Cr3C2-NiCr Thermal Spray Coating

Cr3C2-25NiCr is a hard (900-1100 HV) thermal spray coating composed of chromium carbide particles in a nickel-chromium metallic binder. Applied by HVOF or plasma spray processes. The coating microstructure contains carbide particles, NiCr binder splats, oxide stringers (from in-flight oxidation), and porosity. Cross-section preparation is essential for coating quality evaluation. The extreme hardness difference between the coating (900-1100 HV) and typical steel substrates (200-400 HV) creates differential polishing and relief challenges at the interface.

Use a precision abrasive cut-off saw with an alumina or SiC blade designed for hard materials. Cut perpendicular to the coated surface to produce a true cross-section. Use generous coolant to prevent heat damage to the coating/substrate interface. Cutting speed: 200-300 RPM with moderate feed rate. Do not cut through the coating surface from the top; instead, cut from the substrate side to minimize coating edge damage. The hard coating will wear the blade quickly. Leave 2-3 mm allowance for grinding. For thin coatings, precision wafering with a diamond blade at low speed is preferred.

Edge-retaining mounting is absolutely critical. Use a hard epoxy mounting compound (e.g., epoxy with mineral filler) or a specialized edge-retaining resin. The coating surface must be preserved without rounding for accurate thickness measurement and surface quality assessment. Vacuum impregnation is strongly recommended to fill coating porosity and prevent pullout during grinding and polishing. Orient the sample so the coating surface faces down in the mold for best edge retention. Cold mounting is preferred to avoid thermal stress at the coating/substrate interface.

The hard coating (900-1100 HV) requires diamond grinding. SiC papers are ineffective against the carbide phase. Use rigid diamond grinding discs throughout.

Grinding sequence:

• 70 μm diamond disc: Remove sectioning damage and establish a flat cross-section (30-60 seconds). Moderate pressure (25-35 N). The coating grinds much slower than the substrate, so monitor for planarity.
• 40 μm diamond disc: Remove previous scratches (30-60 seconds). Maintain even pressure across the coating/substrate boundary.
• 15 μm diamond disc: Fine grinding (30-60 seconds). Reduce pressure slightly (20-30 N) to minimize differential relief at the interface.

Disc speed: 250-300 RPM. Use complementary rotation. Thorough ultrasonic cleaning between steps is essential to remove diamond debris that can scratch softer substrate areas. Monitor for coating pullout (indicates insufficient mounting or too-aggressive grinding).

Diamond polishing is required throughout. The hardness mismatch between coating and substrate is the primary challenge.

Diamond polishing sequence:

• 9 μm diamond: 3-5 minutes on a hard composite pad with moderate pressure (25-30 N). Use napless cloth to minimize relief at the interface.
• 3 μm diamond: 3-5 minutes on a medium-hard synthetic pad (20-25 N). Monitor for relief development at coating/substrate boundary.
• 1 μm diamond: 2-3 minutes on a synthetic pad (15-20 N). Check for coating surface edge retention.

Final polishing:

• 0.05 μm colloidal silica: 1-2 minutes on a soft pad with light pressure. Keep time short to avoid excessive relief. Alternatively, 0.05 μm alumina suspension can be used.

If porosity measurement is the primary objective, keep the surface as flat as possible; even minor relief can produce shadow artifacts that inflate apparent porosity in image analysis.

The as-polished condition is often preferred for porosity measurement and coating thickness measurement per ASTM E2109. Etching is used when phase identification or bond quality assessment is needed.

10% Oxalic Acid, Electrolytic (Electrolytic Etching) - Primary choice:

• Composition: 10 g oxalic acid, 100 ml water
• Application: Electrolytic at 6V DC for 10-30 seconds. Stainless steel cathode.
• Reveals: NiCr binder phase, carbide distribution, oxide stringers. Differentiates binder from carbide clearly.
• Rinse: Water, then ethanol. Dry with warm air.

Murakami's Reagent (Chemical Etching) - For carbide phase detail:

• Composition: 10 g K3Fe(CN)6, 10 g KOH, 100 ml water
• Application: Immerse for 10-60 seconds at room temperature, or 3-10 seconds at 80-90°C.
• Reveals: Cr3C2 carbide particles darkened; NiCr binder unaffected. Excellent for carbide distribution analysis.

Note: If the substrate is steel, 2-5% Nital can be used to reveal the substrate microstructure and heat-affected zone, but protect the coating surface during application.