Interfacial deformation and failure mechanisms at the single-splat length scale revealed in-situ by indentation of cold sprayed aluminum microparticles

Abstract

Cold spray deposition exploits the phenomenon of impact bonding for solid-state consolidation of metallic microparticles. However, the particle interfaces in the deposits are susceptible to crack propagation under mechanical stresses, which results in inferior ductility. In this work, we seek to develop insights into splat-substrate interface bonding by in-situ micromechanical investigations. A miniaturized mechanical testing approach is reported here, which relies on micromachining, targeted indentation, and real-time scanning electron microscopy to probe deformation and failure at buried interfaces. Investigations at the “single splat length scale” enabled us to distinguish deformation mechanisms associated with 6061Al splats with globular and pancake-shaped morphologies. We observed a transition from mechanical interlocking to metallurgical bonding with an increase in the degree of particle flattening during deposition. The mechanically interlocked splats debond from the substrate via crack propagation and splat sliding. On the other hand, metallurgically bonded splats do not fail under indentation stresses exceeding 380 MPa; instead, displaying shear band propagation and pile-up mechanisms. A four-fold enhancement in the critical stress for crack propagation in mechanically-interlocked splats is achieved after a two-step annealing-aging heat-treatment cycle. We demonstrate that interface bonding plays a more dominant role than the inherent plasticity of splats in influencing bulk deposits' ductility, underscoring the importance of interface engineering in cold sprayed materials.