Mechanical properties of the alloy Ti-6Al-4V irradiated with swift Kr ion

The radiation damage effect on the friction coefficient, wear, and microhardness of the alloy Ti-6Al-4V after 250 MeV krypton ion irradiation was studied. Tribological measurements were made in air, oxygen, and argon atmospheres and in a vacuum. The smallest friction coefficient for the irradiated samples occurred in argon and the vacuum. The wear of the unirradiated samples and those irradiated with low fluences (<10¹³ cm⁻²) increased in the vacuum and argon atmosphere. Wear was significantly reduced after irradiation with the fluence of 10¹⁴ cm⁻². The microhardness of the alloy Ti-6Al-4V increased by over 25% after irradiation with a large fluence.     

Effect of Hot Extrusion on Electrical Tribological Behavior of Ag-Cu/MoS2 Composite under Air and Vacuum Conditions

The different microstructures of silver–copper/molybdenum disulfide (Ag-Cu/MoS₂) composites were manufactured by hot press and hot extrusion processes to investigate the electrical tribological behaviors of both the hot-pressed and hot-extruded composites under air and vacuum. The results showed that microstructures and properties of Ag-Cu/MoS₂ composites were improved by hot extrusion, which decreased the wear rates rapidly in both air and vacuum. In air, hot extrusion could improve the transfer layer and tribofilm, resulting in a significant decrease in contact voltage drop, which goes from more than 70 mV in the hot-pressed composite to 30 mV at the hot-extruded composite. Under vacuum condition, some wear debris was melted on the worn surface and then transferred to the counterface to form the transfer layers, which led to the lower contact voltage drops under vacuum, about 6 mV in hot-pressed composites and 3 mV in hot-extruded composites. In addition, the severe adhesive and abrasive wear were attributed to the molten wear debris and transfer layer, resulting in a dramatic fluctuation in the friction coefficient in a vacuum.     

Sliding wear-induced microstructure evolution of nanocrystalline and coarse-grained AZ91D Mg alloy

Wear behavior of nanocrystalline (NC) AZ91D Mg alloy generated by surface mechanical attrition treatment and the corresponding coarse-grained (CG) samples subjected to dry sliding wear at different sliding velocities were investigated. The subsurface microstructure evolution of these two samples was analyzed by cross-sectional transmission electron microscope. The microstructure of NC layer was almost unchanged while grain refinement into nanometer scale was identified for CG sample at high velocity, which contributed to the wear behavior of both NC and CG samples.     

Comparisons of dry sliding tribological behaviors between coarse-grained and nanocrystalline copper

Dry sliding tribological behaviors of nanocrystalline (NC) and coarse grained (CG) Cu were studied by using a ball-on-plate tribometer with a counterface ball of cemented tungsten carbide. The results showed that prior to oxidation and delamination, the steady-state friction coefficients (FCs) of NC and CG Cu are comparable (～0.35). As oxidation with delamination of wear surface occur, the FC for either CG or NC Cu increases gradually, approaching a steady-state FC (～0.63). The wear resistance of the NC Cu was enhanced by at least one order of magnitude under the measured loads ranging from 5 N to 25 N in comparison with the CG counterpart, which is mainly attributed to the higher hardness of the NC layer.     

Formation of Friction Layers in Graphene-Reinforced TiAl Matrix Self-Lubricating Composites

The friction layer structure has been proved to be formed during severe plastic deformation and markedly improves the tribological properties of material. The dry friction and wear performance of graphene-reinforced TiAl matrix self-lubricating composites (GTMSC) at different sliding velocities are systematically researched. GTMSC show the best tribological properties and special friction layer structure containing a wear-induced layer and a grain refinement layer with a nanocrystalline (NC) structure under surface after sliding at a sliding speed of 1.1 m/s. Nanoindentation results show that the grain refinement layer has a higher hardness and elastic modulus than the wear-induced layer. This special microstructure of friction layers beneath the surface after sliding leads to a low coefficient of friction and high wear resistance of GTMSC. Moreover, it is deduced that the appearance of an NC structure results in hardening of the material. The formation mechanisms of friction layers are researched in detail. It can be concluded that the formation of a wear-induced layer results from frictional heat and fracture of the counterpart. The formation of a grain refinement layer is due to severe plastic deformation and dynamic recrystallization. Severe plastic deformation results in the formation of an NC structure and dynamic recrystallization leads to grain refinement.     

In Vacuo Tribological Behavior of Polytetrafluoroethylene (PTFE) and Alumina Nanocomposites: The Importance of Water for Ultralow Wear

Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many practical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of magnitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 × 10^?6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1–10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 × 10^?7 mm³/(Nm).     

