MECHANICAL-PROPERTIES AND WEAR-RESISTANCE OF ION-BEAM-ASSISTED SPUTTER-DEPOSITED NITI(N) COATINGS

Thin films are deposited on an austenitic type 304L stainless steel using the sputtering of an Ni49Ti51 target with 1.5 keV N+ ions. The influence of a simultaneous 160 keV Ar+ implantation on hardness and wear resistance is studied. Hardness is measured with a nanoindenter. N+ reactive sputtering forms a coating harder than the substrate. The hardness increases by 80% when the deposit is ion beam assisted. Transmission electron microscopy analyses of the thin films reveal that the N+ sputtered films present a large content of nitrogen which forms with titanium strong disorganized metal-metalloid Ti-N bonds; when ion implantation is simultaneously used, very tiny crystalline TiN precipitates embedded in an amorphous matrix are observed. This could explain the increase in hardness. The simultaneous ion irradiation would favour TiN precipitation through an enhanced diffusion mechanism in the cascades. Subsequent wear resistance of these coatings is measured through three-dimensional profile analyses of the wear tracks obtained after tests performed on a pin-on-disc machine. It is shown that sputtered NiTi(N) coatings largely increase the wear resistance. When the deposit is ion beam assisted, the improvement is much more pronounced; it can be related to either the increase in the hardness or the ion beam mixing of the interface, or both these processes. The influence of the coating thickness and the ratio of the implanted ions to the deposited atoms are also studied.