To systematically investigate the tribo-corrosion performance of high-entropy alloy coatings on marine drilling tools，this review first summarized the preparation technologies and process characteristics of three major categories of high-entropy alloycoatings:(1) cladding technologies，such as laser cladding，plasma cladding，and electron-beam cladding，which are suitable for fabricating thick coatings and provide high bonding strength，although dilution-rate control is critical；(2) spraying technologies，such as plasma spraying，high-velocity oxygen-fuel spraying，cold spraying，and arc spraying，which show high deposition efficiency，although porosity issues need to be addressed through posttreatment；(3) deposition technologies，including magnetron sputtering，arc ion plating，and electrochemical deposition，which are suitable for preparing nanocrystalline or amorphous thin coatings with dense microstructures but limited thickness. Comparative analysis indicated that process parameters，i. e.，preparation methods，directly affect the microstructure，phase composition，and defect density of coatings，thereby determining their corrosion and wear resistance.
Second，the addition of different elements often affects the corrosion resistance of coatings. Therefore，this review summarized the effects of metallic elements，non-metallic elements，and ceramic particles on the corrosion resistance and tribo-corrosion performance of high-entropy alloy coatings in simulated marine environments. The reviewed studies showed that most elements or ceramic particles，when added in appropriate amounts，enhanced the tribo-corrosion resistance of high-entropy alloy coatings through solid-solution strengthening，grain refinement，or the formation of stable and dense passive films.
Based on the above analysis，this review drew the following main conclusions:high-entropy alloy coatings can achieve synergistic optimization of corrosion resistance and wear resistance through rational compositional design，and the stability of passive films and the distribution of hard phases are key factors governing corrosion-wear interactions. However，current studies are still mainly focused on single-performance evaluation，while the dynamic coupling mechanism of corrosion and wear has not been fully elucidated. In addition，the interfacial bonding strength between coatings and substrates and long-term service reliability still require further improvement. Therefore，future research on high-entropy alloy coatings can be carried out in three aspects:(1) compositional design of composite materials before preparation，such as the introduction of reinforcing constituents to construct high-entropy alloy coatings with composite strengthening or amorphous-nanocrystalline composite structures；(2) process-parameter regulation and interfacial optimization during preparation，through targeted and systematic experiments to identify the most suitable coating preparation methods and optimized processes for different key components and elemental systems of marine drilling tools；(3) integration of artificial intelligence and damage models to establish simulation and life-prediction systems for high-entropy alloy-coated drilling tools under actual service conditions. This review aims to provide a systematic reference for further research and engineering applications of high-entropy alloy coatings in the surface protection of marine drilling tools.
The synergistic effect of multi-field coupling is not a simple superposition of individual field effects；rather，the coupling relationship exhibits complex nonlinear characteristics. Its synergistic mechanism can be analyzed mainly from mechanical and thermal dimensions. In the mechanical dimension，pores and elemental segregation can be mitigated by regulating melt flow and particle distribution. In the thermal dimension，cracks can be effectively suppressed by regulating molten-pool morphology and solidification process. Nevertheless，multi-field composite assistance imposes more complex and stringent requirements on the process. Therefore，the scientific selection of external-field assistance combinations，the synergistic matching of composite-field parameters with key laser cladding process parameters，and the clarification of multi-field coupling mechanisms remain core issues that urgently require further investigation in this field.
This review further discussed the future development directions of external-field-assisted laser cladding technology. First，multi-field coupling mechanisms should be investigated in greater depth，and modeling and numerical simulation methods should be combined to realize realtime feedback and dynamic regulation of external-field parameters. Second，new material systems and application scenarios should be further expanded，and highly matched external-field-assisted equipment should be explored. Third，intelligent andspecialized supporting equipment should be developed to achieve independent and precise control of different external-field parameters as well as intelligent coordinated feedback.
