Boron carbon nitride (BCN) coatings were deposited on Si(100) wafers and Si3N4 disks by using ion beam assisted deposition from a boron carbide target. The BCN coatings were synthesized by the reaction between boron and carbon vapor as well as nitrogen ion simultaneously. The influence of deposition parameters such as ion acceleration voltage, ion acceleration current density and deposition ratio on the surface roughness and mechanical properties of the BCN coatings was investigated. The surface roughness was determined by using atomic force microscopy and the mechanical properties of the BCN coatings were evaluated by nano-indentation tests and friction tests in N-2 gas. The composition and chemical bonding of the BCN coatings were analyzed by using X-ray photoelectron spectroscopy. The results showed that the lower deposition rate, the smaller surface roughness and higher nano-hardness the BCN coatings were. The BCN coating with the smoothest surface (R-a= 0.25 nm and RP-V=2.8 nm) and the highest nanohardness of 33 GPa as well as excellent friction property were obtained at 0.5 nm/s and the nitrogen ions were generated at 2.0 W and 60 mu A/cm(2), and the chemical composition of this BCN coating was 49 at.% B, 42 at.% C and 9 at.% N. Moreover, there were several bonding states such as B-N, B-C and C-N with B-C-N hybridization in this BCN coating. ((c) 2005 Elsevier B.V. All rights reserved.