Overview
ISO 28079:2009 specifies a standardized indentation method - the Palmqvist toughness test - for characterizing the toughness of hardmetals and cermets at (normally) room temperature. The method calculates Palmqvist toughness from the total length of cracks that emanate from the four corners of a Vickers hardness indentation. Tests are intended for use in a normal laboratory-air environment (not corrosive media) and can be adapted to other temperatures by agreement.
Keywords: ISO 28079:2009, Palmqvist toughness, hardmetals, cermets, Vickers indentation, fracture toughness test
Key topics and technical requirements
- Test principle: Measure crack lengths from the corners of a Vickers indentation and compute a Palmqvist toughness value (W) - often reported alongside Vickers hardness (HV).
- Indentation practice: Indentations may be made at a single load (commonly 30 kgf) or by using multiple loads and extracting toughness from the slope of crack-length vs. load.
- Sample preparation:
- Flat, parallel faces required; remove ~0.2 mm before final polish to expose bulk material.
- Wet grinding with metal-bonded 40 µm diamond discs; polishing sequence recommended at least 30 µm → 6 µm → 1 µm, finishing on napless cloths.
- Surfaces should be free of residual stresses; annealing (e.g., 800 °C for 1 h in vacuum) is noted as a practice to reduce polishing-induced stresses.
- Verify indentation geometry: diagonal measurements must agree within 1% or the test is invalid.
- Specimen size: Practical guidance suggests specimen thickness should be several times the expected crack length (some recommendations: ≥10× crack length) to avoid substrate effects.
- Environment and limitations: Designed for ambient lab air; not intended for strongly corrosive environments (acids, seawater). Palmqvist results have known variability and are sensitive to surface condition and operator measurement.
- Uncertainty and reporting: The standard includes guidance on test validity, measurement uncertainty and a recommended test-report pro forma.
Applications and users
- Quality control and production testing for cemented carbides, WC/Co hardmetals and related cermets.
- Materials development and R&D where relative toughness ranking is required.
- Failure analysis and comparative assessments where quick, low-cost toughness screening is useful.
- Typical users: materials laboratories, hardmetal manufacturers, tooling producers, and standards/verification labs.
Related standards and references
- ISO 3878 - Vickers hardness test for hardmetals (normative reference used by ISO 28079).
- ISO 28079 complements fracture toughness work and interlaboratory efforts (e.g., VAMAS studies) but is focused specifically on Palmqvist indentation measurements rather than standardized fracture-mechanics specimens.
Practical note: Palmqvist toughness provides a rapid, correlated toughness indicator together with hardness, but it is not a direct substitute for rigorous fracture-mechanics K_Ic tests; use it for comparative testing, QC and material screening.
