Recently, Hay Think entered the ZKBT Rare Earth Research Institute to explore the secrets of making neodymium iron boron materials rejuvenated.
In the Rare Earth Research Institute, we have a close experience of the manufacturing process of high-entropy alloy targets. This is the first step in the production of NdFeB coatings. I saw the research and development personnel sieving and processing the alloy powder, adding it to a fixed-size mold, and loading it into a vacuum hot-pressing sintering furnace. "The R&D personnel are pressing and sintering high-entropy alloy target materials. The alloy powder is high-entropy alloy powder, which needs to be smelted by a special material through a vacuum induction melting furnace to cast ingots and crushed." Lu Fei, senior engineer of the Rare Earth Institute Introduced.
The so-called high-entropy alloy is an alloy formed from five or more metals in equal or approximately equal amounts.
According to Lu Fei, they conducted a high-entropy alloy target system design with the Chinese Academy of Sciences, and carried out a lot of research work in the preparation and application technology of high-entropy alloy films. Research has found that some high-entropy alloys have better strength, hardness, corrosion resistance and oxidation resistance than traditional alloys. It is a multi-principal super solid solution that breaks through the traditional alloy design concept. It is a new material and serves as a target. The material is suitable.
The anti-corrosion technology of NdFeB magnets urgently needs to be upgraded
Neodymium iron boron (NdFeB) permanent magnet material occupies a large market share in the rare earth functional material industry, but some of its own shortcomings also restrict its high-end application.
As we all know, NdFeB magnet is a multi-phase alloy composed of the main phase Nd2Fe14B, boron-rich phase, and neodymium-rich phase. The processing technology is powder metallurgy molding, the structure is relatively loose, and the potential difference between the phases is large. It is easy to produce serious problems during service. Intergranular corrosion will cause damage to the composition and structure in the local area of the surface, which will affect the integrity of the magnet's overall dimensions and the stability of its magnetic properties, and reduce the actual service life of the magnet.
In recent years, through the introduction and absorption of key equipment and the improvement of preparation processes, China’s high-end rare earth permanent magnet materials represented by sintered NdFeB have been comparable to Japanese products in terms of magnetic properties, "but there is still a big gap in the weather resistance of magnets. The research and development speed of new technologies, new coatings, and new coatings for magnet surface protection cannot keep up with the development speed of rare earth magnet application technology. The improvement of existing technologies and the development of new technologies are both lagging behind the international advanced level and Because of industrial demand.” Lu Fei was worried about this.
It can be seen that the anti-corrosion technology of NdFeB has become an important bottleneck restricting the improvement and strengthening of my country's rare earth industry.
At present, the common industrial-scale NdFeB surface protective coating preparation methods in China are electroplating and electroless plating. "Although these methods can improve the corrosion resistance of magnets to a certain extent and make the salt spray resistance of magnets reach about 100 hours, they have their own limitations. Because these methods are used to prepare the NdFeB protective layer, the plating solution It may remain inside NdFeB and NdFeB may undergo hydrogen absorption reaction, which tends to reduce the yield of products and cause serious pollution.” said Xia Yuan, a researcher at the Institute of Mechanics, Chinese Academy of Sciences.
As an environmentally friendly technology, vacuum coating technology has received extensive attention in recent years for its application in surface protection of NdFeB magnets. After research, the key consumable for vacuum coating is aluminum-based high-entropy alloy target.
With the continuous advancement of technology and the transformation and upgrading of related industries, the demand for aluminum-based alloy sputtering targets is increasing day by day, and higher requirements are put forward for the purity, relative density, structure and performance control of target materials and the overall quality of target components.
"This requires breakthroughs in key common technologies to realize the localization of high-performance NdFeB permanent magnet material vacuum coating targets, enhance industrial competitiveness and development autonomy, and ensure the rapid, stable and healthy development of downstream industries." Lu Fei said.
High entropy alloy target
Breakthrough in trial production
In May, the high-entropy alloy target developed by ZKBT Rare Earth Research Institute passed the coating test verification and met the requirements of magnetron sputtering. The project has declared 5 patents, 2 have been authorized, 1 standard has been formulated, and 1 academic paper has been published.
"High-entropy alloys are selected because high mixing entropy reduces the Gibbs free energy of the system, improves compatibility between components, inhibits the formation of ordered phases of intermetallic compounds, and promotes simple solid solution structures and even amorphous phases. Formation, unlike previous alloys, there are a variety of metals that do not embrittle.” Lu Fei said.
The currently developed high-performance amorphous aluminum manganese (AlMn) coating has too high manganese content, high alloy brittleness, difficult target formation, and high manufacturing costs. This has led to the limited promotion of vacuum coating in the field of NdFeB surface protection, requiring AlMn-based coatings. The optimized design of material system improves the processing performance of block materials.
