Which is harder, granite or marble, nickel or aluminum? And what is hardness anyway? We will try to answer these questions in our article. A number of foreign scientists de alt with the problem of determining the hardness of minerals and substances. Among them are Albert Schor, Friedrich Moos, Johan August Brinell, William Vickers and others. However, the only and generally accepted method for calculating hardness in science still does not exist.
What is hardness?
Each of the substances known to science has a number of specific physical properties and qualities. This article will discuss what hardness is. This is the ability of a material to resist the penetration of another, more durable body into it (for example, a cutting or piercing tool).
The hardness of substances is most often measured in special units - kgf/mm2 (kilogram-force per square millimeter of area). It is designated in Latin letters HB, HRC or HRB, depending on the chosen scale.
The hardest mineral on Earth is diamond. If we talk about materials of artificial origin, then the most durable is fullerite. It is a molecular crystal that forms at high temperatures (about 300 degrees Celsius) and extremely high pressures (over 90,000 atmospheres). According to scientists, fullerite is about one and a half times harder than diamond.
What is hardness?
There are three main hardness options:
- Surface (determined by the ratio of the load to the surface area of the print).
- Projection (the ratio of the load to the projection area of the imprint).
- Volume (load to print volume ratio).
Besides this, the hardness of physical bodies is measured in four ranges:
- Nanohardness (Load less than 1 gf).
- Microhardness (1 – 200 gf).
- Hardness at low loads (200 gf - 5 kgf).
- Macrohardness (more than 5 kgf).
Hardness of metals
Out of 104 elements of Mendeleev's periodic system, 82 are metals. And the total number of alloys known to man reaches five thousand! The scope of metals in the modern world is incredibly wide. These are the military and chemical industries, metallurgy, electrical engineering, space industry, jewelry, shipbuilding, medicine, etc.
Among all the physical and chemical characteristics of metals, hardness is far from the lastrole. After all, she clearly demonstrates:
- degree of wear resistance of the metal;
- pressure resistance;
- its ability to cut other materials.
Among other things, the hardness of a metal shows whether it can be processed on certain machines, whether it can be polished and the like. By the way, scientists have long proven that the hardness of a metal largely determines its other mechanical properties.
What is the hardness of iron, copper and aluminum? And which metal is the hardest and most durable?
Magnesium and aluminum are among the softest metals. Their hardness values vary within 5 kgf/mm2. About twice as hard - nickel and copper (about 10 kgf/mm2). The hardness of iron is estimated at 30 kgf/mm2. Well, the hardest metals of natural origin include titanium, osmium and iridium.
Determination of hardness: methods, methods and approaches
How is the hardness of a physical body measured? To do this, a so-called indenter is introduced into the sample. Its role can be played by a heavy-duty metal ball, pyramid or diamond cone. After the direct contact effect of the indenter, an imprint remains on the test sample, the size of which determines the hardness of the material.
In practice, two groups of methods for measuring hardness are used:
- Dynamic.
- Kinetic.
In this case, the applied load during the introduction of the indenter into the body can be carried outby scratching, indenting (most often), cutting or rebounding.
Today there are several different approaches to determining hardness:
- Rockwell;
- Brinell;
- according to Vickers;
- by Shore;
- according to Mohs.
Accordingly, there are a number of different scales of hardness of materials, there is no direct relationship between them. One or another measurement method is chosen based on a number of factors (for example, the properties of a particular material, the conditions of the experiment, the equipment used, etc.). Devices that determine the hardness of metals or minerals are commonly called hardness testers.
Rockwell method
The Rockwell hardness value is determined by the depth of the indentation of a diamond cone or metal ball left on the surface of the test specimen. Moreover, it is dimensionless and is denoted by the letters HR. Materials that are too soft may have negative hardness values.
The so-called Rockwell hardness tester was invented at the beginning of the last century by the Americans Hugh Rockwell and Stanley Rockwell. You can see how it works in the following video. A critical factor for this method is the thickness of the test piece. It should not be less than ten times the penetration depth of the indenter into the test body.
Depending on the type of indenter and applied load, there are three measuring scales. They are designated by three Latin letters: A, B and C. The Rockwell hardness value has a numerical form. For example: 25.5 HRC (lastthe letter indicates the scale that was used in the test).
Brinell method
The Brinell hardness value is determined by the diameter of the impression left by a hardened steel ball on the surface of the metal being tested. The unit of measure is kgf/mm2.
The method was proposed in 1900 by the Swedish engineer Johan August Brinell. The test is carried out as follows: first, the preload of the indenter on the sample is set, and only then - the main one. Moreover, the material under this load is maintained for up to 30 seconds, after which the depth of indentation is measured. Brinell hardness (referred to as HB) is calculated as the ratio of the applied load to the surface area of the resulting print.
Some hardness values for different materials (according to Brinell):
- Wood – 2, 6-7, 0 HB.
- Aluminum - 15 HB.
- Copper – 35 HB.
- Mild steel - 120 HB.
- Glass – 500 HB.
- Tool steel - 650-700 HB.
Vickers method
Hardness according to the Vickers method is determined by pressing a diamond tip into the sample, having the shape of a regular quadrangular pyramid. After removing the load, measure the two diagonals formed on the surface of the material and calculate the arithmetic mean value d (in millimeters).
Vickers hardness tester is quite compact (see photo below). The test is carried out at room temperature (+20 degrees). The hardness value of the body is indicated by the letters HV.
Shor method
This method of measuring hardness was proposed by the American inventor Albert Shor. It is also often referred to as the "rebound method". When measuring Shore hardness, a striker of standard size and weight is dropped from a certain height onto the surface of the material being tested. The key value of this experiment is the rebound height of the striker, measured in conventional units.
Shore hardness is measured in the range from 20 to 140 units. One hundred units corresponds to a rebound height of 13.6 mm (± 0.5 mm). According to the standard, this value is the hardness of hardened carbon steel. A modern device for measuring the Shore hardness of materials is called a scleroscope or durometer (it can be seen in the photo below).
Mohs scale
The Mohs hardness scale is relative and applies exclusively to minerals. Ten minerals were chosen as reference ones, which were arranged in order of increasing hardness (in the photo diagram below). Accordingly, the scale has 10 points (from 1 to 10).
The mineralogical scale of hardness was proposed by the German scientist Friedrich Moos back in 1811. Nevertheless, it is still used in geology.
How to determine the hardness of a particular mineral on the Mohs scale? This can be done by carefully examining the scratch left by the specimen. It is convenient to use a fingernail, a copper coin, a piece of glass or a steel knife.
So ifthe tested mineral writes on paper without scratching it, then its hardness is equal to one. If the stone is easily scratched with a fingernail, its hardness is 2. Three points have minerals that are easily scratched with a knife. If you need to make some effort to leave a mark on the stone, then its hardness is 4 or 5. Minerals with a hardness of 6 or higher themselves leave scratches on the knife blade.
In conclusion…
So what is hardness? This is the ability of a physical body to resist destruction and deformation under the influence of local contact forces. The hardest mineral on Earth is considered to be diamond, and the most durable metal is iridium. In modern science and technology, several hardness measurement methods are used (according to Brinell, Rockwell, Vickers, Shore and Mohs).
