空间摩擦学及其材料的研究进展

综述了空间苛刻服役环境及其对空间摩擦学材料性能影响的研究,深入分析了空间环境对空间摩擦材料、空间耐磨材料和空间减摩材料摩擦磨损机理的影响。空间摩擦材料主要应用于空间对接机构及空间机械臂中,应具有稳定的摩擦力矩与优良的抗黏着磨损性能。空间耐磨材料主要应用于空间轴承、齿轮和密封件等部件中,如FeAl金属间化合物在高温下抗蠕变性急剧下降,常通过添加金属元素(Ce,Cr,Mn,Mo,Nb,W等)及固体润滑剂提高材料抗蠕变性能;Ti及其合金常通过表面改性改善黏着性;与基体结合性良好的耐磨涂层可以较大程度的改善材料的耐磨性。空间减摩材料主要指润滑剂与自润滑材料,如软金属Pb、高分子材料PI和PTFE等,以及某些金属的氧化物,氟化物和硫化物等,能较好地降低材料表面的摩擦因数。随着航天科技的发展,亟须开发新型高性能空间摩擦学材料,建立摩擦学材料数据库,以应对国际航天技术发展的挑战。

塑料及其复合材料的摩擦学

综述了近年来国内外塑料及其复合材料的摩擦学研究的新进展,涉及到PTFE、PEEK、UHMWPE的合金以及塑料复合材料的重点研究领域,指出基于现代材料设计的摩擦学材料设计、塑料在热和应力作用下的蠕变和长期稳定性、塑料梯度功能材料和环保的绿色材料的摩擦学等方面是未来的发展方向。

摩擦学机敏材料中的相变控制润滑剂

探讨了摩擦学机敏材料的核心部件聚乙烯基复合导电自润滑材料的制备工艺与电性能 ,不同方法制得的复合材料的电阻率与导电组分炭黑含量的关系亦表现出很大的差别。与干法相比 ,湿法电阻率较低 ,而过渡区组分范围更宽。据此 ,确定出用作复合导电自润滑材料的炭黑含量宜大于 35 % (质量 ,下同 ) ,但经实验得出可以自加热发生熔融相变的合适的炭黑含量为 40 %。加入润滑剂后构成复合导电自润滑材料 ,通电加热 ,显示出快速、易控、热性能稳定的自加热熔融性能 ,实现了复合导电自润滑材料的固 -液 -固的控制相变。

滑动速度对团球共晶体增强奥氏体钢基自生复合材料摩擦学性能的影响

利用MPX-2000型主轴盘销式磨损实验机和扫描电子显微镜(SEM)研究了相对滑动速度对团球γ+(Fe,Mn)_3C共晶体增强奥氏体钢基自生复合材料(EAMC)摩擦学性能的影响。实验表明,在干摩擦磨损工况下,EAMC对G45钢摩擦系统的摩擦系数随相对滑动速度的增加呈递减趋势;而磨损率呈递增趋势,但始终远低于奥氏体中锰钢(单一奥氏体相);并且,随着相对滑动速度的提高,EAMC与中锰钢磨损量的差值呈递增趋势。通过对磨损表面和磨屑形貌的分析,发现EAMC在低载下主要磨损机制是磨粒磨损与剥层磨损;高载下的磨损机制主要为剥层磨损与氧化磨损。对偶件之间的粘着作用随相对滑动速度的提高而增加。运用临界转变温度理论与Archard磨损理论分析了相对滑动速度对EAMC摩擦学性能影响的机制。

Friction stir keyholeless spot welding of AZ31 Mg alloy-mild steel

Friction stir keyholeless spot welding（FSKSW） using a retractable pin for 1.0 mm thick galvanized mild steel and 3 mm thick AZ31 B magnesium alloy in a lap configuration was investigated.The process variables were optimized in terms of the joint strength.The effects of the stacking sequence on joint formation and the joining mechanism of FSKSW AZ31B-to-mild steel joints were also analyzed.It shows that the process window and joint strength are strongly influenced by the stacking sequence of the workpieces.While the process window is narrow and unstable for FSKSW of a magnesium-to-steel stack-up,a desirable process was established for the steel-to-magnesium stacking sequence,a desirable process and higher strength joint can be got when the steel-to-magnesium stacking sequence.XRD phase and EPMA analyses of the FSKSW joint showed that the intermetallic compounds are formed at the steel-to-magnesium interface,and the element diffusion between the mild steel and AZ31 B magnesium alloy revealed that the joining methods for FSKSW joints is the main mechanical joining along with certain metallurgical bonding.

