Aluminum coatings via kinetic spray with relatively large powder particles

Abstract

Coatings have been produced by entraining relatively large diameter metal powders in a supersonic airflow. For the first time, most of the particles in the powders have diameters >50 μm. Substantial plastic deformation is involved in the conversion of the particle's kinetic energy into heat and strain energy in this kinetic spray process. As suggested by simple estimates and confirmed by coating grain structures, the particles are not melted or thermally softened in this coating process. These coatings have a relatively low oxide content, low thermal stress, high adhesion, low porosity and hardness somewhat higher than those of corresponding bulk materials. Threshold or critical velocities for coating formation are discussed. Critical velocities for the relatively large particles were observed to be substantially less than have been reported earlier for smaller diameter (<50 μm) particles. Coating particle rotation and deformation due to particle impact resulted in a corresponding decrease in porosity. Bond formation, particle deformation and grain deformation were found to be highly anisotropic, depending on the direction of the incident particle velocity. At higher incident velocities, increasing metallic bond formation between particles was observed. This is consistent with a metallic form for stress/strain curves obtained via tensile tests on Al coatings removed from the substrate. The coating elastic modulus was found to be less than half that of bulk Al. Measured ultimate tensile strengths and yield points of Al coatings were comparable to those of bulk Al. This may be due to work hardening resulting from the plastic deformation necessary for coating formation. These tensile test results are consistent with coating cohesive strengths as measured by stud pull tests. Higher powder feed rates produced coatings with higher failure loads in three point bending, higher coating cohesion and lower coating strength anisotropy, presumably due to a peening effect. Four velocity-dependent stages of coating formation are proposed based on observations reported here. Coating properties arise from a competition between these stages. Parallels with models of dynamic (explosive) powder compaction are made. This is the first comprehensive model for kinetic spray coating formation.