Rubber，as an indispensablekey engineering material，plays a central role in dynamic sealing applications in aerospace，automotive manufacturing，and petrochemical industries due to its excellent resilience，gas tightness，and chemical resistance. However，the inherently high friction coefficient (typically around 1) and poor wear resistance of rubber materials often leads to premature component failure，energy loss，and safety risks，which constitute major obstacles to their wider application in high-performance systems. To solve this problem，surface modification using advanced films has become an important research direction. Among various candidate materials，diamond -like carbon(DLC) films，especially their hydrogenated form (a-C:H)，have become the preferred solution. They integrate high hardness，excellent lubricity，chemical inertness，and the potential to achieve strong interfacial bonding on flexible substrates，with significant synergistic performance advantages. This paper systematically reviewed the research progress in the tribology of DLC films on rubber surfaces over the past two decades，providing a comprehensive analysis from the perspectives of fundamental deposition science，advanced interfacial engineering，performance evaluation，and future research directions.Depositing DLC films on thermally sensitive and viscoelastic rubber substrates requires specialized low-temperature techniques to avoid substrate damage. Plasma-enhanced chemical vapor deposition and magnetron sputtering are the two most important and effective methods.These techniques can finely regulate the composition，microstructure，and final properties of the films by precisely controlling key process parameters，such as precursor gas ratios (e. g.，CH4/ Ar/ H2/ N2)，substrate bias voltage，plasma power density，and chamber pressure. Unlike films formed on rigid substrates，a fundamental and distinctive feature of DLC films on rubber surfaces is the intentional formation of microcrack or wrinkle networks. This characteristic morphology is not a process defect，but rather a critical structural adaptation that is essential for relieving the enormous stresses caused by the severe mismatch in thermal expansion coefficients between the hard，brittle carbon film and the soft rubber substrate. By regulating the temperature change process of the substrate during deposition through bias control，this microstructure can be designed to present different states from crack-dominated to wrinkle-dominated，which directly determines the mechanical flexibility，durability，and final adhesion performance of the film.

Achieving strong and durable interfacial bonding strength remains the primary challenge for the reliable application of DLC films on rubber.Research has mainly focused on two engineering strategies that are often complementary. The first is advanced plasma pretreatment of the rubber surface prior to film deposition. Exposing it to high-energy plasmas (such as Ar，O2，and H2) can achieve multiple key functions:thoroughly cleaning the surface，increasing microscopic roughness through etching to promote mechanical interlocking，and most importantly，breaking C－H/ C－C bonds in the polymer chains to generate a high density of active dangling bonds. These active sites are crucial for forming strong covalent bonds with the deposited carbon film，which can significantly enhance chemical bonding force. The second strategy involves the rational design of functional transition interlayers. Interlayer materials such as silicon，titanium-doped carbon，or chromium are introduced between the rubber and the DLC film. These interlayers serve as gradient interfaces，which can effectively alleviate the abrupt mismatch in mechanical properties，release intrinsic residual stresses，and improve chemical compatibility. The effectiveness of a specific interlayer is highly system -dependent，underscoring the necessity of substrate-specific design. For example，an optimized silicon interlayer is most suitable for silicondoped DLC films on nitrile rubber，whereas a chromium interlayer may be more appropriate for certain fluoroelastomers.

A large number of experimental evidences over the past two decades have shown that carefully designed DLC films can significantly change the tribological properties of rubber components. The friction coefficient of uncoated rubber can be continuously reduced from about 1.0 to the range of 0.1－0.3. Advanced film designs，such as doped or multilayer structures，can further achieve ultralow friction coefficients of 0.05－0.10. This significant reduction is accompanied by an improvement in wear resistance by several orders of magnitude. Nitrile rubber has become the most widely studied substrate because its polar－CN groups are conducive to adhesion. On its surface，a variety of films，including pure DLC，silicon-doped DLC，nitrogen-doped DLC，and fluorine-doped DLC，have been successfully integrated. Their properties can be finely regulated through parameters such as hydrogen content，dopant element content，and bias voltage. This technology has also been successfully applied to other important engineering elastomers，including fluoroelastomers，ethylene propylene diene monomer rubber，and hydrogenated nitrile butadiene rubber. This highlights the versatility of this approach and also emphasizes that each substrate requires uniquely optimized deposition processes and interfacial designs.

