Electroplating is the dominant industrial route in China，accounting for more than 90％ of applications，and recently it has focused on upgrading Ni-Co-based composite coatings. Although early Ni-Co coatings improved wear resistance，high-temperature softening remained a serious problem. In recent years，nanoparticle-reinforced systems have emerged as a major breakthrough:relevant nanocomposite coatings can markedly reduce substrate mass loss from wear and mitigate corrosion risk. The dispersion-strengthening concept also provides technical reference for coating optimization across different components. Nevertheless，electroplating still faces constraints such as limited repair thickness and electrolyte-related environmental pollution. Laser surface processing offers a key advantage in achieving metallurgical bonding and achieves high-strength connection between the coating and the substrate through parameter control. Coatings on CuCrZr substrates generally exhibit good thermal-shock resistance in medium-to-high-temperature environments，and the hardness of reinforced composite coatings can be significantly increased. However，the high reflectivity and high thermal conductivity of copper alloys restrict the selectable coating-material systems and processing parameters. Thermal spraying is irreplaceable for rapid repair of large-thickness damage. The associated coatings can effectively extend mold service campaigns，and composite repair layers can both fill surface damage and satisfy the thermal-conductivity requirements of the crystallizer. The primary bottleneck of this technology is the relatively high porosity of the sprayed coatings. Cold spraying，as a solid-state deposition process，avoids thermal damage to the substrate. The resulting coatings can substantially reduce wear under high-temperature service conditions，and the use of transition-layer designs has effectively alleviated interfacial cracking. Laser-assisted cold spraying can further enhance coating performance. However，traditional cold spraying still suffers from insufficient bonding strength. In conclusion，these technologies differ markedly in suitable application scenarios. Electroplating provides a balance between cost and baseline performance and is appropriate for moderate service conditions. Thermal spraying is well suited for rapid restoration of thick damage. Laser surface processing is advantageous primarily because of its high metallurgical bonding strength. Cold spraying is more suitable for precision repair in heat-sensitive regions. Key challenges currently include limited coating-material options for laser processing of copper alloys，high porosity in thermally sprayed coatings，environmental burdens associated with electroplating，and inadequate bonding strength in cold-sprayed coatings. Future research should focus on three directions. First，process bottlenecks in laser treatment of copper alloys should be addressed by improving laser absorptivity through surface pretreatments. Second，new composite coating systems tailored to CuCrZr substrates should be developed，with particular emphasis on optimizing thermal-expansion compatibility. Third，process intelligence should be advanced by establishing a digital correlation framework linking “parameters-microstructure-service performance.”
The DL-EPR method has become the most widely used technique for quantitatively evaluating the degree of sensitization due to its high sensitivity and good repeatability. Related research has gradually expanded from simple sensitization sensitivity tests to the systematic exploration of complex metallurgical variables. Recent research progress has showed the application value of this technique in evaluating the inhibitory effects of alloying elements such as nitrogen and vanadium，which could effectively delay the precipitation of carbides. This method can also meet the testing requirements of highly corrosion-resistant super stainless steels and accurately detect subtle chromium depletion phenomena.
As a non-destructive perturbation analysis technique，EIS has become an indispensable method for exploring the interfacial kinetics of intergranular corrosion. Current research has focused on the dynamic change rules of the passive film resistance and charge transfer resistance during the induction period of intergranular corrosion. By fitting the complex and variable impedance data to a specific equivalent circuit model，the differences in electrochemical responses between the grain interiors and sensitized grain boundaries can be effectively distinguished.
The EN technology，as an in situ real-time monitoring technology，is different from methods that requires external polarization. It could capture the spontaneous fluctuations of potential and current. Researchers can distinguish the unique “continuous random pulse” signals corresponding to general corrosion，pitting corrosion，and intergranular corrosion by calculating characteristic parameters such as noise resistance and localization index，clarify the differences among different corrosion types，and thus achieve accurate detection at the early stage of intergranular corrosion initiation.
