Corrosion resistance and biocompatibility of cold-sprayed titanium on 316L stainless steel

Porous coatings for biomaterials are provided to enhance biological response for bone fixation. Adhesion and cohesion strengths of the porous coatings can be improved by adjusting the porosity levels of the coating system. Different levels of the porous structure can possibly affect other properties such as corrosion resistance and biological properties. In this study, the corrosion resistance and biocompatibility of titanium (Ti) porous coating with high and low porosity gradients on grit-blasted and ground 316L stainless steel by cold spray were studied. The deposition of titanium (Ti) porous coating on all 316L stainless steel substrates was achieved by cold spray. The coatings comprised three layers, i.e. a low porosity layer (the 1st layer) at the interface coating/substrate, and higher porosity layers (the 2nd and 3rd layers) on top. The high and low porosity gradient samples had 8 % and 1 % porosity gradient, respectively. They related to the differing porosity between the 2nd and 3rd layers, deposited by the different process gas pressure used for the 2nd layer. Corrosion resistance was analyzed from the result of the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) test. All EIS results presented capacitive behavior (high corrosion resistance) with phase angle close to −90°. High polarization resistance (R2) in the ground substrate indicated a highly stable passive film at the interface coating/substrate, which was higher than grit-blasted substrate. Surface preparation with grinding also exhibited a lower corrosion rate than grit blasting. However, the high and low porosity gradients hardly affected the EIS result and corrosion rate. Porous structures in all Ti coatings could promote bone cell ingrowth. The biological responses such as alkaline phosphatase (ALP) activity, and alizarin red S staining (ARS) concentration had high values in the ground substrate. The highest ALP and ARS could be found in the high porosity gradient on ground substrate after cell culture for 8 weeks.