CoCrMo Cast (ASTM F75)

CoCrMo cast (ASTM F75) is a hard, ductile cobalt-chromium alloy (25-35 HRC, ~280 HV) with a density of 8.3 g/cm³ and a melting range of 1350-1450°C. The as-cast microstructure features an FCC cobalt-chromium matrix with M23C6 and M7C3 carbides distributed along grain boundaries and within interdendritic regions. The hardness contrast between the matrix and carbide phases is significant. This alloy work-hardens readily, so preparation must use sharp abrasives with moderate pressure to avoid introducing deformation artifacts. Diamond grinding discs are preferred over SiC paper for more efficient material removal with less surface damage.

Use an abrasive cut-off wheel designed for hard non-ferrous or cobalt alloys with continuous coolant flow. An alumina (Al2O3) blade is suitable. Cutting speed: 2500-3500 RPM for standard abrasive wheels. The alloy is tough and work-hardens, so use moderate feed pressure and avoid stalling the blade. For precision work on retrieved implants, a low-speed diamond saw at 200-400 RPM with a diamond wafering blade provides minimal deformation. Leave 2-3 mm allowance for grinding to remove the heat-affected zone from sectioning.

Compression mounting with standard phenolic or epoxy mounting compounds is appropriate. The high melting range (1350-1450°C) makes the alloy completely unaffected by mounting temperatures (150-180°C). For retrieved orthopedic implants or failure analysis specimens where edge retention at the bearing surface is critical, use an edge-retaining mounting compound (mineral-filled epoxy or diallyl phthalate). Cold mounting with castable epoxy is also acceptable and may be preferred for porous-coated implant surfaces to allow vacuum impregnation of the porous coating.

CoCrMo is hard and work-hardens, so use firm pressure with sharp abrasives. Diamond grinding discs are preferred over SiC paper for more consistent results and reduced work hardening. Disc speed: 250-300 RPM. Apply 25-30 N per sample.

Grinding sequence:

• 240 grit: Remove sectioning damage and establish a flat plane (30-60 seconds). Firm, steady pressure.
• 320 grit: Remove previous scratch pattern (30-45 seconds).
• 400 grit: Continue refinement (30-45 seconds).
• 600 grit: Fine grinding (20-40 seconds).
• 800 grit: Final grinding step (20-40 seconds). Ensure all 600-grit scratches are removed before proceeding to polishing.

Rotate specimen 90° between steps. Use complementary rotation (specimen counter to disc). Abundant water lubrication. Check under the microscope before moving to polishing to confirm all grinding scratches are unidirectional from the final step.

Use napless or low-nap pads throughout to control relief between the matrix and carbide phases.

Diamond polishing sequence:

• 9μm diamond: 3-5 minutes on a napless composite pad at 20-25 N per sample. This step removes residual grinding damage and begins to level the surface.
• 3μm diamond: 3-5 minutes on a napless synthetic pad at 20-25 N. Monitor for relief around carbide particles.
• 1μm diamond: 2-4 minutes on a napless pad at 15-20 N. Surface should appear nearly scratch-free at this stage.

Final polishing:

• 0.05μm colloidal silica: 2-4 minutes on a porous chemical-mechanical pad at 10-15 N. The slight chemical action of colloidal silica helps remove the final deformation layer from this work-hardening alloy. Alternatively, vibratory polishing with colloidal silica for 4-8 hours produces excellent results for publication-quality microstructures.

CoCrMo responds well to electrolytic and immersion etching. Electrolytic methods provide the most consistent results for revealing the carbide network and grain structure.

Electrolytic 10% HCl (Electrolytic Etching) - Primary choice:

• Composition: 10 ml HCl (concentrated), 90 ml water
• Application: Electrolytic at 3-6 V DC for 5-15 seconds. Stainless steel cathode. Sample is anode.
• Reveals: M23C6 and M7C3 carbides clearly outlined against the CoCr matrix. Grain boundaries visible. Dendritic structure in as-cast specimens.
• Rinse: Water, then ethanol. Dry with warm air.

Mixed Acid HCl-H2O2 (Chemical Etching) - Alternative immersion etch:

• Composition: 15 ml HCl, 10 ml H2O2 (30%), 100 ml water
• Application: Immerse for 10-30 seconds. Swab for more controlled etching.
• Reveals: Carbide distribution and general microstructure. Less selective than electrolytic method.

Glyceregia (Chemical Etching) - For grain boundaries:

• Composition: 15 ml HCl, 10 ml glycerol, 5 ml HNO₃. Mix fresh before use.
• Application: Immerse for 15-60 seconds. Monitor closely; etch rate accelerates.
• Reveals: Grain boundaries and carbide outlines. Good for grain size measurement.
• Note: Glyceregia is unstable; prepare only the amount needed and discard after use.

Safety: Electrolytic etching requires DC power supply. Use fume hood for all etchants. Standard PPE with acid-resistant gloves.