Surface Treatment: A Comprehensive Guide from Core Definition to Practical Operation

In the process of manufacturing industry transforming from "basic production" to "high-end customization", the surface performance of materials often determines the final value of products. Whether it is the anti-corrosion requirement for metal parts or the wear resistance and aesthetic requirements for plastic casings, "Surface Treatment" plays the dual role of a "material makeup artist" and a "performance enhancer". It is not a single process, but an integrated system covering chemical, physical, mechanical and other fields of technology. By changing the morphology, composition or structure of the material surface, it makes up for the performance defects of the base material itself and expands the application boundaries of materials. This article will comprehensively analyze surface treatment technology from four dimensions: essential definition, process types, industry adaptation, and practical operation, providing references for actual production and selection.

I. What is the Essential Definition of Surface Treatment? How Does Its Core Technical Logic Change Material Performance?

Surface treatment refers to a general term for processes that modify the material surface through physical, chemical or mechanical methods to obtain the required surface properties (such as corrosion resistance, wear resistance, aesthetics, electrical conductivity, etc.). Its core goal is to "promote strengths and make up for weaknesses" - it not only retains the mechanical properties of the base material itself (such as strength and toughness), but also makes up for the performance shortcomings of the base material in specific scenarios (such as the easy corrosion of metals and the easy scratching of plastics) through surface modification.

From the perspective of technical logic, surface treatment mainly improves material performance through three paths: surface coating, surface conversion and surface alloying. Surface coating is the most common path. By forming one or more functional coatings (such as metal coatings, organic coatings, ceramic coatings) on the material surface, the base material is isolated from harsh external environments (such as humidity, chemical reagents, friction). For example, the "cathodic electrophoresis + electrostatic spraying" process for automobile bodies first forms a uniform anti-rust coating (thickness 5-20μm) on the metal surface through electrophoresis, and then covers it with a colored topcoat through electrostatic spraying. This not only achieves anti-corrosion (salt spray test can reach more than 1000 hours), but also meets aesthetic requirements. Surface conversion refers to the formation of a dense conversion film (such as phosphating film and passivation film of metals) on the material surface through chemical or electrochemical reactions. Such films are tightly combined with the base material and can significantly improve surface hardness and corrosion resistance. Taking the phosphating treatment of steel parts as an example, by immersing the parts in a phosphate solution, a phosphating film with a thickness of 1-10μm is formed on the surface, and its adhesion can reach more than 5MPa, which can effectively prevent the coating from falling off during the subsequent painting process. Surface alloying introduces alloying elements into the surface layer of the material through high-temperature diffusion, ion implantation and other methods to form an alloy layer with a gradual composition of the base material, thereby improving the surface wear resistance and high-temperature resistance. For example, the "aluminizing" treatment of aero-engine blades diffuses aluminum elements to the blade surface at high temperature to form an Al₂O₃ protective film, enabling it to work for a long time in a high-temperature environment of 800-1000℃ and avoid oxidation and corrosion.

From the perspective of process characteristics, surface treatment must meet two major requirements: "accuracy" and "compatibility". Accuracy is reflected in the precise control of the treatment effect. For example, the deviation of the coating thickness must be controlled within ±5%, and the porosity of the conversion film must be less than 0.1% to ensure stable performance; compatibility means that the treatment process must match the characteristics of the base material. For example, due to poor heat resistance (usually below 150℃), plastic materials cannot use high-temperature spraying processes and need to choose low-temperature plasma treatment or vacuum coating technology. In addition, surface treatment must also take environmental protection into account. With the tightening of global environmental regulations (such as the EU RoHS directive and China's VOCs emission standards), traditional processes such as chromium-containing passivation and solvent-based spraying are gradually being replaced by environmentally friendly processes such as chromium-free passivation and water-based paint spraying. A home appliance enterprise reduced VOCs emissions by 85% by changing the solvent-based spraying of refrigerator door panels to water-based spraying, and at the same time increased the coating utilization rate from 60% to 92%.

II. What are the Specific Types of Surface Treatment? What are the Differences in Process Characteristics and Performance Between Different Types?

According to technical principles and application scenarios, surface treatment processes can be divided into three categories: chemical surface treatment, physical surface treatment and mechanical surface treatment. Each category includes a variety of subdivided processes. Different processes have significant differences in treatment effects, applicable base materials and costs, and need to be accurately selected according to product requirements.

(I) Chemical Surface Treatment: Realizing Surface Modification Through Chemical Reactions to Adapt to High Anti-Corrosion Requirements

Chemical surface treatment uses chemical reagents as the medium to cause chemical reactions on the material surface through immersion, spraying and other methods to form functional films. Its core advantages are that the film is tightly combined with the base material and has strong corrosion resistance, which is suitable for inorganic materials such as metals and ceramics. Common subdivided processes include phosphating treatment, passivation treatment and electroless plating.

Phosphating treatment is mainly used on the surface of metals such as steel and zinc alloys. Through the reaction between the phosphate solution and the metal surface, a phosphate conversion film (mainly composed of Zn₃(PO₄)₂, FePO₄, etc.) is formed. The film thickness is usually 1-15μm, the hardness can reach 300-500HV, and the salt spray test life can reach 200-500 hours. Its core function is to improve the adhesion of the subsequent coating. For example, auto chassis parts must undergo phosphating treatment before spraying, otherwise the coating adhesion will decrease by more than 40%, and peeling is likely to occur. According to the composition of the phosphating solution, it can be divided into zinc-based phosphating (suitable for normal temperature treatment, uniform film) and manganese-based phosphating (suitable for high-temperature treatment, high film hardness). The hardness of the manganese-based phosphating film can reach more than 500HV, which is often used for wear-resistant parts such as gears and bearings.