Recent progress has mainly focused on three approaches:elemental doping，multilayer/interfacial structural design，and surface wettability regulation. For metallic element doping，elements such as Cr，Ti，and Cu effectively reduced internal stress，improved compactness，and enhanced adhesion. For example，Cr doping promoted the formation of stable Cr-C/ Cr-O phases，blocked the penetration of corrosive media，and produced “super corrosion-resistant” behavior in NaCl solution. Ti doping adjusted the sp2/ sp3 ratio and formed a more uniform and compact amorphous carbon structure，thereby improving mechanical stability and resistance to environmental degradation. Cu doping produced a uniform Cu nanoparticle-amorphous carbon dual-phase structure and significantly increased charge-transfer resistance. However，inappropriate doping，such as Ni doping，introduced electrochemical activity and micro-galvanic effects，thereby impairing corrosion resistance.
For non-metallic element doping，elements such as N，F，and Si mainly optimized chemical bonding and surface energy. N doping induced controllable graphitization and promoted the formation of a graphitized transfer film during friction，reducing the friction coefficient while improving corrosion resistance. F doping increased surface hydrophobicity and chemical inertness，with protection efficiencies exceeding97％in NaCl solution. Si doping enhanced structural densification and formed silicon-containing lubricating films，thereby improving tribo-corrosion resistance in various solutions.
Multi-element co-doping used synergistic effects to overcome the limitations of single-element doping. For example，B and Ni co-doping on magnesium alloys significantly increased the sp3 bond content and formed a dense spherical particle-cluster structure，which effectively hindered chloride ion penetration and resulted in excellent corrosion and wear resistance. Ti，Al，and V co-doping formed metal carbides and oxides，enhanced structural compactness and interfacial bonding strength，and indirectly improved corrosion resistance potential.
Interfacial engineering and chemical bonding were also important strategies for improving DLC coatings. The introduction of specific interlayers，such as Ti，TiCx，and Cu-O-Si-C，formed strong chemical bonds，such as Ti-C and Si-O，at the interface. These bonds acted as“molecular bridges”，significantly improving adhesion and preventing coating delamination，which was essential for long-term coating integrity.In terms of structural densification and defect blocking，multilayer structures disrupted columnar crystal growth，reduced through-thickness defects，and significantly extended the diffusion path of corrosive media through multiple interfaces. The “deflection effect” at interlayer interfaces forced corrosive agents to follow a tortuous path，thereby delaying their arrival at the substrate. Studies on Ti/ Ti-DLC and AlCrCN/ AlCr/ DLC multilayer structures showed that their corrosion current densities were several orders of magnitude lower than those of uncoated substrates.
The performance of multilayer structures also depended strongly on environmental interactions. Studies on Ti/ DLC，TiCx / DLC，and Ti-TiCx / DLC multilayer structures deposited on 316L stainless steel showed that the Ti-TiCx alternating interface provided the best long-term barrier protection in immersion tests and maintained high polarization resistance. Bioinspired gradient structures，such as Si support layers and multilayer Si/ DLC buffer layers，as well as coating/ corrosion-product synergistic strategies，in which corrosion products blocked wear-induced cracks，further enhanced durability in complex oil and gas pipeline environments.
Because intrinsic DLC surfaces are usually hydrophilic，regulating their wettability，especially toward hydrophobicity or superhydrophobicity，is a distinctive strategy for weakening capillary action and interfacial liquid adhesion in humid environments. Surface modification techniques include plasma treatment，such as CF4 plasma treatment，chemical grafting of long-chain molecules，such as fluorinated acrylates and PFPE lubricants，and elemental doping，such as F doping. These methods increased water contact angles，in some cases to more than 150°，and produced superhydrophobic surfaces. The introduced hydrophobic groups or layers，such as C-Fx groups，reduced surface energy. In addition，combining surface patterning，such as laser texturing，with DLC deposition created micro-/ nano-scale hierarchical structures and further enhanced hydrophobicity through the Cassie-Baxter state. Hydrophobic and superhydrophobic DLC coatings exhibited significantly reduced corrosion current densities and lower friction coefficients under high-humidity conditions. The hydrophobic layer minimized the adhesion and shear deformation of microdroplets at the interface，thereby reducing friction. For example，fluorinated long-chain monolayers on DLC reduced the corrosion current density to 8.20×10－9 A/ cm2.
