What is the Hardness of Material?
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.
Macroscopic hardness is generally characterized by strong intermolecular bonds, 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.
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.