In conclusion，DLC films represent a powerful and versatile technological route for overcoming the intrinsic friction and wear limitations of rubber seals. Significant and substantial progress has been made in understanding and controlling film microstructure，enhancing interfacial bonding strength through surface pretreatment and interlayers，and achieving remarkable improvements in tribological performance on different rubber substrates. However，to achieve widespread industrial application，there are still several persistent challenges that need to be addressed.These include the remaining friction performance gap compared with DLC films on rigid substrates，the lack of standardized quantitative methods for characterizing bonding strength and wear on soft materials，and the complexity of predicting long-term service life under real multi-factor operating conditions. Future research will address these challenges through several key frontier directions. These directions include using in situ characterization techniques to deepen the fundamental scientific understanding of tribological interfaces，developing next-generation coating structures such as nanocomposite or bioinspired multilayer films，integrating intelligent functions such as self-healing and condition monitoring，and applying data-driven methods such as machine learning to efficiently explore the vast parameter space and find optimized solutions for specific application scenarios. Continuous progress along these strategic directions is expected to fully unlock the potential of DLC and its related advanced carbon films，thus spurring the emergence of a new generation of high-performance，long-life，and intelligent rubber sealing systems for engineering applications.

This study systematically explored the microstructural evolution law，phase transformation behavior，and surface strengthening mechanism of F53 duplex stainless steel under different nitriding temperatures (475，500，525，550，575 ℃). As a thermochemical treatment process commonly used to improve the wear resistance and corrosion resistance of stainless steel，ion nitriding has a significant enhancement effect on the surface properties of materials. However，for F53 super duplex stainless steel with complex composition and coexisting austenite and ferrite phases，systematic research on its nitriding behavior，evolution of strengthening phases，and elemental diffusion characteristics was still lacking. In particular，the specific influence of phase structure evolution at different temperatures on microstructural stability and mechanical properties remained unclear. To this end，this study comprehensively adopted characterization methods such as X-ray diffraction (XRD)，scanning electron microscopy (SEM)，energy-dispersive spectroscopy (EDS)，and microhardness testing to fully analyze the types of strengthening phases，elemental diffusion behavior，interfacial microstructural morphology，and mechanical response characteristics of the nitrided layer under different temperature conditions. Cross-sectional analysis revealed the evolution laws of the composition，thickness，continuity，and defects of the nitrided layer，which provided a basis for clarifying the regulation mechanism of temperature on the structure-property relationship.The experimental results showed that when treated at475 ℃，the nitrided layer was dominated by CrN，and γ′-Fe4N precipitated synchronously，showing an initial dual-phase coexisting structure. However，the surface compound layer was unevenly distributed，and the interface fluctuated obviously，indicating phase mismatch and local stress accumulation. As the temperature increased to 500－525 ℃，CrN and γ′-Fe4N synergistically formed a composite strengthening layer with dense structure and flat interface. At this stage，nitrogen diffusion was more sufficient，and the diffusion rates between austenite and ferrite were close. This promoted the continuous deposition of strengthening phases and significantly improved the mechanical properties of the nitrided layer. Within this optimal temperature window，the surface hardness exceeded 1 450 HV0.1，and the effective depth of the nitrided layer was about 80 μm. SEM morphologies showed that the strengthening phases were uniformly distributed at the grain boundaries and inside the austenite grains. The diffraction peak intensities of CrN and γ′-Fe4N in the XRD pattern increased significantly，reflecting the active phase formation process and excellent structural stability.

However，when the temperature further increased to 550 ℃ and 575 ℃，the structure of the nitrided layer deteriorated significantly. The strengthening phases showed a tendency of coarsening and agglomeration，and cracks and spalling of the compound layer appeared in the interface region. This was most obvious especially at 575 ℃. XRD analysis showed that the peak intensities of CrN and γ′-Fe4N decreased，and the diffraction peak of carbide CFe15.1(PDF＃00-052-0512) appeared for the first time in the pattern. This indicated that high-temperature treatment induced a secondary phase reaction between residual carbon and iron，forming a new brittle phase. At the same time，the diffraction peak of α-Fe increased，which might indicate that austenite transformed back to ferrite，showing a decrease in microstructural stability. The EDS elemental distribution results further verified the above trends.

