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

WC-Ti _(1-x) Al _x N 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) Al _x N films depend on the Al concentration (x). With increasing Al in the film, the interfaces between WC and TiAIN layers loose their coherency and WC-Ti ₀ .37Al ₀ .57N films shows 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) N 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 /D _nc ) of 0.8. When the D _buffer /D _nc- ratio was 0.3 film adhesion strength achieved a maximum value of 45.5 N while at higher D _buffer /D _nc- ratios than 0.3 the film adhesion strength decreased to 25 N. The microhardness of WC-Ti _(1-x) Al _x N 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-Ti ₀ .43Al ₀ .57N. In the X-ray diffraction analysis (XRD) analysis, the Ti ₀ .43Al ₀ .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-Ti ₀ .43Al ₀ .57N film was composed of very fine ( ~ 10 nm) nanocrystalline grains. So, we believe that the nanocrystalline microstructure of the film is fundamental importance for the dramatic enhancement of film hardness. The plastic deformation resistance factor (H ³ /E ² ) of WC-Ti _(1-x) Al _x N films was calculated to be in a range of 0.27-0.46.