Passivation treatment forms a dense oxide film on the metal surface through the reaction of oxidizing chemical reagents (such as nitric acid, chromate) with the metal surface. It is mainly used for materials such as stainless steel and aluminum alloys to improve their corrosion resistance. For example, stainless steel tableware must undergo nitric acid passivation treatment after production to form a Cr₂O₃ oxide film on the surface. The salt spray test life is increased from 100 hours to more than 500 hours, and metal ion precipitation can be avoided (complying with the food contact material standard GB 4806.9). Traditional passivation processes mostly use chromate, but the hexavalent chromium it contains is toxic. At present, it has been gradually replaced by chromium-free passivation (such as zirconium salt passivation and molybdate passivation). A stainless steel enterprise reduced the heavy metal content of its products to less than 0.001mg/kg by adopting the zirconium salt passivation process, and at the same time, the corrosion resistance is equivalent to that of the traditional process.

Electroless plating deposits metal ions (such as Ni²⁺, Cu²⁺) on the material surface through chemical reducing agents (such as sodium hypophosphite) without external current to form a metal coating. It is suitable for non-conductive base materials such as plastics and ceramics. For example, in the electroless nickel plating process of ABS plastic casings, the plastic surface is first roughened and sensitized to make it conductive, and then a nickel layer with a thickness of 5-20μm is deposited through electroless plating. The coating conductivity can be below 10⁻⁵Ω·cm, and it also has good wear resistance (wear loss is less than 0.1mg per 1000 frictions), which is often used for electronic connectors and electromagnetic shielding parts.

(II) Physical Surface Treatment: Realizing Surface Coating Through Physical Means to Adapt to High Aesthetic and Functional Requirements

Physical surface treatment does not involve chemical reactions. It mainly forms coatings on the material surface through physical deposition, ion bombardment and other methods. Its core advantages are environmental protection and a wide range of coating types (such as metals, ceramics, organic films), which are suitable for various base materials such as metals, plastics and glass. Common subdivided processes include vacuum coating, plasma treatment and spraying.

Vacuum coating deposits coating materials on the base material surface in a vacuum environment through evaporation, sputtering, ion plating and other methods to form an ultra-thin coating (usually 0.1-10μm in thickness). According to the coating material, it can be divided into metal coating (such as aluminum, chromium, titanium) and ceramic coating (such as TiO₂, SiO₂). Metal coating is mainly used to improve aesthetics and conductivity. For example, the vacuum aluminum plating process for mobile phone middle frames can form a mirror effect, and at the same time improve surface wear resistance through subsequent wire drawing treatment; ceramic coating has high hardness and corrosion resistance. For example, the TiN ceramic coating (thickness 2-5μm) of kitchen knives has a hardness of more than 2000HV, and the sharpness retention time is 3 times longer than that of uncoated knives. Ion plating is a high-end process in vacuum coating. It makes the coating more tightly combined with the base material through ion bombardment, and the adhesion can reach more than 10MPa. It is often used for parts in the aerospace field (such as the CrAlY coating of turbine blades), which can maintain stable performance for a long time in a high-temperature environment.

Plasma treatment uses low-temperature plasma (temperature 200-500℃) to modify the material surface. Its main function is to improve surface roughness and hydrophilicity, and it is suitable for polymer materials such as plastics and rubber. For example, before spraying PP plastics, they need to undergo plasma treatment. The surface contact angle is reduced from more than 90° to less than 30°, and the coating adhesion is increased by more than 50% to avoid "paint peeling"; in the medical field, after plasma treatment of silica gel catheters, the surface hydrophilicity is improved, which can reduce the friction resistance when inserted into the human body and improve patient comfort. In addition, plasma treatment can also be used for surface activation. For example, in the chip packaging process, plasma treatment of the chip surface can improve the wettability of the solder and reduce the welding defect rate.

Spraying process atomizes the coating (such as paint, powder coating) through a high-pressure spray gun and sprays it on the material surface to form an organic coating. Its core advantages are low cost and rich colors, which are suitable for products such as household appliances and furniture. According to the type of coating, it can be divided into solvent-based spraying (such as automotive topcoat), water-based spraying (such as refrigerator door panels) and powder spraying (such as aluminum alloy doors and windows). Powder spraying has the best environmental protection due to no VOCs emissions. Its coating thickness is usually 50-150μm, the hardness can reach more than 2H (pencil hardness test), and the impact resistance can reach 50cm·kg (falling ball impact test). It is often used for products such as outdoor furniture and traffic guardrails, and can resist the erosion of ultraviolet rays and rainwater.

(III) Mechanical Surface Treatment: Changing Surface Morphology Through Mechanical Action to Adapt to High Flatness and Wear Resistance Requirements

Mechanical surface treatment changes the surface roughness and flatness of materials through mechanical means such as grinding, polishing and sandblasting. Its core advantages are simple process and low cost, which are suitable for materials such as metals, stones and glass. Common subdivided processes include grinding and polishing, sandblasting treatment and rolling processing.

