Redefining Surface Hardness

Material is a matter that constitutes an object. The performance of a product strongly depends on the material characteristics. 

The ability of a substance to endure localized deformation is measured in hardness. It’s especially crucial when looking for a material to use in an environment with small particles that might cause material wear. Soft materials are prone to indentations, but hard materials resist any form change.

Hardness should be considered along with other qualities such as strength, elasticity, and so on. Many hard materials, for example, are fragile, restricting their use.

We’ll look at what material hardness is, how it’s measured, and more in this article.

What is surface hardness?

From a mechanical aspect, the hardness is a measure of resistance against plastic deformation, and it is used to define whether the product material can be recovered after external deformation. 

The phrase can also refer to the material’s resistance to scratching, abrasion, or cutting. Hardness is most typically used to evaluate a metal’s capacity to withstand persistent deformation caused by concentrated applied stresses. 

The higher the metal’s hardness, the better its capacity to maintain its form in the face of external pressures.

How do hardness tests work?

A hardness test is a technique for determining a material’s hardness. The resistance of a substance to persistent indentation is measured in hardness.

There are several methods for measuring hardness, and each of these tests might reveal different hardness levels for the same material. As a result, the hardness test as a procedure might be variable, and the results of each test must be labelled to establish the type of hardness test utilized.

Hardness tests are commonly used to describe a material and determine if it is suitable for its intended function. Every hardness test requires the use of a specially formed indenter that is harder than the material being tested. 

With a given amount of force, the indenter is pushed against the test surface. To ascertain the hardness value, the size of the indent’s depth is measured.

Types of hardness test

Here are some examples of hardness test that you can conduct:

1. Rockwell hardness test

The Rockwell hardness test determines a material’s hardness by determining the permanent depth of indentation caused by a concentrated force. The harder the material is, the higher the number on the Rockwell hardness scale. 

The test is carried out by exerting a small force of 10 kg to the material’s surface using a diamond cone or a steel ball indenter. This preliminary load’s depth of indentation is recorded and utilised as a reference point.



2. Vickers hardness test

The Vickers hardness test employs a four-sided square-based pyramid indenter with a precisely determined constant force applied to the test specimen to determine the indentation’s surface area. 

First, the specimen is elevated until it makes contact with the indenter. The indenter then applies the test force on the test specimen, gradually increasing it until it achieves the prescribed amount. 

The surface area of the diamond or square shaped indentation is computed once this force is held for the proper dwell duration.



3. Knoop hardness test

A diamond/pyramid shaped indenter is placed to the test material for a predetermined dwell period in the Knoop hardness test, which is quite similar to the Vickers hardness test. 

The Knoop indenter, on the other hand, is an elongated diamond that can test fragile materials and thin layers without splitting.


4. Brinell hardness test

The Brinell hardness test involves measuring the diameter of an indentation created by a continuous focused force applied to a test specimen using a steel or carbide spherical indenter. 

The steel ball indenter is first put in contact with the material, after which a consistent force is applied and maintained for a dwell time of 10 to 15 seconds. 

The spherical indenter is removed once the dwell period is finished, leaving a round-shaped indent on the sample.


Benefits of hardness test

Here are some reasons as to why hardness tests are useful :

  • The hardness test is simple to carry out.
  • Within 30 seconds, you’ll have your results.
  • The cost of tests is pretty low.
  • Tests may be performed on finished components without causing harm.
  • Testing may be done on any shape or size of surface.

How to get a degree of resistance against plastic deformation?

In order to obtain the degree of resistance against plastic deformation, indentation tests are used to characterize the material hardness. 

Indentation tests employ the concept of forcing a hard indenter into a specimen surface. In the first contact between the indenter and specimen, plastic flow is initiated on the specimen surface due to high contact pressure. 

Further loading results in elastoplastic deformation on the material beneath the indenter. After complete loading on the surface, dwell-loading is applied on the surface in order to ensure the material is fully deformed corresponding to the load applied on it. 

The indentation is ended by load removal of the indenter from the material surface. As the material surface is deformed plastically, a residual indent is formed on the specimen surface.

In reality, all materials are characterized by the presence of surface roughness. The inevitable influence of roughness has always been a controversial topic in material hardness research. According to ISO standards, mechanical indentation requires the appliance of stringent sample surface preparation procedures. 

One of the procedure criteria is that the roughness of the surface should be not greater than fifteen percent of the indentation height. The purpose of setting such a condition is to avoid the huge variation of hardness value resulting from the roughness factor. 

However, the variation of hardness value is significant despite the surface roughness fulfilling the prescribed ISO standard. Consequently, it casts doubt on whether all materials’ hardness values should be characterized under such a sample preparation condition. 

The gap of research above motivated the creation of a new platform where all materials can be fairly characterized in indentation tests. In order to achieve it, we developed a roughness dependent mathematical model whose roughness parameter was taken into account. 

With this mathematical model, a roughness independent coefficient c0 plays an important role to define the genuine hardness value of material with zero roughness condition. 

Thus, the hardness of all materials can be characterized under a perfectly flat surface. With the exclusion of the roughness factor, the hardness values of all materials are now can be compared without the influence of roughness. 

Apart from the roughness independent coefficient c0, a dependent hardness coefficient c1 was created in the model development. This coefficient c1 is particularly useful in the evaluation of the increment of hardness value for different materials. 

Attribute to different material nature, the increment of nano-scale roughness creates various increasing hardness values for different materials in small scale indentation tests.  

Photo by Jeswin Thomas from Pexels

Final thoughts

Our current research is progressing in the roughness dependent plastic strain gradient in a small scale indentation study. Apart from that, we are delving into the alternative measurement methods for hardness testing in order to justify the logic of the ISE and seek more possibilities in the measurement.     

To be a part of this fascinating development, join UOW Malaysia KDU’s Engineering programmes, which are exposing students to different types of innovations that can help them in future prospects. 


31 January 2022


Science and Technology