ICMCTF2014 Session A1-2: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

Single crystal nickel-based superalloys are used as structural materials for hot pressure blades in aeronautic gas turbines. Cooled and uncooled blades both include thin-wall sections, sometimes less than 1 mm thick. These parts are always coated with a typically 50 µm thick metallic alloy to improve their resistance to high temperature corrosion and cyclic oxidation. These architectures result in large gradients of microstructure and material properties. Moreover, these systems are subjected to thermal gradients in addition to cyclic temperature, cyclic loading and aggressive atmosphere. The thickness of the area where the substrate microstructure is affected by selective oxidation or by interdiffusion with the coating cannot be neglected anymore for thin components. Interactions among surface reactivity, interdiffusion and creep properties have hence to be considered.

In order to model the lifetime of these components, first generation Ni-based single crystal superalloys were creep tested at high temperature under controlled atmospheres. Flat thin (1mm) and ultra-thin (down to 20 µm) specimens were machined. Aged systems were also machined to extract mechanical samples from the various layers of these architectural materials. The main objective is to characterize the mechanical properties of each individual layer (coating, interdiffusion zone). This approach has been made possible by the development of new test rigs dedicated to high temperature mechanical testing of ultra-thin specimens. In addition, thicker specimens were also creep tested at constant or cyclically changing temperatures, with in some cases a switch of atmospheres during the test (high/low P _O2) in order to understand the nature of the coupling between the various degradation mechanisms. For example, creep tests were performed on coated and uncoated specimens in the exact same conditions to discriminate between the effects of surface reactivity and thermal cycling on creep rate. As an overall result, it was found that two kinds of interactions need to be distinguished. The first “static” interaction ,- well-known in the literature – consists of the formation of a non load-bearing section due to the irreversible evolution of the microstructure affected by oxidation and/or interdiffusion. , The building of creep database for graded materials is required to estimate the impact of the non load-bearing section. The second kind corresponds to the “dynamic” interaction between thermal cycling, “in progress” oxidation or interdiffusion, and creep. One example is the increased creep rate of a superalloy being oxidized with vacancies injection leading to a reversible vacancy super-saturation. Fundamental research is needed to better understand these dynamic coupling effects and some available examples of the research in progress will be shown .

In service, turbine blades are subjected to thermo-mechanical solicitation at elevated temperature and evolve in severe environmental conditions (oxidation and corrosion). Due to these harsh specifications, the use of Ni-based single-crystal superalloys overlaid with Thermal Barrier Coating (TBC) has proven to insure the integrity of this microstructure graded system for several thousands of hours of flights. NiAlPt or MCrAlY alloys generally constitute the bond coating between the superalloy and the thermally insulating zirconia. Nevertheless, microstructure and properties of the different layers constitutive of the TBC system evolve with time due to interdiffusion, oxidation and mechanical solicitations. These evolutions as rumpling/ratcheting, edge delamination, phase transformations, voids formation are deleterious for the lifetime of turbine blades. Therefore, improvements in the prediction of mechanical behavior and the lifetime of turbine blades require a data base related to the intrinsic properties of each layer at different ageing times. Such data are missing for the modeling of the overall behavior of the TBC systems.

A detailed understanding of Al₂O₃/Ni-aluminides interfaces is a key issue for Thermal Barrier Coatings adhesion. Similarly the evolution of the oxide - intermetallic interface during the early stage of oxidation of TiAl is of primary importance in the context of increasing implementation of these alloys for high temperature applications.

The aim of this lecture is to show how advanced surface analytical techniques, Scanning Tunneling Microscopy (STM), X-Ray Photoelectron Spectroscopy (XPS), and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) can be used to address these interfacial problems.

The selected examples will include the question of vacancies generated at the Al₂O₃/TiAl interface, studied by STM, and the Al₂O₃/NiAl interface, studied by XPS and ToF-SIMS.

