Overview
ISO/TR 21555:2019 - Paints and varnishes: Overview of test methods on hardness and wear resistance of coatings is a technical report that helps users select the most suitable test methods for evaluating hardness and wear resistance of dry-film coatings. It summarizes a wide range of indentation, scratch and abrasion tests, describes typical procedure elements and limitations, and provides a compact annex (Annex A) with a summarized list of methods mapped to different mechanical stresses. Note: methods for testing cross‑linking (wear with solvents) and multiple‑impact abrasion are excluded.
Key topics and technical requirements
- Scope & purpose: Guidance for choosing appropriate test methods rather than replacement of the detailed normative test standards.
- Definitions & terminology: Uses ISO 4618 and other standardized terms (hardness, wear, stylus, scratch, abrasion, repeatability/reproducibility).
- Hardness tests:
- Indentation methods with resting indenters (e.g., Buchholz) and oscillating indenters.
- Representative examples include Buchholz, Knoop, Pfund, Vickers. Procedures specify loading, dwell times and measurement approach (e.g., Buchholz indentation length measured after defined resting time; Knoop long‑diagonal used to calculate KHN).
- Example precision data provided (Buchholz repeatability limit r = 0.23 mm, reproducibility R = 0.45 mm; Knoop repeatability ≈ 9%, reproducibility ≈ 24%).
- Wear and abrasion tests:
- Single‑scratch and multiple‑scratch tests with different stylus geometries (pencils, ball, conical, disc, U‑shaped).
- Dry and wet abrasion tests (locked or rotating abrasive wheels, brushes, non‑woven media).
- Falling‑sand tests (corundum, quartz sand).
- Test conditions & influences:
- Emphasizes visco‑elastic behaviour, film thickness, temperature and humidity influence results.
- Recommends conditioning (examples: 23 °C / 50 % RH for defined durations) and performing tests in a climatic chamber when appropriate.
- Evaluation: Typical reporting is mean values from multiple determinations and reference to the applicable normative standards for full procedures and calibration.
Practical applications - who uses this standard
- Coatings manufacturers - method selection for product development and QC.
- Test laboratories - planning appropriate mechanical tests and interpreting limitations.
- R&D teams - comparing formulations and understanding mechanical failure modes.
- Specifiers / procurement - selecting test requirements for tender/specs.
- Standards writers & regulatory bodies - harmonizing test method references.
Related standards (referenced)
- ISO 4618 (terms and definitions)
- ISO 2815 (Buchholz test)
- ISO 22557 (stylus terminology)
- ISO 4545‑1, ASTM D1474 (Knoop references)
- See Annex A of ISO/TR 21555:2019 for a concise mapping of methods to stress types.
Keywords: ISO TR 21555, coatings hardness test, wear resistance tests, Buchholz, Knoop, Vickers, scratch test, abrasion test, paint testing, coatings durability.