Microhardness Testing: Vickers & Knoop Methods per ASTM E384
What Is Micro-Hardness Testing?
Micro-hardness testing measures the hardness of small, localized areas in a material using very light indentation loads (1 gf to 1,000 gf, with test loads below 200 gf classified as micro-hardness per ASTM E384). The resulting indentations are microscopic, requiring optical or digital microscopy for measurement. This makes micro-hardness testing uniquely suited for characterizing the hardness of individual microstructural features, thin surface layers, weld zones, coatings, and small components that cannot be tested by macro-hardness methods.
Micro-hardness testing per ASTM E384 and ISO 6507/4545 is fundamental to the metals, semiconductors, thin films, and precision manufacturing industries.
Two Indenters: Vickers vs. Knoop
Vickers Micro-Hardness (HV)
The Vickers indenter is a square-based diamond pyramid with a 136° included angle between opposite faces. The indentation is square in plan view; both diagonals are measured and averaged.
HV = 1.854 × F / d²
Where F is the applied load (gf) and d is the arithmetic mean of the two diagonals (µm).
Characteristics:
- Geometrically similar at all loads—HV values are theoretically load-independent for homogeneous materials
- Square indentation enables measurement of both diagonals for orientation-sensitive materials
- Applicable to all hardness levels (very soft metals to ceramics and hard coatings)
- Standard for microstructural characterization, case depth measurement, weld HAZ hardness profiles
Knoop Micro-Hardness (HK)
The Knoop indenter is an asymmetric rhomboid pyramid with included angles of 172.5° (long axis) and 130° (short axis), producing an elongated, boat-shaped indentation. Only the long diagonal is measured.
HK = 14.229 × F / L²
Where F is the applied load (gf) and L is the long diagonal (µm).
Characteristics:
- Elongated indentation is much shallower than Vickers at the same load—ideal for thin layers, brittle ceramics, and surface coatings where deep indentation would crack the substrate
- Anisotropy can be detected by rotating the specimen (HK varies with indenter orientation relative to crystal or fiber direction)
- Better suited for very brittle materials (glass, ceramics) where Vickers indentation causes radial cracking
ASTM E384 Requirements
ASTM E384 governs micro-hardness testing of metallic materials and defines:
- Specimen preparation requirements (metallographic polish to 1 µm or better)
- Minimum spacing between indentations and from specimen edges (to avoid plastic zone overlap): typically 2.5× diagonal spacing for Vickers
- Load selection guidelines to ensure indentation is large enough for accurate measurement (d ≥ 20 µm recommended)
- Optical magnification requirements for diagonal measurement accuracy
- Calibration with certified reference hardness blocks
Key Applications
| Application | Preferred Indenter | Typical Load |
| Case depth measurement (carburized/nitrided) | Vickers | 100–500 gf |
| Weld HAZ hardness traverse | Vickers | 100–300 gf |
| Thin film and coating hardness | Knoop or Vickers | 1–25 gf |
| Individual grain hardness | Vickers | 10–50 gf |
| Ceramic and glass hardness | Knoop | 100–500 gf |
| Intermetallic compound hardness | Vickers | 10–25 gf |
Nanoindentation: Beyond ASTM E384
For films thinner than ~1 µm and for direct measurement of elastic modulus alongside hardness, nanoindentation (Oliver-Pharr method, ASTM E2546) uses nano-newton force resolution and nanometer displacement resolution—extending hardness measurement to films and coatings that are inaccessible to conventional micro-hardness methods.
Why Choose Infinita Lab for Micro-Hardness Testing?
Infinita Lab offers Vickers and Knoop micro-hardness testing per ASTM E384, ISO 6507, and ISO 4545 with automated hardness mapping capabilities. Our nationwide accredited laboratory network provides precise, traceable micro-hardness measurements for case depth analysis, weld qualification, coating characterization, and failure analysis programs.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
What is the minimum coating or case depth that can be reliably measured by Vickers micro-hardness? A common rule of thumb is that the indentation depth should not exceed 10% of the coating or case depth (to avoid substrate influence). For a Vickers 100 gf indentation in steel (d ≈ 40–60 µm, depth ≈ d/7 ≈ 6–9 µm), the minimum measurable case depth is approximately 60–90 µm. For thinner cases or coatings, lighter loads (10–25 gf) or nanoindentation must be used.
What surface finish is required for micro-hardness specimens? ASTM E384 requires that the test surface be prepared to a metallographic finish—ground and polished to at least 1 µm diamond or alumina. The surface must be flat, without relief (smearing of soft phases over hard phases) or significant deformation from grinding. Electropolishing may be required for very soft materials to remove deformed surface layers.
Why does Knoop hardness give different values than Vickers hardness for the same material? Knoop and Vickers indenters have different geometries and different stressed volumes. The elongated Knoop indentation loads a smaller volume at equivalent depth compared to the Vickers square pyramid. Knoop values are generally lower than Vickers for the same material and load because the larger elastic recovery in the short axis direction affects the hardness calculation. Conversion tables (ASTM E140) provide approximate conversions, but they are not exact.
How are micro-hardness traverses used to characterize carburized case depth? Micro-hardness traverses measure the Vickers hardness at equally spaced indentation positions from the surface into the core. The case depth is defined as the depth at which hardness falls to a defined threshold—typically 550 HV for effective case depth or 50 HRC equivalent. This depth measurement is reported as the "effective case depth" per SAE J423 or ISO 2639.
What is the purpose of reporting "CHD" (Case Hardening Depth) vs. "TCD" (Total Case Depth)? CHD (Case Hardening Depth, sometimes called Effective Case Depth) is the depth to a defined hardness limit—typically 550 HV for carburized steel. TCD (Total Case Depth) is the depth to which compositional change (carbon enrichment) extends, measured by microstructural examination or chemical analysis rather than hardness. CHD is the engineering-relevant specification parameter; TCD is a process characterization parameter.
