1. Sputtering
1.1 Magnetron sputtering
It is a method of generating glow discharge through voltage, ionizing argon gas, bombarding the target with argon ions to sputter heavy rare earth particles such as dysprosium and terbium, which are deposited on the surface of the magnetic steel to form a film layer. Products applicable to magnetron sputtering include square pieces, tiles, round pieces, rings, etc., and the diffusion source is mostly pure metal targets of dysprosium or terbium.
Magnetron sputtering has the advantages of good coating uniformity, dense film layer, and stable coercive force enhancement effect. Its disadvantages are: 1. The equipment is expensive, and the price of a single continuous equipment is between 3.5 million and 5 million; 2. The target material utilization rate is low, so the relative cost is high.
1.2 Multi-arc sputtering
It is a coating that produces local high-temperature sputtering by field-induced discharge. It has a high coating efficiency target material utilization rate, and good bonding force. But the disadvantages are 1. Large coating particles and rough film surface; 2. General coating uniformity; 3. Unsatisfactory coating stability; 4. Local temperature rise.
2. Evaporation
Evaporation is a method of depositing a film by heating and vaporizing the coating material under vacuum. Evaporation is divided into static evaporation and rotary evaporation, similar to hanging plating and roller plating in the electroplating process and suitable for large and small magnets, respectively.
2.1 Static Evaporation
The advantage is that the evaporation temperature is high, and the sublimation of the evaporation source, deposition on the surface of the magnetic steel, and diffusion in the magnet are carried out simultaneously, so the heavy rare earth diffusion effect is better. Since the quality of the heavy rare earth film after evaporation is related to the evaporation temperature, vacuum degree, and plating distance (the distance between the heavy rare earth target and the magnetic sheet), the magnetic steel sheet needs to be placed in specific tooling to ensure the plating distance, resulting in a more complicated process, low production efficiency, and poor film consistency. Therefore, static evaporation is less used in the domestic magnetic material industry.
2.2 Rotary Evaporation
It is a method of continuously mixing small and micromagnetic steel and heavy rare earth target materials through rotation, and the metal Dy/Tb volatilized after high temperature is deposited on the product's surface. This process is suitable for small products, especially electroacoustic products with a unit weight of less than 1g; the diffusion material cost is low, the weight gain is small, and the HcJ is significantly improved; the HcJ consistency is high. The disadvantage is that the separation process of the product and dysprosium and terbium metals is relatively labor-intensive, and this process is rarely used in the industry, and the equipment usually needs to be customized, which has certain technical barriers.
3. Coating
3-1 Automatic spraying
Put the magnetic sheets in a single layer and place them on the workpiece tray. Use a pneumatic spray gun to spray them in a reciprocating manner to form a film. When the diffusion source is fluoride or oxide, it can be sprayed in the atmosphere. If it is a hydride or heavy rare earth alloy powder, inert gas protection is required.
Advantages: 1. It is applicable to a large number of diffusion sources, including hydrides, fluorides, oxides and alloys; 2. It can diffuse products of various shapes and sizes, including tile-shaped, square pieces, round pieces and rings; 3. The equipment has a high degree of automation, and the workpiece tray can be used universally, and order switching is flexible and convenient; 4. The equipment investment is low and the utilization rate of heavy rare earth is high.
Main disadvantages: There is a patent risk when the diffusion source is fluoride, there are safety hazards when it is hydride and alloy, and the performance consistency of oxides is poor.
3-2 Screen printing
Heavy rare earth powder is prepared into ink. When printing, the ink is poured into one end of the screen printing plate, and a certain pressure is applied to the ink part on the screen printing plate with a scraper. At the same time, the scraper moves at a uniform speed toward the other end of the screen printing plate. The ink is squeezed from the mesh to the magnetic sheet by the scraper to form a film during the movement.
Due to the encapsulation of the organic reagent in the ink, the hydride or heavy rare earth alloy is blocked from contacting the air, and oxidation is avoided. Therefore, heavy rare earth hydride can be used as a diffusion source for coating in the atmospheric environment, effectively avoiding the patent risk of fluoride. At the same time, the screen printing process has the characteristics of high production efficiency, high utilization rate of heavy rare earth, and good consistency of weight gain, which is very suitable for grain boundary diffusion of square magnetic steel. In the past two years, the screen printing process has been rapidly promoted in the industry and has become one of the mainstream processes.
The disadvantages are: 1. The screen and scraper are not suitable for tile-shaped products with uneven surfaces; 2. Products of different sizes and weight gain ratios usually require corresponding inks, screen printing plates, and fixtures, which are suitable for large-scale stable orders. For small batch orders, frequent replacement of screens and fixtures will result in reduced efficiency, increased costs, and low flexibility.
