Home / News & Blog / Abrasive Blog / Comprehensive Guide to Aluminum Oxide Abrasives
Aluminum oxide abrasives are vital materials widely utilized in various industrial applications due to their durability and effectiveness. Available in multiple forms, each type is designed for specific purposes. This article explores the types, properties, selection criteria, applications, and emerging trends in aluminum oxide abrasives.
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Processing Techniques |
Main Abrasives |
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Bonded Abrasives (grinding, honing, super-finishing) |
Super-hard abrasives—diamond and cubic boron nitride (CBN)—alongside traditional abrasives like aluminum oxide (Al2O3) and silicon carbide (SiC) |
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Polishing |
Super-hard abrasives (diamond and CBN), Al2O3, SiC, boron carbide (B4C), chromium oxide (Cr2O3), garnet, and emery |
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Sandblasting |
Al2O3, SiC, B4C, quartz (SiO2), and garnet |
Aluminum oxide (Al2O3) has been mined since 2000 BC, with its rich history beginning on the Greek island of Naxos. The material’s crystal structure includes rhombohedral α-Al2O3 and various impurities. Gem-quality aluminum oxide serves as a base for gemstones like sapphires and rubies. Bauxite is the primary raw material for all molten aluminum oxide, which is produced through electric melting or chemical precipitation and sintering.
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Abrasive Type |
Chemical Composition |
Color |
Knoop Hardness HK |
Relative Toughness (%) |
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Zirconia-Alumina |
60%-75% Al2O3, 25%-40% ZrO2 |
Gray/Brown |
1450-1700 |
50 |
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Semi-fragile Aluminum Oxide |
~98% Al2O3, ~1.5% TiO2 |
Gray |
1950-2000 |
20 |
|
Brown Aluminum Oxide (BC) |
~96% Al2O3, ~3% TiO2 |
Brown |
1950 |
21 |
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Hollow Sphere Aluminum Oxide |
>98% Al2O3, max 1% SiO2 |
White |
~1950 |
> WFC |
|
99.8% Al2O3, 0.2% Na2O |
White |
2000-2160 |
15 |
|
|
Chromium Aluminum Oxide |
99.5% Al2O3, ~0.3% Cr2O3, 0.2% Na2O |
Pink |
2160 |
18 |
|
Ruby Melting Aluminum Oxide (RFC) |
~98% Al2O3, ~2% Cr2O3 |
Ruby/Red |
2150 |
19 |
|
Single Crystal Aluminum Oxide (MCC) |
~99% Al2O3 |
Light Gray |
2300 |
> RFC < BC |
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Sintered Aluminum Oxide |
Al2O3 Nk/Nk + ZrO2 |
Brown |
1300-1400 |
Extremely Tough (>50) |
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Sol-Gel Aluminum Oxide |
95%-99% Al2O3, 0%-5% MgO/Fe2O3 and various additives |
Multicolored |
2300-2400 |
Comparable to single crystal aluminum oxide |
By modifying the starting materials and processing methods, various types of fused alumina can be produced, each exhibiting different properties such as hardness and brittleness:
This type contains 96% – 98% Al2O3 and is known for its relatively high toughness, making it suitable for various applications.
This category includes pink fused alumina, which contains 0.2% – 0.3% chromium oxide (Cr2O3), and ruby (red) fused alumina, which contains 2% Cr2O3. The chromium oxide is integrated into the Al2O3 crystal structure. Chromium alumina is slightly harder than white alumina, and the addition of small amounts of TiO2 enhances its toughness. The final product typically features a medium grain size with an elongated or blocky yet sharp shape. Ruby fused alumina, having a higher chromium oxide content, exhibits greater toughness and hardness, with particles that are blocky, angular, and cooled cut. Adding vanadium oxide can impart a distinctive green hue.
