Freestanding diamond films were fabricated by a new self-separation method. Thick poly-crystalline diamond films were grown on poly-crystalline Ni substrates by microwave plasma-enhanced chemical vapor deposition after the substrates were saturated with carbon via a saturation process using a carbon solid solution. This saturation process suppressed the erosion of diamond nuclei on the Ni substrates. During the cooling process after diamond growth, the carbon atoms dissolved in the Ni substrates became supersaturated and precipitated as graphite interlayers at the interfaces between the diamond films and Ni substrates. The graphite interlayers caused the thick diamond films to spontaneously separate from the Ni substrates without cracking, allowing the Ni substrates to be reused. This self-separation method provides a low-cost way to fabricate freestanding diamond films. (C) 2017 Elsevier B.V. All rights reserved.