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Last updated date: 21st Apr 2024
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Hardness Definition

Hardness is a metric that measures how resistant a material is to localised plastic deformation caused by mechanical indentation or abrasion. It has important diagnostic properties in mineral identification or abrasion. There is a general bounding between hardness and chemical composition, thus most hydrous minerals like halides, carbonates, sulfates, and phosphates are relatively soft. Sulfides are relatively most soft (two exceptions being marcasite and pyrite) and silicates are hard and most anhydrous oxides. In general, the different materials have different hardness. For example, hard metals like titanium and beryllium are harder than soft metals like sodium and metallic tin, or wood and normal plastics. Powerful intermolecular bonds are commonly used to identify macroscopic hardness, but the structure of solid materials under stress is more complicated. In addition, there are different measurements of hardness such as scratch hardness, indentation hardness, and rebound hardness.

Hardness is based on plasticity, ductility, elastic stiffness, strain, strength, toughness, viscosity, and viscoelasticity. For example, polymers and elastomers, it is defined as the resistance to elastic distortion of the surface.

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Scratch Hardness

Scratch hardness is a measurement of a sample's resistance to fracture or permanent plastic deformation caused by pressure from a sharp edge. An object made by a tougher material will scratch an object made by a softer material, according to the theory. Scratch hardness refers to the force used to break through the film to the substrate when examining coatings. Sclerometer is a tool that is used for the measurement of scratch hardness.

Indentation Hardness

The resistance of a sample to material deformation caused by a steady compression load from a sharp object is measured by indentation hardness. Indentation hardness test is primarily used in engineering fields and metallurgy fields. Indentation tests are based on the principle of calculating the essential dimensions of an indentation created by special dimensions and loaded indenter.

Rebound Hardness

Rebound hardness is the type of hardness that is related to elasticity. The height of the "bounce" of a diamond-tipped hammer falling from a set height into a substrate is measured by rebound hardness, also known as dynamic hardness. The rebound hardness test and the bennett hardness scale are two scales that measure rebound hardness. The ultrasonic contact impedance (uci) method determines the hardness by calculating the frequency of an oscillating rod. A metal shaft with a vibrating part and a pyramid-shaped diamond preparation on one end make up the oscillating rod.

Marcasite and Pyrite in Hardness

Marcasite and pyrite are two general minerals. Both of them are fes2 chemicals, making them Polymorphs. Polymorphs are also minerals with the same chemical composition but different crystal structures. Diamond and graphite, both minerals being pure carbon and both are polymorphs. Diamond and graphite have different arrangements of carbon atoms giving these two minerals very distinct physical properties. Marcasite and pyrite, on the other hand, also have identical physical properties, making them tough to tell from each other.

Let's Discuss Their Properties,

  1. Marcasite and pyrite both are metallic and pale yellow to brassy yellow. 

  2. Marcasite and pyrite can tarnish and be iridescent. 

  3. Generally, both have densities of about 5 grams per cubic centimetre (pyrite is a bit denser, but not enough to be detectible without delicate calculation).  

  4. Marcasite and pyrite both can even be found together in the same rock. 


There are five hardening processes which follow as,

  1. Hall-petch strengthening

  2. Work hardening

  3. Solid solution strengthening

  4. Precipitation hardening

  5. Martensitic transformation.

Since there is no universal concept for hardness, it is assumed that it is a composite property that includes contributions from yield power, work hardening, true tensile strength, modulus, and other variables. The hardness of a surface can be determined using several different techniques.

Method of Hardness

The following are a couple of the more traditional approaches.

Mohs Hardness Test

German mineralogist friedrich mohs in 1812 was devised one of the oldest ways of measuring hardness. The mohs hardness test involves observing whether a substance of known or specified hardness scratches the surface of a material. Minerals are graded around the mohs scale, which is made up of ten minerals with arbitrary hardness values, for assigning numerical values to this physical property. Although the mohs hardness test is useful for identifying minerals in the environment, it is not appropriate for determining the hardness of industrial materials such as ceramics or steel. Mohs hardness can be measured on a micro or nanoscale.