In marine equipment，DLC coatings modified by elemental doping，such as W-DLC and Cr-DLC，and multilayer design showed excellent performance in simulated seawater. They greatly reduced corrosion current density，maintained a low and stable friction coefficient of approximately 0.1，and effectively protected key components，such as bearings and hydraulic pump parts，from corrosion-wear synergistic damage. In biomedical applications，DLC coatings deposited on implant alloys，such as Ti6Al4V，CoCrMo，and WE43 magnesium alloy，showed excellent biocompatibility，enhanced cell adhesion，and effective barrier properties against corrosive body fluids. Nitrogen-doped DLC on WE43 magnesium alloy significantly delayed degradation，improved corrosion resistance，and promoted cell proliferation，addressing the rapid degradation of biodegradable metals. In oil and gas as well as chemical industries，multilayer DLC coatings，such as periodic Si/(Si，N)-DLC coatings，provided excellent protection in neutral and alkaline environments by extending corrosion pathways and forming insulating silicon oxide layers in simulated coal mine water or chemical media. DLC coatings on valve surfaces and pump components withstood corrosive coolants and salt spray，significantly improving wear resistance and service life.
Nevertheless，challenges remain in the broad engineering application of highly durable DLC coatings. Future research should focus on three aspects. First，the corrosion-wear coupled failure mechanism should be investigated in greater depth by developing in situ electrochemical tribological testing systems combined with advanced characterization techniques，so as to dynamically capture interfacial reactions and material evolution underreal service conditions. This is crucial for failure warning and life assessment. Second，breakthroughs in engineering-oriented preparation technologies and cost control are required. Low-temperature and high-rate deposition technologies，as well as continuous，large-area，and uniform coating processes，should be developed to ensure batch-production consistency and reduce overall manufacturing costs. Third，artificial intelligence and data-driven design should be introduced. Comprehensive DLC coating performance databases should be established，and machine learning/ deep learning methods should be used to predict optimal elemental doping and structural design schemes for specific substrates and service conditions. These approaches can greatly reduce trial-and-error costs in early engineering development and pave the way for the large-scale application of DLC coatings.
Cold spray technology was selected because it was characterized by solid-state deposition through particle plastic deformation and avoided the disadvantages of conventional thermal processing，such as oxidation，phase transformation，residual stress，and heat-affected zones. The 6061/ Al2O3 composite material system was adopted because 6061 aluminum alloy provided good plasticity and compatibility with the substrate，while the Al2O3 hard phase improved coating hardness and wear resistance and promoted densification of the deposited layer. First，key process parameters，including gas pressure and temperature，were optimized on flat 2A12 and ZM5 specimens to obtain dense coatings without obvious defects. Optical microscopy and scanning electron microscopy were used to characterize the coating microstructure，with emphasis on the flattening deformation of 6061 aluminum particles，interparticle bonding，pore distribution，and the dispersion uniformity of the Al2O3 phase. The microhardness，interfacial bonding strength，and wear resistance of the coatings were then quantitatively evaluated. Subsequently，simulated pit defects were prefabricated on the specimens，and the optimized process was applied for filling repair. Cross-sectional metallographic samples were prepared to investigate the interfacial microstructure between the repair coating and the two dissimilar substrates，namely 2A12 aluminum alloy and ZM5 magnesium alloy，with a focus on interface continuity and coating defects.
