Chromium Aluminum Nitride (CrAlN)
The addition of varying concentrations of aluminum to chromium based coatings yields several important benefits, such as increased hardness of the lattice due to spinodal decomposition (similar to the process encountered during deposition and application of AlTiN coatings), and improved oxidation resistance. As in the case for AlTiN, the improvement in oxidation resistance is the result of conversion of surface residual aluminum (Al) to a protective, dense aluminum oxide (Al2O3) layer by the exposure to atmospheric oxygen of the tool surfaces at high temperatures generated during, e.g., high speed dry machining. This oxide layer serves as a diffusion barrier to atmospheric oxygen, which, once it is formed on the surface, prevents further oxidation of the interior of the hard coating and the substrate.
The corrosion protection of AlTiN is limited due to the disruption of the protective Al2O3 oxide layer by the presence of pre-existing or out-diffusing Ti. At elevated temperatures during cutting, this titanium will also oxidize, and its oxides tend to be porous, with inferior protective properties, thus disrupting the continuity and reducing the protective ability of Al2O3.
Therefore, the logical next step in coating development for metal cutting tools was the search for titanium-free coatings, which would still possess high hardness, high adhesion to the substrates, sufficient toughness, and extended oxidation resistance. The elimination or replacement of titanium by chromium extends the temperature range of applicability of the coating. CrAlN proved to be one of the coatings with extended thermal stability and excellent high temperature performance.
Similar to the AlTiN, the incorporation of Al into the cubic CrN crystal structure substantially increased the hardness of the CrAlN coatings. Because of their superior hardness, CrAlN coatings (especially with higher Al-content) have higher abrasive wear resistance than the TiAlN coatings. Abrasive wear is the major wear mechanism in machining and forming applications, and often the dominant factor limiting the life of the tool. Also, practical metal working applications demonstrated that the tribological properties of CrAlN are superior to those of the AlTiN coatings in sliding wear (with both lower wear rate and lower friction coefficient). As expected, CrAlN coatings also exhibited higher corrosion resistance and better cutting performance under high speed machining conditions than the AlTiN coatings.
It should be noted, that as with AlTiN coatings, the superior performance of CrAlN coatings is also predicated on the exposure to high temperatures to produce age hardening (spinodal decomposition) and for the formation of a dense, continuous protective Al2O3 layer. Consequently, CrAlN coatings are best suited for high speed dry machining of carbons steels where high temperatures are generated during the operation.
Further, CrAlN also possesses self-healing properties; that is, a worn or damaged protective Al2O3 layer will re-grow, “heal” as a result of oxidation of the surface – residual Al upon exposure to atmospheric oxygen at high cutting temperatures.