Different metals are prized for different things – their electrical conductivity, their rarity, their high melting points, their resistance to corrosion etc. In some cases, however, it’s hardness that’s needed. If this is the case, people can turn to the Mohs scale, which is one of the popular scales used to measure hardness. It ranks metals and minerals on a scale of one to 10, with diamond at 10 being the hardest. Hardness in the Mohs scale is essentially measuring how easily a material can be scratched.
Used for making drill bits, other tools, jewellery and sporting equipment, tungsten carbide has a hardness of around 9.5 on the Mohs scale. It is made up of tungsten and carbon in equal parts and is technically a ceramic rather than a metal. However, it is generally treated as a metal.
Chromium has a rating of 8.5 and is responsible for the shiny “chrome” finish on some cars and motorbikes. It forms compounds readily and so can be present in things like dyes, rather than in its metallic form.
At 7.5, tungsten is the third hardest metal on the Mohs scale. It is used to tip bullets and other weapons, in tools and but it is also widely used as an alloy with other metals. These are used in things like halogen light bulbs and high speed steel for tools.
Steel is hardened by heating and then quickly cooling – or “quenching” – in water to obtain a hardness of up to 7.5. Large machine parts and smaller tools like drill bits are often made with hardened steel.
This is often added to steel to make it stronger and in this form it is used for machine parts and vehicle frames. It has a hardness rating of seven. The metal form is not found in nature and has to be industrially produced.
Rhenium is one of the rarest elements to be found on earth, so despite having a hardness measurement of seven it is not widely used in industry. The bit that is used is mainly used in the engines of jet fighters.
Part of the platinum family of metals, osmium has a hardness rating of seven. It is nearly always used as an alloy because when the metal reacts with oxygen in the air is becomes very toxic and volatile. It used to be used for record needles and is sometimes used in the tips of fountain pens.
As well as being very hard, with a rating of around 6.5, tantalum is prized for its high conductivity of electricity. It is used in electrical components like capacitors and resistors as a result.
Iridium is very rare on earth and so is mainly used as an alloy with other metals. It has a rating of 6.5 and the least susceptible metal to corrosion. Underground pipes are sometimes made with an iridium content because of its corrosion resistance.
The metal famously used for nuclear fuel and bombs is also prized for its hardness as well as its radioactive properties. Depleted uranium, with a rating of six is used on the tips of bullets and other projectiles that need to pierce armour. As a result, metal plates made of depleted uranium are also used by some militaries.