Results of the mechanical and structural properties of silicon-incorporated hard hydrogenated amorphous carbon films are reported. A strong reduction of the residual internal stress with an almost constant mechanical hardness was observed and the possible causes of this behavior are investigated, Infrared absorption and elastic recoil hydrogen detection results show that silicon is incorporated replacing carbon atoms in the amorphous network with approximately constant bonded hydrogen density and total hydrogen content. Additionally, Raman experiments indicate that silicon incorporation increases the sp(3) character of the material, while electron spin resonance measurements show a decrease of the number and size of the sp(2) graphitic defects present in the material. Hydrogen effusion experiments indicate that the observed reduction of residual internal stress may be attributed to a less compact material with an increased density of voids in comparison tu pure a-C:H.