Effects of duty cycle and nitrogen flow rate on the mechanical properties of (V,Mo)N coatings deposited by high-power pulsed magnetron sputtering

Abstract

(V,Mo)N is theoretically predicted to have high hardness and fracture toughness and is a promising material for the application on protective hard coatings. However, the toughness enhancement of (V,Mo)N coatings deposited by dc-unbalanced magnetron sputtering (dc-UBMS) was not as remarkable as expected. The issue could be due to insufficient energy delivery to the plasma species in the deposition process such that nitrogen and metal atoms were not fully reacted and led to the degradation of coating quality. Since high-power pulsed magnetron sputtering (HPPMS) can provide high peak power density, the method was selected to deposit (V,Mo)N coatings in this research. The objective of this study was to investigate the effects of duty cycle and nitrogen flow rate on the microstructure and mechanical properties of (V,Mo)N coatings deposited on Si substrates by HPPMS. Four sets of (V,Mo)N coatings were deposited by HPPMS at different durations with two duty cycles, 5% and 3%, and two nitrogen flow rates, 6.0 and 12.0 SCCM. The results showed that the N/metal ratio was mainly affected by the nitrogen flow rate, ranging from 0.70 to 0.96 with increasing nitrogen flow rate. The lattice parameter of the samples linearly increased with the N/metal ratio. The x-ray diffraction (XRD) patterns revealed that all samples tended to approach (200)-preferred orientation with increasing deposition duration. The glancing incident XRD patterns indicated that the samples deposited at 6 SCCM nitrogen flow rate and 3% duty cycle have multiphases. Transmission electron microscopy analysis confirmed that phase separation from (V,Mo)N to (V-rich,Mo)N and (V,Mo-rich)N occurred in those samples. The hardness of the (V,Mo)N coatings decreased with increasing N/metal ratio, which may be related to the N-vacancy hardening effect. The sample deposited at 6 SCCM nitrogen flow rate and 3% duty cycle for 36 h showed the highest hardness of 28.4 GPa, which was possibly associated with the phase separation, and hence plastic deformation became difficult. The fracture toughness (Gc) of the (V,Mo)N coatings was evaluated using the internal energy-induced cracking method. The resultant Gc of the (V,Mo)N coatings, ranging from 36.1 to 43.7 J/m2, was higher than that of the coatings deposited by dc-UBMS in our previous study. The toughness enhancement could be caused by a higher fraction of Mo–N bonding due to the adequate reaction energy provided by the HPPMS process.