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 (lambda). 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 (D-buffer/Dnc-) of 0.8. When the D-buffer/Dnc- ratio was 0.3, film adhesion strength achieved a maximum value of 45.5 N while at higher D-buffer/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 greater than or equal to 40 GPa) phase, which exhibits only TiAlN(111) and (200) reflections. Transmission electron microscopy (TEM) analysis also showed that WC-Ti(0.43)A(0.57)N film was composed of very fine (similar to 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 (H-3/E-2) of WC-Ti(1-x)AlxN films was calculated to be in a range of 0.27-0.46. (C) 2001 Elsevier Science B.V. All rights reserved.