Abstract
Metallic materials have a major role to play in supporting society evolution. In a sense, every significant advance in materials will lead to significant adjustments in social production. Therefore, there has been a constant search for new metal materials with superior properties, and researchers continue to study and attempt to change their chemical composition. High Entropy Alloys (HEAs) have attracted the interest of many researchers due to their unique and outstanding properties. However, the cost of metal powders and extrusions used in bulk HEA fabrication is very expensive from an application perspective. Therefore, HEA coatings on inexpensive substrates can be considered an economically viable way to significantly reduce the manufacturing costs and overcome the limitations of coatings used in industrial applications.
in this study, a nanostructured CoCrCuFeNi-HEAs thin film was deposited on 304 stainless steel and titanium grade 5 (Ti6Al4V) using radio frequency (Rf) magnetron sputtering. The L9 Taguchi experimental design was utilized in this study. Nine sputtering runs were systematically performed by varying the temperature, deposition time, and Rf power. The thin films were characterized using field-emission scanning electron microscopy, atomic Force Microscope, nano-indentations, potentiostat, and ball-on-disk tribometers.
The microstructure evolution of the surface morphology of CoCrCuFeNi sputtered alloy films deposited on 304 stainless steel substrates shows that all coatings have a dense and uniform microstructure and are free of microcracks. This shows the uniformity as well as the chemical stability of the CoCrCuFeNi-HEAs coating and the sputtering processes. The growth pattern of the CoCrCuFeNi high-entropy alloy film coating at RF power 100 W (L1), (L4), and (L7) can be compared with the island growth model showing the early stage of the formation process, with the film showing a formation and nucleation with larger surface particle sizes. This finding may be partly due to the lower energy level at low sputtering power. As the Rf power increases from 100W to 200W, the layer thickens and the particle size decreases. AFM results showed that HEA films with low deposition time exhibited uneven growth and well-dispersed particles. While the samples with the highest deposition time showed well-developed grain structure and better surface coverage, samples at low temperatures showed amorphous structures, while samples at high temperatures showed domed clusters that have a needle-like structure and do not oscillate.
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The obtained XRD results demonstrate that the film coatings exhibit unique BCC and FCC phase diffraction peak patterns for different process parameters, and the XRD analyses confirm the presence of lateral Cu separation. Furthermore, CoCrCuFeNi-HEA thin film coating(s) sputtered onto stainless steel to induce hardness showed that the HEA thin film exhibits a microhardness between 432 and 1850 HV, which is more than three times the hardness of the substrate material of (436.6 HV); and nano-hardness results also indicate that CoCrCuFeNi-HEAs thin-film coatings exhibit nano hardness from 4.66 (GPa) to 19.97 (GPa), more than four times the nano hardness of the as received stainless steel 304 substrate. Hence demonstrated that the HEAs film significantly enhanced the 304 stainless steel substrate(s) hardness. Also, the outcome demonstrates that increasing the sputtering process variables such as the temperature, the Rf power, and the sputtering time results in increases in the material(s) hardness. While the sputter HEA film coating on titanium alloy produced the maximum nano hardness of 5.14 GPa, the HEA thin film coatings significantly improved the nano-hardness properties of the titanium substrate. Thus, the observed results show that the HEA thin film coating on both titanium and stainless-steel substrates significantly improves the physical and mechanical properties of the substrate material, and the choice of the substrate is also very important in the final result for the improvement of the thin film coating. In addition, the tribological result of the sputtered HEAs film coatings on stainless steel, reveals that the coefficient of friction is strongly influenced by the sputtering Rf power and temperature. As the temperature and power increase, the Cofficient of Friction also increases. The wear result shows that samples produced at the shortest sputtering time of 30 min exhibit the highest range of wear rate (0.00288 [mm³/N/m]). In contrast, samples produced at the longest sputtering time of 90 min exhibit the smallest range of wear rate (0.00019 [mm³/N/m]). The sputtered HEAs film coatings on titanium produced the highest wear resistance at the lowest temperature and decrease with an increment in temperature, with the lowest wear rate of 0.00017 [mm³/N/m]); and the wear morphology reveals that all the wear scar are abrasive. Thus, the results demonstrate that both substrate material(s) HEAs thin films are significantly influenced by temperature. It can also be noted that the substrate tribological materials were greatly improved by the thin films of CoCrCuFeNi-HEAs. The results on the corrosion test of the HEAs films on titanium and stainless steel demonstrate that the HEAs thin film coating has a significant impact on both substrate materials, leading to a significant reduction in the average corrosion rates for both substrate materials, and illustrates the
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importance of process parameters such as RF power and temperature in improving the corrosion resistance of substrate materials through thin film coating.
Taguchi analysis was used to examine the effect of radio frequency magnetron sputtering process parameter on HEAs thin films deposition on stainless steel substrates and their output response. The result revealed that the process parameters (deposition time, rf power, and temperature) are all statistically significant in the modification of HEAs thin film properties, and they all influence the quality of the final thin film coating properties. The optimized settings may be recommended for the production of superior materials and ideal parameters using Rf magnetron sputtering, enabling process optimization for each application and increasing the potential for use in the manufacture of microcoatings and breakthrough applications such as biomaterial, aerospace and uses in harsh environment.
Keywords: CoCrCuFeNi, corrosion rate, high entropy alloys, mechanical properties, nanoindentation, radio frequency magnetron sputtering, stainless steel 304L, titanium alloy grade 5, tribological properties