Hardness is a relative term when referring to materials, both metal and non-metal. In general, hardness involves a high melting point, scratch resistance, and high resistance to deforming under pressure. Chromium is among the hardest metallic elements, compared to transition metals such as copper and iron, alkali metals including sodium, and post-transition metals such as lead. However, compounds and alloys of metals and other elements can be harder than those in their pure state.
Hardness is a property that at first seems simple but has complicated aspects that come up under careful study. To rate and compare the hardness of materials, scientists have devised a number of tests and measurement scales. For example, the Mohs scale is a relative rating system that compares the scratch resistance of materials. So if material A can scratch substance B, then A must be harder than B, and A gets a higher Mohs number. The hardest Mohs-rated substance is diamond with a score of 10, and the softest is talc with a rating of 1. The Vickers scale uses a diamond indenter in the shape of a right pyramid, which is then pressed into the test material for 10 to 15 seconds and reported as VHN or Vickers Hardness Number.
Steel is an alloy of iron, carbon and other materials; a range of steels offer a variety of different properties, including hardness. Chromium is added to increase corrosion and chemical resistance as well as hardening and high-temperature strength. Boron, nickel, molybdenum, niobium and titanium can all add strengthening and hardening properties. A combination of these different substances can produce some of the hardest-known metals.
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Tungsten carbide 857 is made of 85.7 percent tungsten carbide, 9.5 percent nickel, 1.8 percent tantalum, 1.5 percent titanium, 1 percent niobium and 0.3 percent chromium. This form of tungsten carbide measures between 8 and 9 on the Mohs scale. It is four times harder than titanium.
With a Mohs rating of 8.5, chromium is the hardest pure elemental metal; however, the steels which use chromium are harder than the element is by itself. Only trace amounts of chromium are required to add significant hardness to steels. In addition to its use in alloys, chrome plating adds a thin coating of the metal to other materials, providing a lustrous, hard outer “shell” that also resists corrosion.
When combined chemically with other elements, some metals can produce extremely hard substances. For example, the rare metals rhenium and osmium combine with boron to make compounds that are much harder than steel; in fact, osmium diboride is known to scratch diamond, the hardest-known substance that occurs naturally.