In situ duplex plasma nitriding and PVD coating was performed using the combined steered arc unbalanced magnetron production scale unit (Hauzer HTC 1000/ABS). The processing procedure customary used was modified and further steps relating to the new plasma nitriding were added. In particular the total pressure during nitriding was reduced from 2−5 mbar, which is typical for pulse plasma nitriding process, to 5−10×10−3 mbar. To enhance ionisation to a sufficiently high level the available four magnetrons were ignited at a low power level (0.5 W/cm2). Furthermore, electromagnetic coils were used to generate a closed magnetic field situation, thus, reducing the loss of electrons to the chamber walls.
Using the duplex plasma nitriding (treatment time 30min.) and PVD treatment, 3 microns thick TiAlCrYN coatings were deposited onto flat HSS coupons. Scratch test results showed a substantial enhancement of the critical load (Lc > 80 N) compared to un-nitrided coated samples (Lc ~ 50 N).
It is important to note that these high adhesion values have been obtained only when the thin build-up in carbon near the surface, which was generated during the plasma nitriding step, was etched away by metal ion etching with Cr+ directly prior to coating.