Figure 1. PVD process by multi-arc evaporation[1]

Question:
I am looking to determine the process/application temperature for titanium-nitride coatings on high-speed steels to determine whether or not it would affect the hardness of a tool steel that was tempered at 1000ºF (540ºC)?

Answer:
Titanium nitride (TiN) coatings can be applied using either Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) methods. For high-speed steel applications, PVD processes (Fig. 1) are normally preferred over CVD processes (Fig. 2) for the very concern you express. However, PVD processes have their own limitations with respect to component geometry limitations, the need for workpiece rotation to achieve uniformity and coating temperatures (for certain materials).

CVD processing temperatures are typically between 1550-2000ºF (850–1100°C). The basic chemical reaction in the CVD coating to produce a layer of TiN is between titanium tetrachloride (TiCl4), nitrogen (N) and hydrogen (H):

2TiCl4+ N2+ 4H 2 ®2TiN + 8HCl

Figure 2. PVD process by reactive sputtering[1]

By contrast, PVD processes operate at much lower temperatures – in the range of between 750-1100ºF (400–600°C). PVD processes rely on ion bombardment instead of high temperatures (as is the case of CVD) as the driving force. The substrate to be coated is placed in a vacuum chamber and is heated to temperature. The coating material (Ti) is vaporized, and a reactive gas such as N2is introduced and ionized. The vaporized titanium atoms then react with the ionized nitrogen to form TiN compound that deposits on the substrate to form the coating. For coating tools, at least, there are three major PVD processes: evaporating (Fig 1), sputtering (Fig. 2) and reactive ion plating, differing primarily in the way the reacting metal is vaporized.

PVD is widely used for high-speed and tool steels since CVD process temperatures fall into the range where certain tool steels are hardened. Post-coating treatments (rehardening and retempering) may be required, which may affect coating adhesion and dimensions.