An investigation of grain size and nitrogen-doping effects on the mechanical properties of ultrananocrystalline diamond films

Due to the lack of thorough understanding of the ultrananocrystalline diamond (UNCD) growth mechanism, a simple procedure is proposed to form a polycrystalline UNCD block with an artificial grain boundary (GB). The mechanical responses of the resulting UNCD films with various grain sizes are investigated by applying displacement-controlled tensile loading in the molecular dynamics simulations. By randomly adding different numbers of nitrogen (N) atoms into the GBs of these polycrystalline UNCD films, the effects of N atom number density and GB width on the mechanical properties of UNCD are also studied. It appears that the initial elastic moduli of pure and N-doped UNCD films are size-insensitive, although their tensile strengths decrease with the specimen size. The initial elastic modulus of N-doped UNCD is insensitive to the GB width, while the tensile strength decreases with both the N atom number density and the GB width.