Corrosion properties of hard PVD nitride coatings (with the emphasis on TiN)

The life-time of various machine parts and tools is greatly affected by the wear process. Since more and more production processes are being automated and interlinked the performance of individual parts or tools is extremely important.Therefore, the wear protection is essential from the technological, as weil as from economic point of view. For this purpose thin (3-5 $\mu$m) layers of mechanically resistant material (hard coatings) have been used in the past years. Materials like nitrides and carbides of transition metals have proved to be very successful. The advantages offered by these materials are an extremely hard surface, while retaining the toughness of the component, low friction coefficient, excellent adhesion, good sliding behaviour, relatively high electrical and thermal conductivity. Some of the characteristic properties are given in Table 1. Last but not least, the attractive appearance of the golden TiN and ZrN or silver erN is an additional advantage. Due to their brittleness and fracture susceptibility - not mentioning the cost - these materials cannot be used in solid form. However, when applied as thin coatings they loose their brittleness and conform the toughness of the base material. Despite that hard coatings are mostly used in tribological systems, other applications should also be mentioned, i.e. decorative coatings, biocompatible materials for medical applications, interconnections and diffusion barriers for VLSI circuits, high temperature sensors, etc. Due to such a diverse range of applications over the past ten years there has been a world-wide interest in new coatings and surface treatment methods. This is illustrated in Figure 1, which shows the increase in the number of papers published on TiN, erN and ZrN from 1980 (1). It is evident that TiN is still the most extensively investigated coating material, although in future other materials, i.e. erN, ZrN and ternary coatings, will play an important role. In many applications coated systems are exposed to hostile environments, including corrosive media, elevated temperatures, stress, or combination of these. Under such conditions significant changes occur at the coating surface which then consequently affect the performance of the system as a whole. For such applications, besides [...