At 500－525 ℃，nitrogen and chromium were significantly enriched within 70－80 μm of the surface layer，forming the main nitrided layer.Beyond this depth，the elemental content decreased rapidly，and the substrate maintained a stable dual-phase structure. However，at 575 ℃，the elemental distribution tended to be uneven，with local segregation and abnormal precipitates appearing. This reflected the decrease in compositional stability and the loss of control of strengthening phases. In summary，the ion nitriding behavior of F53 duplex stainless steel was highly dependent on temperature regulation.500－525 ℃ was the optimal nitriding window，where a nitrided layer with uniform strengthening phases，stable structure，and significantly improved hardness could be obtained. Beyond this temperature range，structural deterioration and performance degradation increased significantly. This study clarified the influence of temperature on the strengthening mechanism，and provided a theoretical basis and process optimization basis for the surface strengthening of duplex stainless steel in high-strength corrosion and high-stress service environments.

To tackle environmental issues caused by traditional oil-lubricated bearings，water-lubricated bearings are increasingly used in clean energy equipment. However，water’ s low viscosity often leads to lubrication film rupture，causing bearings to operate in mixed or boundary lubrication regimes and posing severe challenges to material wear resistance. Current research on water-lubricated bearing materials mainly focuses on thermosetting composites，and there is a lack of systematic evaluation of recyclable and easily processable thermoplastics. This study systematically evaluated the tribological properties of three typical thermoplastic materials，namely polyoxymethylene (POM)，polyethylene terephthalate (PET)，and polyetherimide (PEI)，under water-lubricated conditions through full-scale bearing tests simulating actual operating conditions，providing a basis for their engineering application.A self-developed high-performance comprehensive test bench for water-lubricated bearings was used to conduct systematic full-scale bearing tests on bearings made of three thermoplastic materials (POM，PET，and PEI) with a circumferential eight-narrow-groove structure. The tests strictly simulated the actual operating conditions of horizontal hydro-turbine guide bearings:constant load of 4 050 N (specific pressure 0.5 MPa)，rotational speed of 255 r/ min (corresponding to a journal linear speed of 1.2 m/ s)，and an 8 h formal test under tap water lubrication. An integrated sensor network on the test bench collected real-time data of friction torque and normal load. Based on this data，the friction coefficient curve over time was calculated and plotted. Meanwhile，embedded temperature sensors continuously monitored the temperature evolution of the critical interface of the friction pair，with the water tank temperature as the environmental reference. After the test，a precision electronic balance was used to measure the mass difference of the bearing before and after the test，and a feeler gauge was used to quantitatively detect the change in bearing fit clearance. This method accurately measured the mass loss rate of the material. At the micro level，a scanning electron microscope (SEM) was used to conduct high-resolution morphological observation of the bearing load-bearing surface to reveal the micro characteristics of the worn surface. Further，laser confocal microscopy was used to obtain the 3D topography data of the worn surface，and parameters such as arithmetic mean roughness (Ra) and maximum profile height (Rz) were calculated to quantitatively evaluate the degree of surface damage.

Based on systematic test methods and multi-dimensional data analysis，clear and significantly different quantitative results were obtained.In terms of friction performance:POM and PET showed excellent adaptability. After the initial running-in stage，the friction coefficients of both rapidly decreased and entered a stable stage，with steady-state values of 0.110 and 0.109，respectively. Further analysis of data stability found that POM exhibited better operational stability，with a fluctuation range of its friction coefficient of about 0.1 within each 24 s data cycle at the end of the test. In contrast，the friction coefficient of PEI continued to rise throughout the test，eventually reaching a high value of about 0.300 with severe fluctuations，indicating an extremely unstable friction state. In terms of wear characteristics:comparison of test data showed that PET exhibited the most prominent wear resistance，with a mass loss rate of only 0.71％ and the smallest change in bearing fit clearance. POM was the second (mass loss rate of 1.31％). PEI suffered severe wear，with a mass loss rate as high as 4.68％ and an increase in vertical clearance of 2.27 mm，far exceeding the previous two materials. The results of microscopic observation of surface morphology and quantitative analysis of roughness were consistent with the wear data:the worn surface of PET was the smoothest (Ra value of 4.6 μm)，and SEM observation showed that plastic deformation occurred on the bearing surface，along with the generation of fine wear debris. The Ra value of the POM surface was 5.7 μm，and the morphology showed plastic deformation characteristics and flaky debris adhesion. The PEI surface was the most severely damaged，with deep and wide ploughing grooves，significant brittle material spallation zones and abundant large-size wear debris，and the highest surface roughness Ra value (7.9 μm). In terms of thermal behavior response:the temperature rise data of the friction interface directly reflected the friction interface conditions of different materials. POM had the lowest temperature rise (8.82 ℃)，PET was slightly higher(11.33 ℃)，and PEI had the most significant temperature rise (21.21 ℃)，indicating that its friction process generated a large amount of heat and had poor heat dissipation.

