Deposition and characterisation of RF magnetron sputtered phosphate based glasses

Abstract

Phosphate based glasses are emerging in the field of biomaterials for their potential to resorb in biological environments, fulfilling applications from the fibre reinforcement of resorbable polymeric matrices to carriers for therapeutic drug delivery. Here we show the optimisation and characterisation of thin film glasses deposited by RF magnetron sputtering onto medical implant materials such as Ti6Al4V to function as ion leaching coatings to promote osseointegration or inhibit bacterial attachment. Vaporisation of the target preform occurred by momentum exchange interactions leading to non stoichiometric transfer to the condensed coating, sputtering in the order Na > Mg > Ca > Fe > P. Structural analyses revealed short range variation between compositionally equivalent glasses such that coatings with 32.5, 34 and 37 mol% P2O5 showed bulk polymerisation increasing in Q2 species by (23% to 45%) versus (9% to 32%) in quenched glasses. P O P bridging oxygens on the surface of coatings formed (PO3) metaphosphates (Q2), compared to (PO4)3 orthophosphates (Q0) and (P2O7)4 pyrophosphates (Q1) in quenched glasses. Quinternary coatings of up to 2.67 μm, containing Fe3+ and Ti4+ intermediate and cross linking elements were degraded in distilled water and phosphate buffered saline. Fe2O3 was increased from 4 to 8 mol% to stabilise dissolution, however an observed increase was attributed to variable condensation energies leading to inequivalent enthalpy and internal stress states. A comparison of a compositionally equivalent condensed and quenched glass suggested that the surface ratios of P O P to (P=O and PO ) were 34.2% to 65.8% versus 20.5% to 79.5% respectively leading to more soluble coating surfaces, exhibiting an exponential degradation dependence in the first 2 h in distilled water, followed by a linear profile. Post deposition heat treatments at 500, 550 and 610 °C were employed to stabilise dissolution and to tailor mechanical properties. All phosphate glass coatings showed interfacial tensile adhesion in excess of 73.6 MPa; surpassing ISO and FDA requirements for HA coatings. The initial exponential degradation from 0 2 h was stabilised via heat treatment. From 2 24 h coatings exhibited linear ion release rates ordering P > Na > Mg > Ca > Fe whilst dissolution rates reduced by factors of 2.44 to 4.55, attributed the formation of crystals and the depletion of hydrophilic P O P bonds within the surface layer. Vapour deposition has shown its ability to condense tailorable compositions of glasses, maintaining their amorphous tetrahedral structures whilst demonstrating exceptional adhesion to Ti6Al4V substrates. Coatings have demonstrated linear ion release capabilities and the ability to accommodate a vast array of potentially therapeutic ions to promote osteogenic or antimicrobial capabilities