查看更多>>摘要：? 2021 Elsevier B.V.A plethora of difficulties may appear with brake materials during braking with high-energy dissipation. The tribological properties of a copper-based sintered material for high-energy brakes at different levels of braking energy ranging from 3010 to 31,789 J/cm2 were experimentally evaluated using a reduced-scale dynamometer. Morphological analysis of worn surfaces and wear debris analysis were conducted using scanning electron microscope and three-dimensional video microscope, respectively. Friction and wear behaviour of the copper-based sintered material depended strongly on the contact surface characteristics, being controlled by the dissipation of braking energy. The worn surfaces with low roughness, well-covered by the tribo-films, contributed to the low and stable friction coefficient. Additionally, examination and analysis of worn surfaces and wear debris corroborated the wear mechanisms of the copper-based sintered material. Severe adhesive wear and delamination wear occurred during high-energy braking.
查看更多>>摘要：? 2021 Elsevier B.V.Based on the concept of green design and development, FeS/Cu–Bi lead-free copper matrix composites were prepared by using FeS and Bi powder as lubricating phase to replace the lead in CuPb alloy. Surface modification of FeS powder by electroless plating and mechanical alloying of mixed powder by high-energy ball milling are effective strategies for preparing lead-free copper matrix composites. The effects of surface modification and mechanical alloying on the mechanical and tribological properties of the materials are studied. Results show that electroless nickel plating improves the mechanical locking and metallurgical bonding between mixed powder particles; enhances the interface bonding between FeS particles and copper matrix; and increases the hardness, impact toughness, and crushing strength by 29.7%, 19.4%, and 13%, respectively. The density and hardness of the composite prepared by electroless nickel plating combined with mechanical alloying increases by 11.4% and 40.6%, respectively, average friction coefficient decreases from 0.44 to 0.28, and wear rate decreases by 62%. Electroless plating and mechanical alloying reduce the microploughing and material transfer on the worn surface of the composites, and the abrasive wear and adhesive wear are remarkably improved.
查看更多>>摘要：? 2021 Elsevier B.V.The in-situ carbides reinforced Ni-based composites were fabricated by reactive hot-pressing sintering Ni and Ti2AlC powders. Tribological performance and mechanisms at room temperature and 800 °C were studied. Sintered composites contain Ni-based solid solution, Ni2TiAl, Ni3Al, TiCx, Ti3NiAl2C and Al2O3. At room temperature, the elastic modulus, hardness and H/E of composites enhanced due to the increase of carbides, resulting in wear rate declined with the rise of Ti2AlC. At 800 °C, because of the formation of reducing-friction and anti-wear oxide layers consisting of TiO2, NiO, Al2O3, NiTiO3, and NiAl2O4, wear rate was further decreased to 4.2 × 10?6 mm3/Nm. Composition differences on the surface between worn and static oxidized at 800 °C show that tribo-chemical reaction promotes the generation of NiO and NiTiO3.
查看更多>>摘要：? 2021 Elsevier B.V.A series of TiZrV0.5Nb0.5Alx refractory high entropy alloys were prepared by arc-metling, aiming to solve the problems of high density, poor oxidation and wear resistance of refractory high entropy alloys at elevated temperature. The microstructure, mechanical and tribological properties were investigated. The results indicate that the increase in Al addition could promote precipitation of Laves phase in the BCC matrix, thus regulation the mechanical properties. The tribology behavior is strongly dependent on the temperature. Below 400 °C, the wear mechanism is dominated by mechanical wear. In this case, the TiZrV0.5Nb0.5Al0.25 alloy with high strength and good ductility at the same time, exhibits optimal wear resistance. When the temperature increases above to 600 °C, the coefficient of friction and the corresponding wear rate dramatically decrease with Al addition, which could be attributed to the preformed formation of dense and stable oxide glazed-layer before wear. Accordingly, the main wear mechanism changes from mechanical wear to oxidation wear. This study provides an effective strategy for improving wear resistance and oxidation resistance of refractory high entropy alloys in a wide temperature range.
查看更多>>摘要：? 2021 Elsevier B.V.Based on the advantages of Ni or Cu incorporation to the mechanical and tribological properties of nitride coatings, the co-incorporation of Ni and Cu at different ratios was employed on CrBN coatings. Results showed that Ni (11.01 at%) and Cu (3.41 at%) co-incorporation increased hardness from 24.4 GPa to 26.4 GPa. As a consequence, coatings wear rate decreased from 6.4 × 10?7 mm3/Nm to 5.4 × 10?7 mm3/Nm. At the meantime, friction coefficient declined from 0.30 to 0.28 due to CuO lubricating layer. However, when Cu content was high (10.68 or 15.05 at.%), the hardness of Ni-Cu-CrBN coatings sharply declined, which weakened wear resistance and increased friction coefficient. It indicated that Ni (11.01 at%) and Cu (3.41 at%) was the proper co-incorporation combination.
查看更多>>摘要：? 2021 Elsevier B.V.This study investigates the effect of the contact size and the gross-slip sliding amplitude on the fretting wear rate of a steel flat-on-flat interface. Results confirm an asymptotic decrease of the wear rate with the contact size. This evolution is induced by an easier oxygen access to the interface favoring abrasive wear according to the Contact Oxygenation Concept (COC) and by a faster ejection of the protective debris particles from the interface according to the Third Body Theory (TBT). To decouple these COC and TBT contributions, the effect of the sliding amplitude and the contact orientation with respect to the sliding direction is assessed using an original macro-texturation test strategy. Wear rate evolutions as well as SEM-EDX fretting scar analyses suggested that COC is mainly driven by the minimum distance between the contact center and the open air contact edges (LCOC), whereas TBT wear process is chiefly controlled by the ratio of the collinear contact length to the sliding amplitude (LTBT/δg). Assuming a weighted influence of the contact oxygenation process on the TBT wear process, a simple power-law formulation is introduced to quantify the contact size effect on the energy wear rate fluctuation. A very good correlation with the experiments confirms the stability of the proposal.
