Preparation of hydroxyapatite ceramic powders adapted to cold spray process

Up to date, cold spray has been mostly dedicated to ductile materials (i.e. metals). However, cold spray of brittle materials (i.e. ceramics) has recently gained attention. In the specific case of ceramics, the challenge consists mostly in controlling the powder fragmentation at the impact to optimize the coating quality and its construction rate. Even if it has been shown that below a given dimension, the ceramic elementary particles may exhibit a plastic deformation, the objective remains to work with ceramic agglomerates of nanoparticles, those agglomerates being able to exhibit fragmentation while the elementary particles can be kept almost intact.

This is exactly the strategy we follow here to prepare hydroxyapatite (HAP) coatings using different MEDICOAT HAP agglomerates and a cold spray Impact Spray System 5/11. Three types of HAP agglomerates are compared. The 3 types of agglomerates are comparable in terms of size but the difference stands in their crystallinities, developed specific surface areas and morphologies. A relationship between the characteristics of those agglomerates, the processing parameters and the obtained quality of the coatings is then established demonstrating that controlling the fragmentation is really the key point to get high quality HAP coatings by cold spray.