According to the RAE, “hardness” is “the quality of being hard”. And “hard” can be understood as “the body that resists being worked, scratched, compressed or disfigured, that does not lend itself to being reshaped or makes it very difficult to reshape”. In other words: hardness is a resistance of a material to physical alterations such as scratching or penetration.
Hardness tests is one of the most widely used methods in industry to determine the quality of materials such as metals. In this post we explain what these tests consist of, what the most common types are and how they are carried out in chemical laboratories.
Hardness test: definition
A hardness test is the examination of different materials to evaluate their resistance to scratching or penetration by other substances. In general, the higher the hardness of the surface of the parts, the better their behavior against wear, abrasion or deformation in their interaction with the environment.
It is undoubtedly of great importance to carry out this type of practice to measure the hardness of materials, as it provides in-depth knowledge of the properties of the material and, consequently, whether a particular treatment is suitable or not.
How is the hardness of materials measured?
When asked how hardness is measured, it should be noted that hardness tests can be distinguished according to whether they measure scratch resistance or penetration. In both cases, certain materials are applied to the surface of the sample to estimate its hardness. The difference between the different methods lies in the nature of the procedures used.
How do we know which test method to use?
But how do we choose the hardness test for a material? We find different types of methodologies, so it is important to take into account the following points when selecting them:
- Type of material
- Hardness of the material (approximate)
- Size of the object or part
- Number of samples to be tested
- Whether a certain standard has to be met
- Accuracy of the result
- Homogeneity or heterogeneity of the material
- If deep drawing is required
Remember: at Infinitia we have the necessary equipment to test materials. One of our strengths is that we carry out customized tests, completely personalized for our customers.
Types of hardness testing
Now that you know what a hardness test is, let’s take a look at the main types.
Scratch hardness testing
Did you know that diamond is the hardest natural element? A diamond can only be scratched by another diamond. Let’s get to know the different types of hardness tests to check the scratch hardness unit.
Mohs scale
Talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum and diamond. This is the Mohs scale. The first scale, designed by Mohs himself in 1820, on the value of hardness in minerals.
It is the first method used to measure the hardness of materials. It works with a numerical scale consisting of 10 minerals, each representing a degree of hardness. Different substances are classified on this hardness scale according to the number of minerals that can scratch their surface. However, it is a somewhat imprecise method, as it does not work with metals.
Martens hardness test
The Martens hardness test is a type of hardness test, in which surfaces are scratched with a diamond cone driven by a fixed force, making it suitable for tougher substances. The hardness of materials is calculated by measuring the width of the scratch produced by the diamond tip. Thus, this Martens method makes it possible to measure scratching in metals.
Penetration hardness testing
How is hardness measured? Another way to give an answer is according to the shape of the indenter and its application. In this regard, we can distinguish mainly between the Brinell, Vickers and Rockwell methods.
Brinell hardness test
The Brinell hardness test can be used with extremely soft materials and ferrous metals, provided that their surface is not spherical or cylindrical. Thus, this hardness test allows the use of large specimens with irregular or large grain structure. The indenter object is a small steel ball (1-10 mm) which is applied with a load of a force determined for each type of material. The hardness is obtained by measuring the diameter of the indenter with a microscopic magnifying glass. The name for the Brinell hardness units is HBW.
Vickers hardness test
In this Vickers test, the indenter is a quadrangular diamond pyramid with an angle of 136° between the opposite faces. This makes the Vickers method suitable for use on spherical surfaces and solid materials (including metals) of higher hardness than the Brinell tests. The final result is obtained by dividing the force used by the area of the mark produced by indentation.
Rockwell hardness test
The Rockwell scale is the most common test for defining the hardness of metals because of its simplicity, speed and ease of use. Rockwell hardness scales are calculated with a machine that measures the depth of penetration on the surface of the sample, leaving a smaller trace than the other procedures. For soft materials, the Rockwell method uses a hardened steel ball as an indenter, while for hard materials a 120° diamond cone is used.
When are hardness tests performed?
Although they can be performed on all types of substances and in various production sectors, the most common application of these tests is currently to characterize the degree of strength and hardness of metals in the metallurgical industry, which results in greater reliability for final constructions.
Hardness testing of metals and other materials provides fundamental information on properties such as strength, ductility or resistance to plastic and elastic deformation. In this way, they function as a quality control mechanism in production processes, helping manufacturers to select the most suitable compounds for each use.
If you want to improve the procedures in your company, in our forensic engineering service we carry out the necessary hardness test for your project, in order to get the best materials. And if you want to stay informed about everything that happens in the industry, do not hesitate to keep visiting our blog.