The influence of MoS_2 on tribology characteristic parameter of Ni60A/MoS_2 composite lubricating coating

The influence of MoS2 on the tribology characteristic parameter of Ni60A/MoS2 composite lubricating coating was researched on the UMT-2 fretting abrasion tester （USA） The result shows that with increasing content of MoS2, the hardness curve of the composite coating decreases and the trend accelerates. Under the same experimental conditions, the mass loss of plasma spray composite coating without adding MoS2 iS 1.27×10^-2 mg. When the amount of MoS2 reaches 35%, the mass loss is 0.96×10^-2 mg. It can be seen that adding MoS2 phase can improve the wear resistance, the amplitude of which is close to 30%. The friction coefficient of plasma spray composite coating without adding MoS2 is 0.23. Adding MoSz could decrease the friction coefficient of the coating and presents a downtrend. When the mass fraction is 35%, the friction coefficient is the smallest （0.13）, and the range is doubled.

Effect of Carburization on the Mechanical Properties of Biomedical Grade Titanium Alloys

Titanium cermets were successfully synthesized on the surface of biomedical grade titanium alloys by using sequential carburization method. The mechanical properties such as hardness, fracture toughness and plasticity were measured to estimate the potential application of titanium cermets. The results show that after carburization the surface hardness of titanium cermets was 778 HV, with a significant improvement of 128% compared with that of titanium alloys. In addition, the fracture toughness of titanium cermets was 21.5 × 10^6 Pa.m^1/2, much higher than that of other ceramics. Furthermore, the analysis of the loading-unloading curve in the nanoindentation test also indicates that the plasticity of titanium cermet reached 32.1%, a relatively high value which illustrates the combination of the metal and ceramics properties. The results suggest that sequential carburization should be an efficient way to produce titanium cermets with hard surface, high toughness and plasticity.

The Influence of Normal Load and Sliding Speed on Frictional Properties of Skin

The study of frictional properties of human skin is important for medical research, skin care products and textile exploi- tation. In order to investigate the influence of normal load and sliding speed on the frictional properties of skin and its possible mechanism, tests were carded out on a multi-specimen friction tester. When the normal load increases from 0.1 N to 0.9 N, normal displacement and the friction coefficient of skin increase. The friction coefficient is dependent on the load, indicating that both adhesion and deformation contribute to the friction behaviour. The deformation friction was interpreted using the plough model of friction. When sliding speed increases from 0.5 mm·s^-1 to 4 mm·s^-1, the friction coefficient increases and ＂stick-slip＂ phenomena increase, indicating that hysteretic friction contributes to the friction. The hysteretic friction was in- terpreted using schematic of energy translation during the rigid spherical probe sliding on the soft skin surface, which provides an explanation for the influence of the sliding speed on the frictional characteristics of the skin.

RESEARCH AND SIMULATION OF THE INFLUENCE OF THREE-AXIAL COMPACTION ON POWDER METALLURGIC PRODUCT PROPERTIES

The warm powder compaction process is simulated by the finite element analysis software, MSCJMARC. The thermal mechanically coupled analysis method is applied on the basis of the updated Lagrangian Method, to simulate the warm powder compaction process. The warm powder compaction process is simulated, and the influence of friction condition and pressing styles are researched on the density of powder green and the mechanics behavior at certain temperature. The results indicate that for cylindrical compacts, with the improvement of the friction condition, the uniformity of distribution of green relative density is largely improved, the pressing force and stress decrease, and the nonconforming pressing processes influence the distribution of green density to some degree. The status of stress distribution of the process that punches firstly press and die finally press is different from the other three processes, and presents the figure of ＇flume ＇.

