Microstructural aspects of vacuum annealing‐induced solute depletion in sputter‐deposited 304 stainless‐steel films

Type 304 stainless‐steel (SS) films were dc‐planar‐magnetron sputter deposited onto substrates maintained at 77 K. The as‐deposited ferromagnetic films of bcc symmetry possessed equiaxed and columnar grain morphologies in the size range of 10–20 nm. These nanocrystalline films were found to be compositionally homogeneous and to be identical in composition to the sputtering target. Interest in protective coating application for these films stimulated a thermal stability investigation of their microstructure. Following vacuum annealing at 1146 K at 1.33×10⁻³ Pa for 15 min, analytical electron microscopy was performed on cross‐sectional specimens revealing (1) a significant Cr depletion from 18.1 wt. % at 775 nm to 4.4 wt. % at 20‐nm depth from the film surface, (2) a slight Ni depletion from 8.9 to 8.1 wt. % over the depth (from surface) increment of 75 to 20 nm, and (3) a near‐surface bcc–fcc phase transformation and associated grain size increases. Theoretical weight loss and surface Cr concentration calculations did not take into account the microstructural effects on the solute depletion profile and, therefore, were not in good agreement with the experimental analyses conducted under identical exposure conditions. Stereological analyses revealed that the nanocrystalline microstructure contained approximately 10¹⁷ interfaces/cm³ which comprised over 10⁶ cm²/cm³ (≊100 m²/cm³) of interfacial material. The grain size–grain morphology relationship was quantified and results indicated it to be complex; very small multifaceted grains comprising a film which possessed a large volume fraction of interfacial material. Calculations of grain matrix (lattice) and interfacial (grain boundary) Cr flux have shown the interfacial component to be significantly greater than the flux emerging from grain matrix regions. The disordered characteristics of the interfacial material relaxed the requirements for transport along, and evaporation from, these interfacial regions thereby significantly enhancing the Cr loss from these films.