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.     

Microstructure and wear performance of Al5083/CeO2/SiC mono and hybrid surface composites fabricated by friction stir processing

Friction stir processing (FSP) was utilized to produce surface composites by incorporating nano-sized cerium oxide (CeO₂) and silicon carbide (SiC) particles individually and in combined form into the Al5083 alloy matrix. The study signified the role of these reinforcements on microstructure and wear behavior of the resultant surface composite layers. The wear characteristics of the resultant mono and hybrid surface composite layers were investigated using a pin-on-disc wear tester at room temperature. The microstructural observations of FSPed regions and the worn out surfaces were performed by optical and scanning electron microscopy. Considerable grain refinement and uniform distribution of reinforcement particles were achieved inside the nugget zone. All the composite samples showed higher hardness and wear resistance compared to the base metal. Among the composite samples, the hybrid composite (Al5083/CeO₂/SiC) revealed the highest wear resistance and the lowest friction coefficient, whereas the Al5083/SiC composite exhibited the highest hardness, i.e., 1.5 times as hard as that of the Al5083 base metal. The enhancement in wear behavior of the hybrid composites was attributed to the solid lubrication effect provided by CeO₂ particles. The predominant wear mechanism was identified as severe adhesive in non-composite samples, which changed to abrasive wear and delamination in the presence of reinforcing particles.     

Effect of Fe-impurity on tribological properties of Al-15Mg2Si composite

The effect of Fe-impurity (0.2%–2%, mass fraction) on the microstructure, dry sliding wear, and friction properties of Al-15Mg₂Si 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 Mg₂Si particles from irregular dendritic form to smaller particles with polyhedral shapes, refined the pseudo-eutectic structure, and led to the formation of hard β-Al₅FeSi 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 β-phase on the tribological properties. It was found that β-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.     

Microstructure and Dry Sliding Wear Behavior of Mg-Y-Zn Alloy Modified by Laser Surface Melting

Mg-11Y-2.5Zn alloy was surface-melted using a 6.0 kW continuous wave CO₂ laser as a heat-generating source. X-ray diffractometer, laser optical microscopy, and Vickers hardness indentation were used to characterize the microstructure and hardness of the Mg-11Y-2.5Zn alloy. The results show that the microstructure in the laser-melted zone can be greatly refined and hardness is slightly improved. Dry sliding tests were performed on the as cast and laser surface-melted Mg-11Y-2.5Zn alloys using a pin-on-disk configuration. Coefficients of friction and wear rates were measured within a load range of 20-320 N at a sliding velocity of 0.785 m/s. Laser surface-melted Mg-11Y-2.5Zn alloy exhibited good wear resistance when compared with the as cast one under given applied load conditions, which has been explained by refining of the microstructure in the melted zone. Morphologies of worn surface on the as cast and laser surface-melted Mg-11Y-2.5Zn alloys were examined using scanning electron microscopy. Four wear mechanisms, namely abrasion, delamination, thermal softening, and melting, have operated.     

Effect of multi-pass multi-directional forging on tribological properties of Si-rich eutectoid ZA alloys

The effect of multi-pass multi-directional forge (MDF) on the tribological properties of ZA22−xSi alloy (x=0, 4, 8 wt.%) was investigated. The results indicate that MDF breaks down the cast microstructure of alloys and produces a well-modified microstructure comprising finely distributed α- and η-phases and primary Si particles. It is also found that, despite the matrix work softening, MDFed ZA22−xSi alloys show high wear resistance. The maximum wear resistance is observed in the five-pass MDFed ZA22−4Si sample, at the applied loads of 10 and 30 N, and its wear rates are lower than the wear rate of the as-cast ZA22 alloy by about 80% and 75%, respectively. MDF also significantly decreases both average friction coefficient and friction coefficient fluctuation of the sample. The high resistance of the substrate to microcracking, formation of hard Si reinforcements, fine redistribution of α- and η-phases in the microstructure, and formation of tribolayers rich in Al and Zn oxides can be considered as the main factors improving the tribological properties.     