POM and PET have appropriate hardness and significant crystalline structures. Under the action of water lubrication and frictional heat generated during the test，the surface layer of the materials could undergo a certain degree of viscoelastic deformation，effectively dissipating part of the mechanical energy. Among them，the POM bearing was mainly dominated by abrasive wear and plastic deformation，and the PET bearing was mainly dominated by fatigue wear，with hydrolytic wear generated due to local high temperature. In contrast，PEI has the highest hardness (85.5 HD) and an extremely high glass transition temperature (217 ℃). This high hardness and high rigidity made it difficult for PEI to adapt and buffer through viscoelastic deformation under cyclic contact stress and shear force，and it was more prone to brittle fracture，leading to flaky or blocky spallation of the surface layer material.

Bearing tests confirmed that POM and PET exhibited excellent comprehensive tribological performance under water-lubricated conditions:low friction coefficient (about 0.11)，low wear rate (mass loss rate≤1.31％)，moderate temperature rise (≤11.33 ℃)，and stable operation. This made them suitable for horizontal hydro-turbine water-lubricated guide bearings. In contrast，due to its brittle delamination wear mechanism，PEI exhibited high friction，high wear，and significant heat accumulation，making it unsuitable for this operating condition. The research results provided direct experimental basis for the selection of thermoplastic materials for water-lubricated bearings and had practical significance for promoting green bearing technology. Future research can focus on the composite modification of POM and PET to improve their adaptability to extreme operating conditions.

With the deepening exploitation of marine resources，engineering equipment serving for long periods in marine environments is exposed to complex coupled corrosion and friction，resulting in severe tribocorrosion problems that shorten the service life of key components and give rise to safety risks and economic losses. Existing studies have shown that WC-10Co-4Cr cermet coatings prepared by high-velocity air fuel(HVAF) spraying exhibit excellent corrosion resistance and wear resistance in marine environments. To further investigate their tribocorrosion behavior in marine environments and analyze the influence of different substrates on coating performance，this study deposited WC-10Co-4Cr coatings by HVAF on the surfaces of 316 stainless steel，40Cr steel，and SKD61 steel，and systematically studied their tribocorrosion performance in a simulated marine environment to reveal the mechanism by which substrate materials affect the protective performance of the coatings.

Cross-sectional microstructural observations of the WC316，WC40，and WC61 coatings showed that the coatings prepared under the same spraying parameters all had thicknesses of about 200 μm，with microstructures characterized by WC particles wrapped in the metallic binder phase，and the coatings were overall dense and free of obvious cracks，indicating that substrate type had no significant effect on coating structure. Phase analysis of the powder and coatings showed that the phase compositions of the coatings were basically consistent with those of the feedstock powder，with only a small amount of decarburization observed. Tribological tests in seawater revealed that the friction coefficients of the three substrates followed the order SKD61 steel>40Cr steel>316 stainless steel，whereas the wear volume showed the opposite trend，namely 316 stainless steel>40Cr steel>SKD61 steel. In comparison，the average friction coefficients and wear volumes of the three coating systems were basically consistent and were all significantly lower than those of their corresponding substrates. Specifically，the friction coefficients of the WC316，WC40，and WC61 coatings were reduced by 28.94％，49.36％，and47.20％，respectively，while the wear rates were reduced by 99.20％，97.74％，and 92.26％，respectively，indicating that the WC-10Co-4Cr coating could effectively improve the tribological performance of the substrates in seawater environments.

