Corrosion protection of advanced surface coatings for decorative applications

If Australian manufacturing industries are to be sustainable into the 21st century, they will have to compete in a global market. This means they will be challenged by increasing cost pressures arising from dwindling material resources and tighter environmental controls. The latter is being driven by a growing public awareness and concern, leading to ever more stringent legislation. These issues have already impacted significantly on the decorative coating industry, particularly in regard to its management of toxic waste. Over the last ten years, this industry has sought to improve the longevity of its product by offering lifetime guarantees. The approach has been to apply a physical vapour deposition (PVD) coating, with the opportunity of depositing coatings with a range of attractive colours, combined with abrasion and corrosion resistance, to many of its electroplated low cost components such as high pressure die-cast zinc or brass. To be successful in this endeavour it is essential that the PVD coatings be deposited at as low a substrate temperature as possible to avoid microstructural damage of the temperature sensitive substrates for decorative applications. This restriction leads to some fundamental concerns that the integrity of PVD coatings may be comprised and hence, limits their ability to provide the necessary protection for applications in the harsher, longer term environments. Furthermore, when such PVD coated metal substrates are exposed to the environment; they may have their electrochemical behaviour affected by the properties of the coating–substrate interface, and by the microstructure of the coatings. In addition, the presence of defects, such as, pores and pinholes in the coating can affect the corrosion protection performance of the coating. In view of the above considerations, the aim of the present investigation was to study inherent corrosion properties of TiN coatings and the corrosion protection obtained by depositing TiN coating on substrates used in the decorative coating industry, such as brass and high pressure die-cast zinc. This included a study of the development of micro-structural features of decorative TiN, cathodic arc evaporated (CAE) coatings deposited at the low substrate temperature and its effect on corrosion protection performance, decorative properties (colour and optical reflectivity) and tribological behaviour (abrasive wear). The work is presented in two parts: the first relates to the fundamental corrosion properties of TiN as a PVD coating, while the second relates to the effect of substrate deposition temperature, reactive gas pressure and multilayer formation on corrosion protection performance, decorative properties (colour and optical reflectivity) and tribological performance. In the first part of the work, the fundamental corrosion properties of TiN coatings were investigated on inert substrates. The study revealed a very slow rate of corrosion (nAcm-2) for TiN which indicated the chemical inertness of TiN in sodium chloride solution. The results of further investigation to evaluate the corrosion behaviour of galvanically-coupled TiN coating with typical substrate materials suggested that galvanic corrosion may not be significant for a TiN-substrate system in sodium chloride solution. In the second part of the work, the effect of substrate deposition temperature on the coating microstructure was evaluated by depositing TiN coatings with no prior substrate heating (referred to as TL); rather, the intention was to rely on the energy of ion bombardment, through the selection of a relatively high substrate bias potential, to achieve a dense microstructure. For comparison purposes, TiN coatings were deposited under the same conditions, however, with substrate heating (referred to as TH). The study revealed that the TL TiN coatings exhibited improved decorative properties, corrosion protection and tribological performance than the TH TiN coatings. With respect to the reactive gas pressure, a significant reduction in the macroparticle content of the coating was obtained with increasing deposition pressure. The findings further revealed that the choice of the deposition pressure for decorative coatings is a key issue, since decorative properties and corrosion performance were directly influenced by the deposition pressure. Specifically, decorative properties and tribological performance were found to improve for the coatings deposited at the higher deposition pressure, whilst corrosion protection performance was improved for the coatings deposited at the lower deposition pressure. Although macroparticles have been considered an unfavourable feature of CAE coatings, the results presented in this investigation demonstrated that their effect on the corrosion protection performance was not significant. Finally, the deposition of TiN/Cr sequential multilayer coatings led to significant improvements in the decorative properties, corrosion protection and tribological performance. The findings show that advanced surface engineering may well be a potential strategy for improvement of the performance of decorative TiN CAE coatings through the deposition of multilayers of TiN and Cr. In terms of commercial exploitation, these coatings present a valuable multilayer system in which the tribological performance and corrosion protection performance of PVD TiN, decorative coatings can be enhanced significantly.