Methods Used to Measure Material Hardness
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,
Marcasite and pyrite both are metallic and pale yellow to brassy yellow.
Marcasite and pyrite can tarnish and be iridescent.
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).
Marcasite and pyrite both can even be found together in the same rock.
Hardening
There are five hardening processes which follow as,
Hall-petch strengthening
Work hardening
Solid solution strengthening
Precipitation hardening
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 What Is Hardness in Physics?
1. What is the definition of hardness in physics?
In physics, hardness is defined as a material's ability to resist localised plastic deformation, such as scratching, abrasion, or indentation. It is primarily a measure of a single property rather than a combination of properties. Essentially, when we say a material is 'hard,' we mean it is difficult to scratch or dent its surface.
2. How is the hardness of a material measured?
The hardness of a material is measured using specific tests that apply a controlled force to an indenter on the material's surface. The resulting indentation or scratch is then measured. Common methods include:
- Mohs Scale: A qualitative scale that ranks materials based on their ability to scratch one another.
- Vickers Hardness Test: Uses a diamond pyramid indenter to create an indentation, which is then measured optically.
- Rockwell Hardness Test: Measures the depth of penetration of an indenter under a large load compared to a preload.
- Brinell Hardness Test: Uses a hard metal ball as an indenter.
3. What are the units of hardness?
Hardness is not a fundamental physical property with a standard SI unit. Instead, its value is expressed in terms of the specific measurement scale used. For example, hardness can be given as a Brinell Hardness Number (HB), a Vickers Hardness Number (HV), or on the Rockwell scale (e.g., HRA, HRB, HRC). The Mohs scale is a unitless, relative ranking from 1 (Talc) to 10 (Diamond).
4. What are some real-world examples that illustrate the importance of hardness?
The property of hardness is critical in many applications. For instance:
- Cutting Tools: The tip of a drill bit must be significantly harder than the material it is drilling. This is why many tools are tipped with tungsten carbide or diamond.
- Protective Screens: The glass on a smartphone (e.g., Gorilla Glass) is engineered to have high surface hardness to resist scratches from everyday objects like keys and coins.
- Bearings and Gears: Components in engines and machinery need to be hard to resist wear and tear from constant friction.
5. How does hardness differ from toughness?
Hardness and toughness are often confused but describe different properties. Hardness is the resistance to surface scratching and indentation. In contrast, toughness is a material's ability to absorb energy and deform plastically without fracturing. For example, a diamond is extremely hard (difficult to scratch) but not very tough (it can be shattered with a hammer). Conversely, steel is very tough but less hard than a diamond.
6. What is the difference between the strength and hardness of a material?
The key difference lies in the nature of the force application. Hardness refers to resistance against a concentrated surface force (like a scratch or dent). Strength (specifically tensile strength) refers to the ability of the bulk material to withstand a distributed load without breaking or permanently deforming. A material can be strong but not particularly hard. For example, a steel rope is very strong under tension but its surface can be easily scratched.
7. What determines a material's hardness at the microscopic level?
At the microscopic level, a material's hardness is primarily determined by two factors:
- Interatomic Bond Strength: Materials with very strong covalent bonds, like diamond, are extremely hard because a large amount of energy is required to break these bonds and displace atoms.
- Crystal Structure: The arrangement of atoms in a crystal lattice plays a crucial role. Hardness is related to the difficulty of moving dislocations (defects in the crystal) through the material. Materials with complex structures or features that pin these dislocations tend to be harder.
