Tribological Behavior of Ni-based Self-lubricating Composites with the Addition of Ti3SiC2 and Ag2W2O7

The tribological properties of Nickel-based composites containing Ti3SiC2 and Ag2W2O7 fabricated by spark plasma sintering against Si3N4 balls were investigated using a ball-on-disk tribometer from room temperature to 600℃. The tribolayers formed on the friction surface and their effects on the tribological properties of composites at different temperatures were discussed based on the worn surface characterization. The results show that Ag2W2O7 is decomposed into metallic silver and CrWO4 during the high-temperature fabrication process. The composite with the addition of 20 wt% Ti3SiC2 and 5 wt% Ag2W2O7 exhibits a friction coefficient of 0.33-0.49 and a wear rate of 7.07x 10~5-9.89x 10~5 mm3/(Nm) over a wide temperature range from room temperature to 600℃. The excellent tribological properties at a wide temperature range are attributed to the formation of a glaze layer at low temperature and a tribooxide layer at high temperature, which can provide a low shearing strength for the synergistic effects of Ag and tribooxides.

Effects of Friction Heat on the Tribological Properties of the Woven Self-lubricating Liner

In the dry-sliding process of the woven self-lubricating liner which is used in the self-lubricating spherical plain bearing, the friction heat plays an important role in the tribological performances of the liner. It has important value to study on the relationship between tribological performances of the liner and the friction heat. Unforttmately, up to now, published work on this relationship is quite scarce. Therefore, the effect of friction heat on the tribological performances of the liner was investigated in the present work. The tribological behaviors of the liner were evaluated by using the high temperature end surface wear tester. Scanning electron microscopy （SEM） was utilized to examine the morphologies of worn surfaces of the liner and study the failure modes. Differential scanning calorimetry （DSC） measurement and X-ray diffraction （XRD） analysis were performed to study the behaviors of the wear debris. The temperature rise on the worn surface was calculated according to classical models. SEM observation shows that the dominating wear mechanism for the liner is mainly affected by friction shear force, contact pressure and friction heat. Higher fusion heat for the wear debris than that for the pure polytetrafluroethylene （PTFE） indicates that the PTFE is the main portion of the wear debris, and, the PTFE in the wear debris shows a higher crystallisation degree owing to the effects of friction shear force and the friction heat. Combining the calculated temperature rise results with the wear rate of the liner, it can be concluded that the effects of temperature rise o n the tribological performances of the liner become more obvious when the temperature rise exceeds the glass transition temperature （Tg） of the PTFE. The wear resistance of the liner deteriorates dramatically when the temperature rise approaches to the melting point （Ton） of the PTFE. The tribological performances of the liner can be improved when the temperature rise exceeds Tg but is far lower than Ton- The present study on the relationship between the temperature rise and the tribological performances of the liner may provide the basis for further understanding of the wear mechanisms of the liner as well as the relationship between the formation of the PTFE transfer film and the friction heat during the dry-sliding of the Finer.

The Friction and Wear Properties of the Spherical Plain Bearings with Self-lubricating Composite Liner in Oscillatory Movement

A test method based on the condition simulation and a friction and wear test machine featuring in oscillatory movement were set up for self-lubricating spherical plain bearings (SPB). In the machine the condition parameters such as load, angle and frequency of oscillation and number of test cycles can be properly controlled. The data relating to the tribological properties of the bearing, in terms of friction coefficient, linear wear amount, temperature near friction surface and applied load, can be monitored and recorded simultaneously during test process by a computerized measuring system of the machine. Efforts were made to improve the measurement technology of the friction coefficient in oscillating motion. In result, a well-designed bearing torque mechanism was developed, which could reveal the relation between the friction coefficient and the displacement of oscillating angle in any defined cycle while the curve of friction coefficient vs number of testing cycles was continuously plotted. The tribological properties and service life of four kinds of the bearings, i.e, the sampleⅠ-Ⅳ with different self-lubricating composite liners, including three kinds of polytetrafluoroethylene (PTFE) fiber weave/epoxy resin composite liners and a PTFE plastic/copper grid composite liner, were evaluated by testing, and the wear mechanisms of the liner materials were analyzed.

