Materials Hardness :Definition, Types and Testing method
- Written By: Gavin Leo
- Updated By: CoCo
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Material hardness is one of the fundamental properties of a material. It tells you if the material can resist deformation. Understanding this property is crucial for manufacturing companies like proto labs ltd.
It’s important because surface hardness and durability determines how well the product will perform for a wide range of materials.
This article explores the hardness of a material in detail, along with a couple of hardness testing methods, their applications, and how to choose the right material hardness test according to your requirements.
Material Hardness: Definition
By definition, it is any material’s ability to resist deformation, indentation or scratches. Here, deformation means permanent deformation, also known as plastic deformation. A material does not return back to its original shape in plastic deformation when a force is applied.
You can also refer to it as hardness value of a material. Also, measuring each type of hardness, i.e, scratch, rebound and indentation requires different set of tools.
Hardness is essential for the structure of the material because it impacts wear resistance, durability and performance. Hard materials have a range of materials including diamond, quartz or concrete. Materials can be naturally hard, like tungsten which is used in tool steels.
For instance, hardened steel components in machining and tooling maintain their shape and effectiveness longer than softer materials like polymers, colloids or surfactants.
Or, they can be soft metals. Like pure gold, you can bend or scratch it easily. However, surface hardening can be improved by adding silver or copper to create an alloy. Mixing soft metals together improves their hardness value.
You can also improve hardness value with heat, which works with thermosets. harden cnc metal materials. To enhance material hardness and wear resistance, the metal part is heated in processes like carburising, nitriding, carbonitriding, induction hardening.
In today’s age, modern digital networks offer carburising solutions that integrate real-time hardness data for better process optimization.
Hardness and wear resistance are key considerations in selecting materials for tools, machine parts, and protective coatings.
Different Hardness Units
Industries like aerospace, automotive and construction test a material for its hardness to ensure it can withstand extreme conditions. These industries use various scales like Rockwell HRC, Vickers HV, and Brinell HB to compare material hardness.
The type of hardness testing results in different hardness units.
Hardness is often measured in mm Newton per square (N/mm²) or n mm Newton per (N/mm²), depending on the test method. It is similar to that of pressure. So, you can use Pascal for material hardness too. Some commonly used hardness units include:
Brinell Hardness Number (BHN): You can calculate the brinell hardness number by dividing the load applied with the surface area of the indentation.
Rockwell Hardness Number (HRA, HRB, HRC, etc): HR determines hardness by measuring net increase in indentation depth of a major and minor load.
Vickers Hardness Number (HV): Uses a diamond pyramid-shaped indenter to measure small materials. It is typically reported in units of kgf/mm² or N/mm².
Knoop Hardness Number (HK): Similar to Vickers but designed for very delicate materials and coatings.
Leeb Hardness Value (HLD, HLS, HLE, etc): A dynamic test that measures rebound velocity to determine hardness, useful for on-site testing of large components.
Each of these units provides valuable insights into a material’s mechanical properties, helping engineers determine the best materials for specific applications.
Equivalency Table of Material Hardness
You can compare different hardness scales using an equivalency table. Hardness conversions between HB, Rockwell HRC, and Vickers HV allow for standardization across industries.
This table provides approximate conversions between Brinell, Rockwell, Vickers, and Leeb hardness values for various materials.
| Brinell Hardness (HB) | Rockwell C (HRC) | Vickers Hardness (HV) | Leeb Hardness (HL) |
| 100 | 10 | 105 | 500 |
| 150 | 15 | 160 | 550 |
| 200 | 20 | 210 | 600 |
| 300 | 30 | 310 | 700 |
| 350 | 35 | 360 | 750 |
| 250 | 25 | 260 | 650 |
This table serves as a quick reference to compare different hardness values and helps in selecting the most suitable materials for industrial and engineering applications. And a larger number, regardless of unit, will show greater material hardness.
Types of Hardness Test Methods
There are several established methods for testing material hardness, each suited to different materials and applications. All of these methods indent the material and measure plastic deformation.
If a material has high material hardness, it will have lower deformation than a material with low material hardness. One common thing between all these methods is that you use a certain indenter to check ultimate tensile strength of any test subject.
Brinell Hardness Test
A hardened steel ball with an indentation diameter of around 10 mm (for softer materials) or a tungsten carbide ball (for harder materials) is the indenter. The unit of brinell hardness value changes from HB/HBN to HBW when you use tungsten.
The indenter is then pushed into into the material’s surface with a heavy force, for about 30 seconds. Force applied depends on your test material. The dent caused is then observed with a microscope and you determine the size by measuring the indentation as an average of lengths taken at right angles.
Then, you calculate the hb brinell hardness number (BHN) as mentioned previously. The HB Brinell hardness kg/mm² value helps determine load capacity for engineering applications.
Brinell hardness test is suitable for softer materials like cast iron and aluminum, as well as materials with rough surfaces. It is commonly used in industrial applications that require measuring large components.
Rockwell Hardness Test
Rockwell hardness test is the most commonly used method to measure the hardness of a material because there are plenty of scales to choose from.
The Rockwell C HRC hardness scale is commonly used for testing hardened steel components in industrial applications. Rockwell hardness scales mean the various types of the combination of indenter and load used.
The HRB Rockwell B MPa scale is used for softer materials, while HRC Rockwell C MPa is ideal for hardened steels.
To improve accuracy of the rockwell hardness test, you can remove any irregular surface on the indenter by applying a small load first. The minor load helps to connect the indenter with the test material perfectly.
Then, just like you did before, in the rockwell test, force is applied on the major load to test your part’s hardness and material hardness is measured. The rockwell hardness values are then read directly from a testing machine known as dial gouge.
The rockwell hardness test is widely used for metals and plastics due to its speed, simplicity, and accuracy.