Grinding and polishing polish the material surface through abrasives (such as sandpaper, grinding wheels, polishing pastes) to reduce the surface roughness (Ra) and improve flatness and gloss. For example, in the production process of stainless steel sinks, multiple processes such as rough grinding, fine grinding and polishing are required. The surface Ra value is reduced from more than 5μm to less than 0.1μm to form a mirror effect; in the field of precision machinery, after grinding and polishing of bearing balls, the surface Ra value can be reduced to less than 0.02μm, which can reduce friction loss and improve service life. According to the polishing accuracy, it can be divided into rough polishing (Ra 0.8-1.6μm), fine polishing (Ra 0.1-0.8μm) and ultra-fine polishing (Ra <0.1μm). Ultra-fine polishing is often used for high-precision products such as optical lenses and semiconductor wafers.

Sandblasting treatment sprays abrasives (such as quartz sand, alumina sand) onto the material surface through high-pressure air flow to form a rough surface. Its core functions are to remove surface oxide scale and oil, or to obtain a matte effect. For example, before anodizing aluminum alloy profiles, they need to undergo sandblasting treatment to remove the surface oxide film and ensure the uniformity of the anodized film; in the construction field, after sandblasting treatment of stones, a matte effect is formed on the surface, which can avoid glare and improve anti-skid performance. According to the abrasive particle size, sandblasting can be divided into coarse sandblasting (particle size 0.5-2mm, surface Ra 10-20μm) and fine sandblasting (particle size 0.1-0.5mm, surface Ra 1-10μm). The selection of different particle sizes depends on the surface requirements of the product. For example, fine sand is mostly used for sandblasting of medical devices to avoid excessive surface roughness leading to bacterial growth.

Rolling processing uses rolling tools to cold extrude the metal surface, causing plastic deformation on the surface to form a dense metal layer. Its core advantage is to improve surface hardness and wear resistance. For example, after rolling processing of the inner hole of the hydraulic cylinder, the surface Ra value is reduced from 1.6μm to less than 0.2μm, the hardness is increased by 20%-30%, and at the same time, the sealing performance of the inner hole is improved to reduce hydraulic oil leakage; in the automotive field, after rolling processing of the main journal of the engine crankshaft, the fatigue life can be extended by more than 50%, which can withstand higher speed and load.

To intuitively show the differences between different types of surface treatment processes, a comparison can be made through the following table:

III. How Does Surface Treatment Adapt to the Special Needs of Different Industries? What are the Application Focus and Technical Difficulties of Each Industry?

Due to differences in product usage scenarios and performance requirements, different industries have significant "customized" demands for surface treatment. The selection of surface treatment processes must be closely combined with industry pain points, such as the anti-corrosion and aesthetic requirements of the automotive industry, the biocompatibility and sterility requirements of the medical industry, and the conductivity and precision requirements of the electronics industry, to maximize the process value.

(I) Automotive Industry: Balancing Anti-Corrosion, Aesthetics and High-Temperature Resistance to Cope with Complex Working Conditions

Automotive products need to be exposed to outdoor environments (ultraviolet rays, rainwater, salt spray) for a long time, and at the same time, components such as the engine compartment need to withstand high temperatures (100-200℃). Surface treatment must meet three core requirements: anti-corrosion, aesthetics and high-temperature resistance.

In the field of vehicle bodies, the surface treatment adopts a three-layer system of "cathodic electrophoresis + intermediate coating + topcoat": the cathodic electrophoresis layer (thickness 15-25μm) serves as the base layer, forming a uniform anti-rust coating through electrophoretic deposition. Its salt spray test life can reach over 1000 hours, resisting erosion from rainwater and deicing agents. The intermediate coating (thickness 30-40μm) mainly functions to fill tiny defects on the vehicle body surface, improve flatness, and enhance the adhesion of the topcoat. The topcoat layer (thickness 20-30μm) is divided into metallic paint and solid-color paint. Metallic paint incorporates aluminum flakes or mica particles to create rich visual effects, while solid-color paint focuses on color uniformity and weather resistance (ultraviolet aging test can reach over 1000 hours with a color difference ΔE < 1). An automotive manufacturer optimized electrophoretic process parameters (such as voltage and temperature), increasing the throwing power of the electrophoretic layer to over 95%, ensuring that hidden areas like the vehicle body cavity and welds also form a complete coating to avoid "local rusting".

In the field of engine compartment components, surface treatment focuses on high-temperature resistance and oil resistance. For example, engine brackets adopt the "high-temperature phosphating + silicone spraying" process: the high-temperature phosphating layer (thickness 5-10μm) can remain stable at 200℃, and the silicone coating (thickness 20-30μm) has excellent oil resistance, resisting erosion from engine oil with a service life of over 5 years. Exhaust pipes undergo "high-temperature enamel" treatment: enamel coating is sprayed on the metal surface and sintered at high temperature (800-900℃) to form an enamel layer with a thickness of 50-100μm, which has a high-temperature resistance of over 600℃ and prevents the exhaust pipe from oxidative rusting at high temperatures.