An Overview of Bearing Candidates for the Next Generation of Reusable Liquid Rocket Turbopumps

There is a consensus in the aerospace field that the development of reusable liquid rockets can effectively reduce the launch expense.The pursuit of a long service life and reutilization highly depends on the bearing components.However,the rolling element bearings(REBs)used in the existing rocket turbopumps present obvious and increasing limitations due to their mechanical contacting mode.For REBs,high rotational speed and long service life are two performance indexes that mutually restrict each other.To go beyond the DN value(the product of the bearing bore and rotational speed)limit of REBs,the major space powers have conducted substantial explorations on the use of new types of bearings to replace the REB.This review discusses,first,the crucial role of bearings in rocket turbopumps and the related structural improvements of REBs.Then,with the prospect of application to the next generation of reusable liquid rocket turbopumps,the bearing candidates investigated by major space powers are summarized comprehensively.These promising alternatives to REBs include fluid-film,foil,and magnetic bearings,together with the novel superconducting compound bearings recently proposed by our team.Our more than ten years of relevant research on fluid-film and magnetic bearings are also introduced.This review is meaningful for the development of long-life and highly reliable bearings to be used in future reusable rocket turbopumps.

Morphology-Controlled Growth of A1N One-Dimensional Nanostructures

Aluminum nitride （AIN） nanowires, serrated nanoribbons, and nanoribbons were selectively obtained through a simple chloride assisted chemical vapor deposition process. The morphologies of the products could be controlled by adjusting the deposition position and the flux of the reactant gas. The morphologies and structures of the AIN products were investigated in detail. The formation mechanism of the as-prepared different morphologies of AIN one-dimensional （ID） nanostructures was discussed on the basis of the experimental results.

Chloride Assisted Growth of Aluminum Nitride Nanobelts and Their Enhanced Dielectric Responses

Aluminum nitride （AlN） nanobelts were successfully synthesized in high yield through a chloride assisted vapor-solid process. X-ray diffraction, transmission electron microscopy, and selected area electronic diffraction demonstrate that the as-prepared nanobelts are pure, structurally uniform and single crystalline, and can be indexed to hexagonal wurtzite structure. The micro observations show that there exist no defects in the obtained nanobelts. The growth direction of the nanobelts is along [0001]. The frequency spectra of the relative dielectric constant and of the dielectric loss were measured in the frequency range of 50 Hz to 5 MHz. Analysis of these spectra indicates that the interface in samples has great influence on the dielectric behavior of samples. As compared with AlN micropowders, AlN nanobelts have much higher relative dielectric constant, especially at low frequencies at room temperature.

Effects of macrosegregation on mechanical and tribological properties of squeeze casting immiscible bearing alloys

The macrosegregation behaviors of Al-Sn-Cu ternary immiscible alloy castings and their effects on mechanical and tribological properties were investigated.The results demonstrate that Sn and Cu segregate in the casting simultaneously,and the mass fraction of the two elements has a"U"shaped distribution.Significantly,positive and negative segregation occur in the casting,with positive segregation appearing on the top and lower surfaces and negative segregation on the remaining surfaces,with the 1/2 surface(hot node location)having the highest degree of negative segregation.Furthermore,the results of Vickers hardness,tensile strength,and elongation show that Sn and Cu cooperatively affect the mechanical properties of castings.The higher the mass fraction of Sn and Cu elements,the higher the hardness,the greater the tensile strength,and the better the elongation.The findings of the step-by-step loading tests demonstrate that the segregation of Sn and Cu significantly impacts the tribological characteristics of the castings.The higher the mass fraction of Sn and Cu on the sample surface,the better the tribological characteristics.

Modeling of Rolling Contact Friction of Ball Bearings at Elevated Temperature

In order to predict the rolling friction coefficient and analyze the effects of material compatibility on the friction coefficient at elevated temperature(from room temperature to 600℃),a theoretic computation model was developed based on bearing's elastic contact hysteresis and related material characteristic parameters.Some examples were carried out to verify the feasibility of the model.The research results show that the material compatibility has significant effects on rolling friction coefficient,especially the modulus of elasticity and the expansion coefficient of material,and results also show the rolling friction coefficient of the matched pair between GCr15 and Si3N4 is the smallest comparing with that between GCr15 and M50(or ASP-23,or GCr15) when making GCr15,M50,ASP-23,and Si3N4 materials as bearing's rolling balls and GCr15 material as bearing's inner and outer rings.Further research indicates that the working temperature also has played an important role in the rolling friction coefficient of the ball bearing,moreover,the friction coefficient of the matched pair of M50 and Si3N4 behaves most stably at elevated temperature.

