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Boronizing treatment has been considered as a promising strategy to enhance the surface hardness, self-lubrication and wear resistance of high entropy alloy (HEA), endowing the advancements of HEA in engineering applications. However, the anti-friction, anti-wear and corrosion resistance properties of the boronized HEA in sulfuric acid and seawater environments remain unclear. Here, the self-lubrication and corrosion resistance of the boronized CoCrFeNiMn HEA in these corrosive media were studied to assess the applicable environments. Firstly, a boronized layer with thickness of approximately 25 μm on the CoCrFeNiMn HEA surface was fabricated using the powder-pack boronizing treatment. Tribological assessments demonstrated exceptional self-lubricating properties of boronized HEA in various liquid conditions, with over 30 % reduced friction coefficients and over 80 % decreased wear rates. XPS analysis confirmed the formation of a boron-containing tribofilm, enhancing self-lubrication and reducing friction and wear in liquid environments. However, electrochemical tests indicated that the corrosion resistance of the boronized HEA was significantly affected by the liquid medium. The impedance value (|Z|) of boronized HEA decreased to 4.6 % of that of the original HEA in 3.5 wt% NaCl solution, revealing poor corrosion resistance. Conversely, an enhancement of corrosion resistance was observed in 0.05 M H 2 SO 4 solutions with the increase of the |Z| value by 8 times. Therefore, for applications requiring service safety, boronizing HEAs are more suitable for acidic environments, but not in seawater environments, offering valuable insights for the industrial application of CoCrFeNiMn HEA in reducing friction and wear. • The COF of boronized HEA is over 30 % lower than original HEA in liquid conditions. • The tribofilm (composed of H 3 BO 3 and B 2 O 3) significantly reduce friction and wear. • Boronized HEAs are more suitable for acidic environments, but not in seawater. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide (MoS) nanosheets are a promising lubricant additive for enhanced engine efficiency in cars. However, high-cost production methods and poor dispersion have limited their application in industry. In this study, the ball milling process is demonstrated as a low-cost and high-efficient method for fabrication of oil-dispersible MoS nanosheet, and the ball milling parameters are optimized. Moreover, the lubrication effectiveness of ball-milled MoS nanosheet was also evaluated. Results indicated that well-dispersed MoS nanosheets with a size of 250 nm can be manufactured with optimized surfactants of zinc dialkyldithiphosphates (ZDDP) and polyisobutylene succinimide (PIBS) after being ball milled for 36 h. Tribological results revealed that a friction coefficient of white oil with 0.25% MoS nanosheets reached 0.075, much lower than that of lubricant without nanosheets (0.16). The wear scar radius of 0.015% MoS nanosheets was similar with that of Hertz contact, and the wear scar radius reduction reached 20% compared with that of 1% ZDDP. In addition, EDS and XPS results indicated the formation of a MoS and FeS tribofilm on the wear surface. [ABSTRACT FROM AUTHOR]

The CoCrNi medium-entropy alloy (MEA) exhibits an impressive ductility, but its low hardness (∼352 HV) and poor wear resistance limit its engineering applications. In this study, a significant increase of surface hardness and enhancement of self-lubrication were achieved for the CoCrNi MEA using powder-pack boronizing. Results indicate that dual reaction layers with a total thickness of about 50 μm were formed on the surface. These two strengthened layers are mainly consisted of silicide (Ni 2 Si) and borides (CoB, Cr 5 B 3). The hardness of the affected region reaches to 11–21 GPa dependence on the cross-sectional depth, being about 6 times higher than that of untreated MEA. In the deionized water condition, the friction and wear rate reductions of the boronizing treated specimen reach to about 25% and 63%, respectively. Energy Dispersive Spectrometer (EDS) and X-ray Photoelectron Spectroscopy (XPS) analysis on the wear scars showed that a silicon/boron-containing tribofilm was formed under the water environment, which reduces the friction coefficient and enhances the wear resistance. Our study demonstrates that powder-pack boronizing is a powerful way to enhance surface hardness and self-lubrication properties of CoCrNi MEA. • Double-strengthened layers were observed on the boronized CoCrNi alloy surface. • An obvious improvement of surface hardness was obtained. • The self-lubrication performance of boronized CoCrNi MEAs in the water is excellent. • The self-lubrication enhancement was due to the formation of silicon-boron based tribofilm. [ABSTRACT FROM AUTHOR]

