Coatings & Surface Treatments
Class 7 covers all coating and surface treatment specimens prepared in cross-section, where the coating-substrate interface is the primary region of interest. The class spans thermal spray coatings (WC-Co, NiCr, YSZ, Cr₃C₂-NiCr, NiAl bond coats), electroplated layers (hard chrome), conversion coatings (anodized aluminum), and hot-dip coatings (galvanized zinc). The defining preparation challenge is the extreme hardness mismatch between coating and substrate: a WC-Co coating at 1300 HV bonded to a steel substrate at 200 HV grinds at completely different rates, creating edge rounding and relief at the very interface you need to evaluate. Porosity preservation, delamination prevention, and edge retention dominate every step from sectioning through final polish.
Overview
Class 7 specimens are almost always prepared in cross-section, and every preparation step must preserve the coating-substrate interface, coating porosity, and edge geometry. The extreme hardness differences within a single specimen create unique challenges not found in bulk material preparation.
Preparation Challenges
Seven properties drive the prep procedure. Tap a card for full detail.
Coating-Substrate Hardness Mismatch 1300 HV coatings on 200 HV substrates grind at completely different rates.
A WC-Co coating (1300 HV) bonded to a mild steel substrate (200 HV) grinds at completely different rates, causing the softer substrate to undercut the interface. This creates edge rounding at the exact location that needs to be evaluated. Napless polishing cloths and edge-retention mounting compounds are essential to maintain a flat interface.
Porosity Preservation Pores collapse or smear without vacuum-impregnation epoxy support.
Thermal spray coatings contain 1-15% porosity that must remain open and unsmeared for accurate porosity measurement by image analysis. Without vacuum impregnation during mounting, pores collapse or fill with grinding debris, and soft coating material smears across pore openings. Ultrasonic cleaning between each preparation step helps keep pores clear.
Coating Delamination Mechanical-bonded thermal spray shears loose under lateral grinding forces.
Thermal spray coatings bond mechanically (not metallurgically) to the substrate, so lateral forces during grinding can shear the coating loose. Electroplated and hot-dip coatings have stronger bonds but can still separate at the interface if loads are too high. Vacuum impregnation with epoxy fills gaps at the interface and stabilizes the coating before any grinding begins.
Hard Phase Pull-Out WC and Cr₃C₂ particles rip from the binder on napped cloths.
WC particles in WC-Co coatings and Cr₃C₂ particles in cermet coatings can pull out during polishing, leaving voids that inflate porosity measurements and misrepresent coating quality. Napless cloths with low applied pressure (15-20 N) retain hard particles best. Napped cloths catch particles and rip them from the binder.
Cobalt Oxidation Sensitivity Water staining on WC-Co binder; use oil or alcohol suspensions throughout.
The cobalt binder in WC-Co coatings oxidizes rapidly when exposed to water-based lubricants, producing dark staining that obscures the true microstructure and makes porosity measurement unreliable. Oil-based or alcohol-based diamond suspensions must be used for all grinding and polishing steps on cobalt-containing coatings.
Splat Boundary & Oxide Preservation Thin inter-splat oxide lamellae vanish under aggressive polishing.
Thermal spray coatings are built from individual splats (flattened droplets), with thin oxide layers between them. These inter-splat oxides define the coating's structural character and affect its mechanical properties. Aggressive polishing removes these thin oxide lamellae, misrepresenting coating quality and making splat structure invisible.
Edge Retention at Coating Surface Both critical edges sit at the specimen perimeter and need filled-resin support.
The outer coating surface and the coating-substrate interface are the two most critical evaluation areas, and both sit at the edge of the specimen. Standard mounting without edge support rounds these edges during grinding. Castable mounting with edge-retention fillers (alumina or glass beads mixed into the resin) provides support on both sides of the coating cross-section.
Class 7 Materials
Eight coating types across two families. All evaluated in cross-section.
Thermal Spray
- NiCr Thermal Spray Coating
- WC-Co Thermal Spray Coating
- YSZ Thermal Spray Coating
- Cr3C2-NiCr Thermal Spray Coating
- NiAl Bond Coat
Surface Coatings
- Hot-Dip Galvanized Coating
- Hard Chrome Plating
- Anodized Aluminum Coating
Recommended Procedure
Five-stage workflow optimized for cross-sectional coating analysis.
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1
Sectioning
Cross-section perpendicular to the coating; precision diamond wafering with a backing block prevents delamination at the cut edge.