查看更多>>摘要：? 2021 Elsevier B.V.Increase in edge radius, decrease in diameter, and growth of flank wear are three commonly used indices for quantitative assessment of the micro-milling tool wear. Worn-out tool diameter is usually measured by observing it under a microscope at low magnification, and this leads to significant measurement error (up to 20% of the tool life). When edge geometry changes with progressive wear, the outer diameter also decreases synchronously. In this article, a correlation between the changes in edge features and the corresponding reduction in outer diameter is developed to calculate the latter more precisely. For this purpose, first, analytical expressions are derived for different wear scenarios (uniform abrasion, non-uniform abrasion, adhesion, and edge-chipping). Validation of these expressions is further carried out through sustainable minimum quantity lubrication (MQL) assisted micro-milling of Ti–6Al–4V using TiAlN-coated WC/6Co micro-mills. Various stages of tool wear are characterized, and suitable parameters for quantifying the same are assessed. While the directly measured diameter values are affected by inconsistency and negative wear rate, the corresponding estimated values vary consistently and reliably with positive wear rate across different regimes. The diameter reduction is further established as the preferred variable for drawing tool life curve in micro-milling.
查看更多>>摘要：? 2021 Elsevier B.V.MoN–Ag coatings with different silver contents were synthesized using DC and RF magnetron sputtering. The microstructure and mechanical performance of the as-prepared coatings were investigated in detail. The friction and wear behaviors of the MoN–Ag coatings sliding against Si3N4 ceramic ball at RT and elevated temperature were also evaluated by a ball-on-disc tester with rotation method. The results showed that the MoN–Ag coatings were polycrystalline structures composed of γ-Mo2N and c-Ag. And the MoN–Ag coating with a 2.2% Ag content had the highest hardness and elastic modulus. The friction interface between the MoN–Ag coating and Si3N4 ball presented distinct oxidation after sliding at RT. In addition, the temperature greatly impacted the friction and wear behaviors of the MoN and MoN–Ag coatings. The friction coefficients of the investigated coatings decreased gradually when the temperature rose from 100 °C to 700 °C. However, the wear rates displayed opposite trends. The decrease in the friction coefficient for the coatings could be ascribed to the formation of lubricating phases (MoO3, Ag2MoO4, Ag2Mo4O13), while the increases in the wear rate were proven to be due to decreases in the residual compressive stresses of the coatings with increasing temperature. The MoN–Ag coating with 2.2% Ag had excellent tribological properties and an extremely low friction coefficient of 0.27 at 700 °C, which was superior to that of the pure MoN coating.
查看更多>>摘要：? 2021 Elsevier B.V.Nickel-based superalloys have become one of the most promising and potential materials in gas turbine engine components contributing to aerospace industry due to their outstanding mechanical properties (high ductility, high temperate strength, etc.). However, the poor machinability (lower thermal conductivity, high work hardening, etc.) and low machining efficiency of nickel-based superalloys have further hampered it to be widely applied in the potential aerospace applications. In this work, we systematically investigated the effects of various cutting speeds (i.e. 100–400 m/min) on tool wear and failure mechanisms of the SiAlON ceramic tools in the dry cutting of nickel-based superalloys by performing high-speed turning experiments. The tool life and wear evolution with the cutting distance at various cutting speeds were studied; the response relationship of tool flank wear, cutting speed, and cutting distance was visually presented. The optimal cutting speed range could be 200 m/min～300 m/min in our cutting conditions considering the tool life and the response relationship. Further, tool wear and failure mechanisms are demonstrated to be the adhesion, catastrophic fracture, and microcracks at flank wear land; and the micro-chipping is observed at rake face at a low cutting speed of 100 m/min. Additionally, at a higher cutting speed of 400 m/min, the wear and failure mechanisms are severe adhesion, cutting edge micro-chipping, microcracks, but none of catastrophic fracture failure. This disparity can be attributed to the different alternating dominant effects in strain hardening and thermal softening mechanisms behind the machining process.
查看更多>>摘要：? 2021 Elsevier B.V.Grid-to-rod-fretting (GTRF) in pressurized water reactors (PWRs) is known to cause wear and surface damage on the fuel claddings, potentially leading to radioactive leakage. One of the accident-tolerant fuel (ATF) concepts is to use advanced cladding materials that could withstand higher temperatures. This study investigated the wear behavior of candidate silicon carbide (SiC)-based composite claddings with different levels of surface finish in fretting against a commercial ZIRLO alloy grid using a unique bench-scale autoclave GTRF rig. The experiments mimicked the environment in an industrial full-assembly PWR simulator. Fretting tests were conducted with a realistic load (～0.5 N) in deionized water under a pressure of 20–23 bar at 204 °C for 100 h. While the SiC/SiC composite claddings showed significantly higher wear resistance than the commercial ZIRLO alloy cladding as expected, the smoother versions experienced surprisingly higher wear than the much softer counterface, ZIRLO grid. The wear mechanism of the SiC/SiC cladding was attributed to the SiC wear debris that was trapped at the fretting interface causing both 3-body and 2-body (embedded into the grid surface) abrasion of the cladding. Rougher SiC/SiC claddings had less material loss but caused more wear on the ZIRLO grid. Pre-oxidized ZIRLO grid showed better compatibility with the SiC/SiC cladding to protect both the cladding and grid as a result of reduced wear debris trapping.
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