Thermal Spray Coatings Sample Preparation

Introduction

Thermal spray coatings are important in aerospace, power generation, and industrial applications for wear resistance, thermal barrier, and corrosion protection. These coatings include WC-Co and chromium carbide cermet coatings, alumina and YSZ ceramic coatings, and various alloy coatings. Proper preparation is essential to reveal the true coating microstructure, interface with substrate, and any defects without introducing artifacts. Thermal spray coatings are particularly challenging due to their layered structure, varying hardness, and potential for delamination.

Common thermal spray coatings include WC-Co (tungsten carbide-cobalt), chromium carbide (Cr₃C₂), alumina (Al₂O₃), yttria-stabilized zirconia (YSZ), and various NiCr, NiAl, and Fe-based alloy coatings. These coatings can vary significantly in hardness (200-1500 HV) and thickness (50-500 μm). The key to successful preparation is preserving the coating-substrate interface, avoiding delamination, and revealing the coating structure while managing the different polishing rates between coating and substrate that can cause relief.

Sectioning

When sectioning thermal spray coatings, use a slow cutting speed to minimize heat generation and prevent delamination. Cut perpendicular to the coating surface to reveal the cross-section and coating-substrate interface. These coatings can be prone to delamination, especially at the interface, requiring careful handling and adequate cooling.

MAX-D or MAX-VHS series abrasive cut-off blades depending on coating hardness. For hard coatings (WC-Co, Cr₃C₂), use MAX-VHS. For softer coatings, use MAX-D. Cut slowly and use adequate coolant to preserve the coating-substrate interface.

• Use MAX-D series blades for softer coatings, or MAX-VHS series for hard coatings (WC-Co, Cr₃C₂)
• Cut perpendicular to the coating surface to reveal the cross-section
• Apply steady, moderate pressure - avoid sudden changes that could cause delamination
• Use adequate coolant continuously to prevent overheating and delamination
• Cut slowly to preserve the coating-substrate interface
• Clean sample immediately after sectioning to remove cutting fluid and debris
• Inspect the cut surface for any signs of delamination or interface damage

For more information on sectioning blades, visit our Abrasive Blades collection.

Mounting

Mounting is critical for thermal spray coatings to protect the coating edge and preserve the coating-substrate interface. Vacuum impregnation with epoxy is highly recommended to fill any porosity in the coating and prevent pullout during preparation. Compression mounting can also be used, but care must be taken with temperature to avoid affecting the coating structure.

Vacuum Impregnation (Strongly Recommended)

1. Place sample with coating facing up in vacuum chamber
2. Apply vacuum (typically 25-30 inHg) to remove air from coating porosity - hold for 5-10 minutes
3. Introduce low-viscosity epoxy under vacuum to ensure it flows into all pores
4. Release vacuum slowly to allow atmospheric pressure to force epoxy into pores
5. Allow to cure at room temperature (typically 4-8 hours) or according to resin specifications

This ensures coating integrity is preserved and prevents pullout during grinding and polishing.

Compression Mounting (Alternative)

1. Clean the sample thoroughly to remove cutting fluid and debris
2. Place sample in mounting press with low-viscosity epoxy resin
3. Apply pressure: 2000-3000 psi
4. Heat to 120-150°C (lower than standard mounting temperatures) and hold for 5-8 minutes
5. Cool under pressure to room temperature

Important: Avoid high temperatures that could affect the coating structure or cause delamination. Vacuum impregnation is strongly recommended as it ensures complete pore filling without the heat of compression mounting. The coating-substrate interface must be preserved during mounting.

For more information on mounting equipment, visit our Compression Mounting Equipment page.

Grinding

Grinding removes sectioning damage and prepares the surface for polishing. Start with fine grits to avoid damage to the coating. For thermal spray coatings, we recommend starting with 240 or 320 grit rather than coarse grits. Maintain consistent pressure and grind parallel to the coating surface to avoid delamination. The coating-substrate interface is particularly vulnerable during grinding, so careful handling is essential.

Silicon carbide (SiC) grinding papers in various grit sizes for progressive grinding. For thermal spray coatings, start with 240 or 320 grit to minimize damage to the coating. Grind parallel to the coating surface to avoid delamination.

Grinding Sequence

1. 240 grit: Remove sectioning damage (30-60 seconds per step) - start here for most thermal spray coatings
2. 400 grit: Remove previous scratches (30-60 seconds)
3. 600 grit: Final grinding step (30-60 seconds)

Important: Grind parallel to the coating surface (not perpendicular) to avoid delamination. Rotate the sample 90° between each grit, but maintain the grinding direction parallel to the coating. Use water as a lubricant and maintain consistent, moderate pressure. Avoid excessive pressure that could cause delamination, especially at the coating-substrate interface. Clean the sample thoroughly between steps to prevent contamination.

Note: For very hard coatings (WC-Co, Cr₃C₂), you may need to extend grinding times slightly. Monitor the coating-substrate interface carefully for any signs of delamination or damage.

For more information on grinding supplies, visit our Silicon Carbide Grinding Papers collection.

Polishing

Polishing removes grinding scratches and prepares a surface suitable for analysis. For thermal spray coatings, diamond polishing is essential, especially for hard coatings. Use appropriate pads and maintain consistent pressure to avoid relief between coating and substrate. The different polishing rates of coating and substrate can cause relief, which must be minimized through careful pressure control and appropriate pad selection.

