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ISO 26443: Rockwell Indentation Test for Adhesion Evaluation of Ceramic Coatings
Qualitative Assessment of Coating Adhesion Using Standard Rockwell Hardness Testing Equipment
Introduction to the Rockwell Indentation Adhesion Test
ISO 26443:2023 specifies a qualitative method for evaluating the adhesion of ceramic coatings (up to 20 micrometers thick) using a Rockwell diamond indenter. This test, applicable also to metallic coatings, provides a rapid and cost-effective way to assess coating quality without specialized adhesion testing equipment. The principle is elegantly simple: a Rockwell indentation is made into the coated surface, and the resulting damage patterns around the indent reveal whether the coating has adequate adhesion to its substrate.
The test is classified by TC 206 (Fine ceramics) and was developed in collaboration with CEN/TC 184. It is not suitable for elastic coatings on hard substrates, where the coating simply stretches rather than delaminates under indentation.
Test Procedure and Classification System
The indentation is made perpendicular to the specimen surface using a standard Rockwell hardness testing machine conforming to ISO 6508-1 and ISO 6508-2. The applied load depends on the substrate hardness: 1471.5 N (Rockwell C) for substrates harder than 54 HRC, 981 N (Rockwell D) for substrates softer than 54 HRC and medium case-hardened steel, and 588.6 N (Rockwell A) for all other substrates including cemented carbides, solid ceramics, and cermets.
After indentation, the area is examined under an optical microscope at 100x magnification. Results are classified into four classes based on the observed failure pattern:
| Class | Observation | Adhesion Assessment |
|---|---|---|
| Class 0 | No cracking and no adhesive delamination | Acceptable |
| Class 1 | Cracking without adhesive delamination | Acceptable |
| Class 2 | Partial adhesive delamination, with or without cracking | Unacceptable |
| Class 3 | Complete adhesive delamination | Unacceptable |
Engineering Insights and Practical Guidance
For class 2 failures, an estimate of the percentage of delamination relative to the indent surface area must be reported, calculated from the sum of individual delamination areas. For class 3 failures, the ratio (r/a) of the maximum delamination radius to the indent radius must be reported. At least three measurements at representative locations are recommended. The diamond indenter contour must be checked regularly by optical means at 200x magnification or higher to detect ring cracks or microwear that could invalidate test results.
Results shall only be compared when using a similar substrate/coating combination and coating thickness. Class designations must be linked to the specific load used comparisons across different loads are not valid. This is because the stress distribution at the coating-substrate interface scales with load, so a class 2 result at 1471.5 N does not imply the same adhesion quality as a class 2 result at 588.6 N.
The standard notes that cohesive failure (cracking within the coating) can also be revealed and may be observed using optical contrasting techniques such as Nomarski interference contrast microscopy. For multilayer coatings, delamination can occur between sublayers rather than at the coating-substrate interface. The standard carefully distinguishes between adhesive delamination (where substrate becomes visible), cohesive delamination (where substrate remains covered), and complete delamination (uninterrupted removal along the indent circumference).
FAQs
Q: Can this test be used for quality control in production?
A: Yes. The Rockwell indentation test is rapid, uses equipment already available in most hardness testing laboratories, and provides immediate visual results. It is well-suited as a production QC tool for ceramic-coated components in cutting tools, automotive engine parts, and medical implants. However, it is qualitative it does not provide a numerical adhesion strength value.
A: Yes. The Rockwell indentation test is rapid, uses equipment already available in most hardness testing laboratories, and provides immediate visual results. It is well-suited as a production QC tool for ceramic-coated components in cutting tools, automotive engine parts, and medical implants. However, it is qualitative it does not provide a numerical adhesion strength value.
Q: What is the difference between adhesive and cohesive failure?
A: Adhesive failure occurs at the coating-substrate interface, where the coating separates from the substrate and the underlying surface becomes visible. Cohesive failure occurs within the coating itself (cracking or internal delamination) where the substrate remains covered. Both types can occur simultaneously.
A: Adhesive failure occurs at the coating-substrate interface, where the coating separates from the substrate and the underlying surface becomes visible. Cohesive failure occurs within the coating itself (cracking or internal delamination) where the substrate remains covered. Both types can occur simultaneously.
Q: Why are elastic coatings on hard substrates excluded?
A: Elastic coatings on hard substrates deform rather than fracture or delaminate under indentation. The Rockwell indent simply stretches the coating without creating the crack patterns or flaking that the classification system relies on. Such coatings require alternative adhesion test methods such as scratch testing or pull-off testing.
A: Elastic coatings on hard substrates deform rather than fracture or delaminate under indentation. The Rockwell indent simply stretches the coating without creating the crack patterns or flaking that the classification system relies on. Such coatings require alternative adhesion test methods such as scratch testing or pull-off testing.
