Coating became one of the most significant developments in the history of cemented carbides, starting in the early 1960s (TiC, TiN) and still progressing today. In coated parts, the carbide plays a different role than in non-coated parts, because it is no longer the active component. The main requirements for protective coatings are high hardness, high wear resistance, low friction values, high thermal stability and high oxidation resistance. The carbide part in turn has to supply the best mechanical support for the coating (rigidity, creep resistance, toughness, thermal properties) and allow perfect bonding to the coating (good adhesion) in order to resist spalling. Coatings are made by CVD (chemical vapour deposition), PVD (physical vapour deposition), medium-temperature CVD and plasma-activated CVD. The latter technique is now successfully used for producing diamond layers onto the cemented carbide substrate. The keenest edges are today produced by PVD coatings. The thickness of the coatings is in the range of 5-20μm (CVD) and 2-8μm (PVD).
Today, more than 80% of all turning inserts and about 70% of milling inserts are coated. Also the proportion of PVD-coated drilling tools is steadily increasing. The newest generation are multi-layer coatings with exactly tailored properties for the respective application (material), including true nanocrystalline layer sequences. Several hard and superhard coatings are today state-of-the art in the manufacturing industry: TiC, TiN, Ti(C,N), Al2O3, TiB2, TiAlN, AlCrN and several new tailor made quaternary coatings TiAlMeN, AlCrMeN (Me = Si, B, V, Ta, Mo, Nb...), diamond, diamond-like carbon, and most recently also PVD Al2O3.