NiAl Bond Coat

NiAl bond coats (250-350 HV) are medium-hardness metallic coatings applied by plasma spray, HVOF, or EB-PVD as part of thermal barrier coating (TBC) systems for gas turbines. The full TBC system presents a unique preparation challenge: three materials of vastly different hardness (superalloy substrate ~350-450 HV, metallic bond coat ~250-350 HV, YSZ ceramic top coat ~1200 HV) must be prepared flat and without relief at the interfaces. The thermally grown oxide (TGO) at the bond coat/top coat interface is typically 1-10 μm thick and is the most critical feature to preserve.

Use a precision low-speed diamond wafering saw with continuous coolant. Cut perpendicular to the coated surface. The brittle YSZ ceramic top coat (if present) is prone to cracking and delamination during cutting; use very low feed rates and a thin diamond blade (0.3-0.5 mm). Cutting speed: 100-200 RPM. If the top coat delaminates during cutting, the section is unusable for interface analysis. Some labs embed the sample in epoxy before cutting to support the top coat. Leave 2-3 mm allowance for grinding.

Cold mounting with edge-retaining epoxy is essential. Vacuum impregnation is critical to fill porosity in all three layers and to support the brittle YSZ top coat during grinding. Without vacuum impregnation, the porous YSZ will experience grain pullout, and gaps at interfaces will fill with grinding debris creating false porosity readings. Orient the sample with the coating surface facing down. Use a hard epoxy formulation with mineral filler for maximum edge support. Allow full cure (8-12 hours minimum) before grinding.

The three-layer system requires careful grinding to maintain planarity across all interfaces. Diamond grinding is preferred for the hard YSZ layer.

Grinding sequence:

• 40 μm diamond disc: Remove sectioning damage (30-60 seconds). Moderate pressure (20-30 N). Monitor for YSZ pullout or cracking.
• 15 μm diamond disc: Intermediate grinding (30-60 seconds). Reduce pressure (15-25 N). Check all interfaces for planarity.
• 9 μm diamond disc: Fine grinding (30-60 seconds). Light pressure (15-20 N).

Disc speed: 200-300 RPM. The YSZ grinds slower than the metallic layers, so the YSZ may stand slightly proud. Use complementary rotation and consistent pressure. Thorough ultrasonic cleaning between each step. SiC papers can be used for the initial planarization if YSZ is not present, but diamond discs are preferred for the full TBC system.

Diamond polishing is essential for the full TBC system. The hardness difference between YSZ (~1200 HV) and bond coat (~300 HV) creates relief risk.

Diamond polishing sequence:

• 6 μm diamond: 3-5 minutes on a hard napless synthetic pad with moderate pressure (20-25 N). Monitor bond coat/YSZ interface for relief.
• 3 μm diamond: 3-5 minutes on a medium-hard synthetic pad (15-20 N). The TGO layer should begin to be visible at this stage.
• 1 μm diamond: 2-4 minutes on a synthetic pad (12-18 N). Continue monitoring all interfaces.

Final polishing:

• 0.05 μm colloidal silica: 1-2 minutes on a soft pad with light pressure. Keep time minimal to prevent relief buildup. The TGO layer at the bond coat/YSZ interface should be clearly visible as a thin, bright oxide band.

Vibratory polishing with colloidal silica (1-2 hours) can produce excellent results for TBC cross-sections by removing deformation uniformly.

The as-polished condition is often sufficient for TBC system analysis, especially for TGO thickness measurement, porosity assessment, and interface quality evaluation. Etching is used for phase identification within individual layers.

Waterless Kalling's Reagent (Chemical Etching) - For NiAl bond coat:

• Composition: 5 g CuCl2, 100 ml HCl, 100 ml ethanol
• Application: Swab for 5-15 seconds on bond coat area only.
• Reveals: Splat boundaries, intermetallic phases, interdiffusion zone in the bond coat.
• Rinse: Ethanol, dry with warm air.

Glyceregia (Chemical Etching) - Alternative for Ni-based bond coat:

• Composition: 15 ml HCl, 10 ml glycerol, 5 ml HNO3
• Application: Swab for 10-30 seconds. Use fresh solution only (degrades within minutes of mixing).
• Reveals: Grain boundaries and phase distribution in the bond coat.

For the full TBC system: Combine selective etching. Murakami's reagent can be used on the YSZ layer to reveal grain structure. The superalloy substrate can be etched with its appropriate etchant (e.g., Marble's for Ni-base superalloys). Apply each etchant selectively with a cotton swab to avoid damaging other layers.