Effect of arc re-melting on microstructure,mechanical and tribological properties of commercial 390A alloy

To investigate the effect of the arc re-melting on the microstructure, mechanical and tribological properties of the 390A alloy, its ingot produced by the conventional induction melting method was subjected to the arc re-melting process. The microstructure of the 390A alloy was examined by OM and SEM. Mechanical properties of the 390A alloy were determined by the Brinell method and tensile tests. Tribological properties were investigated with a ball-on-disc type tester. It was observed that the microstructure of both conventional induction melted and arc re-melted 390A alloys consisted of α(Al), eutectic Al–12Si, primary silicon particles, θ-CuAl₂, β-Al₅FeSi, δ-Al₄FeSi₂, and α-Al₁₅(FeMnCu)₃Si₂ phases. Re-melting with the arc process caused grain refinement in these phases. In addition, after this process, the α(Al) phase and primary silicon particles were dispersed more uniformly, and sharp edges of primary silicon particles became round. The arc re-melting process resulted in an increase in the hardness of the 390A alloy produced by the conventional method from 102 HB to 118 HB and the tensile strength from 130 to 240 MPa. It also caused an increase in the wear resistance of the 390A alloy and a decrease in the friction coefficient.     

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

Silver based composites containing different amounts of WS₂ were prepared by hot-pressing method and their tribological behaviors were investigated against coin silver under humid air, dry nitrogen and vacuum on a ball-on-disk tester with normal load of 5 N. 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 friction coefficient is the highest in humid air, and the lowest in dry nitrogen. It is found that the friction and wear behavior of the composites are strongly depended on the characteristics of the lubrication film forming in different operating environments, such as thickness and composition. In addition, it is indicated that the dominant wear mechanisms of silver based composites are abrasive wear and delamination under different conditions.     

Tribological behaviors of Zr-based bulk metallic glass versus Zr-based bulk metallic glass under relative heavy loads

As a novel engineering material, bulk metallic glass (BMG) has received much attention. However, the knowledge concerning the tribological behavior of BMG versus BMG under relatively heavy loads is still insufficient. In this study, Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 metallic glass pins and discs were prepared by copper-mold suction casting. The dry sliding friction and the wear characteristics of the as-cast Zr-based BMG versus Zr-based BMG were tested under loads of 100, 125 and 150 N, respectively, using a pin-on-disc tribological apparatus at room temperature. The worn surfaces were studied by scanning electron microscopy in order to identify the wear mechanisms. The results showed that both the coefficient of friction and the wear rate increased with both the normal load and the rotational sliding velocity. X-ray diffraction patterns recorded after the tribological experiments indicated that no sliding-induced crystallization occurred. Transmission electron microscopy was also used to confirm the amorphous of the BMGs after sliding tests. In addition, the wear mechanisms changed with the experimental conditions. For a normal load of 100 N, the main mechanisms were abrasive wear, slight grooves and micro-cracks. For higher loads, adhesive wear was predominant, accompanied by abrasive wear and deeper grooves and more micro-cracks. When the rotational sliding velocity was increased, the dominant wear changed from slight grooves to viscous flow and adhesive wear.     

Wear resistance of NiTi alloy after surface mechanical attrition treatment

An ultrafine grain layer consisting of nanocrystallites as well as submicrometer grains is produced on NiTi shape memory alloy by surface mechanical attrition treatment (SMAT) and the effects of the ultrafine grain layer on the tribological properties are investigated under dry sliding conditions. Compared to the coarse grain (CG) NiTi, the SMAT NiTi has smaller friction coefficients and improved wear resistance at applied loads from 5 to 15 N due to the grain refinement effect. Examination of the worn surfaces indicates that materials delamination and particles co-exist on both the CG and SMAT NiTi samples. Our results indicate that delamination is the main wear mechanism on CG NiTi whereas abrasive particles dominate the wear process on SMAT NiTi.     

