Material Hardness


There are different ways to define hardness depending on the material. For minerals, it is the property of resistance of a material to scratching by another material. Metallurgy defines hardness as the ability of a material to resist significant plastic deformation. The greater the hardness of the material, the greater resistance it has to deformation. The term hardness also refers to resistance to scratching, bending, or cutting. In this article, you will learn about the types of hardness, common tests, and industry applications.

Types of Hardness

The three common types are scratch, rebound, and indentation hardness. Measurement of each type of hardness requires a different set of procedures and tools.  The same material may have different hardness values for each of these hardness types.

Scratch Hardness

As the name implies this type of hardness refers to a material’s ability to resist surface scratches. The scratches constituing sharp narrow lines of indentations in the upper layer of the object when it rubs with a harder, sharp object. Scratch testing often applies to brittle materials such as ceramics as they are easy to scratch but do not undergo significant plastic deformation.

scratch hardness machine
Courtesy: Gardco

Rebound Hardness

Rebound hardness, also known as dynamic hardness, measures the ability of the material to absorb the energy on impact and return it to the indenter. This type of hardness is often measured by striking a diamond-tipped hammer on the test object and then measuring the bounce-back of the hammer. The nearer the height to the original striking height, the higher is the value for rebound hardness.

rebound hammer test machine
Courtesy: Gharpedia

Indentation Hardness

Indentation hardness gives the measure of the resistance to permanent deformation when subjecting an object to a continuous load.  Measuring indentation hardness is of primary interest to engineers as continuous loading is the most common form of loading metal objects in real-world applications.  

indentation hardness machine
Courtesy: Research Gate

Common Hardness Tests

There are many types of hardness tests. Each possesses advantages and disadvantages depending on the applications. Each test has different standards which give the procedures and tools for the test in detail. The Rockwell, Vickers, Knoop, and Brinell test are four common tests.

Rockwell Test

The metal manufacturing processes commonly use the Rockwell test to determine the hardness of the material. To calculate the Rockwell hardness value, technicians measure the depth of an indent after load applies to the object.  This is a quick test that is applicable for metallic objects of large geometries.

The Rockwell Hardness value represents the difference in depth from the zero load position to the high load position. The Rockwell Hardness test provides economical and reliable hardness results for metals, alloys, and plastics. 

rockwell hardness test
Courtesy: Buehler

Vickers Test

Vickers hardness test is suitable for all types of solids including metal objects. This test measures the diagonal of an indent made by a diamond-tipped indenter with a given load. Vickers is a macro-indentation test and applies to a wide range of materials.  Measurement of the diagonal indent determines the value of hardness. This test is popular for hardness testing of all solid material, including metal objects, for a wide range of applications, including evaluating the hardness of welds.

vickers hardness test
Courtesy: Tec Science

Knoop Test

The Knoop test is mainly used to measure cracking tendency in brittle materials. The indenter is an asymmetrical pyramidal diamond tip. Measuring the diagonal indent by a high-powered microscope or by automatic image analyzers gives good results. These results are particularly useful for hard and brittle materials, such as ceramics, and other surface coatings.

knoop test
Courtesy: Buehler

Brinell Test

This test is useful for materials with a rough grain structure. The Brinell Hardness test leaves quite a relatively large mark, which uses a tungsten carbide ball. The most convenient and quick way is to read the indent is by optical instruments. This test is suitable for forgings and castings of large structural objects such as large diameter piping and engine castings.

After the application of the load, measuring the resultant recovered round impression by using an optical microscope or automatic measuring device gives the hardness value.

Brinell testing is usually used in testing aluminum, copper alloys, cast irons, and steel objects. . Hardened steel or other materials are not tested by the Brinell method.

brinell test
Courtesy: Chegg

Applications in Industry

Materials behave differently under different types of loading. For example, some metals may take a huge one-time impact extremely well but may not act in the same way during continuous loading. For each case, hardness testing is necessary to ascertain the material strength for a particular object.

Metals must often withstand high pressure or extreme temperature during their use, and hardness testing provides an ideal way to determine whether the component can perform in such extreme conditions. For components such as bearings and gears, the hardness, and the corresponding compressive strength, of the surface material, must be above a critical value for them to have a long working life. The component with the surface, which extends outside the close contact area, needs to be the hardest of the two, to avoid any indentation developing at the edge of the contact. Shafts and thrust collars must therefore generally be harder than their associated support bearings.

Hardness testing is quite critical for ensuring quality control during manufacturing. Knowing a material’s hardness helps to determine if additional heat treatment is necessary to strengthen the object.

Consider the case of an engine cylinder liner. Scratching of a cylinder liner occurs because the liner’s surface comes in contact with a variety of materials such as grime, foreign objects, piston rings, or contamination in lubrication oil. Good hardness of the liner will ensure a long life of the engine. Another example is that of the screwdriver tip which is made of hard steel so that it does not deform easily.