A brief introduction to the self-developed advanced composite electroplating coating
Composite electroplating is a new type of electroplating developed in the 1920s, and the first patent did not appear until 1949. This is the diamond composite of American Simos (Simos) using diamond and nickel co-deposition to make cutting tools. plating technology. Since then, composite plating has won the attention of electroplating technicians in various countries, and research and development have been very active. Today, it has become a very important branch of electroplating technology.
The characteristic of composite electroplating is to deposit particles with various functionalities into the coating layer as the matrix to obtain the coating layer with the characteristic functions of the particles. The different particles used, there are wear-resistant coating, anti-friction coating, high-hardness cutting coating, fluorescent coating, special material composite coating, nano-composite coating, etc.
Almost all kinds of plating can be used as the base bath for composite plating, including single metal plating and alloy plating. However, the commonly used base baths for composite plating are mostly nickel plating. Recently, there are also composite coatings based on zinc and alloy electroplating for actual production.
In the early days, composite particles were mainly wear-resistant materials, such as silicon carbide, alumina, etc., and now they are developed into composite coatings with multiple functions. Especially since the emergence of the concept of nanometers, composite coatings called nanocomposite materials have appeared from time to time. This is where composite coatings have great potential.
2. Principle of composite electroplating
Composite electroplating, also known as cladding plating and inlay plating, is a new process for coating solid particles in a metal coating to improve the performance of the coating. According to the properties of the coated solid particles, composite coatings with different functions are produced.
In the process of studying composite electroplating co-deposition, Renxin has proposed three co-deposition mechanisms, namely mechanical co-deposition, electrophoretic co-deposition, and adsorption co-deposition. At present, the two-stage adsorption theory proposed by NGuglielmi in 1972 is generally accepted. The model proposed by Guglielmi believes that the surface of the particles in the plating solution is surrounded by ions. After reaching the surface of the cathode, they are first loosely adsorbed (weakly adsorbed) on the surface of the cathode. This is physical adsorption and a reversible process. The particles gradually enter the surface of the cathode and are then buried by the deposited metal.
The mathematical treatment of the weak adsorption step in this model takes the form of the Langmuir adsorption isotherm. For the strong adsorption step, it is considered that the strong adsorption rate of the particles is related to the coverage of the weak adsorption and the electric field at the interface between the electrode and the solution. Some studies on the co-deposition process of wear-resistant nickel-diamond composite coatings show that the nickel-diamond co-deposition mechanism conforms to Guglielmi's two-step adsorption model, and the speed-controlling step is a strong adsorption step. So far, composite electrodeposition, like other new technologies and new technologies, is far ahead of the theory in practice, and the research on its mechanism is constantly developing.
3. Additives for composite electroplating
The matrix coating of composite electroplating can often use the original additive series of this kind of plating, such as the composite coating with nickel plating as the carrier, and the low-stress nickel plating brightener can be used. However, according to the principle of composite electroplating, composite electroplating itself also needs to use some additives to promote the co-deposition of composite and particles. These additives include particle electrical performance adjusters, surfactants, antioxidants, stabilizers, etc. according to their functions.
(1). Charge adjuster
Because the co-deposition of particles and the coating under the action of an electric field is an important process of composite plating, making the particles with a positive charge is beneficial to the co-deposition, but most particles are electrically neutral and need to be treated to make the surface adsorb positively charged particles. Some metal ions such as Ti+, Rb+, etc. can be adsorbed on the surface of alumina to form particles with a positive charge, which is beneficial to co-deposition with the coating. Certain complex salts and macromolecular compounds also have the function of adjusting the charge of particles. In order to fully combine the surface energy of the particles with the corresponding compounds, all-composite plating requires the particles added to the plating solution to undergo surface treatment, similar to degreasing and surface activation in the electroplating process, in order to obtain favorable co-deposition. electrical properties.
(2). Surfactant
In the composite plating with silicon carbide as composite particles, fluorocarbon surfactant is added to facilitate the co-deposition of particles. Therefore, some surfactants are also potential modifiers. But the surfactant also acts as a dispersant, which is also important for the uniform distribution of the particles in the bath. There are also some surfactants that have obvious potential characteristics and have an obvious effect at a specific potential, which is beneficial to the composite plating of the gradient structure.