Cubic Boron Nitride (cBN)

Cubic boron nitride (cBN) at 4000-5000 HV is the second hardest material after diamond. Unlike PCD, cBN can be ground and polished with diamond abrasive because diamond (8000-10000 HV) is significantly harder than cBN. This makes preparation challenging but more practical than PCD. Material removal rates are still very low compared to metals or even cemented carbides. The preparation objective is revealing cBN grain size, binder distribution, phase composition, and any defects. The binder phase (metallic or ceramic) is softer than cBN, creating relief challenges similar to WC-Co but at a higher hardness level.

Use EDM wire cutting for metallic-binder grades (electrically conductive). For ceramic-binder grades, use a diamond wire saw or resin-bonded diamond blade on a precision wafering saw at very low speed (50-100 RPM) with continuous coolant and minimal feed rate. Standard abrasive blades are ineffective. Laser cutting is also used commercially. Leave 0.5-1 mm allowance for grinding. The material is very brittle; avoid thermal shock and mechanical impact during handling.

Hot compression mounting or cold mounting with epoxy are both suitable. The PCBN material is thermally stable at all mounting temperatures. For small PCBN cutting tool tips (often brazed onto a WC-Co substrate), mount the full tool tip to show the cBN layer, braze joint, and WC-Co substrate in one cross-section. This multi-material mount requires careful preparation to balance the different hardnesses. Vacuum impregnation helps fill porosity in the binder phase.

Diamond grinding is required. Diamond (8000-10000 HV) is harder than cBN (4000-5000 HV), making conventional diamond grinding effective, though slow.

Grinding sequence:

• 40 μm diamond disc: Remove sectioning damage (60-180 seconds). Firm pressure (30-45 N). Material removal is slow but significantly faster than PCD.
• 15 μm diamond disc: Remove previous scratches (60-180 seconds). Moderate pressure (25-40 N).
• 6 μm diamond disc: Fine grinding (60-120 seconds). Moderate pressure (20-35 N).

Disc speed: 250-300 RPM. Use rigid diamond grinding discs. The binder phase grinds faster than the cBN grains, so monitor for relief. Thorough ultrasonic cleaning between steps.

Diamond polishing is required throughout. Preparation is demanding but feasible with standard diamond polishing equipment (unlike PCD which requires specialized lapping).

Diamond polishing sequence:

• 6 μm diamond: 10-20 minutes on a hard composite pad with firm pressure (25-40 N). cBN polishes slowly but steadily.
• 3 μm diamond: 10-20 minutes on a medium-hard synthetic pad (20-35 N). Monitor for binder smearing and relief.
• 1 μm diamond: 10-15 minutes on a synthetic pad (15-30 N). cBN grain boundaries should become visible.

Final polishing:

• 0.25 μm diamond or 0.05 μm colloidal alumina: 30-60 minutes on a chemically resistant pad, or vibratory polishing for 4-8 hours. The surface should be highly reflective with sharp grain boundaries. For highest quality, ion milling or broad ion beam (BIB) polishing produces the ideal surface for SEM/EBSD analysis.

Chemical etching of cBN requires aggressive reagents due to the material's chemical stability. The key advantage for cBN (vs. diamond) is that molten alkali selectively attacks cBN.

Molten KOH (Chemical Etching) - Primary choice for cBN grain boundary revelation:

• Composition: KOH pellets, melted
• Application: Immerse polished surface in molten KOH at 400-500°C for 1-5 minutes using a nickel crucible.
• Reveals: cBN grain boundaries by selective attack on cBN grain surfaces. Binder phase is also attacked. Creates topographic contrast excellent for SEM grain size analysis. Etch pits on cBN grains reveal crystallographic orientation.
• Safety: Molten KOH at 400-500°C is extremely corrosive and dangerous. Use long-handled nickel tongs, full face shield, heat-resistant gloves, and leather apron. Work in a well-ventilated area or fume hood.

Murakami's Reagent (Chemical Etching) - For metallic binder etching:

• Composition: 10 g K3Fe(CN)6, 10 g KOH, 100 ml water
• Application: Immerse for 1-10 minutes at room temperature.
• Reveals: Metallic binder phase (Co, Ni) selectively etched. cBN grains and ceramic binder phases (TiN, TiC) remain unetched. Good for binder distribution analysis without attacking the cBN grains.

10% HCl (Chemical Etching) - For metallic binder removal:

• Composition: 10 ml HCl, 90 ml water
• Application: Immerse for 30-120 seconds.
• Reveals: Dissolves metallic binder (Co, Ni) selectively. Creates topographic relief for SEM grain size measurement. Does not attack cBN or ceramic binder phases.