Hybrid carbon coating was developed for the modification of diecast mold surfaces to expand the application fields of diecasting to the production of high performance automotive components. However, nitride decomposition and/or denitriding occur on the mold surface after several ten thousand shots in the practical diecast process, resulting in shortened mold life. In order to generated the self-healing of diecast mold surfaces during the process, pollution-free soft-nitriding was fundamentally studied by heating a pure iron substrate contacting urea at 500℃ for 48h in an oxygen-free atmosphere. It was confirmed that the modified surface layer composed of nitrides and cementite formed on a pure iron substrate by soft-nitriding with urea. The gas species of retained ammonia and carbon monoxide decomposed from urea, which work as nitrogen and carbon sources in soft-nitriding, were detected at 500℃ by gas chromatography. Soft-nitriding with urea was also carried out on a hybrid carbon coated SKD61 substrate. The hardened depth increased mainly due to the diffusion of nitrogen into the substrate. When hybrid carbon coated SKD61 substrate in contact with a small amount of oil-typed lubricant containing 25wt% of urea was heated, a soft-nitrided surface layer formed on the substrate. Hence the self-healing of diecast mold surfaces can be expected through in-process soft-nitriding by spraying the lubricant containing urea.
