ICMCTF2019 Session B5-2-FrM: Hard and Multifunctional Nanostructured Coatings II

Combining the high hot-hardness to enhanced toughness and good oxidation resistance is one of the greatest challenges in material-design of transition metal nitride (TMN) ceramics, which are widely employed as protective coatings in machining industry.

In Ti-Al-N and Cr-Al-N, well-known Al-containing TMNs, the thermally-induced decomposition of the cubic solid solution via spinodal mechanism or precipitation leads to the formation of fine-grained nanostructures and the associated age hardening. However, continuous thermal load/unload cycling during industrial machining cause transformation into thermodynamically more stable coarse-grained structures containing hexagonal AlN and cubic TiN or hexagonal CrN_x phases resulting in degradation of mechanical properties. In addition, brittleness limits the use of TMN-AlN alloys in applications demanding a high fracture-resistance.

The concept of multicomponent alloying with elements from groups IIIB – VIB (Y, Zr, Hf V, Nb, Ta, Mo, and W) represents a suitable way to improve the properties of Al-containing TMNs. From this group, pentavalent tantalum is a very attractive substitutional element which is used, for example, in Ti-Al superalloys to improve ductility, and to reduce oxidation at high temperatures. Moreover, the Ta-N phase diagram exhibits a large variety of Ta_yN_x structures, characterized by different electronic properties.

Here, the role of tantalum as a substitution atom improving thermal stability and mechanical properties is presented in several ternary and quaternary systems. In the first case, we combine experiments and ab initio calculations to investigate thermally induced age hardening in tough Ta-Al-N coatings via spinodal decomposition. The increase in hardness from 29 GPa to 35 GPa was observed during early stages of phases separations when temperature exceeded 1000°C. In the next case, a significant improvement in toughness of nitride coatings was observed in highly TaN-alloyed Ti-Al-N. While the hardness of Ti_0.46Al_0.54N (32.5 GPa) is not significantly affected by alloying with TaN, the elastic stiffness monotonically decreases from 442 to 354 GPa with increasing Ta contents indicating enhanced toughness in TiAlTaN. In the last presented Cr-Al-Ta-Y-N system, the presence of Ta in the solid solution shifts the decomposition process to higher temperatures (>1000°C) compared to Cr-Al-Y-N (~900°C), thus enhancing the alloy thermal stability. The improved thermal stability may be attributed to increased cohesive energie, as revealed from ab initio calculations.

This work was supported by the Slovak Research and Development Agency [Grant No. APVV-17-0320]

Most of the hard coatings have good corrosion resistance on certain applications. The traditional way for corrosion resistance accepted by industries are electroplated hard chrome. However, in some severe environment, hard chrome can not pass corrosion tests, as a result, PVD hard coatings are applied on hard chrome to even further enhance corrosion resistance in some special applications. In this research, we are investigating the unique application of cathodic arc technology in depositing AlTiN hard coating film onto electroplated hard chrome PH17-4 stainless steel 1-inch OD shafts. The requirement for the AlTiN multi-layer coating is to take into consideration of residual stress between the hard chrome layer and AlTiN coating to ensure great adhesion between two materials. Also, the residual chemicals from hard chroming process will be analyzed and the removal of chromium oxide will be addressed in both pre-coating cleaning and in-chamber cleaning processes. This research will reveals the effect of coating process on the sensitive to temperature of the duplex coatings which related to the residual stress. Meanwhile, it is critical to monitor whether any released on the hard chrome surface, causing contamination of chrome surface in the coating process. Results of this research will enhance corrosion resistance by applying both hard chrome and hard coating by PVD cathodic arc process.