Nano-indentation on amorphous calcium phosphate splats: effect of droplet size on mechanical properties

Droplet processing technologies and many biological processes use disk-like or hemispherical shapes for construction or the design of surfaces. The ability to tune the characteristics and properties of a surface is important at the micro- and nano-scale. The influence of size on the mechanical properties is presently unknown. This work set out to produce splats from different droplet sizes (20-40 μm, 40-60 μm and 60-80 μm), and then determine the deposit characteristics and mechanical properties. All splats produced by melting particles in a flame and depositing onto a polished titanium surface were amorphous, as determined by Raman micro-spectrometry. The topography shown in an optical and scanning electron microscope and topographically mapped using the scanning mode of the nano-indenter revealed a flattened hemispherical deposit. The critical nano-indentation load for determining the true hardness decreased with increasing splat size; for 20-40 μm, 40-60 μm and 60-80 μm splats the critical load was 19, 16, 11 mN respectively compared to 30 mN for sintered hydroxyapatite. Higher loads are required to cause cracking and delamination in smaller splats. A load between 40 and 60 mN was required for delamination of the splat. Delamination of the splats could offer a new means to determine the adhesion of splats on low roughness surfaces.