Microstructural analysis showed that the 6061 aluminum alloy particles underwent sufficient plastic deformation and exhibited a typical flattened lamellar structure. The interparticle interfaces were tightly bonded. The porosity was low (0.05％)，and a highly dense deposit was formed. The Al2O3 reinforcing phase was uniformly dispersed in the aluminum matrix without obvious agglomeration，providing effective strengthening and wear resistance. Continuous and contamination-free interfacial bonding was formed between the repair layer and both the 2A12 aluminum alloy and ZM5 magnesium alloy substrates. No cracks or oxide layers were observed at the interfaces，indicating typical mechanical interlocking and local metallurgical bonding driven by severe plastic deformation. The interfacial bonding strength between the repair coating and both substrates exceeded 50 MPa. The microhardness of the repair coating was higher than 100 HV0.1，significantly higher than those of the 2A12 aluminum alloy and ZM5 magnesium alloy substrates，which was mainly attributed to the work-hardening effect of the 6061 aluminum alloy and the dispersion strengthening of Al2O3 particles. The repair coating exhibited significantly better wear resistance than the aluminum and magnesium alloy substrates，with a greatly reduced wear volume，demonstrating excellent anti-wear capability. In the simulated defect-filling experiments，cold spray technology showed good repair capability，completed defect filling，and effectively restored the original geometric profile and dimensions of the components.
The above results confirmed the feasibility of using portable cold spray technology combined with 6061/ Al2O3 composite materials for the in situ repair of aviation aluminum and magnesium alloy components. Under low-temperature solid-state deposition conditions，this technology produced repair deposits with high density，high bonding strength，high hardness，and excellent wear resistance. It not only efficiently restored the geometric integrity of damaged components but also achieved surface functional enhancement. This study provides a feasible process scheme and parameter basis for rapid and high-quality on-site repair of aviation lightweight alloy components，and also offers an important theoretical reference for the broader application of cold spray technology in material repair.
At present，studies on the preparation，microstructure，and properties of high-entropy alloy coatings are mostly based on laser additive manufacturing technology. However，because the poor room-temperature plasticity of high-entropy alloys makes wire fabrication difficult，most related studies have used powder materials，and relatively few studies have focused on the preparation of high-entropy alloy coatings by wire arc additive manufacturing. In comparison，wire arc additive manufacturing has the advantages of high deposition efficiency，strong bonding strength，and low cost. Therefore，in this study，a self-designed stranded high-entropy alloy welding wire was used as the raw material，and an NbMoTaNiCr refractory high-entropy alloy coating was prepared on TC4 titanium alloy by TIG rotating arc cladding technology，which is efficient，low-cost，and stable in forming quality. The microstructure and properties of the coating were investigated to reveal the structure-property relationship between its microstructure and wear resistance. This study aimed to provide a key theoretical basis and data support for the engineering application of this refractory high-entropy alloy as a wear－resistant component or protective coating.
The NbMoTaNiCr refractory high-entropy alloy (RHEA) exhibits excellent mechanical properties and superior wear resistance，showing significant potential for applications in aerospace，nuclear energy，and advanced equipment manufacturing. In this work，a tungsten inert gas(TIG) rotating wire-arc additive manufacturing system was employed to fabricate NbMoTaNiCr RHEA coatings on TC4 titanium alloy substrates. The microstructure，hardness，and tribological properties of the deposited layer were systematically investigated. The experimental results revealed that the room-temperaturephase structure of the as-deposited RHEA layer was dominated by a disordered face-centered cubic(FCC) matrix with a small quantity of μ-phase precipitates. The microstructure was characterized by fine and uniformly distributed dendrites，with elemental microsegregation observed between dendritic and inter-dendritic regions. The average microhardness of the alloy layer reached approximately 913 HV. Under identical dry sliding wear test conditions，the deposited RHEA layer exhibited a wear resistance approximately 42 times higher than that of the TC4 titanium alloy substrate. This exceptional performance wasmainly attributed to the synergistic effects of several strengthening mechanisms inherent in the deposited layer. The refined dendritic microstructure significantlyimproved the strength through grain boundary hardening，which conformed to the Hall-Petch relationship. Furthermore，the severe lattice distortion，as a fundamentalcharacteristic of the high-entropy alloy，generated effective solid-solution strengthening and createdconsiderable barriers to dislocation motion. The hard and brittleμ-phase particles belonging to topologically close-packed (TCP) phases further enhanced the wear resistance of the coating. These dispersed particles effectively inhibited plastic deformation and mitigated abrasive wear during sliding friction. This study verifiedthe feasibility of fabrication high-performance RHEA coatings via the TIG-based wire-arc additive manufacturing and clarified the correlation between the microstructural characteristics and excellent mechanicaland tribological properties of the coatings.
