ICMCTF2016 Session H3: Characterization of Coatings in Harsh Environments

Thermal barrier coatings (TBCs) have been widely used in combustion and turbine engines to provide thermal protection for the metallic components and allow higher operating temperatures. However, TBC failure during service strongly limits component lifetime and efficiency. Further improvements in TBCs require a comprehensive understanding of the correlation between microstructure and failure events.

In this study, we track the microstructural evolution of an air plasma sprayed (APS) TBC during thermal cycling by X-ray computed tomography (XCT). The coating thickness, pore fraction and crack damage is quantified by segmentation of the registered greyscale images. Evolution of these microstructural features are recorded at the same sample location to allow direct correspondence of the quantification with the local evolution of a single volumetric region. Digital volume correlation (DVC) is performed on the image sequences to map the deformation during thermal cycling and to calculate the opening displacements of the crack surfaces.

TerraPower’s traveling wave reactor (TWR) is a sodium cooled fast reactor that utilizes a breed and burn fuel management process to convert depleted uranium into fissile fuel in the core. This process eliminates the need for reprocessing and significantly reduces the long-term need for enrichment plants, thereby alleviating proliferation concerns and lowering the cost of the nuclear energy process.

In a sodium cooled nuclear reactor such as the TWR, there are numerous high stress interfaces between components where relative motion can lead to undesirable friction and wear. Additionally, the use of high temperature (180°C to 510°C) liquid sodium with a low (~1 wppm) oxygen concentration aggressively removes the oxide layer from the surfaces of metals, promoting adhesion of mated surfaces leading to severe galling and self welding. A complete understanding of the tribological performance of materials in the core is necessary to mitigate accelerated deterioration of contact surfaces in the core that could lead to unexpected reactor shutdown from component malfunction or failure due to galling and seizure.

Extensive materials testing for tribological applications was performed in support of the FFTF by the U.S. Department of Energy (DOE) under the National Friction, Wear, and Self-Welding Program; however, additional testing is necessary to bolster the data available for these materials and qualify them for the extended lifetime expected in the TWR. Additionally, new materials and methods have been developed in the intervening years that warrant investigation for sodium cooled reactor applications.

In order to qualify candidate hard face coatings and surface treatments for the operating conditions and lifetime expected in the TWR, TerraPower is constructing a high temperature sodium loop to test for friction and wear, self welding, corrosion, and thermal shock performance in 180°C to 650°C, ~1 wppm liquid sodium. Initial testing includes structural materials (e.g. 316 SS and HT9) as well as hard face coatings of chromium carbide and Tribaloy 700 applied by electrospark deposition. The results from tribological testing at TerraPower coupled with irradiation testing will inform material selection and coating development for the traveling wave reactor.

A common goal for materials employed in nuclear environments is to exhibit the highest radiation tolerance. The lifetimes of current and even more of future reactors are largely determined by materials issues such as embrittlement and swelling. In the process of energy production via fission and fusion, structural materials are subject to substantial radiation damage, which appears in the form of point defects and their agglomeration to form dislocation loops and vacancy clusters. Combination of vacancy clusters with transmutation products such as helium (He) promotes the formation of He bubbles. These bubbles cause swelling, embrittlement and dimensional instabilities in structural metals, which represent a real challenge for application of metals in nuclear industry.

It is well known that surfaces, grain boundaries and heterointerfaces are good sinks for radiation-induced point defects and traps for implanted He. Composite materials with a high interface density distribution showed enhanced radiation tolerance compared to conventional single phase metals. In spite of this beneficial effect, the role of He bubbles on the mechanical properties and structural integrity of nanostructured materials is still to be understood.

This study is aimed at evaluating and correlating the effects of He bubbles formation with structural and mechanical properties of nanomaterials with high interface density distributions such as nanoscale metallic multilayers. With this aim, Cu/W multilayers were deposited by magnetron sputtering and subjected to He ion implantation (1 MeV) with two different fluences (1.1 and 3.2 10¹⁶ cm^-2) and incident angles. Structure of pristine and irradiated multilayers was investigated by XRD and FIB/TEM analyses, while mechanical properties changes were evaluated by nanoindentation, through which possible deformation mechanisms in multilayers with He bubble-decorated interfaces were also investigated.

