NiCr Thermal Spray Coating

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

Use abrasive cut-off wheel designed for nickel alloys (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 medium-hardness (300 HB) of NiCr 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 (20-30 N per 30 mm sample) - excessive pressure can cause coating delamination. Use sharp, fresh grinding papers to minimize deformation.

Grinding sequence:

• 240 grit: Remove sectioning damage (30-60 seconds). Use moderate pressure to remove heat-affected zone. Monitor for coating delamination.
• 320 grit: Remove previous scratches (30-60 seconds). Ensure complete scratch removal.
• 400 grit: Remove previous scratches (30-60 seconds). Ensure complete scratch removal.
• 600 grit: Remove previous scratches (30-60 seconds). Ensure complete scratch removal.
• 800 grit: Final grinding step (30-60 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 medium-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: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light to moderate pressure (20-30 N per 30 mm sample). Start with 6μm to minimize damage. Monitor for coating delamination.
• 3μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure (15-25 N). Continue removing scratches from previous step.
• 1μm diamond: 2-3 minutes on a soft synthetic pad with lighter pressure (15-20 N). These pads provide gentle material removal suitable for coatings.
• 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 medium-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, oxide inclusions, or excessive relief.

NiCr thermal spray coatings respond well to standard etchants for nickel alloys. The nickel-chromium matrix will reveal clearly with appropriate etchants.

Glyceregia (Chemical Etching) - Primary choice for nickel alloys:

• Composition: 10ml glycerol, 15ml HCl, 5ml HNO₃
• Preparation: Add acids to glycerol slowly with stirring. Prepare fresh for best results. The glycerol moderates the reaction rate.
• Application: Immerse sample or swab for 10-30 seconds. Classic general-purpose micro-etchant for nickel alloys. Provides good grain boundary contrast.
• Reveals: Grain boundaries, grain structure, porosity, and oxide inclusions 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 hours. Use in fume hood. The glycerol moderates the reaction - check progress frequently.

Etching Strategy:

• Start with Glyceregia 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, oxide inclusions, and coating thickness

Safety: All etchants are highly corrosive and produce toxic fumes – full PPE (gloves, safety glasses, lab coat), proper fume hood, and appropriate safety measures are mandatory.