In this review，the current development status of coating technologies for metal bipolar plates in hydrogen energy systems was systematically reviewed from the perspective ofengineering service environments. Unlike most previous reviews that focused on a single system or classified coatings mainly according to material categories，a parallel analytical framework for PEMFC and PEMWE application scenarios was established in this work. The differences between the two devices in terms of service environment，failure mechanism，and performance requirements were systematically compared，thereby clarifying the engineering adaptation logic of different coating design strategies. First，physical vapor deposition (PVD)，chemical vapor deposition (CVD)，electrochemical deposition (ECD)，and other commonly used MBPP coating preparation technologies were comparatively analyzed. The relationships among deposition particle energy，thin-film growth kinetics，and microstructural evolution were discussed，together with their effects on coating compactness and interfacial bonding strength.
Subsequently，PEMFC and PEMWE application scenarios were analyzed in parallel from the perspectives of coating type，performance，and application. In PEMFC environments，coatings are required to balance electrical conductivity，corrosion resistance，hydrophobicity，and mechanical stability. Metallic coatings，carbon-based coatings，and conductive polymer coatings were systematically analyzed in terms of their optimization strategies and practical performance in interfacial resistance control，passive film suppression，and long -term durability. In PEMWE environments，the anode side is exposed to high potentials and strongly oxidative conditions for extended periods，imposing more stringent requirements on coating stability. Therefore，research progress on noble metal coatings，metal compound coatings，and multilayer composite structures was reviewed，with emphasis on high-potential corrosion resistance，interfacial stability，and failure mechanisms. The trade-off between cost and engineering feasibility was also discussed.
Considering the differences in testing methods and evaluation systems among different studies，an engineering-application-oriented evaluation framework was constructed to summarize different coatings and enhance the engineering guidance value of this review. Overall，coating technologies for metal bipolar plates are developing from single-performance optimization toward multifunctional synergistic design. Future research should focus on interfacial synergistic regulation，multilayer gradient structure design，standardized durability evaluation systems，and improved scalable preparation capability，thereby promoting the transition of bipolar plate coating technologies from laboratory research to engineering application and industrial implementation.
To further investigate the effects of haze weather and moisture on the electrochemical corrosion mechanism of X80steel in sandy soil，the electrochemical impedance spectroscopy，polarization curves，corrosion products，and corrosion morphology of X80steel in sandy soil containing simulated haze solution (HA solution) under natural air-drying conditions were tested using a CS350H electrochemical workstation，an optical digital microscope，scanning electron microscopy (SEM)，an energy dispersive spectrometer (EDS)，and X-ray photoelectron spectroscopy (XPS). Combined with the physicochemical properties of sandy soil measured using a soil moisture meter and electrochemical impedance spectroscopy theory，the electrochemical corrosion behavior of X80 steel in sandy soil containing simulated haze solution was comprehensively investigated.
The results showed that the pH value of the sandy soil medium fluctuated within the range of 6.51－7.03，indicating an approximately neutral environment，while the temperature fluctuated around 20 ℃. The water content of the sandy soil gradually decreased from the initial 30％saturation state to 0 within 14 d. The impedance spectra did not exhibit a single-time-constant characteristic，and all impedance spectra in the low-frequency region consisted of flattened capacitive arcs. With increasing time，the radius of the capacitive arc first decreased and then increased，indicating that the corrosivity of the sandy soil toward X80 steel first increased and then decreased. The variations in impedance modulus and phase angle with sandy soil water content were consistent with the evolution of the impedance spectra. After 6 d，the impedance spectra，impedance modulus，and phase angle fluctuated significantly in the frequency range of 103－105 Hz.
