Improved mechanical properties of nanocrystalline hydroxyapatite coating for dental and orthopedic implants

Hydroxyapatite (HA) has been widely used as a coating material for orthopedic/dental applications due to its similar chemical composition to natural bone mineral and its capability to promote bone regeneration. It has been reported that HA with nano-scale crystalline features and controlled porosity and pore size could promote osseointegration (that is, direct bonding to natural bone). So far, a number of methods have been developed or used commercially to deposit HA on metal implants, such as electrophoretic deposition, sputter, dip coating, spin coating and plasma spray. It is, however, very challenging to produce a nanocrystalline HA coating with desirable nano-features and surface roughness as well as controlled pore size and porosity for dental/orthopedic implants. It is also necessary for nano-HA coating to have good adhesion strength to metallic substrates and sufficient mechanical properties for load-bearing conditions. Therefore, a novel hybrid coating process, combing electrostatic spray coating (ESC) technique with a novel non-conventional sintering, was developed to meet requirements for dental implants in this study. Specifically, HA nanoparticles were deposited on titanium substrates using ESC technique and the green HA coating was then sintered in a controlled condition. The produced HA coating were characterized for grain size and pore size using an environmental scanning electron microscope (ESEM), the composition and Ca/P ratio using Energy Dispersive X-ray (EDX) analysis, and crystalline phases using X-ray diffraction (XRD). The results demonstrated that a nanocrystalline HA coating with a grain size from 50 to 300 nm and a gradient of nano-to-micron pore sizes were fabricated successfully using this novel coating process. The controlled nano-scale grain size and a gradient of pore sizes are expected to promote bone cell functions and facilitate bone healing. Besides biological properties, such HA coating was also characterized for its mechanical properties, such as adhesion strength, hardness and toughness. Microscratch test results showed that the critical load of coating de-lamination reached as high as 10 N. In conclusion, this study demonstrated that it is very promising to scale up this novel hybrid coating process (ESC followed by a novel sintering process) for dental/orthopedic implant applications.