Ultra-precision polishing powders are key fundamental functional materials for high-end precision manufacturing. For a long time, the core technology for high-precision, low-damage high-end polished alumina powders has been monopolized by overseas enterprises, becoming a critical bottleneck constraining the independent and controllable development of China's precision manufacturing supply chain. Traditional domestically produced alumina polishing powders are mostly general-purpose spherical or block-shaped particles. They either suffer from low efficiency, failing to meet mass production requirements, or readily produce micro-scratches, haze, pits, and other defects on the workpiece surface, making them unsuitable for the processing standards of ultra-smooth substrates such as wafers, sapphire lenses, and high-end touch glass.
Against this backdrop, flake-shaped alumina polishing powders with a unique hexagonal plate-like micromorphology have broken through the performance limitations of conventional abrasives. With their distinctive planar sliding polishing mechanism, uniform stress distribution characteristics, and low-damage grinding advantages, they have emerged as an important class of functional powder materials in the field of ultra-precision polishing. Based on the microstructural characteristics of flake-shaped alumina, this article systematically analyzes its polishing mechanism, core performance advantages, and mainstream application scenarios.
Unique Advantages of Flake-Shaped Alumina Polishing Powders
Flake-shaped alumina polishing powders feature high-purity α-Al₂O₃ as the core crystalline phase, exhibiting a regular hexagonal plate-like flake structure with a large aspect ratio, distinguishing them from traditional spherical or irregular block-shaped alumina abrasives. Their polishing mechanism is based on planar uniform sliding micro-grinding, rather than the rolling rigid cutting of conventional abrasives. Based on this mechanism, flake-shaped alumina offers the following unique advantages in polishing:
01 Low-Damage Processing
During the polishing process, the flat geometric characteristics of the flake particles allow them to spontaneously align parallel to the workpiece surface, forming extensive face-to-face contact with the substrate. Instead of concentrated cutting by sharp edges and corners, the particles remove microscopic protrusions, oxide layers, and burr defects through smooth, uniform, and minimal material removal. After polishing, the workpiece surface exhibits no haze, no hidden micro-cracks, and extremely high surface flatness.
02 Stable Grinding Performance
During polishing, the applied pressure is evenly distributed across the entire flat surface of the flake particles, which not only avoids localized stress concentration that can lead to random scratches, micro-cracks, and other machining defects, but also significantly reduces the probability of particle fracture. The abrasive morphology is better preserved throughout the process, ensuring consistently stable micro-grinding performance.
03 Stable Physicochemical Properties and Good Wear Resistance
Flake-shaped alumina maintains a Mohs hardness of 9.0, making it suitable for fine polishing of various hard and brittle materials such as sapphire, silicon carbide, silicon wafers, and optical glass. It also exhibits high temperature resistance, acid/alkali corrosion resistance, and chemical inertness, causing no chemical reaction with the substrate during polishing and no secondary contamination, making it compatible with various complex polishing conditions.
Preparation of Flake-Shaped Alumina Polishing Powders
In the ultra-precision polishing process system, the micromorphology and structural state of the polishing powder are key factors determining polishing performance. An ideal polishing powder must possess a regular hexagonal plate-like structure, free from irregular blocky, needle-like, or fragmented impurity particles. The particle surfaces should be flat, edges smooth, and free from sharp burrs or chipping defects. At the same time, the aspect ratio, as a critical parameter, must be controlled within a reasonable range-too thin leads to insufficient toughness and susceptibility to bending/fracture, while too thick compromises the advantage of planar contact. This imposes stringent requirements on controllable powder preparation technology.
Currently, the main preparation techniques for flake-shaped alumina include the molten salt method, hydrothermal synthesis method, solid-state high-temperature calcination method, and sol-gel method. These four mainstream processes differ significantly in crystal growth mechanisms, morphology control capabilities, and product performance, directly determining the flake integrity, aspect ratio uniformity, and purity grade of the polishing powder.
01 Molten Salt Method
The molten salt method is the mainstream process for industrial-scale production of highly regular flake-shaped alumina polishing powders. Typically, soluble aluminum salts (such as aluminum sulfate) are mixed with low-melting-point salts (such as sodium sulfate or potassium sulfate) and heated to form a molten salt melt, in which the aluminum source dissolves, diffuses, and crystallizes. By controlling parameters such as temperature, time, and additives, the growth of alumina along specific crystal planes is promoted to form a flake structure.
This method features a simple process, controllable morphology-allowing precise control over particle size, thickness, and aspect ratio-and good dispersibility, making it suitable for fields such as polishing and pearlescent pigments that demand high powder performance. Currently, Germany's Merck & Co. is a representative producer. However, some molten salts (e.g., fluoride-containing salts) may be toxic, and volatiles may corrode furnace equipment or pollute the environment. Additionally, post-calcination steps such as water washing and filtration are required to remove the molten salt, generating wastewater and increasing environmental costs.
Currently, representative companies producing flake-shaped alumina using this technology include Germany's Merck, although as a world-leading pearlescent pigment manufacturer, they primarily use it as a substrate for pearlescent pigments in cosmetics, automotive coatings, and similar applications. In China, Minjiang University has also produced flake-shaped alumina pearlescent pigment substrates with performance parameters comparable to Merck's products using this technology, and has completed hundred-kilogram-scale preparation and mass production process validation.
02 Solid-State Sintering Method
At high temperatures, the growth rates of different crystal planes of alumina crystals vary. The solid-state sintering method involves high-temperature sintering of alumina precursors (such as aluminum hydroxide or alumina powder) with the addition of additives to regulate the growth rates of different crystal planes, thereby producing flake-shaped alumina. Typically, fluorine-containing additives react with the alumina precursor to form gaseous intermediate compounds. These compounds adsorb less on the (0001) crystal plane, causing slower growth along the c-axis, while the (1010) crystal plane grows relatively faster, promoting planar expansion of the crystal and ultimately forming a flake morphology.
This method offers low cost and high efficiency, making it suitable for large-scale production. Currently, Japan's Fujimi Corporation's PWA series of flake calcined α-Al₂O₃, the USA's Micro Abrasives Corporation's WCA series of flake α-Al₂O₃, and Japan's DIC Corporation's CeramNex™ AP10 plate-like α-Al₂O₃ are all produced using this method. However, because sintering tends to cause agglomeration and additives may introduce impurities, products from this method are mainly used as general-purpose polishing materials.