Although the above-mentioned macroscopic electrochemical methods had many advantages，they still faced challenges. DL-EPR and EIS provided the macroscopic average response of the entire exposed surface，which might underestimate the risks brought by non-uniform sensitization or local deep-seated corrosion. In addition，these methods were highly sensitive to surface treatment (such as roughness and cleanliness)and often had difficulty accurately simulating complex actual service environments. Although the EN technology performed well in monitoring，there was currently a lack of a unified criterion for judging intergranular corrosion，and its signals were easily interfered by the environment and the system，resulting in a complex interpretation of its physical meaning. To overcome the existing limitations，the future trend of intergranular corrosion evaluation was the integration of multi-scale and multi-modality. Adopting micro area electrochemical technology was crucial for achieving high-spatial-resolution visualization of corrosion morphology and revealing the mechanisms at the microstructural level. Combining electrochemical data with advanced characterization tools and artificial intelligence would help achieve high-throughput screening of materials and develop more accurate service life prediction models. In addition，formulating standard test protocols closer to actual working conditions was decisive for the scientific application and reliable protection of stainless steel materials in key infrastructure.
FeCoNiCrTi HEA coatings with different Er2O3contents (0，1.0％，2.0％，and 3.0％，mass fraction，the same below) were fabricated on H13 tool steel under argon protection using a Raycus RFL-C4000 laser processing system equipped with a KUKA robot and a coaxial powderfeeding system. The optimized processing parameters were as follows:laser power of 1 700 W，scanning speed of 8 mm/ s，spot diameter of 3 mm，overlap ratio of 35％，and powder feed rate of 15 g/ min. The feedstock powders consisted of equiatomic Fe，Co，Ni，Cr，and Ti elemental powders (purity＞99.9％) as the base HEA powder，mixed with high-purity Er2O3 powder (particle size～5 μm). The samples were denoted as Er0，Er1，Er2，and Er3，respectively. Phase constituents were analyzed by X -ray diffraction (XRD；D8 Advance，20°－90°，5(°)/ min). Microstructural morphologies were observed by field-emission scanning electron microscopy (SEM). Cross-sectional hardness distributions were measured using a microhardness tester (load:500 g；dwell time:15 s). Wear performance was evaluated using a pin-ondisk tribometer with a Si3N4 ceramic ball as the counterpart (load:5 N；rotational speed:200 r/ min；duration:30 min). Potentiodynamic polarization tests were conducted in 3.5％ NaCl solution using an electrochemical workstation (CHI660E；potential range from－0.8 V to＋0.5 V vs. SCE at a scan rate of 1 mV/ s).
The XRD results indicated that all coatings were composed of a single FCC phase. The addition of Er2O3did not alter the primary crystal structure but systematically shifted diffraction peaks toward higher angles，suggesting that the dissolved large Er atoms induced lattice distortion and reduced the interplanar spacing. With increasing Er2O3 content，peak broadening and decreased intensity were observed，implying grain refinement and increased microstrain. Microstructural observations showed that the undoped Er0 coating exhibited a mixed structure of coarse columnar and equiaxed grains，accompanied by pores and microcracks. After addition of Er2O3，the microstructure was significantly refined. The Er1 coating exhibited finer and more uniform grains. The Er2 coating presented the finest and most uniform equiaxed-grain structure with high density and clean grain boundaries，indicating the most effective grain-refinement behavior. However，the Er3 coating showed reduced uniformity and slight grain coarsening，which was likely attributed to Er segregation when excessive Er exceeded its solid-solubility limit.
The microhardness increased with Er2O3 content and then slightly decreased，with the Er2 coating reaching a peak value of～650 HV，which was an increase of～85.7％ compared with the Er0 coating (350 HV). This strengthening was attributed to the synergistic effect of grainrefinement strengthening (Hall-Petch mechanism) and solid-solution strengthening induced by lattice distortion. The wear test results showed that the Er2 coating exhibited the lowest and most stable average friction coefficient (0.18) and the lowest wear rate (3.2×10－5 mm3/(N·m))，which was reduced by～63％ compared with that of the Er0 coating (8.7×10－5 mm3/(N·m)). The superior wear resistance of the Er2 coating was ascribed to its fine and dense microstructure and high hardness，which effectively suppressed crack initiation and propagation and promoted stable wear. The performance of the Er3 coating was slightly inferior to that of the Er2 coating.
