Microstructure and mechanical properties of (HfVTiZrW)B₂ High-Entropy Alloy Diboride films prepared by HiPIMS at different temperatures

Abstract

This study investigates the microstructural evolution and tribological performance of (HfVTiZrW)B₂ High-Entropy Alloy Diboride coatings deposited using High Power Impulse Magnetron Sputtering (HiPIMS). The coatings were applied to various substrates, including P-type single crystal (100) silicon wafers, 304 stainless steel, and 420 stainless steels, at different temperatures, with the aim of exploring their mechanical and physical properties for potential use as protective coatings for cutting tools and forming dies. The composition of the High-Entropy Alloy (HEA) Diboride thin films remained stable across varying process temperatures, indicating strong thermal stability. X-ray diffraction (XRD) analysis confirmed that all films exhibited a consistent hexagonal close-packed (hcp) structure. High-resolution transmission electron microscopy (TEM) revealed that the coatings contained both amorphous and crystalline regions, with the degree of crystallization and grain size increasing with deposition temperature, from 11.39 ± 0.84 nm to 27.25 ± 3.59 nm. Tribological tests demonstrated a significant correlation between deposition temperature and frictional performance. The coefficient of friction (COF) increased from 0.33 ± 0.02 for samples deposited at 200°C to 0.57 ± 0.01 for samples deposited at 500°C. This rise in COF values suggests that increased crystallization and grain size enhance the tribological performance of the coatings. Hardness measurements showed an increase from 34.8 ± 0.3 GPa at lower temperatures to 38.0 ± 0.6 GPa at 500°C, indicating the coatings potential to withstand high-stress environments. Adhesion quality was evaluated using Rockwell-C indentation (HRC-DB) tests, where the sample deposited at 500°C exhibited the highest hardness and adequate adhesion properties, making it particularly suitable for applications requiring enhanced wear resistance and durability. Scratch tests performed with an RST 300 instrument indicated variable adhesion and scratch resistance among the samples, with critical loads (Lc) ranging from 3.9 N to 21.8 N. The sample deposited at 500°C demonstrated the highest resistance to scratch-induced failure, indicating superior mechanical performance under high-stress conditions. The (HfVTiZrW)B2 Diboride film grown at 500°C exhibited the highest hardness of 38.0 ± 0.6 GPa and the best corrosion resistance in 0.5 M sulfuric acid solution, which is 38 times higher than 304 stainless steels, demonstrating their potential as protective coatings for cutting tools. In summary, this study provides comprehensive insights into the microstructural and tribological behavior of (HfVTiZrW)B2 coatings deposited via HiPIMS. The findings underscore the critical influence of microstructural features, such as crystallization and grain size, on the frictional, mechanical, and corrosion-resistant properties of these coatings, making them promising candidates for industrial applications requiring robust performance.