Duration: 1998-2001

The cutting edges of milling tools reach very high temperatures, especially under dry, high-speed machining conditions. Tool failure may then be initiated by oxidation of the tool coating. This project seeks to gain fundamental understanding of the oxidation process of TiN, TiAlN and TiAlCrYN based hard coatings at elevated temperatures, in the range of 700-950 degrees celsius.

High speed cutting operations demand good oxidation resistance from the coatings that are deposited on cemented carbide tools. A multi-component TiAlCrYN coating increases the temperature resistance of the well-known and commonly used TiAlN coating from 700 degrees celsius to 950 degrees celsius. A series of transmission electron microscope (TEM) images demonstrates which methods are used to establish the oxidation mechanism of TiAlCrYN with a subsequently deposited oxy-nitride layer.

From these investigations it is known that the structure of the coating changes from fine grained interrupted columnar growth to a competitive growth structure. Y was found to diffuse to the grain boundaries and to prevent further internal oxidation of the coating. The substrate/coating interface is unaffected by a five-hour heat treatment at 900 degrees celsius.

The scale formation is rather complicated and disturbed because different mechanism are competing against each other, like different expansion coefficients from the oxides, different free energy of formation and activation energies.

Project staff: Professor P Eh Hovsepian, M Lembke