Understanding the connection between the mechanical properties of cold spray aggregates and the morphology and strength of its interfaces

Cold spray is a process in which a metal substrate is exposed to a highvelocity (3001200 m/s) jet of feedstock particles (5-40 μm). The particles from a layer of coating on the substrate. The deposited particles deform plastically and form bonded interfaces with each other. We present results from a simple particle-based model to study the mechanical response of the aggregate. The model takes the geometry of bonded interfaces directly from electron micrographs of sectioned aggregates, and the only tunable parameter, α, represents the cohesive strength of the bonded interfaces, and we perform a parametric study in which we vary this parameter. We study i) the linear elastic response to homogeneous strain, ii) the ultimate strength under various types of homogeneous loading, iii) micro-indentation. The shear modulus, μ, is reduced by 40% and the compression modulus, K, is reduced by 15% as α is tuned from 1 (monolithic material with perfectly bonded interfaces) to 0 (perfectly weak interfaces with no cohesive strength). For failure under uniaxial compression, the samples remain surprisingly strong even for completely weak interfaces with no cohesive strength. In all cases except for perfect interfaces (monolithic material), the shear becomes strongly localized along a single path connecting weak interfaces along the surface of maximum shear stress 45 degrees from the loading axes, and the residual strength comes, effectively, from frictional sliding at the interfaces under compressive normal stress. For failure under uniaxial tension, the samples fail catastrophically with the percolation of a mode-1 type fracture for all cases except the monolithic material, with the ultimate strength depending monotonically on the strength of the interfaces. For indentation, we see a large site-to-site variance in the loading curves. The site-to-site variation increases when the interfaces are weaker and decreases when the indenter is flatter or the indentation depth is deeper. These results should provide useful guidance for interpreting various mechanical tests performed on cold spray aggregates.