Which is the Second Hardest Substance in Nature?
Corundum, primarily composed of aluminum oxide (Al2O3), stands as the second hardest natural substance in the world. Its unique composition and crystalline form make it an essential material in various applications across industries. Depending on the presence of trace elements, corundum can form sapphires and rubies, contributing to its value and beauty. However, it is crucial to understand the other contenders for the title of second hardest substance, particularly cubic boron nitride (cBN), to gain a comprehensive view of nature's toughest materials.
The Second Hardest Natural Substance: Corundum
Corundum, which ranks 9 on the Mohs hardness scale, is celebrated for its impressive hardness. This crystalline form of aluminum oxide is not only valued for its beauty but also for its durability. Its remarkable ability to scratch most materials on the Mohs scale, aside from diamond, makes it an indispensable component in industrial applications, such as grinding and cutting tools.
Cubic Boron Nitride: The Close Runner-Up
Though cubic boron nitride (cBN) is still the world's second hardest material with a Vickers hardness of approximately 50 GPa, it falls slightly short of diamond's perpetually unyielding hardness. When it comes to hardness, cBN, a hexagonal form of boron nitride, represented a promising alternative to diamond. Its early theoretical predictions suggested a potential indentation strength 18 times greater than that of diamond. However, these initial results were based on theoretical simulations, and subsequent experimental evidence did not consistently back up these extraordinarily high claims.
The Rarest of Second Hardest Materials
Beyond diamond and cBN, there are other contenders for the title of second hardest material. For instance, wurtzite boron nitride, with its similar crystal structure to diamond but different atomic composition, offers a fascinating alternative. Additionally, minerals like lonsdaleite, often colloquially referred to as hexagonal diamond, are made from carbon atoms just like diamond but with a distinct arrangement that affects their hardness. Another contender is rhenium diboride, which, despite its name, is a crystalline compound known for its remarkable hardness.
Applications of Second Hardest Substances
The second hardest substances, such as corundum and cBN, find application in a wide range of industries. For example, in the music industry, diamonds are famously used in turntable styli (often referred to as needles) due to their ability to withstand the vibrations and heat generated by playing records. As alternative materials, ceramic is also used in more cost-effective turntable styli.
Understanding the Mohs Scale of Hardness
To better understand the hardness scale of these materials, the Mohs scale is an essential tool. This scale, first introduced by German mineralogist Friedrich Mohs in 1812, ranks minerals based on their scratch resistance. According to the Mohs scale, diamond is the hardest at 10, and corundum ranks at 9, signifying that it can scratch all materials ranked below it. Other contenders for the second spot include wurtzite boron nitride and lonsdaleite, making them suitable for various industrial applications requiring extreme durability.
While corundum and cBN may not surpass the unyielding hardness of diamond, their exceptional hardness and durability make them indispensable in many industrial and technological sectors. Understanding the properties of these materials is crucial for their continued application and innovation.