Hardness Definition

What is Hardness?

A measure of resistance to localized plastic deformation brought on by either mechanical indentation or abrasion, hardness (the opposite of softness), is used in materials research. The hardness of various materials varies; for instance, hard metals like titanium and beryllium are harder than soft metals like sodium and metallic tin or wood and everyday plastics. Strong intermolecular connections are a common feature of macroscopic hardness. Still, because the behavior of solid materials under stress is complicated, hardness may be assessed in various methods, including scratch hardness, indentation hardness, and rebound hardness. Ductility, elastic stiffness, plasticity, strength, strain, toughness, viscoelasticity, and viscosity are all factors that affect hardness. Compared to soft matter, ceramics, concrete, certain metals, and superhard materials are typical examples of hard matter.

How to Measure Hardness

Indentation, Scratch, and Rebound measures are the three basic forms of hardness evaluations. There are specific measuring scales for each of these measurement classes. Conversion tables are employed for practical purposes when converting between scales.

1. Indentation Hardness

The capacity of a material to resist deformation or penetration by a hard object under a certain force is referred to as indentation hardness. It is often determined by pressing a hard object, such as a steel ball or a diamond indenter, into the material's surface while applying a controlled load for a predetermined amount of time. The Vickers hardness test, the Brinell hardness test, and the Rockwell hardness test are three of the most popular indentation hardness tests. The Brinell test employs a hardened steel ball to produce a circular impression on the material's surface, whereas the Vickers test uses a diamond indenter to generate a square impression. A steel ball, a diamond cone, or a disc is used in the Rockwell test. to make a circle. The Rockwell test evaluates the depth of the indentation using a steel ball or a diamond cone as the indenter.

An essential mechanical characteristic of materials, indentation hardness, is frequently used to describe and compare the hardness of various materials. It is especially helpful for materials that cannot be evaluated for hardness using conventional methods, such as tensile or flexural strength, due to their fragility or thinness.

2. Scratch Hardness

The capacity of a material to withstand scratching or abrasion by a tougher material or item is referred to as scratch hardness. In contrast to indentation hardness, scratch hardness assesses a material's capacity to withstand scratching or abrasion over a greater surface area instead of merely at a single point of contact. The Mohs scale of mineral hardness, the Knoop hardness test, and the Rockwell scratch test are a few techniques that may be used to quantify scratch hardness. The Mohs scale rates minerals on a qualitative range from 1 to 10, with 10 being the hardest mineral, according to how resistant they are to scratch (diamond). The Knoop test employs an indenter with a diamond or pyramid form to scrape a tiny, predetermined area.

In contrast, the Rockwell scratch test scratches the material's surface with a tungsten carbide stylus. Scratch resistance is a crucial attribute of materials, particularly for those that experience abrasive or scratching pressures during processing or usage. For instance, their ability to withstand scratches significantly influences coatings and surface treatments' longevity and performance. The design and production of many items, including electronic displays, automotive components, and medical equipment, where the material's surface must be able to withstand wear and damage, depending on the scratch resistance of the materials.

3. Rebound Hardness

Rebound hardness is a measurement of a material's hardness based on the rebound energy of a tiny item placed onto the material's surface. It is also referred to as dynamic hardness or shore hardness. This hardness measurement method was developed based on the idea that a harder surface can bounce a dropped object with more energy than a softer surface. The Shore durometer test, frequently used to gauge the hardness of rubber, plastics, and elastomers, is the most used rebound hardness test. The Shore durometer test uses a portable device with a spring-loaded indenter and a thin, pointed tip. When the indenter is released after the instrument has been pushed against the material's surface, it bounces off. The quantity of rebound energy is measured to determine the hardness value, which is then represented as a numerical number on the Shore hardness scale. For assessing the hardness of materials that are too thin, too tiny, or too fragile to be examined using standard hardness testing techniques, rebound hardness is frequently utilized as a rapid and non-destructive alternative. Rebound hardness should only be used as a screening technique to identify materials with comparable hardness values since it is less accurate than indentation hardness or scratch hardness testing methods.

Physics Behind the Hardness

The capacity of a substance to resist deformation, scratching, or penetration by another material is referred to as its hardness. The complicated physics behind hardness depends on several variables, including the material's strength and structure, the kind and shape of the indenter or scratcher, and the applied stress or force. A substance's hardness is frequently correlated with its interior atomic structure and the tenacity of the bonds that hold its atoms together. Because breaking covalent connections is challenging, materials with strong covalent bonds, like diamonds, have great hardness. The hardness of materials having weaker metallic connections, like copper or aluminum, is lower. A material's internal structure deforms in response to a load or force, and the material resists this deformation by undergoing either elastic or plastic deformation.

When a substance deforms elastically, it does so temporarily before returning to its original shape when the load is removed, whereas a material deforms permanently in a plastic way. Indentation hardness tests, scratch hardness tests, and rebound hardness tests, based on the ideas of the material's deformation and its capacity to withstand deformation or penetration, are only a few of the methods used to gauge a material's hardness. Because different indenters or scratchers might result in various forms of deformation or damage, the kind and shape of the indenter or scratchers employed in these tests can also alter the measured hardness value. The physics of hardness is a key attribute for comprehending and defining a material's mechanical behavior. It involves a complicated interplay between the structure of the material, the kind of deformation, and the applied load or stress.

Relation Between Hardness and Stress-Strain Curve

The stress-strain curve, in contrast, illustrates a material's reaction to an external force and details its general mechanical behavior. The curve depicts the connection between the applied stress and the resultant strain in the material. It can provide details about the material's yield strength, ultimate strength, fracture behavior, and elastic and plastic deformation behavior. There can be some association between hardness and stress-strain curves, even though they characterize separate elements of a material's mechanical behavior. As an illustration, materials with higher hardness levels typically have greater yield, ultimate strengths, and potential greater resistance to fracture or plastic deformation. Nevertheless, depending on the material's structure, composition, and other characteristics, the connection between hardness and stress-strain behavior exists.