Ultrahigh tribocorrosion resistance of metals enabled by nano-layering

Abstract

Tribocorrosion damage on metal surfaces imposes a great challenge to their reliable long-term performance in corrosive environment. In the present work, we showed that nanostructured metallic multilayers (NMMs) exhibited ultrahigh tribocorrosion resistance owing to abundant interfaces and nanoscale chemical modulation that effectively restricted plastic deformation, reduced micro-galvanic corrosion and surface reactivity. Specifically, the tribocorrosion behaviors of equal-spaced Al/X (X = Ti, Mg and Cu) NMMs with ~ 3 nm individual layer thickness were studied in 0.6 M NaCl aqueous solution under room temperature. Nanomechanical and electrochemical measurements were coupled with advanced material characterization tools to study the effects of constituting materials on the deformation and degradation mechanisms. It was found that while corrosion dominated in Al/Mg and Al/Cu NMMs, severe plastic deformation dominated in Al/Ti during tribocorrosion due to sustained surface passivity. A finite element (FE) based computational model was developed and validated to quantify the tribocorrosion behavior of all NMMs, which showed accelerated material loss at layer interfaces as well as wear track edge resulting from the synergistic effects of wear and corrosion. Finally, density functional theory (DFT) calculations were carried out to uncover the origin of corrosion resistance in NMM. It was found that via nanolayering, the surface work function of Al was increased while Cl adatoms adsorb less strongly than that on pure Al, thus reducing the surface reactivity and pitting susceptibility. The combined experimental and computational study provides a guideline for future material selection and design of multilayered and multi-phase metals for use under extreme environment.

Introduction

The design of new metals and alloys resistant to both wear damage and corrosion degradation becomes increasingly demanded in complex service conditions, such as biomedical implants, hydraulic systems, nuclear power plants, marine and offshore infrastructures, etc. [1], [2], [3], [4], [5], [6], [7]. Recently, nanostructured metallic multilayers (NMMs) consisting of alternating layers of different metals or alloys have attracted significant interests due to their potential to simultaneously achieve improved mechanical and electrochemical properties compared to their monolithic counterparts [8,9]. Hence it is critical to understand how the structure and properties of the constituting materials and interfaces of NMMs can be tailored to simultaneously optimize their wear and corrosion resistance, i.e. tribocorrosion resistance. Previous research shows that the individual layer thickness ($h$) of NMMs governs their mechanical and tribological behavior [10], [11], [12]. The wear rate of NMMs typically increases with decreasing hardness per Archard's law [13], [14], [15], although an optimum individual layer thickness of NMM for the highest wear resistance is not known a priori [16,17]. In addition to layer thickness effect, the elastic/plastic incompatibility between the constituting materials [18], the structure and properties of the heterophase interfaces [14,19], as well as residual stress [16,20] also play important roles on the tribological response of NMMs.

Apart from their excellent mechanical and tribological properties, many NMMs exhibit improved corrosion resistance compared to their monolithic counterpart of equal thickness due to the formation of less susceptible microstructure (e.g. finer grains and smoother surfaces) and the ability to effectively block ionic diffusion due to the presence of interfaces [8,9,21,22]. However, such low corrosion rate is often short-lived. As soon as localized corrosion penetrates through the top layer, the ensuring galvanic coupling between adjacent layers often leads to preferred corrosion of the more active layers thus destructing the overall structural integrity. For example, in Zn/Ni NMMs, only Ni layers could be found after extended corrosion testing in 5 wt.% sodium chloride solution while all Zn sublayers were dissolved [22]. Hence optimizing corrosion resistance of NMMs requires an effective strategy that minimizes the micro-galvanic coupling between the constituting layers [23].

Even though there is no unified theory at the moment, the abundant evidence above indicates exciting opportunities to optimize tribocorrosion resistance of metals by forming nanostructured multilayers. For NMMs, the individual layer thickness not only controls its mechanical response due to length-scale dependent dislocation-mediated plasticity, but also represents the wavelength of chemical modulation, hence governs its electrochemical kinetics. Recent studies show that materials’ corrosion resistance could be enhanced by controlling the chemical modulation below a critical length scale on the order of a few nm. For example, Ralston et al. [24] showed that when the precipitate size is below ~ 4 nm in Al-Cu-Mg alloys, the precipitates become ‘invisible’ during corrosion, i.e., they harden the material without forming unfavorable micro-galvanic coupling with the matrix. It was suggested that a continuous protective passive film could not be formed if the chemical heterogeneity length scale exceeds this threshold. Based on these separate observations on mechanical and corrosion properties, NMMs with individual layer thickness of a few nm, can potentially allow for achieving extensive ultra-fine chemical modulation across adjacent layers and hence represent a new class of material system for exploring fundamental chemistry and physics at the scale of a few atoms. However, the underlying deformation and degradation mechanisms of NMMs, and how the synergistic effects of the mechanical and electrochemical properties of the individual layers govern the overall tribocorrosion behavior, remains largely unknown.

This work aims to develop a fundamental understanding of equal-spaced Al/X (X=Mg, Ti, and Cu) NMMs, their structure, and their deformation and degradation mechanisms during tribocorrosion. Aluminum (Al) is chosen as the top layer for its high corrosion resistance and passivity in 0.6 M NaCl aqueous solution, while Mg, Ti, and Cu were chosen to represent increasing thermodynamic driving force for corrosion, with more negative (-2.37 V vs. SHE), comparable (-1.63 V vs. SHE) and more positive (+0.34 V vs. SHE) standard electromotive force potential than that of Al (-1.66 V vs. SHE) [25]. Notably, the galvanic series of the four metals in seawater follows a slightly different trend: ranking from Mg, Al, Cu, to Ti with increasing corrosion potential (i.e. from active to noble) [26]. An individual layer thickness of ~ 3 nm was chosen for all samples. Specifically, nanoindentation and nanowear, potentiodynamic polarization, and tribocorrosion tests were performed to evaluate the effects of constituting materials on the tribological, corrosion, and tribocorrosion properties of Al/X NMMs. The microstructure of NMMs before and after tribocorrosion were characterized using scanning and transmission electron microscopies. Finally, finite element simulations and density functional theory calculations were performed to offer more insight on the localized corrosion and tribocorrosion mechanisms of NMMs.