Rhenium

Rhenium is an extremely hard (1350 HV) refractory metal with HCP crystal structure. The extreme hardness makes it resistant to grinding with SiC papers; diamond grinding is required throughout. Standard SiC abrasives wear out rapidly and are ineffective. The material is brittle at room temperature, so excessive pressure during cutting and grinding can cause fracture or chipping. Specimens are typically small due to the rarity and cost of the material.

Use a precision diamond wafering saw with a thin diamond blade (metal-bonded or resin-bonded diamond) and continuous coolant. The extreme hardness (1350 HV) makes cutting very slow; allow adequate time and avoid forcing the cut. Cutting speed: 100-200 RPM with very low feed rate. Standard abrasive cut-off wheels are ineffective and will wear rapidly. For very small specimens, electric discharge machining (EDM) or diamond wire cutting are alternatives. The material is brittle; minimize vibration and avoid impact loading during clamping. Leave 1-2 mm allowance for grinding.

Cold mounting with castable epoxy is standard. Hot compression mounting is also acceptable, as the extreme melting point (3186 degrees C) makes temperature effects irrelevant. Use a mineral-filled epoxy for cold mounts to provide a hard support base during the aggressive diamond grinding required. For small or irregularly shaped rhenium specimens (common due to high cost), ensure proper sample orientation before mounting. Edge retention is not typically a concern due to the extreme hardness.

Diamond grinding is mandatory. SiC papers are ineffective against this 1350 HV material and wear out immediately. Use rigid diamond grinding discs (metal-bonded or resin-bonded diamond). Apply moderate to firm pressure (30-50 N per 30 mm sample); the extreme hardness tolerates higher forces. Disc speed: 200-300 RPM. The material removes slowly; patience is required at each step.

Grinding sequence:

• 70 micrometer diamond disc: Remove sectioning damage (60-120 seconds). Firm pressure. Monitor for edge chipping due to brittleness.
• 40 micrometer diamond disc: Remove previous damage (45-90 seconds). Moderate pressure.
• 15 micrometer diamond disc: Refinement (30-60 seconds).
• 9 micrometer diamond disc: Final grinding step (30-60 seconds). Ensure all previous scratches are removed.

Use water-based diamond lubricant. Rotate specimen 90 degrees between steps. Clean thoroughly between steps to prevent carryover of coarse diamond particles.

Diamond polishing on hard, napless pads. The extreme hardness means polishing times are longer than typical metals.

Diamond polishing sequence:

• 6 micrometer diamond: 3-5 minutes on a hard napless synthetic pad with moderate pressure (25-35 N per 30 mm sample). The hardness means relief is not a concern; focus on scratch removal.
• 3 micrometer diamond: 3-5 minutes on a hard napless pad (25-30 N).
• 1 micrometer diamond: 2-4 minutes on a napless pad (20-30 N).

Final polishing:

• 0.05 micrometer colloidal silica: 2-5 minutes on a medium-soft pad with moderate pressure. Colloidal silica provides slight chemical-mechanical action. Alternatively, 0.05 micrometer alumina suspension can be used. Vibratory polishing for 4-8 hours with colloidal silica produces the best possible surface finish on this difficult material.

Rhenium is chemically resistant to most single acids. Specialized etchants or electrolytic techniques are required.

Murakami's Reagent (Chemical Etching) - Primary choice:

• Composition: 10 g K3Fe(CN)6, 10 g KOH, 100 ml water. Mix fresh before use.
• Application: Immerse for 15-60 seconds at room temperature, or warm to 50-70 degrees C for faster action.
• Reveals: Grain boundaries and grain structure. Some crystallographic orientation contrast visible.
• Rinse: Water, then ethanol. Dry with warm air.

Electrolytic Etching with NaOH (Electrolytic Etching) - For superior grain boundary delineation:

• Composition: 10% NaOH in water (electrolyte)
• Application: 3-6 V DC, 5-30 seconds. Rhenium as anode, stainless steel cathode. Adjust voltage and time to avoid pitting.
• Reveals: Grain boundaries with excellent clarity. Preferred for grain size measurement and texture analysis. Polarized light can be used after etching to reveal grain orientation in this HCP metal.

Safety: Murakami's reagent contains potassium ferricyanide; while not as toxic as free cyanide, avoid contact with strong acids which could release HCN. Work in a fume hood. Standard PPE required.