Effect of surface roughness on the surface lubrication performance of galvanized steel sheets with a self-lubricated coating

Four kinds of galvanized steel sheets having different surface roughness values were used to prepare the steel sheets with a self-lubricated coating. The effects of surface roughness on the surface lubrication performance of the steel sheets were examined using a friction coefficient tester. Results revealed large dynamic friction coefficients for the galvanized steel sheets, which increased remarkably with surface roughness. Once the self-lubricated coating was applied, significant drops in the dynamic friction coefficients were measured. After the first stage of the friction test,the coefficients were almost unchanged, which reflected a weak dependence on the surface roughness of the self-lubricated steel sheets. However, the dynamic friction coefficients gradually increased as the test progressed, where these increase clearly correlated with the surface roughness of the self-lubricated steel sheets.

Tribological Properties of Self-lubricating Laminated Ceramic Materials

In order to improve the tribological properties of ceramic composites, Al2O3/TiC-Al2O3/ TiC/CaF2 self-lubricating laminated ceramic composites were prepared by vacuum hot pressing sintering. Experiments were conducted to get mechanical properties and the friction and wear properties were also measured with friction and wear tester. The worn surfaces were observed by scanning electron microscope （SEM） and energy dispersion spectrum （EDS）. The wear resistance properties and the self-lubricating effect of ceramic composites were analyzed. Results show that the Al2O3/TiC-Al2O3/TiC/CaF2 self-lubricating laminated ceramic composites layers are well-defined with a higher bonding strength and the mechanical performances are uniform enough to overcome the anisotropy of weak laminated ceramic composites. In addition, the fracture toughness of Al2O3/TiC layers is also improved. Its friction coefficient and wear rates decrease with the increase of rotation speed and load. Al2O3/TiC-Al2O3/TiC/CaF2 self-lubricating laminated ceramic composites have good wear resistance because of the tribofilm formed by the CaF2 solid lubricants. The wear mechanisms of Al2O3/TiC/ CaF2 layers are abrasive wear and Al2O3/TiC layers are adhesive wear.

Study on Solid Self-Lubricating Material for Plasma Facing Components in EAST

In this study, the friction performance of self-lubricating material with the counterpart steel ball-plate rubbing was investigated in vacuum conditions and the thermal distortion of the heat sink sample was tested. The analysis and test results show that the self-lubricating ma- terial has excellent anti-friction properties in high vacuum condition and can decrease the thermal stress and avoid damage to the PFCs during physical experiments.

Progresses on cryo-tribology:lubrication mechanisms,detection methods and applications

Tribology at cryogenic temperatures has attracted much attention since the 1950s with the acceleration of its applications in high-tech equipment such as cryogenic wind tunnels,liquid fuel rockets,space infrared telescopes,superconducting devices,and planetary exploration,which require solid lubrication for moving parts at low temperatures down to 4 K in cryogenic liquid,gaseous,or vacuum environments.Herein,the research progress regarding cryo-tribology is reviewed.The tribological properties and mechanisms of solid lubricants listed as carbon materials,molybdenum disulfide,polymers,and polymer-based composites with decreasing temperature are summarized.The friction coefficient increases with decreasing temperature induced by thermally activated processes.The mechanism of transfer film formation should be considered as a significant way to enhance the tribological properties of solid lubricants.In addition,applications of solid lubrication on moving parts under cryogenic conditions,such as spherical plain bearings and roller bearings,are introduced.The technology for tribological testing of materials and bearings at cryogenic temperatures is summarized,where the environmental control,motion and loading realization,as well as friction and wear measurement together in a low-temperature environment,result in the difficulties and challenges of the low-temperature tribotester.In particular,novel technologies and tribotesters have been developed for tribotests and tribological studies of solid lubricants,spherical plain bearings,and roller bearings,overcoming limitations regarding cooling in vacuum and resolution of friction measurement,among others,and concentrating on in-situ observation of friction interface.These not only promote a deep understanding of friction and wear mechanism at low temperatures,but also provide insights into the performance of moving parts or components in cryogenic applications.