Tribological Properties of Borided AISI 4140 Steel with the Powder Pack-Boriding Method

The present study evaluates the tribological properties of boride layers on the surface of AISI 4140 steel, formed using the pack-boriding method. Commercial EKabor^®2 was used as the boronizing agent and the treatment was carried out at 900, 950, 1000, and 1050 °C for 2, 4, and 6 h, respectively. X-ray diffraction (XRD), scanning electron microscopy (SEM), and microhardness tests were used to characterize the phase composition, microstructure, and local hardness, respectively, of the borided steel samples. Block-on-disc tests were used to investigate tribological properties. Abrasive wear tests were carried out using emery paper at a fixed sliding speed and three different loads. Adhesive wear tests were executed against AISI 52100 steel at a fixed load and distance. The coefficient of friction values (COF) of the samples were determined simultaneously during the tests. The weight loss and COF of the borided samples were compared with untreated samples and the results suggest that both wear resistance and friction properties of the AISI 4140 steel improve with boriding.     

Effect of Load and Composition on Friction and Dry Sliding Wear Behavior of Tungsten Carbide Particle-Reinforced Iron Composites

Investigations on the dry sliding wear behavior of tungsten carbide (WC)-reinforced iron matrix composites were carried out at room temperature. Three sets of samples (unreinforced iron, 4 wt% micrometer-size (～5–15 μm) WC-reinforced iron and 4 wt% nanosize (～30 nm) WC-reinforced iron were prepared using a powder metallurgy route to assess their friction and wear behaviors under two different loads. The relative dry sliding wear performances of the micrometer-size and nanosize WC-reinforced composites were compared with unreinforced matrix. An increase in microhardness of the order of 2.5 times was observed in the case of 4 wt% nanosize WC-reinforced iron matrix compared to the unreinforced iron matrix. The wear rate was 1.35 to 1.45 times lower in the case of nanocomposites compared to the unreinforced iron matrix (under different experimental conditions). The values of the coefficient of friction (COF) of composites were found to decrease with increase in load. Nanocomposites showed lower COF, surface roughness, and fractal dimension (D) values than micrometer-size WC-reinforced composites and the unreinforced iron matrix.     

Sliding behavior of dual phase steels in vacuum and in air

While the importance of fatigue failure is well established in rolling systems, the evidence for a correlation of fatigue with sliding wear is less convincing. McEvily has reported striking differences in the fatigue properties of two dual phase steels, one containing martensite islands in a ferrite matrix (type A) and one containing ferrite islands in a martensite matrix (type B). We have examined the sliding behavior of these same materials in vacuum and in air for various sliding distances. The pin-on-disk tests included self-mated pairs and tests with molybdenum as a counterface. Post-test analysis included the use of microhardness testing, scanning electron microscopy (including local chemical analysis) and X-ray diffraction analysis of the wear surfaces, sample cross-sections and wear debris. Transitions from low to high friction were associated with transfer processes and mechanical mixing. The wear rates of the type A and B samples were similar in vacuum. However, in air, the wear rates of the type B samples were increased by about 100 times. The lack of correlation with fatigue properties is not yet understood. Any of the following could be responsible: short crack (initiation) phenomena; large local stress intensities; surface material associated with transfer.     

Effect of electrical current on the tribological behavior of the Cu-WS2-G composites in air and vacuum

As the traditional graphite-based composites cannot meet the requirement of rapid developing modern industry, novel sliding electrical contact materials with high self-lubricating performance in multiple environments are eagerly required. Herein a copper-based composite with WS2 and graphite as solid lubricant are fabricated by powder metallurgy hot-pressed method. The friction and wear behaviors of the composites with and without current are investigated under the condition with sliding velocity of 10 m/s and normal load of 2.5N/cm 2 in both air and vacuum. Morphologies of the worn surfaces are observed by optical microscope and compositions of the lubricating films are analyzed by XPS. Surface profile curves and roughness of the worn surfaces are obtained by 2205 surface profiler. The results of wear tests show that the friction coefficient and wear volume loss of the composites with current are greater than that without current in both air and vacuum due to the adverse effects of electrical current which damaged the lubricating film partially and roughed the worn surfaces. XPS results demonstrate that the lubricating film formed in air is composed of oxides of Cu, WS2 , elemental S and graphite, while the lubricating film formed in vacuum is composed of Cu, WS2 and graphite. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composites show low friction coefficient and wear volume loss in air condition. Owing to the fact that graphite loses its lubricity which makes WS2 become the only lubricant, severe adhesive and abrasive wear occur and result in a high value of wear rate in vacuum condition. The formation of the lubricating film on the contact interface between the brush and ring is one of the factors which can greatly affect the wear performance of the brushes. The low contact voltage drop of the composites in vacuum condition is attributed to the high content of Cu in the surface film. This study fabricated a kind of new sliding electrical contact self-lubricating composite with dual-lubricant which can work well in both air and vacuum environments and provides a comprehensive analysis on the lubrication mechanisms of the composite.     

