Interfacial bond strength of electrophoretically deposited hydroxyapatite coatings on metals

Hydroxyapatite (HAp) coatings were deposited onto substrates of metal biomaterials (Ti, Ti6Al4V, and 316L stainless steel) by electrophoretic deposition (EPD). Only ultra-high surface area HAp powder, prepared by the metathesis method (10Ca(NO₃)₂ + 6(NH₄)₂HPO₄ + 8NH₄OH), could produce dense coatings when sintered at 875-1000°C. Single EPD coatings cracked during sintering owing to the 15-18% sintering shrinkage, but the HAp did not decompose. The use of dual coatings (coat, sinter, coat, sinter) resolved the cracking problem. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) inspection revealed that the second coating filled in the 'valleys' in the cracks of the first coating. The interfacial shear strength of the dual coatings was found, by ASTM F1044-87, to be ~ 12 MPa on a titanium substrate and ~ 22 MPa on 316L stainless steel, comparing quite favorably with the 34 MPa benchmark (the shear strength of bovine cortical bone was found to be 34 MPa). Stainless steel gave the better result since α-316L (20.5 μm mK^-1) > α-HAp (~ 14 μm mK^-1), resulting in residual compressive stresses in the coating, whereas α-titanium (~ 10.3 μm mK^-1) < α-HAp, resulting in residual tensile stresses in the coating.