Sol–gel hydroxyapatite coatings on stainless steel substrates

Thin film hydroxyapatite deposits onto sandblasted 316L stainless steel substrates were prepared using water-based sol–gel technique recently developed in our lab. The coatings were annealed in air at 375°C, 400°C, and 500°C. Phase formation, surface morphology, interfacial microstructure, and interfacial bonding strength of the coatings were investigated. Apatitic structure developed within the coatings while, annealing at temperatures 400°C, while those heat-treated at 375°C showed poor crystallinity. The coatings were dense and firmly attached to the underlying substrates, reaching an average bonding strength (as determined through the pull-out test) of 44 MPa. Nano-porous structure was found for the coatings annealed at 500°C, believed to result from grain growth, and causing a slight decrease in the bonding strength. Surface microcracking, although not extensive, occurred after annealing at temperatures 400°C, and was linked to non-uniform thickness of the coating due to roughness of the substrate. A contraction of the coatings as a result of sintering, and phase transition from amorphous (or poor crystalline) to reasonably good crystalline apatite, may be responsible for the loss of structural integrity of the thicker sections of the coatings. It seems quite promising that a dense and adhesive apatite coating can be achieved through water-based sol–gel technology after short-term annealing at around 400°C in air.