WC-Co Thermal Spray Coating

WC-Co thermal spray coatings require careful preparation to preserve the coating-substrate interface and reveal microstructural features such as porosity, flow, and coating thickness. The coating may be porous and brittle, requiring gentle handling throughout the preparation process.

Use abrasive cut-off wheel designed for hard materials (Al₂O₃ or SiC abrasive). Standard cut-off wheel (1.0-1.5 mm thickness) is appropriate. Use adequate coolant flow to prevent overheating - excessive heat can affect the coating microstructure and cause delamination. Cutting speed: 200-300 RPM for most cut-off saws. Apply steady, moderate pressure - avoid forcing the cut which can cause coating delamination. Leave adequate allowance (~2-3 mm) for grinding away the heat-affected zone from cutting. Care must be taken to preserve the coating-substrate interface.

Cold mounting with epoxy resin is preferred to avoid heat that could affect the coating microstructure or cause delamination. Use a low-shrinkage epoxy resin for best edge retention. Vacuum impregnation may be necessary to fill porosity in the coating. Ensure complete cure before grinding to prevent edge rounding and maintain sample integrity. The coating-substrate interface requires careful handling during mounting to avoid separation.

The hardness (1300 HV) of WC-Co Thermal Spray Coating requires careful grinding. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (25-35 N per 30 mm sample) - the hard material may require longer grinding times, but excessive pressure can cause coating delamination. Use sharp, fresh grinding papers to minimize deformation.

Grinding sequence:

• 240 grit: Remove sectioning damage (40-90 seconds). Use moderate pressure to remove heat-affected zone. Monitor for coating delamination.
• 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.
• 800 grit: Final grinding step (40-90 seconds). Prepare surface for polishing.

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 hardness requires careful polishing with diamond abrasives. Use diamond polishing with appropriate polishing pads for each stage. Apply light to moderate pressure throughout to prevent coating delamination and deformation.

Diamond polishing sequence:

• 6μm diamond: 3-5 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light to moderate pressure (25-35 N per 30 mm sample). Start with 6μm to minimize damage. The hard material may require longer polishing times. Monitor for coating delamination.
• 3μm diamond: 3-5 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure (20-30 N). Continue removing scratches from previous step.
• 1μm diamond: 2-4 minutes on a medium-hard synthetic pad with lighter pressure (20-30 N). These pads provide gentle material removal.
• 0.05μm colloidal silica: 1-2 minutes on a soft final polishing pad (e.g., MICROPAD) with very light pressure. This removes any remaining fine scratches and prepares the surface for examination. Monitor for relief between coating and substrate - reduce polishing time if excessive relief develops.

Use appropriate polishing lubricants. The hard material means polishing times should be sufficient but not excessive - avoid over-polishing which can cause relief and affect the coating-substrate interface. Monitor the surface frequently under the microscope to check for delamination, porosity, or excessive relief.

WC-Co thermal spray coatings respond well to standard etchants for cemented carbides. The WC particles in cobalt matrix will reveal clearly with appropriate etchants.

Murakami's Reagent (Chemical Etching) - Primary choice for WC-Co materials:

• Composition: 10g K₃Fe(CN)₆, 10g KOH, 100ml H₂O
• Preparation: Dissolve K₃Fe(CN)₆ and KOH in water. Solution is stable for several weeks if stored properly.
• Application: Immerse sample or swab for 10-30 seconds. Standard etchant for WC-Co materials.
• Reveals: WC particles, cobalt matrix, and porosity clearly. Excellent for general microstructure examination.
• Rinse: Immediately with water, then ethanol. Dry with compressed air or warm air to avoid staining.
• Note: Prepare fresh when needed. Shelf life: several weeks if stored properly. Use in fume hood.

Etching Strategy:

• Start with Murakami's reagent for general microstructure examination
• Always clean and degrease before etching
• Use short initial etch times (a few seconds), check under the microscope, repeat if needed
• Check etching progress frequently - over-etching can obscure fine details
• Pay attention to porosity and coating thickness

Safety: All etchants require proper PPE (gloves, safety glasses, lab coat), proper fume hood, and appropriate safety measures. Handle with care.