Search Results (22)

In hybrid rolling bearings operating under extreme high-temperature and high-load conditions, steel rolling elements are prone to early failure, which has accelerated the widespread adoption of ceramic materials. To address the limitations of conventional studies, which have focused mainly on macroscopic wear parameters while neglecting subsurface failure mechanisms and the relationship among sintering process, microstructure, and fatigue performance, this work systematically compares the tribological behavior of Si₃N₄ ceramic balls fabricated by high-pressure electric resistance hot-pressing (REHP) and B₄C ceramic balls prepared by conventional hot pressing (HP) against 52100 steel counterparts. The central innovation of this study lies in clarifying, based on Hertzian contact theory and Lundberg-Palmgren life theory, that subsurface orthogonal shear stress, rather than surface compressive stress, is the fundamental driving force for contact fatigue failure of ceramic balls. In addition, two distinct damage evolution modes are revealed: B₄C exhibits early-stage brittle fracture and large-scale spalling, whereas REHP-Si₃N₄ is characterized by microcrack initiation and slow crack propagation. Moreover, the intrinsic mechanism by which the REHP process significantly enhances the contact fatigue life of ceramics is elucidated; namely, it refines grain size, eliminates residual porosity, and increases densification. The results show that, under the same high-load conditions, the mass loss of REHP-Si₃N₄ ceramic balls is only 35.7% of that of HP-B₄C, while the service life is extended by 20%. This work provides a key theoretical basis for ceramic material selection and sintering process optimization in high-performance hybrid bearings. Full article

The hydrogen embrittlement (HE) behavior of an Fe–18Mn–8Al–1C–5Ni lightweight steel containing a fine and uniformly distributed B2 phase and κ-carbide was investigated by slow strain rate tensile testing with in situ hydrogen charging. Hydrogen charging reduces the elongation from 28.2% to 11.2%, while preserving an ultimate tensile strength above 1100 MPa and yielding an HE index of 60.2%. A thermal desorption analysis reveals a multi-peak desorption curve corresponding to diffusible hydrogen, hydrogen reversibly trapped at κ-carbides, and hydrogen strongly bound at the B2/γ interfaces, revealing a hierarchical hydrogen trapping behavior. Electron backscatter diffraction and electron channeling contrast imaging analyses near the fracture head region further reveal that localized hydrogen enrichment at the B2/γ boundaries induces severe stress concentration and interfacial weakening, shifting the fracture mode from ductile micro-void coalescence in air to hydrogen assisted intergranular and interphase cracking. This study clarifies the distinct roles of coherent κ-carbide and B2/γ interfaces in hydrogen trapping and crack initiation, offering a microstructure-based perspective for designing high-strength, HE resistant lightweight steels. Full article

To investigate the mechanical bearing characteristics of the “excavation-backfill” composite after the excavation of coal pillars and backfill replacement with gangue-based cemented paste backfill, mechanical bearing characteristic experiments are conducted on a series of coal samples with rectangular “excavation-backfill” roadways under uniaxial loading, covering the full deformation and failure process. The MTS universal testing machine and DS5-type acoustic emission signal acquisition system are employed, and a high-speed camera is adopted to monitor and record the full failure process. The mechanical bearing characteristics and crack evolution mechanisms of unfilled coal pillar (U-C) and backfill coal pillar (B-C) samples are explored. The results show that with the increase in “excavation-backfill” width, the uniaxial compressive strength and elastic modulus of U-C samples decrease significantly, and the samples exhibit brittle–ductile failure. When the “excavation-backfill” width is 60 mm, the backfill can distinctly improve the strength and elastic modulus of B-C samples, showing a strong strength recovery effect. The temporal characteristics of AE signals indicate that both U-C and B-C samples experience four stages subjected to uniaxial compression: quiet period, rising period, active period, and post-peak rising period. In the quiet period and rising period, the b-value fluctuates upward with energy release; in the active period, the b-value decreases significantly with large energy release; in the post-peak rising period, crack propagation and frictional slip increase, leading to an enlarged fluctuation amplitude of the b-value. Based on the location of AE sources, the three-dimensional crack chain evolution is inverted. The crack chain evolution of the U-C is mainly distributed along the dip direction (75°~90°, 255°~270°) and vertical direction (165°~180°) of the coal bedding plane, while the B-C is more uniform, indicating that the backfill evidently affects the crack distribution. This study provides new insights for predicting the crack evolution and failure mode of coal–rock composites. Full article

