Hardness is a material’s quality to withstand localised deformation. It may be especially important when looking for a suitable material for an environment that includes little particles that can induce material wear. Soft materials suffer indentations while hard ones resist to any change in shape.
Hardness should be viewed in context with other material properties like strength, elasticity, etc. For example, many hard materials tend to be brittle, limiting their use-cases.
In this article, we will take a closer look at what material hardness is, how it is measured and how to compare the different units.
What Is Material Hardness?
Hardness is the measure of a material’s resistance to localised permanent deformation. Permanent deformation is also called plastic deformation. While elastic deformation means that a material changes its shape only during the application of force, a resulting plastic deformation means that the material will not return to its original shape.
Some materials are naturally hard. For example, tungsten is an incredibly hard metal that finds use as an alloying element in tool steels. This makes sure that this group of steels can resist wear even at high temperatures during cutting operations.
On the other hand, some materials, including metals, are soft to the point that renders them useless for many applications. Pure gold is so soft that scratching or bending it does not need much effort. Therefore, adding other metals like silver, copper and aluminium is essential to improve its hardness.
With some materials, heat treatment is a possibility to induce greater surface hardness while maintaining the other qualities of the metal in its core. Machine shafts often undergo this process to guarantee a longer lifespan.
An engineer must also consider the ratio of hardness when creating a design concept. For example, in a bearing and shaft fit, the bearing has to be softer because they are easier to replace. With constant movement, one part has to wear and the choice is up to the engineer.
Types of Hardness
Materials behave differently under different types of loading. For example, a metal that can take a huge one time impact extremely well may not act the same during continuous loading.
Hardness testing must be carried out for each case so that a well-informed choice can be made for the application.
The three types of hardness are scratch, rebound, and indentation hardness. Measuring each type of hardness requires a different set of tools. Also, the same material will have different hardness values for each of the above-mentioned types.
This hardness type refers to the resistance to permanent deformation when subjecting a material to a continuous load.
Indentation hardness is what engineers and metallurgists usually refer to when they talk about hardness. Measuring its value is of primary interest as continuous loading is the most common form of loading metals are subjected to.
This type of hardness refers to a material’s ability to resist scratches on the surface. Scratches are narrow continuous indentations in the upper layer due to contact with a sharp, harder material.
Scratch testing is also commonly used for brittle materials such as ceramics as they do not undergo significant plastic deformation. It is important to consider scratch hardness as some material applications are highly sensitive to scoring.
Consider the case of an engine cylinder liner as an example. Scratching or scoring can take place due to various reasons. The liner’s surface comes in contact with a variety of metals such as piston rings, foreign particles in fuel or lubrication oil. Sometimes improper seating of liner can contribute to it.
The abrasive particles can cause scratches that ultimately degrade the engine performance and cost more in maintenance, spare parts, and fuel consumption in the long run.
At the design stage, the right metal selection considers the hardness of materials it will come into contact with. The hardness of the liner must be greater than the materials it will interact with. This helps to avert many possible issues.
Rebound or Dynamic Hardness
Rebound hardness has more to do with elastic hardness than plastic hardness. The material absorbs the energy on impact and returns it to the indenter.
An indenter is a reference material used for hardness testing. Dynamic hardness is usually measured by dropping a diamond-tipped hammer on the test piece and recording the hammer’s bounce after it strikes the surface.
The closer the height to the original dropping height, the higher the value for rebound hardness.
Different Hardness Units
The SI unit of hardness is N/mm². The unit Pascal is thus used for hardness as well but hardness must not be confused with pressure.
The different types of hardness discussed above have different scales of measurement. For scratch, indentation, and rebound hardness, the measurement methods vary (e.g. Brinell, Rockwell, Knoop, Leeb, and Meyer). As the units are derived from these measurement methods, they are not suitable for direct comparison.
However, you can always use a conversion table for comparing the Rockwell (B & C), Vickers and Brinell values. Tables like these are not 100% accurate but give a good indication.
In all of these scales, a hard material will have a higher hardness number.
The commonly used units for hardness measurement are:
- Brinell Hardness Number (HB)
- Vickers hardness number (HV)
- Rockwell hardness number (HRA, HRB, HRC, etc)
- Leeb hardness value (HLD, HLS, HLE, etc)
The various types of hardness are measured using different testing methods. A commonality among all methods is the use of an indenter to create the indentation on the test piece surface area. The indentation provides a tangible representation of the hardness of materials and it is easy to measure and replicate.