Modeling and prediction of tribological properties of copper/aluminum-graphite self-lubricating composites using machine learning algorithms

The tribological properties of self-lubricating composites are influenced by many variables and complex mechanisms.Data-driven methods,including machine learning(ML)algorithms,can yield a better comprehensive understanding of complex problems under the influence of multiple parameters,typically for how tribological performances and material properties correlate.Correlation of friction coefficients and wear rates of copper/aluminum-graphite(Cu/Al-graphite)self-lubricating composites with their inherent material properties(composition,lubricant content,particle size,processing process,and interfacial bonding strength)and the variables related to the testing method(normal load,sliding speed,and sliding distance)were analyzed using traditional approaches,followed by modeling and prediction of tribological properties through five different ML algorithms,namely support vector machine(SVM),K-Nearest neighbor(KNN),random forest(RF),eXtreme gradient boosting(XGBoost),and least-squares boosting(LSBoost),based on the tribology experimental data.Results demonstrated that ML models could satisfactorily predict friction coefficient and wear rate from the material properties and testing method variables data.Herein,the LSBoost model based on the integrated learning algorithm presented the best prediction performance for friction coefficients and wear rates,with R^(2) of 0.9219 and 0.9243,respectively.Feature importance analysis also revealed that the content of graphite and the hardness of the matrix have the greatest influence on the friction coefficients,and the normal load,the content of graphite,and the hardness of the matrix influence the wear rates the most.

Fabrication of in-situ synthesized ceramic reinforced Ni-based alloy composite coatings by reactive braze coating processing

In-situ synthesized ceramic such as TiC,Cr7C3 and Cr5B3 reinforced Ni-based alloy composite coating was fabricated on the surface of mild steel substrate by reactive braze coating processing with colloidal graphite,Cr,Ni,ferro-boron,Si and titanium powders as the raw materials at low temperature of 1000℃,and a new kind of coating materials was developed.By means of SEM,EDS,XRD and surface hardness tester,the microstructures,phases,hardness and wear-resistance of the coating were analyzed,respectively.The results revealed that the coating was mainly composed of the ceramic in-situ synthesized reinforcement phases of TiC,Cr7C3 and Cr5B3 and the binder phases in-situ synthesized of Ni31Si12 and(Ni,Fe)solid solution;The ceramic reinforcement phases of TiC,Cr7C3 and Cr5B3 were randomly distributed in the binder phases of Ni31Si12 and(Ni,Fe)solid solution;The coating had about 15vol%pores and can possibly be applied as a self-lubrication coating;The coating and the substrate were integrated together by metallurgical bonding;The coating had a hardness up to 91-94HR15N.

Effect of graphite contents on friction and wear properties of Ni-Cr-W composites

Ni-based alloy/W/graphite self-lubricating composites with different graphite addition amounts were prepared using PM （powder metallurgy） method. It is found that the addtion of graphite can increase the tensile strength and hardness, while the bending strength decreases. The tribological properties of the composites when rubbing with Al2O3 ceramic disc were investigated in the temperature range of 20600℃. The results show that the tribological properties of the alloy are improved by adding graphite. When the graphite content is up to 9%, the friction coefficient at 600℃ is about 0.2, which is one third of that with no graphite. At elevated temperature, the friction coefficients change a little with velocity and load. At room temperature, graphite extruded from the composite plays the the role of lubrication. The synergistic effect of oxide films and graphite are responsible for the reduction of friction coefficient at high temperature.