In this study, a boride cladding layer with Mo₂FeB₂ hard phase was prepared on the GCr15 steel via plasma cladding. The phase composition, microstructure, thermal fatigue resistance, microhardness, and wear resistance of the boride cladding layer were investigated. The results revealed that the hard phases in the boride cladding layer were Mo₂FeB₂ and (Cr,Fe)₂₃(C,B)₆, while the binder phase consisted of α-Fe martensite. When the thermal fatigue times increased, the indentation crack length extended in a quadratic pattern, and the crack propagation rate increased. Crack propagation in the cladding layer occurred via both transgranular and intergranular modes. When the thermal fatigue temperature was below 600 °C, the cladding layer exhibited good thermal stability, and a reliable metallurgical bond was formed between the cladding layer and the GCr15 steel substrate. The microhardness of the cladding layer reached 1022.1 HV_0.5, approximately 2.6 times that of the GCr15 steel. The mass loss of the cladding layer increased with the increase in wear load and wear time. The wear of the cladding layer was mainly three-body abrasion wear, resulting from brittle spalling of the hard phase on the worn surface. This study demonstrates the potential of Mo₂FeB₂-based cladding layers for extending the service life of high-value industrial components. Full article

This study investigates the bending performance of web-embedded double inverted T-shaped steel–concrete composite beams (WDTSCBs) through experimental testing and finite element analysis (FEA). A novel composite beam structure was developed, where double inverted T-shaped steel beams are interconnected by slotted web plates and embedded in concrete flanges, aiming to enhance load-bearing capacity and ductility. Four WDTSCB specimens, utilizing C40 concrete and Q345qB steel, were tested under static loading to analyze failure modes, load-deflection behavior, and crack development. The experimental results were validated by FEA in ABAQUS, achieving an error margin of less than 5%. The findings indicate that WDTSCBs exhibit superior flexural performance compared to traditional composite beams, with higher bending resistance and reduced steel consumption. These results provide valuable insights into the design and optimization of steel–concrete composite structures, promoting their application in civil engineering. Full article

Boron carbide (B₄C) is an essential material in various high-performance applications due to its light weight and hardness. In this work, B₄C-based composites were fabricated via a powder route consisting of powder mixing, precursor preparation, and hot-pressing under vacuum. The composites’ mechanical properties and microstructure were analyzed to investigate the effect of adding minor second-phase particles. In addition to homogenizing the grain size, the addition of SiC (≤10 wt%) to B₄C increased its strength and improved its fracture toughness, with values reaching 551 MPa and 3.22 MPa m^1/2, respectively. Meanwhile, the addition of TiB₂ (≤10 wt%) significantly improved the strength and fracture toughness only, with values reaching 548 MPa and 3.92 MPa m^1/2, respectively, with only a minimal decrease in hardness. Microstructural analysis revealed that the second-phase particles were uniformly distributed and reduced the average grain size, contributing to the increase in strength. Additionally, the TiB₂ particles impeded crack propagation and induced crack deflection at the interface, indicating the formation of an intergranular fracture mode. On the contrary, the addition of SiC primarily resulted in transgranular fracture behavior, though it still improved the toughness of the B₄C. These results suggest that small amounts of SiC and TiB₂ can effectively enhance the mechanical properties of B₄C ceramics while maintaining the lightweight characteristics critical for military and aerospace applications. Full article

Ultra-high-performance concrete is widely used in bridge strengthening to improve mechanical performance and bridge durability. Interfacial bonding performance is a key factor in ensuring the effectiveness of ultra-high-performance concrete strengthening. The bending test of the UHPC–NC composite structure was carried out in this article. The effects of groove treatment type and epoxy resin bonding were considered to discuss the damage modes, load–deflection relationships, and strengths. The interfacial tensile strength of the UHPC–NC composite structure and the distribution pattern of cracks were clarified. The results of the test showed that (a) only 22.2% of the groove-treated specimens failed due to bonding surface failure, indicating that the UHPC–NC bonding surface has a high degree of reliability; (b) the strength of specimens with an epoxy adhesive interface was the lowest. It was only 21% higher than the pure normal concrete specimen, and the effective synergistic force of UHPC–NC cannot be achieved; (c) the specimens treated with a positive trapezoidal keyway exhibited the highest strength, with an increase of approximately 200% compared to the pure normal concrete specimens. The strength of bending specimens with right-angled and inverted trapezoidal grooves increased by approximately 100% compared with pure normal concrete specimens. Based on the established three-dimensional numerical model and the analysis of test results under economic and safe conditions, the positive trapezoidal keyway specimen exhibits superior interfacial bonding–tensile performances. Full article