Surface modification of Al-Pb alloy by high current pulsed electron beam

Al-Pb alloy was modified by high current pulsed electron beam and the microstructure, hardness and tribological characteristics were characterized by scanning electron microscopy, electronic microanalysis probe microanalysis, Knoop hardness indentation and pin-on-disc type wear testing machine. The results show that the microstructure and hardness can be greatly improved, and the modification layer consists of a molten zone, an overlapped zone of heat-affected and quasistatic thermal stress-affected zone and a transition zone followed by the substrate. The tribological properties of high current pulsed electron beam irradiated Al-Pb alloy are correspondingly improved largely. Optical observation and scanning electron microscopy analysis reveal that the low wear rate and lowest level in coefficient of friction at high load level for irradiated Al-Pb alloy are due to the formation of a lubricious tribolayer covering the worn surface, which is a mixture of Al2O3, Pb3O4 and silicate. The wear mode varies from oxidative wear at low load to film spalling at high load and, finally, adhesive wear.     

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

The AA6061−10wt.%B₄C mono composite, AA6061−10wt.%B₄C−Gr (Gr: graphite) hybrid composites containing 2.5, 5, and 7.5 wt.% Gr particles, and AA6061−10wt.%B₄C−MoS₂ hybrid composites containing 2.5, 5, and 7.5 wt.% MoS₂ 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 MoS₂-reinforced hybrid composites decreased in the temperature range of 30−100 °C 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 °C, respectively. At 150 °C, 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.     

Wear Performance of A356 Matrix Composites Reinforced with Different Types of Reinforcing Particles

To improve the wear resistance of Al-Si alloys, different types of reinforcing particles such as SiC, TiC, ZrO₂, and B₄C were used to produce matrix composites by friction stir processing (FSP). First, microstructural properties of different locations of stir zone (SZ) in the FSPed specimens such as advancing side, retreating side, shoulder-affected area, and pin-affected area were investigated. The results demonstrate that Si particles size is not the same in different SZ subdomains. SEM investigation was performed in order to investigate the particles distribution in different areas of the SZ as well as bonding quality between particles and metal matrix. Hardness and wear tests were carried out to determine mechanical and wear properties of the composites. The pin-on-disk wear tests were performed at room temperature, with the normal applied loads of 5, 10, and 20 N and sliding speed of 1 and 2 m/s. All fabricated composites show higher resistance in wear than A356 alloy. Wear test results show, by increasing the normal load and sliding velocity, the wear loss weight of all composites increased gradually.     

Effects of Mg content on microstructure and mechanical properties of low Zn-containing Al−xMg−3Zn−1Cu cast alloys

Effects of Mg content on the microstructure and mechanical properties of low Zn-containing Al?xMg? 3Zn?1Cu cast alloys (x=3?5, wt.%) were investigated. As Mg content increased in the as-cast alloys, the grains were refined due to enhanced growth restriction, and the formation of η-Mg(AlZnCu)₂ and S-Al₂CuMg phases was inhibited while the formation of T-Mg₃₂(AlZnCu)₄₉ phase was promoted when Mg content exceeded 4 wt.%. The increase of Mg content encumbered the solution kinetics by increasing the size of eutectic phase but accelerated and enhanced the age-hardening through expediting precipitation kinetics and elevating the number density of the precipitates. As Mg content increased, the yield strength and tensile strength of the as-cast, solution-treated and peak-aged alloys were severally improved, while the elongation of the alloys decreased. The tensile strength and elongation of the peak-aged Al?5Mg?3Zn?1Cu alloy exceed 500 MPa and 5%, respectively. Precipitation strengthening implemented by T′ precipitates is the predominant strengthening mechanism in the peak-aged alloys and is enhanced by increasing Mg content.     

Sliding Wear Performance of Plasma-Sprayed Al2O3-Cr2O3 Composite Coatings Against Graphite under Severe Conditions

Al₂O₃, Cr₂O₃, and Al₂O₃-Cr₂O₃ composite coatings were produced by plasma spraying. Their tribological properties were evaluated at high load conditions. The average friction coefficients, wear rates, and worn surface temperatures of the coating/graphite pairs were measured. Compared with the single coating/graphite pairs, the friction coefficients of composite coating/graphite pairs are more stable. The corresponding wear rates and worn surface temperatures are lower, which may be conducive to the formation of more effective and stable graphite transfer film on the surface of the coating subjected to abrasion. Especially, 10wt.%Al₂O₃-90wt.%Cr₂O₃ (AC₉₀) composite coating shows better anti-wear performance, which may be attributed to its higher thermal conduction.     

