Reach Compliant Nickel-Based Nanocomposite Coatings for Tribological Applications
Authors: Zoia Filippo , Afzali Pooria , Bernasconi Roberto , Magagnin Luca 
Published: December 2024

Chromium trioxide (CrO3) is used for functional hard chromium plating in order to deposit thick layers of metallic chromium to improve corrosion, wear resistance and tribological properties of a component. However, safety issues are related to the use of concentrated solutions of chromium oxide, which dissociates into hexavalent chromium species (Cr6+). Chromium trioxide is classified as carcinogenic (category 1A) and mutagenic (category 1B). Moreover, since 2013 it has been listed as a Substance of Very High Concern (SVHC) in the Annex XIV REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) authorization list by European Chemicals Agency (ECHA) [1].

In the framework of finding an alternative to hard chromium, the recent achievements in the development of new nickel-based coatings seem promising. Both nickel alloys (NiCo, NiP, NiW) and nickel-based nanocomposite coatings have been investigated as viable alternatives to hard chromium plating. Among the latter category, two different types of particles can be co-deposited with nickel, namely hard ceramic particles (B4C, Al3O3, TiO2) and solid lubricant particles (MoS2, WSe2) to confer high hardness and low friction coefficient, respectively [2].

Despite their potential for hard chromium replacement, currently employed nickel and nickel alloys plating electrolytes still present applicative issues. Indeed, boric acid (H3BO3), which is commonly added to the nickel plating solution as a pH buffer, needs to be replaced as this substance has also been nominated as a candidate SVHC. According to the harmonised classification and labelling (ATP20) approved by the European Union, boric acid may damage fertility and may damage the unborn child [3]. In the past years different organic compounds, such as citric, succinic and malonic acid, have been proposed as alternatives to boric acid, showing promising results [4].

The necessity to develop REACH compliant formulations for the electrodeposition of functional coatings is supported also by the European Union, which has been funding projects on the topic. An example of such projects is MOZART, which has the ambitious purpose of assisting the fulfilment of REACH requirements to deposit tribo-coatings for wear reduction or self-lubrication [5]. This can offer less harmful and less toxic alternative to the painting and coating industry, following the Safe and Sustainable by Design (SSbD) principles.

In the context of the MOZART project, the present research aims at developing a nickel nanocomposite coating produced using an innovative boric-free REACH compliant solution. Initially, the pH stability of the nanoparticles-free electrolyte and the properties of the pure Ni layers obtained are investigated and compared with a standard boric acid buffered solution. Then, boron carbide nanoparticles are dispersed in the bath and Ni/B4C nanocomposites are deposited at various nanoparticles concentrations and current densities. The morphological and mechanical properties of the composites are investigated.

[1] European Chemical Agency, European Chemical Agency Chromium oxide info-card [Online]. Accessed on 12th February, 2023. Available at https://www.echa.europa.eu/web/guest/substance-information/-/substanceinfo/100.014.189

[2] Mahidashti, Z., Aliofkhazraei, M. & Lotfi, N. Review of Nickel-Based Electrodeposited Tribo-Coatings. Transactions of the Indian Institute of Metals
71, 257–295 (2018)

[3] European Chemical Agency, European Chemical Agency Boric acid info-card [Online]. Accessed on 12th February, 2023. Available at https://www.echa.europa.eu/substance-information/-/substanceinfo/100.030.114

[4] Gamburg, Yu. D., Grosheva, M. Yu., Biallozor, S. & Hass, M. The electrochemical deposition of nickel from electrolytes containing malonic acid. Surf Coat Technol
150, 95–100 (2002)

[5] European Union, Mozart project (Horizon Europe; Grant Agreement: 101058450). Website: https://www.mozart-project.eu