Corrosion induced diffusion pathways in PVD Al1-xCrxN coatings investigated by atom probe tomography

Abstract

Corrosion processes are common phenomena in fields of engineering and there is nearly never an instance, where a material is totally inert to its environment. Therefore, corrosion and corrosion-resistance are essential variables that play a pivotal role in the development of protective coatings. Ingenuity of next generation PVD coatings has given rise to a wide range of material concepts set out to withstand all kinds of corrosive attacks (e.g. NaCl, HCl, SO3 and O2). While their performance is mostly assessed on descriptors such as mass change, impairment of mechanical properties, or variance in electrochemical surface potential, little work has been dedicated to understand corrosion driven diffusion pathways, specifically on an atomic scale.

Particularly the production of metallic-spits or “droplets” during PVD processes poses a significant drawback in light of the coating’s corrosion resistive capabilities. In many regards, embedded macro particles, logged within the deposited coating matrix, serve a beneficial cause, when it comes to improved mechanical properties, such as hardness, fatigue resistance and fracture toughness. However, in light of corrosion behavior, macro-particles provide formations of voids and rugged grain boundaries that allow for fast-track diffusion of corrosive media to the substrate-coating interface.

This study showcases a systematic approach on highlighting preferred diffusion pathways of corrosive NaCl-rich media in PVD thin films. Intended as a model system, arc-evaporated- as well as sputtered AlCrN coatings were deposited on low alloy steel substrates and electrochemically investigated using a three-electrode set-up. With a Ag/AgCl reference electrode (RE), a Pt-counter electrode (CE) and the coated-steel sample as working electrode (WE), linear potentiodynamic polarization experiments were conducted in a 1M NaCl solution. Next to SEM investigations, high resolution analytical techniques such as APT and TEM were consulted to help identify preferential diffusion paths, and highlight differences in the corrosion behavior of arced- and sputtered coatings.