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

Per Don §11.7.1, thermal spray coatings should be sectioned with precision wafering using diamond or cubic boron nitride (CBN) blades, not abrasive cut-off wheels. Abrasive wheels generate enough heat and lateral force to delaminate the coating from the substrate at the interface. This is the single most damaging artifact for thermal spray analysis, since the coating- substrate bond is exactly what most QC examinations are trying to evaluate.

Precision diamond wafering blades for thermal spray coatings. CBN blades are an alternative when the substrate is a ferrous alloy and reaction between diamond and iron at the cut interface is a concern.

• Use a precision wafering saw with a diamond or CBN blade. Resin-bonded diamond is the workhorse; CBN avoids the diamond-iron reaction that dulls diamond blades quickly when sectioning ferrous substrates.
• Cut perpendicular to the coating surface to reveal the cross-section.
• Use slow, controlled feed; let the blade cut. Excessive force at the coating/substrate interface is the dominant cause of delamination at section time.
• Flood coolant continuously to prevent heat-induced delamination.
• For very thin or fragile coatings, vacuum-impregnate the sample in epoxy before sectioning, then cut through the mounted assembly.
• Inspect the cut surface immediately for delamination, edge chipping, or interface separation. If present, the rest of the prep cannot recover the as-deposited microstructure.

For more information on sectioning blades, visit our Diamond & CBN Cut-Off Blades collection.

Mounting

Per Don §11.7.1, use castable epoxies or acrylics only. Compression mounting is not appropriate; the combination of 150-180°C heat and 2000-4000 psi pressure causes interface delamination, especially for as-sprayed coatings where the coating-substrate bond is mechanical rather than metallurgical. Vacuum impregnation is strongly recommended for any porous coating (most thermal spray coatings have 1-15% porosity).

Vacuum Impregnation with Castable Epoxy (Standard)

1. Place the sample with the coating surface facing up in the mounting cup.
2. Place the cup in a vacuum impregnation chamber. Draw vacuum to 25-100 mbar (about 25-30 inHg).
3. Hold under vacuum for 5-10 minutes and watch for bubble evolution from the coating porosity. When bubbling slows, the pore network is degassed.
4. Introduce low-viscosity epoxy under vacuum so it flows into all open pores.
5. Slowly release vacuum to atmospheric. The pressure differential drives resin into the evacuated pores.
6. Cure at room temperature (typically 4-8 hours) or per resin specification.

Castable Mounting Without Vacuum (Acceptable for Dense Coatings)

For dense coatings (HVOF WC-Co, dense ceramic top coats) where porosity is <2%, vacuum impregnation may be unnecessary. Mix low-viscosity castable epoxy or acrylic per manufacturer instructions, pour over the sample with the coating facing the polished surface direction, and cure at room temperature.

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

Grinding

Don §11.7.1 specifies a diamond-on-metal-mesh plane-grinding step instead of SiC paper. CERMESH cloth is rigid enough to maintain flatness across the coating-substrate interface and traps diamond particles instead of releasing loose abrasive into porosity (where SiC paper would shed grit into pores and create false porosity readings). This step is the most important in the entire procedure; it must remove sectioning damage without pulling out poorly-bonded coating material.

Plane Grinding

1. 30 µm DIAMAT diamond on CERMESH metal mesh cloth: DIALUBE Extender, 5-10 lb, 100/100 rpm head/base, until plane.

Use moderate, steady force. Excessive pressure at this step is the leading cause of edge delamination and coating chipping at the cross-section edge. Inspect the coating-substrate interface frequently.

For more information, see our CERMESH and grinding supplies collection.

Polishing

Per Don §11.7.1, thermal spray polishing uses rigid composite discs (SIRIUS, ORION) for the diamond steps to maintain flatness across the coating-substrate hardness mismatch. Conventional woven cloths at coarse diamond sizes round the interface edge and produce relief between coating and substrate. The two materials have very different polishing rates. The composite discs hold diamond particles rigidly, preventing the rounding that woven cloths cause.

DIAMAT polycrystalline diamond suspension, required for the diamond polishing steps on SIRIUS and ORION composite discs.

ATLANTIS for the 1 µm DIAMAT step; TRICOTE for the 0.05 µm Nanometer alumina final polish.