Achieving Macroscale Ultra-low Friction via the Synthesis of Dioctyl Sebacate using Catalytic Esterification Using MoS_(2)/C_(60)

Dioctyl sebacate(DOS)is an organic ester compound,mainly used as a low-temperature plasticizer and synthetic antirust and cold-resistant ester lubricant base oil.This study discusses the use of synthesized molybdenum disulfide/C60(MoS_(2)/C_(60))composite particles as catalysts to synthesize a new type of DOS endowed with high lubricity.This was achieved through a catalytic esterification reaction between sebacic acid and 1-octanol.After the reaction,MoS_(2)/C_(60) was dispersed in situ in this novel DOS to form a suspension(MoS_(2)/C_(60)/DOS).The tribological properties of MoS_(2)/C_(60)/DOS were examined through high-frequency reciprocating friction and wear experiments,and the friction and wear mechanisms were analyzed.The results show that MoS_(2)/C_(60)/DOS can significantly enhance the antiwear and friction reduction performance compared to commercial DOS by 91% and 95%,respectively,achieving an ultra-low friction state with an average friction coefficient of 0.006.A friction film containing elements such as Fe,O,C,Mo,and S forms on the friction surface,significantly improving the lubrication state of the friction interface and achieving low friction.

EXPERIMENTAL INVESTIGATION ON PARTICLE EFFECT IN PISTON RING-CYLINDER LUBRICATION

To study the tribological properties of the piston ring-cylinder liner in liquid-solid lubrication, the experiment is carried out on a modified piston ring-cylinder liner tester. Two kinds of liquid-solid lubricants are used, one with ultra-dispersed diamond （UDD） nano-particles suspending in pure lubricant, the other with micro-sized MoS2 particles. The particle concentrations are 0%, 0.02% and 0.1% by weight. The experimental temperature is 30℃ and 75℃ respectively. The results show that with the presence of ultra-dispersed diamond particles, the load when scuffing failure occurs is increased. For the lubricant contains MoS2 particles, the scuffing load is decreased. The liquid-solid lubricant also affects the thermal behavior of piston ring-cylinder liner. The surface bulk temperatures of cylinder liner specimen are measured. It has been seen that liquid-solid lubricant used in this research tends to improve the thermal properties generally and the measured friction forces also decreases with the presence of UDD nano-particles. The surface bulk temperature when scuffing occurs is also measured. The results show that the size effect and environment temperature have obvious influence on scuffing load and scuffing temperature. With some new findings, this work is an important complement to the existing research on particle effect on lubrication, because the existing results only show one aspect of this problem.

Bioinspired micro/nanostructured surfaces prepared by femtosecond laser direct writing for multi-functional applications

manufacturing of biomimetic micro/nanostructures due to its specific advantages including high precision,simplicity,and compatibility for diverse materials in comparison with other methods(e.g.ion etching,sol-gel process,chemical vapor deposition,template method,and self-assembly).These biomimetic micro/nanostructured surfaces are of significant interest for academic and industrial research due to their wide range of potential applications,including self-cleaning surfaces,oil-water separation,and fog collection.This review presents the inherent relationship between natural organisms,fabrication methods,micro/nanostructures and their potential applications.Thereafter,we throw a list of current fabrication strategies so as to highlight the advantages of FLDW in manufacturing bioinspired microstructured surfaces.Subsequently,we summarize a variety of typical bioinspired designs(e.g.lotus leaf,pitcher plant,rice leaf,butterfly wings,etc)for diverse multifunctional micro/nanostructures through extreme femtosecond laser processing technology.Based on the principle of interfacial chemistry and geometrical optics,we discuss the potential applications of these functional micro/nanostructures and assess the underlying challenges and opportunities in the extreme fabrication of bioinspired micro/nanostructures by FLDW.This review concludes with a follow up and an outlook of femtosecond laser processing in biomimetic domains.

