Ssnipp>>Ion nitriding (also known as plasma ion nitriding) is a surface hardening heat treatment that uses diffused nitrogen to form various nitrogen compounds. The nitrogen ions are diffused into a part (or sample) in a vacuum through the use of high voltage electrical energy.
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Black Nitriding is also available, also Black or clear PVD
They are very hard but very thin processes, unlikely to scratch easily.
Black and clear PVD for instance, was used on high end mobile phones on a softer substrate such as stainless steel which would easily scratch in normal use. It lasts for years but eventually wears through in say 1 - 3 years under normal use.
Hardness:
PVD 67 = HRC typically
Nitriding = 76 HRC typically
Case hardened steel might be HRC 62 but the depth of hardness would be much thicker