Abstract: The interlayers of chemical vapour deposition (CVD) of TiN on three iron and steel samples were studied. It noted that the interlayers were formed by diffusion of Ti and N which could penetrate into iron and steel across the TiN coatings. Carbon, contained in steels, also diffused towards the outer coatings, and this process could increase the depositing rate of TiN. Because the diffusing rate of N in γ-Fe was much higher than that of Ti, Fe₂N+x-Fe and Fe₂N+Fe₂Ti+TiN+x-Fe were formed while TiN was depositing on pure iron.