In high-speed and heavy-load mechanical operations, gears are susceptible to pitting, adhesion, and wear, which resulting tooth breakage and wear failure. In this paper, failed gears are subjected to repair experiments through the integration of laser cladding and boriding, and the tribological performances and post-repair vibration characteristics are also systematically investigated. In comparison to the base material, the average hardness of the boride surface exhibited an increase of up to 8.37 times, while the wear resistance increased to 15.60 times. The wear resistance of the repaired gear is 2.62 times that of the normal gear, and its vibration parameters are commensurate with that of the normal gear, which offers empirical substantiation for the enhancement of gear repair and remanufacturing strategies. • The hardness and wear resistance were increased at least 8 times after situ-boriding laser cladding treatment. • After situ-boriding laser cladding, the vibration and impact of the repaired gear were weakened. • Compared with the normal gear, the wear resistance of the repaired gear increased by 2.62 times. [ABSTRACT FROM AUTHOR]

Fretting wear is one of the main factors restricting the fastener service reliability for the high-temperature titanium alloys. Enhancing the high-temperature fretting wear resistance of titanium alloys with laser cladding coatings is an effective method. Thus, in this work, a self-lubricating composite coating was designed and fabricated on the surface of TC21 titanium alloy with laser cladding. Special attentions were made to the effect of the fractions of the hard and self-lubricating phases on high-temperature fretting resistance. In addition, the contribution of laser cladding power was also concerned. The primary phases of the self-lubricating composite coatings are α-Co, CaF 2 , WC, TiC and Cr 23 C 6 , and the reasons for the formation of TiC and Cr 23 C 6 are investigated from a thermodynamic point. The prepared composite coatings are dense and uniform. The crystallographic characteristics and the volume fractions of the hard phases (WC, TiC and Cr 23 C 6) and self-lubricating phase (CaF 2) were revealed, and the effect of grain size on the coating properties was analyzed. The hardness of the coatings were all maintained around 700 HV 0.2 , which was significantly higher than that of the substrate. The composite coating significantly improves the friction and wear resistance at 500 ℃. The hard phase was also found to be the most resistant to friction and wear. The frictional performance of the coating gets better when the hard phase and the self-lubricating phase increase. However, the hard phase contributes more to the high-temperature friction and wear resistance than the self-lubricating phase and the wear volume is 1/40 that of substrate. For the laser cladding power, the higher power can lead to a higher fraction of CaF 2 but a lower fraction of hard phase, causing a decline in COF but an increase in wear volume. The wear mechanisms of the coating and the substrate were analyzed, the subsurface stress distribution state of the wear scars were further analyzed. The current results present new insights on the anti-wear design strategy to the composite coatings consist of hard and self-lubricating phase. [Display omitted] • The high-temperature fretting wear volume is reduced about 40 times. • The strategy to the addition of the hard and lubricating phase were revealed. • The hard phase has a greater impact on the high-temperature fretting wear. • This work guide for the protection of the titanium alloys. [ABSTRACT FROM AUTHOR]