Enhanced wear resistivity of a Zr-based bulk metallic glass processed by high-pressure torsion under reciprocating dry conditions

Wear properties of bulk metallic glasses (BMGs) are important for industrial applications as much as strength and ductility. Free volume of BMGs is a significant factor which decides wear mechanism and resistance. Increased free volume of a Zr₅₅Al₁₀Ni₅Cu₃₀ BMG processed by high-pressure torsion (HPT) affected wear resistance under dry reciprocating conditions. Two- and three-body abrasive wear as well as the delamination of oxide layers simultaneously operated during the wear tests of both as-cast and HPT-processed BMG (HPT-BMG). However, the HPT- BMG had a larger area of the oxide layers on a worn surface compared to the as-cast BMG at the early stage of the wear tests. The increased free volume by the HPT process resulted in ductile plastic deformation, prohibited crack propagation, and delayed delamination of the oxide layers. Therefore, the HPT-BMG had thicker oxide layers, which acted as an adequate protection and increased wear properties of the Zr-based BMG.     

Effects of SiC Particle Size and Process Parameters on the Microstructure and Hardness of AZ91/SiC Composite Layer Fabricated by FSP

In this study, friction stir processing (FSP) was employed to develop a composite layer on the surface of as-cast AZ91 magnesium alloy using SiC particles (5 μm and 30 nm). The effects of the rotational and traverse speeds and the FSP pass number on the microstructure and microhardness of the friction stir processed (FSPed) layer with and without SiC particles were investigated. Optical microscopy and scanning electron microscopy (SEM) were employed for microstructural analysis. FSP produces a homogeneous microstructure by eliminating the precipitates near the grain boundaries. The analyses showed that the effects of the rotational and traverse speeds on the microstructure of specimens produced without nano-sized SiC particles are considerable; however, they are negligible in the specimens with particles. While the second FSP pass enhances the microstructure and microhardness of the samples with SiC particles, it has no significant effect on such properties in the samples without SiC particles.     

Dry Sliding Tribological Studies of AA6061-B<Subscript>4</Subscript>C-Gr Hybrid Composites

The dry sliding behavior of stir-cast AA6061-10 wt.% B₄C composites containing 2.5, 5 and 7.5 wt.% graphite particles was studied as a function of applied load, sliding speed and sliding distance on a pin-on-disk tribotester. The wear rate and friction coefficient increased with increase in applied load and sliding distance. The increase in graphite addition reduced the increase in wear rate and friction coefficient in the sliding speed range 2-2.5 m/s. Scanning electron microscopy of the worn pin revealed a graphite tribolayer, and transmission electron microscopy revealed overlapping deformation bands under 30 N applied load. Upon increasing the applied load to 40 N, welded region with fine crystalline structure was formed due to dynamic recrystallization of AA6061 alloy matrix.     

石墨/CaF2/TiC协同改善镍基合金喷涂层的摩擦磨损行为与机理（英文）

为了降低机械零件在强烈摩擦磨损条件下的摩擦因数,提高其耐磨性,制备了等离子喷涂石墨/CaF2/TiC/镍基合金复合涂层,研究其摩擦学行为及机理。结果表明,石墨/CaF2/TiC/镍基合金复合涂层的摩擦因数为0.22~0.288,较纯镍基合金涂层的降低了25.9%~53%,磨损率较之降低18.6%~70.1%。与GCr15钢球对摩时,复合涂层的磨损表面逐渐形成了由铁氧化物、石墨和CaF2组成的转移层,使GCr15钢球与复合涂层的摩擦转变为钢球与转移层的摩擦。由于转移层起到固体润滑作用,复合涂层的摩擦因数和磨损率大幅度降低。复合涂层的主要磨损机理是转移层在载荷的反复作用下而产生的层脱剥落。