PVD Coatings as Protective Films in Fashion Industry
Authors: Innocenti Massimo , Vorobyova Mariya Vorobyova , Donati Lorenzo , Cioppi Elia , Giurlani Walter , Martinuzzi Stefano Mauro , Giovani Claudia , Bettoni Diana , Caneschi Andrea 
Published: February 2024

Modification of surface properties with the purpose of improving resistance, hardness, and endurance is a common practice in most industrial applications. However, the fashion industry has a more complex goal of meeting customers' aesthetic tastes while maintaining quality and mechanical properties with safety characteristics. The use of galvanic coatings on metal substrates allows for a good compromise among all these requirements, but toxic solvents combined with nickel and chromium(VI) baths heavily impact the environment and human health. For this reason, alternative approaches have been studied, and we propose Physical Vapor Deposition (PVD) coatings as a good alternative.

The objective of this study is to develop protective coatings in Cr/CrN for brass substrates using the PVD technique. PVD is an mature and very stable technique, the use of which was limited until now by the high costs of the vacuum system and targets. The environmental impact of PVD is very low, and a wide range of coatings with elevated purity can be prepared, such as metallic, ceramic, nitrides, and oxides.

Brass substrate is commonly used in the manufacturing industry because of some interesting properties, such as ductility, resistance, and a low melting point, which allows for low-temperature processing and low costs. However, brass is known to have inadequate corrosion resistance, which negatively impacts the quality of the final products. For this reason, there is a need to look for coatings that improve the corrosion resistance of brass, keeping a shiny and smooth aspect with desirable colors.

Chromium Nitride is an extremely hard, inert, thin film that is applied primarily to precision metal parts. CrNx also performs well in corrosive environments and in sliding wear applications. Precisely, CrNx is an interstitial compound with nitrogen atoms occupying the octahedral holes in the chromium lattice. This characteristic opens the door to interesting investigations: by modifying the chamber pressure and N2/Ar flow, one can obtain films with different stoichiometry. In this study, five thin films of 350 nm of Cr/CrN were prepared by changing the ratio of N2/Ar in the chamber from 0.2 to 0.6. Using Scanning Electron Microscopy (SEM), we investigated the homogeneity of the coatings and estimated the percentage of defects formation. The chemical composition was analyzed with Energy Dispersive X-ray Spectroscopy (EDS-SEM), while the color coordinates were estimated with a Spectrophotometer. In parallel, we investigated the thickness and structure combining SEM with XRF technique [1]. As a result of this study, we selected the 0.4 ratio N2/Ar for CrNx layers, which agreed with literature [2], and created a multilayer film of Cr/CrN. Finally, we tested different combinations of engineered multilayer films with electrochemical tests such as free corrosion test OCP, Electrochemical Impedance Spectroscopy (EIS), and neutral salt spray (NSS) test (ISO 9227:2017). Then, we compared the results to understand and optimize which condition combinations are more suitable for protective applications. In conclusion, by deepening the knowledge on this topic, it might be possible to play with the pressure conditions in the chamber to open new design possibilities.

References:

[1] Giurlani et al. Coatings 2018, 8, 84.

[2] Grzegorz Greczynski et al. IEEE Transactions on plasma science, vol. 38, no. 11, November 2010