Salt-induced corrosion phenomena in physical vapor deposited nitride-based coatings

Abstract

The impact of corrosion phenomena is significant across a wide range of modern technological fields. To address this challenge, protective physical vapor deposited (PVD) coatings and surface modifications present new opportunities for enhancing material properties and extending the operational lifespan of machining components. However, a lacking understanding of corrosion processes within PVD coatings has led to a limited availability of high-performance corrosion resistant thin films in industrial settings. Particularly salt-rich environments represent regimes, where PVD coated machining elements continue to suffer accelerated breakdown. Factors contributing to this issue include inherent coating defects, porosity, the presence of embedded macroparticles, and the overall columnar growth orientation, which make PVD coatings especially vulnerable to the infiltration of corrosive substances.The first part of this thesis provides a systematic approach on highlighting preferred diffusion pathways of Cl--rich aqueous media in Al0.7Cr0.3N PVD thin films. Intended as a model system, the study was designed to discern differences in the passive behavior between cathodic arc evaporated (CAE) and direct current magnetron sputtered (DCMS) Al0.7Cr0.3N coatings. Through use of a three-electrode electrochemical setup in conjunction with an array of high-resolution analytical techniques (e.g., transmission electron microscopy (TEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), atom probe tomography (APT), and transmission electron backscatter diffraction (t-EBSD)), the study presents a meticulous breakdown of the most prevalent diffusion pathways taken by chemically active species. The results emphasize that accelerated infiltration occurs along embedded macroparticles in CAE Al0.7Cr0.3N coatings, whereas preferential diffusion along under dense column boundaries constitute the predominant weak points in DCMS coatings. Subsequent investigations were directed towards assessing the level of coating passivity of pristine and unimpaired coating sites. Notably, ToF-SIMS was employed to collect Cl- diffusion profiles, which unveiled an intrinsically inferior passive behavior of DCMS coatings in comparison to their CAE counterparts. Thus, a dense coating morphology with a randomized growth orientation and minimal fast-track diffusion routes along embedded macroparticles are viewed as beneficial attributes that corrosion resistant coatings should possess.Based on these findings, two approaches were chosen to counter the prevailing diffusion routes that were identified earlier and improve the corrosion behavior of the Al0.7Cr0.3N system: (i) a grain-refining doping strategy for imparting complexity to the diffusion routs along grain boundaries using vanadium (V), and (ii) a subsequent annealing treatment for generating a surface-oxide that functions as a sealing layer. It was found that both, grain-refinement of the Al0.7Cr0.3-xVxN coatings and the development of a continuous and dense top-oxide were directly related to the amount of doped V. Through extensive electrochemical analysis, using potentiodynamic polarization techniques, it was shown that both, grain-refining effects and the formation of a dense V-rich top oxide layer translated to an improved corrosion behavior of the initial AlCrN base-system.The second part of the thesis focuses on hot corrosion (HC) phenomena and proposes CAE Ti1-xAlxN as a potential candidate for providing corrosion protection in high-temperature settings. Ti1-xAlxN coatings with varying metal content ratios were deposited on an industrially established NiCoCr-based superalloy (alloy c-263) and investigated in an in-house built HC testing rig. After applying a Na2SO4-MgSO4 salt mixture onto both Ti1-xAlxN coated and uncoated samples, HC experiments were conducted in a SOx-rich atmosphere under two distinct sets of conditions: low-temperature hot corrosion (LTHC) settings at 700 °C, and high-temperature hot corrosion (HTHC) settings at 850 °C. Here, the primary objective was to extend the understanding of the corrosion mechanisms that govern Ti1-xAlxN coatings when investigated under LTHC and HTHC parameters, and to directly compare the corrosion resistance between Ti1-xAlxN coated samples and the bare c-263 alloy. Notably, the Ti1-xAlxN coatings demonstrated superior resistance to both LTHC and HTHC compared to the bare c-263 alloy.Regarding the LTHC behavior of Ti1-xAlxN, a highly accelerated and localized attack was observed. Initiated by a nitride-to-sulphate transformation and formation of a low-melting liquid salt interface, a synergistic fluxing mechanism was found to be the dominant degradation process. Due to the acidic nature of the liquid salt interface under LTHC conditions, the development of a porous Al2O3-dominated top-oxide was observed, which is known for its enhanced stability under acidic conditions. Underneath, the corrosion scale exhibited a lamellar structure composed of TiO2- and Al2O3-rich oxide domains below which a more globular scale morphology was featured.Conversely, under HTHC conditions, a more uniform development of the corrosion scale was noted. Similar to the corrosion behavior observed under LTHC conditions, a sequential fluxing mechanism was found to be the predominant corrosion process, resulting in the formation of a layered corrosion scale. Owing to the higher melt basicity under HTHC, a porous TiO2 scale was primarily formed at the salt-scale interface, followed by a layered corrosion scale of Al2O3-rich and TiO2-rich domains.In summary, this thesis emphasizes the importance of developing a fundamental understanding of the governing corrosion mechanisms in thin films, in order to help establish PVD coatings as a viable solution for highly demanding environments.