Role of PEEK as Intermediate Layer in the Cold Spray of Hydroxyapatite Coating

In the last 15 years, the cold spray process has demonstrated a great efficiency for the deposition of metallic powders. In this case, the consolidation of coatings is achieved thanks to the high kinetic energy of unmelted particles exhibiting a ductile behaviour. Cold spray is generally of great interest because properties that are not reachable with classical thermal spray technologies ca be expected thanks to lower involved temperatures (typically below several hundred degrees), such as the conservation of both the characteristics of starting powders and substrate properties after spraying. However, in the case of ceramics, cold spray still remains challenging because of the lack of plastic deformation. Here, in the specific case of hydroxyapatite (HAP) and to overcome this brittleness issue, we investigate the role of an intermediate PolyEtherEtherKetone (PEEK) layer between the substrate and the deposit. In a first time, we highlight how the sublayer properties are different, according to the technology used for the deposition (flame spray or air plasma spray). Especially, the influence of the pore volume fraction in the sublayer, on the mechanisms involved in the deposition of HAP is discussed. Indeed, the porosity can help improving the building-up and consolidation of the HAP coating, especially when the deposition efficiency of the powder is very low. In this case, the sublayer is kept intact, and the rebound of particles are promoted, inducing a complete filling of the PEEK porosity with HAP. As a consequence, a thick and cohesive hybrid coating is created, containing PEEK and HAP.