Microstructural evolution and deposition mechanism of cold-sprayed Cr coatings on the Al and Ti substrates

Microstructural evolution and nanoindentation analyses were employed to investigate the deposition behavior of Cr particles on the Al and Ti substrates during cold spraying. The results indicate that substrate nanohardness plays an important role in determining both the thickness of the Cr coating and the bonding mechanism between the Cr particles and the substrates. In the initial coating layer, the rigid Cr particles are mechanically embedded into the softer Al substrate with minimal deformation, resulting in a dense and continuous coating composed of a few deformed particles. In contrast, deposition on the harder Ti substrate induces coordinated plastic deformation between the Cr particles and the substrate with mechanical interlocking; however, the resulting coating is discontinuous, exhibiting a thickness equivalent to a single deposited particle. The primary limitation in further increasing coating thickness arises during the stage of interaction between subsequent particles and the pre-deposited coating. Here, the uneven surface of the deposited layer reduces the effective impact velocity of incoming particles and, coupled with continuous work hardening, prevents synergistic plastic deformation with subsequent particles. Based on the observations of microstructural evolution and nanohardness distribution, the hypothesis regarding the formation mechanism of Cr coatings on the Al and Ti substrates is proposed.