HARDNESS

The Vickers hardness is the first hardness method introducing the notion of micro-hardness. It was invented within Vickers Ltd in 1921 by R. SMITH and G. SANDLAND. At the time, the Vickers company, initially a foundry specialising in bells, was a well-known company in the field of naval industry and was taking part in the early days of aeronautics.

Compared to the Brinell hardness, the Vickers hardness has the advantage that, regardless of the material tested and the load used, the indenter remains the same, regardless of the hardness of the material.

The Vickers hardness uses a square-based diamond pyramid as an indenter.

The Knoop hardness was invented in 1939, almost 20 years after the Vickers hardness. It was invented by Mr. Frédéric Knoop and his collaborators from the National Bureau of Standards of the United States. This is a micro-hardness test method only (test < 1kgf).

The Knoop hardness uses a diamond indenter leaving an indentation in the shape of a stretched rhombus.

Thanks to the shape of its indenter, the Knoop test method is particularly suitable for:

• Thin layer hardness measurements,
• Fragile materials (such as glass/ceramics) – the only method suitable for this type of material.

Like the Vickers hardness, the Knoop hardness uses a single indenter for all measurements.

The Knoop hardness is not widespread in Europe.

The Rockwell hardness is based on the principle of differential hardness. The principle of differential hardness was theorised in Austria in 1908 by Paul Ludwik. The Rockwell brothers filed a first patent of application on July 15th 1914, which was accepted at the end of the First World War, in 1919 (1 294 171). They both worked for the New Departure Manufacturing Co (Connecticut), a ball bearing manufacturer. This first patent was to be followed by two improvements before ending up with a Rockwell Hardness Tester close to those we currently know.

The Rockwell hardness measures the residual deformation from a force applied via a spherical or conical indenter.

Unlike all other types of hardness, the Rockwell hardness does not employ the principle of optical reading of the indent; therefore, this method escapes surface conditions.

These are simple tests, requiring little preparation of the sample surface. Rockwell tests are therefore fast. That is why the Rockwell hardness is now widely used in industry.

However, Rockwell tests suffer from a lower accuracy than the previously listed tests and from the fact that many factors can distort the measurement; the regular use of a test block is strongly recommended.

The Brinell hardness is the first hardness testing method widely used in industry. It was invented in Sweden in 1900 by Mr. Johan August Brinell.

This method largely inspired first of all the Vickers method and then the Knoop method because all three are based on the same principle: the observation of the deformation of materials when facing a force applied by means of a standardised indenter.

Brinell tests strongly mark the materials and require several spherical indenters, depending on the loads used. These last two characteristics make Brinell tests rather inconvenient to use in an industrial environment.

PRESI offers a wide range of Hardness Testers, Vickers, Knoop and Rockwell.

Regarding the machines themselves, we use several types of technology:

• Load cell: the force is applied by an electric motor and is controlled via a load cell: the HZ Hardness Tester range and the Matsuzawa Rockwell Hardness Tester range.
• Mechanical weight: the force is applied by physical masses: the Vickers Hardness Tester range and Knoop Matsuzawa Hardness Tester range.

Depending on the set-up, the Hardness Tester can be used autonomously (without a computer) or automatically.

Automation means the use of an image processing software and the motorisation of the axes (the XY table, the vertical Z axis and turret).