EN 843-4 Advanced Technical Ceramics - Mechanical Properties of Monolithic Ceramics at Room Temperature - Part 4: Vickers, Knoop and Rockwell Superficial Hardness

The European standard "EN 843-4 Advanced technical ceramics - Mechanical properties of monolithic ceramics at room temperature - Part 4: Vickers, Knoop and Rockwell superficial hardness" describes the guidelines and conditions of execution regarding the value that can be attributed to the results of standard hardness tests when applied to advanced monolithic technical ceramics. The calibration and testing procedures used are assumed to be exactly those for metallic materials.

This standard defines three test methods used to determine the hardness of ceramic materials:

• The Vickers hardness test is used to measure the ability of a material to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales of hardness tests.
• The Knoop hardness test is a microhardness test. This is a mechanical hardness test used especially for very brittle materials or thin sheets, where only a small indentation can be made for testing purposes.
• The Rockwell scale is a hardness scale based on the indentation hardness of a material. The Rockwell hardness test compares the depth of penetration of an indentation under a large load to the penetration made by a preload.

All three test methods use an indenter to apply local plastic deformation. The indentation point is measured and hardness values ​​are calculated. Due to the brittle behavior of ceramic materials, an experienced user of the standard is required to obtain reliable results.

For the purposes of this standard, the following terms and definitions apply:

• Hardness refers to the resistance of a material to being forced into a test surface by a hard indenter of defined geometry.
• The hardness number refers to the hardness value calculated in a specified hardness test, usually without unit specification, derived from the penetration depth of the indentation or the lateral dimension of the indentation and the applied force.
• The hardness indenter is a hard indenter device with a defined geometry and is used for testing ceramics, usually made from single crystal diamond.

The three types of tests defined in this standard are standardized for metallic materials and are widely used as a guide to the state of thermal processing or work hardening. These test methods are also widely used in advanced technical ceramics, particularly to identify materials for applications in wear environments. While a hardness test in a metal is a measure of yield stress, in a brittle material deformation tends to be non-uniform. In addition to plastic flow, some degree of cracking and spalling usually occurs, and the extent of these has a significant effect on apparent hardness and the ability to make meaningful measurements.

Hardness testing on a range of quite different ceramics allows them to be ranked in order of resistance to local penetration. This can be related to other behavioural properties of a similar type, for example abrasive wear or erosion resistance. If the materials exhibit similar properties, such an interpretation may not be possible because the discrimination shown by the hardness tests may be inadequate. Uses beyond this application should be treated with caution. For example, it is recommended that hardness tests not be used as pass/fail criteria in a specification. Potential differences between observers or testing machines are too great for a high degree of confidence in the test results and lead to possible disagreement between the parties to the specification.

Hardness standard test blocks are usually supplied with the testing machine. They are essential for checking the behaviour of the machine and, in the case of Vickers and Knoop tests, also the visual criteria used by the operator for measurement. The test block should also be used to ensure that it is free from nicks or cracks which can easily occur when used on very hard materials. Very high hardness calibration blocks are recommended when testing ceramics.

Among the numerous tests, measurements, analyses and evaluation studies that our organization has carried out for businesses in various sectors, our organization also provides testing services defined in the standard “EN 843-4 Advanced technical ceramics - Mechanical properties of monolithic ceramics at room temperature - Part 4: Vickers, Knoop and Rockwell superficial hardness” with its trained and expert staff and advanced technological equipment.