Boron carbonitride thin films were deposited by sputtering of a B4C target with AT-N-2 ion assistance. BCxNy films were grown onto Si (001) at room temperature. The chemical composition and the type of bonding were determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The hardness of the films was measured with a nanoindenter. The chemical analysis of the samples indicates the formation of two different compounds, a ternary BCxNy and a binary carbonitride CNx. All the films showed high hardness, in the range 16-33 GPa, which clearly increases as the BCxNy content in the sample increases. In this study the highest hardness (i.e. 33 GPa) was obtained when the BCxNy content in the sample was 50%. The average composition of this BCxNy was estimated by XPS as 20 at.% carbon and 12 at.% nitrogen. (c) 2005 Elsevier B.V. All rights reserved.