The effect of Al composition on the microstructure and mechanical properties of WC–TiAlN superhard composite coating

WC–Ti(1−x)AlxN nc-films were deposited on WC–Co and Si substrates using a multi-cathode arc ion-plating system. The microstructure and mechanical properties of the films were investigated to find out the nanostructured film growth mechanism. The microstructure of the WC–Ti(1−x)AlxN films depend on the Al concentration (x). With increasing Al in the film, the interfaces between WC and TiAlN layers loose their coherency and WC–Ti0.37Al0.57N films show a completely nanocrystalline structure with a grain size of 10 nm, which is in agreement with the superlattice period (λ). The residual stress in WC–Ti(1−x)AlxN films was independent of the x value and measured to be approximately 6.5 GPa. This high stress of the films was reduced to a value of 4.7G Pa by introducing Ti–WC buffer layers periodically with a thickness ratio (Dbuffer/Dnc-) of 0.8. When the Dbuffer/Dnc- ratio was 0.3, film adhesion strength achieved a maximum value of 45.5 N while at higher Dbuffer/Dnc- ratios than 0.3 the film adhesion strength decreased to 25 N. The microhardness of WC–Ti(1−x)AlxN film was measured to be in the range of 38–50 GPa. The highest value of film hardness was obtained from the nanocomposite film of WC–Ti0.43Al0.57N. In the X-ray diffraction analysis (XRD) analysis, the Ti0.43Al0.57N film exhibited the same structure as the superhard (H≥40 GPa) phase, which exhibits only TiAlN(111) and (200) reflections. Transmission electron microscopy (TEM) analysis also showed that WC–Ti0.43Al0.57N film was composed of very fine (∼10 nm) nanocrystalline grains. So, we believe that the nanocrystalline microstructure of the film is of fundamental importance for the dramatic enhancement of film hardness. The plastic deformation resistance factor (H3/E2) of WC–Ti(1−x)AlxN films was calculated to be in a range of 0.27–0.46.