Structure and morphology of stainless steel coatings sputter-deposited in a nitrogen/argon atmosphere

Abstract

Reactive magnetron sputtering was used to deposit coatings from a 304 stainless steel target (nominal composition 18%Cr, 8% Ni, balance Fe). Deposition was carried out in a mixed argon/nitrogen atmosphere with Ar:N2 ratios of 4, 1.5 and 1, and a total gas flow of 25sccm for all cases. Substrate temperatures ranged from 150 to 600°C, along with substrate bias levels from −100V to −140V. X-ray diffraction analyses showed the structure of the coatings were strongly temperature dependent: above 450°C, the films were a mixture of CrN, bcc-Fe and Ni; below 450°C the S-phase (a N-supersaturated fcc structure) was observed. While this structure is nominally cubic, the commonly observed anomaly of (hkl)-dependent lattice constants, where a200>a111, was also found in the present study. Area-detector based X-ray diffraction studies, which allowed peak position measurements as a function of the inclination of the diffraction vector (angle ψ), showed a200 declined with increasing ψ, but always remained greater than a111, which was relatively constant with ψ. SEM cross-sections for samples deposited below 450°C had discontinuous, angular crystallites, whereas at higher substrate temperatures the structure had the appearance of a loose particle aggregate. At higher nitrogen has concentrations (Ar:N2 of 1:1) a more typical columnar structure was found. Hardness testing gave values between 450 and 1968kg/mm2, with higher bias, temperatures and N2 gas concentrations promoting higher hardness levels.