Development of Multi-Functional Flame Sprayed High Entropy Alloy (HEA) Coatings
Creators: Pal, Sanhita
Publisher: name: University of Alberta Library
Year: 2023

<p>High entropy alloys (HEAs) are a new class of advanced metallic materials that have received significant attention in recent years due to their stable microstructures and promising properties. They are characterized by their peculiar mixing of five or more principal elements in equimolar concentrations. It has been argued that HEAs exhibit high configurational entropy features that arise from compositionally complex mixing, which enable them to stabilize a single-phase solid solution structure. Owing to the solid solution phase formations, these alloys have shown immense<br>
structural properties, including excellent strength and fracture toughness, which make them potential candidates for extreme environmental conditions.</p>

<p>Surface modification allows the possibility of combining bulk properties of the substrate with the<br>
tailored capabilities of coatings, thereby creating a new range of possibilities. Smart functional coatings incorporating new functionalities and coherent responses have surpassed the traditional capabilities of coatings and have taken surface technologies to new heights. Presently, the potential<br>
benefits of HEAs are being extended to development of coatings via thermal spraying. The exploration of high entropy alloys in conjunction with thermal spraying techniques to address numerous challenges in extreme engineering environments is still in their early stage of development. Only a few studies were attempted to develop thermal-sprayed HEA coatings to protect against wear and corrosion. However, limited, if any, studies on functional properties such<br>
as thermal, electrical, and magnetic properties of thermal sprayed HEA coatings have been pursued.</p>

<p>Solid particle erosion is a typical wear mode that negatively impacts the longevity of parts in many<br>
sectors like aerospace, marine, mining, wind energy, and oil and gas; thus improving the erosion resistance of the components in this sector is economically important. The second aspect of this work revolves around studying the solid particle erosion properties of these coatings. </p>

<p>The goal of this study was to develop novel AlCoCrFeMo, AlCoCrFeMoW and AlCoCrFeMoV HEA compositions to understand how tungsten (W) and vanadium (V) additions in AlCoCrFeMo influence the evolution of microstructures, phase formations, microhardness, electrical resistivity and solid particle erosion properties. </p>

<p>A cost-effective flame spray technique was utilized to produce three different equiatomic AlCoCrFeMo, AlCoCrFeMoW, and AlCoCrFeMoV HEA coatings on stainless steel substrates. To avoid conductivity and short-circuiting during Joule heating experiments, an insulating layer of alumina was deposited on to the substrates before coating depositions. The coatings were characterized using XRD and SEM. The Vickers microhardness technique was used to quantify hardness. A custom assembly was used to determine the electrical resistivity and analyze the Joule heating performance of the coatings. The Joule heating performance was compared on the basis of the rate of increase in surface temperature for a given power input. Solid particle erosion studies were performed using a modified version of the ASTM G-76 standard with a low-pressure cold spray unit, using garnet sand as the erodent and the surface morphology was studied.</p>

<p>The microstructure of the HEA coatings showed the presence of multiple oxide regions along with solid-solution phases. The HEA coatings had an average thickness of approximately 153 ± 14 μm with porosity between 2 to 3%. High hardness values were recorded for all coatings with AlCoCrFeMoV showing the highest hardness. The electrical resistivity values were higher for all the HEA coatings compared to flame-sprayed Ni-20Cr and NiCrAlY coatings and AlCoCrFeNi HEA thin film, which may be attributed to the characteristics of HEAs, such as severe lattice distortion and solute segregations. The coatings have shown improved Joule heating performance when compared with conventional Ni-20Cr flame-sprayed coatings, as indicated by the higher rate of increase in surface temperature for a given power input. Solid particle erosion studies indicated that the coatings underwent a brittle mode of failure. The erosion rate of all three coatings was found to be independent of temperature on testing up to 250 °C. It was statistically determined that AlCoCrFeMo and AlCoCrFeMoW coatings have better solid particle erosion resistance than the AlCoCrFeMoV coatings. The erosion rate of the coatings was found to be linearly dependent on the H⁄E (elastic strain to break) ratio and decreased with increasing value of H⁄E ratio. The combined interaction of high hardness, increased electrical resistivity and improved erosion resistance properties suggests that the flame-sprayed HEA coatings can be used as multi-functional wear-resistant materials for energy generation applications.<br>
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