Silicon Carbide (SiC)

Use diamond saw with continuous coolant flow. Diamond saws are essential for cutting hard ceramic materials. Standard diamond blade (0.3-0.5 mm thickness) is appropriate. Use adequate coolant flow to prevent overheating and minimize thermal shock. Cutting speed: 100-200 RPM for most diamond saws. Apply light to moderate pressure - the very hard material requires careful handling to avoid cracking. Avoid forcing the cut which can cause blade damage and sample cracking. Leave adequate allowance (~1-2 mm) for grinding away any damage from cutting.

Cold mounting with epoxy resin is preferred to avoid heat that could affect the microstructure. Use a low-shrinkage epoxy resin for best edge retention. Ensure complete cure before grinding to prevent edge rounding and maintain sample integrity. The very hard material requires careful handling during mounting to avoid cracking. For cutting tool and wear-resistant applications, ensure the mounting material provides adequate edge retention.

The very-hardness (2800 HV) of Silicon Carbide (SiC) requires careful grinding. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (30-40 N per 30 mm sample) - the hard material may require longer grinding times. Use sharp, fresh grinding papers to minimize deformation.

Grinding sequence:

• 120 grit: Remove sectioning damage (40-90 seconds). Use moderate pressure to remove heat-affected zone.
• 240 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 320 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 400 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 600 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.

Always rotate the specimen holder 90° between steps to ensure complete scratch removal. Use complementary rotation (platen and holder same direction, different speeds) rather than contra-rotation to minimize deformation. Adequate water lubrication is critical - avoid drying during grinding which can cause smearing.

The very-hardness requires careful polishing. Use diamond polishing with appropriate polishing pads for each stage. Apply light to moderate pressure throughout to prevent deformation.

Diamond polishing sequence:

• 6μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light to moderate pressure (30-40 N per 30 mm sample). Start with 6μm to minimize damage. The hard material may require longer polishing times.
• 3μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure (25-35 N). Continue removing scratches from previous step.
• 1μm diamond: 2-3 minutes on a medium-hard synthetic pad with lighter pressure (25-35 N). These pads provide gentle material removal.
• 0.05μm colloidal silica: 1-2 minutes on a high-napped final polishing pad (e.g., MICROPAD) with very light pressure. This removes any remaining fine scratches and prepares the surface for etching. Monitor for relief - reduce polishing time if excessive relief develops.

Use appropriate polishing lubricants. The very-hard material means polishing times should be sufficient but not excessive - avoid over-polishing which can cause relief and affect grain boundary revelation. Monitor the surface frequently under the microscope to check for smearing or excessive relief.

Silicon carbide is extremely chemically resistant. Most acids and alkalis have no effect at room temperature. Thermal etching and molten salt etching are the primary methods. SiC will oxidize in air at high temperature, so inert atmosphere is mandatory for thermal etching. Thermal Etching - Primary method for grain boundary revelation:

• Conditions: 1800–1900°C for 30–60 min in inert atmosphere (Ar or N₂). Do NOT use air — SiC oxidizes above ~1200°C.
• Reveals: Grain boundaries by thermal grooving. Polytypes (α-SiC hexagonal, β-SiC cubic) may show different contrast.
• Note: Requires high-temperature furnace with atmosphere control. Ensure sample is free of polishing debris.

Molten Na/K Bicarbonate - Standard molten salt etchant for SiC:

• Composition: Sodium bicarbonate (NaHCO₃) or potassium bicarbonate (KHCO₃) melt.
• Application: Melt salt in a platinum crucible, immerse polished sample for approximately 10 min.
• Reveals: Grain boundaries and polytypes. Preferentially attacks grain boundaries and reveals SiC grain structure.
• Rinse: Cool, then dissolve residual salt with warm water. Rinse with ethanol, dry with compressed air.
• Note: Platinum crucible required — molten carbonates attack most other crucible materials.

Etching Strategy:

• Examine as-polished first — porosity, secondary phases, and free carbon/Si are visible without etching
• Thermal etching produces the cleanest grain boundary delineation for grain size measurement
• Molten bicarbonate is the standard chemical alternative when atmosphere-controlled furnaces are unavailable
• Polarized light microscopy is useful for revealing SiC polytypes without etching

Safety: Thermal etching at 1800°C+ requires strict high-temperature furnace safety. Molten salts cause severe burns — use appropriate tongs, face shield, and heat-resistant gloves. Platinum crucible required for molten salt methods.