Open circuit potential (EOCP) measurements during the tribocorrosion process showed that，for 316 stainless steel，EOCP decreased during the friction stage because of passive film breakdown and gradually recovered after friction stopped，exhibiting a characteristic “passivationbreakdown-repassivation” behavior. In contrast，for 40Cr steel and SKD61 steel，EOCP increased during the friction stage because the loose rust layer was removed，exposing a fresher and more corrosion-resistant surface，and then decreased again after friction ceased. The overall EOCP order was 316 stainless steel>SKD61 steel>40Cr steel. For all coatings，EOCP decreased during the friction stage and recovered after friction stopped，also showing a “passivation-breakdown-repassivation” behavior. Their EOCP ranking remained WC61>WC40>WC316 throughout，which could be attributed to differences in galvanic effects between the coatings and the respective substrates. Potentiodynamic polarization results indicated that friction aggravated the corrosion of 316 stainless steel，as reflected by a lower corrosion potential and a higher corrosion current density. For 40Cr steel and SKD61 steel，however，friction removed the surface rust layer and thereby reduced their corrosion tendency；their corrosion potential and current density were both higher than those under pure corrosion conditions. For the coatings，the corrosion potential under tribocorrosion conditions was higher than that under pure corrosion conditions，which might be due to wear debris filling surface defects and slowing down the corrosion tendency. Nevertheless，their corrosion current density remained relatively high because wear promoted the electrochemical process. Quantitative analysis of the proportion of each damage factor in tribocorrosion showed that the tribocorrosion damage of both coatings and substrates mainly originated from wear itself and the synergistic effect between corrosion and wear.

Based on the prepared coatings，this study systematically examined the influence of different substrates on the tribocorrosion performance of WC-10Co-4Cr coatings in a simulated seawater environment and clarified the underlying mechanisms. The results showed that the coatings significantly improved the tribological performance of all substrates. In particular，the friction coefficient of the WC40 coating decreased by 49.36％，while the WC316 coating exhibited especially outstanding wear resistance，with a wear-rate reduction of up to 99.20％. The electrochemical performance was strongly affected by the substrate. The coating deposited on SKD61 steel (WC61) exhibited the highest open circuit potential and the lowest corrosion current density，demonstrating the best electrochemical stability. The protective mechanism of the coating arose from the synergy of its mechanical wear resistance，corrosion-barrier effect，and coating-substrate galvanic effect，which could effectively delay material failure in tribocorrosion environments. This study provided a theoretical basis for material selection and performance optimization of surface coatings for key components of marine equipment.

With the continuous advancement of military and civil aviation technologies，the service temperatures of key moving components in aero-engines have gradually increased. This leads to prominent friction and wear problems，which severely affect the service life of components and bring severe challenges to the high-performance service of moving components. Therefore，aiming at the high-temperature friction and wear problems of key moving components in aero-engines，this study carried out the design of high-temperature friction-reducing and wear-resistant coatings，and investigated the evolution laws and wear mechanisms of their tribological behaviors under different service temperature conditions，so as to ensure the service performance of aero-engine moving components at high temperatures.Firstly，according to the material composition of friction pairs of key moving components in aero-engines，this study prepared YSZ-CaF2 high-temperature self-lubricating and wear-resistant coatings on GH5188 alloy substrates by atmospheric plasma spraying，and processed IC21 alloy balls as counterpart samples. Then，this study carried out reciprocating friction and wear tests of IC21/ YSZ-CaF2 coating friction pairs at 26，200，400，600，800 ℃ by using a self-developed high-temperature friction and wear test device. On this basis，this study analyzed the wear morphology，microstructure and tribological properties of the coatings by scanning electron microscopy (SEM)，X-ray diffraction (XRD)and white light interferometry. Finally，this study revealed the evolution laws and mechanisms of friction and wear behavior of IC21/ YSZ-CaF2 coating friction pairs with temperature.

The experimental results showed that the friction coefficient and wear rate of YSZ-CaF2 coating first increased and then decreased with the increase of temperature. At 26 ℃，the coating showed good wear resistance and low friction coefficient. However，with the increase of temperature，the friction coefficient and wear rate began to increase and reached the peak values at 400 ℃，which were 0.211 and 4.3×10－4 mm3/(N•m)，respectively. From 400 ℃ to 800 ℃，the friction coefficient and wear rate decreased rapidly and reached the minimum values at 800 ℃，which were 0.109 and 0.27×10－4 mm3/(N•m)，respectively.

