Polycrystalline CrN/AlN multilayer coatings were deposited by RE magnetron sputtering on silicon (001) substrates. The bilayer periods of CrN/AlN were controlled from 4 nm to 20 nm by the use of shutters, which were adjusted by a programmable logic control (PLC). To evaluate the thermal stability, the films were annealed at 500 degrees C, 600 degrees C, 700 degrees C, 800 degrees C, and 850 degrees C, for I h in both vacuum and air environments. The phase transformation during thermal evolution was studied by X-ray diffraction (XRD). The microstructure of CrN/AlN multilayer coatings as-deposited and after annealing was observed by transmission electron microscopy (TEM). The hardness of as-deposited CrN/AlN coating with a period of 4 nm was 28.2 GPa, which was 60% higher than that predicted by the rule of mixtures. The hardness of CrN/AlN multilayer coatings annealed at 850 degrees C in vacuum remained similar to the as-deposited state, and the nano-layered structure still persisted. The thermal stability of CrN/AlN coatings was better than that of CrN coating. The hardness degradation ratio of CrN/AlN coating with modulation period of 4 nm was only 8.1% at 700 degrees C, which was superior to that of a simple CrN coating. (c) 2005 Elsevier B.V. All rights reserved.