The use of DFT for modeling the interface at the same scale as STM data (i. e. the atomistic scale) will also be discussed (vacancies and nanovoids).

Zinc-based coatings effectively protect steel against corrosion but have some drawbacks related to environmental issues arising from zinc dissolution and to scarcity of zinc resources. Consequently, aluminium-based coatings gain importance both from a scientific as an industrial point of view. Despite the fact that these Al-based coatings are already being applied industrially, the electrochemical behaviour of the different intermetallic layers formed during the hot dipping process still remains under debate.

When the hot dipping process is performed in a pure Al bath, it gives rise to the formation of different intermetallic layers. The outer layer comprises nearly pure aluminium. This layer is on top of a first intermetallic layer commonly referred to as FeAl₃. The second intermetallic layer is a Fe₂Al₅ layer and adjacent to the steel substrate. In case Si is added to the bath, this element also affects the intermetallic layers.

In order to clarify the electrochemical nature of the different layers with respect to each other, both macroscopic and local electrochemical measurements were performed. To evaluate the performance of the different layers, craters with different depths were produced using a GDOES system, to expose the different layers to the atmosphere. Within those craters the electrochemical behaviour of each specific layer was analysed by immersion in a chloride solution. Afterwards a characterisation of the craters was performed by material characterization techniques such as SEM-EDX and XRD.

Recently, surface modification of AISI 316-NiTi with elements such as Nb, Co, Mo and ZrO₂ have been attempted. The aim of the study is to investigate the influence of ruthenium (Ru) additions and laser speed on the corrosion behaviour of AISI 316 austenitic stainless steel (ASS) reinforced with NiTi. Ni_54.6Ti_45.4 and Ni_54.60Ti_45.32Ru_0.08 feedstock powders blended in a Turbula mixer were laser deposited onto 316 ASS using a 4.4 kW Nd:YAG laser. Their corrosion behaviour was investigated in 0.9 % NaCl solution using potentiodynamic polarisation technique. It is well known that the corrosion performance of NiTi based alloys, especially the passivation depends on the surface conditions. Corrosion potential, E_corr, calculated for 316 ASSS, 316-NiTi and 316-NiTiRu were -0.38, -0.28 and -0.26 V respectively. A positive shift in E_corr and negative shift in I_corr shows a lower tendency and high resistance to corrosion for the 316-NiTiRu alloys as compared to 316 ASS. Ruthenium affects the cathodic Tafel constant (β_c) in the chloride solution, which indicates that it influences the cathodic part of the corrosion reaction. SEM micrographs revealed that the alloys were attacked by chloride ions, with different pit morphologies and depths.

Recently thin films metallic glasses (TFMGs) represent a class of promising engineering materials for structural applications. Lots of efforts have been done on the research and development of TFMG materials. Nevertheless, the Iron-based thin film metallic glasses have rarely been investigated. In this work, the Iron-based Fe-Zr-Ti thin film metallic glasses with different Fe contents were prepared by magnetron co-sputtering system using pure Zr, Ti and Fe targets. The thermal behavior of each TFMG was determined using a differential scanning calorimeter (DSC). The crystal structure of the samples was determined by a grazing incidence X-ray diffractometer (GIXRD). Compositions of thin films were analysed by a field emission electron probe microanalyzer (FE-EPMA). Surface and cross-sectional microstructures of thin films were observed by scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. The surface roughness of thin films was explored by atomic force microscopy (AFM). A nanoindenter and scratch tester were used to evaluate the hardness and adhesion properties of TFMGs, respectively. The potentiodynamic polarization test in sodium chloride aqueous solution was conducted for each TFMG. It was discovered that the mechanical property of TFMG was enhanced as Fe content increased. The influence of Fe concentration on the amorphous state, microstructure, mechanical and anti-corrosion properties of Iron-based thin film metallic glasses was further discussed in this work.

Keywords: Iron-based, thin film metal glass, scratch tester, nanoindenter, potentiodynamic polarization test