With prolonged exposure time and the continuous decrease in water content in the sandy soil，the polarization curves of X80 steel shifted upward as a whole. The corrosion current density first increased and then decreased，reaching a peak value of 2.72×10－5 A/ cm2 on the 3rd day.According to Jcorr，the corrosion grade of X80 steel in sandy soil was moderate or above from 1 to 12 d，and decreased to slight corrosion from 13 to 14 d. The electrochemical corrosion process of X80 steel was jointly affected by the HA solution and the pore structure of the sandy soil.The overall corrosion rate first increased and then decreased，with an order of magnitude ranging from 10－1 to 10－2 mm/ a. The average corrosion rate was 1.26×10－1 mm/ a，indicating that the simulated haze solution accelerated the corrosion of X80 steel in sandy soil.
The surface of X80 steel was covered with uneven dark brownish-yellow corrosion products，which were identified as iron oxides，including Fe2O3 and FeO. The average thickness of the corrosion products was approximately 150 μm，and the thickness in local areas exceeded 600μm. The corrosion products exhibited leopard-like，flocculent，and rice-grain-like morphologies. Severe localized corrosion occurred in some areas，resulting in the formation of numerous corrosion pits. This study clarified the electrochemical corrosion behavior and mechanism of X80 steel in sandy soil containing simulated haze solution under natural air-drying conditions. The results provided important experimental evidence and theoretical support for corrosion prediction，safety protection，and engineering application of buried X80 pipeline steel in sandy soil strata.
The effects of key ball milling variables were systematically investigated，including the milling medium，namely air，deionized water，and 95％(volume fraction) ethanol；the CNTs-to-ethanol mass ratio，ranging from 1∶0 to 1∶20；and the milling time，ranging from 3 to 12 h.The dispersion quality was evaluated using a particle size analyzer，ultraviolet-visible spectrophotometry (UV-vis)，and scanning electron microscopy (SEM)，while the structural integrity was evaluated using Raman spectroscopy and X-ray diffraction (XRD). The results identified an optimal dispersion condition，namely a CNTs-to-ethanol mass ratio of 1∶15 and a ball milling time of 6 h. Under this condition，the particle size of CNTs was mainly distributed in the range of 140－280 nm. Stability tests showed that the absorbance of the CNT aqueous suspension decreased by 3.0％ and 22.0％ after standing for 4 h and 24 h，respectively. SEM observations showed that CNTs were uniformly distributed as individually separated tubular structures or very small bundles. Raman spectroscopy and XRD results indicated that the CNTs had few defects and good crystalline order.
The ball-milled CNTs were then used to prepare CNTs/ Ni composite coatings by composite electrodeposition. Morphological analysis，tribological performance testing，and corrosion resistance testing of the coatings were performed using electron microscopy，a friction testing machine，and an electrochemical workstation，respectively. The test results showed that the CNTs/ Ni composite coating prepared under the above optimal dispersion condition exhibited lower surface roughness and better anti-friction and wear-resistant properties. Specifically，at a sliding speed of 0.084 m/ s in air，the average friction coefficient between the coating and the GCr15 ball was as low as 0.170 8. The wear scar had the smallest width and height，demonstrating the excellent anti-friction and wear-resistant properties of the coating under this condition.
Interestingly，the corrosion resistance test in 3.5％(mass fraction) NaCl solution revealed another set of optimal parameters for CNTs in the coating:a CNTs-to-ethanol mass ratio of 1∶4 and a ball milling time of 6 h. Under this condition，the lowest corrosion rate of 0.108 79 mm/ a was obtained. This study demonstrated that ball milling parameters strongly affected the dispersion state and structural preservation of CNTs，thereby determining the functional properties of the final composite coating. A CNTs-to-ethanol mass ratio of 1∶15 and a ball milling time of 6 h were established as the optimal process parameters for achieving excellent CNT dispersion. The CNTs/ Ni coating prepared under this condition exhibited lower surface roughness and better anti-friction and wear-resistant properties，as evidenced by a friction coefficient of 0.170 8.
