Evaluation of interfacial shear strength and residual stress of sol-gel derived fluoridated hydroxyapatite coatings on Ti6Al4V substrates

Interfacial shear strength is one of the critical properties in bioceramic coatings on metal implants because it directly affects the success of implantation and long-term stability. In this study, shear strain lag method was employed to evaluate the interfacial shear strength of sol-gel derived fluoridated hydroxyapatite (FHA) coatings on Ti6Al4V substrates. The residual stresses were measured using the "wafer curvature method". The resultant interfacial shear strength increased from pure HA's ∼393-459 MPa as fluorine was increased to 1.96 at% and further increased to ∼572 MPa as fluorine increased to 3.29 at%. The residual stresses in the coating also decreased from pure HA's ∼273-190 MPa and further to ∼137 MPa as fluorine composition in the coating increased. The reduction in the residual stress mainly comes from the reduction in the difference in coefficient of thermal expansion between the coating and the titanium alloy substrate.