Lubrication antagonism mechanism of nano-MoS_(2)and soot particles in ester base oil

Spherical nano-MoS_(2)(S-MoS_(2))has excellent lubricating properties and potential application value in engine oil additives.Engine soot can enter the engine oil,so the tribological interaction between S-MoS_(2)and diesel combustion soot(DCS)should be investigated.In this study,DCS was used to simulate engine soot.The interaction was investigated in dioctyl sebacate(DOS),and the interaction mechanism was full characterized.Results showed that S-MoS_(2)and DCS had obvious antagonism effects on lubrication.The 0.5%S-MoS_(2)exhibited good lubricating properties in DOS,which could reduce friction by~22%and wear by~54%.However,after 0.5%S-MoS_(2)was added to the 0.5%DCS contaminated DOS,the lubrication performance was not improved and was even worse than that without S-MoS_(2).When S-MoS_(2)was added for DOS lubrication,a tribofilm containing MoS_(2)formed on the friction surface,but simultaneously adding 0.5%DCS resulted in the disappearance of the MoS_(2)tribofilm.Moreover,under the action of friction heat,DCS and S-MoS_(2)could form hard Mo_(x)C_(y),thereby increasing abrasive wear.Finally,a preliminary deantagonism method was provided.After 2.0%zinc isooctyl dithiophosphate was added to the above antagonistic system,the friction coefficient did not show visible changes,but the wear recovered to a level close to that when only S-MoS_(2)was added.The antiantagonism method is not very satisfactory and some more efficient methods need to be further explored.

Role of temperature in tribolayers in fretting wear ofγ-TiAl alloy

The formation of tribolayers may play significant influences on fretting wear.At elevated temperature,the adhesion among wear debris and the increased diffusion rate facilitate the formation of tribolayers.However,the intensification of oxidation at elevated temperature and the low diffusion rate in oxides may play an adverse role.The present study aims to investigate the role of temperature in tribolayers in fretting wear using aγ-TiAl alloy.Scanning electron microscope,energy dispersive spectrometer,Raman spectrometer,transmission electron microscope and nanoindentation were utilized to investigate the wear debris,tribolayers,and wear scars.The fretting tests showed that,compared with that at room temperature(RT)and 350℃,significant reduction in wear rate and decrease in the fluctuation of friction coefficient occurred at 550 and 750℃.It was further revealed that when temperature raised from room temperature(RT)to 750℃,the oxidation of the wear debris increased slightly and the diffusion coefficients increased prominently,which facilities the formation of well tribo-sintered tribolayers.The well tribo-sintered tribolayers presented homogenous structure,nanocrystalline grains with excellent mechanical properties,and resulted in the improvement in the fretting wear resistance of theγ-TiAl alloy at 550 and 750℃.

Tribological behaviors of Ni-modified citric acid carbon quantum dot particles as a green additive in polyethylene glycol

A novel green lubricating oil additive(carbon quantum dot(CQD)particle‐doped nickel(Ni‐CQD))was synthesized from citric acid and nickel acetate.The effects of CQD and Ni‐CQD nanoparticles on the tribological behaviors of polyethylene glycol(PEG200)were investigated under different loads and reciprocation speeds.The results indicate that CQD and Ni‐CQD particles can both enhance the lubrication properties of PEG200.However,the Ni‐CQD nanoparticles enhanced the lubrication properties more than the plain CQD particles did.The average friction coefficient and wear rate of PEG200 containing 2 wt%Ni‐CQDs were reduced by 35.5%and 36.4%,respectively,compared to PEG200 containing pure CQDs under a load of 8 N and reciprocation speed of 25 mm/s over 60 min.The friction and wear mechanisms are attributed to the fact that friction induces the Ni‐CQDs to participate in the formation of a tribofilm,resulting in a low friction coefficient and wear rate.

