How do hardness tests work?

A hardness test is typically performed by pressing a specifically dimensioned and loaded object (indenter) into the surface of the material you are testing. The hardness is determined by measuring the depth of indenter penetration or by measuring the size of the impression left by an indenter.

• Hardness tests that measure the depth of indenter penetration include: Rockwell, Instrumented Indentation Testing, and Ball Indentation Hardness
• Hardness tests that measure the size of the impression left by the indenter include: Vickers, Knoop, and Brinell

The four most common indentation hardness tests

HOW TO ENSURE ACCURACY AND REPEATABILITY IN HARDNESS TESTING

The correct application of hardness testing requires careful preparation and execution. However, once you have the basics in place, most hardness tests offer good accuracy and repeatability.

Factors that influence hardness testing

A number of factors influence hardness tests results. As a general rule, the lower the load you use in the hardness test, the more factors that need to be controlled to ensure an accurate conclusion of the hardness test.

Here are a few of the most important factors to consider to ensure an accurate conclusion from a hardness test.

• External factors such as light, dirt, vibrations, temperature, and humidity should be controlled
• The tester and stage should be secured on a solid horizontal table, and the sample should be clamped or held in a holder or anvil
• The indenter should be perpendicular to the tested surface
• Illumination settings should be constant during the test when using Vickers, Knoop, or Brinell
• The tester should be recalibrated/verified every time you change the indenter or objective lens

Surface preparation requirements for hardness testing

Before hardness testing of metallic materials or other materials, you must prepare the surface. The required surface condition depends on the type of test and load used. In general, the quality of surface preparation has a direct effect on the hardness test result, so you should consider the trade-off between surface quality and test result variation before deciding on an inferior surface preparation.

Macro hardness testing
A ground surface is usually sufficient, and sometimes no preparation is required.

Micro hardness testing
Due to the lower loads used during hardness testing, micro hardness testing requires a polished or electropolished surface. It is important that the borders/corners of an optically evaluated impression are clearly visible. This can be performed mechanically, chemically, or electrochemically. It is important that heating or cold working does not change the surface properties of the specimen.

Deformations

Cutting and grinding may introduce deformations. These must be removed by polishing down to 6.0, 3.0, or 1.0 μm, depending on the hardness test load.

For small loads (less than 300 gf1) the surface needs to be completely free of deformations, and the specimens require oxide or electrolytic polishing to obtain a completely damage-free surface. You should also take into account that soft and/or ductile materials (i.e. for HV less than 120-150) are more sensitive when it comes to introducing preparation artefacts.

In the table below, you can see the surface preparation requirements for the different hardness tests.

DEFINITION OF HARDNESS TESTING LOADS

Officially, hardness testing loads are expressed in Newton (N). However, historically, loads were expressed in kilogram-force (kgf), gram-force (gf), or pond (p). The correlation between kgf, kp, and N is: 1.0 kgf = 1,000 gf = 1.0 kp = 9.81 N.

• The term micro hardness testing is usually used when indentation loads are below or equal to 1 kgf
• The term macro hardness testing is used when loads are higher than 1 kgf

If standards permit, use the highest possible load/force for largest indent to ensure the most accurate results.

The loads used by each of the four methods for hardness testing of metallic materials* comply with the different ISO and ASTM standards.