Al2O3 Polishing Powder: Technical Guide for Industrial and Laboratory Use

Introduction

In polishing work — whether it is bringing a telescope mirror into optical tolerance or cleaning up a titanium orthopedic implant — aluminium oxide polishing powder is a staple. Known chemically as Al2O3, this ceramic abrasive comes in multiple grades and phases. The right choice can mean the difference between a uniform, high-gloss finish and a surface riddled with micro-scratches.

Crystal Phase Selection

Two crystal forms dominate the polishing use:

• α-Al2O3 (alpha alumina): Dense corundum structure, hardness close to sapphire. Stable under heat and mechanical stress, making it a good match for glass, hard metals, and ceramics.

• γ-Al2O3 (gamma alumina): Softer, more friable. Breaks down during use, which can help achieve a finer finish on softer metals or polymer composites.

Phase is not just a lab curiosity — it directly affects cutting rate, slurry behaviour, and the final roughness you will achieve.

Particle Size and Application Matching

Industry often works with particle sizes from 5 µm down to 0.05 µm. For example:

A three-step optical sequence might go 3 µm → 1 µm → 0.05 µm, with pad changes between stages. Skipping an intermediate step usually costs more time than it saves.

Slurry Preparation and Process Control

For most polishing setups:

• Concentration: 5–20 wt% Al2O3 powder in deionised water.

• pH: Neutral for ceramics and glass; slightly acidic (pH 4–6) for metals prone to oxidation.

• Pad choice: Felt or polyurethane for brittle substrates; woven cloth for ductile metals.

• Pressure: Around 20–80 kPa for delicate parts; up to 150 kPa for heavy stock removal.

• Rotation: 30–90 rpm platen speed is typical; higher risks of overheating thin parts.

Some optics shops use ASTM E112 particle size verification before charging a polishing lap to ensure no oversized grit enters the process.

Application Notes

• Metals: For stainless steel finishing, γ-Al2O3 at 3 µm followed by α-Al2O3 at 1 µm works well. Avoid alkaline slurry on aluminium alloys to prevent surface etching.

• Glass and optics: Use α-Al2O3 with filtered slurry (0.2 µm filtration) to prevent random scratches. Keep work under 30 °C to avoid thermal distortion.

• Ceramics: Continuous slurry flow prevents localised heating and microcracking in alumina or zirconia parts.

• Semiconductors: Ultra-high-purity (>99.99%) α-Al2O3, often in 0.05 µm grade, is standard for final wafer planarisation.

Troubleshooting Common Issues

Safety and Storage

Even though alumina ceramic is inert, its fine dust can irritate the respiratory system. Work under extraction or wear a particulate mask. For high-purity polishing, store powder in sealed containers with desiccant — Al2O3 will absorb ambient moisture over time, changing slurry behaviour.

Final Remarks

Choosing the right aluminium oxide polishing powder is not just about selecting a grit size from a catalogue. The crystal phase, particle distribution, purity, and process parameters all contribute to the final surface. In demanding work — optical flats, precision medical devices, semiconductor wafers — those details decide whether the part meets specification.

If you need consistent, high-purity Al2O3 powder with verified specifications, Stanford Advanced Materials supplies multiple grades tailored for both laboratory and production-scale polishing. Our technical team can match the powder characteristics to your exact process, helping you reach your surface finish targets faster and with fewer rejects.