The Importance of a Hardness Test in Materials Production

The hardness of a material is the property that enables it to resist deformation, bending, scratching, abrasion, or cutting. If you work in materials manufacturing, you probably already know that a hardness test is a crucial step in the production process for your products and the products you receive from third-party suppliers. You need to set baseline measurements and ensure that your products meet specific standards for strength. But if you're not a materials engineer, you may be surprised to know that the hardness of any material is not an intrinsic property dictated by precise definitions and fundamental units of mass, length, or time. Instead, the value of a material's hardness is the result of a specific measurement procedure.

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For most materials of the modern age, a hardness test has usually been performed through a device that measures depth of an indentation left by a point of a specific shape, with a specific force, and for a specific amount of time. The most common hardness tests that measure the relationship between hardness and the size of the impression left by the test are the Rockwell, Brinell, and Vickers hardness tests.

Rockwell hardness tests are the most common type of hardness test available, and for good reason: it's an incredibly fast and reliable method to determine the hardness of metals, alloys, and plastics, when compared to other methods. In the test, a diamond cone or hardened steel ball indenter is forced into the material being tested with a preliminary force that is usally 10 kgf. After equilibrium is reached, an additional load is applied which causes greater penetration of the material. Once equilibrium is reached again, the additional load is removed and the material rebounds slightly. The permanent depth of the indentation that remains is then used to calculate the Rockwell hardness number.

We offer many testers at Berg Engineering which are used to measure not only Rockwell hardness, but also Brinell, Vickers, and Knoop tests. Give us a call or use our convenient online ordering to get the tester you need today. Browse our entire selection of hardness testers.

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Berg Engineering and Sales Company has been supplying nondestructive and material testing equipment since . We represent the world's leading manufacturers of nondestructive testing (NDT) testing devices. We publish articles that cover topics of interest for current and future members of the NDT community.

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Hardness is a material's ability to resist deformation at its surface. A measurement is taken of the resulting indentation and converted to a hardness value relative to the hardness scale of the particular test. Many different standardized hardness tests exist, with each having an area of applicability to different types of materials, such as metals, ceramics, or rubbers. Measuring hardness has a number of benefits in comparing materials and providing quality control in manufacturing and hardening processes. This article will discuss hardness testing, its definition, how it works, its types, and benefits.

An Overview of Hardness Testing and How it Works

Hardness testing measures a material's resistance to permanent deformation at its surface, by pressing a harder material into it. It is used in a number of industries for material comparison and selection, as well as quality control of a manufacturing or hardening process. A specifically manufactured indenter, with chosen dimensions, is used to press into the material being tested, with a prescribed force. The pressing time is also important in the hardness test. 

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Hardness is not a fundamental physical property of a material, but rather a measured characteristic. It can however provide some valuable information about the strength and durability of a material, depending on the application it is intended for.

Hardness testing works by pressing a standardized, hard instrument into the material that is being tested. The instrument, called an indenter, is pressed into the sample for a standardized length of time (a number of seconds), and with a specific load (force) behind it. When the indenter is pressed into the material, it deforms the surface of the material by a measurable amount. This deformation is then measured, and the measurement is used to assign a hardness value to the material according to the test's specific scale.

Hardness Testing Materials

Hardness testing can be applied to various types of materials, some of which are listed below:

1. Metals and alloys.
2. Ceramics.
3. Elastomers.
4. Polymers and plastics.
5. Films.
6. Rocks and minerals.

Importance of Hardness Testing

Hardness testing is important for a number of different reasons. The hardness of a material can be a critical parameter in its application. This can be the case for metal components used in cutting and grinding or on the opposite extreme, for rubbers used for shock absorption. It is important to be able to measure and compare the hardness of materials objectively. 

Having an objective, standardized hardness value for a material is important to allow quality control during manufacturing. Each unit would need to be within an acceptable range of the specified hardness. Furthermore, these objective hardness values allow for the comparison of different materials and are therefore important for material selection to match the needs of an application.

Benefits of Hardness Testing

Hardness testing provides a valuable measure of a material characteristic with a number of benefits, including:

1. An automated hardness testing machine allows hardness to be measured relatively simply.
2. To provide meaningful hardness readings for a wide variety of materials, from soft to hard.
3. For some applications, the hardness of a material is the key characteristic that is valuable to know. This can be measured directly, rather than inferred by a physical property.

Challenges of Hardness Testing

Hardness testing does have some limitations and challenges that can affect the accuracy and repeatability of hardness results, some of which are listed below:

1. It is necessary to have properly prepared the surface of the sample to be measured, usually by grinding and polishing. The surface condition has a significant influence on the hardness reading, so improper surface preparation can give inaccurate or spurious readings.
2. Most hardness tests are designed for flat surfaces, so curved surfaces do present a challenge. There are some correction factors that need to be applied to compensate for an indentation on a curved surface.
3. The size of the indentation can be difficult to ascertain objectively, particularly when manually reading the size of micro impressions through an eyepiece. This is mainly an issue with older machines. The latest models have cameras and software packages to automatically focus correctly, and display the edge of the impression clearly on a connected computer screen.
4. If the prescribed minimum distances between indentations are not followed according to the testing standards, the results of the hardness test will be affected. This is also true of an indentation too close to the edge of a sample.

Types of Hardness Test Methods

There are a number of different hardness test methods, each suited to a certain scale of measurement, or type of material. Therefore, different methods should be selected according to the substance being tested. The most common hardness tests are explained below:

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1. Rockwell: Developed to give a quick reading, mainly for metallic samples. The depth of the deformation made by the indenter is measured, and a deeper indentation is assigned a lower number, indicating a softer material. Two broad types of tests are used: the Rockwell superficial hardness and the Rockwell regular hardness tests.
2. Brinell: A spherical indenter is used, and the diameter of the resulting indentation is measured to give the hardness value. It is generally used for testing larger samples, or samples that are not perfectly homogenous since it causes a larger indentation that is less sensitive to local variations in material hardness.
3. Vickers: Uses a diamond pyramid to make a square indentation on the surface of the material being tested. The diagonal of the indentation is measured optically and is converted to the hardness value. The Vickers hardness test can be applied for micro-testing.
4. Knoop: Specifically used to test thin or brittle materials, as it gives a meaningful reading with a shallow indentation. It uses an elongated pyramid to make the impression, with the measurement of the long diagonal used to calculate the Knoop hardness. It is used for micro-hardness testing.
5. Mohs: A scale from 1 to 10 that provides an indication of the relative hardness of minerals based on how easily they can be scratched. A mineral's hardness is measured by attempting to scratch it with materials of known Mohs hardness and ranking it accordingly.
6. Shore: Is used for softer materials such as elastomers and plastics. A spring-loaded indenter (called a durometer) is pressed into the material sample, and the depth of penetration is converted to a hardness value.