Material hardness is an essential property that indicates how well a material can withstand mechanical forces without undergoing significant deformation. It is a critical attribute in manufacturing and engineering, affecting not only the performance and lifespan of products but also directly influencing the efficiency of the production process and the quality of the final products. In this article, we will take a closer look at what material hardness is and how it is measured.
What Is Material Hardness?
Material hardness refers to the ability of a material to resist localized plastic deformation (permanent deformation). In other words, it measures how resistant a material is to shape changes in the presence of external forces like scratching, indentation, and abrasion.
As shown in the diagram below, most plastic materials have lower hardness, while it is extremely difficult to dent or scratch a diamond. Most other materials fall somewhere in between.
Hardness can be easily confused with other material properties, such as toughness and strength, yet they belongs to three different properties of a material. Specifically, hardness signifies the surface's resistance to deformation and wear, whereas toughness refers to a material's ability to absorb energy and plastically deform without fracturing, and strength concerns the material's overall capacity to withstand tensile, compressive, and bending forces. A simple way to distinguish these properties is to remember that hardness is about surface resistance, toughness is about energy absorption and deformation, and strength is about overall load-bearing capacity.
At the same time, these properties are interconnected. For example, materials with high hardness typically have better wear resistance, meaning that using harder materials can reduce part wear and significantly extend the lifespan of components. Hardness is also closely related to a material's ability to resist deformation and fracture. Generally, materials with high hardness also have higher tensile and compressive strengths, which are often used in many engineering applications to ensure structural stability and safety. However, it is important to note that materials with high hardness may be more brittle and have lower toughness, while materials with high toughness may have lower hardness.
Types of Hardness
After understanding the definition of hardness, we can’t wait to clarify how it comes into play. Now, let’s explore the three main types: indentation hardness, scratch hardness, and rebound hardness.
Indentation Hardness
Indentation hardness refers to a material's resistance to permanent deformation when subjected to a continuous load, which is the most common loading form applied to metals. Therefore, when discussing hardness, it usually indicates indentation hardness.
It is commonly assessed using tests that measure the depth or size of an indentation left by a hard object under a specific force. The most widely used methods for measuring indentation hardness include Rockwell, Brinell , and Vickers hardness tests.
Scratch Hardness
Scratch hardness describes a material's ability to resist scratching of its surface due to contact with another metal. It is typically measured using the Mohs hardness test, which is primarily used to test the hardness of brittle materials such as minerals and ceramics.
Rebound Hardness
Rebound hardness, also known as dynamic hardness, relates to a material's elasticity and describes its ability to absorb energy upon impact and return it to the indenter.
Rebound hardness is assessed by measuring the height at which a standard hammer rebounds from the surface of the material. The Leeb hardness tester (Leeb hardness) is an instrument based on the principle of rebound hardness.
How Can Material Hardness Be Measured?
Each type of hardness provides a different perspective on a material's resistance to deformation and wear, and can be measured using various methods and scales. Before going into the measurement methods, let’s first have a general understanding of hardness units.
Different Hardness Units
As shown in the table above, we see various hardness designations like “HV”, “HB” , and “HRB.” What do they mean? Hardness values themselves do not have traditional physical units like newtons or pascals. Instead, they are given with test method designations (like HV or HB), which effectively serve as their "units" in practical use. Below are the commonly used units for hardness measurement:
HB: Brinell hardness number
HRA, HRB, HRC, etc.: Rockwell hardness number
HV: Vickers hardness number
HLD, HLS, HLE, etc.: Leeb hardness value
Common Methods for Testing Material Hardness
As mentioned earlier, hardness reflects a material's ability to resist scratching, cutting, or wearing away on its surface. It is typically measured by applying a specified load to the surface of the material. Below are the common methods for testing hardness. We outline each method's principles and primary characteristics, highlighting the advantages and limitations of each method in measuring hardness.
Method #1: Brinell Hardness Test
The Brinell hardness test involves pressing a hard steel or carbide ball, typically 10 mm in diameter, into the surface of the material under a specified load, which varies depending on the material you’re going to test. The standard load is 3000 kg (or 29420 N), usually applied to hard metals like iron and steel. But for softer materials like copper and brass, the applied load is 500 kg (or 4905 N). This load is applied for a predetermined amount of time and then removed. The diameter of the indentation left on the material's surface is measured using a microscope. The Brinell hardness number (HB) is then calculated using the following formula:
Where, F – the applied load in kilograms-force (kgf). In the SI system, Newtons (N) are commonly used as the unit of load. In this case, HB can also be denoted as HBN D – the diameter of the ball , mm d – the diameter of the indentation , mm
The Brinell hardness test is commonly used for testing large or thick materials with low to medium hardness, such as cast iron, steel, non-ferrous metals, and their alloys. It is particularly suitable for testing materials with coarse or uneven grain structures, as the large indentation averages out local inconsistencies. The test is relatively simple to perform and requires minimal sample preparation. However, the testing equipment is usually bulky and less portable compared to other hardness testing methods like Rockwell or Vickers hardness tests.
Method #2: Rockwell Hardness Test
This testing method first involves pressing an indenter into the material surface under a preload (minor load), typically 10 kgf, which helps to seat the indenter and eliminate any surface irregularities. Then, an additional major load is applied, with the total load (minor plus major) ranging from 60 to 150 kgf, depending on the scale being used. After a specified dwell time, the major load is removed, and the indentation depth is measured while still under the minor load.