By combining calculated He concentration profiles, throughout the multilayer thickness and TEM images, it is found that in low He concentrations regions, bubbles formed mostly along interfaces, while more homogeneously distributed bubbles were found in Cu layers and along columnar grain boundaries in higher He concentrations regions. It is suggested that the capability of interfaces to annihilate point defects is weakened by the He bubbles shielding effect. Nanoindentation tests revealed a hardness decrease amounting to ~ 0.5 and ~ 1 GPa for low and high fluences, respectively. The observed softening effect is mostly attributed to He storage induced changes in residual stresses, and columnar grain boundary sliding facilitated by He bubbles.

Graphene films is a good coating material which can effectively protect metal surfaces from corrosion. However, recent studies show that graphene may promote metal corrosion. The conductivity of graphene will accelerate the charge transfer between graphene and metals then increasing the corrosion process. In this article, a new alumina-graphene oxide nanosheets (Al-GO) has been synthesized and characterized by SEM, TEM and AFM.

The hydroxyl groups on GO react with aluminum isopropoxide via sol−gel process to obtain Al-GO and incorporating into polyimide. The alumina-graphene oxide nanosheets polyimide (Al-GO/PI) coating was found to revealed superior long-term a nticorrosion properties on cold-rolled steel (CRS) electrodes compared with a corresponding GO/PI and PI coating. The mechanism may due to the extremely low conductivity of alumina hydroxide on GO can reduce the charge transfer between GO and metal substrates and improve the anticorrosion property.

Al-GO/PI composites were identified by a series of electrochemical measurements such as corrosion potential (E_corr), polarization resistance (R_p), corrosion current (I_corr) and electrochemical impedance spectroscopy (EIS) studied in 3.5 wt% NaCl electrolyte.

Taiwan is an island country surrounded by seas. Economically, transporting imported and exported goods and supplies is an indispensable part. However, sound absorption materials of underwater ambience related literature is not easy to get, therefore, to fabricate an underwater cladding materials with function of sound absorption and corrosion resistibility is the purpose of our research.

Microbial adhesion and corrosion are universal phenomena in salt water media such as seawater and wastewater environments. To mitigate the adhesion and corrosion induced by microorganisms on marine steel constructions, a kind of organic composite biocide 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) was added into Zn-Ni electrolytes as functional ingredient to prepare Zn-Ni-DCOIT composite films. It was found that the electrodeposition potential, Ni content, phase structure and surface morphology of the composite films were strongly influenced by DCOIT. Microbial adhesion and corrosion mitigation were conducted by Escherichia coli suspended phosphate buffer saline medium immersion and sulfate-reducing bacteria medium exposure. It was found that the antibacterial properties and microbial corrosion resistance of the composite films were improved. As a kind of antibacterial component, DCOIT showed remarkably enhanced anticorrosion and antibacterial effect.

One of the primary motivations for development of instrumented indentation was to measure the mechanical properties of thin films. Characterization of thin film mechanical properties as a function of temperature is of immense industrial and scientific interest. The major bottlenecks in variable temperature measurements have been thermal drift, signal stability (noise) and oxidation of/condensation on the surfaces. Thermal drift is a measurement artifact that arises due to thermal expansion/contraction of indenter tip and loading column. This gets superimposed on the mechanical behavior data precluding accurate extraction of mechanical properties of the sample at elevated/cryogenic temperatures. Vacuum is essential to prevent sample/tip oxidation at elevated temperatures and condensation at cryogenic temperatures.

In this talk, the design and development of a novel nanoindentation system that can perform reliable load-displacement measurements over a wide temperature ranges (from -150 to 700 °C) will be presented emphasizing the procedures and techniques for carrying out accurate nanomechanical measurements. This system is based on the Ultra Nanoindentation Tester (UNHT) that utilizes an active surface referencing technique comprising of two independent axes, one for surface referencing and another for indentation. The differential depth measurement technology results in negligible compliance of the system and very low thermal drift rates at high temperatures. The sample, indenter and reference tip are heated/cooled separately and the surface temperatures matched to obtain drift rates as low as 5nm/min at 700 °C. Instrumentation development, system characterization, experimental protocol, operational refinements and thermal drift characteristics over the temperature range will be presented. Extensive test results on standard calibration materials like fused silica, standard metals and thin films, used for validating the system, will be shown. Finally, the current status and future roadmap for variable temperature nanoindentation testing will be summarized.