Effect of electrical current on tribological property of Cu matrix composite reinforced by carbon nanotubes

Cu matrix composite reinforced with 10%（volume fraction） carbon nanotubes（CNTs/Cu） and pure Cu bulk were prepared by powder metallurgy techniques under the same consolidation processing condition.The effect of electrical current on tribological property of the materials was investigated by using a pin-on-disk friction and wear tester.The results show that the friction coefficient and wear rate of CNTs/Cu composite as well as those of pure Cu bulk increase with increasing the electrical current without exception,and the effect of electrical current is more obvious on tribological property of pure Cu bulk than on that of CNTs/Cu composite;the dominant wear mechanisms are arc erosion wear and plastic flow deformation,respectively;CNTs can improve tribological property of Cu matrix composites with electrical current.

Microstructure and mechanical properties of dissimilar Al-Cu joints by friction stir welding

Dissimilar friction stir welding between 1060 aluminum alloy and annealed pure copper sheet with a thickness of 3 mm was investigated. Sound weld was obtained at a rotational speed of 1050 r/min and a welding speed of 30 mm/min. Intercalation structure formed at the crown and Cu/weld nugget （WN） area promotes interracial diffusion and metallurgical bonding of aluminum and copper. However, corrosion morphology reveals the weak bonding mechanism of internal interface, which causes the joint failing across the interface with a brittle-ductile mixed fracture mode. The tensile strength of the joint is 148 MPa, which is higher than that of the aluminum matrix. Crystal defects and grain refinement by severely plastic deformation during friction stir welding facilitate short circuit diffusion and thus accelerate the formation of A14Cu9 and A12Cu intermetallic compounds （IMCs）. XRD results show that A14Cu9 is mainly in Cu/WN transition zone. The high dislocation density and formation of dislocation loops are the major reasons of hardness increase in the WN.

Tribological behavior of CNTs-Cu and graphite-Cu composites with electric current

CNTs-Cu and graphite-Cu composites were separately prepared by powder metallurgy technique under the same consolidation processing. Tribological behavior of the composites with electric current was investigated by using a pin-on-disk friction and wear tester. The results show that the friction coefficient and wear rate of the composites decrease with increasing the reinforcement content, and increase with increasing the electric current density; the effects of electric current are more obvious on tribological properties of graphite-Cu composites than on CNTs-Cu composites; for graphite-Cu composites the dominant wear mechanisms are electric arc erosion and adhesive wear, while for CNTs-Cu composites are adhesive wear.

Effects of environment on dry sliding wear behavior of silver-copper based composites containing tungsten disulfide

Silver based composites containing different amounts of WS2were prepared by hot-pressing method and their tribologicalbehaviors were investigated against coin silver under humid air,dry nitrogen and vacuum on a ball-on-disk tester with normal load of5N.The components of composites,microstructure of debris and worn surface were characterized using XRD SEM,EDS and XPS.It is demonstrated that environmental conditions significantly affect the tribological behavior of silver based composites.The frictioncoefficient is the highest in humid air,and the lowest in dry nitrogen.It is found that the friction and wear behavior of the compositesare strongly depended on the characteristics of the lubrication film forming in different operating environments,such as thicknessand composition.In addition,it is indicated that the dominant wear mechanisms of silver based composites are abrasive wear anddelamination under different conditions.

Effect of Fe-impurity on tribological properties of Al-15Mg_2Si composite

The effect of Fe-impurity（0.2%-2%, mass fraction） on the microstructure, dry sliding wear, and friction properties of Al-15 Mg2 Si composite was investigated using a pin-on-disk tester under the applied pressures of 0.25, 0.5 and 1 MPa at a constant sliding speed of 0.13 m/s. According to the results, Fe modified the primary Mg2 Si particles from irregular dendritic form to smaller particles with polyhedral shapes, refined the pseudo-eutectic structure, and led to the formation of hard b-Al5 Fe Si platelets in the matrix. In spite of hardness improvement by these microstructural changes, the resistance of the composite against dry sliding wear was impaired. SEM examination of the worn surfaces, wear debris, and subsurface regions confirmed the negative effect of b-phase on the tribological properties. It was found that b-particles were fractured easily, thereby decreasing the potential of the substrate to resist against sliding stresses and giving rise to the instability and easy detachment of tribolayer as large delaminated debris. The friction results also revealed that Fe slightly decreased the average friction coefficient, but increased the fluctuation in friction.

Tribological behavior of Cu-15Ni-8Sn/graphite under sea water, distilled water and dry-sliding conditions

The tribological behaviors of Cu-15Ni-8Sn/graphite composites with the graphite content of 38 vol.%against AISI321 stainless steel under dry-sliding,deionized water and sea water were investigated on a block-on-ring configuration.The results indicated that the friction coefficient was the lowest under dry-sliding,and the highest in deionized water.The wear rate decreased to reach the minimum value of 1.39×10-15 m3/(N·m)in sea water and in deionized water,it increased to the maximum value of 5.56×10-15 m3/(N·m).The deionized water hindered the formation of tribo-oxide layer and lubricating film,which resulted in the largest friction coefficient and wear rate.In sea water,however,the corrosion products comprised of oxides,hydroxides and chlorides were found on the worn surface,and the compacted layer composed of corrosion products and graphite played an important role in keeping the excellent wear resistance.It was elucidated that the tribological behaviors of Cu-15Ni-8Sn/graphite composite were powerful influenced by the friction environments.

