A CrAlN/18 vol.% BN nanocomposite film was deposited on substrate by reactive co-sputtering. The films showed an increase of about 30% in indentation hardness and achieved a maximum hardness of approximately 50 GPa after annealing at 800 degrees C in air. In contrast, the indentation hardness barely changed when the film sample was annealed at 800 degrees C in nitrogen and argon atmosphere. High-resolution transmission electron microscopy (HRTEM) images revealed that the uppermost layer was characterized by amorphous materials with embedded nanocrystalline particles (occurring at less than similar to 50 nm below surface). Energy dispersive X-ray spectroscopy (EDS) line profiles of cross-sectional thin films showed a high concentration of oxygen in the uppermost layer of the annealed sample. The indentation hardness of the air-annealed sample was measured by Ar+ ion sputtering before and after etching to the depth at 200 nm from the annealed surface. The hardness decreased from approximately 48 GPa to 43 GPa, which was the same level as the as-deposited films. These results indicate that oxidization of the film surface could be one of the factors responsible for the self-hardening of the CrAlN/BN film. (C) 2012 Elsevier B. V. All rights reserved.