Hard Chrome Plating

Hard chrome plating is a very hard (900-1100 HV) electrolytic chromium deposit on a softer substrate (typically steel at 200-400 HV). The extreme hardness mismatch between coating and substrate is the primary preparation challenge, causing severe differential polishing and relief at the interface. The microcrack network within the chrome is a critical quality feature that must be preserved and revealed. Cross-section preparation is standard for thickness measurement, crack density assessment, and interface bond quality evaluation.

Use a precision abrasive cut-off saw with an alumina blade designed for hard materials. Cut perpendicular to the plated surface. The very hard chrome coating will accelerate blade wear. Use generous coolant to prevent heating the interface. Cutting speed: 200-300 RPM with moderate to low feed rate. Do not force the cut through the hard coating. For cylindrical parts (hydraulic rods, piston rings), clamp securely and cut a diametral cross-section. Leave 2-3 mm allowance for grinding away sectioning damage.

Edge-retaining mounting is absolutely critical. Use a hard epoxy mounting compound with mineral filler, or specialized edge-retaining resin. The chrome plating surface must be perfectly preserved for thickness measurement. Vacuum impregnation is recommended to fill the microcrack network and any interface gaps, preventing crack closure during polishing and preserving crack morphology for analysis. Orient the plated surface face-down in the mold. Cold mounting is preferred. For the best edge retention, apply a thin electroless nickel plating over the chrome surface before mounting to create a supportive metallic edge.

The very hard chrome coating (900-1100 HV) requires diamond grinding. SiC papers are inefficient and create excessive relief at the coating/substrate boundary because the substrate grinds much faster than the chrome.

Grinding sequence:

• 70 μm diamond disc: Remove sectioning damage (30-60 seconds). Moderate pressure (25-35 N). The coating grinds much slower than the substrate; monitor planarity.
• 40 μm diamond disc: Remove previous scratches (30-60 seconds). Adjust pressure to balance grinding rates between coating and substrate.
• 15 μm diamond disc: Fine grinding (30-60 seconds). Reduce pressure (20-30 N).

Disc speed: 250-300 RPM. If SiC papers must be used (diamond discs unavailable), use 240-600 grit sequence with very light pressure and accept longer grinding times on the chrome. Thorough ultrasonic cleaning between steps.

Diamond polishing is required. The hardness mismatch between chrome coating and steel substrate demands careful technique to minimize relief.

Diamond polishing sequence:

• 9 μm diamond: 3-5 minutes on a hard napless composite pad with moderate pressure (25-30 N). Use napless cloth exclusively to prevent rounding at the interface.
• 3 μm diamond: 3-5 minutes on a medium-hard synthetic pad (20-25 N). The microcrack network should begin to be visible. Monitor for relief at the coating/substrate interface.
• 1 μm diamond: 2-3 minutes on a synthetic pad (15-20 N). Microcracks should be clearly visible.

Final polishing:

• 0.05 μm colloidal silica: 1-2 minutes on a soft pad with light pressure. Keep time short. The microcrack network must remain sharp and open, not rounded or smeared closed.

If the microcracks appear smeared or closed after polishing, the sample is over-polished or insufficient vacuum impregnation was performed.

The microcrack network is typically visible in the as-polished condition. Etching reveals additional detail about the chrome structure and the substrate heat-affected zone.

10% NaOH Electrolytic (Electrolytic Etching) - Primary choice for chrome structure:

• Composition: 10 g NaOH, 100 ml water
• Application: Electrolytic at 3-6V DC for 5-15 seconds.
• Reveals: Microcrack network with enhanced contrast, columnar grain structure, and any nodular defects in the chrome deposit.
• Rinse: Water, then ethanol. Dry with warm air.

10% Oxalic Acid Electrolytic (Electrolytic Etching) - Alternative:

• Composition: 10 g oxalic acid, 100 ml water
• Application: Electrolytic at 6V DC for 10-30 seconds.
• Reveals: Chrome crack network, deposit structure, and interface bond quality.

Villella's Reagent (Chemical Etching) - For substrate HAZ:

• Composition: 1 g picric acid, 5 ml HCl, 100 ml ethanol
• Application: Swab on substrate area for 10-30 seconds.
• Reveals: Substrate microstructure and any hydrogen embrittlement effects near the interface.