Tribological behavior of friction stir processed SiP/ZA40 in-situ composites

The effect of friction stir processing(FSP)at different rotation speeds(400,630,800,and 1000 r/min)and traverse speeds(25 and 50 mm/min)on the tribological properties of a Si particle reinforced Zn−40Al−2Cu-based in-situ composite was investigated.After preliminary optimization,800 r/min and 25 mm/min were selected as optimum FSP parameters.According to the results,multi-pass FSP improved the tribological properties.For instance,at an applied pressure of 0.75 MPa,the wear rate and average coefficient of friction(COF)of four-pass FSPed composite were lower than those of base composite by 53%and 50%,respectively.SEM examinations of worn surfaces,wear debris,and worn subsurfaces revealed that the intensive refinement and uniform distribution of microstructural phases,especially the coarse Si particles,reduced Si particles interspacing,and elimination of casting defects were the most important factors enhancing the substrate resistance against sliding-induced deformation.This led to the formation of stable tribolayers that improved the tribological properties.

High temperature tribological behaviors of aluminum matrix composites reinforced with solid lubricant particles

The AA6061-10 wt.%B4 C mono composite, AA6061-10 wt.%B4 C-Gr(Gr: graphite) hybrid composites containing 2.5, 5, and 7.5 wt.% Gr particles, and AA6061-10 wt.%B4 C-Mo S2 hybrid composites containing 2.5, 5, and 7.5 wt.% Mo S2 particles were fabricated through stir casting. The dry sliding tribological behaviors of the mono composite and hybrid composites were studied as a function of temperature on high temperature pin-on-disc tribotester against EN 31 counterface. The wear rate and friction coefficient of the Gr-reinforced and Mo S2-reinforced hybrid composites decreased in the temperature range of 30-100 ℃ due to the combined lubrication offered by the wear protective layer and its solid lubricant phase. Scanning electron microscopy(SEM) observation of the worn pin surface revealed severe adhesion, delamination, and abrasion wear mechanisms at temperatures of 150, 200, and 250 ℃, respectively. At 150 ℃, transmission electron microscopy(TEM) observation of the hybrid composites revealed the formation of deformation bands due to severe plastic deformation and fine crystalline structure due to dynamic recrystallization.

Reciprocating sliding tribology of brake oil treated carbon fiber reinforced ceramic matrix composites

The effect of brake oil on sliding behavior of carbon/carbon(C/C) and carbon/carbon-silicon carbide(C/C-SiC) composites was investigated with the variation of laminate orientation and surface conformity. The partial and low conformity contacts with the normal and parallel orientations of laminates were considered. The normal load was varied from 50 to 90 N in a step of 10 N. The friction and wear behavior was investigated under reciprocating sliding conditions. The results showed that friction coefficient and wear loss of composites with normal orientation of laminates were larger as compared to those of composites with parallel orientation of laminates. C/C composites with normal orientation of laminates yielded the highest value of friction coefficient. Wear loss decreased by a maximum of 78%, and friction coefficient decreased by a maximum of 49% in low conformity contacts as compared to partial conformity contacts. The presence of brake oil reduced the adhesion tendency of compacted wear debris because the formation of friction film was difficult, and thus, friction behavior was affected. The wear debris retention between the contact surfaces due to confined area motion in reciprocating sliding depicted the tribological behavior.

Mechanical properties and wear resistance of medium entropy Fe40Mn40Cr10Co10/TiC composites

The Fe40Mn40Cr10Co10/TiC (volume fraction of TiC, 10%) composites were synthesized in combination of ball milling and spark plasma sintering (SPS) in the present work. Mechanical properties and wear resistance of the Fe40Mn40Cr10Co10/TiC composites were individually investigated. It was found that TiC particles homogenously distributed in the Fe40Mn40Cr10Co10/TiC composite after being sintered at 1373 K for 15 min. Meanwhile, grain refinement was observed in the as-sintered composite. Compared with the pure Fe40Mn40Cr10Co10 medium entropy alloy (MEA) matrix grain, addition of 10% TiC particles resulted in an increase in the compressive strength from 1.571 to 2.174 GPa, and the hardness from HV 320 to HV 872. Wear resistance results demonstrated that the friction coefficient, wear depth and width of the composite decreased in comparison with the Fe40Mn40Cr10Co10 MEA matrix. Excellent mechanical properties and wear resistance could offer the Fe40Mn40Cr10Co10/TiC composite a very promising candidate for engineering applications.

