Indentation Size Scale Effect on Mechanical Properties of Cold SprayCoating

For well over a hundred years, hardness testing has provided engineers a quick measure of the mechanical properties of a material or coating. However, the technique has also been fraught with potential artifacts, many of which are related to a phenomenon known as the indentationsize effect . Unlike bulk materials, experimental studies on the hardness measurements of cold spray coatings in different load regimes shows strong dependency on the indentation size in a manner different from the Nix–Gao model. In cold spray coating additional parameters such asporosity and cohesive strength between cold sprayed particles affect the hardness measurements. As a result the hardness loss was observed by increasing the indentation load. To interpret the experimental observation, a two dimensional model was developed taking into consideration the inter particle damage. Ductile damage initiation in combination with the linear damage evolution models has been used. The deviation of load-displacement curves in the material with inter particle defects in comparison to bulk material was studied to explain the mechanism involved in hardness loss. Furthermore, microstructural characterization before and after indentation, was used to examine the relationships between processing conditions, coating microstructure and mechanical behavior of coatings fabricated with cold spray process.