Harder materials will have shallow indentations and softer materials will have deeper indentations.
Brinell Hardness Test
The Brinell test was one of the first widely accepted hardness tests for indentation hardness measurement. In Brinell test, a steel ball of 10 mm diameter is used as an indenter to create an impression on the test piece to calculate its Brinell hardness number.
The ball is held in place for a predetermined time, usually 30 seconds, and a force is applied on the ball. This force will vary depending on the test metal being measured.
The standard load is 3000 kg, but it may be reduced to 500 kg for softer metals. For harder metals, a tungsten carbide ball may be used to prevent distortion of the ball. The hardness unit HB (or HBN) will be changed to HBW in case of tungsten to notify its use (Tungsten=Wolfram in German/Swedish).
On removal of the indenter, the dent is observed with a low-power microscope and the size is calculated by taking the average of the measurements at right angles.
On completion of Brinell test, the hardness number is calculated as follows:
F – force, N
D – indenter diameter, mm
d – indentation diameter, mm
Rockwell Hardness Test
Rockwell hardness test is the most commonly used method for indentation hardness measurements. The value of Rockwell hardness is accompanied by the scale used.
Depending on the material being tested, an appropriate scale must be selected. This hardness scale gives information on the type of indenter-load combination used.
There are a total of 30 scales available to choose from. This is what makes Rockwell a yardstick test to measure the hardness of a wide array of materials. Even ceramic and composite material hardness measurements are possible. The most commonly used scales are “B” and ”C”.
In Rockwell hardness test, prior to applying the testing load, a small minor load is applied to seat the indenter into the test piece and remove the effect of any surface irregularities. This provides better accuracy.
Then similar to the Brinell test, the indenter is used to create an impression on the material by applying the testing load also known as a major load. The impression is then measured for determining the hardness. A dial gauge is used to record the deformation.
The net increase in dent dimension (between the application of minor and major load) is considered for calculating the hardness value.
Specifying the speed of loading is necessary. In soft metals, varying speeds of load application can produce an appreciable difference in the final value. It is important to carefully monitor that the rate of loading is according to the standard.
The formula for Rockwell hardness is:
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
Vickers Hardness Test
The third way for measuring material hardness is using the Vickers test. This is especially suitable for softer materials that do not need high loads. With soft materials, the Vickers method provides better accuracy.
Also, calculating the hardness value is easier, as Vickers uses the same diamond indenter for all materials. Thus, tweaking the formula is not necessary.
Another important feature is the use of a magnifier, making it possible to test areas with a specific microstructure.
First, the tester has to place the part onto the machine and use the microscope to find the suitable height. Then, using the images, the correct place is determined.
The diamond indenter is in the shape of a four-sided pyramid. After touching the part, the machine soon reaches the pre-determined force value. It stays at the same load for a certain time.
Then, the measuring of the indentation takes place. Calculating the Vickers hardness value uses the following formula:
F – force, N
d – indentation diagonal, mm
Mohs Hardness Test
German mineralogist Mohs first devised the Mohs hardness test to measure the scratch hardness of materials. In this test, the material is scratched with a reference material which has a defined hardness.
A numerical hardness value is assigned to the test material based on the results of the test. Mohs hardness test uses 10 reference materials of varying hardness as the scale for the test.
The softest material used is talc (value=1) and the hardest material is diamond (value=10). Given that the references used for the Mohs scale do not have a graded increase, the Mohs scale lacks accuracy and is only a rough measure of hardness.
Today, modern scratch tests are carried out using Rockwell diamond indenters by scratching the test piece for a specific length pressed by selected load value.
A scleroscope is a device used to measure the rebound or dynamic hardness of materials. The setup consists of a hollow vertical glass tube connected to a stand. Through this tube, a diamond hammer is dropped onto the test piece and the bounce of the hammer is recorded.
The diamond hammer is dropped from a fixed height under its own weight. On coming in contact with the test piece, the hammer bounces back. This bounce will be higher for materials with higher hardness.
The bounce will be lower for a soft metal as a portion of the impact energy will be exhausted in creating a dent on the test surface. The glass tube has gradients to measure the height of the bounce. Rebound hardness is measured in shore units.