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Brinell Hardness Test

The brinell hardness test is the most popular hardness test tool used on engineering materials. Dr j. A. Brinell was discovered the brinell test in sweden in 1900. The brinell test uses laptop computers to applying a specified load to a hardened sphere of a specified diameter. The brinell test number, or simply called the brinell number, is obtained by dividing the load used, in kilograms, by the measured surface area of the indentation, in square millimetres, left on the brinell hardness test surface. The brinell hardness test gives measurement over a fairly large area that is less affected by the coarse grain structure of these materials this is rockwell hardness or vickers hardness tests.

Rockwell Hardness Test

The rockwell hardness test uses a machine to apply a specific load and then measure the depth of the resulting impression. The indenter may either be a steel ball of some fixed diameter or a spherical diamond-tipped cone of  0.3 mm tip radius, and 120° angle called a brale. A light load of 10 kg is added first, causing a slight initial penetration to seat the indenter and eliminate any surface irregularities. The main load is added after the dial is reset to 0. The depth reading is taken when the main load is still on after the major load has been removed.

Rockwell Superficial Hardness Test

The rockwell superficial hardness test is used to test thin materials, lightly carbonised steel surfaces or parts that might be angled or mangle under the conditions of the regular test. This test uses the same indenters as the standard rockwell hardness test but the loads are reduced. A minor load of 3 kilograms is used and the major load is either 15 or 40 kilograms depending on the indenter used.

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Vickers and Knoop Microhardness Test

The vickers and knoop hardness tests are an updated version of the brinell test and this is used to measure the hardness of thin-film coatings or the surface hardness of case-hardened parts. Vickers and knoop microhardness test consist small diamond pyramid is pressed into the sample under loads that are less than those used in the brinell hardness test. The shape of the diamond pyramid indenter is the only distinction between the vickers and knoop experiments. A square pyramidal indenter is used in the vickers test, which is resistant to cracking brittle materials. On the other hands, the knoop test using a rhombic-based pyramidal indenter which produces longer but shallower indentations.

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Durometer Hardness Test

A durometer is a device that is simply used for measuring the indentation hardness of elastomers or rubber and soft plastics such as polyolefin, vinyl, and fluoropolymer. A durometer hardness test commonly uses a calibrated spring to apply specific pressure to an indenter foot. The indenter foot can be either cone-shaped or sphere-shaped. An appropriate device measures the depth of indentation. Durometers are always available in a variety of models and the most popular testers.

Barcol Hardness Test

The barcol hardness test obtains a hardness value by calculating the penetration of a sharp steel point under a spring load. The uniform pressure is applied until the dial indicator reaches a maximum. The barcol hardness test method is used to decide the hardness of both reinforced and non-reinforced rigid plastics and to determine the degree of cure of resins and plastics.

FAQs on Hardness

Q.1) What does Hardness Mean in Physics?

Answer: hardness is a measure of a material’s resistance to being scratched and is measured using various experimental techniques, including the rockwell tests, barcol test, durometer test, brinell test, mohs test, and vickers and knoop tests. Therefore, the values obtained often depend on the testing method of the knoop diamond. For example, koop diamond is sharper than the vicker’s and gives a lower hardness.

Q.2) What is the Difference Between Hardness and Toughness?

Answer: hardness refers to a material's ability to tolerate friction or abrasion resistance. Diamonds are the hardest substances known to man, it is incredibly hard to scratch a diamond. Therefore, while a diamond is hard but it is not tough. If you took a hammer to a diamond it would crack, which demonstrates that not all materials that are hard are also tough.

Whereas, toughness requires strength and ductility, which allows a material to deform before fracturing. It is, believe it or not, relatively tough in these words, since it can stretch and twist rather than crack. Commonly, a single material will have more than one of these properties.

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