Titanium doped CVD alumina coatings

Chemical vapor deposited alumina coatings are successfully applied for high-performance cemented carbide cutting tools. The aim of this work was to investigate in detail the influence of a varied TiCl4 flow rate on the deposition of alumina by low-pressure chemical vapor deposition (LPCVD). The coatings were deposited onto a TiN–TiCN base-layer at 1005 °C using a precursor gas mixture of AlCl3, TiCl4, CO2, HCl, H2S, and H2. Coating characterization was conducted by scanning electron microscopy (SEM), glancing angle X-ray diffraction (GAXRD), glow discharge optical emission spectroscopy (GDOES), nanoindentation, biaxial stress measurements, and ball-on-disc tests. GAXRD investigations showed the α-Al2O3 phase for all coatings beside different titanium compounds. SEM top-view images indicated that low titanium contents modify the polygonal grain shape of the α-Al2O3 phase to a more cuboid-like form, while the highest titanium content applied (4.2 at.% Ti) causes a less dense coating topography. The hardness values remain constant at ~ 26 GPa for pure α-Al2O3 and low Ti doping contents and decrease slightly to ~ 23 GPa for 4.2 at.% Ti. However, with increasing Ti-content the tribological behavior at 700 °C in terms of both friction and wear resistance is significantly improved.