Quaternary TiZrAlN nanocomposite thin films were deposited by Closed-Field Unbalanced Magnetron Sputtering (CFUBMS), and their microstructure and mechanical properties were examined. The grain refinement of the TiZrAlN nanocomposite thin films was controlled by adjusting the N-2 partial pressure. The hardness of the film varied with the N-2 partial pressure with a maximum value of approximately 47 GPa. A critical grain size, d(c), was found to be essential for maximizing the hardness. (c) 2007 Elsevier B.V. All rights reserved.