• The Co-based alloy/WC/CaF 2 composite coating is fabricated by laser cladding to improve the fretting wear resistance. • The surface microhardness of the composite coating can reach to 600 HV. • The fretting wear resistance are significantly improved, which can lead to the decrease of the COF and the wear loss. • This work guide for the protection of heat transfer tube in nuclear power plants. The coating fabricated by laser cladding on the surface has been an effective way to strengthen and enhance the wear resistance of alloys. However, the traditional Co-based alloy coatings rarely pay attention to the addition of self-lubrication constituents to form the composite coating. In addition, the effects of the Co-based composite coating on fretting wear resistance were few addressed. In this study, a Co-based alloy/WC/CaF 2 composite coating was fabricated by laser cladding on the surface of 316 stainless steel to improve the surface hardness and self-lubrication performance. The microstructure, phase constituents and fretting wear resistance at both room temperature (RT) and 500 °C were investigated. Results confirm that the composite coating is consisting of α-Co, Cr 23 C 6 and WC phases. And the F element is effectively dissolved into the α-Co phase. The fabricated composite coating is dense and uniform. The microhardness of the composite coating can reach to 600 HV 0.2 due to the presence of WC and Cr 23 C 6 phase. In addition, the grain refinement is another reason resulting in the higher microhardness. Fretting tests demonstrate that the fretting wear resistance at both RT and 500 °C are significantly improved by the composite coating, which can lead to the decrease of the COF and the wear loss. The lower COF can be ascribed to the dissolved F element in α-Co phase, especially at high fretting temperature. Additionally, the composite coating can effectively prevent the oxidation during fretting both at RT and 500 °C. This work provides an excellent method for the metallic materials to improve fretting wear resistance at both RT and high temperature. [ABSTRACT FROM AUTHOR]

TiZrHfTa 0.5 refractory high-entropy alloy (RHEA) shows great strength and ductility trade-off at ambient temperature, and has a lot of potential for use at elevated temperature. In this work, the effect of B element addition on the wear properties at different temperatures was thoroughly investigated. The results revealed that 1 at% B addition sample presents the best tribological properties at elevated temperatures, and borides exhibit a morphology evolution from blocky to acicular with the increasing B content. The wear rates at ambient and elevated temperatures successfully decrease with the increasing B content, which is ascribed to the increasing hardness, the solution strengthening, and the pinning effect at the grain boundaries induced by the second phase. In addition to the B content, the wear test temperature also significantly affects wear properties. With the analysis of the wear behavior and worn surface, it is demonstrated that the lowest wear rate at 600 ℃ is attributed to the continuous and densified oxide layer, while the serious wear at 400 ℃ results from the uncovered matrix and loose oxide layer. This work offers a new method to improve the wear properties of RHEAs at ambient and elevated temperatures. • The addition of B element for TiZrHfTa 0.5 RHEA results in the preferable wear performance at RT, 400 ℃ and 600 ℃. • The wear performance at 600 ℃ for four types of samples present the lowest wear rate. • The continuous and densified oxide layer formed at 600 ℃ contributes to the lowest wear rate. • The loose oxide layer formed in 400 ℃ leads to more metal fragments and more serious wear. [ABSTRACT FROM AUTHOR]

Heat transfer tubes in nuclear power plants are easily damaged by fretting wear. The fretting behaviour of the heat transfer tube related to the lead-bismuth eutectic (LBE) alloys needs to be investigated. In this work, the effect of pre-immersion corrosion in LBE of AISI 316 stainless steel on fretting behaviour is explored in detail. Following short-duration corrosion (specimens denoted C0 and PC1), no significant change occurred during corrosion, and after the fretting test, the wear scars were shallow, with adhesive wear being the dominant wear mechanism. Following long-duration corrosion (specimens denoted PC2 and PC3), the dissolution of Ni and the formation of an oxide layer were the main corrosion mechanisms. After the fretting tests, the wear scars were deep, demonstrating a degraded anti-wear property, and fretting fatigue wear was dominant. The degradation of the fretting wear resistance in specimens PC2 and PC3 can be attributed to the absence of the protection of NiO in the tribolayer, the large grain size, and the weak plastic deformation accommodation ability. The underlying mechanism is the dissolution of Ni induced by pre-immersion corrosion in the LBE. The present work provides useful information on the fretting behaviour of heat-transfer tubes in nuclear power plants related to the LBE environment. • The dissolution of Ni element and the formation of oxide layer are main corrosion mechanisms in LBE. • The pre-immersion corrosion behaviour results in a larger wear loss in the fretting behaviour. • The effect of pre-immersion corrosion on fretting wear mechanisms are illustrated systematically. • This work guide for the protection of heat transfer tube in nuclear power plants. [ABSTRACT FROM AUTHOR]

