Common surface treatment technical knowledge

There are three main types of surface technology, each of which can be divided into a variety of technologies. The following is a brief introduction to several common surface technologies and applications using chemical or physical chemistry principles.

 

1. Paint

Coating is a kind of coating used to coat the surface of objects to form a coating film to protect, decorate signs and other special functions (such as insulation, anti-fouling, drag reduction, heat insulation, radiation resistance, electrical conductivity, electromagnetic, etc.). Material.

 

 

A long time ago, paints were processed with vegetable oils and natural resins, so they were commonly called paints. In 1790, the first paint factory was established in the United Kingdom. With the development of the industry, synthetic resins have gradually replaced vegetable oils, so now the various materials on the surface of the coating are collectively referred to as coatings. Coatings as an industrial sector have a history of only a hundred years, but their applications are very extensive. All around us, paint can be said to be everywhere, from indoor walls, refrigerators, cabinets and furniture, and even audio and video tapes and CDs, to outdoor houses, cars, ships, bridges, beverage cans, and chewing gum packaging. Places are everywhere. The current coatings market is around $60 billion. Since my country's reform and opening up for more than 20 years, the annual growth rate of coating production is about 10%. The current production capacity has exceeded 2.5 million tons, ranking third in the world. The following is a brief introduction to the composition, classification and basic principles of film formation.

 

Composition of paint

The composition of the coating includes 4 parts by function: film-forming substances, pigments, solvents and additives.

Film-forming substance: It is the main substance that can form the coating film in the coating and is the main factor determining the performance of the coating, sometimes also called the base material or paint. There are mainly oil esters, natural resins, natural polymers and synthetic resins.

Pigment: Pigment is usually a solid powder. Although it cannot form a film by itself, it always remains in the coating film, which can make the coating film show color and hiding power, and can also enhance the aging resistance and abrasion resistance of the coating film and enhance the film. anti-corrosion, anti-fouling and other capabilities. There are many types of pigments, such as white pigments including titanium dioxide (TiO2), zinc white (zinc oxide), etc., black pigments including carbon black, iron oxide black (Fe3O4), color pigments including chrome yellow (PbCrO4), iron red (Fe2O3) , iron blue (Plutu blue FeK[Fe(CN)6]•H2O), etc., as well as metal pigments such as aluminum powder (commonly known as silver powder), anti-rust pigments such as zinc phosphate and so on. In addition to these, there are also a large number of organic pigments.

Solvent: Solvent is added to the coating in order to dissolve or disperse the film-forming material and reduce the viscosity of the film-forming material, so that the construction can be uniform with organic solvents and water.

Additives: There are more and more additives in the application of coatings, and their dosage is often very small, accounting for a few percent or even a few thousandths of the total formulation, but they are improving performance, extending storage life, expanding application scope and Ease of construction and other aspects often play a big role. Including driers, thickeners, thixotropic agents, matting agents, antistatic agents, UV absorbers, defoaming agents, leveling agents, etc.

 

Classification of coatings

After long-term development, the varieties of coatings are particularly complex and their uses are also different. There are many classification methods, and the following three are mainly introduced:

According to the main film-forming substances, it can be divided into organic coatings and inorganic coatings, and organic coatings can be divided into alkyd, epoxy, and acrylic coatings according to the type of film-forming resin.

According to the type of solvent, it can be divided into solvent-based (with organic solvent as dispersant, and divided into low-curing and high-curing) coatings, water-based (water as solvent) coatings, finishing coatings, radiation curing coatings, etc. The future development direction of coatings will be reflected in the following two aspects: one is to develop in a more environmentally friendly and green direction, and will continue to develop high-solid, water-based, powder and radiation-cured coatings; the other is to develop in a more functional direction, Various functionalized special coating products will continue to come out. Table 1 shows the paint product structure in the world's major industrialized regions.

