Structure, mechanical properties and thermal stability of nitrogen-doped TaNbSiZrCr high entropy alloy coatings and their application to glass moulding and micro-drills

TaNbSiZrCr high entropy alloy (HEA) coatings were deposited on WC substrates using a radio frequency (RF) unbalanced magnetron sputtering system. To explore the effects of nitrogen addition on the coating properties, the coatings were deposited with various nitrogen-to-argon ratios R_Nx (where x = N₂/Ar and lies in the range of 0– 0.5). Furthermore, to examine the effects of heat treatment, half of the coatings were retained in an as-deposited condition, while the remainder were annealed at 750 °C for 60 min. The microstructures, mechanical properties and anti-sticking properties of the various coatings were evaluated and compared. The as-deposited coatings were all found to have an amorphous structure. By contrast, the annealed samples had a BCC structure at low R_Nx ratios (≤0.3), but a FCC structure at HR_N0.5. All of the coatings showed excellent adhesion and mechanical properties in both the as-deposited condition and the annealed condition. The hardness of the as-deposited coatings increased with an increasing nitrogen content. However, the annealing process produced no significant change in the hardness. Moderate nitrogen addition (R_Nx ≤ 0.4) was found to be beneficial in preventing the adhesion of the HEA coating to the glass interface during simulated moulding tests. However, the R_N0.5 coating showed a poor anti-glass-sticking performance due its columnar structure and high nitrogen content, which prompted a strong interfacial interaction. No significant change was observed in the surface energies of the coatings after the annealing process. Thus, it was inferred that all of the coatings had excellent thermal stability. Finally, the micro-drilling test results showed that the R_N0.5 coating, with the highest hardness of all the coatings, improved the quality of the drilled holes and increased the micro-drill lifetime to more than 6000 holes; representing a three-fold improvement over that of an uncoated micro-drill.