The fretting behaviors of a metastable β titanium alloy with three kinds of microstructures were studied. Special attentions have been paid to the underlying wear mechanism mediated by fretting conditions and microstructural features. The wear volume is more sensitive to the stroke amplitude rather than the normal loading. The mixed abrasive-adhesive wear is dominated in all fretting samples. The microstructural features make a significant contribution to the difference in the wear resistance and subsurface microstructure stability. Due to a large difference in the grain size and the ability of plastic deformation in adjacent grains, the SM (single-phase microstructure) sample is more favorable to initiation and propagation of cracks. The DM (duplex microstructure) and BM (basketweave microstructure) samples, however, have a high resistance ability to fretting wear and subsurface microstructure stability. This can be attributed to the large amounts of the β/α interfaces and the dispersed orientation distribution of α precipitates. The present work offers valuable experimental data for the prediction of fatigue life after fretting. • The fretting behaviors are evidenced to connect with coupling effect of microstructural features and fretting conditions. • The fretting wear volume is more sensitive to the stroke amplitude rather than the normal loading. • The microstructural origins of fretting wear mechanisms are illustrated systematically. • Microstructure features dependence on the subsurface microstructure stability is revealed in detail. [ABSTRACT FROM AUTHOR]

The wear behaviors of γ-TiAl alloy with two typical microstructures, i.e., the lamellar colony microstructure (denoted as the HIPed sample) and the fine equiaxed microstructure (denoted as the heat-treated sample), were thoroughly studied during dry sliding test to resolve the microstructural feature dependence of wear mechanisms. Special efforts have been made to the subsurface stability during sliding by using electron backscatter diffraction (EBSD) techniques. It is demonstrated that both cases present a similarity in the friction coefficients but a disparity in the wear loss. The lower wear loss was achieved in the case of HIPed sample. This can be attributed to the various directions in the local strain induced by plastic deformation during sliding, the crystallographic structure and the coherent interface in the lamellar colony, leading to high resistance ability to the wear. For the case of heat-treated sample, the high plastic deformation ability in the α 2 grains along the surface and the equiaxed morphology generate to high ability to plastic deformation and sliding during friction, contributing to the high wear loss. However, the heat-treated sample presents a high subsurface structure stability according to the local misorientation analysis. The present results provide new information on wear mechanisms of γ-TiAl alloy and may offer a method to enhance its wear resistance by tailoring the microstructural features. • The wear loss in a dry sliding of a γ-TiAl alloy is evidenced to connect with microstructural feature. • Special efforts have been made to the subsurface stability during sliding by using EBSD techniques. • The lower wear loss was achieved in the case of the lamellar colony microstructure. • The fine globular microstructure presents a high subsurface structure stability. [ABSTRACT FROM AUTHOR]

Bulk metallic glasses (BMGs) are potential candidate materials for numerous structural applications. However, this class of materials has a major limitation for engineering applications due to their inferior resistance to fatigue wear. In present work, a novel micro-scale repetitive impact test was utilized for fatigue testing of ZrCu-based BMGs (Zr 63.6- x Cu 18 Ni 10.4 Al 8 Fe x , x = 0, 3, 5 at.%), and specific attention was paid to the micro-alloying effect on wear resistance. Transmission electron microscopy of as-cast Zr 58.6 Cu 18 Ni 10.4 Al 8 Fe 5 revealed amorphous structure with the formation of a second glassy phase of few nanometers in diameter. Although these structural changes have minor effects on hardness and modulus, they have a substantial influence on fatigue wear resistance. Phase separation was found to promote numerous shear bands, which induced strain hardening, and improved the crack resistance. Consequently, all these features are considered to contribute the improved wear resistance. On the contrary, a monolithic glass without Fe addition lacks any microstructural features that can provide means for the local arrest of shear bands and incipient fatigue cracks. These findings improve the understanding of the impact fatigue mechanisms and provide suggestions for the future design of BMGs with excellent wear performance. • A micro-scale repetitive impact test has been exploited to explore the fatigue and wear behavior of BMGs. • Minor Fe substitution for Zr leads to the formation of second glassy phase. • Phase separation can have a dramatic effect on the wear resistance. [ABSTRACT FROM AUTHOR]