Sapphire (Al₂O₃, 9 on Mohs scale), fused silica, flint (high-index) glass, and IR germanium windows are materials that cannot be polished with conventional oxides. Cerium oxide only works on soft glass (borosilicate, 5–6 Mohs); aluminum oxide and silica are 2–4 times softer than sapphire — they "rub against air" and remove nothing. The physics is simple: the abrasive must be harder than the workpiece. The only industrial abrasive harder than sapphire is diamond.
Mechanism of Diamond Finishing
Unlike grinding, where a grain in bond acts like a cutting edge, polishing uses free diamond particles in a slurry or on a lap (pitch, tin, resin). They roll across the surface, micro-indent, and remove a layer of tenths of a micron per pass.
Three Stages of the Process
| Stage | Grit | Goal | Target Ra |
|---|---|---|---|
| 1. Rough lapping | W14–W28 | Remove sawing damage | 0.1–0.2 мкм |
| 2. Semi-finish | W3.5–W7 | Remove previous-stage scratches | 0.02–0.05 мкм |
| 3. Finish polishing | М1 / АСМ 0–1 | Scatter-free mirror surface | <1 нм |
Typical Defects and Causes
Scratches — a D97 Oversight
The most common cause of parallel scratches on sapphire substrates is too broad a distribution tail at the finishing stage. A powder with D50=1 μm but D97=4 μm leaves fine but visible scratches. The fix is switching to powder with D97 ≤ 2 μm and thoroughly cleaning the lap between stages.
Matte Haze — Grain Too Fine
If the finishing stage uses grain too fine (ASM 0/1), it does not remove material but only plastically deforms the surface. After an hour of polishing the part goes matte. Step back 1–2 stages and remove the damaged layer with coarser grain.
"Orange Peel" — Lap Too Hard
With a tin or hard pitch lap, individual grains embed too deeply and act like cutting tools — the surface becomes textured. Switch to a softer wax or resin lap.
Which Powder Your Process Needs
- LED sapphire substrates, watch glass → semi-rounded M40/M42 for finish
- Fused silica optics (UV lenses, mirrors) → sharp W7 semi-finish, ASM 0/1 finish
- Germanium and silicon IR windows → W14 rough, M28 finish
DIAMTECH supplies all types of diamond micron powder for optical finishing, with full D97 and TAPS certification. Request a trial batch and compare with your current supplier.