The finding that the optimal ball milling parameters for corrosion resistance and wear resistance were different revealed a performance trade-off and highlighted the importance of application-oriented and purpose-specific optimization based on CNT pretreatment parameters. This study provided a comprehensive and data-supported framework for tailoring CNT-reinforced nickel-based coatings，enabling a balance between excellent wear resistance and superior corrosion protection according to the target service requirements.
To enhance the wear resistance of tubular copper components，plasma cladding was used to strengthen the surface of a small copper tube.The microstructure and properties of the cladding layer were systematically investigated using scanning electron microscopy (SEM)，X-ray diffraction (XRD)，an energy dispersive spectrometer (EDS)，a microhardness tester，and a friction and wear testing machine. Based on two criteria，namely surface morphology and microhardness，the optimal process parameter combination for preparing the nickel-based cladding layer on the copper tube surface was determined through orthogonal experiments:a preheating temperature of 600 ℃，a plasmaarc current of 140 A，and an arc swing speed of 3 000 mm/ min. The obtained cladding layer had a relatively smooth and flat surface，and its average hardness reached up to 616.9 HV0.1.
However，because of the limited volume of the small copper tube，the surface temperature of the copper substrate increased rapidly，which easily led to inconsistency between the front and rear sections of the cladding layer，although this inconsistency was not obvious in appearance.Specifically，the rear section showed a higher dilution rate and lower performance. Therefore，the cladding layer prepared under the optimal parameter combination obtained from the orthogonal experiment was further optimized. After the innovative segmented variable-current method was adopted，with the current changed from 140 A in the front section to 130 A in the rear section，the temperature rise of the copper surface during the cladding process was reduced. As a result，the uniformity between the front and rear sections of the cladding layer was significantly improved. The average dilution rate was approximately 10％，and the average microhardness was approximately 620 HV0.1.
Microstructural characterization showed that significant elemental diffusion occurred on both sides of the interface of the cladding layer，indicating the formation of good metallurgical bonding. The cladding layer contained a γ-(Cu，Fe，Ni) solid-solution matrix phase，an interdendritic Ni3Si phase，and a large number of dispersed Cr-rich phases，including Cr23C6 and CrB. These phases played roles in solid-solution strengthening and dispersion strengthening，respectively，resulting in significantly better performance than that of the copper substrate.
Wear resistance tests showed that the copper substrate had a high and severely fluctuating friction coefficient because of its low hardness and poor resistance to plastic deformation. Under shear force，large-area detachment easily occurred on the copper substrate surface，and the wear mechanism was adhesive wear. In contrast，the cladding layer exhibited a lower and more stable friction coefficient. The wear weight loss was reduced by 75.2％ compared with that of the copper substrate. The worn surface was mainly characterized by ploughing grooves，and the wear mechanism was abrasive wear.
In summary，the preparation of nickel-based cladding layers by plasma cladding can significantly improve the surface properties of small copper tubes，providing theoretical and technical guidance for the surface strengthening of large tubular copper components. In the actual cladding process of large tubular copper components，the variable-current regulation method can also be adopted. According to practical conditions，the current can be set as a multi-stage gradient to ensure consistency between the front and rear sections of the cladding layer. In addition，for copper substrates with different volumes，a similar cladding effect can be achieved through simple adjustments of process parameters based on the results of this study.
