Materials Hardness :Definition, Types and Testing method
Material hardness determines the applicability of a material for a project. Your project sometimes requires a high hardness like steel or softer materials. This is determined by the comparative ability of materials to scratch or be scratched by others.
We all know that using the wrong materials in a project could have a disastrous effect especially when the produced part is part of a larger project. Therefore, evaluating the hardness of your material before production is indispensable.
Let’s say that If you need a material to withstand high indentation or abrasion without deformation, you should choose hard materials. However, materials with high hardness are not necessarily applicable for all projects. You need to consult an expert machining company to know which is applicable for your project.
What Is Material Hardness?
Material Hardness testing measures a material’s resistance to deformation, by pressing a harder material into its surface. It is used in a number of industries for material comparison and selection, as well as quality standards 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.
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. The evaluation of hardness involves the measured resistance of the material to indention, abrasion, deformation, or machining.
Now let’s take a look at how does hardness testing works:
It 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 evaluate a hardness value of the material.
Types of Material Hardness
There are different hardness test methods, each applicable to a certain scale of measurement, or type of material. so, different methods should be selected according to the material being tested. The most common hardness tests are explained below:
Scratch hardness
The Mohs hardness scale can be used to evaluate the hardness of the material by comparing it to the list of standard materials hardness. The hardness of the material is measured on a scale between the material you barely scratch and the material you can’t scratch.
Rebound hardness
The Leeb Rebound Hardness Test (LRHT), invented by Swiss firm Proceq SA, is one of the four most commonly used methods for metal hardness testing. This portable method is mainly used to test large enough workpieces (basically more than 1kg). It is a kind of non-destructive test used to test the large workpieces weighing more than 1kg. It also measures the response factor. Rapid measurement is a key advantage of the Leeb Rebound Hardness Test, allowing for quick determination of material hardness.
It evaluate how high a diamond-tipped hammer will “bounce” off of a material after being dropped from a predetermined height. Elasticity and this kind of hardness are connected. Two tests that evaluate rebound hardness are the Leeb rebound hardness test and the Bennett hardness test.
Indentation hardness
The measured resistance of a material to deformation resulting from a continual compressive load from a sharp item is measured by indentation hardness. Rockwell, Vickers, Shore, and Brinell hardness tests are typical indentation hardness tests.
The Rockwell test measures a material’s resistance to indentation as a measure of hardness. The Rockwell test gauges how deeply an indenter penetrates a material under a heavy load (major load) in comparison to how deeply a preload penetrates (minor load). Different scales that employ various loads or indenters are identified by a single letter. The outcome is a dimensionless number denoted by the letters HRA, HRB, HRC, etc., where the last letter stands for the appropriate Rockwell scale.
Types of Hardware Unit And Methods to Measure Hardness
There are several methods to test water hardness, including titration, colorimetric, and electronic methods. Each method has its own advantages and disadvantages, depending on the specific requirements and resources available. Advanced hardness testing methods often utilize ion selective electrodes, which allow for rapid measurement and can determine changes in water hardness.
Rockwell Hardness
Rockwell hardness test is a fast testing method found for production control, with direct reading, used primarily for metallic materials. Rockwell hardness (HR) is calculated by measuring the depth of the indentation after inserting the indenter into the sample material at a given load, which reflects the measured resistance of the metal to indentation. Generally, it is used for larger geometric samples. The fast Test is mainly used for metallic materials. It can be used in advanced tests such as the Jominy (final suppression) (HRC) test.
The Rockwell hardness test is based on an inverse relationship with the additional depth measurement forced by the indenter due to the heavy total load (primary) exceeding the depth generated by the previously applied primary (secondary) load. Initially, a simple load is applied, and a zero data position is created. The primary load is then applied for a specified period of time before being removed, leaving the secondary load in place. The resulting Rockwell number represents the depth difference from the zero datum position resulting from the application of the main load.
The entire procedure can take from a few seconds to 15 minutes for plastics. Rockwell test results are obtained quickly and directly, without the need for secondary dimensional measurement requirements. The most common type of indenter is a diamond cone ground at 120 degrees for testing hardened steels and carbides. Softer materials are typically tested using tungsten carbide balls that range from 1/16 inch to 1/2 inch in diameter. The Rockwell scale is a combination of indenter force and test force.
Brinell Hardness
Brinell Hardness involves applying a constant load or force, usually between 500 and 3,000 kg, for a specified period (10 to 30 seconds) using a 5 or 10 mm diameter tungsten carbide ball. This process measures the metal’s hardness by evaluating its measured resistance to indentation, abrasion, deformation, or machining. A loading period is required to ensure that plastic flow of the metal stops. In some cases, lower forces and smaller diameter balls are used. The Brinell test, like the Knope and Vickers tests, uses only one testing power. After the load is removed, the recovered round impression is measured in millimeters using a low-power microscope or an automatic measuring device.
The Brinell test is commonly used in testing aluminum and copper alloys (at lower forces) and steel and cast iron (at higher forces). High-strength steels or other materials are not typically tested using the Brinell method, but the Brinell test is particularly useful in certain material finishes because the indentation size and heavy applied force are more tolerant of surface conditions. Brinell testers are often designed to accommodate large parts such as engine castings and large diameter tubing.
Vickers Hardness
Vickers Hardness test is a hardness test for all solid materials, including metallic materials. Vickers hardness (HV) is calculated by measuring the diagonal length of the indentation in the remaining sample material by inserting a diamond pyramid-shaped indentation with a given load, which evaluates the measured resistance of the material to indentation. Indentation diameters are measured visually to determine hardness using a table or formula.
It is used to test the hardness of all solid materials, including metallic materials.
It is suitable for a wide range of applications.
This includes a subset of weld hardness tests.
Vickers tests, also known as total indent distance tests, are used on a wide range of materials, including solid and steel components. Vickers’ distances are also less sensitive to surface conditions than Knoop test.
Knoop Hardness
The Knoop (HK) is an alternative to the Vickers test in the microhardness testing range. It is mainly used to overcome cracks in brittle materials and also to facilitate hardness testing of thin layers. The Knoop Hardness Test measures the hardness by evaluating the measured resistance of a material to indentation, abrasion, deformation, or machining. An indentation is an asymmetric pyramidal diamond, and the indentation is measured by visually measuring the long diameter.
It is used for hard and brittle materials, such as ceramics.
Suitable for small elongated areas, such as coatings.
Mohs Hardness (Mohs Scale)
You can adopt this method of testing to measure the scratch hardness. The Mohs Hardness Test, based on the Mohs scale, is performed by scratching the material against a reference material with a known hardness. Mohs hardness test uses ten reference materials with different hardness. The kind of material under test determines which kind of reference to adopt. The result is then obtained by evaluating a numerical hardness value to the test material.
Talk is the softest material used, with a value of 1. The hardest material, on the other hand, is diamond with a value of 10.
A Rockwell diamond indenter is used for the modern Mohs hardness test. The working principle of the machine is similar to this of the traditional method. It involves scratching the test piece for a specified period by a suitable load.
Shore Hardness
Shore hardness test is a method used to test the measured resistance of materials to localized deformation or indentation. It is one of many scales used to measure the durometer (or hardness) of a material, typically of polymeric materials. Shore scale measures the hardness of a variety of materials — from soft rubbers, TPEs (thermoplastic elastomers), and silicones to semirigid plastics.
Hardness values determined using the Shore A scale are relative to the hardness of other materials that have been measured using the same scale. An empirical relationship between the depth of the indentation made in one material compared to a standard of 0 to 100% of the possible indentation depth, is used to report an empirical number known as Shore A hardness.
Material Hardness Chart
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.