The results of wear morphology and composition analysis showed that at 26 ℃，the coating surface was mainly composed of ZrO2 wear-resistant phase and CaF2 lubricating phase，and its hardness was as high as 656.4 HV，thus showing good wear resistance and low friction coefficient. However，at 400 ℃，the coating hardness decreased to 324.6 HV. The hard Cr2O3 and NiO abrasive particles formed by interfacial oxidation cut and damaged the coating surface during reciprocating friction，thus aggravating wear. At the same time，the contents of ZrO2 wear-resistant phase and CaF2 lubricating phase decreased，which weakened the wear resistance and lubrication performance of the coating.

Above 400 ℃，the lubricating effect of CaF2 was enhanced，and the hard Cr2O3 and NiO abrasive particles reacted to form lubricating NiCr2O4，which greatly improved the interfacial lubrication performance of the coating. At the same time，the content of ZrO2 wear-resistant phase increased，enhancing the wear resistance. The further decrease in coating hardness led to surface softening and enhanced material flowability，and a continuous and dense tribofilm was formed during friction，which effectively protected the coating surface and significantly reduced wear. Therefore，the coating exhibited good friction-reducing and wear-resistant properties at 800 ℃.

In summary，the YSZ-CaF2 coating had excellent high-temperature friction-reducing and wear-resistant properties，which could effectively guarantee the high-performance service of key moving components in aero-engines. With the continuous increase of aero-engine service temperature and frequent vibration and impact，the long-term service behavior of friction-reducing and wear-resistant coatings under the coupling of high temperature and vibration should be further studied，so as to support the reliable application of coatings in extreme service environments.

Gold-tin (Au-Sn) alloy electroplating is an important surface treatment technology in the electronics industry. The eutectic Au-Sn solder layer，as a preferred alternative to high-melting-point lead-based solders，is widely used in electronic packaging and other fields. Cyanide-based baths dominated early research，but environmental concerns over highly toxic cyanides have shifted the focus to cyanide-free systems. Cyanide-free technology is eco-friendly and meets global regulatory requirements，yet it still faces challenges such as poor bath stability and low deposition rates，necessitating the development of new baths and additives.

The composition of Au-Sn electroplating baths plays a decisive role in coating performance. Gold and tin salts constitute the primary sources of metal ions，while various additives—including complexing agents，buffering agents，brighteners，leveling agents，and surfactants—are incorporated to optimize the electroplating process and enhance coating quality.

Gold salt selection:Common cyanide-free gold salts include sulfite gold salts (sodium/ potassium gold sulfite)，thiosulfate gold salts (sodium/ potassium gold thiosulfate)，chloroaurates (sodium/ potassium chloroaurate)，and citrate gold salts (sodium/ potassium gold citrate). In sulfite systems，gold exists stably as [Au(SO3)2]3－. However，under acidic or neutral conditions，sulfite decomposition leads to the oxidation and precipitation of gold ions. The addition of stabilizers such as hydroxyethylidene diphosphonic acid (HEDP) effectively prevents gold ion oxidation and significantly enhances bath stability. Thiosulfate gold salts were first reported by IBM researchers in the early 1990s，but they suffer from poor bath stability at pH<8. Thiosulfate disproportionation produces colloidal sulfur，which adsorbs onto the electrode surface and causes coating embrittlement. Research by Harbin Institute of Technology demonstrated that the addition of stabilizers such as ammonium citrate effectively maintains long-term bath stability. Citrate gold salts form stable complexes with citrate ions and exhibit good bath stability，operational simplicity，and high impurity tolerance，and the ability to produce dense，strongly adherent，glossy coatings with good uniformity and corrosion resistance. Chloroaurates provide high-purity Au3＋ions，enabling precise control of Au-Sn alloy composition by adjusting the ratio of chloroaurates to tin salts.

