Chemical and mechanical performances of CVD‑silicon oxycarbonitride films for corrosion protection applications: Towards inert coatings in aggressive aqueous media

Abstract

The chemical and mechanical performances of a set of silicon oxycarbonitride thin films, deposited via three different CVD chemical pathways at 550 °C and 650 °C namely from TEOS (Tetraethyl orthosilicate), TDMSA (tris(dimethylsilyl)amine) and BMDSD (N,N-bis(1-methylethyl)-N′,N′-disilyl-silanediamine), were tested after an exposure to an aqueous solution with an alkaline pH = 8 at 80 °C for one month. Their potential as an effective solution to address the issue of glass delamination in the pharmaceutical industry has been explored. Thanks to the combination of RBS, NRA and ERDA, the thickness loss and the evolution of the elemental composition throughout alteration were successfully trucked. Based on these parameters alongside the structural and bonding state investigations, the films were classified into 3 groups, each exhibiting distinct behavior. The first group, with zero (silica) or very low concentrations of N and C, completely dissolved into the solution via hydrolysis. The second group consisted of films with higher rates of O substitution by N and C ranging between 0.25 and 0.77, which experienced partial thickness loss and depletion of N and O from the network. This led to a relative increase in C concentration beneficial to the films alteration resistance. The final group included the most durable films, characterized by a substitution rate higher than 1. These films did not experience any thickness loss despite the aggressiveness of the chemical medium. For (N + C)/O ratios of ≥1.2, there was no alteration in composition, structure and bonding state. For lower values, oxidation starts to manifest, which seemed to be slowed down by the further presence of carbon in the film. These characterizations enabled also the provision of logical explanations for the complex variations in hardness and elasticity observed post-alteration, as measured by nanoindentation. In parallel with these chemical performances, the SiOxNyCz systems exhibit a range of Young's modulus and hardness that can reach 127.5 and 9.2 GPa respectively, depending of the oxygen substitution level. Films with higher nitrogen and carbon content show no reduction in hardness and maintain better elastic properties after exposure to the aqueous solution. Films depleted from N and O after alteration become harder when they dispose of an initial high C concentration. This comprehensive approach not only advances our understanding of the durability of silicon oxycarbonitride films but also underscores the significant influence of initial compositional and structural attributes on their performances under extreme conditions. By classifying the tested films into different families based on their degradation behaviors, we were able to offer a clear perspective on the factors that contribute to the major robustness of these materials. These innovative and promising results will enable these silicon oxycarbonitride thin films to be used in new applications in corrosive environment.