The technical difficulties of surface treatment in the automotive industry lie in "multi-process coordination" and "cost control": multi-process coordination requires ensuring adhesion matching between coatings. For example, the adhesion between the intermediate coating and the topcoat must reach over 10MPa to avoid "interlayer peeling"; cost control requires selecting efficient and low-cost processes due to the large output of automobiles (annual output of a single model can reach over 100,000 units). For instance, the bath solution of cathodic electrophoresis can be recycled with a utilization rate of over 95%, effectively reducing unit costs.

(II) Medical Industry: Focusing on Biocompatibility and Sterility to Ensure Usage Safety

Medical products are in direct contact with human tissues or body fluids. Surface treatment must meet three core requirements: biocompatibility (non-toxicity, non-sensitization), sterility (withstanding high-temperature sterilization or chemical sterilization), and corrosion resistance (withstanding disinfection solution cleaning), while complying with strict industry standards (such as ISO 10993 and GB/T 16886).

In the field of implantable medical devices (such as artificial joints and cardiac stents), the core goal of surface treatment is to improve biocompatibility and osseointegration ability. For example, titanium alloy artificial joints adopt the "hydroxyapatite (HA) coating" treatment: HA powder is deposited on the joint surface through plasma spraying to form a coating with a thickness of 50-100μm. The HA component is similar to human bone, promoting the adhesion and proliferation of osteoblasts, increasing the bonding strength between the artificial joint and bone by over 30%. At the same time, the HA coating has good biocompatibility, non-toxicity, and non-sensitization, complying with the ISO 10993-1 biocompatibility standard. Cardiac stents adopt "drug-coated" surface treatment: a polymer drug-loaded layer (such as paclitaxel and rapamycin) with a thickness of 1-5μm is coated on the metal stent surface. After stent implantation, the drug is slowly released, inhibiting the proliferation of vascular smooth muscle cells and reducing the in-stent restenosis rate from 30%-40% (for bare metal stents) to below 5% (for drug-coated stents). Such coatings need to have good biodegradability, which can be metabolized and absorbed by the human body after drug release, avoiding long-term retention that may cause inflammatory reactions. A medical enterprise has developed a degradable drug-coated stent that achieves a 90% drug release rate and a controllable degradation cycle of 6-12 months, which is currently in the clinical trial stage.

In the field of non-implantable medical devices (such as surgical instruments and disinfection containers), surface treatment focuses on solving the problems of "sterility" and "corrosion resistance". Stainless steel surgical scissors adopt the "electropolishing + passivation" combined process: electropolishing removes tiny burrs on the surface through electrochemical action, reducing the surface Ra value to below 0.05μm and reducing bacterial adhesion sites; subsequent passivation treatment forms a Cr₂O₃ oxide film with a salt spray test life of over 1000 hours, which can withstand high-temperature and high-pressure sterilization (134℃, 0.2MPa steam) and erosion from chlorine-containing disinfection solutions (such as 84 disinfectant), ensuring safety during repeated use. The surface treatment of dental handpieces (high-speed instruments for tooth grinding) is more precise: their metal shells adopt the "vacuum titanium plating" process to form a titanium coating with a thickness of 2-5μm, which has a hardness of over 1500HV and can resist high-frequency friction during dental grinding (rotational speed up to 400,000 r/min). At the same time, the titanium coating has good biocompatibility, avoiding metal ion precipitation that may irritate oral mucosa.

The technical difficulty of surface treatment in the medical industry lies in the "balance between performance and safety": on the one hand, the coating needs to have excellent functionality (such as drug release and wear resistance); on the other hand, the risk of coating detachment must be strictly controlled (such as HA coating detachment may cause thrombosis). Therefore, strict adhesion tests (such as cross-cut test with adhesion ≥ 5B grade) and in vitro degradation tests (such as immersion in simulated body fluid for 30 days with a coating weight loss rate ≤ 1%) are required to ensure safety. In addition, the surface treatment process of medical products must pass GMP (Good Manufacturing Practice) certification. The cleanliness of the production environment (such as a Class 10,000 clean workshop) and the purity of raw materials (such as medical-grade titanium powder with a purity ≥ 99.99%) must comply with strict standards, which also increases process costs and technical thresholds.

(III) Electronics Industry: Pursuing Precision and Functionality to Adapt to Miniaturization and High Reliability Requirements

Electronic products (such as chips, circuit boards, and connectors) exhibit "miniaturization" and "high integration" characteristics. Surface treatment must meet three core requirements: high precision (coating thickness deviation ≤ 0.1μm), high conductivity (resistivity ≤ 10⁻⁶Ω·cm), and high reliability (stable performance in high-low temperature and humid-heat environments), while adapting to the processing requirements of ultra-small sizes (such as chip pin pitch ≤ 0.1mm).

In the field of chip manufacturing, surface treatment runs through the entire "wafer manufacturing - packaging and testing" process. In the wafer manufacturing stage, the silicon wafer surface undergoes "oxide layer growth" treatment: a SiO₂ insulating layer with a thickness of 10-100nm is formed through high-temperature (1000-1200℃) oxidation, serving as the gate insulating layer of chip transistors. The thickness uniformity deviation must be controlled within ±5%; otherwise, the transistor threshold voltage will fluctuate (deviation exceeding 0.1V), affecting chip performance. In the chip packaging stage, pins (such as QFP packaging pins) adopt the "electroplated nickel-gold" process: a nickel layer with a thickness of 1-3μm is first electroplated (to improve adhesion and wear resistance), and then a gold layer with a thickness of 0.1-0.5μm is electroplated (to reduce contact resistance). The resistivity of the gold layer must be ≤ 2.4×10⁻⁸Ω·cm to ensure stable conductivity between the chip and the circuit board. In addition, the chip surface also undergoes "underfill coating" treatment: epoxy resin is filled between the chip and the substrate through a dispensing process to form a glue layer with a thickness of 50-100μm, improving the chip's anti-drop performance (able to withstand a 1.5m drop onto a concrete floor without damage). A chip manufacturer's test shows that the drop failure rate of chips adopting this process is reduced from 15% to below 2%.