Friction and Wear Behavior of Irradiated Polyethylene Sliding Against a Rough Steel Surface

The aim of this study was to evaluate the tribological behavior of polyethylene crosslinked by gamma radiation sliding against a steel surface. Two high-density polyethylenes were irradiated to a total dose in the range of 2?20 Mrad under vacuum and at room temperature. After irradiation, the materials were annealed at 423 K and then cooled slowly to room temperature. The same thermal treatment was applied to the non-irradiated polymer. The wear behavior of the polymers was determined under controlled ambient temperature of 298 and 333 K using a homemade tribometer. Sheet-shaped specimens were loaded against the surface of a steel disc with different normal loads to generate nominal contact pressures in the range of 0.25–1.5 MPa. The tests were performed under dry conditions using a disc rotation to produce an average sliding speed of 0.6 m/s and during a period of time to provide an average sliding distance of 1,080 m. The wear rate was obtained as the mass loss by the sample divided by the sliding distance, and the friction coefficient was determined by measuring the friction force. The results indicate that the wear rate increases with load in the case of non-irradiated polyethylene and low-dose irradiated polymers, while the wear rate reaches a maximum value with the load in the case of the irradiated samples with high doses. The samples irradiated with a dose of 10 Mrad showed the lowest wear. The coefficient of friction (COF) increases slightly with the load in all the cases. Most irradiated polymers show higher COF than the non-irradiated material when compared at a given load. The results show that the irradiation dose applied to the polyethylenes produced no noticeable effect on the COF values when a comparison was made at a given applied load.     

Tribological characterization of electroless Ni–10% P coatings at elevated test temperature under dry conditions

An experimental study of wear characteristics of electroless Ni–10% P coating sliding against hard AISI 52100 steel pin is investigated. Experiments are carried out at room and 550°C temperatures. Heat treatment effects on tribological behavior of this coating are studied. The wear surface and the microstructure of the coatings are analyzed using optical microscopy, scanning electron microscopy, energy dispersion analysis X-ray, and microhardness testing equipment. It is observed that the forming of continuous oxide film on contacting surfaces of pin and disk improves wear resistance and decreases friction coefficient of the Ni–10% P coating. The results indicate that the wear resistance of electroless Ni–10% P coating has improved with heat treatment at room temperature wear test, but it reverses in the wear test at 550°C. In addition, specimens without heat treatment have the highest wear resistance and the lowest friction for wear tests at elevated temperatures.     

沉积气压对W-S-C复合薄膜结构和摩擦学性能的影响

采用磁控溅射方法制备W-S-C复合薄膜,研究沉积气压对薄膜结构和摩擦学性能的影响。结果表明,复合膜以非晶或纳米晶结构生长,沉积气压低时薄膜中C含量高,薄膜结构致密;沉积气压高时薄膜中WSx含量高,薄膜致密性下降。复合膜硬度在HV420～500之间,并且随着沉积气压的增加,硬度逐渐下降。在潮湿大气中的摩擦磨损实验表明,实验载荷越大摩擦因数越小;随着沉积气压的增加,复合膜的摩擦因数先降低后增加;当沉积气压在0.45～0.55 Pa时,复合膜的摩擦因数最低约为0.1,耐磨性能最好。     

Significant improvement of wear properties by creating micro/nano dual-scale structure in Al–Sn alloys

We present in this work evidence that significant improvement of wear behavior can be achieved by creating a dual-scale structure. An Al–12 wt%Sn bearing alloy consisting of mixtures of nanocrystalline (NC) Al–Sn powder and coarse-grained (CG) Al–Sn powder was produced by a combination of mechanical alloying and conventional powder sintering. The extent of the improvement in wear properties was related to the ratio of NC/UFG to CG, and the best properties were achieved at a ratio of 30 wt% CG. With this ratio, this dual-scale Al–Sn (CG-30) alloy had a wear resistance about 1.5 times greater than that of the monolithic NC/UFC alloy prepared by mechanical alloying, about twice that of the monolithic CG alloy. The friction coefficient decreased by nearly 13% of the CG alloy. The detailed wear mechanism investigation revealed that an optimized combination of hardness and toughness is a key for the improvement of wear properties in the dual-scale material.     

