and contact geometries was undertaken. The study showed that different friction responses could be activated depending on contact pressure and sliding amplitude conditions. Low pressures and small sliding amplitudes induced a lubrication plateau friction response (I) whereas high pressures and large sliding amplitudes induced continuously rising friction combined with titanium transfer (II). The transition from friction response (I) to friction response (II) could be formalised using an “effective” pv eff factor. Moreover, bonded-MoS 2 coating endurance could be predicted by an “effective friction” energy density parameter expressed as the difference between the nominal friction energy density imputed in the interface and an energy contribution related to the titanium transfer activation. Using this φ – N chart representation, all the experimental friction endurance values followed a single endurance master curve, formalised as an inverse function of the “effective” friction energy density parameter. An equivalent “effective” Archard work density approach is also introduced. Based on a pressure work density parameter, this formulation is easier to apply but displays a wider scatter because friction fluctuations are not taken into consideration. [ABSTRACT FROM AUTHOR]