Electrochemical polarization tests demonstrated that Er2O3 addition significantly improved corrosion resistance in 3.5％ NaCl solution. The corrosion potential (Ecorr) shifted positively，and the corrosion current density (Jcorr) decreased markedly. The Er2 coating exhibited the best corrosion resistance with the most positive Ecorr(－0.469 V) and the lowest Jcorr(1.089×10－6 A/ cm2)，compared with Er0(Ecorr＝－0.420 V，Jcorr＝6.500×10－6 A/ cm2). The enhancement mechanisms included:(1) grain refinement and homogenization，which reduced the driving force for micro-galvanic corrosion；(2) a purification effect，whereby reactive Er reacted with impurities (S and O) and purified grain boundaries；(3) promotion of a denser and more stable Cr2O3/ TiO2 passive film. The corrosion resistance of the Er3 coating was slightly worse than that of the Er2 coating，possibly due to intensified micro-galvanic effects associated with an increased Er-rich secondary phase；nevertheless，it remained superior to that of the Er0 coating.
Conclusions:(1) Er2O3-modified FeCoNiCrTi HEA coatings were successfully fabricated by laser cladding，and Er2O3 addition significantly refined the coating microstructure.(2) Er2O3 improved the mechanical and tribological properties mainly through grain-refinement strengthening and solid-solution strengthening. When the Er2O3 content was 2.0％，the comprehensive performance was optimal，and both microhardness and wear resistance reached their best levels.(3) The introduction of Er2O3 significantly enhanced corrosion resistance in Cl－-containing environments by refining the microstructure，purifying grain boundaries，and promoting the formation of a more stable passive film；the 2.0％ Er2O3 coating exhibited the best corrosion resistance.(4) An appropriate Er2O3 addition (2.0％) was critical for achieving a synergistic improvement in strength，wear resistance，and corrosion resistance of FeCoNiCrTi HEA coatings. This work provided an important reference for designing high-performance laser-cladded coatings using rare-earth oxides.
In this study，MAO treatment was performed on SiCp / Al composites in an aluminate electrolyte system under a bidirectional constant-voltage power supply mode. Scanning electron microscopy (SEM)，X-ray diffraction (XRD)，a scratch tester for coating adhesion，a high-resistance meter，and an electrochemical workstation were employed to systematically analyze the effects of negative voltage on the coating micromorphology，porosity，phase composition，bonding strength，insulation performance，and corrosion resistance. Results showed that as the negative voltage increased，the thickness of the MAO coating on the SiCp / Al composite surface exhibited an increasing trend，whereas the surface porosity exhibited a decreasing trend. When the negative voltage was 170 V，obvious ablation was observed on the coating. The coatings prepared at different negative voltages were composed of γ-Al2O3，mullite，and an amorphous phase. With increasing negative voltage，the bonding strength of the coatings changed only slightly. When the negative voltage was 20 V，the insulation resistance of the coating reached a maximum value of 1.68×1010 Ω. The corrosion potential first increased and then decreased with increasing negative voltage，while the corrosion current density first decreased and then increased. In conclusion，in the aluminate electrolyte system，negative voltage strengthened the effect of positive voltage，thereby promoting the growth of MAO coatings on SiCp / Al composites. Moderately increasing the negative voltage improved coating uniformity and compactness；however，excessively high negative voltage should be avoided to prevent crack formation and ablation. The insulation resistance was closely related to coating compactness and thickness，and MAO coatings with improved insulation performance were obtained by selecting an appropriate negative voltage. Excessive negative voltage led to reduced corrosion resistance.