Mo离子注入提高TC4合金微动磨损抗力的研究

对TC4合金进行了Mo离子注入表面改性处理,利用摩擦磨损试验机进行了点接触微动磨损试验,借助读数显微镜和表面粗糙度仪测量出有关参数,计算出试样的微动磨损体积。结果表明,Mo离子注入使试样表面硬度提高,微动磨损体积明显降低。在微动磨损初期,Mo离子注入具有较好的减摩效果。Mo离子注入带来的表面强化效应是基体合金的微动磨损抗力得以提高的主要原因。     

Tribological Properties of Double-Glow Plasma Surface Niobizing on Low-Carbon Steel

The niobized layer was formed on Q235 low-carbon steel by double-glow plasma surface niobizing to improve its wear resistance. The microstructure, phase composition, and microhardness were determined. The friction and wear properties of the niobized samples and the untreated alloys were tested on a ball-on-disk tribometer by rubbing against GCr15 and silicon nitride (Si₃N₄) balls at room temperature and 400°C, respectively. The results indicated that the alloyed layer that contained a sediment layer and diffusion layer is about 35 μm in thickness, metallurgically adhered to the base metal. Niobium content was gradually decreased along the depth direction from the surface, which was similar to the change in the microhardness. The alloying layer mainly consisted of Nb, Fe₂Nb, and FeNb phases. Under unlubricated sliding conditions, the friction coefficients and the specific wear rates were lower than those of the untreated carbon steel at room and high temperatures. The wear mechanism of the niobized specimen at room temperature is dominated by slightly abrasive wear, whereas the predominant wear mechanism is abrasive wear and fatigue delamination at high temperature.     

纯钛表面等离子渗镍合金层的显微组织及摩擦学性能

采用等离子表面合金化技术对纯钛进行表面渗镍处理,对渗镍后形成的合金层显微组织、物相组成及硬度进行了分析,并对其摩擦学性能进行了研究。结果表明:渗镍后在纯钛表面形成了与基体结合良好的钛-镍合金层,合金层主要由Ti2Ni、TiNi及钛相组成,厚约30μm,显微硬度约为570HV,合金层的干滑动摩擦磨损性能较纯钛基体的有很大提高。     

Tribological Behavior of NiAl–1.5 wt% Graphene Composite Under Different Velocities

The progress in aerospace field requires a new NiAl matrix composite that can stand against wear and decrease the energy dissipation through decreasing friction. In this study, the tribological behavior of NiAl–1.5 wt% graphene composite is investigated at room temperature under a constant load of 12 N and different sliding velocities. The results show that the friction coefficient and wear rate increase with increasing sliding velocity from 0.2 to 0.4 m/s due to the adhesion between the sliding bodies and tearing of the graphene layer. The friction coefficient and wear rate tend to decrease at a sliding velocity of 0.6 m/s as a result of severe plastic deformation and grain refinement of the worn surface. However, at 0.8 m/s the friction coefficient reaches a minimum value and the wear rate increases and changes the wear mechanism to fatigue wear. It can be concluded that various wear mechanisms lead to different tribological performance of NiAl–1.5 wt% graphene composite.     

Fretting wear behavior of nanocrystalline surface layer of copper under dry condition

Unlubricated fretting tests were performed with a nanocrystalline surface layer of a 99.99 wt.% copper fabricated by means of surface mechanical attrition treatment (SMAT), in comparison with a coarse-grained (CG) copper. The measured friction and wear data show that the fretting wear resistance is markedly enhanced with the nanocrystalline surface layer relative to the CG counterpart. The friction coefficient and wear volume of the SMAT Cu are lower than that of the CG Cu. For both samples, the friction coefficients and wear volumes increase with an increasing applied load and fretting frequency. A rapid increase of the friction coefficient and wear volume under an applied load above a critical value (30 N for the SMAT Cu and 20 N for the CG Cu) is noticed, corresponding to the formation of a continuous oxide layer between two contact surfaces. Also two sharp increases of the friction coefficient and wear volume at fretting frequencies of 50 Hz and 175 Hz were observed for the SMAT and the CG Cu. The former is correlated with the formation of a continuous oxide layer, while the latter corresponds to wearing away of the oxide layer.     

单相硼化物的制备及摩擦磨损性能研究

为了深入研究渗硼层中硼化物的性能,采用真空感应熔炼法制备单相硼化物材料。观察分析制备的硼化物微观组织,测试其力学性能。采用MMU-5G型销-盘式摩擦磨损试验机,在干摩擦条件下,研究了不同载荷下单相硼化物的摩擦学性能,观察其磨损表面形貌特征,探讨其磨损方式。结果表明：制备的硼化物为单一相,试样纯度高,试样的平均显微硬度为HV2065,平均断裂韧性值为1.68 MPa·m1/2;硼化物的断口处没有宏观的塑性变形,断口齐平光亮,表现为脆性断裂特征;干摩擦条件下随着载荷从10 N增加到30 N,硼化物的摩擦因数先降低后增加,20 N载荷时达到最小值,而其磨损量随着载荷的增加不断上升;随着载荷从10 N增加到30 N,磨损表面的粗糙度先逐渐上升后急剧上升; 10~20 N载荷下,硼化物的磨损以磨粒磨损为主,而25~30 N载荷下,硼化物的主要磨损方式从磨粒磨损转变为脆性破损。