YSZ Thermal Spray Coating

YSZ thermal spray coatings require careful preparation to preserve the coating-substrate interface and reveal microstructural features such as porosity, microcracks, and coating thickness. The coating is very hard and brittle, requiring careful handling throughout the preparation process.

Use precision diamond saw with continuous coolant flow. Diamond saws are essential for cutting hard, brittle YSZ ceramic coatings. 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 brittle material requires careful handling to avoid cracking and coating delamination. 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. Care must be taken to preserve the coating-substrate interface.

Cold mounting with epoxy resin is essential 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 and microcracks 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 YSZ Thermal Spray Coating requires careful grinding with diamond abrasives. Use diamond grinding papers or diamond lapping films with adequate water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (25-35 N per 30 mm sample) - the hard, brittle material may require longer grinding times, but excessive pressure can cause coating delamination or microcrack propagation. Use sharp, fresh diamond papers to minimize deformation.

Grinding sequence:

• 240 grit diamond: Remove sectioning damage (40-90 seconds). Use moderate pressure to remove any damage from cutting. Monitor for coating delamination.
• 320 grit diamond: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 400 grit diamond: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 600 grit diamond: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 800 grit diamond: 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 chipping or microcrack propagation.

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, microcrack propagation, 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 and microcrack propagation.
• 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, microcracks, or excessive relief.

YSZ thermal spray coatings typically do not require etching for general microstructural examination. The porosity and microcracks are usually visible in the as-polished condition. For specific analysis, thermal etching may be used:

Thermal Etching (Heat Treatment) - For grain structure revelation:

• Application: Heat sample in air or controlled atmosphere at 1400-1500°C for 1-2 hours. Use appropriate furnace with temperature control.
• Reveals: Grain boundaries and grain structure in the zirconia coating.
• Note: Thermal etching may affect the microstructure and should be used only when necessary. Ensure proper temperature control to avoid excessive grain growth.

Etching Strategy:

• Most YSZ coatings do not require etching for general examination - porosity and microcracks are visible as-polished
• Use thermal etching only when grain structure analysis is required
• Always clean and degrease before thermal etching
• Monitor temperature carefully to avoid excessive grain growth
• Pay attention to porosity, microcracks, and coating thickness

Safety: Thermal etching requires proper furnace operation and temperature control. Use appropriate safety measures for high-temperature operations.