Recent progress in large scale manufacturing of multilayer/superlattice hard coatings

Since the early fundamental research on superlattice structured hard coatin gs in the late 1980s, rapid progress has been achieved to produce nanoscale compositionally modulated multilayer structures. It has been shown that th e periodicity of the multilayers is strongly controlled by the substrate ro tation and the actual deposition rate. Appropriate multi-target geometry an d controlled target poisoning by optimised pumping conditions lead to depos ition conditions similar in their economy to the deposition of typical mono lithically grown binary hard coatings. The combined steered cathodic are/un balanced magnetron technology guaranties sufficient adhesion (L-C > 50 N) o f the usually highly stressed coatings as well as smooth surfaces due to UB M deposition (R-a < 0.04 <mu>m). This paper has described the properties of coatings dedicated to high temperature performance: TiAlN/CrN (period 3.03 nm), to tribological applications: TiAlYN/VN (period 3.62 nm) and combined wear and corrosion resistance CrN/NbN (period 3.2 nm). All the coatings in vestigated were found to crystallise into B1 NaCl f.c.c. structures, and ex hibited {110} and {111} or {100} preferred orientations for TiAl/CrN, TiAlY N/VN and CrN/NbN superlattice coatings, respectively. The residual stress w as found to be compressive in the range of -4.0 to -8.5 GPa for TiAlYN/VN a nd between -1.8 and -7.5 GPa for CrN/NbN, depending on the stoichiometry an d the bias voltage during coating deposition. Corresponding to the high str ess values, the plastic hardness of the coatings was measured to be 55-60 G Pa for TiAlN/CrN, 42-78 GPa for the TiAlYN/VN system and between 42 and 56 GPa for CrN/NbN, depending on the bias voltage. Oxidation resistance at tem peratures exceeding 900 degreesC was typical for TiAlN/CrN. The TiAlYN/VN c oating showed superior tribological properties with a coefficient of fricti on mu = 0.4 and low sliding wear of 1.26 X 10(-17) m(2) N-1 after 1.1 milli on cycles against an Al2O3 ball in a pin-on-disc test. CrN/NbN exhibited tw o orders of magnitude lower passive current densities than electroplated ha rd Cr and a pitting potential of 450 mV during polarisation in acetate buff er solution. When Nb+ ion etching was used, the CrN/NbN superlattice coatin g deposited on 304L, stainless steel showed high pitting potentials in the range of 750-1000 mV in the same corrosive medium. (C) 2000 Elsevier Scienc e B.V. All rights reserved.