Material Hardness – Types, Testing Methods

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 a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example, hard metals such as titanium and beryllium are harder than soft metals such as sodium and metallic tin, or wood and common plastics.

Strong intermolecular bonds generally characterize macroscopic hardness, but the behavior of solid materials under force is complex; therefore, there are different measurements of hardness: scratch hardness, indentation hardness, and rebound hardness.

Hardness is dependent on ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity, and viscosity.

Common examples of hard matter are ceramics, concrete, certain metals, and super hard materials, which can be contrasted with soft matter.

What Is Material Hardness

Introduction Of Hardness

The hardness of a material is defined as its ability to withstand localized permanent deformation, typically by indentation. Hardness may also be used to describe a material’s resistance to deformation due to other actions, such as:

  • Cutting
  • Abrasion
  • Penetration
  • Scratching

Hardness is the measure of a material’s resistance to localized 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.

Cemented carbide, which finds much use in milling cutters, also often includes tungsten. These replaceable cutting tool bits lengthen the lifetime of cutting tools considerably.

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 aluminum 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 product 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

Hardness is the ability of a material to resist deformation, which is determined by a standard test where the surface resistance to indentation is measured. The most commonly used hardness tests are defined by the shape or type of indent, the size, and the amount of load applied.

There are three main types of hardness measurements: scratch, indentation, and rebound. Within each of these classes of measurement, there are individual measurement scales. For practical reasons, conversion tables are used to convert between one scale and another.

#1. Scratch Hardness.

Scratch hardness is the measure of how resistant a sample is to fracture or permanent plastic deformation due to friction from a sharp object. The principle is that an object made of a harder material will scratch an object made of a softer material.

When testing coatings, scratch hardness refers to the force necessary to cut through the film to the substrate. The most common test is the Mohs scale, which is used in mineralogy. One tool to make this measurement is the sclerometer.

Another tool used to make these tests is the pocket hardness tester. This tool consists of a scale arm with graduated markings attached to a four-wheeled carriage. A scratch tool with a sharp rim is mounted at a predetermined angle to the testing surface.

In order to use it a weight of the known mass is added to the scale arm at one of the graduated markings, the tool is then drawn across the test surface. The use of the weight and markings allows a known pressure to be applied without the need for complicated machinery.

#2. Indentation Hardness.

Indentation hardness measures the resistance of a sample to material deformation due to a constant compression load from a sharp object. Tests for indentation hardness are primarily used in engineering and metallurgy.

The tests work on the basic premise of measuring the critical dimensions of an indentation left by a specifically dimensioned and loaded indenter.

Common indentation hardness scales are Rockwell, Vickers, Shore, and Brinell, amongst others.

#3. Rebound hardness.

Rebound hardness, also known as dynamic hardness, measures the height of the “bounce” of a diamond-tipped hammer dropped from a fixed height onto a material. This type of hardness is related to elasticity. The device used to take this measurement is known as a scleroscope.

Two scales that measure rebound hardness are the Leeb rebound hardness test and the Bennett hardness scale.

Ultrasonic Contact Impedance (UCI) method determines the hardness by measuring the frequency of an oscillating rod. The rod consists of a metal shaft with a vibrating element and a pyramid-shaped diamond mounted on one end.

What Are the Different Types of Hardness Test Methods?

There are a number of different hardness test methods, each suited to a certain scale of measurement, or type of material. Therefore, different methods should be selected according to the substance being tested. The most common hardness tests are explained below:

  1. Rockwell: Developed to give a quick reading, mainly for metallic samples. The depth of the deformation made by the indenter is measured, and a deeper indentation is assigned a lower number, indicating a softer material. Two broad types of tests are used: the Rockwell superficial hardness and the Rockwell regular hardness tests.
  2. Brinell: A spherical indenter is used, and the diameter of the resulting indentation is measured to give the hardness value. It is generally used for testing larger samples, or samples that are not perfectly homogenous since it causes a larger indentation that is less sensitive to local variations in material hardness.
  3. Vickers: Uses a diamond pyramid to make a square indentation on the surface of the material being tested. The diagonal of the indentation is measured optically and is converted to the hardness value. The Vickers hardness test can be applied for micro-testing.
  4. Knoop: Specifically used to test thin or brittle materials, as it gives a meaningful reading with a shallow indentation. It uses an elongated pyramid to make the impression, with the measurement of the long diagonal used to calculate the Knoop hardness. It is used for micro-hardness testing.
  5. Mohs: A scale from 1 to 10 that provides an indication of the relative hardness of minerals based on how easily they can be scratched. A mineral’s hardness is measured by attempting to scratch it with materials of known Mohs hardness and ranking it accordingly.
  6. Shore: Is used for softer materials such as elastomers and plastics. A spring-loaded indenter (called a durometer) is pressed into the material sample, and the depth of penetration is converted to a hardness value.