A novel Ni-Cr-Si-B filler metal (JNi-5) was designed and further fabricated into the amorphous brazing filler metal for joining the GH4169 alloy. The effect of brazing temperature on the microstructure and mechanical properties of GH4169 joints was investigated. The typical microstructure of the joint at 1030 °C is composed of four specific zones: the base metal (BM), heat-affected zone (HAZ), isothermal solidification zone (ISZ), and athermal solidification zone (ASZ). The typical microstructure of the joint is GH4169/(Nb, Mo)-rich boride+(Cr, Nb, Mo)-rich boride/γ(Ni)/Ni-rich boride+γ(Ni)/γ(Ni)/(Cr, Nb, Mo)-rich boride+(Nb, Mo)-rich boride/GH4169. As the temperature increased, the HAZ continued to widen and the ASZ depleted at 1090 °C and 1120 °C. Additionally, the borides within the HAZ coarsened at temperatures of 1090 °C and 1120 °C. At 1030 °C, the fracture path is in the ASZ, and the existence of the brittle phase in the ASZ provides the potential origin for crack growth. The fracture mode is a quasi-cleavage fracture. At 1060 °C, 1090 °C, and 1120 °C, the fracture behavior mainly happened in the HAZ, and the existence of borides in the HAZ provides the potential origin for crack growth. Namely, the shear strength of joints was principally dominated by the brittle precipitations in the HAZ. The fracture mode of these joints is the hybrid ductile. At 1060 °C, the shear strength of the obtained joint is the highest value (693.78 MPa) due to the volume fraction increase in the Ni-based solid solution. Finally, the optimized brazing parameter of 1060 °C/10 min was determined, and the corresponding highest shear strength of 693.78 MPa was obtained owing to the increased content of the Ni-based solid solution in the joint. Full article

Eutectic high-entropy alloys (EHEAs) can achieve a balance of high strength and ductility. It has been found that the mechanical properties of hypoeutectic high-entropy alloys are superior to those of EHEAs. In this work, hypereutectic Al_1.1CoCrFeNi_2.1 alloy was prepared, and the mechanical properties in a wide temperature range were studied. The presence of both soft ordered L1₂ and hard BCC (B₂) phases results in a combination of ductile and brittle fracture modes. The Al_1.1CoCrFeNi_2.1 hypereutectic high-entropy alloy contains more primary soft L1₂ phases, which ensure excellent ductility. Moreover, the Orowan by-passing mechanism caused by the B₂ precipitates increases in the strength of the alloy for low-temperature tensile tests (−100 °C and 23 ± 2 °C). The −100 °C test exhibits a dimple morphology and demonstrates the highest ultimate tensile strength of 1231 MPa, along with an excellent elongation of 44%. At high tensile temperatures (650 °C, 750 °C, and 850 °C), the dislocation cutting mechanism and dynamic recrystallization increase the plasticity. However, the presence of a large number of cracks near the spherical primary L1₂ phase significantly reduces the ductility and strength. The results show that the hypereutectic Al_1.1CoCrFeNi_2.1 exhibits superior plasticity and strength properties at low temperatures. The findings of the article provide a new approach to enhancing the comprehensive mechanical properties of hypereutectic alloys. Full article