Sliding Wear of the Hybrid Kevlar/PTFE Fabric Reinforced Phenolic Composite Filled with Nano-titania

The Kevlar/polytetrafluroethylene（Kevlar/PTFE） fabric composite can be used as a self-lubricating liner of the self-lubricating bearing.Many types of nano-particles can improve the tribological performance of the polymer-based composite.Unfortunately,up to now,published work on the effect of nano-particles on the tribological performance of the fabric composite which can be used as a self-lubricating liner is quite scarce.Therefore,for the purpose of exploring a way to significantly improve the tribological performance of the fabric composite,the tribological performance of the Kevlar/PTFE fabric composite filled with nano-titania is evaluated by using the block-on-ring wear tester.The scanning electron microscopy is utilized to observe the morphologies of worn surfaces of the fabric composites and the counterparts.The tensile properties of the composites are evaluated on the universal material testing machine.The test results show that the addition of nano-titania at a proper mass fraction of the matrix resin improves the wear resistance and the tensile strength,decreases the friction coefficient,and makes the wear volume of the composite reach a relative steady state more quickly;plastic deformation and microcutting are important for the wear of the fabric composite;a lubricating layer is formed on the worn surface of the composite during sliding,and the lubricating layer is critical for the tribological performance of the composite;the formation and properties of the lubricating layer are influenced by the nano-titania particles.The proposed study on the effect of nano-titania on the tribological performance of the Kevlar/PTFE fabric composite,especially on the evolution of the worn surface of the composite,provides the basis for further understanding of the influence mechanism of the nano-particles on the tribological performance of the composite and explores a method of improving the tribological performance of the composite.

Effect of Carbon Nanotube on the Oscillating Wear Behaviour of Metal-PTFE Multilayer Composites

Two kinds of metal-PTFE multilayer composites, which were composed of a steel backing, a middle layer of sintered porous bronze and a surface layer of polytetrafluoroethylene（PTFE） filled by carbon nanotubes（CNTs） or not, were prepared. The wear properties of metal-PTFE multilayer composites oscillating against 45 carbon steel under dry condition were evaluated on an oscillating wear tester, and the effect of CNTs on wear behaviour of metal-PTFE multilayer composites was studied. The results showed that the worn surface of metal-PTFE multilayer composites was characterized by adhesive wear, abrasive wear and fatigue wear. The CNTs greatly increased the adhesion strength of PTFE in the metal-PTFE composites and thereby greatly reduced puck, ploughing, and fatigue failure of PTFE during wearing. The PTFE filled with CNTs prevented direct contact between the mating surfaces and served as fine self-lubricating film, in which the oscillating wear mechanism of the composites was changed to a slightly adhesive wear. Therefore, the CNTs significantly decreased the weight loss and obviously increased the wear resistance of metal-PTFE multilayer composites.

Computer Aided Modeling and Deign of a New Magnetic Sealing Mechanism in Engineering Applications

This article introduces a new type of magnetic sealing mechanism that reduces the lubrication oil pollution and media gaseous leakage in general reciprocating machinery including air compressors and refrigerators. The feasible function and reliable performance of this new sealing mechanism are introduced and analyzed in this paper. The computer aided design, modeling and analysis are being used to study this new sealing mechanism, and the prototype of this sealing mechanism is being tested. The study indicated the proper function of this sealing mechanism. The major advantages of this sealing mechanism include: improved sealing capacity to prevent the gaseous leakage and oil leakage, simple and compact in structure, lower precision requirement on surfaces of reciprocating pistons and shafts in production and manufacturing, and longer services in sealing life span. Also there is almost no frictional loss during the reciprocating motion of piston or shaft.

In-situ preparation of robust self-lubricating composite coating from thermally sprayed ceramic template

The self-lubricating ceramic coatings that can control friction and wear have attracted researchers’widespread attention.However,the poor interfacial bonding between lubricants and ceramics and the deterioration of mechanical properties due to a tribological design limit their practical applications.Here,a robust self-lubricating coating was fabricated by an in-situ synthesis of MoS_(2)/C within inherent defects of thermally sprayed yttria-stabilized zirconia(YSZ)coatings.The edge-pinning by noncoherent endows hybrid coatings with excellent interfacial strength,increasing their hardness(HV)and cohesive strength.Furthermore,owing to the formation of a well-covered robust lubricating film at a frictional interface,a coefficient of friction(COF)can be reduced by 79.6%to 0.15,and a specific wear rate(W)drops from 1.36×10^(−3) to 6.27×10^(−7) mm^(3)·N^(−1)·m^(−1).Combining outstanding mechanical properties and tribological performance,the hybrid coating exhibits great application potential in controlling friction and wear.Importantly,this strategy of introducing the target materials into the inherent defects of the raw materials to improve the relevant properties opens new avenues for the design and preparation of composite materials.

