Influence of glass contact induced changes in surface composition of Pt, PtIr and Ir protective coatings on glass adhesion

Highlights: • Pt, PtIr and Ir protective coatings are investigated before and after glass contact. • Ramifications thereof for glass adhesion is rationalized based on bonding analysis. • IrO₂ appears to provide passivation and thus inhibits glass adhesion. • The presence of Pt instigates glass adhesion by Pt-O-Si bond formation. The previously overlooked influence of glass contact-induced changes in surface composition of Pt, PtIr and Ir protective coatings on glass adhesion is investigated. All coatings, deposited onto sapphire, are subjected to contact with B270® glass at the molding temperature of 680 °C for four hours in vacuum (≤5 $\times$ 10⁻⁴ Pa). Optical microscopy, electron microscopy and energy-dispersive X-ray spectroscopy reveal an increase in macroscopic glass adhesion which is concurrent with the increase in Pt concentration. Consistent with these macro-scale observations, X-ray photoelectron spectroscopy (XPS) indicates not only the presence of Pt-O-Si bonds but also an increased Si concentration as the Pt concentration is increased. Furthermore, as a consequence of glass contact, a local increase in Pt concentration and hence evidence for surface Pt-enrichment is obtained. Based on thermogravimetry and XPS data, the as-deposited Ir coating exhibits the presence of a surface oxide (IrO₂) that dissociates at the glass molding temperature at a rate of 3.5 wt.% per hour. Hence, IrO₂ appears to provide passivation and thus inhibits glass adhesion while the presence of Pt, instigates glass adhesion by Pt-O-Si bond formation.