Diamond Polishing on Rigid Composite Discs

1. 9 µm DIAMAT diamond on SIRIUS composite disk: 5-10 lb, 100/100 rpm, 3 minutes.
2. 3 µm DIAMAT diamond on ORION composite disk: 5-10 lb, 100/100 rpm, 3 minutes.
3. 1 µm DIAMAT diamond on ATLANTIS polishing pad: DIALUBE Purple Extender, 5-10 lb, 100/100 rpm, 2 minutes.

Final Polishing

1. 0.05 µm Nanometer alumina on TRICOTE polishing pad: 5-10 lb, 100/100 rpm, 1 minute.
2. Rinse thoroughly with water, then ethanol; dry with compressed air. Do not wipe.

Pad Map & Parameters Summary

Critical considerations: The rigid composite discs (SIRIUS, ORION) at the diamond steps are what eliminate edge rounding and interface relief. Substituting woven cloths at these steps (even firm low-nap cloths) produces noticeable relief between coating and substrate within the first minute. Hard coatings (WC-Co, Cr₃C₂) may require slightly extended diamond times (4-5 min instead of 3); soft coatings (NiAl bond coats, polymer-modified sprays) finish faster. Use ASTM E2109 image-analysis methods to measure porosity; this is the primary QC application for thermal spray cross-sections.

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

HVOF OEM Coating Systems

High-velocity oxy-fuel (HVOF) spray produces dense coatings with comparatively low porosity and minimal oxide content. Three OEM coating compositions in particular are PACE's most frequently submitted thermal-spray samples: nickel-aluminum (Ni-Al), nickel-aluminum-molybdenum (Ni-Al-Mo), and tungsten-cobalt (W-Co). All three use the same PACE Class 7 thermal-spray procedure documented above; the difference is in application context and the specific failure modes each coating presents.

Ni-Al HVOF

Nickel-aluminum HVOF bond coats and standalone wear coatings. Used as the bond layer under most ceramic thermal-barrier coating systems and as a corrosion-resistant overlay on combustion chamber and turbine components. Microstructural analysis target is porosity, flow (lamellar splat structure), thickness uniformity, and inclusion count. Standard Class 7 procedure: MAXCUT abrasive blade, EPOCOMP epoxy compression mount, SiC paper progression 240 grit (P280) through 1200 grit (P4000) at 200/200 RPM, 3 µm DIAMAT on GOLDPAD, 1 µm DIAMAT on ATLANTIS with SIAMAT colloidal silica, then 15-30 min vibratory final polish with 0.05 µm nanometer alumina on MICROPAD.

Ni-Al-Mo HVOF

Nickel-aluminum-molybdenum HVOF coatings, used where additional wear resistance or tribological performance is needed over Ni-Al alone. Common on shaft journals, pump sleeves, and high-temperature seal faces. Same Class 7 procedure as Ni-Al. The Mo-rich phases produce slightly higher hardness contrast with the Ni-Al matrix, but the standard procedure handles this within tolerable relief limits; the vibratory final step is what flattens it.

W-Co HVOF

Tungsten-cobalt HVOF coatings are the workhorse hard-wear coating used on landing-gear slipper bearings, hydraulic actuator rods, and other high-stress wear surfaces. Hardness is in the 1000-1300 HV range, much harder than Ni-Al systems. The standard Class 7 procedure applies. PACE's documentation flags that some W-Co coatings can be porous and brittle due to inadequate spray processing; the vibratory polish reveals such defects that mechanical polish alone would smear over.

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.

Additional Reading

• Zipperian, D.C. Metallographic Handbook, §11.7 (Class 7 Thermal Spray Materials) and §11.7.1 (Thermal Spray Coatings). PACE Technologies, Tucson, AZ. The house reference for the CERMESH/SIRIUS/ORION/ATLANTIS/TRICOTE sequence used in this guide.
• Weidmann, E., Guesnier, A., and Duclos, B. "Metallographic Preparation of Thermal Spray Coatings." Struers Application Notes. Alternative perspective and additional troubleshooting guidance.
• ASM Handbook, Vol. 9: Metallography and Microstructures. ASM International. Thermal spray coating preparation, microstructure interpretation, and porosity measurement.
• ASTM E1920 — Standard Guide for Metallographic Preparation of Thermal Sprayed Coatings.
• ASTM E2109 — Standard Test Methods for Determining Area Percentage Porosity in Thermal Sprayed Coatings (the primary QC application).
• ASTM C633 — Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings.
• ASTM E3 — Standard Guide for Preparation of Metallographic Specimens.
• ASTM E407 — Standard Practice for Microetching Metals and Alloys.