In this study, Mo-(Y,Zr)-Si-B coatings were obtained by direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HIPIMS) using mosaic targets. The results showed that the addition of Y and Zr into the composition of Mo-Si-B coatings led to the suppression of columnar grain growth, a decrease in the crystallite size of h-MoSi₂ phase from ~50 to ~5 nm, and an increase in the amorphous to crystalline phases ratio Doping of the Mo-Si-B coating with Y and Zr promoted an increase in oxidation resistance at a temperature of 1000 °C. The introduction of yttrium into the composition of Mo-Si-B contributed to an increase in their crack resistance when heated to 1300 °C. High oxidation resistance of the coatings was provided by a defect-free SiO₂ + MoO₃ + Y₂O₃ surface layer. The transition from the DCMS mode to HIPIMS decreased the texture of the Mo-Si-B coatings. The use of an HIPIMS mode led to a decrease in the oxidation rate of Mo-(Y)-Si-B coatings at T = 1000 °C by 1.6–4.5 times compared to DCMS. In the case of Mo-Y-Si-B coatings, the use of HIPIMS led to a decrease of more than 50% in the thickness of the oxide layer at a temperature of 1300 °C. Full article

The present work mainly focuses on a comparative study of the individual and combined effect of reinforcements on tensile strength and fracture surface analysis of Al6061 alloy and its composites during artificial aging. SiC and B₄C are the two reinforcements used in the present work for the preparation of Al6061 composites by the stir casting process, and the reinforcement percentage from 2, 4, and 6 wt.% varied. Both Al6061 alloy and its composites are solution-treated at 558 °C/2 h and artificially aged at 100 and 200 °C for different time intervals to achieve peak aging. The results show substantial improvement in ultimate tensile strength during low temperature aging at 100 °C. Approximately 80–110% increase in UTS value is observed in both individual and hybrid composites compared to Al6061 alloy. The mechanism of failure governing the tensile strength for both alloy and its composites is thoroughly analyzed and discussed using a scanning electron microscope. The morphology of crack propagation is also studied to determine the mechanism of failure. Al6061 alloy shows ductile failure due to coarser dimples. Al6061-SiC composites show particle-matrix interface cracking and shear failure. Al6061-B₄C composites show elongated dimple rupture mode of failure, whereas Al6061-SiC + B₄C hybrid composites fail due to nucleation growth and mixed fracture mode. Full article

The freeform capability additive manufacturing (AM) technique and the magnetic efficiency of Fe-6.5Si steel have the potential for the development of electromechanical component designs with thin body sections. Moreover, the directional anisotropy of the material, which is formed during growth, improves the magnetic and electrical properties of Fe-6.5 wt%Si. We obtained the range of optimal technological modes of Laser Power Bed Fusion process (volume energy density (VED) of 100–140 J/mm³, scanning speed of 750–500 mm/s) to produce the samples from Fe-6.5 wt%Si powder, but even at the best of them cracks may appear. The optical microscopy and SEM with EDX analysis of the laser-fabricated structures are applied for investigation of this phenomena. We detected a carbon content at the boundaries of the cracks. This suggests that one of the reasons for the crack formation is the presence of Fe₃C in the area of the ordered $\mathsf{\alpha }’\mathrm{FeSi} \left(\mathrm{B}2\right)+{\mathrm{Fe}}_3\mathrm{Si}\left(\mathrm{D}{0}_3\right)$ phases. Quantitative analysis based on crack initiation criteria (CIC) showed that the safe level of internal stresses in terms of the CIC criteria in the area of discontinuities is exceeded by almost 190%. Local precipitates of carbides in the area of cracks are explained by the heterogeneity and high dynamics of temperature fields, as well as the transfer of substances due to Marangoni convection, which, as a result, contributes to a significant segregation of elements and the formation of precipitate phases. Full article