An elastic mechanics model and computation method for geotechnical material

Internal friction characteristic is one of the basic properties of geotechnical materials and it exists in mechanical elements all the time. However,until now internal friction is only considered in limit analysis and plastic mechanics but not included in elastic theory for rocks and soils. We consider that internal friction exists in both elastic state and plastic state of geotechnical materials,so the mechanical unit of friction material is constituted. Based on study results of soil tests,the paper also proposes that cohesion takes effect first and internal friction works gradually with the increment of deformation. By assuming that the friction coefficient is proportional to the strain,the internal friction is computed. At last,by imitating the linear elastic mechanics,the nonlinear elastic mechanics model of friction material is established,where the shear modulus G is not a constant. The new model and the traditional elastic model are used simultaneously to analyze an elastic foundation. The results indicate that the displacements computed by the new model are less than those from the traditional method,which agrees with the fact and shows that the mechanical units of friction material are suitable for geotechnical material.

Tribological behavior and energy dissipation characteristics of nano-Al_2O_3-reinforced PTFE-PPS composites in sliding system

Nanoparticles are increasingly being used to improve the friction and wear performance of polymers. In this study, we investigated the tribological behavior and energy dissipation characteristics of nano-Al_2O_3-reinforced polytetrafluoroethylenepolyphenylene sulfide(PTFE-PPS) composites in a sliding system. The tribological behaviors of the composites were evaluated under different normal loads(100–300 N) at a high linear velocity(2 m/s) using a block-on-ring tester. Addition of the nano-Al_2O_3 filler improved the antiwear performance of the PTFE-PPS composites, and the friction coefficient increased slightly. The lowest wear rate was obtained when the nano-Al_2O_3 content was 3%(volume fraction). Further, the results indicated a linear correlation between wear and the amount of energy dissipated, even though the wear mechanism changed with the nano-Al_2O_3 content, independent of the normal load applied.

Effects of h-BN content on properties of Ni-Cr/h-BN composite

Ni-Cr/h-BN self-lubricating composities were prepared by powder metallurgy (P/M) method.The effects of hexagonal boron nitride (h-BN) content on the mechanical and tribological properties of the Ni-Cr/h-BN composites were investigated.The corresponding frictional models were established to analyze the formation of the lubricant h-BN films on the surfaces of the Ni-Cr/h-BN composites.The results show that,when the content of h-BN increases from 5% to 15% (mass fraction),the bending strength of the Ni-Cr/h-BN composite decreases from 96.670 MPa to 17.319 MPa,and the hardness (HB) decreases from 33 to 14.The friction coefficient of the Ni-Cr/h-BN composite decreases firstly from 0.385 to 0.216,and then increases to 0.284,while the wear rate decreases firstly from 4.14×10-9 kg/(N·m) to 1.35×10-9 kg/(N·m),then increases to 2.36×10-9 kg/(N·m).The best comprehensive mechanical and tribological properties can be obtained between 10% and 12% h-BN addition.

Tribology Performance and Adhesive Strength Evaluation of TiAlSiN Coating

Scratch test and friction test were performed to evaluate the internal and external interface behaviors of TiAISiN coating, respectively. The critical compressive and shearing stress of coating failure during scratch test were calculated and the values are 30.84 MPa and 4.98 MPa respectively. The average friction coefficients of TiAISiN coat- ing against 2Crl2Ni4Mo3VNbN steel are 0.70 （sliding speed 50 m/rain）, 0.63 （sliding speed 100 m/min）, and 0.81 （sliding speed 150 m/min）. The elements diffusion was analyzed by EDS. A1 and Si element of coating material dif- fuse to the steel disc, except Ti element. The oxidation decreases with the increase of sliding speed, but the adhesion increases with the increase of sliding speed. More A1 element diffuses to the steel disc at the high sliding speed, but the diffusion of Si element keeps almost constant at dlfferent sliding speeds.

Mechanical and tribological behaviors of graphene/Inconel 718 composites

Graphene/Inconel 718 composites were innovatively synthesized through selective laser melting,and the mechanical and tribological performances of the grapheme-reinforced Inconel 718 matrix composites were evaluated.The composite microstructures were characterized by XRD,SEM and Raman spectroscopy.The results show that selective laser melting is a viable method to fabricate Inconel 718 matrix composite and the addition of graphene nanoplatelets leads to a significant strengthening of Inconel 718 alloy,as well as the improvement of tribological performance.The yield strength and ultimate tensile strength of 1.0%graphene/Inconel 718 composites(mass fraction)are 42%and 53%higher than those of pure material,and the friction coefficient and wear rate are 22.4%and 66.8%lower than those of pure material.The decrease of fraction coefficient and wear rate is attributed to the improved hardness of composites and the formation of graphene nanoplatelet protective layer on the worn surfaces.