What is hardness?
Hardness is the property of a material to resist surface indentation (and scratches).
Some materials are harder than other materials. For example, metals are harder than plastics, wood etc.
Difference between hardness and toughness
Hardness is a property which is related to the surface of a material while toughness is a property which is related to whole material.
Toughness is the ability of a material to absorb energy until ultimate failure happens. It means more tough material will be fractured after less tough material.
There are several tests to measure hardness of a material, some of them are.
- Brinell Hardness Test
- Rockwell Hardness Test
- Knoop Hardness Test
- Vickers Hardness Test
Brinell Hardness Test
Brinell Hardness Test is most commonly used to test materials that have a structure that is too coarse or that have a surface that is too rough to be tested using another test method (for example parts made by castings and forging). Brinell testing often use a very high-test load (3000 kgf) and a 10mm diameter indenter so that the resulting indentation averages out most surface and sub-surface inconsistencies.
Rockwell Hardness Test
Image attributions: By Three-quarter-ten – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11461977
Rockwell method measures the permanent depth of indentation produced by a force/load on an indenter. First, a preliminary test force (commonly referred to as preload or minor load) is applied to a sample using a diamond or ball indenter. This preload breaks through the surface to reduce the effects of surface finish. After holding the preliminary test force for a specified dwell time, the baseline depth of indentation is measured.
Knoop Hardness Test
In Knoop hardness test a pyramidal diamond point is pressed into the polished surface of the test material with a known (often 100g) load, for a specified dwell time, and the resulting indentation is measured using a microscope.
Vickers Hardness Test
Image attributions: By Freundchen – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=19350656
In Vickers Hardness Test a diamond pyramid is pressed against the solid with a certain normal load and the hardness is calculated based on the imprint left on the surface.
Brinell Hardness Test: https://www.hardnesstesters.com/test-types/brinell-hardness-testing
Rockwell Hardness Test: https://www.hardnesstesters.com/test-types/rockwell-hardness-testing
Knoop Hardness Test: https://en.wikipedia.org/wiki/Knoop_hardness_test
Vickers Hardness Test: http://www.tribonet.org/hardness-calculation-vickers-indenter/