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.

Self-lubricating behavior of Fe_(22)Co_(26)Cr_(20)Ni_(22)Ta_(10)high-entropy alloy matrix composites

Eutectic high entropy alloys(EHEAs)have high temperature stability,good mechanical properties,and are promising for tribological applications at high temperatures.To study the high temperature lubrication behavior,Fe_(22)Co_(26)Cr_(20)Ni_(22)Ta_(10)−(BaF_(2)/CaF_(2))x(x=3−20,wt.%)composites were prepared by spark plasma sintering(SPS),with BaF_(2)/CaF_(2) eutectic powder used as solid lubricant.The lubrication behavior and mechanical properties were studied at both room and high temperatures.With the increase of the content of BaF_(2)/CaF_(2) eutectic powder,the friction coefficients and the wear rates of the composites at 600 and 800℃ decrease significantly.The composites with eutectic powder content of 15 and 20 wt.%have the best lubricating performance at 600℃,with low friction coefficient and wear rates,mainly due to the good mechanical properties of EHEA matrix,the lubrication effect of BaF_(2)/CaF_(2) phase and the oxides formed on the worn surface.

Tensile,compressive and wear behaviour of self-lubricating sintered magnesium based composites

The graphite （Gr）/MoS2 reinforced Mg self-lubricating composites were prepared through powder metallurgy. The composites were characterized for microstructure, physical, mechanical and wear properties. Gr/MoS2 phase in the composites was identified by XRD analysis. Microstructural observation showed that the Gr/MoS2 particles were homogeneously dispersed within the magnesium matrix. Micro-hardness was measured using an applied load of 5 g with a dwell time of 15 s at room temperature. Hardness of all the composites was measured to be in the range of VHN 29？34. The mechanical properties were studied using micro-hardness, tensile and compression tests. A fractographic analysis was performed using scanning electron microscope. The highest values of hardness, compressive strength and tensile strength were attained using Mg-10MoS2 composite. A pin-on-disk tribometer was used to measure the friction coefficient and the wear loss of the sintered composites. In addition to that, the friction and wear mechanism of the composites were systematically studied by worn surface characterization and wear debris studies using SEM analysis. The reduced friction coefficient and wear loss were achieved in MoS2 rather than Gr.

Effects of copper-coated MoS_(2) on friction performance of bronze-graphite-MoS_(2) self-lubricating materials

Two kinds of bronze-graphite-MoS_(2) self-lubricating materials with copper-coated MoS_(2) and uncoated MoS_(2) were prepared by powder metallurgy.Friction and wear experiments were carried out under 4 N and 10 N loads respectively,and the effects of copper-coated MoS_(2) on the friction performances of the materials were studied.Results showed that the way of copper-coated on the surface of MoS_(2) could reinforce the bonding between MoS_(2) and matrix,and inhibited the formation of MoO_(2).Moreover,both materials formed a MoS_(2) lubricating film on the surface during the friction process.While the lubricating film formed after copper coating on MoS_(2) was thicker and had uneven morphology,it was more conducive to improving the friction performance of the material.Compared with conventional materials,the wear rate of copper-coated materials was reduced by one order of magnitude,and the friction coefficient was also reduced by 22.44% and 22.53%,respectively,when sliding under 4 N and 10 N loads.It shows that copper-coated MoS_(2)can improve friction properties of bronze-graphite-MoS_(2)self-lubricating materials furtherly.

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.

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

The tribological properties of Nickel-based composites containing Ti3 SiC2 and Ag2 W2 O7 fabricated by spark plasma sintering against Si3 N4 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 Ag2 W2 O7 is decomposed into metallic silver and CrWO4 during the high-temperature fabrication process. The composite with the addition of 20 wt% Ti3 SiC2 and 5 wt% Ag2 W2 O7 exhibits a friction coefficient of 0.33-0.49 and a wear rate of 7.07×10-5-9.89×10-5 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.

High temperature self-lubricating Ti-Mo-Ag composites with exceptional high mechanical strength and wear resistance