In order to optimize the properties of this alloy, the Rare Earth Research Institute has carried out the preparation of AlMn planar targets. The Rare Earth Institute uses different process technologies to prepare AlMn alloy targets for different manganese content. Among them, the Al-Mn alloy target with low manganese content is mainly produced by vacuum melting and ingot slab vacuum hot pressing deformation process. High manganese content Al-Mn alloy target material, through vacuum hot pressing sintering, an Al-Mn alloy target material assembly composed of high manganese content Al-Mn alloy target material body and Cu back plate composite is obtained at one time.
"The process is good, but during the sputtering process, the target will also have problems. In this regard, we have done a lot of scientific research." Li Hui, a senior engineer of the Rare Earth Institute, introduced.
Li Hui said that for Al-Mn alloys with high manganese content, with the increase of Mn content, Al-Mn intermetallic phases increase, resulting in higher alloy brittleness and high internal stress, leading to cracking problems during the sputtering process. Not available.
According to the analysis of the research group, the cracking of the AlMn target during sputtering is due to the internal thermal stress of the target on the one hand, and the intrinsic brittleness of the target on the other hand. To this end, the AlMn target samples were subjected to vacuum annealing and bonding back plate material treatment.
After annealing treatment, the problem of sputtering cracking of the target material was improved. After sputtering used for 1 hour, cracks appeared, but the whole did not crack off the target. After bonding the Al backplane, sputtering used for 2 hours, cracks appeared on the sputtering surface of the target, and the sputtering effect was significantly improved. After bonding the Cu backplane, the sputtering process of the target is intact without cracks, which can meet the requirements of use.
"Although the cracking problem of the Al-Mn target material during sputtering is solved after bonding the Cu backplane, the prepared target material has serious outgassing during the sputtering process." Li Hui said.
After research, the preliminary judgment is due to the low density of the prepared target material. Combined with the DSC curve of AlMn alloy powder, the research group improved the microstructure compactness of the target body by adjusting the hot pressing process parameters. The relative density of the prepared Al-Mn target component can reach more than 99%. After the coating test of the Institute of Mechanics, Chinese Academy of Sciences, the outgassing phenomenon of the target material has been significantly improved.
"The low-manganese content aluminum-manganese target ingot blank is subjected to vacuum hot pressing deformation treatment to obtain a high-density target product. The relative density of the aluminum-manganese alloy target prepared by this method is above 99%, with no internal pores and no external cracks , Which can meet the demand for high-quality aluminum and manganese targets for vacuum coating." Lu Fei said excitedly.
Magnetron sputtering
Let the NdFeB film finally form
According to Lu Fei's recollection, on December 26, 2019, he and Li Hui went to Zhongketai Magnetic Coating Technology Co., Ltd. (hereinafter referred to as "Zhongketai Magnetics") to study and visit. Xu Yi, a doctor from the Institute of Mechanics, Chinese Academy of Sciences, introduced Zhongketai For the magnetic magnet coating production line and the existing magnet vacuum coating protective coating system, the two parties conducted in-depth communication on the progress of the project and the exchange of visits.
"Currently, the performance of the product is relatively stable. In 2019, the company's total output value was 80,300 yuan, and 11 people were employed. In 2020, financing and share expansion will begin." said Zhang Yanjie, general manager of Zhongketai Magnetics.
Zhongketai Magnetics focuses on the development of high-energy pulsed magnetron sputtering (HiPIMS) technology on the surface of neodymium iron boron magnets. With "sintered NdFeB magnet high-strength corrosion-resistant rare earth coating" as its core product, it forms corrosion-resistant coatings for NdFeB magnets for mobile phones and motors. , Complete sets of equipment and systematic solutions for high-temperature coatings for automotive molds.
"The magnetron sputtering coating process is to make the material to be deposited into a thin film into a target material, and fix it on the cathode of the sputtering deposition system, and place the substrate to be deposited on the anode facing the target surface. On the surface of the target material Establishing a magnetic field orthogonal to the electric field allows the secondary electrons to ionize more ions and improves the sputtering efficiency." Xia Yuan said.
Then use the sputtering system to pump to high vacuum and then fill with argon gas, etc., load high voltage between the cathode and the anode, and a low-pressure glow discharge will be generated between the cathode and the anode. In the plasma generated by the discharge, argon positive ions move to the cathode under the action of an electric field, collide with the surface of the target, and the target atoms sputtered from the surface of the target due to the collision are called sputtering atoms. The energy of the sputtering atoms is general In the range of one to tens of electron volts, sputtered atoms are deposited on the surface of the substrate and then form a film.
"We combine high-energy pulsed magnetron sputtering technology with plasma feedback control technology to form an advanced film formation process and quality control technology. Through effective control of the incident particle energy and distribution during the film formation process, a high film base Cohesion, high-quality, high-uniformity film preparation and deposition rate have been greatly improved." Xia Yuan said.
"The next step is to pay attention to the use of targets in the vacuum coating deposition system and the problems in the use of target sputtering, to improve the target manufacturing technology, or to combine target research and vacuum coating applications. In terms of performance and coating correlation, we still need to work closely with coating application units." Lu Fei said.