FeCrNi alloy cladding layers were prepared at three scanning speeds of 90，110，130 mm/ min under constant process parameters，namely a cladding current of 110 A，ashielding gas flow rate of 15 L/ min，and an arc length of 15 mm. The powder particle size was 40－90 μm，and the powder was identified as a single face-centered cubic (FCC) phase. With increasing scanning speed，the macroscopic forming behavior followed the trend of decreased heat input，reduced molten pool size，and thinner and narrower bead geometry. At 90 mm/ min，the molten pool had a relatively high heat input and a long liquid residence time，resulting in a wide and thick bead with coarse surface ripples. At 110 mm/min，the bead morphology was the most regular，the fish-scale ripples were continuous and uniform，the thickness distribution was more consistent，and the forming stability was the best. At 130 mm/ min，insufficient heat input further reduced the bead width and thickness.
Interfacial scanning electron microscopy (SEM) observations showed that，at 90 mm/ min，the thermal effect was strong and the metallurgical reaction was sufficient，while local interfacial discontinuities or metallurgical defects were not readily observed. At 110 mm/ min，the interface was the straightest and most continuous，and the best metallurgical bonding was obtained. At 130 mm/ min，interfacial waviness was intensified，and signs of locally incomplete metallurgical fusion were observed.
In terms of microstructure，the coating was composed of columnar grains extending along the deposition direction，accompanied by some equiaxed grains. As the scanning speed increased from 90 to 130 mm/ min，the heat input per unit length decreased，and the cooling rate was significantly increased. As a result，the microstructure was gradually transformed from coarse dendrites into fine and compact cellular/ fine columnar structures，and the sizes of grains and dendritic/ cellular structures were markedly reduced. Microstructural refinement led to an increase in hardness. The average Vickers hardness of the coating increased from 237 HV at 90 mm/ min to 248 HV at 110 mm/ min and further to 261 HV at 130 mm/ min.
To characterize the mechanical differences in the interfacial microregion，nanoindentation tests were performed at 110 mm/ min. The reduced elastic modulus E*at seven test positions ranged from 262.45 to 337.55 GPa，the indentation elastic modulus EIT ranged from 238.83 to 307.17 GPa，and the indentation hardness HIT ranged from 2 110.81 to 5 112.33 MPa. A representative interfacial point showed E*＝293.66 GPa，EIT＝267.23 GPa，and HIT＝3 712.72 MPa，indicating a significant microscale property gradient near the interface.
Corrosion behavior was evaluated in 3.5％(mass fraction) NaCl solution by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). It was shown by PDP that，with increasing scanning speed，the corrosion current density generally decreased，and the current density in the anodic passivation region decreased and tended to stabilize，indicating an overall improvement in corrosion resistance. However，the electrochemical responses of the samples prepared at 90 and 110 mm/ min were relatively close，suggesting that the effect of scanning speed on corrosion resistance was not strictly linear.
The EIS results provided a clearer distinction. The radius of the Nyquist capacitive arc increased significantly with increasing scanning speed，indicating that the charge transfer process was increasingly suppressed. Equivalent-circuit fitting showed that the total impedance parameter Rtotal(R2＋R3) was 2.109 0×103 Ω·cm2 at 90 mm/ min，3.952 0×103 Ω·cm2 at 110 mm/ min，and 1.908 1×104 Ω·cm2 at 130 mm/ min，demonstrating that the interfacial charge-transfer resistance and film resistance were significantly improved for the sample prepared at 130 mm/ min. Combined with microstructural evolution，it was inferred that microstructural refinement and compositional homogenization induced by increased scanning speed weakened interdendritic segregation and micro-galvanic effects，which was beneficial for the formation of a more continuous and compact passive film，thereby improving impedance and corrosion resistance. Scanning speed significantly affected the forming quality，interfacial metallurgical bonding state，and microstructural refinement of the FeCrNi cladding layers by regulating heat input，and further influenced their mechanical properties and corrosion resistance. Overall，with increasing scanning speed，the microstructure tended to become refined，and the properties were synergistically improved. However，the evolution of corrosion resistance did not follow a completely linear trend. These results provided a reference for process parameter optimization and property regulation，while also indicating that the evolution mechanisms of microstructure and properties under the coupled effects of multiple factors require further investigation.
