Polishing Abrasives for Metallographic Sample Preparation

Polishing is a critical step in metallographic sample preparation, ensuring specimens are free from scratches, deformations, and surface damage, all of which are essential for accurate analysis. Obtaining a flawless surface requires precision abrasives like diamond, aluminum oxide (Al₂O₃), and amorphous silicon dioxide (SiO₂) in colloidal suspension. At OnPoint Abrasives, we offer a range of highly engineered polishing abrasives designed to meet the specific demands of different materials and applications.

Diamond Abrasives: The Pinnacle in Hardness and Performance

Diamonds are known for their exceptional hardness and outstanding efficiency in material removal. They are an ideal choice for producing extremely flat, scratch-free, and deformation-free surfaces across a variety of materials. Originally introduced as a carrier paste, diamond abrasives have evolved into more efficient slurry forms, enhancing their utility and effectiveness. Synthetic diamonds, now the industry standard, offer more consistent performance compared to natural diamonds.

At OnPoint Abrasives, we offer two premium types of diamond suspensions:

• Diabrasive Choice: Featuring synthetic monocrystalline diamonds, Diabrasive Choice suspensions provide excellent cutting performance and longevity. The uniform morphology of these diamonds results in predictable and reliable material removal, particularly suitable for harder materials.
• Diabrasive Premium: For faster cutting rates and a finer surface finish, Diabrasive Premium suspensions contain synthetic polycrystalline diamonds. Studies show that polycrystalline diamonds exhibit higher cutting rates due to their more fragmented structure, which results in more active cutting surfaces compared to monocrystalline diamonds. Polycrystalline diamonds also yield a finer finish, making them ideal for samples requiring the utmost surface quality.

Technical Advantages:

• Hardness: Diamond (10 Mohs) ensures rapid material removal.
• Available Formats: Offered in suspensions and pastes for greater application flexibility.
• Grain Sizes: Various sizes from 30 µm down to 0.25 µm for precision polishing stages.

Using diamond abrasives allows you to maximize material removal efficiency while achieving a planar surface, crucial for detailed microstructural analysis.

Colloidal Silica: Chemical Mechanical Action for Precision Finishing

Colloidal silica is widely used for final polishing in metallographic sample preparation. Its unique polishing mechanism combines mechanical abrasion with chemical activity, providing the ability to achieve nearly perfect surfaces that are free from both scratches and deformation.

At OnPoint Abrasives, we offer three types of colloidal silica suspensions:

• ProPrep: Our standard colloidal silica suspension, ProPrep is free of additives and offers reliable polishing with a basic pH of ~9.8. It is ideal for final polishing of softer metals and non-metals, providing smooth, scratch-free surfaces.
• ProPrep 2: This low-crystallizing colloidal silica suspension includes proprietary additives to significantly minimize the risk of silica crystallization, which can lead to surface scratching. ProPrep 2 is ideal for highly precise polishing where long polishing cycles are required.
• ProPrep Blue: With the same properties as ProPrep, ProPrep Blue adds a blue dye for easy lab identification, allowing for better process control and ensuring the right product is used for each application.

Technical Insights:

• Grain Size: A fine particle size of approximately 0.06 µm enables precise polishing.
• pH: The basic pH (~9.8) of colloidal silica enhances its chemical polishing action, which is especially effective on ductile materials.
• Chemical-Mechanical Polishing: The combined action allows for the removal of deformation and minor surface imperfections without aggressive mechanical abrasion, ideal for final polishing steps before high-resolution imaging or analysis.

To prevent silica crystallization, which can cause surface damage, it is essential to manage the suspension carefully. ProPrep 2’s additives reduce crystallization, ensuring safe, high-quality polishing throughout extended use.

Aluminum Oxide: Versatile and Reliable for Routine Polishing

Aluminum oxide (Al₂O₃) is a highly versatile abrasive frequently used in routine metallographic sample preparation. It provides a balance of mechanical strength and surface finish quality, making it suitable for a wide range of materials.

At OnPoint Abrasives, we offer three aluminum oxide products:

• Alumabrasive: Our traditional alpha alumina suspension, designed for routine polishing applications. It is effective in both intermediate and final polishing steps, offering high-quality surface finishes at economical costs.
• AlumaPro: This premium product is produced using a wet-milling process, which ensures complete deagglomeration of the alumina particles. Wet-milled alumina provides superior surface finishes compared to calcined alumina, with fewer surface imperfections and more consistent performance.
• Calcined Alumina Powder: For those who prefer traditional calcined alumina abrasives, we offer a range of particle sizes. Although calcined alumina may exhibit some agglomeration, it remains a cost-effective option for certain applications.

Technical Considerations:

• Grain Sizes: Available in 0.05 µm, 0.3 µm, 1.0 and 3.0 µm for various stages of sample preparation.
• Deagglomeration: AlumaPro’s wet-milling process eliminates agglomerates, providing a more uniform and defect-free surface. Available only in 0.05 µm
• Versatility: Suitable for a wide range of materials, from hard steels to non-ferrous alloys and ceramics.

For final polishing, these aluminum oxide products ensure scratch-free surfaces, particularly in sequential polishing steps leading to high-quality specimen surfaces.

Partner with OnPoint Abrasives for Precision Polishing

At OnPoint Abrasives, we understand that every sample preparation process is unique. Whether you’re preparing hard metals, delicate materials, or complex surfaces, our range of diamond, colloidal silica, and aluminum oxide abrasives are designed to provide you with the highest quality polish in the most efficient manner. Our consumable supplies support your metallographic sample preparation, ensuring accurate and reproducible results every time.