The Surface Engineers

Low Pressure CVD Technology for TiAlN Coatings

15 September 2013
Category: Cutting Tools

The development of new and more efficient wear resistant coating is motivated by increasing demands on cutting tools which result from the increase of high-speed and dry cutting processes as well as the cutting of hard materials. Such materials include, for example, high-strength steel and cast materials as well as high-strength alloys for automotive and aircraft engineering. At high cutting speeds, temperatures of more than 1000 °C are obtained on the cutting edge. This is why modern wear resistant layers must have - aside from high hardness or even toughness - good oxidation resistance and to be chemically inert against the counterpart to be machined.

Over the last decades, Ti1-xAlxN with face center cubic (fcc) structure has become an important standard layer for wear resistant applications. To date, it could only be produced using coating methods that work at comparatively low temperatures (PVD) as it is a metastable material. But PVD methods only allow the deposition of fcc structure Ti1-xAlxN coatings with an aluminum content limited to x = 0.67 and thus oxidation resistance is also limited. There have been different development works in order to increase the Al content in such coatings. IHI Ionbond has developed a new industrial scale CVD technology for the deposition of high aluminum containing Ti1-xAlxN coatings with stoichiometry coefficient up to x = 0.91 and face centered cubic structure. The cubic Ti1-xAlxN phase, however, can only be produced in a certain process window, such as temperature below 850 °C, low pressures and using certain ratios of AlCl3/TiCl4. In case of a ratio that is too low, TiN is additionally deposited. In case of ratios that are too high, AlN is deposited in the wurtzite structure. Usually at temperatures above 850 °C, TiN and AlN are co-deposited. High contents of these secondary phases are not desired, as hardness, oxidation resistance and wear resistance of the layer are reduced.

Aluminum chlorides AlCl3 and Titanium chlorides TiCl4 are used as starting materials. For this TiCl4 liquid is evaporated and AlCl3 is generated in-situ before flowing in the CVD reactor. Additional ammonia NH3 is used as nitrogen source. For this purpose a special NH3 module was developed by IHI Ionbond AG in order to make the supply of the precursor possible. A further step for making this process industrial scale capable was an optimized gas mixing and distribution system and specific tools loading procedure. The last process characteristic is the low pressure conditions, that are required and favor the formation of fcc Ti1-xAlxN. For this purpose IHI Ionbond AG has developed a special pumping system for the CVD equipment that allows the operation at pressures around 10 mbar.

An example here is showing Ti1-xAlxN coating with a high aluminum content of x = 0.83 and almost solely consists of the cubic phase “fcc-TiAlN”. This layer also demonstrate high hardness values of approx. 29 GPa at room temperature and moderate residual compressive stress up to -3 GPa.

For further questions please contact Hristo Strakov