The Rockwell hardness number can be read using different Rockwell hardness scales, with A, B, and C scales (HRA, HRB, HRC) being the most commonly used. Different scales are suitable for various types of materials and hardness ranges, each using different loads and indenter types. For example, to measure the hardness of harder steels and hard alloys, a diamond cone indenter with a major load up to 150 kgf is used, and the hardness number is read on the "C" scale. For softer materials like copper alloys, aluminum, and softer steels, a 1/16-inch diameter steel ball with a major load up to 100 kgf is typically used, and the hardness value is read on the "B" scale.
The formula for Rockwell hardness is:
Where， N – scale factor depending on the scale used s – scale factor depending on the scale used d – depth of permanent indentation compared to minor load, mm
This method is quick and easy to perform, and the hardness results can be directly read from Rockwell hardness scales. There are a total of 30 scales available to test different materials and hardness ranges. Therefore, this method is commonly used in quality control and material testing across various industries. However, the Rockwell test may not be ideal for extremely hard materials (e.g., ceramics) or very soft materials (e.g., rubbers).
Method #3: Vickers Hardness Test
This test uses a diamond pyramid-shaped indenter with a square base and an angle of 136 degrees between opposite faces. The indenter is pressed into the material with a specific load and held for a certain period. After unloading, a square indentation is left. The lengths of the two diagonals of the indentation are measured using a microscope or other precision measuring equipment. These measurements are then used to calculate the Vickers Hardness Number (HV) using the following formula:
Where, F – the applied load , kgf d – the average length of the diagonals , mm
The Vickers hardness test is versatile and can be used to test virtually all materials, from very soft metals to extremely hard ceramics. It provides precise hardness values and can be performed on very small areas or thin layers. Additionally, calculating the hardness value is straightforward since the same diamond indenter is used for all materials. However, this test method requires specialized equipment, such as a microhardness tester and a high-quality microscope, which are costly. Moreover, measuring the indentation diagonals is time-consuming, especially for multiple tests.
Method #4: Mohs Hardness Test
The Mohs hardness test is a scratch test. It is a qualitative method that compares a material's resistance to scratching by minerals of known hardness on a scale from 1 (softest) to 10 (hardest). Talc is the softest (1), and diamond is the hardest (10).
This method is simple and quick to perform without the need for sophisticated equipment. It’s very useful for fieldwork and quick identification of minerals and other materials. However, it only provides relative hardness values and relies on the skill and experience of the operator.
Modern scratch tests have evolved from the simple Mohs hardness test to more sophisticated methods that often use precise equipment, such as diamond indenters and controlled loading mechanisms, to provide more accurate and reproducible measurements of material hardness and wear resistance. These tests can quantify the scratch resistance of materials and are widely used in various industries, including coatings, metals, ceramics, and composites.
Method #5: Scleroscope Test
The Scleroscope test measures the rebound hardness of a material. It involves using a diamond-tipped hammer being dropped from a fixed height onto the material, and the rebound height is measured. The higher the rebound, the harder the material.
This test is a traditional but direct and non-destructive measurement, now often found in more specialized or historical contexts. In contrast, the Leeb hardness test, which is also a non-destructive test based on the rebound velocity of a test mass, has largely replaced the Scleroscope test in modern applications. The Leeb hardness test is more widely used, especially for field testing and large-scale industrial use, due to its portability, versatility, and often includes digital readings.
Material Hardness Chart
Once you know the type of load conditions that the part will undergo, you can consider any of the above hardness test methods to compare your material options. Alternatively, you can rely on the information provided by suppliers and online resources such as Matweb. Additionally, material hardness chart is also a vital reference tool for understanding and comparing the hardness of various materials. The following chart provides the hardness values of these different materials using various measurement methods.
Work with Chiggo for Hardness Test and Material Selection
If you want to choose the right material for your project, you must understand the hardness of the material. As a CNC machining manufacturer with over 10 years of experience and the capability to process a wide range of materials, we can obtain the most accurate hardness data from our reliable raw material suppliers. We also have an in-house factory and conduct regular hardness testing as an essential part of our quality control procedures.
Of course, if you have specific requirements, we can provide professional and detailed test reports. Not only do we have a team of quality control experts who can test the hardness of materials, but we also have a professional team of engineers and machinists to ensure that each project meets the highest quality standards every time.
FAQ
How can we select appropriate hardness testing method?
When selecting an appropriate hardness testing method, we should consider factors such as the type of material, its thickness, surface condition, and the required testing accuracy. Below points can be as an important reference:
For thick materials or large materials with coarse microstructures – the Brinell hardness test is suitable.
If you need a quick, direct, non-destructive measurement for a variety of materials in a manufacturing setting – the Rockwell hardness test is suitable.
When you require precise measurements for thin materials or small parts – the Vickers hardness test is suitable.
When needing to conduct on-site or non-destructive testing – the Leeb hardness test provides portability and convenience.
What’s the relation between hardness number and stress-strain curve?
The hardness number provides a quantitative indicator of a material's resistance to indentation or scratching, while the stress-strain curve offers a comprehensive demonstration of a material's deformation behavior under varying stress levels.
Although there is no direct mathematical correlation between hardness and stress-strain parameters, the hardness number can often serve as an indirect indication of a material's elastic modulus and yield strength. A material with a high hardness number generally exhibit a steeper elastic region and a higher yield point on the stress-strain curve, indicating its higher yield strength and greater resistance to deformation. In contrast, a material with a low hardness number may exhibit a more ductile behavior with a longer plastic region on the curve. To comprehensively assess the mechanical properties of materials, it is necessary to understand the relation between these two factors.
