WC-Co Cemented Carbide (Hardmetal)

WC-Co cemented carbide is an ultra-hard material (87-93 HRA, ~1300 HV for typical grades) consisting of hard WC grains (2600 HV) in a softer Co binder (100-300 HV). The extreme hardness contrast between the WC grains and the Co binder is the fundamental preparation challenge. The cobalt binder smears easily and can be preferentially removed during polishing, leaving WC grains standing proud. Diamond grinding and polishing are mandatory; SiC is ineffective against WC. The goal is a flat surface with both WC grains and Co binder in the same focal plane for accurate grain size measurement.

Use a diamond wafering saw with a thin resin-bonded diamond blade (not electroplated, which wears too quickly). Cutting speed: 100-200 RPM with very low feed rate and continuous coolant. The extreme hardness means cutting is slow; do not force the cut. Abrasive cut-off wheels designed for hard materials (reinforced alumina or diamond) can be used for rough sectioning. The material is brittle and can chip at edges. Leave 1-2 mm allowance for grinding. For small inserts, mount first and then grind to the desired cross-section plane.

Hot compression mounting with phenolic or Bakelite resin is standard. The cemented carbide is thermally stable at all mounting temperatures. For porosity analysis per ASTM B276, vacuum impregnation with epoxy is essential to distinguish true porosity from pullout artifacts. Cold mounting with epoxy is also suitable. Edge retention is not typically a concern since the carbide is harder than any mounting material. For small inserts, multiple samples can be mounted together for batch processing.

Diamond grinding is MANDATORY. SiC grinding papers are ineffective because SiC (~2500 HV) is softer than WC (~2600 HV). Use rigid diamond grinding discs with diamond abrasive bonded to a metal or resin disc.

Grinding sequence:

• 70 μm diamond disc: Remove sectioning damage (60-120 seconds). Moderate to firm pressure (30-45 N). The material is very hard and removes slowly.
• 40 μm diamond disc: Remove previous scratches (60-120 seconds). Maintain firm pressure (25-40 N).
• 15 μm diamond disc: Fine grinding (60-120 seconds). Reduce pressure slightly (20-35 N).

Disc speed: 250-300 RPM. Use complementary rotation. The cobalt binder grinds much faster than the WC; rigid discs minimize this differential by maintaining planarity. Thorough ultrasonic cleaning between steps is critical to remove diamond particles that can cause deep scratches in subsequent steps.

Diamond polishing is required throughout. The key challenge is achieving a flat surface where the Co binder is coplanar with the WC grains (no relief).

Diamond polishing sequence:

• 6 μm diamond: 5-10 minutes on a hard composite or napless synthetic pad with firm pressure (25-35 N). The WC grains polish slowly; allow adequate time.
• 3 μm diamond: 5-10 minutes on a medium-hard synthetic pad (20-30 N). Monitor for Co binder smearing or preferential removal.
• 1 μm diamond: 3-5 minutes on a synthetic pad (15-25 N). The WC/Co boundary should be becoming sharp and clear.

Final polishing:

• 0.25 μm diamond or 0.05 μm colloidal alumina: 2-5 minutes on a soft pad with moderate pressure. Use colloidal alumina (NOT colloidal silica) for the final step because silica contamination can interfere with WC grain boundary analysis. Alternatively, 0.25 μm diamond paste on a hard cloth produces excellent results for grain size measurement.

For the highest quality preparation (grain size measurement per ASTM B390), vibratory polishing with 0.05 μm alumina for 2-4 hours produces the flattest surface with the sharpest WC/Co boundaries.

Etching is essential for WC grain size measurement per ASTM B390. The as-polished surface shows poor contrast between WC and Co; etching selectively attacks one phase to create contrast.

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

• Composition: 10 g K3Fe(CN)6, 10 g KOH, 100 ml water
• Application: Immerse for 3-10 minutes at room temperature. Fresh solution required (degrades with use).
• Reveals: WC grain boundaries by etching the Co binder and WC grain surfaces. WC grains appear as angular, geometric shapes with clear boundaries. Also reveals eta phase (W3Co3C or W6Co6C) as a distinct dark-etching phase, and free carbon (graphite) as dark spots.
• Rinse: Water, then ethanol. Dry with warm air.

Alkaline K3Fe(CN)6, Hot (Chemical Etching) - For selective WC etching:

• Composition: 10 g K3Fe(CN)6, 10 g NaOH, 100 ml water
• Application: Immerse at 80-100°C for 2-5 minutes.
• Reveals: Selectively attacks WC grains, leaving Co binder bright. Useful for binder distribution analysis and binder mean free path measurement.

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

• Composition: 10 ml HCl, 90 ml water or ethanol
• Application: Immerse for 30-120 seconds.
• Reveals: Dissolves Co binder selectively, creating relief and revealing WC grain shapes. More aggressive than Murakami's. Useful for SEM grain size analysis where topographic contrast is preferred.