Titanium alloys are of keen interest as lightweight structural materials for aerospace and automotive in-dustries.However,a longstanding problem for these materials is their poor tribological performances.Herein,we designed and fabricated a multiphase Ti-Mo-Ag composite(TMA)with heterogeneous triple-phase precipitation(TPP)structure by spark plasma sintering.A lamellarα-phase(αL)precipitates from theβ-phase under furnace cooling conditions and maintains a Burgers orientation relationship(BOR)withβ-matrix.An active eutectic transition also occurs in the titanium matrix,resulting in TiAg phase.The intersecting acicular TiAg and lamellarαL cutβgrains into fine blocks and the primary equiaxedαphase also provides many interfaces withβphase,which together effectively impede dislocation move-ment and increase strength.Compared with other titanium composites,TMA with TPP microstructure gets an excellent combination of strength(yield strength 1205 MPa)and toughness(fracture strain 27%).Furthermore,the TPP structure endows TMA with strong cracking resistance,which aids in reducing abra-sive debris at high temperatures during sliding and obtaining a low wear rate.Simultaneously,Ag parti-cles distributed at grain boundaries will easily diffuse to the wear surface,in situ forming the necessary lubricating phase Ag_(2)MoO_(4) with Mo-rich matrix debris via oxidation.TMA possesses excellent tribologi-cal properties with especially low wear rate of 8.0 x 10^(-6)mm^(3)N^(-1)m^(-1) and friction coefficient(CoF)of merely 0.20 at 600℃.Unlike other self-lubricating composites with high volume fraction of soft ceramic lubricants,which inevitably sacrifice their mechanical strength and ductility,the composite TMA pos-sesses well-balanced strength,toughness and self-lubricating properties.It holds important implications to design other metal matrix self-lubricating composites(MMSCs)used for load-bearing moving parts.

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.

Flexible metal–organic frameworks based self-lubricating composite

To expand the use of metal–organic frameworks(MOFs)based self-lubricating composite,flexible MOFs MIL-88D has been studied as a nanocontainer for loading lubricant.In this work,the mechanism of oleamine adsorption and desorption by MIL-88D was investigated through molecular simulations and experiments.Molecular simulations showed that the oleamines can be physically adsorbed into open MIL-88Ds with the Fe and O atoms of MIL-88D interacting with oleamine NH2-group.Higher temperature can cause Ole@MIL-88D to release more oleamines,while higher pressure on Ole@MIL-88D caused less oleamines released.Moreover the Ole@MIL-88D was incorporated into epoxy resin(EP)for friction tests.The optimum mass ratio of MIL-88D to EP is 15 wt%,and the EP/Ole@MIL-88D prefers light load and high frequency friction.This work suggests that flexible MOFs can be used as a nanocontainer for loading lubricant,and can be used as a new self-lubricating composite.

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.

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.

Self-lubricating epoxy-based composite abradable seal coating eliminating adhesive transfer via hierarchical design

Aluminum-based composite abradable seal coatings are pivotal to improving the efficiency of aero engines or gas turbines.However,the adhesive transfer frequently occurs between metallic blade tips and aluminum-based composite coatings,resulting in engine vibration and even jam.Many past studies had tried to solve this problem by reducing coating hardness,improving lubrication,or strengthening blade tips,but all had failed.In this paper,we proposed a novel epoxy-based composite abradable seal coating,eliminating adhesive transfer by changing metal-to-metal scraping pair to metal-to-polymer scraping pair.The coating was developed via a hierarchical structure design.Large spherical pores were uniformly distributed in the continuous epoxy matrix with fine graphite dispersion.By adding 20 vol.%graphite and 50 vol.%hollow microspheres,a self-lubricating epoxy-based coating of 0.26 friction coefficient with thermal conductivity of 0.28 W/(m·K),coating HR15 Y hardness at 54.8,and bonding strength at 18.7 MPa can be reached.When the metallic blades scrape the epoxy-based composite coating,no adhesive transfer occurs.Besides,a smooth scraped surface is formed by pseudoplastic deformation.This epoxy-based composite abradable seal coating opens a new way to improve the efficiency and reliable operations of air engine compressors.

Design and preparation of gradient graphite/cermets self-lubricating composites

Based on the functionally graded materials （FGMs） design concept, the laminated-graded graphite/cermets self-lubricating composite was prepared to achieve the integration of mechanical prop- erties and lubrication performance of the cermet. The effects of the layer number and thickness of graded structure on residual stresses in the gradient composites were investigated by finite element method （FEM）. From the FEM analyses, the optimal gradient structure design was obtained correspond- ing to the following parameters： the number of graded layers n = 2 and the thickness of graded structure t = I ram. According to the optimum design, a graded graphite/cermets self-lubricating material with two layers was fabricated by a typical powder metallurgy technique. Compared with the homogenous graphite/cermets composite, the surface hardness and indentation fracture toughness of graded compos- ite were increased by approximately 15.9% and 6.3%, respectively. The results of X-ray diffraction （XRD） stress measurement identified the existence of residual compressive stress on the surface of graded com- posite. Additionally, the friction and wear tests revealed that the wear resistance of the graphite/cermets self-lubricating composite was improved significantly via the graded structural design, whereas the coefficient of friction changed slightly.

