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

Since the early fundamental research on superlattice structured hard coatings in the late 1980s, rapid progress has been achieved to produce nanoscale compositionally modulated multilayer structures. It has been shown that the periodicity of the multilayers is strongly controlled by the substrate rotation and the actual deposition rate. Appropriate multi-target geometry and controlled target poisoning by optimised pumping conditions lead to deposition conditions similar in their economy to the deposition of typical monolithically grown binary hard coatings. The combined steered cathodic are/unbalanced magnetron technology guaranties sufficient adhesion (L-C > 50 N) of the usually highly stressed coatings as well as smooth surfaces due to UBM deposition (R-a < 0.04 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 investigated were found to crystallise into B1 NaCl f.c.c. structures, and exhibited {110} and {111} or {100} preferred orientations for TiAl/CrN, TiAlYN/VN and CrN/NbN superlattice coatings, respectively. The residual stress was found to be compressive in the range of -4.0 to -8.5 GPa for TiAlYN/VN and between -1.8 and -7.5 GPa for CrN/NbN, depending on the stoichiometry and the bias voltage during coating deposition. Corresponding to the high stress values, the plastic hardness of the coatings was measured to be 55-60 GPa 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 temperatures exceeding 900 degreesC was typical for TiAlN/CrN. The TiAlYN/VN coating showed superior tribological properties with a coefficient of friction mu = 0.4 and low sliding wear of 1.26 X 10(-17) m(2) N-1 after 1.1 million cycles against an Al2O3 ball in a pin-on-disc test. CrN/NbN exhibited two orders of magnitude lower passive current densities than electroplated hard Cr and a pitting potential of 450 mV during polarisation in acetate buffer solution. When Nb+ ion etching was used, the CrN/NbN superlattice coating deposited on 304L, stainless steel showed high pitting potentials in the range of 750-1000 mV in the same corrosive medium. (C) 2000 Elsevier Science B.V. All rights reserved.