Amorphous carbon nitride (CNx) films were deposited on plasma nitrided Ti–6Al–4V substrate in order to improve the adhesion strength and tribological behaviour. Scratch and ball-on-disk wear tests were performed to evaluate the load bearing capacity, wear and friction characteristics of the duplex-treated coatings. Compared with a CNx film deposited on Ti–6Al–4V substrate, the load bearing capacity of a CNx film deposited on plasma nitrided layer was improved dramatically. Results showed that under dry sliding condition, the duplex-treated system was more effective in maintaining a favourable low and stable coefficient of friction and improving the wear resistance than both individual plasma nitriding and CNx film on Ti–6Al–4V substrate. The reasons for this significant improvement in tribological behaviour with the application of duplex treatment can be attributed to the combined benefits from both plasma nitriding and CNx films. (1) Plasma nitriding of Ti–6Al–4V produces a graded hardened case which serves as an excellent supporting and load bearing layer for hard CNx films. (2) The CNx film deposited at low temperature can produce a wear resistant and low-friction surface without impairing the beneficial effects from plasma nitriding treatment, and the smooth CNx films could effectively reduce both the interfacial stresses and the stresses near the surface thus providing a good tribological behaviour. (3) The graphitisation of the wear debris during dry sliding condition can help to decrease the coefficient of friction and improve the wear resistance.