The hardness of a material is a measure of how hard it is to change its shape. A hard material is difficult to scratch or dent, but it will scratch or dent a softer material.
The relative hardness of some materials are:
Picture 4.5 Measuring hardness using Brinell test.
Material
Relative hardness
Diamond
10.0
Steel
7.0
Glass
5.5
Tin
1.5
Brinell hardness test
In the Brinell hardness test a hard steel ball with a diameter of between 1mm and 10mm is forced onto the surface of the sample of steel under test for 20 seconds. The force used in the test is measured in units of kilogramforce. The force applied is usually between 500kgf and 3000kgf. [1kgf = 9.8newtons].
The area of the indentation made by the steel ball is measured in square millimetres. The Brinell hardness number (HB), is equal to the applied force divided by the area of indentation.
Picture 4.6 A hardness tester forces a diamond into the surface of the sample.
Vickers hardness test
In the Vickers hardness test the surface of the sample is indented by a diamond which makes a pyramid shaped hole. A force of between 5kgf and 120kgf is applied for 15 seconds.
The Vickers hardness number is found by dividing the applied force by the surface area of the indentation. This method is very accurate and, since it uses a diamond, it can be used with all metals.
The Vickers and Brinell hardness numbers of a particular type of steel are very similar.
Picture 4.7 Using a microscope to take measurements from the results of a Vickers hardness test.
Impact tests
Impact tests measure how a material behaves when subjected to a sudden force. In the Izod impact test, a heavy pendulum swings through an arc and strikes a test piece. The test piece has a notch cut into it near its base so that it is fairly easily broken. The pendulum continues its swing but does not go as high as the point where it started because it has lost some energy in breaking the test piece. The energy lost on impact can be calculated and is known as the impact energy.