In the field of printed circuit boards (PCBs), the core of surface treatment is to improve the solderability and corrosion resistance of pads. Common processes include "Hot Air Solder Leveling (HASL)", "Electroless Nickel Immersion Gold (ENIG)", and "Immersion Silver". The HASL process immerses the PCB in molten tin-lead alloy (230-250℃), then uses hot air to blow off excess solder, forming a tin-lead coating with a thickness of 5-20μm on the pad surface. It has low cost (approximately 0.2 CNY/cm²) and good solderability, suitable for PCBs of consumer electronics (such as TVs and routers); however, its poor surface flatness (Ra value ≥ 1μm) makes it unable to adapt to high-density packaging with chip pin pitch ≤ 0.3mm. The ENIG process forms a "nickel layer (5-10μm) + gold layer (0.05-0.1μm)" structure on the pad surface, with high surface flatness (Ra value ≤ 0.1μm) and strong corrosion resistance (salt spray test life ≥ 500 hours), suitable for high-density PCBs of mobile phones and laptops; however, its process is complex, and the cost is 3-5 times that of HASL (approximately 0.8 CNY/cm²). The immersion silver process forms a silver layer with a thickness of 0.1-0.3μm on the pad surface through chemical replacement reaction, with excellent surface flatness and solderability, and no "black pad effect" of the gold layer (solder joint failure caused by the reaction between the gold layer and the nickel layer). It is suitable for PCBs of automotive electronics (such as in-vehicle navigation) and can withstand high-low temperature cycle environments (-40℃ to 125℃) with no solder joint detachment after 1000 cycles.

In the field of electronic connectors (such as USB interfaces and RF connectors), surface treatment must balance conductivity and wear resistance. Connector pins mostly adopt a three-layer structure of "electroplated copper + electroplated nickel + electroplated gold": the copper layer (thickness 10-20μm) ensures high conductivity, the nickel layer (thickness 1-3μm) improves wear resistance, and the gold layer (thickness 0.1-0.5μm) reduces contact resistance. For example, the gold layer thickness of USB Type-C connector pins must be ≥ 0.15μm, with a plug-in life of over 10,000 times and a contact resistance change of ≤ 10mΩ after each plug-in. Some high-end RF connectors (such as those for 5G base stations) also adopt the "electroplated palladium-nickel alloy" process. The palladium-nickel alloy layer (thickness 1-2μm) has 5-10 times the wear resistance of the gold layer and a lower cost (approximately 60% of the gold layer cost), which can meet the long-term stable operation (service life ≥ 5 years) of 5G equipment.

The technical difficulties of surface treatment in the electronics industry lie in "miniaturized processing" and "environmental adaptability": miniaturized processing requires achieving uniform coatings on ultra-small size substrates (such as chip pins with a width ≤ 0.05mm), which requires high-precision electroplating equipment (such as vertical continuous electroplating lines) to control the current density deviation ≤ 1%; environmental adaptability requires the coating to have stable performance in extreme environments (such as high-low temperature cycles of -55℃ to 150℃ and 95% humidity). For example, the surface treatment of automotive electronic PCBs must pass 1000 high-low temperature cycle tests without coating detachment or solder joint failure.

(IV) Aerospace Industry: Breaking Through Extreme Environment Limitations to Adapt to High-Temperature, High-Pressure and High-Radiation Requirements

Aerospace products (such as engine blades, satellite casings, and rocket fuel tanks) work in extreme environments for a long time (such as engine combustion chamber temperature ≥ 1500℃, satellite orbit vacuum and high radiation, and high-pressure impact during rocket launch). Surface treatment must have ultra-high temperature resistance (long-term service temperature ≥ 1000℃), ultra-high corrosion resistance (withstanding space plasma erosion), and ultra-high mechanical properties (impact strength ≥ 100MPa), making it a "high-end test ground" for surface treatment technology.

In the field of aero-engines, the surface treatment of high-temperature components is a core technical difficulty. Aero-engine turbine blades (operating temperature 1200-1500℃) adopt the "Thermal Barrier Coating (TBC)" treatment, with a typical structure of "metal bond coat (MCrAlY, thickness 50-100μm) + ceramic topcoat (YSZ, yttria-stabilized zirconia, thickness 100-300μm)". The metal bond coat is prepared by plasma spraying, which can form an Al₂O₃ oxide film at high temperature to prevent oxidation of the base alloy (such as nickel-based superalloy); the ceramic topcoat has a low thermal conductivity (≤ 1.5W/(m·K)), which can reduce the blade base temperature by 100-200℃ and extend the blade service life from 1000 hours (without coating) to over 3000 hours (with coating). To further improve high-temperature resistance, some advanced engine blades also use "Electron Beam Physical Vapor Deposition (EB-PVD)" to prepare the ceramic topcoat, forming a columnar crystal structure. Its thermal shock resistance (no cracking when rapidly cooling from 1500℃ to room temperature) is 2-3 times that of the plasma-sprayed coating, suitable for ultra-high temperature areas such as combustion chambers. An aero-engine enterprise's test shows that blades adopting the EB-PVD coating can withstand short-term high-temperature impact of 1600℃.