Vickers Hardness Test
The vickers test is mainly used for softer materials because it provides better accuracy with them. Calculating the hardness of materials through the vicker test is easier because the same indenter (diamond) is used for all test loads.
For areas with microscopic structure, a magnifier is used. The diamond indenter is a pyramid with 4 sides and is inserted into the part, after which the machine reaches a predetermined force value. Then, the indentation depth is measured and vickers hardness value is calculated.
Knoop Hardness Test
The Knoop hardness test, also known as microhardness test method, is similar to the Vickers method but uses an elongated diamond-shaped indenter to determine the hardness of materials.
It is specifically designed for testing very thin materials, coatings, and micro-components. This material hardness test is used when indentations are close to each other.
The Knoop hardness number (HK) is calculated by measuring the indentation’s long diagonal, making it ideal for low-force testing on delicate materials.
Leeb Hardness Test
The Leeb test is a dynamic hardness test that uses an impact device to measure the rebound velocity of an indenter. It is also called leeb rebound hardness testing. The Leeb hardness value (HL) is calculated based on the energy loss upon impact.
In the Leeb hardness test, an impact device stimulates an impact body towards the test surface using spring force. This process is divided into three key phases:
Approach Phase – The spring force accelerates the impact body in the direction of the test surface.
Impact Phase – The impact body makes contact with the test specimen, causing plastic or elastic deformation. The impact body comes to a complete stop as the material absorbs the energy.
Rebound Phase – The elastic properties of both the impact body and the specimen cause the impact body to bounce back.
A magnet inside the impact device generates an induced voltage as it moves through a coil, enabling a contact-free velocity measurement. The electronic system records the peak values at both the impact and rebound phases.
The hardness number is then calculated using the ratio of rebound velocity (vr) to impact velocity (vi), multiplied by a factor of 1000. This measurement provides an accurate assessment of a material’s hardness.
This method is ideal for large, heavy components and on-site hardness testing, especially when portability is required.
Scleroscope Test
Scelerscope is a device used to measure dynamic hardness of a material. In this test, a hammer is dropped onto the material surface, and the height of the hammer’s bounce is measured to determine hardness.
For harder materials, bounce will be higher. Soft metals have a smaller bounce because a dent is created on the test surface as impact energy is ending.
The scleroscope test is primarily used for surface hardness assessments, particularly for materials that cannot be tested using standard indentation methods.
Each hardness test has its own advantages and applications. Choosing the right test, , whether its brinell, rockwell or vickers, depends on factors such as material type, surface condition, sample size, and accuracy requirements.
Hardness Testing Loads
The amount of force applied during a hardness test is called ‘load’. A hardness testing device is used to apply force, f n/kgf. The units are test force f n and kilograms force kgf. Applied force in kilograms significantly affects the accuracy and reliability of the results.
Different hardness tests require specific load levels to give accurate results. However, vickers hardness test is used on both macro and micro scales.
Brinell hardness test: Uses heavy loads (500–3,000 kgf) applied through a hardened steel ball to test materials like cast iron and aluminum alloys.
Rockwell hardness test: Applies a minor load (15 kgf) followed by a major load (60, 100, or 150 kgf), making it ideal for various metals and plastics.
Vickers test: Uses very light loads (1 gf–1 kgf 1 gf–120 kgf 10 gf–100 kgf mm or n) to measure small materials.
Knoop hardness test: Use very light loads (from a few grams to 50 kgf) to measure small or thin materials precisely.
Leeb hardness test: Uses an impact device with adjustable force, depending on the material and size of the tested part.
Selecting the right testing load ensures accurate and repeatable hardness values, allowing engineers to compare materials reliably.
Hardness Testing Standards
Various organizations have established standards to ensure consistency and accuracy in hardness testing. Two widely accepted standards are ASTM (American Society for Testing Materials) and ISO (International Organization for Standardization)
These standards help manufacturers and laboratories follow uniform procedures for measuring material hardness. Some common ASTM and ISO standards are:
ASTM E10: Specifies the method for the brinell hardness test, ensuring accurate results for cast metals and heavy components.
ASTM E18: Outlines procedures for the Rockwell hardness test, commonly used in industrial applications.
ASTM E92: Defines guidelines for the Vickers hardness test, ideal for thin materials and microhardness testing.
ISO 6506: Similar to ASTM E10, it is a standard method for the brinnell hardness test.
ISO 6508: Establishes international standards for Rockwell hardness testing, ensuring global compatibility.
ISO 6507: Sets standards for the Vickers hardness test, focusing on precision and consistency.
ISO 4545: Covers knoop hardness testing.
Following these testing standards ensures that hardness measurements are accurate, reproducible, and comparable across different industries and testing facilities.
Why You Should Perform Hardness Tests?
Performing hardness tests is essential for ensuring that materials meet performance and safety standards. Testing before and after induction hardening and flame treatments helps verify the material’s structural integrity. Here are key reasons why hardness testing is important:
Quality Control: Hardness testing helps manufacturers ensure their materials meet the required specifications before production.
Material Selection: Engineers can compare different materials and select the best one based on its hardness value.
Wear Resistance Assessment: Hardness tests provide insight into how well a material will withstand friction and wear over time.
Failure Prevention: Testing helps identify weak materials that may fail under stress, reducing the risk of unexpected failures in machinery and structures.
Process Optimization: Heat treatment processes like carburising, nitriding, and induction hardening can be adjusted to achieve the desired hardness levels.
Repeated hardness checks enable industrial operations to increase material durability through process optimization which leads to superior product performance.
Conclusion
Industries using durable materials must perform correct hardness tests in order to understand material resistance. These tests help engineers select the best materials, optimize manufacturing processes, and prevent failures in critical components.