More detail
All coated specimens must be cross-sectioned perpendicular to the coating surface. Use precision wafering saws with diamond blades for best edge retention on thin coatings (anodized layers, galvanized). Clamp the coated surface against a backing material (steel plate or sacrificial block) during cutting to prevent delamination and edge chipping. Abrasive blades work for thicker thermal spray coatings but leave more subsurface damage. For WC-Co coatings, use diamond wafering blades exclusively because the carbide phase rapidly destroys abrasive wheels.
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2
Mounting
Vacuum-impregnate porous coatings with low-viscosity epoxy; use edge-retention filler for dense coatings; never compression-mount.
More detail
Vacuum impregnation with low-viscosity epoxy is required for all porous coatings (thermal spray). The epoxy infiltrates pores and stabilizes the coating structure, preventing smearing and collapse during grinding. For dense coatings (hard chrome, anodized aluminum), castable mounting with edge-retention filler (alumina or glass beads mixed into the resin) provides the edge support needed for accurate thickness and interface measurements. Never use compression mounting for coating specimens; the heat (150-180°C) and pressure can damage, separate, or crack coatings.
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3
Grinding
Per Don §11.7.1: 30 µm DIAMAT diamond on CERMESH metal mesh cloth. DIALUBE Extender, 5-10 lb, 100/100 rpm, until plane.
More detail
Don §11.7.1 replaces conventional SiC paper grinding with 30 µm DIAMAT diamond on CERMESH metal mesh cloth for the plane-grinding step (DIALUBE Extender, 5-10 lb, 100/100 rpm, until plane). CERMESH holds diamond particles rigidly and prevents loose abrasive from shedding into coating porosity (which SiC paper does, creating false porosity readings). The rigid mesh also maintains flatness across the coating-substrate interface during this critical first material-removal step. For WC-Co and other cobalt-containing coatings, oil-based lubricants can substitute for water-based to prevent cobalt oxidation staining if the analysis requires it.
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4
Polishing
9 µm DIAMAT on SIRIUS → 3 µm DIAMAT on ORION → 1 µm DIAMAT on ATLANTIS → 0.05 µm Nanometer alumina on TRICOTE. Rigid composite discs eliminate coating-substrate edge relief.
More detail
Don §11.7.1 polishing sequence: 9 µm DIAMAT on SIRIUS composite disc (5-10 lb, 100/100 rpm, 3 min) → 3 µm DIAMAT on ORION composite disc (5-10 lb, 100/100 rpm, 3 min) → 1 µm DIAMAT on ATLANTIS polishing pad (DIALUBE Purple Extender, 5-10 lb, 100/100 rpm, 2 min) → 0.05 µm Nanometer alumina on TRICOTE polishing pad (5-10 lb, 100/100 rpm, 1 min). The rigid composite discs (SIRIUS, ORION) at the diamond steps are what eliminate the coating-substrate edge relief that woven cloths produce. Substituting softer cloths at these steps reintroduces relief within minutes. Ultrasonic cleaning between each step is critical to remove abrasive trapped in pores.
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5
Etching
Most coatings examined as-polished; etch only the substrate, or use light electrolytic CrO₃ for splat structure when needed.
More detail
Most coating evaluations are performed as-polished (unetched) to measure porosity, coating thickness, oxide content, and interface quality. When the substrate microstructure must be revealed, etch only the substrate region: nital (2-5%) for steel substrates, Keller's reagent for aluminum substrates. For thermal spray coatings where splat structure needs emphasis, light electrolytic etching with 2% CrO₃ at 2-3V DC for 5-10 seconds reveals splat boundaries without damaging the coating. Anodized layers and galvanized coatings are typically examined entirely as-polished, often with polarized light to enhance layer contrast.
Common etchants by coating type
- WC-Co thermal spray
- Murakami's (carbides); 50/50 Marble's + Murakami's blend
- NiCr / Cr₃C₂-NiCr coatings
- Glyceregia; Murakami's for carbide phase ID
- YSZ thermal barrier
- No chemical etch; polished surface only; porosity by image analysis
- Hard chrome plating
- Boiling 50% HCl (cracks & column structure); alkali ferricyanide
- Anodized aluminum
- Usually as-polished; phosphochromic for porous structure if needed
- Hot-dip galvanized
- 4% picral for Fe-Zn intermetallics; CrO₃ + Na₂SO₄ for zinc layer
- Substrate (typically steel)
- 2% nital after coating evaluation is complete
Steel substrate etchant guide → Aluminum etchant guide → Zinc etchant guide → Learn about etchants → Shop etchants →
Quality Checks
- Coating-substrate interface intact with no delamination or separation
- Porosity appears open and unsmeared, suitable for image analysis
- No cobalt staining or oxidation artifacts in WC-Co coatings
- Hard phases (WC, Cr₃C₂) retained in place with no pull-out voids
- Coating edges sharp and well-defined with no rounding at outer surface or interface