Nb, NbXAl, NbXAlN and Nb2AlN-CHX coatings (where x indicates the niobium target current or acetylene flow rate) were deposited on SKH51 substrates using a radio frequency unbalanced magnetron sputtering system. The addition of Al and N to the Nb coating decreased the cluster size and prompted a solid solution strengthening effect, which improved the hardness, hardness/elastic modulus (H/E) ratio, adhesion strength and tribological properties. Among all the Nb, NbXAl and NbXAlN coatings, the Nb2AlN coating showed the best hardness (34.5 GPa), adhesion strength (Lc > 100 N) and tribological properties. Nb2AlN coatings were thus deposited with acetylene fluxes of 6 to 18 sccm. Acetylene addition prompted a change in the coating structure from metal nitride crystalline to diamond-like carbon. The Nb2AlN-CH12 coating showed a high H/E value of 0.094 and produced a carbon-rich transfer layer, which provided a solid lubricant effect at the contact interface during sliding. Consequently, the Nb2AlN-CH12 coating exhibited the best tribological performance among all the tested coatings.
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