Aluminum-rich TiAlCN coatings by Low Pressure CVD

Abstract

Ti 1 − x Al xN is a well established material for cutting tool applications exhibiting a high hardness and an excellent oxidation resistance. A main route for increasing the performance of Ti 1 − x Al xN is the incorporation of further elements. Therefore the main objective of this work is to improve the properties and wear resistance of aluminum-rich CVD–TiAlN coatings by incorporating carbon. A new Low Pressure CVD process was employed for the deposition of a very aluminum-rich TiAlCN layers. The process works with a gas mixture of TiCl 4, AlCl 3, NH 3, H 2, N 2, Ar and ethylene as carbon source. In this work microstructure, composition, properties and cutting performance of CVD–TiAlCN coatings were investigated. Hard aluminum-rich TiAlCN coatings were obtained at 800 °C and 850 °C consisting of a composite of fcc-Ti 1 − x Al xN and minor phases of TiN, h-AlN and amorphous carbon. WDX analysis indicates only a low carbon content < 2 at.%. Lattice constant calculations suggest that carbon atoms should not be incorporated in the Ti 1 − x Al xN lattice. From TEM analysis and Raman spectroscopy it is evident that carbon is mainly located at the grain boundaries as a-C phase. Therefore these fcc-Ti 1 − x Al xN(C) coatings with low carbon content are rather a composite of fcc-Ti 1 − x Al xN and an amorphous carbon phase (a-C). At 900 °C the metastable fcc-Ti 1 − x Al xN nearly disappears and co-deposition of TiN and h-AlN occurs. The layers deposited at 800 °C and 850 °C possess a high hardness around 3000 HV and compressive stress. CVD–TiAlCN coatings prepared at 850 °C shows also an amazing thermal stability under high vacuum conditions up to 1200 °C. Aluminum-rich composites fcc-Ti 1 − x Al xN/a-C with x > 0.8 exhibit a superior cutting performance in different milling tests.