Accelerated pavement testing (APT) is an effective method to study the long-term performance of pavement. Therefore, the dynamic strain behavior analysis of asphalt pavement has important guiding significance in the study of pavement failure modes. To explore the dynamic response of a high-content plant-mixed hot-reclaimed asphalt mixture under a dynamic load of vehicles, a full-scale test road was paved, and ALT biaxial accelerated loading test equipment was used to simulate the dynamic loads of vehicles. Based on parameters such as axle load, temperature, speed, and loading times, the development law for the bottom strain of the three pavement structures was analyzed. The test results show that the most unfavorable position of the asphalt pavement load is located just below the centerline of the wheel track on one side, and the damage effect of a single double-axle wheel load is far greater than that of two single-axle wheel loads. Then, the longitudinal tensile strain of the pavement bottom always maintains the alternating state of compression-tension and compression. The longitudinal tensile strain of the pavement bottom is larger than the transverse tensile strain, and transverse fatigue cracks appear first. Under normal temperature conditions, the bottom tensile strains of the three composite pavement structures under different axial loads are close, and the pavement performance of the hot-recycled asphalt pavement of structure A and structure B can meet the specification requirements. The relationship between the bottom strain and axle load is nonlinear and is directly related to the tire ground pressure, and the difference in the tensile and compressive strain values of the bottom of the three composite pavement structures is small. Under high temperature conditions, the bottom layer temperature of structure A and structure B is lower than that of structure C, and the thermal heat transfer efficiency of hot-recycled asphalt pavement is lower than that of ordinary asphalt pavement. Additionally, the longitudinal tensile strain is about 1–1.5 times that of the transverse tensile strain. Based on the Boltzmann function, the accumulative tensile strain prediction model was established to reflect the relationship between the cumulative strain at the bottom and the number of loads. Full article

Epoxy varnishes are of high relevance to advanced steel laminates for the transformation of electric energy. Structure–property correlations of epoxy varnishes, coil coatings and electrical steel laminates are poorly described. Hence, the main objective of this paper was to develop, implement and evaluate well-defined waterborne model epoxy varnishes for electrical steel laminates, and to elucidate structure–property correlations. Adhesives with systematically varied equivalent epoxy weight (EEW) based on bisphenol-A-diglycidyl ether (DGEBA) were investigated and used to formulate waterborne varnishes. Crosslinking agent dicyandiamide (DICY) was added in an over-stoichiometric ratio. The waterborne model varnishes were prepared by shear emulsification at elevated temperatures. The model varnishes in the A-stage were applied to electrical steel using a doctoral blade. At a peak metal temperature of 210 °C, the coatings were cured to the partly crosslinked B-stage. Coated steel sheets were stacked, laminated and fully cured to C-stage at 180 °C for 2 h. For laminates with an epoxy adhesive layer in the C-stage, glass transition temperatures (T_G) in the range of 81 to 102 °C were obtained by dynamic mechanical analysis in torsional mode. Within the investigated EEW range, a negative linear correlation of EEW and T_G was ascertained. Presumably, higher EEW of the varnish is associated with a less densely crosslinked network in the fully cured state. Roll peel testing of laminates at ambient and elevated temperatures up to 140 °C confirmed the effect of EEW. However, no clear correlation of roll peel strength and glass transition temperature was discernible. In contrast, fatigue fracture mechanics investigations revealed that hydroxyl functionality and crosslinking density were affecting the crack growth resistance of laminates in a contrary manner. The energy-based fracture mechanics approach was much more sensitive than monotonic peel testing. Full article

The aim of this study was to evaluate the moisture absorption behaviour and its influence on the mechanical properties of newly developed sandwich biocomposites with flax fibre-reinforced poly-lactic acid (PLA) face sheets and soft cork as the core material. Three different types of sandwich biocomposite laminates comprised of different layup configurations, namely, non-woven flax/PLA (Sample A), non-woven flax/PLA and cork as core (Sample B) and non-woven flax/paper backing/PLA, cork as core (Sample C), were fabricated. In order to evaluate the influence of moisture ingress on the mechanical properties, the biocomposites were immersed in seawater for a period of 1200 h. The biocomposites (both dry and water immersed) were then subjected to tensile, flexural and low-velocity falling weight impact tests. It was observed from the experimental results that the moisture uptake significantly influenced the mechanical properties of the biocomposites. The presence of the cork and paper in sample C made it more susceptible to water absorption, reaching a value of 34.33%. The presence of cork in the core also has a considerable effect on the mechanical, as well as energy dissipation, behaviours. The results of sample A exhibited improved mechanical performance in both dry and wet conditions compared to samples B and C. Sample A exhibits 32.6% more tensile strength and 81.4% more flexural strength in dry conditions than that in sample C. The scanning electron microscopy (SEM) and X-ray micro-CT images revealed that the failure modes observed are a combination of matrix cracking, core crushing and face core debonding. The results from this study suggest that flax/PLA sandwich biocomposites can be used in various lightweight applications with improved environmental benefits. Full article