Complete Workflow for Cross-Section Evaluation of Coatings and Diffusion Layers

Applicable Samples and Objectives

This workflow applies to cross-section evaluation of thermal spray coatings, electroplated/electroless coatings, PVD/CVD coatings and surface-modified layers such as nitrided and chromized layers. The objective is to measure coating thickness and uniformity on a perpendicular cross-section, examine porosity, interfacial bonding and defects, and determine the microhardness of the coating and interface, comprehensively assessing coating quality.

Complete Workflow

1. Sampling and Cutting

   Cut a cross-section perpendicular to the coating surface. Brittle coatings (ceramic sprayed layers) and thin platings must be cut with a precision cut-off machine at low speed, cutting from the coating toward the substrate to avoid peeling the coating in tension.

2. Mounting

   Always use cold mounting (low-exotherm, low-shrinkage epoxy) to prevent the temperature and pressure of hot mounting from damaging the coating; add ceramic or glass filler to the resin and ensure edge encapsulation to prevent edge rounding and coating chipping during grinding. Multiple thin coatings may be clamped and mounted together.

3. Grinding and Polishing

   Low speed and light pressure are essential. Grind stepwise with SiC papers from P400 to P2000, rotating 90 degrees between steps with short grinding times; polish with 3 μm and 1 μm diamond, finishing brittle coatings with a short 0.05 μm colloidal silica polish. Continuously check that the coating edge shows no spalling, crack propagation or comet tails.

4. Surface Treatment

   Thickness, porosity and bonding are always examined in the unetched condition; selective etching with material-specific reagents is used only when intra-layer structures (e.g. between plating layers or in diffusion layers) must be revealed.

5. Microscopic Measurement and Observation

   Measure coating thickness with a metallographic microscope equipped with a micrometer scale or image analysis: measure local thickness at 10 or more evenly distributed positions along the section and report the mean and minimum values. In the same fields, examine pores, unmelted particles, oxide interlayers and interfacial bonding; porosity can be quantified by image analysis as an area percentage.

6. Microhardness Testing

   Place Vickers or Knoop indents at mid-thickness of the coating, selecting the load so that the indent diagonal does not exceed one tenth of the coating thickness, and keeping indent edges at least 2.5 diagonals from the interface and the surface; when required, make an interfacial hardness profile across the section to observe the coating-substrate transition.

7. Data Processing and Determination

   Summarize mean/minimum thickness, porosity, mean hardness and a description of the interfacial condition, and judge against drawing or process specification requirements.

8. Recording and Reporting

   The report includes an overview of the section and local micrographs, a schematic of thickness measurement positions, indent locations and data tables, and states the coating process and material system.

Key Control Points

• All thickness measurements must be made on a strictly perpendicular section; oblique cutting enlarges the measured thickness.
• Penetration of cold mounting resin into open coating pores is normal; verify porosity readings against the unimpregnated condition.
• Edge chipping during grinding is the most common failure mode in coating evaluation; stop immediately and re-prepare when chipping is found.
• For thin coatings (below 50 μm), prefer the Knoop indenter with reduced load to avoid the indent penetrating into the substrate.
• Indents near the interface must be placed on each side of the interface separately, never directly on the boundary.
• Unify the threshold segmentation criteria before porosity quantification to avoid manipulating results by adjusting thresholds.

Common Deviations and Remedies

• Rounded coating edges causing low thickness readings: strengthen edge-protection filler and reduce pressure during regrinding.
• Chipping of brittle coatings during cutting: use a lower feed rate and a thin cut-off wheel, entering from the coating side.
• Crack propagation from indents affecting readings: reduce the load, switch to the Knoop indenter, or record the crack condition separately.
• Pores filled with polishing media misjudged as dense coating: clean and dry thoroughly after polishing; verify against vacuum impregnation if necessary.
• Unclear interfacial contamination: observe under dark field or differential interference contrast, or lightly etch the interfacial region.

Reference Standards

• GB/T 6462 Metallic and oxide coatings — Measurement of coating thickness — Microscopical method
• ISO 1463 Metallic and oxide coatings — Measurement of coating thickness — Microscopical method
• ASTM B487 Standard Test Method for Measurement of Metal and Oxide Coating Thickness by Microscopical Examination of Cross Section
• ASTM E2109 Standard Test Methods for Determining Area Percentage Porosity in Thermal Sprayed Coatings
• ISO 4516 Metallic and other inorganic coatings — Vickers and Knoop microhardness tests

The above is a methodological example for technical communication and process discussion, not actual measured data from a specific customer; the actual test plan, sampling locations and acceptance limits are subject to sample evaluation and the procedure confirmed by both parties.