What is harder - granite or marble, nickel or aluminum? And what is hardness in general? We will try to answer these questions in our article. The problem of determining the hardness of minerals and substances has been dealt with by a number of foreign scientists. 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 particular material to resist the introduction of another, more solid body (for example, a cutting or pricking tool) into it.
The hardness of substances is most often measured in special units - kgf / mm 2 (kilogram-force per square millimeter of area). It is indicated by the Latin letters HB, HRC or HRB, depending on the selected scale.
The hardest mineral on Earth is considered diamond. If we talk about materials of artificial origin, then fullerite is the most durable. This is a molecular crystal that forms at high temperatures (about 300 degrees Celsius) and extremely high pressure (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 options for hardness:
- 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 fingerprint).
- Volumetric (the ratio of the load to the volume of the print).
In addition, the hardness of physical bodies is measured in four ranges:
- Nanohardness (load value is less than 1 gf).
- Microhardness (1 - 200 gf).
- Hardness at low loads (200 gf - 5 kgf).
- Macrohardness (more than 5 kgf).
Metal hardness
Of the 104 elements of the periodic table, 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. This is the military and chemical industry, metallurgy, electrical engineering, the space industry, jewelry, shipbuilding, medicine, etc.
Among all the physicochemical characteristics of metals, hardness plays an important role. After all, she clearly demonstrates:
- degree of metal wear resistance;
- resistance to pressure;
- its ability to cut other materials.
Among other things, the hardness of the metal shows whether it can be processed on various machines, whether it lends itself to grinding and the like. By the way, scientists have long proved that the hardness of a metal in many ways determines its other mechanical properties.
What is the hardness of iron, copper and aluminum? And which metal is the hardest and most durable?
The softest metals include magnesium and aluminum. The values of their hardness range from 5 kgf / mm 2 . About twice as hard - nickel and copper (about 10 kgf / mm 2 ). The hardness of iron is estimated at 30 kgf / mm 2 . Well, the hardest metals of natural origin include titanium, osmium and iridium.
Determination of hardness: methods, methods and approaches
How is the physical body hardness measured? For this, a so-called indenter is introduced into the sample. Its role can be played by a heavy-duty metal ball, a pyramid or a diamond cone. After the direct contact action 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 out by scratching, indenting (most often), cutting or rebound.
Today, there are several different approaches for determining hardness:
- according to Rockwell;
- according to Brinell;
- according to Vickers;
- along shore;
- according to moos.
Accordingly, there are a number of different hardness scales of materials, there is no direct relationship between them. A particular 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 imprint depth of the diamond cone or metal ball left on the surface of the test sample. Moreover, it is dimensionless and is denoted by the letters HR. Materials that are too soft can 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. In the next video you can see how it works. A critical factor for this method is the thickness of the test sample. It should not be less than ten times the indenter penetration depth into the test body.
Three measuring scales exist depending on the type of indenter and the applied load. They are designated by three Latin letters: A, B and C. The Rockwell hardness value is numerical. For example: 25.5 HRC (the last letter indicates the scale that was used in the test).
Brinell method
The Brinell hardness value is determined by the diameter of the imprint left by the steel hardened ball on the surface of the test metal. The unit of measure is kgf / mm 2 .
The method was proposed in 1900 by the Swedish engineer Johan August Brinell. The test is carried out as follows: first, the indenter preload on the sample is set, and then the main load. Moreover, the material under this load can withstand up to 30 seconds, after which the depth of indentation is measured. Brinell hardness (indicated as HB) is calculated as the ratio of the applied load to the surface area of the resulting print.
Some hardness values for various 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
Vickers hardness is determined by pressing a diamond tip in the shape of a regular quadrangular pyramid into the sample. After removing the load, two diagonals formed on the surface of the material are measured, and the arithmetic mean value d (in millimeters) is calculated.
The Vickers hardness tester is quite compact (see photo below). The test is carried out at room temperature (+20 degrees). The value of body hardness is indicated by the letters HV.
Shore Method
This method of measuring hardness was proposed by the American inventor Albert Shore. It is often called the "rebound method." When measuring Shore hardness, a firing pin of standard size and mass falls from a certain height onto the surface of the test material. The key value of this experience is the height of the bounce of the striker, measured in arbitrary units.
Shore hardness is measured in the range from 20 to 140 units. One hundred units corresponds to a bounce height of 13.6 mm (± 0.5 mm). By standard, this value is the hardness of hardened carbon steel. A modern instrument for measuring the hardness of materials according to Shore 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 selected as reference ones, which were arranged in order of increasing their hardness (in the photo diagram below). Accordingly, the scale has 10 points (from 1 to 10).
The mineralogical hardness scale was proposed by the German scientist Friedrich Moos back in 1811. Nevertheless, in geology it is still used.
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 sample. It is convenient to use a fingernail, a copper coin, a piece of glass or a steel knife.
So, if the test mineral writes on paper without scratching it, then its hardness is one. If a 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 efforts 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.
Finally…
So what is hardness? This is the ability of the physical body to withstand destruction and deformation when exposed to local contact forces. The hardest mineral on Earth is diamond, and iridium is the strongest metal. In modern science and technology, several methods of measuring hardness are used (according to Brinell, Rockwell, Vickers, Shore, and Mohs).