Ternary and quarternary TiSiN and TiSiCN nanocomposite coatings obtained by Chemical Vapor Deposition

Abstract

TiN and TiCxNy are commercial CVD coatings widely used for cutting tools. A promising route for improving their performance is the addition of silicon. Therefore the main objective of this work is the preparation of TiSiN and TiSiCN coatings with a nanocomposite structure offering a higher hardness as well as an improved oxidation and wear resistance. New low pressure CVD processes in the range 800°C–900°C were developed for the successful deposition of nanocomposite coatings of TiSiN and TiSiCN on hard metal substrates. TiSiN nanocomposites are obtained with a mixture of the precursors TiCl4, SiCl4 and NH3 whereas for TiSiCN coatings acetonitrile is added instead of NH3. In this work microstructure, composition, properties and oxidation behavior of the nanocomposite coatings were investigated. TiSiN coatings consist of the crystalline phases TiN, Ti5Si3 and amorphous silicon nitride. The hardness correlates well with silicon content and crystallite size. A maximum hardness of about 3700 HV[0.01] was observed at a silicon content between 6at.% and 8at.% and a TiN crystallite size about 15nm. TiSiCN coatings prepared with the acetonitrile process exhibit a nanocomposite structure of nanocrystalline TiCxNy and amorphous SiCxNy. Compared to TiSiN a still higher hardness up to 4100 HV[0.01] was measured for a silicon content of 11.7at.%. TiSiN coatings with a silicon content between 5.5 and 7.7at.% offer an oxidation resistance up to 700°C. But TiSiCN with 2.2at.%–11.7at.% silicon is even more stable up to 900°C. At these temperatures TiO2 is formed at the surface.The examination of the adherence showed that a diffusion barrier is necessary for suppressing the cobalt diffusion from cemented carbide substrate into the layers. If interlayers of TiN or TiCxNy were applied critical loads of 80N for TiSiN and 43N for TiSiCN coatings were obtained by scratch tests.