Spatial microstructure in cold sprayed coatings: Influence of material properties

Abstract

Due to the nature of cold spray process, most materials sprayed are subjected to high strain and high strain rate deformation. Interestingly, this technique results in a strong gradient in the strain, strain rate and temperature rise across an individual particle and or multiple particles with the maximum always near interfaces. The extent of this gradient depends majorly on material being sprayed and process conditions. This gradient trickles down to the resultant final microstructure of the coatings or individual splats. This may manifest in shear localization, high level grain refinement, defect density and deformation twinning. In the present work, for the first time ever, influence of material property such as Stacking fault energy (SFE or γ) and Thermal diffusivity (α) on the final resultant microstructure has been rigorously studied with the former. The synergistic influence of these two properties has been studied extensively with the aid of Electron Back Scattered Diffraction across (EBSD) and Focussed Ion Beam (FIB) across 7 materials with a wide variation in SFE and diffusivity. Silver which has low SFE and high thermal diffusivity shows a highly uniform and refined microstructure and Nickel with relatively high SFE and low thermal conductivity shows localised and confined refinement with other materials showing a mix of microstructures. An attempt was also made to estimate the fraction of recrystallized grains with material properties. The observed results also correlate well to the multi-particle simulation data showing spatial temperature profiles across all the coatings.