Aluminum Titanium Nitride (AlTiN)
In order to extend the temperature range of applicability of TiN, a ternary composition, TiAlN or AlTiN was developed. The difference between the two is the Al content, which is higher in AlTiN, with the exact composition depending on the vendor of a specific coating. This coating withstands the conditions emerging during high speed, dry machining of, e.g., carbon steels.
A coating for this class of applications must be sufficiently hard but still resistant to brittle fracture, adherent to its substrate, and chemically inert in a broad range of reactive environments to be effective to extend tool life, and be able to survive exposure to aggressive oxidative exposure at high temperatures encountered during high speed dry machining of carbon steels and other tough metal.
During AlTiN deposition, a metastable solution of Ti-N and Al-N is formed (Al1−xTixN). Upon exposure to high temperatures, it undergoes spinodal decomposition: as a result of localized solid state diffusional mass transport, individual atoms diffuse to coalesce into nanometer-size domains, small conglomerates or crystallites. This structural change will increase the hardness both at low and elevated temperatures. Because of the metastability of the as-deposited coating, spinodal decomposition will occur only at elevated temperatures.
Upon exposure to atmospheric air at high temperatures, the surface residual aluminum oxidizes to form a dense, continuous layer of Al2O3 which is an effective high temperature oxygen diffusion barrier, protecting the interior of the coating and improving oxidation resistance. This layer extends the temperature range of utility of AlTiN-coated tools up to ~ 800°C (1450°F).
Above this temperature, there is an increasingly strong influence of the surface residual titanium which, due to its oxidation, transforms into porous, low density titanium oxide, TiO2, thus providing a low impedance mass transport path of oxygen into the interior of the film. Upon further heating to above ~ 800°C, rapid loss of nitrogen from the underlying lattice and conversion of the surface to a soft, porous, mechanically inferior structure occurs.
Note that both oxidation resistance and superior hardness of this coating is predicated by exposure to high temperature, and unless this condition is fulfilled, the performance of AlTiN coatings will be mediocre at best. It also should be mentioned, that when properly used, there is a self-restoration effect during its application: when the surface Al2O3 layer wears off, new layer is formed by further oxidation.
We recommend AlTiN only be used in applications that will oxidize the surface residual aluminum, however there have been applications that theoretically should have yielded mediocre results with AlTiN but had instead superior results. For this reason we recommend that our customers who want to fully explore their options try AlTiN.