Evaluating sol-gel ceramic thin films for metal implant applications: A study on the processing and mechanical properties of zirconia films on Ti6Al4V

摘要

Surface modification of metallic bone-interfacing implants by a ceramic coating or thin film can potentially improve the performance of these implants with respect to corrosion, wear or implant fixation provided that reliable coating adhesion can be achieved. Using a "model" system consisting of a pure sol-gel zirconia film on Ti6Al4V, a polymeric alkoxide-based sol-gel thin film process was investigated. Initial processing studies enabled dipcoating of uniform ZrO$sb2$ films on the order of 100 nm thick using substrate withdrawal speeds ranging from 2 to 8 cm/min and a nominal sol viscosity of 6 cps. These films were essentially free of surface macrodefects except for random submicron "pinholes", and were found by x-ray diffraction to be at least partially crystalline, with "metastable" cubic and/or tetragonal phases, after annealing for one hour at 500$spcirc.$.;Rigorous application of a previously reported shear lag-based strain approach to adhesion measurement suggested an interfacial shear strength value of approximately 275 MPa for this ZrO$sb2$/Ti6Al4V system. Small variations in film thickness and substrate surface polishing had little effect on shear strength determinations, though arguably the present experimental protocol did not allow this strain method to definitively discern shear strength differences of less than 15-20%. Introduction of a platinum interlayer did dramatically reduce zirconia film adhesion, suggesting the important role of surface oxide layers for bonding in this alkoxide-based deposition. Limited diffusion was also considered a potential contributor to the high bond strength obtained; a higher temperature anneal (750$spcirc$C) in air resulted, however, in an apparent decrease in adhesion presumably due to excessive growth of the passivated substrate surface oxide layer.;Subsequent fatigue testing of these ZrO$sb2$ films in air using novel tapered rotating beam and standard axial fatigue Ti6Al4V samples demonstrated as well their excellent fatigue characteristics, with films surviving to 10$sp7$ cycles up to the endurance limit of the Ti alloy ($sim$635 MPa).;Finally, exploratory studies to assess the relevance of sol-gel films for practical applications based on the ZrO$sb2$/Ti6Al4V model system highlighted their potential to line porous metal coated implant systems despite some film cracking and delamination at sinterneck regions within this porous layer. Results from preliminary corrosion studies of the ZrO$sb2$/Ti6Al4V system showed no benefit from the film, though the lack of film optimization and consideration of less corrosion-resistant metals justify additional studies on the protective role of sol-gel coatings.;Overall, the excellent mechanical properties of this ZrO$sb2$/Ti6Al4V system along with the inherent advantages of sol-gel processing support continued research in this area.