Tin salt selection:In micro-cyanide baths，organic tin salts such as tin citrate and tin pyrophosphate serve as the primary tin sources. Cyanide-free baths widely employ stannous sulfate，stannous methanesulfonate，stannous citrate，and stannous chloride. Stannous sulfate systems are cost-effective but show relatively poor throwing power，with an optimal concentration range of 30－40 g/ L. Stannous methanesulfonate systems exhibit good solderability，conductivity，and corrosion resistance. Their current efficiency reaches 95％，and their throwing power reaches 94.57％. They can completely cover the inner surface of copper tubes. The methanesulfonic acid (MSA) electrolyte exhibits favorable biodegradability. The chemical oxygen demand (COD) of the MSA electrolyte wastewater is significantly lower than that of conventional systems containing aromatic additives. Stannous citrate dissociates to release Sn2＋and citrate ions. The high stability of the complex maintains an extremely low concentration of free Sn2＋，thereby inhibiting hydrolysis and enhancing bath stability. In citrate systems，the optimal brightener concentration ranges from 0.50 to 5.00 g/ L at pH 4－8. Tin pyrophosphate avoids the use of toxic cyanides while providing excellent thermal stability(melting point:1 080 ℃). Its Sn-P covalent network structure (bond energy:532 kJ/ mol) imparts high hardness and strength to the coatings.

Complexing agent selection:Complexing agents could improve bath stability，buffer pH value，enhance coating brightness，and strengthen adhesion to the substrate. Ammonium citrate functions as both a buffer and a complexing agent，forming soluble complexes with Au3＋and Sn2＋to prevent hydrolysis and precipitation. Research indicates that dual-complexing-agent systems (e. g.，citrate-tartrate) significantly improve the uniformity of Au -Sn co -deposition. Potassium pyrophosphate serves as a complexing agent specifically for tin ions，forming stable[Sn(P2O7)]2－complexes that prevent the oxidation or hydrolysis of Sn2＋. However，its relatively high cost and the need for precise parameter control limit its widespread application. Methanesulfonic acid，as a strong organic acid with complete ionization，enhances conductivity and inhibits Sn2＋hydrolysis while reducing electrode polarization and improving plating efficiency and coating uniformity.

Additive selection:Additives critically influence electroplating parameters，coating quality，bath stability，and service life. Brighteners，including organic compounds such as polyamines，aliphatic aldehydes，and aromatic carboxylic acids，can eliminate dendritic crystallization and produce glossy coatings. Graphene quantum dots (GQDs)，used as auxiliary brighteners，reduce coating roughness to below 50 nm through grain refinement. Surfactants，exemplified by sodium dodecyl sulfate (SDS)，could improve coating uniformity and reduce pinhole defects by decreasing surface tension at an addition level of 0.05－0.20 g/ L. Leveling agents，particularly thiourea and its derivatives (0.10－0.50 g/ L)，achieve microscopic leveling through selective adsorption on surface protrusions，thereby reducing surface roughness and improving coating quality. Buffering agents，including boric acid，ammonium citrate，and amino acids such as glycine (optimal pH:4－6)，maintain stable bath pH and prevent metal ion hydrolysis.

Application status:In power chip soldering，stannous methanesulfonate systems，with a current efficiency of 95％，significantly reduce coating porosity，lowering the soldering void rate from 12％ to below 5％ and thereby meeting the reliability requirements of automotive-grade IGBT modules. In optoelectronic device packaging，coatings prepared by citrate systems exhibits a surface roughness of<0.1 μm，a 30％ increase in interfacial bonding strength，and a shear strength of 58 MPa. A leveling agent concentration of 0.30 g/ L enables a bump height deviation of<±3％，supports 5 nm process technology，and yields a solder ball coplanarity compliance rate of 99.2％. In the medical field，the biodegradability of stannous methanesulfonate baths meets medical safety standards，with metal ion release below 0.1 mg/ L. The baths passed ISO 10993 biocompatibility certification. In automotive electronics，the high-temperature resistance of tin pyrophosphate is utilized in oxygen sensors，with coating service life exceeding 5 000 h in a 900 ℃ exhaust environment.

Cyanide-free Au-Sn electroplating technology has achieved remarkable progress in bath development and additive optimization. After composition optimization，the coating performance can be comparable to that of cyanide-based processes，while offering prominent environmental advantages. Various combinations of gold salts and tin salts are suitable for different application scenarios. Dual-complexing-agent systems and novel additives further improve bath stability and coating quality.

Future research should focus on developing high-performance，eco-friendly complexing agents and intelligent additives. It should also focus on establishing theoretical models correlating bath composition with coating performance. The integration of computational materials science and machine learning can accelerate the development of optimal formulations. In addition，promoting process standardization and expanding applications into emerging fields will facilitate technological progress and industrial application.