Zirconium alumina is produced by incorporating 10% – 40% zirconium dioxide (ZrO2) into alumina during manufacturing. There are at least three common combinations used in grinding wheels: 75% Al2O3 and 25% ZrO2, 60% Al2O3 and 40% ZrO2, and 65% Al2O3, 30% ZrO2, and 5% TiO2. The rapid solidification process creates fine, tough structures that enhance the abrasives’ ductility and lifespan, making them suitable for medium-heavy material removal and high-pressure grinding applications, such as foundry ingot grinding.
The inclusion of titanium dioxide improves the ductility of alumina, making this material ideal for handling large and variable mechanical loads. Brown fused alumina generally contains 2% – 4% rutile, while semi-brittle fused alumina features 1% – 2% titanium dioxide, resulting in a material that is harder than white alumina but more brittle than brown fused alumina.
In this process, grain growth occurs within a sulfide matrix under controlled conditions, followed by acid leaching to separate the particles, eliminating the need for crushing. This method yields nodule-shaped particles that enhance binding strength, reduce breakage, and minimize mechanical defects. The defect-free crystal structure provides higher toughness compared to traditional fused alumina.
The fragmentation method used after melting significantly affects particle shape. Impact crushers, such as hammer mills, create blocky shapes, while roller crushers can lead to fractures. Electrostatic separation can distinguish sharp shapes from blocky particles, allowing for different cutting actions with the same composition.
With over 80 years of history and more than 30 years of commercial production, hollow spherical alumina is created by blowing liquid alumina cast flow under reducing conditions using compressed air or steam. Polycrystalline hollow spherical alumina abrasives can also be produced by dispersing molten alumina into fine droplets and cooling them with ultrasound. These abrasives are primarily used to create closed pores in products, facilitating control over shape and size while maintaining low cutting temperatures.
Heat treatment can alter the properties of alumina, particularly brown alumina. The abrasive is heated to 1100 – 1300°C to anneal cracks and defects from the crushing process, potentially increasing toughness by 25% – 40%.
Generally, larger crystals lead to more brittle particles, and slower cooling results in larger crystals. To achieve very fine crystals, the charge in the furnace is cooled rapidly, while coarse crystalline abrasives are obtained by slowly cooling larger charge ingots.
Fused alumina types vary in chemical composition, with trace amounts of chromium imparting a red hue, iron creating a black appearance, and titanium contributing a blue color.
Additionally, various coating processes can enhance the binding strength of alumina particles in grinding tools. For instance, red Fe2O3 can be coated at high temperatures to increase surface area, improving binding in resin cutting wheels. Silane is also utilized in resin-bonded wheel applications to prevent coolant penetration between the binder and abrasive, protecting the resin binder.
When choosing an aluminum oxide abrasive, consider these key factors:
Application: Identify the specific task—grinding, polishing, or cutting—to choose the right type.
Material Compatibility: Ensure the abrasive suits the materials to prevent damage.
Particle Size: Select grit size based on the desired finish and material removal rate.
Bond Type: Decide between bonded and coated abrasives based on your application.
Performance Needs: Consider durability and cutting speed to meet production demands.
Manufacturing: Extensively used in grinding wheels, sandpaper, and other abrasive tools.
Automotive Industry: Critical for grinding and finishing metal components.
Woodworking: Essential for sanding and finishing wood products.
Metal Fabrication: Utilized in processes like deburring and polishing.
Electronics: Employed in manufacturing components requiring precise tolerances.
Eco-Friendly Options: Growing demand for sustainable abrasives with minimal environmental impact.
Advanced Processing Techniques: Innovations enhancing performance and longevity.
Customization: Tailored abrasives to meet specific industrial needs.
Smart Abrasives: Incorporating technology to monitor wear and optimize performance.
Aluminum oxide abrasives play an indispensable role across industries, offering diverse applications and significant benefits. Understanding the types, properties, and selection criteria is crucial for optimizing manufacturing processes. As technology progresses, the future of aluminum oxide abrasives appears bright, with innovations set to enhance their effectiveness and sustainability. Whether you operate in manufacturing, automotive, or woodworking, selecting the right aluminum oxide abrasive is essential for achieving optimal results.
