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ISO 27307:2015 — Adhesion/Cohesion Evaluation of Thermal Sprayed Ceramic Coatings by Transverse Scratch Testing
Standardized Method for Assessing Coating-Substrate Interfacial Bond Strength
1. Standard Scope and Test Principle
ISO 27307:2015 specifies a transverse scratch testing method for evaluating the adhesion and cohesion of thermal sprayed ceramic coatings. The standard applies to coatings with thickness from 50 μm to over 1000 μm, including plasma-sprayed, HVOF, cold spray, and aerosol deposition coatings. The test involves drawing a Rockwell conical diamond indenter (20 μm tip radius) across a polished cross-section of the coated specimen under a constant applied load.
For coating engineers: This transverse scratch method evaluates coating-substrate adhesion directly on the cross-section, unlike conventional face-side scratch testing. This provides a more representative assessment of interfacial bond strength in thick ceramic coatings.
2. Test Equipment and Procedure
The scratch testing machine must provide rigid indenter mounting, controlled normal load application, and acoustic emission (AE) monitoring capability. Specimens are sectioned perpendicular to the coating surface, embedded in cold-mounting acrylic resin, and polished to Ra < 0.5 μm. Testing is performed under controlled environmental conditions (22 °C ± 2 °C, 50% ± 5% RH) with at least 4 scratches per specimen and 3 specimens per test condition.
| Parameter | Requirement | Purpose |
|---|---|---|
| Indenter geometry | Rockwell C, 20 μm tip radius | Standardized stress field |
| Surface finish | Ra < 0.5 μm | Eliminate roughness artifacts |
| Temperature | 22 °C ± 2 °C | Environmental control |
| Relative humidity | 50% ± 5% | Environmental control |
| Scratches per specimen | ≥ 4 | Statistical significance |
| Specimens per condition | ≥ 3 | Statistical significance |
3. Failure Mode Classification
Three primary failure modes are identified: (1) no crack — coating remains intact; (2) cohesive crack — failure within the coating layer; (3) adhesive crack — failure at the coating-substrate interface. The critical load for adhesive failure characterizes the practical coating adhesion strength. Acoustic emission and tangential force monitoring provide real-time detection during testing.
Research findings from the standard’s interlaboratory testing: Plasma-sprayed TiO₂ coatings tested at 3 N showed predominantly no-crack or cohesive failure, while increasing to 20 N caused transition to adhesive failure. YSZ coatings exhibited similar behavior, confirming the test’s ability to discriminate coating quality.
4. Engineering Applications and Interpretation
The transverse scratch test enables objective comparison of coating adhesion across different spray parameters, material systems, and substrate preparations. This is particularly valuable for quality control in aerospace, power generation, and biomedical coating applications. The standard includes classification criteria and optical micrograph reference examples for failure mode identification.
The test evaluates adhesion on a metallographic cross-section, which represents only a 2D slice of the 3D coating system. Results should be correlated with application-specific performance testing for comprehensive coating qualification.
Coating thickness below 50 μm may produce substrate-dominated failure modes that do not represent true coating adhesion. For thin films (< 50 μm), refer to ISO 20502 (conventional face-side scratch testing).
5. Frequently Asked Questions
Q: How does transverse scratch testing compare with conventional face-side scratch testing?
A: Transverse testing evaluates interface adhesion directly on the cross-section, avoiding complications from coating surface roughness and through-thickness property gradients. It is preferred for thick (>100 μm) ceramic coatings.
A: Transverse testing evaluates interface adhesion directly on the cross-section, avoiding complications from coating surface roughness and through-thickness property gradients. It is preferred for thick (>100 μm) ceramic coatings.
Q: Can this method be used for metallic or polymeric thermal spray coatings?
A: The standard is written for ceramic coatings. Metallic coatings typically exhibit different failure mechanisms (plastic deformation-dominated) that may not be adequately captured by this test method.
A: The standard is written for ceramic coatings. Metallic coatings typically exhibit different failure mechanisms (plastic deformation-dominated) that may not be adequately captured by this test method.
Q: What is the typical reproducibility of this test?
A: Round-robin testing results in the standard show that cohesive/adhesive transition loads are reproducible within approximately ±20% when the specified procedures are followed.
A: Round-robin testing results in the standard show that cohesive/adhesive transition loads are reproducible within approximately ±20% when the specified procedures are followed.
