Knoop hardness testing is a microindentation technique widely used to evaluate the hardness of thin, brittle, or very small materials with high precision. Governed by standards such as ASTM E384 and ISO 4545, the method uses an elongated diamond indenter that makes extremely shallow impressions, making it ideal for coatings, ceramics, thin foils, and microscopic material features. This article explains the testing principle, applicable sample types, test procedures, and practical applications of Knoop hardness determination.
Technical Overview
The Knoop hardness test is a microindentation hardness measurement method developed in 1939 at the U.S. National Bureau of Standards. It uses a rhombic-based pyramidal diamond indenter with longitudinal edge angles of 172.5 degrees and 130 degrees, producing an elongated indentation with a long-to-short diagonal ratio of approximately 7:1. Unlike the Vickers test, only the long diagonal of the indentation is measured, which improves measurement accuracy at very small indentation sizes.
The Knoop hardness number (HK) is calculated from the applied test force and the length of the long diagonal, following the relationship HK = 14.229 × F / d², where F is the test force in kilogram-force and d is the long diagonal in millimeters. One of the defining characteristics of the Knoop method is its shallow penetration depth, which is only about 1/30 of the long diagonal. This makes the technique particularly suitable for evaluating very thin materials, surface layers, and brittle substances that would crack under deeper indentation.
The test is typically performed at low loads ranging from 10 gram-force to 1 kilogram-force, placing it firmly within the microhardness testing domain. Because the indentation is extremely small, the method can measure the hardness of individual microstructural constituents, such as specific phases in an alloy or individual grains in a ceramic.
Testing Samples
Knoop hardness testing accommodates a broad range of sample geometries and material classes. Because indentation dimensions are microscopic, samples generally require careful sectioning and polishing before testing.
Metallic Materials
Metallic samples are the most common subject of Knoop testing, including thin sheet, foil, fine wire cross-sections, and small precision components. The method is frequently applied to evaluate case-hardened layers, nitrided surfaces, carburized zones, and weld cross-sections where hardness gradients must be mapped across narrow regions.
Ceramics and Glass
The shallow penetration of the Knoop indenter minimizes crack formation in brittle materials, making it the preferred microhardness method for ceramics, optical glass, enamels, and refractory materials. The low indentation depth allows valid hardness measurements even in materials with limited fracture toughness.
Coatings and Thin Films
Electroplated layers, chemical conversion coatings, physical vapor deposition (PVD) and chemical vapor deposition (CVD) films, anodized surfaces, and diffusion-treated layers can all be evaluated. The Knoop geometry ensures that the substrate does not influence the measured hardness, provided the coating thickness exceeds approximately ten times the indentation depth.
Polymers and Elastomers
With appropriately reduced test forces and extended dwell times, Knoop testing can characterize rigid polymers, dental composites, and organic surface layers that exhibit viscoelastic behavior.
Electronic and Medical Components
Semiconductor wafers, microelectronics assemblies, solder joints, dental restorative materials, and implant surfaces are routinely tested due to their small size and thin functional layers.
Testing Items
Typical test items evaluated through Knoop hardness determination include:
- Knoop hardness number (HK) at specified test forces, for example HK0.05, HK0.1, or HK0.5
- Effective case depth and hardened layer depth in heat-treated steels
- Microhardness gradients across coatings, welds, and diffusion zones
- Hardness of individual phases, inclusions, or grains in multiphase microstructures
- Comparative surface hardness before and after surface treatment or environmental exposure
- Hardness homogeneity and quality consistency checks across production batches
Testing Methods
Knoop hardness testing is performed in accredited third-party laboratories according to internationally recognized standards. The primary governing documents are ASTM E384, Standard Test Method for Microindentation Hardness of Materials, and the ISO 4545 series for metallic materials, which covers the test method, verification of testing machines, calibration of reference blocks, and tables of hardness values.
The general test procedure involves the following steps:
- Sectioning the specimen and mounting it, typically in a polymer resin, with the region of interest exposed
- Grinding and polishing the surface to a mirror finish, usually with abrasives of one micrometer or finer, since surface roughness directly affects diagonal measurement accuracy
- Selecting an appropriate test force, balancing the need for a measurable indentation against the requirement that the layer being tested is sufficiently thick
- Applying the force smoothly and without shock, maintaining it for a dwell time typically between 10 and 15 seconds, or longer for materials exhibiting time-dependent deformation
- Measuring the long diagonal of the indentation using a calibrated optical microscope and computing the Knoop hardness number
- Observing indentation spacing rules, keeping indentations sufficiently far from edges and from each other to avoid interaction effects
Key factors influencing result reliability include indentation size effects at very low loads, elastic recovery of the indentation, mechanical vibration, microscope calibration, and operator skill in diagonal measurement. Verification of the testing machine and indirect verification using calibrated reference blocks are required by both ASTM E384 and ISO 4545-2.
Applications
Knoop hardness determination serves numerous practical purposes across industries:
- Heat treatment verification: confirming case depth and surface hardness of carburized, nitrided, or induction-hardened components in automotive and machinery manufacturing
- Coating development and qualification: measuring the hardness of thin functional coatings in tooling, decorative, and protective applications
- Failure analysis: detecting softening, decarburization, overheating, or embrittlement in fractured parts examined by metallurgical laboratories
- Materials research: characterizing new alloys, ceramics, composites, and additively manufactured materials at the microstructural scale
- Electronics manufacturing: evaluating thin films, bond pads, solder interconnects, and wafer surface layers in semiconductor production
- Medical and dental fields: assessing dental composites, ceramics, and implant surface treatments where small sample dimensions preclude macro-scale testing
- Glass and optics production: quality control of hardness and surface integrity in optical components
- Anisotropy studies: because the elongated Knoop indentation is directional, rotating the indenter orientation can reveal hardness differences related to crystallographic or processing direction
FAQ
What is the difference between Knoop and Vickers hardness testing? Both use a diamond pyramidal indenter, but the Knoop indenter is elongated and produces a shallower indentation. Knoop is better suited to very thin layers and brittle materials, while Vickers produces a symmetric square indentation better suited to bulk hardness and direct comparison across a wider load range.
What test forces are used in Knoop hardness testing? Typical forces range from 10 gram-force to 1 kilogram-force. Lower forces are used for thin coatings and brittle ceramics, while higher forces are used for bulk metallic samples, in accordance with ASTM E384 and ISO 4545-1.
Can Knoop hardness values be converted to other hardness scales? There is no universal, exact conversion between Knoop and other scales such as Rockwell or Vickers. Approximate conversions exist for specific material classes, but direct comparison should be avoided in specification or acceptance contexts.
What surface preparation is required? The test surface must be polished to a mirror finish, generally one micrometer or finer, because the long diagonal measurement is highly sensitive to surface roughness and preparation artifacts.
How thin can a sample be for valid Knoop testing? The specimen or layer thickness should be at least ten times the indentation depth. Since Knoop penetration depth is roughly 1/30 of the long diagonal, the method can evaluate layers considerably thinner than those measurable by Vickers testing at the same force.
Which standards govern Knoop hardness testing? The principal standards are ASTM E384 and the ISO 4545 series. Related national and regional standards, including GB/T 4340.1 for microindentation practice and JIS Z 2244-type microhardness procedures, provide comparable technical requirements.