In the field of spacecraft (such as satellites and space stations), surface treatment needs to solve the problems of "performance stability in vacuum environment" and "radiation resistance". Satellite casings adopt the "anodization + Electrostatic Discharge (ESD) coating" treatment: the aluminum alloy casing first forms an Al₂O₃ film layer with a thickness of 10-20μm through anodization to improve the resistance to space plasma erosion (no obvious corrosion after 5 years of exposure in space); then an ESD coating (such as epoxy coating doped with carbon nanotubes) with a thickness of 5-10μm is coated, and the surface resistance is controlled at 10⁶-10⁹Ω to avoid electrostatic accumulation and discharge in the vacuum environment, which may damage satellite electronic equipment. The surface of the space station's solar panels adopts "anti-radiation coating" treatment: a SiO₂-TiO₂ composite coating with a thickness of 0.1-0.5μm is deposited on the solar panel glass surface through vacuum coating, which can resist space ultraviolet (UV) and high-energy particle radiation. The conversion efficiency attenuation rate of solar cells is reduced from 20%/year (without coating) to below 5%/year, ensuring long-term energy supply for the space station (power supply stability ≥ 99.9%).

In the field of rocket fuel tanks (such as liquid hydrogen tanks, operating temperature -253℃), surface treatment needs to solve the problems of "low-temperature toughness" and "sealing performance". The tank material is mostly aluminum alloy, adopting the "chemical milling + passivation" process: chemical milling removes surface stress concentration areas by controlling the corrosion depth (5-10μm) to improve the low-temperature toughness of the material (impact toughness ≥ 50J/cm² at -253℃); passivation treatment forms a dense Cr₂O₃ film layer to prevent chemical reactions between liquid hydrogen and aluminum alloy, while improving the sealing performance of welds to avoid liquid hydrogen leakage (leakage rate ≤ 1×10⁻⁹Pa·m³/s). The liquid oxygen tanks of some heavy rockets also adopt "shot peening" surface treatment: high-speed steel shots (diameter 0.1-0.3mm) are sprayed on the inner wall of the tank to form a residual compressive stress layer with a depth of 50-100μm, improving the fatigue resistance of the tank and enabling it to withstand multiple launch and recovery pressure cycles (cycle times ≥ 10).

The technical difficulties of surface treatment in the aerospace industry lie in "extreme performance breakthroughs" and "reliability verification": extreme performance breakthroughs require the development of new coating materials (such as high-temperature ceramics and radiation-resistant composites). For example, the ceramic topcoat of thermal barrier coatings needs to maintain structural stability above 1500℃. The current mainstream YSZ coating has approached its performance limit, and the next-generation "rare earth zirconate" coating (such as La₂Zr₂O₇) is in the R&D stage, with high-temperature resistance that can be increased to 1700℃; reliability verification requires passing strict environmental tests (such as 1000 high-temperature cycles and 10,000 hours of space environment simulation) to ensure that the coating does not fail during the entire life cycle of the spacecraft (usually 10-20 years), which puts extremely high requirements on process stability and quality control.

IV. Practical Operation Guide for Surface Treatment: Process Selection, Problem Solving and Safety Maintenance

(I) Process Selection: Four-Step Screening for Adaptive

Solutions

In practical production, the selection of surface treatment processes must consider base material characteristics, performance requirements, cost budgets, and environmental protection requirements, following the four-step process below:

Step 1: Clarify Core Requirements and Base Material Characteristics

First, determine the product’s core performance requirements (e.g., corrosion resistance, electrical conductivity, aesthetics) and application scenarios (e.g., outdoor, high-temperature, medical), then narrow down the process scope based on base material properties (e.g., metal/plastic, heat resistance, conductivity). For example:

Requirement: Corrosion resistance + food contact safety for stainless steel tableware; Base material: 304 stainless steel (weak corrosion resistance, no heavy metals allowed) → Chromium-containing passivation is excluded; Chromium-free zirconium salt passivation is optional.

Requirement: Conductivity + electromagnetic shielding for ABS plastic casings; Base material: ABS plastic (insulating, heat resistance ≤ 80℃) → High-temperature electroplating is excluded; Electroless nickel plating (low temperature ≤ 60℃, conductivity 10⁻⁵Ω·cm) is optional.

Step 2: Compare Process Performance and Costs

Based on core requirements, compare candidate processes in terms of performance indicators (e.g., salt spray life, coating hardness) and costs (equipment investment, unit cost). Taking "outdoor corrosion resistance + aesthetics for aluminum alloy doors and windows" as an example, the comparison of candidate processes is as follows:

If the budget is limited and environmental friendliness is a priority, powder spraying is the optimal choice; if higher hardness is required (e.g., for door handles), anodization is preferred.

Step 3: Verify Process Compatibility

Some products require multi-process combinations (e.g., "phosphating + spraying"), so it is necessary to verify the compatibility of pre-treatment and post-treatment to avoid coating detachment or performance failure. For example:

"Phosphating + powder spraying" for steel parts: The phosphating film thickness must be controlled at 1-5μm (excessive thickness may reduce coating adhesion), and spraying must be completed within 4 hours after phosphating (to prevent phosphating film rusting due to moisture).