Tribological performance of iron- and nickel-base self-lubricating claddings containing metal sulfides at high temperature

Iron-based coatings with the incorporation of solid lubricants have been prepared by means of laser cladding,in an effort to control friction and decrease tool wear at high temperatures during metal forming applications.The choice of a Fe-based powder has been considered advantageous,as it can lead to decreased costs compared to nickel-based claddings previously studied by the authors,in addition to having a lower environmental impact.In particular,the incorporation of transition metal dichalcogenides such as MoS_(2) as precursors leads to the encapsulation of silver in Fe-based self-lubricating claddings,resulting in a uniform distribution of the soft metal across the thickness of the coating.Subsequent tribological evaluation of the claddings at high temperatures shows that the addition of lubricious compounds leads to lower friction at room temperature and significantly decreased wear up to 600℃ compared to the unmodified iron-based reference alloy,although higher than similar self-lubricating Ni-based claddings.In order to cast light into these observed differences,the corresponding microstructures,phase composition,and self-lubricating mechanisms have been studied and compared for Fe-and Ni-based claddings having both of them the addition of silver and MoS_(2).The results suggest a key role of the formation of protective tribolayers on the counter body during high temperature sliding contact.Additional simulation of the phase evolution during solidification reveals that the formation of different chromium-and nickel-based metal sulfides in Fe-and Ni-claddings during laser cladding by the decomposition of MoS_(2) plays a key role in determining their tribological behaviour at high temperatures.

Friction of metal-matrix self-lubricating composites: Relationships among lubricant content, lubricating film coverage, and friction coefficient

Metal-matrix self-lubricating composites can exhibit excellent tribological properties owing to the release of solid lubricant from the matrix and the formation of a lubricating film on the tribosurface.The coverage of the lubricating film on a worn surface significantly influences the sliding process.However,it is difficult to quantify the film coverage owing to the thin and discontinuous character of the lubricating film and the high roughness of the worn surface.A quantitative characterization of the lubricating film coverage based on X-ray photoelectron spectroscopy(XPS)analysis was developed in this study.The friction tests of Cu-MoS2 composites with a MoS2 content of 0-40 vol%were conducted,and the worn surfaces of the composites were observed and analyzed.Further,the influence of the MoS2 volume content on the coverage of the lubricating film on the worn surface was investigated.The relationships among the volume fraction of the lubricant,coverage of the lubricating film,and the friction coefficient were established.The friction model for the metal matrix self-lubricating composites was developed and verified to facilitate the composition design and friction coefficient prediction of self-lubricating composites.

Self-lubricate and anisotropic wear behavior of AZ91D magnesium alloy reinforced with ternary Ti_2AlC MAX phases

The dry sliding wear behavior of Ti_2AlC reinforced AZ91 magnesium composites was investigated at sliding velocity of 0.5 m/s under loads of 10, 20, 40 and 80 N using pin-on-disk configuration against a Cr15 steel disc. Wear rates and friction coefficients were registered during wear tests. Worn tracks and wear debris were examined by scanning electron microscopy, energy dispersive X-ray spectrometry and transmission electron microscopy in order to obtain the wear mechanisms of the studied materials. The main mechanisms were characterized as the magnesium matrix oxidation and self-lubrication of Ti_2AlC MAX phase. In all conditions, the composites exhibit superior wear resistance and self-lubricated ability than the AZ91 Mg alloy. In addition, the anisotropic mechanisms in tribological properties of textured Ti_2AlC-Mg composites were confirmed and discussed.