Coatings can be divided into three categories according to their application: architectural coatings, product coatings and special coatings. Architectural coatings mainly include coatings used to decorate and protect the outer and inner walls of buildings, accounting for about 50% of the total coatings, of which more than 70% are latex paints. Product coatings, also called industrial coatings, are coatings applied to automobiles, home appliances, magnet wires, airplanes, furniture, metal cans, etc. in factories, accounting for about 30% of the total output of coatings. The remaining 20% ​​or so are special coatings, which refer to industrial coatings and some other coatings constructed outside the factory, such as fire-retardant coatings, waterproof coatings, ceramic coatings that are resistant to high temperature corrosion, temperature-indicating coatings, conductive coatings, magnetic coatings, road markings Coatings, anti-mildew and insect-killing coatings, wear-resistant coatings, marine heavy-duty anti-corrosion coatings, etc. Special coatings must not only meet the special needs of national defense cutting-edge products and high-tech development, but also meet the special requirements of new developments in various sectors of the national economy.

 

Basic Principles of Coating Film Formation

The purpose of producing and using coatings is to obtain a coating film that meets the needs. The process of forming a coating film directly affects whether the flip cover can fully exert the predetermined effect and whether the various properties of the obtained coating can be fully surfaced. There are generally three types of curing mechanism of the coating film.

Physical mechanism drying only depends on the evaporation of the coating body (solvent or disperse phase) in the coating to obtain dry and hard rice grassland. The drying process is physical mechanism drying. During the drying process, the polymer does not react chemically.

Cross-linking curing of coatings reacting with air. Therefore, during storage, the paint tank must be well sealed and isolated from the air.

The cross-linked cured coating that reacts between the coating components must remain stable during storage. It can be packaged by two-component coating method or selected to not react with each other at room temperature, but only react at high temperature or when exposed to radiation. Components, for example, UV-curing flip cover is a one-component, solvent-free coating that is exposed to UV light when applied, and the liquid Xing flip cover will cure within a few tenths of a second to tens of seconds to form a film .

In recent years, researchers are developing a co-curing technique that combines several clamshell curing techniques to overcome the shortcomings of a single curing technique.

 

2. Electrochemical and chemical deposition

Among the electrochemical and chemical deposition surface technologies, electroplating, brush plating and electroless plating are common.

 

plating

Electroplating is the process of applying electrolysis to coat one metal surface on another metal surface, which needs to be carried out in an electrolytic bath. The process is to use the metal plated parts that have undergone degreasing and rust removal as the cathode, the plated metal as the anode, and the salt solution of the plated metal as the electrolyte, and electrolysis is performed after electrification. Such as electro-galvanized, electroplated copper, etc. In order to make the coating crystallized, uniform in thickness, and firmly bonded to the substrate, the electroplating solution usually cannot use a simple metal salt solution, but a solution that forms a complex, such as alkaline zincate or zinc cyanide. as an electrolyte.

Popular alloy plating in recent years, such as brass plating (copper-zinc alloy), solderable alloy (tin-lead alloy), corrosion-resistant alloy (zinc-nickel alloy), imitation gold plating (copper-zinc alloy or copper-zinc-tin alloy), etc. , the basic principle is to control the concentration of different metal ions in the solution by adding a complexing agent to generate different complexes, so that the metal ion concentration with a larger electrode potential algebraic value is slightly lower, and the electrode potential algebraic value is slightly lower. The smaller ion concentration is higher, so that different metal ions have the same or similar potential, so that a variety of metal ions get electrons on the cathode at the same time and deposit on the surface of the workpiece as the cathode at the same time, forming an alloy coating.

 

brush plating

Brush plating is a new technology for repairing mechanical parts developed in recent years. Its basic principle is the same as that of electroplating, except that instead of a plating tank, the electrolyte is immersed in a cotton sleeve that wraps the anode (called a plating pen). That is, the plating pen of the electrolyte) is in direct contact with the workpiece (cathode), and in the relative motion of the anode and the cathode, the coating can be obtained, and the coating thickens with the extension of the brushing time, but it is only removed by leaching with the plating pen. Oil or rust remover for treatment. Brush plating is mainly used to repair worn or processed parts, and can also be used for the maintenance and protection of printed boards and electrical contacts. Because brush plating technology can repair local damaged parts of high-value machinery at a small cost, it is widely used as a "mechanical resurrection technique".

 

Electroless plating

Electroless plating is a metal deposition process carried out by a controlled chemical reduction method under the catalysis of the metal surface without external power supply. Because no external power supply is required, it is also called electroless plating (Non electrolytic). Since the reaction must be carried out on the surface of the material with autocatalytic properties, the American Society for Testing and Materials recommends the term Autocatalytic plating. Since the metal deposition process is a passive chemical reaction, it is most appropriate to call this metal deposition process "electroless plating", so that it can fully reflect the essence of the process. So the three words Chemical, Non electrolytic and Electroless are one meaning. At present, the term "chemical plating" has been adopted by everyone at home and abroad.