"Plasma treatment + vacuum aluminum plating" for plastics: The plasma treatment power must be controlled (500-800W) to ensure a surface roughness Ra of 0.5-1μm (too low leads to insufficient coating adhesion; too high affects appearance).

Step 4: Small-Scale Trial Production and Testing

After confirming the process, conduct small-scale trial production (50-100 pieces recommended) and verify performance through professional testing:

Corrosion resistance: Neutral salt spray test (GB/T 10125) to record the time when rust appears.

Adhesion: Cross-cut test (GB/T 9286); no coating detachment after tape adhesion is qualified (≥ 5B grade).

Electrical conductivity: Four-probe method to test resistivity, ensuring compliance with design requirements (e.g., ≤ 10⁻⁶Ω·cm for electronic connectors).

(II) Solutions to Common Problems: From Defect Analysis to Optimization Measures

During surface treatment, problems such as coating detachment, surface defects, and substandard performance often occur, which need to be solved based on process principles:

1. Coating Detachment (Poor Adhesion)

Common Causes: Oil/oxide scale not removed from the base material surface; improper pre-treatment process parameters (e.g., low phosphating temperature); incompatibility between coating and base material.

Solutions:

Pre-treatment optimization: Metal base materials must go through the process of "degreasing (alkaline degreaser, temperature 50-60℃, time 10-15min) → derusting (hydrochloric acid 15%-20%, temperature 20-30℃, time 5-10min) → surface adjustment (titanium phosphate, time 1-2min) → phosphating" to ensure an oil removal rate of ≥ 99%.

Process parameter adjustment: For cathodic electrophoresis, voltage (150-200V) and temperature (25-30℃) must be controlled; too low voltage results in thin coatings and poor adhesion, while too high voltage causes coating cracking.

Compatibility verification: Before spraying plastic base materials, an "adhesion test" is required. For example, PP plastics must first undergo plasma treatment (time 3-5min) and then be sprayed with special PP coatings to avoid using general acrylic coatings.

2. Surface Defects (Bubbles, Pinholes, Color Difference)

Bubbles/Pinholes:

Causes: Moisture/impurities in the coating; oil/water in compressed air during spraying; excessive curing temperature (too fast solvent volatilization).

Solutions: Filter the coating through a 100-200 mesh filter and let it stand for defoaming (2-4h) before use; treat compressed air with an "oil-water separator" (moisture content ≤ 0.1g/m³); use stepwise heating for curing (e.g., pre-bake powder coatings at 60-80℃ for 10min, then cure at 180-200℃ for 20min).

Color Difference:

Causes: Batch differences in coatings; uneven spraying thickness; fluctuations in curing temperature.

Solutions: Use coatings from the same batch for products of the same batch; control the spray gun distance (15-25cm) and moving speed (30-50cm/s) during spraying to ensure a coating thickness deviation of ≤ 5%; use zoned temperature control for curing ovens (temperature difference ≤ ±2℃).

3. Substandard Performance (Poor Corrosion Resistance, Low Hardness)

Poor Corrosion Resistance:

Causes: Insufficient coating thickness; high porosity of the conversion film; coating damage during subsequent processing.

Solutions: For example, the zinc layer thickness of galvanized parts must be controlled at ≥ 8μm (salt spray life ≥ 500h); the porosity of the phosphating film must be controlled at ≤ 0.1% (detectable via oil immersion test, where pores absorb oil stains; adjust phosphating solution concentration and temperature if necessary); avoid coating areas during subsequent processing (e.g., bending, welding); if unavoidable, touch up damaged areas after processing (e.g., using special repair paint to ensure the touch-up thickness matches the original coating).

Low Hardness:

Causes: Inadequate coating curing (low temperature, insufficient time); improper coating formulation (e.g., low resin content); insufficient base material hardness (e.g., soft plastics).

Solutions: Adjust curing parameters according to coating requirements (e.g., epoxy powder coatings require curing at 180℃ for 20min to ensure a cross-linking degree of ≥ 90%); replace with high-hardness coatings (e.g., modified coatings with nano-alumina, which can increase hardness by 30%); perform surface hardening treatment on soft base materials (e.g., PP plastics) first (e.g., plasma-enhanced chemical vapor deposition to form a 1-3μm thick SiO₂ hardened layer with a hardness of up to 5H).

(III) Safety Maintenance: Equipment, Personnel, and Environmental Management

Surface treatment involves chemical reagents (e.g., acids, alkalis, heavy metal salts) and high-temperature equipment (e.g., curing ovens, vacuum coating machines). A comprehensive safety maintenance system must be established to avoid safety accidents and environmental pollution.

1. Equipment Maintenance: Regular Inspection and Preventive Maintenance

Different surface treatment equipment has different maintenance priorities, and targeted maintenance plans must be developed (monthly minor inspections and quarterly major inspections recommended):

Electroplating Equipment: Regularly clean oxide layers from anodes (e.g., nickel anodes, copper anodes) (soak in 10% sulfuric acid solution for 5-10min) to ensure stable current conduction; test the pH value and metal ion concentration of the plating solution weekly (e.g., pH of nickel plating solution must be controlled at 4.0-4.5, nickel ion concentration at 80-100g/L) and supplement if insufficient; replace the filtration system (e.g., filter elements) monthly to avoid impurities affecting coating quality.

