A-C:H films with high hardness (>3500 kp/mm2 ) and high internal stress have been bombarded with 4-MeV Ni+ ions at several doses. Hardness, internal stress, and hydrogen content were measured as a function of Ni+ dose. It was found that at doses between 3×1012 and 3×1013 ions/cm2 the hardness decreases. At doses below 1013 ions/cm2 the internal stress increases above its original value. Hydrogen concentration and internal stress decrease in the ion range between 3×1013 and 1015 ions/cm2. For large doses the three properties reach a constant value.