Polycrystalline diamond compound is essential for thermal spray coatings, especially hard coatings like WC-Co. Use with appropriate pads and consistent pressure to minimize relief between coating and substrate.

Various polishing pads for different polishing stages. Select pad hardness based on coating type and polishing stage. Consistent pressure is critical to avoid relief between coating and substrate.

Diamond Polishing

1. 9 μm diamond: 3-5 minutes on a medium-hard pad (e.g., Texpan)
2. 3 μm diamond: 3-5 minutes on a medium-soft pad
3. 1 μm diamond: 2-3 minutes on a soft pad

Final Polishing

1. 0.05 μm colloidal silica: 1-2 minutes on a soft pad
2. Rinse thoroughly with water and dry with compressed air

Critical Considerations: Use consistent pressure throughout all polishing steps to minimize relief between coating and substrate. Different polishing rates can cause the coating or substrate to polish faster, creating relief. Monitor the interface carefully and adjust pressure or pad hardness if relief becomes excessive. Hard coatings (WC-Co, Cr₃C₂) may require longer polishing times. Clean the sample thoroughly between steps to prevent contamination. Avoid excessive polishing that could cause delamination at the interface.

Tip: If significant relief develops between coating and substrate, try using a slightly harder pad for the substrate area or adjust polishing times. The goal is to achieve a flat surface with minimal relief at the interface.

For more information on polishing supplies, visit our Diamond Abrasives and Polishing Pads collections.

Etching

Etching reveals the coating microstructure, interface, and any defects. Thermal spray coatings typically use etchants appropriate for their composition. The choice of etchant depends on the coating material and what features you want to reveal. Some coatings, particularly ceramic coatings, may not require etching as their structure is visible in the as-polished condition.

Common Etchants for Thermal Spray Coatings

• Murakami's Reagent: For WC-Co coatings. Mix 10g K₃Fe(CN)₆, 10g KOH, 100ml H₂O. Etching time: 10-30 seconds. Most commonly used for tungsten carbide-cobalt coatings.
• 10% Nital: For alloy coatings (NiCr, NiAl, Fe-based). Mix 10ml concentrated HNO₃ with 100ml ethanol. Etching time: 5-20 seconds. Effective for revealing grain boundaries in alloy coatings.
• Vilella's Reagent: For some alloy coatings. Mix 1g picric acid, 5ml concentrated HCl, 100ml ethanol. Etching time: 5-20 seconds. Useful for high-alloy coatings.
• Glyceregia: For stainless steel-based coatings. Mix 15ml concentrated HCl, 10ml glycerol, 5ml concentrated HNO₃. Etching time: 10-30 seconds.
• No Etching: Ceramic coatings (Al₂O₃, YSZ) typically do not require etching as their structure is visible in the as-polished condition.

Etching solutions and reagents for thermal spray coatings. Common etchants include Murakami's reagent (WC-Co), nital (alloy coatings), and Vilella's reagent (high-alloy coatings). Ceramic coatings typically do not require etching. Etching time typically ranges from 5-30 seconds. Warning: Many coating etchants are hazardous and require proper safety equipment.

Etching Procedure

1. Ensure sample is clean and dry
2. Apply etchant with cotton swab or immerse sample (depending on etchant)
3. Etch for 5-30 seconds (time varies by etchant and coating type)
4. Immediately rinse with water, then alcohol
5. Dry with compressed air

Tip: Start with shorter etching times (5-10 seconds) and increase if needed. Some coatings may not etch well due to their structure or composition. For WC-Co coatings, Murakami's reagent is the most commonly used etchant and typically provides good results. Ceramic coatings (Al₂O₃, YSZ) usually do not require etching as their microstructure is visible in the as-polished condition.

For more information on etchants, visit our Etchants collection.

Troubleshooting

Common Issues and Solutions

• Coating delamination: Excessive pressure or heat during sectioning/mounting. Use lower pressure, very slow cutting speeds, and lower mounting temperatures (120-150°C). Ensure adequate coolant during sectioning. Vacuum impregnation is preferred over compression mounting to avoid heat-related delamination.
• Relief between coating and substrate: Different polishing rates between coating and substrate. Use consistent pressure throughout polishing and select appropriate pads. If relief is excessive, try using a slightly harder pad or adjust polishing times. Monitor the interface carefully during polishing.
• Coating pullout: Insufficient mounting or porosity not properly filled. Ensure proper vacuum impregnation with adequate vacuum time (5-10 minutes). Use low-viscosity epoxy resins designed for vacuum impregnation. If pullout occurs, the mounting may need to be redone.
• Scratches remaining: Insufficient polishing time, especially for hard coatings. Increase polishing time, particularly for hard coatings like WC-Co which may require 50-100% longer times. Ensure complete scratch removal at each step before proceeding.
• Interface not visible: Use appropriate etching for the coating type, or adjust polishing to reveal interface. Some coatings may require specific etchants to reveal the interface. If the interface is obscured by relief, adjust polishing technique to minimize relief.
• Coating damage during grinding: Too aggressive grinding or wrong grinding direction. Start with finer grits (240 or 320) and grind parallel to the coating surface, not perpendicular. Avoid excessive pressure that could cause delamination.
• Contamination: Clean between steps thoroughly. Use fresh abrasives and separate polishing stations if possible. Contamination can obscure the coating structure and interface.
• Inconsistent etching: Ensure sample is clean and dry before etching. Surface contamination can cause uneven etching. Some coatings may not etch well due to their structure - ceramic coatings typically do not require etching.