Properties and Microstructure of Plasma Sprayed Hydroxyapatite Coatings Produced with Different Powder Feedstocks

摘要

Hydroxyapatite (HA) is a bioactive material with Ca to P ratio that is comparable to that of natural bone. This similarity encourages rapid bone growth and bonding between bone tissue and the implant surface. Plasma spraying is an efficient coating method of depositing HA onto biomedical implants. However, the biocompatibility of HA changes after plasma spraying. This paper reports the preparation and characterisation of HA coatings using different feedstocks such as agglomerated calcined HA (CHA) and dense flame spheroidised HA (SHA). The results indicate that the state of the starting powder in terms of geometry, shape and structure can produce efficacious effects on the coating characteristics. Both calcined HA and flame spheroidised HA powders were plasma sprayed onto metallic substrates and stubs. Coatings generated from spheroidised HA (SHA) powders have unique surface and microstructure characteristics while the coatings produced with CHA are porous and contain unmelted particles. SEM observation of the coating surface reveals well-formed splats that spread and flatten to a disc configuration without disintegration. This reflects adequate melting of the HA in the plasma with good deposition consistency. The surface topography is generally flat with considerable overlapping of subsequent spreading droplets. Porosity in the form of macropores is substantially reduced compared to CHA coatings. The cross-section microstructure reveals a highly dense coating comprising of randomly stacked lamellae. The tensile adhesion test (TAT) recorded the highest cohesive strength in the flame spheroidised HA coating. This suggests that having a structurally stable powder that is both dense and spherical promotes mechanically strong and coherent coatings.