Overview of hardness testing methods

Classification, process, differentiation and practical recommendations.

Here you will find useful information on the basics of hardness testing, the most common standardized test methods Rockwell, Brinell, Vickers and Knoop as well as other test methods such as carbon, plastic and Leeb testing.

Rockwell

This is a depth difference method.

Rockwell

Vickers

It is one of the optical processes.

Vickers

Knoop

Developed as an alternative to the Vickers method.

Knoop

Brinell

Has a test load range from 1 to 3000 kgf.

Brinell

The Rockwell hardness test method

The Rockwell method is a static hardness test method in accordance with ISO 6508 and ASTM E18 and is used in the macro range with a test force of 29.42 to 1471 N. This hardness test method measures the permanent indentation depth of an indenter, depending on the Rockwell method, either a diamond cone or a hard metal ball.

The procedure begins with the measurement of the indentation depth under a defined test force. The deeper the indenter penetrates, the softer the material. The Rockwell hardness (HR) is calculated from this penetration depth. In contrast to Brinell, Vickers and Knoop, which measure the indentation optically, this hardness test method offers direct readability of the hardness value.

There are various Rockwell methods that differ in indenter, test force and scale division. This variety allows the hardness test method to be used in different areas of application. It requires no sample preparation and is fast and cost-effective, as no complex optics are required. The test is non-destructive, which makes the test piece reusable.

Disadvantages of the hardness test method are possible inaccuracies due to measurement errors and soiling. Worn indenters can also falsify the results. Nevertheless, the Rockwell method remains widely used in many industrial applications due to its speed and simplicity. The versatility of Rockwell methods emphasises its importance and adaptability to different materials and testing requirements.

To the Rockwell hardness test

The Vickers hardness test method

The Vickers method is a versatile, static hardness testing method that is increasingly being used in practice. It is standardised in accordance with ISO 6507 and ASTM E92/E384 and is suitable for all load ranges (micro, small load and macro range). The indenter, a diamond pyramid with a dihedral angle of 136°, leaves an indentation whose size is measured optically. The larger the indentation, the softer the material.

A high-quality surface preparation is required, as the sample should be polished or finely ground. The sample must be firmly clamped to prevent movement during the test and vibrations must be ruled out.

The Vickers method is divided into different load ranges depending on the test load (e.g. HV 0.01; HV 1; HV 10), which enables a wide range of applications. A Vickers hardness value consists of the numerical hardness value, the letters HV, the test load in kgf and, if applicable, the exposure time.

The Vickers method offers numerous advantages, including universal applicability to all materials and hardness ranges and the use of a single indenter for all methods. It also enables non-destructive testing, allowing the sample to be reused. However, the method requires high-quality surface preparation and is slower in the test procedure compared to Rockwell. The necessary optics also make it more expensive to purchase. Despite these disadvantages, the Vickers method remains a widely used and proven method in the industry due to its precision, versatility and accuracy.

To the Vickers hardness test

The Knoop hardness test method

The Knoop method is a static hardness testing method, particularly suitable for thin layers and brittle materials. It is standardised in accordance with ISO 4545 and ASTM E92/E384 and works with test loads of 1 gf to 2 kgf. As an optical method, it measures the indentation size left by the indenter, a diamond pyramid with a rhombic base.

The Knoop method measures the indentation size that the indenter leaves on the sample under a defined test force. The larger the indentation, the softer the material.

The sample must be polished or finely ground and firmly clamped to prevent movement during the test. Shocks or vibrations must be avoided to ensure precise results.

The elongated and narrow Knoop indentation is particularly suitable for thin layers and brittle materials such as glass and ceramics. It is frequently used in the USA, but is less common in Europe.

A Knoop hardness value consists of the numerical hardness value, the letters HK and the test load in kgf.

The Knoop method is universally applicable, requires only one indenter and offers precise evaluation. It is particularly suitable for brittle materials and thin layers. Disadvantages are the surface preparation required, the slow test procedure and higher acquisition costs. It is less common in Europe.

To the Knoop hardness test

The Brinell hardness test method

The Brinell method is a static hardness test method standardised according to ISO 6506 and ASTM E10. It is mainly used in the low load and macro range, with a test load range of 1 to 3000 kgf. The method uses a spherical indenter made of tungsten carbide with diameters of 1, 2.5, 5 or 10 mm. This is an optical process in which the indentation size of the indenter is measured.

The Brinell method measures the size of the indentation that the spherical indenter leaves on the surface of a sample. The larger the indentation, the softer the material. This differs from the Vickers method, which uses a pyramid-shaped indenter.

The Brinell method offers several advantages. It can also test inhomogeneous materials such as castings, as the large sphere hits many crystals and forms a mechanical mean value. There is a wide range of test forces and ball diameters for different applications. The comparatively large test indentations are easier to measure and the sample surface can be rough.

However, there are also disadvantages. A good surface quality of the sample is required as the indentation is measured optically. High test loads in the macro range can deform the material and lead to measurement errors. Good illumination of the test indentation is important. The method is also slower than the Rockwell method, with a test sequence of 30 to 60 seconds without taking the sample preparation into account. It is also less suitable for very hard materials and thin samples.

To the Brinell hardness test

Further test methods

Carbon testing

learn more

Plastic testing

learn more

Leeb test

The determination of the hardness of metallic materials according to Leeb is defined in the ISO 16859 and ASTM A956 standards. In this dynamic test method, the ratio of rebound velocity to impact velocity of a moving impact body is used to determine the hardness.

Legend:

A Peak value of approach phase
B Peak value of rebound phase
t Time
U Voltage