Overview
ISO 9385:1990 - Glass and glass-ceramics: Knoop hardness test specifies a standardized procedure for determining the Knoop hardness number (HK) of glass and glass‑ceramic materials. Developed by ISO/TC 172 (Optics and optical instruments), the standard defines specimen preparation, apparatus requirements, indentation procedure, measurement rules and reporting conventions. The Knoop test is recommended over Vickers for glass based on interlaboratory comparisons (ICG round robin).
Key topics and requirements
- Test principle: A diamond Knoop indenter produces a shallow, rhomboid indentation; HK is proportional to test force divided by the projected indentation area (force / projected area).
- Notation: HK is supplemented by the test force and duration (e.g., 490 HK 0,1/20 denotes force converted to kgf and duration in seconds).
- Recommended test force: 0.9807 N is used for HK0,1/20; additional forces may be applied to check consistency and reduce uncertainty.
- Indenter and apparatus:
- Use a certified Knoop indenter; inspect optically for damage.
- Hardness tester must have relative force error ≤ 1% and load application rate of (0.20 ± 0.05) mm/min.
- Verify tester performance with standardized reference plates.
- Specimen preparation:
- Test surface must be flat, fire‑polished or optically ground and polished; finish with water and cerium oxide immediately before testing.
- Minimum specimen thickness: 0.5 mm.
- Test procedure and measurement:
- Apply force for 20 s; measure the long diagonal within 6 min of indentation.
- Measure the long diagonal twice (values should not differ by more than 2.0 µm).
- Microscope requirements: effective numerical aperture around 0.7 and yellow‑green illumination recommended; long diagonal measurable with uncertainty
