title: Degradation and Characterization of Resorbable Phosphate-Based Glass Thin-Film Coatings Applied by Radio-Frequency Magnetron Sputtering.
abstract: Quinternary phosphate-based glasses of up to 2.67 μm, deposited by radio-frequency magnetron sputtering, were degraded in distilled water and phosphate-buffered saline (PBS) to investigate their degradation characteristics. Magnetron-sputtered coatings have been structurally compared to their compositionally equivalent melt-quenched bulk glass counterparts. The coatings were found to have structurally variable surfaces to melt-quenched glass such that the respective bridging oxygen to nonbridging oxygen bonds were 34.2% to 65.8% versus 20.5% to 79.5%, forming metaphosphate (PO3)(-) (Q(2)) versus less soluble (P2O7)(4-) (Q(1)) and (PO4)(3-) (Q(0)), respectively. This factor led to highly soluble coatings, exhibiting a t(1/2) degradation dependence in the first 2 h in distilled water, followed by a more characteristic linear profile because the subsequent layers were less soluble. Degradation was observed to preferentially occur, forming voids characteristic of pitting corrosion, which was confirmed by the use of a focused ion beam. Coating degradation in PBS precipitated a (PO3)(-) metaphosphate, an X-ray amorphous layer, which remained adherent to the substrate and seemingly formed a protective diffusion barrier, which inhibited further coating degradation. The implications are that while compositionally similar, sputter-deposited coatings and melt-quenched glasses are structurally dissimilar, most notably, with regard to the surface layer. This factor has been attributed to surface etching of the as-deposited coating layer during deposition and variation in the thermal history between the processes of magnetron sputtering and melt quenching.
authors.affiliation: Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham , Nottingham NG7 2RD, United Kingdom.
authors.forename: Bryan W
authors.initials: BW
authors.lastname: Stuart
authors.affiliation: Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham , Nottingham NG7 2RD, United Kingdom.
authors.forename: Miquel
authors.initials: M
authors.lastname: Gimeno-Fabra
authors.affiliation: Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham , Nottingham NG7 2RD, United Kingdom.
authors.forename: Joel
authors.initials: J
authors.lastname: Segal
authors.affiliation: Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham , Nottingham NG7 2RD, United Kingdom.
authors.forename: Ifty
authors.initials: I
authors.lastname: Ahmed
authors.affiliation: Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham , Nottingham NG7 2RD, United Kingdom.
authors.forename: David M
authors.initials: DM
authors.lastname: Grant
keywords: PVD; bioactive thin films; bioresorbable; dissolution; osseointegration; phosphate glass; sputtering