Adhesive and bending failure of thermal sprayed hydroxyapatite coatings: Effect of nanostructures at interface and crack propagation phenomenon during bending

Hydroxyapatite (HA) coatings have shown promising effects on rapid bone remodeling and suitable functional life in orthopedic and dental applications. However, the major problem encountered by the HA-coated implants is the failure of the coating due to its insufficient mechanical properties. The present study investigated the influence of the microstructure near to the coating/substrate interface on the adhesion of the coatings. In addition, the crack propagation behavior within the coatings was studied through 4-point bend test. Results showed that nanostructures (30-110 nm) within the HA coatings were achieved by high velocity oxy-fuel (HVOF) spraying. Comparison among HVOF HA coatings, which were deposited using different starting feedstock, suggests detrimental effect of the perpendicular-to-substrate nano-cuboids on adhesion of the coatings. The presence of the grains with hexagonal shape (<250 nm in length and <50 nm in diameter) triggered a deteriorated adhesion. Granular nanosized grains at the interface give rise to enhanced adhesion through improved mechanical interlocking. Formation mechanism of the nanosized grains was discussed in this paper. Furthermore, the 4-point bend test revealed consistent crack propagation path that the cracks actually grow within the coating with a direction parallel to the interface, and approximately several to 20 microns thick coatings were remained on the substrate. The critical strain energy release rate exhibited a value of ∼1.15 kJm^-2. During the crack propagation, kinking and trapping of the bending cracks were decided by the flaws within the coating, which were mainly located at splats' interface. The interface between the first layer (with one splat thickness) and the second is believed to be the weakest zone in the nanostructured coating.