Compositionally graded hydroxyapatite/tricalcium phosphate coating on Ti by laser and induction plasma

In this study we report the fabrication of compositionally graded hydroxyapatite (HA) coatings on Ti by combining laser engineering net shaping (LENS™) and radio frequency induction plasma spraying processes. Initially, HA powder was embedded in the Ti substrates using LENS™, forming a Ti–HA composite layer. Later, RF induction plasma spraying was used to deposit HA on these Ti substrates with a Ti–HA composite layer on top. Phase analysis by X-ray diffraction indicated phase transformation of HA to β-tricalcium phosphate in the laser processed coating. Laser processed coatings showed the formation of a metallurgically sound and diffused substrate–coating interface, which significantly increased the coating hardness to 922 ± 183 Hv from that of the base metal hardness of 189 ± 22 Hv. In the laser processed multilayer coating a compositionally graded nature was successfully achieved, however, with severe cracking and a consequent decrease in the flexural strength of the coating. To obtain a structurally stable coating with a composition gradient across the coating thickness a phase pure HA layer was sprayed on top of the laser processed single layer coatings using induction plasma spray. The plasma sprayed HA coatings were strongly adherent to the LENS™–TCP coatings, with adhesive bond strength of 21 MPa. In vitro biocompatibility of these coatings, using human fetal osteoblast cells, showed a clear improvement in cellular activity from uncoated Ti compared with LENS™–TCP-coated Ti and reached a maximum in the plasma sprayed HA coating.