Sulfuric acid is an important basic chemical feedstock in the national economy. Equipment used for its production，storage，and transportation typically operates for long periods in sulfuric acid media with varying concentrations and wide temperature fluctuations，and relative motion often exists between components. As a result，such equipment inevitably suffers from the combined effects of corrosion and frictional wear. Corrosion-friction coupling failure has become a critical bottleneck leading to unexpected equipment shutdown，shortened service life，and increased safety risks，and it remains a common technical problem that urgently needs to be addressed in the chemical industry. Hastelloy C-276，owing to its excellent corrosion resistance，is regarded as a preferred material for harsh corrosive environments and has broad application prospects in the sulfuric acid industry. However，existing studies have mainly focused on the static corrosion behavior or dry friction and wear behavior of C-276 in sulfuric acid environments. Comprehensive studies on the combined effects of sulfuric acid temperature and concentration on its static corrosion，electrochemical characteristics，and tribocorrosion performance remain limited. In particular，the interaction mechanism between corrosion and wear has not yet been clarified，making it difficult to provide technical support for material selection，failure prediction，and operating condition optimization under multiparameter coupling conditions in practical industrial applications.

To investigate the service suitability of Hastelloy C-276 under corrosion-friction coupling conditions in the sulfuric acid industry，the corrosion and tribological behaviors of the alloy in 5％－60％ sulfuric acid (mass fraction，the same below) were systematically investigated by means of corrosion weight loss tests，electrochemical tests，and friction and wear tests. The wear scar micromorphologies were characterized using scanning electron microscopy and a wear scar profilometer. Results showed that sulfuric acid concentration and temperature played key roles in the corrosion behavior of the alloy by regulating the stability of the passive film on its surface. In a 5％ sulfuric acid environment at 25－80 ℃，a dense and stable passive film was formed on the alloy surface，and excellent corrosion resistance was exhibited. In 20％ sulfuric acid at 25－40 ℃，corrosion was accelerated because the dissolution-reformation cycle of the passive film became unbalanced，and a maximum corrosion weight loss of 6.2×10－4 g/ cm2 was reached within 72 h. At 80 ℃，the corrosion rate increased stepwise with increasing sulfuric acid concentration. When the concentration was≤40％，the corrosion rate increased slowly and remained below 1×10－3 g/(cm2•h)；when the concentration increased to 60％，the corrosion rate sharply rose to nearly 8×10－3 g/(cm2•h)，which was attributed to the accelerated dissolution of the passive film under high-temperature and high-concentration sulfuric acid conditions. Electrochemical tests conducted at 25 ℃showed that Hastelloy C-276 exhibited the optimal corrosion resistance in 40％ sulfuric acid，where it had the lowest corrosion current density(2.27×10－5 A/ cm2) and the highest polarization resistance (478 Ω•cm2)，confirming its excellent electrochemical stability. In contrast，in 20％ sulfuric acid，the alloy exhibited the highest corrosion current density (7.52×10－5 A/ cm2)，the lowest polarization resistance (211Ω•cm2)，and a relatively low impedance modulus｜Z｜at 0.01 Hz (1.74×105 Ω•cm2)，indicating that the passive film was unstable in this environment and that the corrosion rate was the highest. In terms of tribological performance，the tribological performance of the alloy was significantly improved in a 40％ sulfuric acid environment at 25 ℃. The average friction coefficient was reduced from approximately 0.79 to approximately 0.22，and the volumetric wear rate was decreased from approximately 20.48×10－5 mm3/(N•m) to approximately 5.24×10－5 mm3/(N•m). This improvement was attributed to the synergistic effects of solid lubrication and dynamic repair of the passive film in the sulfuric acid environment. This synergistic effect suppressed adhesive wear between the alloy and the grinding ball. It also transformed the wear mechanism into mild abrasive wear or corrosive wear. As a result，shallow U-shaped wear scars were formed，and damage to the counter ball was minimized.

In summary，Hastelloy C-276 exhibited excellent corrosion resistance and tribological performance in40％ sulfuric acid at 25 ℃ and could satisfy the service requirements under corrosion-friction coupling conditions at this concentration in the sulfuric acid industry. The results of this study could provide technical support for material selection and operation optimization of equipment used in the sulfuric acid industry.