Abstract When it comes to the hardest materials on earth, most people may answer diamonds. The name of the diamond comes from Greek, meaning "unbreakable" or "invincible". The word in English means...
When it comes to the hardest materials on earth, most people may answer diamonds. The name of the diamond comes from the Greek word meaning "unbreakable" or "invincible". The word in English means "hard things." The hardness of the diamond gives it an unparalleled ability to cut, and it also has a high "face value", which has made diamonds highly respected for thousands of years. Modern scientists have spent decades looking for cheaper, harder, and more practical alternatives, and every few years there is news that has created a new "hardest substance in the world." But are these challengers really successful?
Although diamonds are dazzling, in essence, it is simply a special form or an allotrope called carbon. The carbon family has several allotropes, including carbon nanotubes, amorphous carbon, diamonds, and graphite. All of these materials are made up of carbon atoms, but the types of atomic bonds between them are different, resulting in different material structures and properties.
There are four electrons in the outermost layer of each carbon atom. In diamonds, these electrons share with the other four carbon atoms, forming a very strong chemical bond, creating a very solid tetrahedral crystal structure. It is such a simple, dense structure that makes diamonds the hardest substance on earth.
How hard is it?
Hardness is an important property of materials, which often determines where they are used, but hardness is difficult to define. For matter, the scratch hardness is used to measure the hardness of the material, and the other mineral is used to scratch the surface of the material to determine the softness and hardness of the material based on the scratch.
There are also several methods for measuring hardness. Instruments are usually used to locally deform the surface of the material. The ratio of the area of ​​plastic deformation to the pressure load is used to compare the hardness of the material to be tested. The harder the material, the greater the ratio. The Vickers hardness is also calculated from the load per unit area of ​​the indentation. The difference is that the indenter of the Vickers hardness test is a regular quadrilateral pyramid of diamond.
Mild steel has a Vickers hardness of about 9 GPa and a diamond has a Vickers hardness of about 70–100 GPa. The wear resistance of diamonds is unique in the world. Today, 70% of the world's natural diamonds are used in the manufacture of wear-resistant coatings for cutting, drilling and grinding, or as abrasive abrasive additives.
Diamonds are not perfect, although they are very hard, but they are also very unstable. When a diamond is heated above 800 degrees Celsius in air, its chemical properties change, which affects its strength and allows it to react with iron, making it unsuitable for processing steel.
These aspects limit its application, so scientists have been developing new, chemically stable, superhard materials as an alternative. Better wear-resistant coatings allow industrial tools to reduce the need to replace worn parts, extend their life, and reduce the need for coolants that pollute the environment. To date, scientists have developed several potential competitors for diamonds.
Boron nitride
The synthetic material boron nitride, born in 1957, is similar to carbon, and it also has several allotropes. Cubic boron nitride (c-BN) has the same crystal structure as diamond, but it does not contain carbon atoms, but a crystal structure composed of boron and nitrogen atoms. Cubic boron nitride is chemically and thermally stable and is currently used as a coating for superhard mechanical tools primarily in the automotive and aerospace industries.
However, at present, cubic boron nitride is only the second hardest material in the world, and its Vickers hardness is about 50 GPa. Wurtzite boron nitride (w-BN) was originally thought to be harder than diamonds, and this conclusion is based on theoretical simulations (predicting its indentation strength is 18% higher than diamonds). Unfortunately, w-BN is very rare in nature, and it is difficult to develop enough amounts to experiment with this claim.
artificial diamond
Reports on synthetic diamonds have been around since the 1950s, and because of the different crystal structures, they are harder than natural diamonds. Synthetic diamonds are made by placing the graphite in a high-temperature, high-pressure environment where the carbon atoms are rearranged to form new diamonds, but the process is time consuming and expensive. Another manufacturing method is to extract carbon atoms from a heated hydrocarbon gas, but the use of such a substrate is limited.
Synthetic diamond is a polycrystalline structure, a polycrystalline diamond crystallized from microcrystals and "grains" of a few microns to a few nanometers. Unlike most large single crystal structures used to process jewelry. The smaller the grain size, the larger the grain boundary and the harder the material. Recent research on synthetic diamonds shows that it has a Vickers hardness of up to 200 GPa.
Q-carbon
Recently, researchers at North Carolina State University have created a new form of carbon that is different from other allotropes in that it is harder than diamonds. The new material is heated to 3700 ° C with high-energy fast laser pulses and then rapidly cooled or "quenched" to form micron - sized diamonds, hence the name "Q-carbon".
Scientists have found that Q-carbon is 60% stiffer than diamond-like films (an amorphous carbon that is similar in characteristics to diamonds). This makes scientists look forward to the hardness comparison of Q-carbon and diamond, although this still needs to be proved experimentally. Q-carbon has some unique properties, it is magnetic, it will sparkle even when exposed to lower energy. However, so far, it is only an intermediate step for producing tiny synthetic diamonds at normal temperature and pressure. These nanodiamonds are too small to be jewelled, but are an inexpensive coating material for cutting and polishing tools.
Expanded metal is sometimes considered an alternative to sheet metal or wire mesh. It combines some of the best features of both-providing a material that is stronger than thin wire mesh and has better air flow and drainage than sheet metal.
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