Influence of Surface Roughness of Galvanized Steel Sheet on Self-lubricated Coating

Three different thicknesses of self-lubricated coatings were prepared on galvanized steel sheets with differ- ent surface roughness （Ra）. Performances of the coatings were evaluated by various laboratory tests. Scanning elec- tron microscopy （SEM）, atomic force microscope （AFM）, neutral salt spraying test （NSST）, reciprocating friction and wear test were taken in order to characterize the coatings. Results show that the self-lubricated coating has good corrosion resistance and lubricating property. In 0. 857--1. 629 /~m of Ra value, the relationship between friction co- efficients of the self lubricated steel sheets and the surface roughness of the galvanized steel sheets likes a parabolic curve, and has a peak value of friction coefficient in certain range of Ra. Affected by the hereditary surface topogra- phy of the galvanized steel sheet, dents on the surface of sel^lubricated steel become deeper and larger with the in crease of the surface roughness of the galvanized steel sheets. The influence of the surface roughness of the galvanized steel sheet on the corrosion resistance of the self lubricated coating steel is similar to that on friction coefficient of selPlubricated coatings, like a parabolic curve.

Microstructures and high-temperature self-lubricating wearresistance mechanisms of graphene-modified WC–12Co coatings

To reduce the friction coefficient of cobalt-cemented tungsten carbide(WC–12Co)wear-resistant coatings,graphene was compounded into WC–12Co powder via wet ball milling and spray granulation.Selflubricating and wear-resistant graphene coatings were prepared via detonation gun spraying.The presence,morphologies,and phase compositions of graphene in the powders and coatings that are obtained through different powder preparation processes were analyzed.The analysis was performed using the following technologies:energy-dispersive X-ray-spectroscopy(EDXS),scanning electron microscopy(SEM),transmission electron microscopy(TEM),and Raman spectroscopy.The mechanical properties of the coatings were studied using a microhardness tester and a universal drawing machine.The friction and wear properties of the coatings were studied using an SRV-4 friction and wear tester.The results showed that the graphene content in the WC–12Co coating modified with graphene was higher than that without modification;graphene was embedded in the structure in a transparent and thin-layer state.The adhesive strength of this coating at approximately 25°C was approximately 60.33 MPa,and the hardness was approximately 984 HV0.3.After high-temperature treatment,the adhesive strength and hardness of the graphene oxide(GO)/WC–12Co coating decreased slightly(the lowest adhesive strength of 53.16 MPa was observed after treatment at 400°C,and the lowest hardness of approximately 837 HV0.3 was observed after treatment at 300°C).Compared to the friction coefficient(0.6)of the WC–12Co coating obtained at room temperature,the friction coefficient of the GO/WC–12Co coating was decreased by approximately 50%of that value.The graphene-modified coating was continuously exposed to the wear tracks on the surface of the contacting materials during friction,and a lubricating film was formed in the microareas in which the wear tracks were present.The coating exhibited improved self-lubricating and wear-resistant effects compared to the unmodified WC–12Co coating.The results of this study demonstrated that graphene could be effective in self-lubrication and wear-reduction in a temperature range of 100–200°C,as a friction coefficient of 0.3 was maintained.

Friction and wear properties of Cu-based self-lubricating composites in air and vacuum conditions

Cu-based self-lubricating materials containing two different solid lubricants （graphite and MoSs） were fabricated by P/M hot pressing techniques. Physical and mechan- ical properties of the samples were examined. The effects of graphite and MoSs contents on friction coefficient and wear rate were investigated by a ring-on-disc wear machine in air and vacuum conditions, respectively. Tribo-films formed on the worn surfaces were characterized by scanning electron microscopy （SEM） and X-ray photo- electron spectroscopy （XPS）. The results indicated that density, hardness and bending strength all increased with the increasing content of MoS2, while the relative density was opposite. Sample B containing 15 vol. pct graphite and 15 vol. pct MoS2 pos- sessed superior tribological properties both in air and vacuum conditions. However, the tribo-films formed on the worn surfaces of the sample B were greatly discrepant in composition at different testing conditions. In air, the volume ratio of MoS2 and graphite in the tribo-films is 0.31：1 whereas the ratio in vacuum is 1.07：1.