Tribological behaviour of sintered iron based self-lubricating composites

This work is a review of previous works,presenting and discussing the most important results obtained by an ongoing research program towards the development of innovative,low-cost,self-lubricating composites with a low friction coefficient and high mechanical strength and wear resistance.Special emphasis is given to uniaxial die pressing of solid lubricant particles mixed with matrix powders and to metal injection moulding associated with in situ generation of solid lubricant particles.Initially,a microstructural model/processing route (powder injection moulding followed by plasma-assisted debinding and sintering) produced a homogeneous dispersion of in situ generated solid lubricant particles.Micrometric nodules of graphite with diameter smaller than 20 μm were formed,constituting a nanostructured stacking of graphite foils with nanometric thickness.Micro Raman analysis indicated that the graphite nodules were composed of turbostratic 2D graphite having highly misaligned graphene planes separated by large interlamellae distance.Large interplanar distance between the graphene foils and misalignment of these foils were confirmed by transmission electron microscopy and were,probably,the origin of the outstandingly low dry friction coefficient (0.04).The effect of sintering temperature,precursor content,metallic matrix composition and surface finish is also reported.Furthermore,the influence of a double-pressing/double-sintering (DPDS) technique on the tribological performance of self-lubricating uniaxially die-pressed hBN + graphite-Fe-Si-C-Mo composite is also investigated.Moreover,the tribological behaviour of die-pressed Fe-Si-C matrix composites containing 5,7.5 and 10 wt％ solid lubricants (hBN and graphite) added during the mixing step is analysed in terms of mechanical properties and wear mechanisms.Finally,the synergy between solid lubricant particles dispersed in a metallic matrix and fluid lubricants in a cooperative mixed lubrication regime is presented.

Study on the Corrosion Properties of Several Kinds of High-temperature Solid Self-lubricating Material

In some adjusting mechanism,solid self-lubricating material that has hot-resistant and corrosion-resistant is need for adapting the work condition.In the present article,by the comparison study of mass loss and surface topography after corrosion in acid solution and vapour of the four kinds of material,graphite,polymer,BN composite and cermet,it was found that,graphite has good corrosion-resistant to acid solution and vapour under 200℃,but the corrosion-resistant will become worse dramatically under 400℃,polymer has worse corrosion-resistant behavior above 200℃.By comparison,BN self-lubricating composite has better hot-resistant and corrosion-resistant,which can meet the demand of the work condition.

High Temperature Oxidation and Tribological Behaviors of NiCrAl-Graphite Self-Lubricating Composites

Nickel-graphite self-lubricating composites are a promising candidate to be used in turbine constructions that are usually exposed to high temperature oxidation and wear.However,the high-temperature stability of graphite as well as the effect that the oxide scale will play on the following wear process are still yet in debate.In this work,oxidation behavior of a NiCrAl-graphite composite and the subsequent friction and wear performances were studied.Results indicate that graphite is stable in the composites after oxidation at T≤400℃ for 300 h,which contributes synergistically with the thin oxide film to self-lubrication.The friction coefficient is below 0.20 and the wear rate is~1.43×10^(-5) mm³ N-1 m-1.The composite has the highest friction coefficient and wear rate when it was suffered from the high temperature oxidation at 500℃.Once it was oxidized at 600℃,a glaze layer would develop during the subsequent sliding.It plays a positive role in improving tribological properties though in the absence of lubricant phase of graphite,with to be exactly the friction coefficient and wear rate reduced by 13%and 21%,respectively,in comparison with the case of oxidation at 500℃.

Improving high-temperature wear resistance of NiCr matrix self-lubricating composites by controlling oxidation and surface texturing

Self-lubricating composites(SLCs)are widely used in the fields of aerospace and marine,but the conventional NiCr matrix SLCs with sulfide as solid lubricant often suffer from low wear resistance at high temperatures.In view of its high affinity with oxygen and also the high oxidation rate,appropriate amount of nano Ti was added to NiCr-WS_(2)composites prepared by spark plasma sintering(SPS)to adjust the oxidation behavior and surface texture.When exposed to high temperature,Ti was preferentially oxidized in comparison to Ni and Cr,resulting in abundant TiO_(2)protrusions and microdimples on the surface,i.e.in situ surface texturing.Besides,TiO_(2)was of high chemical activity and readily to react with other oxide debris during high temperature sliding process to form compounds of NiTiO_(3)and CrTi_(2)O_(5).The high chemical activity of oxide debris that was conducive to sintering,combining with the special surface texture that stores as many wear debris as possible,promoted the rapid formation of a protective glaze layer on the sliding surface.The NiCr-WS_(2)-Ti composite exhibited low friction coefficient but high wear resistance at elevated temperatures.Especially at 800℃,it presented a wear rate of as low as(2.1±0.3)×10^(-5)mm^3N-1m^(-1),accounting for only 2.7%of that of NiCr-WS_(2)composite.