Carbon doped α-Al2O3 coatings grown by chemical vapor deposition

Abstract

Alumina (Al2O3) coatings deposited by chemical vapor deposition (CVD) with different modifications and dopants are widely applied as wear resistant coatings on cemented carbide cutting tools. The aim of this work was to investigate the influence of CH4 addition on the deposition of α-Al2O3 by low-pressure chemical vapor deposition (LPCVD). The coatings were deposited at 1005°C on a TiN–TiCN base layer using a precursor gas mixture of AlCl3, CH4, CO2, HCl, H2S, and H2. Coating characterization was conducted by scanning electron microscopy (SEM), glow discharge optical emission spectroscopy (GDOES), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), glancing angle X-ray diffraction (GAXRD), nanoindentation and tribological ball-on-disk tests against Al2O3 balls. Additionally, the ball-on-disk wear tracks were investigated by Raman spectroscopy.The highest carbon doping content achieved in this study was in the range of 0.5–1.0at.%. SEM top-view images indicate a less facetted coating topography with slightly larger grains for C-doped coatings. GAXRD patterns show the α-Al2O3 modification for undoped and C-doped coatings with only minor differences concerning lattice parameters, preferred orientation and stresses. The hardness values remain at ~25GPa for both coating types. The friction coefficient decreases from 0.7 for undoped Al2O3 to 0.5 and 0.4 for the C-doped coating at room temperature and 700°C, respectively, which is attributed to the formation of a lubricious nanocrystalline graphite layer in the sliding contact. At 900°C, both coatings show a further reduction of the friction coefficient to 0.35 due to out-diffusion of titanium through the thermal crack network of the Al2O3 layer and formation of rutile.