Wear resistance and structure of (Ti,Al)N coatings

Abstract

Wear resistance of cemented carbide tools with TiN and (Ti, Al)N coatings in lathe turning of steel was investigated. WC-15%TiC-6%Co and WC-3%Co cemented carbides were used in experiments. The thickness of coatings in all cases was 5 µm. Cutting speed in the tests ranged from 70 to 220 m min-1, from 0.1 to 0.2 mm rev-1 and depth from 0.25 to 0.5 mm. All coated inserts outperformed uncoated ones. In the case of WC-15%TiC-6%Co inserts (Ti,Al)N outperformed TiN, but for WC-3%Co inserts TiN coatings demonstrated better wear resistance than (Ti,Al)N. The method of arc vapour deposition was selected for experiments with titanium and Ti-Al alloy cathodes. Ti-Al cathodes were made using electron beam remelting of a powder metallic mixture in vacuum. The compositions of Ti-Al cathodes were 70Ti-30Al (wt%) and 50Ti-50Al. Al/Ti ratios (at.%) in (Ti,Al)N coatings, based upon energy dispersive x-ray microprobe measurements, were 0.1 and 0.3 respectively. From x-ray diffraction data all coatings were found to have an NaCl structure. The preferred orientation for TiN coating was (111); for both (Ti, Al)N coatings it was (200). Lattice parameters calculated from TiN(200) reflections via Al/Ti ratio were as follows: a = 0.4219 nm (for TiN), a = 0.4215 nm and a = 0.4191 nm. The stresses in coatings were determined assuming the plane stress state and an equality of stresses in substrate and coating. It was demonstrated that growth of Al content in the coating led to increased stresses. Investigations of surface topography were performed using scanning electron microscopy. The main roughness parameters (asperity height, average radius of separate asperities, angle of obliquity etc) of (Ti,Al)N coating were higher in comparison with TiN because of a great number of microdrops on the coating surface. The turning tests demonstrated that there was only a slight difference in wear resistance between the two (Ti,Al)N coatings. Probably, the principal reason for improvement of tribological properties was in stronger links between atoms in the (Ti, Al)N structure. The effect of microdrops was harmful, but in oxidizing conditions synthesis of Al2O3 on the surface was a favourable factor.