Why Material Matters When Choosing Diamond Powder

Drawing dies are made from different hard materials – tungsten carbide (WC), polycrystalline diamond (PCD), CVD diamond, or natural single‑crystal diamond. Each has distinct hardness, toughness, and microstructure. Using the same diamond powder for all will lead to poor efficiency, excessive scratches, or die damage. This guide explains how to match diamond powder properties to each die material.

1. Tungsten Carbide (WC) Drawing Dies

Material characteristics: Hardness ~1100–1600 HV, relatively tough, prone to binder (cobalt) pull‑out if over‑polished. Less demanding than diamond dies, but still requires fine surface finish for wire drawing.

• Recommended diamond powder type: Monocrystalline diamond – sharp edges provide efficient cutting.
• Grit sequence: Coarse: 15–25 μm → Medium: 6–10 μm → Fine: 1–3 μm → Final polish: 0.5–1 μm.
• Particle shape: Blocky with moderate sharpness – no need for highly rounded grains.
• Concentration in paste: Medium (15–25 vol%) for roughing; lower for finishing.
• Carrier / lubricant: Oil‑based pastes work well; water‑based may cause rust on steel shanks.

2. Polycrystalline Diamond (PCD) Drawing Dies

Material characteristics: Hardness ~5000–8000 HV, anisotropic, composed of diamond grains with cobalt binder. Polishing is difficult – the binder smears and diamond grains can pull out.

• Recommended diamond powder type: Polycrystalline diamond (self‑sharpening, rounded shape) – produces finer, more uniform scratches and reduces binder smearing.
• Grit sequence: Coarse: 9–15 μm → Medium: 3–6 μm → Fine: 1–2 μm → Mirror: 0.25–0.5 μm.
• Particle shape: Highly rounded, equiaxed – avoid sharp monocrystalline grains which cause deep gouges.
• Concentration in paste: Moderate (15–25 vol%) – too high leads to overheating and cobalt smearing.
• Carrier: Oil‑based with extreme pressure additives; avoid water‑based carriers that may attack cobalt binder.
• Special tip: Use short polishing intervals and frequent cleaning to prevent binder smearing across the surface.

3. CVD Diamond (Uncoated) Drawing Dies

Material characteristics: Pure diamond film, hardness ~10,000 HV, no binder, highly uniform but can be brittle. Polishing is extremely slow.

• Recommended diamond powder type: Polycrystalline diamond for better self‑sharpening; or a mix of monocrystalline (for stock removal) followed by polycrystalline (for finish).
• Grit sequence: Start with 6–9 μm (no coarser, to avoid chipping) → 3 μm → 1 μm → 0.25 μm.
• Particle shape: Rounded for finishing; sharp monocrystalline only for initial roughing if needed.
• Concentration: Low (10–15 vol%) – high concentration generates excessive heat and may cause micro‑chipping.
• Carrier: Oil‑based with excellent cooling properties; water‑based not recommended.
• Special tip: Very light pressure and constant movement – CVD diamond is hard but can fracture at the atomic scale under heavy load.

4. Natural Single‑Crystal Diamond Drawing Dies

Material characteristics: Hard, anisotropic (hardness varies with crystallographic direction), prone to cleavage if stress is misdirected. Used for ultra‑fine wires.

• Recommended diamond powder type: Soft‑grade monocrystalline diamond (less aggressive) or polycrystalline diamond for final smoothing.
• Grit sequence: Very fine only: start ≤ 3 μm, never coarser than 6 μm to avoid cracking the die.
• Particle shape: Rounded or friable polycrystalline – avoid sharp, hard‑cutting grains.
• Concentration: Very low (5–10 vol%).
• Carrier: Light oil or even glycerin‑based paste to minimise friction heat.
• Special tip: Always use extremely light pressure and the finest grits (≤3 μm); crystal orientation is generally unknown for drawing dies, so do not rely on directional polishing.

Comparison Table: Diamond Powder Selection by Die Material

Practical Decision Tree

1. Identify die material – ask your supplier or test with a hardness file if unknown.
2. Select diamond powder type: WC → monocrystalline; PCD → polycrystalline rounded; CVD → polycrystalline; Natural → soft grade.
3. Choose starting grit – based on current surface roughness: rougher surface requires coarser start grit (up to 15 μm for PCD, but only 3 μm for CVD).
4. Match carrier – oil‑based for all diamond dies; water‑based only for WC if no steel parts.
5. Adjust concentration – higher for tough materials (WC), lower for brittle or pure diamond materials.

Common Mistakes to Avoid

• Using the same powder for WC and PCD: Monocrystalline sharp powder on PCD causes deep random scratches; polycrystalline rounded powder on WC cuts too slowly.
• Starting too coarse on CVD diamond: 15 μm or larger can cause chipping – never exceed 9 μm for CVD.
• Ignoring binder smearing on PCD: Use low concentration and frequent cleaning to remove smeared cobalt.
• Using water‑based paste on steel‑shanked dies: Causes rust that contaminates the polishing process.

Conclusion

Matching diamond powder to the specific die material is not optional – it directly affects polishing speed, surface quality, and die life. Use the guidelines and table above as a quick reference. For best results, test a small sample before full production.

Once you have selected the correct powder, refer back to the first article in this series for proper polishing procedures, and the second article for incoming quality inspection of the diamond powder itself.