Alumina polishing powder is the workhorse abrasive for precision finishing of hard materials: sapphire, silicon wafers, optical lenses, ceramic substrates, and hard disk media. Its combination of hardness (Mohs 9), controllable particle size (sub-micron to 30 micrometer), and chemical inertness makes it the abrasive of choice where surface quality cannot be compromised.
At Aluminaworld, we supply approximately 1,200 MT/year of polishing-grade alumina, supporting precision optics manufacturers in Japan, Germany, Israel, and the United States. The data below reflects our standard grades and the polishing performance our customers achieve.
This guide covers grade selection for the most common precision polishing applications, with specific recommendations for sapphire, wafer, lens, and metal polishing.
1. Alumina vs Other Abrasives: Why It Wins for Precision
The main abrasives used in precision polishing are diamond, silicon carbide (SiC), cerium oxide (CeO2), and alumina (Al2O3). Each has its niche:
| Abrasive | Mohs Hardness | Best For |
|---|---|---|
| Diamond | 10 | Hardest materials (SiC, sintered ceramics) |
| Alumina | 9 | Sapphire, wafers, glass, metals |
| Silicon Carbide | 9.5 | Rough lapping, cost-sensitive applications |
| Cerium Oxide | 6 | Glass polishing (final finish) |
Alumina is the best balance of hardness, controllability, and cost for most precision applications. Diamond is harder but produces sub-surface damage on softer materials and is significantly more expensive. Cerium oxide is gentler but cannot finish sapphire or other hard materials efficiently.
2. Sapphire Polishing: LED and Watch Crystal Applications
Sapphire (single crystal Al2O3) is widely used for LED substrates, watch crystals, and smartphone camera windows. The polishing process typically uses three stages with progressively finer alumina:
Stage 1: Rough Polishing (Material Removal)
5 to 15 micrometer alumina slurry on a cast iron or copper lap. Removal rate: 10 to 30 micrometer per minute. Surface finish after stage 1: Ra 50 to 100 nm. Used to remove saw damage and bring the part to near-final dimension.
Stage 2: Pre-Final Polishing
1 to 3 micrometer alumina slurry on a soft polyurethane lap. Removal rate: 1 to 3 micrometer per minute. Surface finish: Ra 5 to 15 nm. Used to remove the damage layer from stage 1.
Stage 3: Final Polishing
0.05 to 0.3 micrometer alumina on a flocked or foam lap. Removal rate: 50 to 200 nm per minute. Surface finish: Ra below 0.5 nm (atomic-scale smoothness). This is the optical-grade finish.
For sapphire LED substrates, the most common spec is Ra below 0.2 nm on the epitaxial side. Achieving this consistently requires our finest grade (D50 of 0.1 to 0.3 micrometer) with tight particle size distribution (D90/D10 below 2).
3. Wafer and Lens Polishing: Specific Grade Recommendations
Silicon Wafer Polishing (CMP Slurries)
For chemical-mechanical polishing of silicon wafers, alumina is used at 0.05 to 0.2 micrometer with tight distribution. The slurry pH is adjusted to 9 to 11 for chemical action; alumina provides mechanical action. Typical concentration: 1 to 5 wt% alumina in slurry. Surface finish after CMP: Ra below 0.1 nm.
Optical Lens Polishing (Glass)
For glass lenses, cerium oxide is more common because it gives faster polishing rates on glass. However, alumina is used for the final 30 seconds to achieve the lowest possible surface roughness. Grade: 0.05 to 0.3 micrometer.
Metal Mirror Polishing
For aluminum, copper, and stainless steel mirrors, alumina at 0.3 to 1 micrometer is the standard. The polishing compound is usually oil-based or water-based with pH 7 to 9. Surface finish achievable: Ra below 1 nm for high-quality optical mirrors.
For specialized polishing applications (semiconductor wafers, precision molds, laser crystals), our technical team can recommend the optimal grade and slurry formulation. We supply free 100 g samples for lab evaluation.
Frequently Asked Questions
What is the difference between calcined and fused alumina for polishing?
Fused alumina (white or pink) is made by melting calcined alumina. It has higher hardness and sharper edges, used for rough polishing. Calcined alumina has rounded particles, used for final polishing where surface quality matters.
Can I reuse polishing alumina slurry?
Yes, with proper filtration and pH control. Most precision polishing shops recycle slurry 5 to 10 times before disposal. We can advise on recycling protocols for your specific application.
What mesh size for plastic lens polishing?
Plastic (acrylic, polycarbonate) is softer than glass. Use 0.3 to 1 micrometer alumina to avoid scratching. For very soft plastics, consider colloidal silica instead.
Is your alumina slurry ready-to-use or concentrate?
Both. We ship ready-to-use slurry at 1 to 5% concentration in 25 L drums, and concentrate slurry at 30 to 50% in 200 L drums for dilution at the customer site.
What is the price for polishing-grade alumina?
$3 to $8 per kg depending on grade and purity. Sub-micron grades are 3 to 5x more expensive than coarse grades due to more complex manufacturing.
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