Spraying Equipment: Clean the spray gun nozzle with solvent after each use (e.g., water for water-based coatings, special thinners for solvent-based coatings) to prevent clogging and uneven spraying; drain water from the air compressor tank weekly (to avoid water in compressed air) and inspect the pressure valve quarterly (to ensure stable pressure at 0.5-0.8MPa).

High-Temperature Equipment (e.g., curing ovens, vacuum coating machines): Calibrate the temperature control system of curing ovens monthly (temperature difference ≤ ±2℃) and inspect heating tubes quarterly, replacing them if aged; replace the vacuum pump oil of vacuum coating machines every six months and clean the vacuum chamber monthly (wipe the inner wall with alcohol to remove residual coating materials) to ensure the vacuum degree meets requirements (≤ 1×10⁻³Pa).

2. Personnel Protection: Standardized Operation and Protective Equipment

Operators must receive professional training, be familiar with the properties of chemical reagents and emergency response procedures, and be equipped with complete protective equipment:

Protective Equipment: Wear acid- and alkali-resistant gloves (e.g., nitrile gloves), protective clothing, and goggles when handling acid/alkali reagents; wear high-temperature-resistant gloves (e.g., aramid gloves) when operating high-temperature equipment to avoid burns; turn on ventilation systems (e.g., fume hoods, fresh air systems) when working in enclosed environments (e.g., electroplating workshops, vacuum coating chambers); wear gas masks if necessary (e.g., organic vapor masks for solvent-based spraying).

Standardized Operation: Store chemical reagents separately (e.g., separate acids and alkalis, isolate oxidizers and reducers) with clear labels (indicating name, concentration,  validity period); follow the principle of "adding acid to water" when preparing chemical solutions (e.g., when diluting sulfuric acid, slowly pour sulfuric acid into water and stir to avoid splashing); in case of reagent leakage, immediately treat with corresponding absorbent materials (e.g., calcium carbonate powder for acid leakage, boric acid solution for alkali leakage) and activate emergency ventilation.

3. Environmental Management: Treatment of Wastewater, Waste Gas, and Solid Waste

Wastewater (e.g., electroplating wastewater, phosphating wastewater), waste gas (e.g., spraying VOCs, pickling waste gas), and solid waste (e.g., waste paint buckets, waste filter elements) generated from surface treatment must be disposed of in compliance with national environmental standards (e.g., GB 21900-2008 Discharge Standard of Pollutants for Electroplating; GB 16297-1996 Integrated Emission Standard of Air Pollutants):

Wastewater Treatment: Treat electroplating wastewater separately; treat heavy metal-containing wastewater (e.g., chromium-containing, nickel-containing wastewater) through the process of "chemical precipitation (adjust pH to 8-9 with alkali to form hydroxide precipitates) → filtration → ion exchange" to ensure the heavy metal concentration is ≤ 0.1mg/L; first remove phosphating slag from phosphating wastewater (precipitate in a sedimentation tank and clean regularly), then adjust the pH to neutral (6-9) and discharge or reuse after ensuring COD ≤ 500mg/L.

Waste Gas Treatment: Treat spraying VOCs through the "activated carbon adsorption + catalytic combustion" process with a removal rate of ≥ 90% and an emission concentration of ≤ 60mg/m³; treat pickling waste gas (e.g., hydrochloric acid mist) through a spray tower (absorb with alkali solution, pH controlled at 8-9) with an emission concentration of ≤ 10mg/m³.

Solid Waste Treatment: Dispose of waste paint buckets and waste filter elements through qualified hazardous waste treatment enterprises; do not discard them randomly; collect hazardous wastes such as phosphating slag and electroplating sludge separately, attach hazardous waste labels, and store them for no more than 90 days to avoid secondary pollution.

V. Conclusion: Core Value and Application Principles of Surface Treatment Technology

As a "basic supporting technology" in the manufacturing industry, the core value of surface treatment lies in enabling ordinary materials to possess "customized performance" through precise surface modification. It can make stainless steel tableware meet food contact safety and long-term rust prevention requirements, allow aero-engine blades to work stably at 1500℃, and enable electronic chips to maintain high reliability in the trend of miniaturization.

In practical applications, three core principles must be followed:

1.Demand-Oriented: Always focus on the product’s application scenarios and performance requirements; avoid blindly choosing high-end processes (e.g., ordinary household hardware does not require aerospace-grade thermal barrier coatings).

2.Compatibility Priority: Ensure the compatibility of pre-treatment, coating processes, and base materials, as well as the synergy of multi-process combinations (e.g., parameter matching between phosphating and spraying), which is key to avoiding coating failure.

3.Safety and Compliance: While pursuing a balance between performance and cost, do not neglect equipment maintenance, personnel protection, and environmental management, which are the foundation for the sustainable development of the surface treatment industry.

With the continuous iteration of new materials and technologies, surface treatment technology will continue to develop in the direction of "greener, more functional, and more intelligent". However, regardless of technological upgrades, "solving practical problems and improving product value" will always be its unchanging core goal. For manufacturing enterprises, mastering the core logic and practical operation methods of surface treatment will become an important support for enhancing product competitiveness and expanding market boundaries.