In this study，the room-temperature bending strength and failure causes of SiCf / SiC specimens with Si/ Yb2Si2O7/ Yb2SiO5 environmental barrier coatings fabricated by vacuum plasma spraying were investigated when the coating was under compression and tension. The specimens were characterized using an extended-depth-of-field 3D microscope，SEM (Scanning Electron Microscope)，and the coating porosity was measured. The bonding strength of the coating was tested by tensile method，while three-point bending tests were performed to test the roomtemperature bending strength of the bare material，the specimen with coating under compression and the specimen with coating under tension.The results showed that the bonding strength between the Si/ Yb2Si2O7/ Yb2SiO5 coatings and the SiCf / SiC composites was 10.13 MPa，and tensile fracture mainly occurred in the surface layer of the composite. When the coating thickness was considered，the bending strength of the coated material under compression (503.21 MPa) was nearly identical to that of the bare material (506.79 MPa). When the coating thickness was not considered，the bending strength of the specimen with coating under tension (499.77 MPa) was consistent with that of the bare material. Overall，the EBC coating had a certain compressive strength，and its overall tensile strength was much lower than that of the composite itself. When the coating was under compression，as the load increased to near the maximum load，gradual interlayer failure occurred in the composite，followed by coating spallation，and the load dropped rapidly，at which point the test was stopped. When the coating was under tension，the coating was pulled apart at a smallload (about 64 N)，and then as the load increased to near the maximum load，gradual interlayer failure occurred. During bending tests，the main failure modes of the composite were SiC fiber fracture，SiC matrix cracking，interlaminar tearing，and longitudinal cracks in the thickness direction of the composite on the tensile side of the composite. The failure modes of the SiC film deposited on the surface of the composite were mainly cracking and spallation. Under compression，the EBC coatings showed overall spallation and cracking；under tension，they fractured into two parts at the maximum stress. This study had reference significance for the design of the maximum bending load of SiCf / SiC composites with EBC coatings. When the EBC coating was only subjected to pressure during operation，the thicknesses of the EBC coating and the SiCf / SiC composite could be calculated together，the maximum bending load of the workpiece could be calculated based on the total thickness of the coating and the composite，and the bending strength of the composite. When the EBC coating was only subjected to tension during operation，or was subjected to both pressure and tension，the thickness of the EBC coating could be omitted，and the maximum bending load of the workpiece could be obtained only by calculating the thickness of the composite.
Results showed that the aging degree of the silicone sealant gradually decreased with the increase of the sealant layer depth under corrosive environments. After 1 440 h of long-term immersion and 1 200 cycles of wet-dry alternation，the aging depths of the silicone sealant were 3.5 mm and 2.0 mm，respectively. Compared with the sealant surface，the peak intensities in the infrared spectra of the silicone sealant-steel plate interface at 1 260 cm－1(Si－C absorption peak) and 790 cm－1(Si－O absorption peak) did not change significantly，indicating that water molecules had better penetration ability than corrosive ions. No new aging products were generated at the silicone sealant-steel plate paint interface，whereas the aging products on the outer surface of the silicone sealant were polymethylphenylvinyl silicone rubber，which was non-corrosive.The failure mode of the composite specimens was mainly cohesive failure within the sealant，indicating that the bonding strength at the interface between the sealant and the steel plate was greater than the cohesive strength of the sealant itself. Under eccentric loading，the steel plate surface at the bonded interface was more severely exposed. After 1 440 h of long-term immersion under eccentric loading，the bonding strength at the sealant-steel plate interface decreased the most，by approximately 14.3％. The sealant at the bonded interface of the specimens was distributed in sheet-like or wavy patterns，mainly due to the uneven thickness of the sealant under the canvas and internal defects，which led to complex crack propagation modes such as crack migration and multiple cracking. Compared with the wet-dry alternation environment，long-term immersion had a more severe impact on the aging and erosion of the sealant，and corrosive media essentially penetrated the bonding interface between the silicone sealant and the steel plate. These findings provided a theoretical basis for the durability design of anchor bolts in anchorages of long-span suspension bridges.