The history of electroless plating is relatively short, and it was first invented by Brenner and Riddell of the American Bureau of Standards in 1946. As soon as electroless nickel plating came out, it immediately attracted people's attention, but it has been nearly 20 years that electroless nickel plating has been widely valued by the industry. Since the 1980s, with the rapid development of electronics, computers, petrochemicals, automobile industries, etc., electroless nickel plating has been developing at an annual growth rate of more than 15%. At present, electroless nickel plating is widely used in almost all industrial sectors. It has become one of the fastest developing processes in the field of surface technology in recent years.

Electroless plating is not through the exchange of metal atoms and ions between solid-liquid two phases at the interface, but the reduction and deposition of liquid-phase ions Mn+ on the metal surface or the surface of other materials through the reducing agent R in the liquid phase:

M^(n+)+R(→┴Surface Catalysis) M+R^(n+)

The electroless plating process must have a catalyst. The substrate can often act as a catalyst, but when the substrate is completely covered, the catalyst for the deposition process to continue can only be the deposited metal itself. Therefore, electroless plating can be said to be a controllable, autocatalytic chemical reaction process for depositing metal. The thickness of electroless plating is proportional to time, and it is theoretically believed that very thick deposits can be produced.

A solution capable of electroless plating must contain the following substances:

The source of the element to be plated - usually a metal salt;

A reducing agent that reduces the ions to be plated into elemental substances;

One or more complexing agents added in order to maintain the stability of the chemical solution, they form complexes with the ions of the metal to be plated, and can also prevent the formation of hydroxide and phosphite precipitation.

Contains a buffer to maintain a constant pH of the plating solution and a stabilizer to improve the stability of the plating solution and increase the life of the plating solution.

Taking the most widely used electroless nickel-phosphorus alloy as an example, the metal salt is generally nickel sulfate, and the commonly used reducing agent is sodium hyposulfite (NaH2PO2). In addition to nickel, the electroless nickel plating layer using sodium hypophosphite as a reducing agent also contains a part of phosphorus to form a Ni-P alloy, so it is called an electroless Ni-P alloy. It is an electroless Ni-P alloy.

The pretreatment of chemical plating is the same as that of electroplating, and degreasing and rust removal are also required. The general process flow is as follows:

Cleaning → Degreasing → Water Washing → Acid Etching → Water Washing → Electroless Plating → Water Washing → Post-plating Treatment

The purpose of degreasing is to remove grease or dirt such as lubricating oil, anti-rust oil, polishing paste, etc. remaining in the machining or storage process of the workpiece surface to avoid uneven plating or leakage. The main degreasing methods include organic solvent degreasing, lye degreasing, electrochemical degreasing, emulsifier degreasing, ultrasonic degreasing, etc.

Acid etching, also known as pickling, is a process of immersing a metal workpiece in acid or an acid solution to remove the oxide film, scale and rust on the metal surface. The plated parts after the pretreatment can be put into the electroless plating solution for plating.

Compared with electroplating, electroless nickel-phosphorus plating has the following advantages:

A The ability of throwing and deep plating is good, and it can produce a coating of uniform thickness on the complex surface of the switch; including blind holes, deep hole parts and pipe fittings with a large aspect ratio, it has the characteristic of "pervasive penetration".

B The coating is dense and has few pores;

C No power supply required, simple equipment and easy operation;

D can be plated on other non-metallic surfaces in addition to metals;

E Amorphous alloy, the nickel-plated layer has special mechanical, physical and chemical properties, its hardness index far exceeds that of electroplated nickel, and it has high wear resistance and corrosion resistance.

The F coating has a good bonding force with the substrate.

Although the pace of application of chemical plating technology in my country is also very fast, due to the late start, the largest users of chemical nickel plating in my country are currently the oil extraction industry and petrochemical industry, which is still far from the international level, and there is a huge market waiting for us. development.

