A new low pressure plasma nitriding and PVD coating duplex treatment of HSS substrates

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 5-10x10(-3) mbar. To enhance ionisation to a sufficiently high level the available four magnetrons were ignited at a low power level (0.5 W/cm(2)). 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 (L-C > 80 N) compared to un-nitrided coated samples (Lc similar to 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.