The connections between powder variability and coating microstructures for cold spray deposition of austenitic stainless steel

This article explores the variability in austenitic stainless steel feed-stock powders and its effect upon low-pressure cold spray deposition. Four, commercial, austenitic stainless steel powders were deposited using cold spray deposition with helium gas at a temperature of 230 °C and a pressure of 1.7 MPa. Electron microscopy and X-ray diffraction were used to characterize the microstructure of both the powders and the resultant coatings. The particle size for the powders ranged from 17 to 40 μm, while the crystallites ranged from 2 to 6 μm in size. Only the 304L stainless steel powder was purely austenite in phase; while the three 316L powders had ferrite contents ranging from 23 to 50%. The deposition efficiency of the four powders varied considerably from 12 to 42%, but this variation did not correlate with the ferrite content alone. The spatial location of the ferrite in the cold sprayed material was highly dependent upon the powder type used: ferrite primarily on the outside of austenite particles, mixtures of wholly ferrite and wholly austenite particles, and particles with both ferrite and austenite internally. After cold spray deposition, all of the coatings exhibited a composite microstructure containing highly deformed prior particle centers with small (300 nm) crystallites near the prior particle boundaries.