Carbon nitride based hard multilayer films prepared by closed field unbalanced magnetron sputtering

Abstract

Single layer a-CNx films as well as CNx/TiN multilayer films were deposited onto Si (100) and M42 high speed steel substrates by closed field unbalanced magnetron sputtering. In CNx/TiN multilayers, different bilayer (CNx + TiN layer) thickness could be attained in two ways: by changing the substrate rotation speed (1·3–20 rev min−1) and by varying the C target current only with substrate rotation speed fixed at 4 rev min−1 during the deposition process. The films were characterised by employing atomic force microscopy (AFM), nano/microindentation measurements, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), pin on disc wear test, scratch tester and Rockwell C tester. The XPS results of CNx films suggest the formation of a major fraction of N–C sp2 bonds and a few N–C sp3 bonds. The nanohardness of CNx film about 11·7–20·8 GPa depends on the N/C ratio. In CNx/TiN multilayers, TiN, TiO2, TiC, C—N and C=N chemical and bonding states exist. The mechanical and tribological properties are influenced by different bilayer periods (about 10–1 nm). The root mean square (RMS) surface roughness measured by AFM was between 0·7 and 1·9 nm. High microhardness (∼50 GPa) of the multilayered structure depends on the bilayer thickness. The internal compressive stress is associated with the hardness of the multilayer which falls to about 3·3–7·7 GPa. The pin on disc tests under dry condition show a low friction coefficient between 0·11 and 0·15. Rockwell C and scratch tests show excellent adhesion results for the coating prepared at C target current of 0·5 A.