Polycrystalline Diamond (PCD)

Polycrystalline diamond (PCD) at 8000-10000 HV is the hardest material in metallographic preparation. Standard grinding and polishing methods used for metals and even cemented carbides are completely inadequate. Only diamond abrasive can cut diamond, and material removal rates are extremely low. The preparation objective is revealing diamond grain boundaries, binder phase distribution, and any defects (cracks, voids, graphitization). The Co binder (when present) is much softer than the diamond grains, creating extreme relief challenges. Extended preparation times (hours to days for final polish) are normal.

Use electrical discharge machining (EDM) wire cutting for Co-bonded PCD (the Co binder is electrically conductive). EDM is the most practical method for sectioning PCD. For SiC-bonded PCD (non-conductive), use a diamond wire saw or specialized diamond cut-off wheel at very low speed. Conventional abrasive cut-off wheels and even standard diamond blades wear out rapidly. Laser cutting is also used commercially. Leave 0.5-1 mm allowance for lapping. The cut surface will have significant heat-affected zone from EDM that must be removed.

Hot compression mounting or cold mounting with epoxy are both suitable. PCD is thermally stable to temperatures well above mounting conditions (though Co-bonded PCD may begin to graphitize above 700°C, which is far above mounting temperatures). For small PCD discs (cutting tool tips), mount with the cross-section face exposed. The extreme hardness means the mount material has no effect on preparation quality; any resin is acceptable. For best results when analyzing the binder phase, vacuum impregnation with epoxy can help fill any voids.

Diamond lapping is required. Standard grinding papers and even diamond grinding discs used for ceramics are too slow for PCD. Specialized preparation is needed.

Lapping sequence:

• 15 μm diamond paste on cast iron lap: Remove sectioning damage (30-120 minutes or longer). Firm pressure (40-60 N). The cast iron lap holds the diamond abrasive and provides a rigid, flat surface. Material removal rate is extremely slow; patience is essential.
• 6 μm diamond paste on cast iron lap: Remove previous scratches (30-60 minutes). Moderate to firm pressure (30-50 N).
• 3 μm diamond paste on cast iron lap: Fine lapping (30-60 minutes). Moderate pressure (25-40 N).

Lap speed: 50-150 RPM. The crystal orientation of individual diamond grains means some grains polish faster than others, creating unavoidable slight relief. Harder crystallographic directions resist polishing. Thorough cleaning between steps to remove lapping debris.

Final polishing of PCD is extremely challenging and time-consuming. The goal is a surface smooth enough to resolve grain boundaries.

Diamond polishing sequence:

• 1 μm diamond paste on hard polishing pad or tin lap: 2-8 hours of continuous polishing with firm pressure (25-40 N). A tin lap charged with diamond paste can be more effective than a cloth pad for PCD.
• 0.25 μm diamond paste on tin lap: 2-8 hours with moderate pressure (20-35 N). The surface should become highly reflective.

Final polishing:

• 0.05 μm diamond or colloidal silica on chemically resistant pad: 4-24 hours (vibratory polishing preferred). Some labs use proprietary CMP slurries developed for diamond polishing. Ion milling or plasma etching as a final step produces the best surface for SEM analysis by removing the amorphous deformation layer left by mechanical polishing.

For routine quality control (rather than research), polishing to the 1 μm diamond stage may be sufficient for SEM grain boundary analysis, especially if chemical or plasma etching is used to enhance contrast.

Standard chemical etchants are ineffective against diamond. Specialized techniques are required to reveal grain boundaries and binder distribution.

Plasma Etching (Physical/Chemical Etching) - Best technique:

• Application: Oxygen or Ar/O2 plasma at 100-500W for 5-30 minutes.
• Reveals: Diamond grain boundaries by preferential removal of the binder phase and graphitized carbon at grain boundaries. Creates topographic contrast ideal for SEM imaging. The most effective technique for PCD grain size analysis.
• Equipment: Requires reactive ion etching (RIE) or plasma etching chamber.

Focused Ion Beam (FIB) Milling - For high-resolution analysis:

• Application: Ga+ ion beam milling at 30 kV in FIB-SEM.
• Reveals: Diamond grain structure, binder distribution, and intergranular features at nanometer resolution. Can prepare site-specific TEM specimens.

Boiling Acid Mixture (Chemical Etching) - For Co binder removal:

• Composition: Hot concentrated HNO3 + HCl (aqua regia) or hot H2SO4 + HNO3
• Application: Boil for 30-120 minutes.
• Reveals: Dissolves Co binder selectively, leaving diamond skeleton with topographic relief at grain boundaries. Only effective for Co-bonded PCD. Does not attack diamond.
• Safety: Boiling concentrated acids. Extreme caution required. Full acid-resistant PPE. Fume hood mandatory.