Oxygen-free aluminum carbonitride thin films were synthesized by reactive magnetron sputtering of Al target with a gas mixture of Ar, CH4 and N-2. The effect of substrate temperature varying from the room temperature to 400 degrees C was investigated, since the crystalline Al-C-N compounds can be modeled as the stacking of zigzag building blocks Al2C2, Al2C and AlN. Generally, film growth proceeds preferably along the [0 0 0 1] direction, but in-plane growth makes its competitive presence in the temperature range from 50 to 260 degrees C, as revealed by X-ray diffraction. For the samples with a typical composition of Al50C13N37, the size of crystallites, and thus the root-mean-square roughness of the film, becomes larger with increasing substrate temperature. Berkovich hardness is over 27.0 GPa for all as-deposited films and a maximum value of 33.6 GPa was measured in the sample prepared at 300 degrees C. These results indicated that hard Al-C-N coatings with well-controlled orientation can be fabricated by reactive magnetron sputtering at moderate substrate temperatures. (c) 2005 Elsevier B.V. All rights reserved.