Surface characterization of steel/steel contact lubricated by PA06 with novel black phosphorus nanocomposites

In the present work,two types of novel nano additives,titanium sulfonate ligand/black phosphorus(TiLi/BP)and titanium dioxide/black phosphorus(TiCVBP)nanocomposites,were prepared.The tribological behavior of the steel/steel friction pairs lubricated by polyalphaolefins type 6(PA06)containing the nanocomposites under boundary lubrication was studied.The worn surfaces were analyzed using modem surface techniques.The experimental results show that the rubbed surfaces became smooth and showed little wear with the addition of the nanocomposites.TiO_(2)/BP nanocomposites can significantly improve the lubricity of BP nanosheets under high contact stress.The synergistic roles of the load-bearing abilities and rolling effect of TiO_(2) nanoparticles,the slip induced by the BP with its layered structure,and the establishment of a tribofilm on the sliding interface are the basis of the tribological mechanisms.

Relationship between the real contact behavior and tribological characteristics of cotton fabric

The tribological characteristics of cotton fibers play an important role in engineering and materials science,and real contact behavior is a significant aspect in the friction behavior of cotton fibers.In this study,the tribological characteristics of cotton fibers and their relationship with the real contact behavior are investigated through reciprocating linear tribotesting and real contact analysis.Results show that the friction coefficient decreases with a general increase in load or velocity,and the load and velocity exhibit a co‐influence on the friction coefficient.The dynamic change in the real contact area is recorded clearly during the experiments and corresponds to the fluctuations observed in the friction coefficient.Moreover,the friction coefficient is positively correlated with the real contact area based on a quantitative analysis of the evolution of friction behavior and the real contact area at different loads and velocities.This correlation is evident at low velocities and medium load.

Nonlinear dynamic analysis of low viscosity fluid-lubricated tilting-pad journal bearing for different design parameters

To reveal nonlinear dynamic rules of low viscosity fluid-lubricated tilting-pad journal bearings(TPJBs),the effects of design parameters on journal center orbits and dynamic minimum film thicknesses of water-lubricated TPJBs with and without static loads are investigated.The hydrodynamic bearing force used in the nonlinear dynamic analysis is an approximate analytical solution including the turbulence effect.The results reveal the methods for vibration suppression and load capacity improvement and give an optimal pivot offset and clearance ratio that can maximize the minimum film thickness.The results also show that four-pad TPJBs with loads between pads are preferred due to good dynamic performance and load capacity.This study would provide some guidance for nonlinear design of low viscosity fluid-lubricated TPJBs under dynamic loads.

Role of capillary adhesion in the friction peak during the tacky transition

The friction peak that occurs in tire–road sliding when the contact changes from wet to dry was previously attributed to capillary cohesion,van der Waals attraction,and surface roughness,but the detailed mechanisms have yet to be revealed.In this study,friction and static contact experiments were conducted using a custom-built in situ optical microtribometer,which allowed us to investigate the evolution of the friction,normal load,and contact area between a polydimethylsiloxane(PDMS)film and a silicon nitride ball during water volatilization.The friction coefficient increased by 100%,and the normal force dropped by 30%relative to those in the dry condition during the wet-to-dry transition.In static contact experiments,the probe indentation depth increased,and the normal load decreased by~60%as the water evaporated.Combining the friction and static contact results,we propose that the large friction peak that appeared in this study can be attributed to the combined effects of increased adhesive capillary force and increased plowing during the wet-to-dry transition.

Numerical Analysis of Frictional Heat-Stress Coupled Field at Dynamic Contact

A new analysis method was developed to simulate the dynamic process of a frictional heat-stress coupled field. The relationship between the frictional heat and the thermal stress was investigated for concave cylinder contact conditions. The results show that, as a nonlinear contact problem, the frictional heat at the contact areas changes with moving velocity in both value and distribution, and that the transient frictional heat at the dynamic condition has a peak within a cycle. The dynamic process of friction heat and thermal stresses affects diffusion of the frictional effects. The result can be helpful for dynamic simulation of diffusion lubrication of elements at elevated temperatures.