In addition to electroless nickel-phosphorus alloy plating, there are other types of plating, such as electroless copper plating, electroless cobalt plating, electroless precious metal plating, electroless plating without alloys, chemical composite plating, etc., which will not be described in detail here.

 

3. Surface modification

There are many techniques for surface modification, only the principles and applications of anodizing and chemical heat treatment are introduced here.

 

Anodizing

Some metals can form a protective oxide film in the air, and the internal metal will be corroded under normal circumstances. For example, it is easy to form a very thin oxide film (0.01-1μm) on the surface of aluminum and its alloys, which has a certain anti-corrosion ability in the atmosphere, but because this oxide film is amorphous, it makes the surface of the aluminum part lose its original luster. . In addition, the oxide film is loose and porous, non-uniform, weak in corrosion resistance, and easily contaminated with stains, so aluminum and its alloy products usually need to be oxidized.

Anodizing is an electrochemical oxidation treatment method. It is a process in which metal is placed in an electrolyte as an anode to form an oxide film of tens to hundreds of microns on the surface of the metal. The formation of this oxide film has better corrosion resistance than the oxide film formed by natural oxidation of metal in the air. , Wear resistance.

For example, in the anodization of aluminum and its alloys, the workpiece is used as the anode, the other aluminum plate is used as the cathode, and the dilute sulfuric acid (or chromic acid) solution is used as the electrolyte. After power-on, the following electrolytic reactions occur:

Anode (workpiece):

2Al+6OH-(aq) = Al2O3+3H2O+6e (main reaction)

4OH-(aq) = 2H2O + 2O2(g) + 4e (minor reaction)

Cathode: 2H+ + 2e = H2

Thus, an Al2O3 oxide film that is firmly bonded to the aluminum substrate is formed on the surface of the aluminum and its alloy workpieces.

 

Chemical heat treatment

Chemical heat treatment is a process in which a metal or alloy workpiece is placed in an active medium at a certain temperature for heat preservation, so that one or several elements penetrate into its surface layer to change its chemical composition, structure and performance. There are many types of chemical heat treatment, including carburizing, nitriding, carbonitriding, boronizing and different metals. This process can not only be used to protect and strengthen materials, improve corrosion resistance, wear resistance, oxidation resistance and fatigue resistance, but also endow materials and their products with optical, magnetic, thermal insulation, decoration and other functions as required. , and can also provide a series of new materials and composite metal sheets for the development of high-tech and products.

 

The basic principle of chemical heat treatment is briefly introduced below by taking carburizing as an example.

Carburizing is the process of infiltrating carbon atoms into the metal surface to form metal carbides. There are many working media for carburizing, but the carburizing process includes three basic steps: ① The working medium (carburizing agent) is decomposed at high temperature to generate activated carbon atoms, such as carburizing of carburizing agent CH4 at high temperature of 1173-120K After a series of reactions in the furnace, activated carbon atoms are generated; ② the active carbohydrate atoms are absorbed by the metal surface; ③ the carbon atoms diffuse to the inner metal layer to form a carburized layer.

 

4. Chemical Vapor Deposition

The chemical vapor deposition process is the process of depositing metal carbides, nitrides, borides or oxides directly on the surface of metal materials. For example, in the chemical vapor deposition process of depositing TiC on steel workpieces, TiC4 vapor can be passed into a high temperature (generally 900-1200°C) reactor equipped with workpieces under the hydrogen carrier, and a series of reactions with hydrocarbons can be carried out. The reaction finally produces TiC deposited on the surface of the workpiece.

The purpose achieved by the chemical vapor deposition process is similar to that of chemical heat treatment, but the deposited layer produced by chemical vapor deposition has the advantages of firm bonding with the base metal, uniform thickness of the deposited layer, compact structure, and stable quality. In addition, such deposited layers also act like lubricating inks. Therefore, it can be used as an oil-free lubrication anti-friction layer, which greatly reduces the wear of the workpiece.

With the advent of chemical vapor deposition and laser-assisted chemical vapor deposition, the deposition temperature that can be achieved is gradually increasing, the types of materials that can be deposited are expanding, and the range of deposition properties is also gradually expanding. It can be said that surface technology has achieved not only changing the chemical composition and organizational structure of the surface of the object, but also giving the object a brand new surface, and more impressively, the deposition layer can be arbitrarily selected according to the performance needs.