Alumina is a crystal that is hexagonally close-packed and has a large lattice energy, a high melting point, a high level of hardness, large mechanical strength, and its products are resistant to both acids and alkalis. Alumina has a wide range of applications in the polishing industry due to its exceptional physical and chemical properties. In the realm of polishing, there are several specific applications, including automotive paint polishing, sapphire polishing, glass lens polishing, wafer polishing, etc. The requirements for the particle size of a-alumina as an abrasive vary for the four applications above.
The following is the polishing diagram:
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α-Alumina in Car Paint Polishing
To prevent agglomeration, the edges and corners of a-alumina should typically be spherical or cylindrical, not too sharp, and as uniform as possible. Depending on the degree of damage to the paint surface, different polishing processes are required, which can be classified as rough polishing, medium polishing, and fine polishing.
In general, the larger the particle size of a-alumina powder, the greater its machinability and the lower its smoothness. In contrast, the smaller the particle size, the worse the machinability and the higher the smoothness. Consequently, for the particle size of a-alumina, the rough polishing of the car paint surface should be 45 to 60 microns; the medium polishing should be 5 to 5.9 microns; and the fine polishing should be 1 to 2 microns.
α-Alumina in Sapphire Polishing
Sapphire is single crystal alumina. With the rapid growth of optoelectronic technology, the need for sapphire substrate materials for optoelectronic products continues to rise. At the same time, sapphire has emerged as one of the most significant substrate materials due to the ongoing proliferation of LED components. It is in high demand both domestically and internationally. The polishing of sapphire's surface has become a current research hotspot since sapphire needs to be flat when utilized as a substrate.
The Al on the sapphire surface reacts with the hydroxyl groups in the polishing slurry during the polishing process to generate a boehmite hydration layer with a Mohs hardness of 3. Abrasives with small particle sizes may not be able to completely penetrate the hydration layer, which prevents the polishing abrasive from taking part in mechanical polishing effectively, resulting in a low rate of polishing material removal. The material removal rate gradually increases and the surface roughness drops to a minimum as the alumina particle size increases up to 360 nm.
The particle size of alumina increases, as do the number of effective abrasives used in polishing and the rate of material removal. A dynamic equilibrium is achieved between chemical and mechanical polishing, that is, after polishing, all damage to the sapphire wafer has been eliminated, the surface is flat, and the surface quality is relatively high, resulting in a low surface roughness in the material. The material removal rate reaches its maximum when the abrasive particle size approaches 560 nm; however, the material's surface roughness also dramatically increases, making the polishing effect not ideal.
The mechanical polishing effect is enhanced when the polishing abrasive's particle size is large, which raises the rate of material removal. Grinding removes material at a different depth depending on the abrasive's particle size. The larger the particle size, the greater the depth, and the more severe the wafer damage. As the particle size reaches 1.5 um, the polishing fluid's stability deteriorates, and there is a slight stratification during polishing. As a result, the dispersion of abrasive is poor, so do the polishing effect. The impact of various a-alumina particle sizes on the rate of material removal and surface roughness is as follows, and the overall polishing effect is optimal when the a-alumina particle size is 360nm.
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The AFM image of the surface morphology of the sapphire wafer before and after polishing by spherical alumina with a particle size of 360 nm is as follows:
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It can be seen that before polishing, the surface of the sapphire wafer is uneven and contains numerous rough peaks; after polishing, the scratch condition is greatly enhanced, and numerous rough peaks on the surface of the sapphire wafer are removed, and the surface becomes smooth and flat, with microscopic undulations on the surface becoming smaller.
α-Alumina in Wafer Polishing
It is recommended to make a-alumina powder into flat plate for silicon wafer polishing so that the particles can adhere to the workpiece's surface during grinding, creating a sliding grinding effect and preventing scratches on the workpiece's surface from the sharp corners of the particles. As a-alumina only has a Mohs hardness of 8.8, it needs to be combined with diamond (Mohs hardness of 10) powder to create abrasives when polishing silicon carbide wafers (Mohs hardness of 9.2). However, because of the irregular morphology and high hardness of the diamond abrasive, the surface roughness of the silicon carbide wafer after mechanical polishing is between 10nm and 20nm, and many scratches of varying depths are visible under the microscope, indicating that the surface damage layer is deep, which will cause the following chemical mechanical polishing to be challenging to entirely eliminate the damage layer induced by mechanical polishing.
In order to lower the surface roughness of the silicon carbide wafer and reduce the impact of the depth of the subsurface damage layer, the particle size of a-alumina micropowder should be made between 0.5 um and 5um, and its specific surface area should between 100 and 250 m2/g. It can help to solve the issues of scratches and damaged layers brought on by the use of diamond micropowder as the abrasive during the polishing process, produce a silicon carbide wafer surface with almost no scratches, small damaged layers, and low roughness, and create the right conditions for further chemical mechanical polishing. If the acquired a-alumina micropowder's particle size exceeds the aforementioned range, it is simple to generate scratches and splits; contrarily, if it is smaller than the mentioned range, it will not be able to eliminate the damage brought on by the diamond micropowders.
α-Alumina in Glass Lens Polishing
Glass lens polishing powder is typically made up of cerium oxide, aluminum oxide, silicon oxide, iron oxide, zirconia, chromium oxide, etc. Because cerium oxide and silicate glass have comparable chemical activity and hardness, cerium oxide is commonly employed as an abrasive in the polishing of glass lenses. Some cerium oxide polishing powders contain a harder substance, like a-alumina, to boost the wear resistance and rate of grinding. Because the Mohs hardness of silicate glass is typically between 6 and 6.5, and the proportion of a-alumina as an abrasive is rather small in this instance, there are no special requirements for the microstructure of a-alumina crystals. Normally, they can meet the requirements as long as the particle size not be excessive, and the edges and corners not be too sharp.
It is worth mentioning that aluminum doped onto the surface of cerium oxide can considerably increase the amount of polishing optical glass. Aluminum nitrate, ammonia water, and hydrated cerium carbonate are mechanically milled together so that the newly generated amorphous aluminum hydroxide is coated on the surface of the tiny cerium carbonate particles. After dehydration, drying, and calcination, cerium oxide doped with alumina on the surface can be prepared.
According to the experimental findings, hydrate cerium carbonate is still the primary intermediate product during ball milling, and the formationh of aluminum hydroxide prevents cerium carbonate from becoming amorphous and from turning into cerium hydroxide. The calcined products all have a cubic fluorite structure, provided that the alumina doping quantity does not exceed 10%. All aluminum-doped cerium oxide powders polish ZF7 and K9 optical glasses at substantially higher rates than pure cerium oxide, demonstrating that the doping of aluminum can enhance cerium oxide's polishing capabilities. The ideal doping amount is 0.6%, and the calcination condition is 1000°C for 2 hours. If so, the MRR value will be twice or more than that of pure cerium oxide.