Quantum mechanically guided design of noble metal coatings for precision glass molding technology

Saksena, Aparna; Schneider, Jochen M. (Thesis advisor); Mitterer, Christian (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

In: Materials chemistry dissertation 38 (2020)
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

This thesis critically appraises the notion of employing noble metal coatings in precision glass molding (PGM) and aims to further the understanding of the underlying fundamental mechanisms. Pt-and Ir-based coatings are investigated experimentally and theoretically regarding their phase formation as binary and quinary systems. Moreover, these coatings are examined after glass contact to unravel the role of chemical composition on the mechanisms governing glass adhesion in PGM.In the first part of the thesis, the dependence of phase formation and mechanical properties on the chemical composition has been investigated for Pt-Ir and Pt-Au combinatorial thin films. Composition spreads are deposited at substrate temperatures ranging from room temperature to 950 °C and are subsequently characterized using X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning transmission electron microscopy (STEM), atom probe tomography (APT) and nanoindentation. The formation of a single, metastable Pt-Ir solid solution phase has been observed for all experimentally probed compositions and growth temperatures. Upon Ir addition to Pt, the experimentally determined changes in the lattice parameter and Young’s modulus display the expected rule of mixture behavior which is in very good agreement with the ab initio data. Whereas, in the Pt-Au system, the single metastable solid solution phase is seen to decompose into two solid solution phases as the growth temperature is raised to ≥ 600 °C. The lattice parameters of the single metastable phase grown at temperatures < 600 °C increase linearly as Au is added, showing the rule of mixture behavior in good agreement with ab initio predictions. However, the lattice parameters of the phases in the dual-phase region are independent of chemical composition, displaying phase formation behavior consistent with the CALPHAD results. The substrate temperature and chemical composition-dependent phase formation in Pt-Ir and Pt-Au thin films can be rationalized based on CALPHAD calculations combined with estimations of the activation energy required for surface diffusion: The metastable phase formation during film growth is caused by kinetic limitations, where Ir atoms (in Pt-Ir) need to overcome an activation energy barrier up to six times higher than that for Au (in Pt-Au) to enable surface diffusion.In the second part, the phase formation of PtIrCuAuX (X = Ag, Pd) compositionally complex thin films is investigated to critically appraise the criteria employed to predict the formation of high entropy alloys. The formation of a single-phase high entropy alloy is predicted if the following requirements are fulfilled: 12 J K-1 mol-1 ≤ configurational entropy ≤ 17.5 J K-1 mol-1, −10 kJ mol-1 ≤ enthalpy of mixing ≤ 5 kJ mol-1 and atomic size difference ≤ 5%. Equiatomic PtIrCuAuX (X = Ag, Pd) fulfill all of these requirements. Based on XRD and EDX data, near-equiatomic Pt22Ir23Cu18Au18Pd19 thin films form a single-phase solid solution while near-equiatomic Pt22Ir23Cu20Au17Ag18 thin films exhibit the formation of two phases. The latter observation is clearly in conflict with the design rules for high entropy alloys. However, the observed phase formation can be rationalized by considering bond strengths and differences in activation energy barriers for surface diffusion. Integrated crystal orbital Hamilton population values per bond imply a decrease in bond strength for all the interactions when Pd is substituted by Ag in PtIrCuAuX, which lowers the surface diffusion activation energy barrier by 35% on average for each constituent. This enables the surface diffusion-mediated formation of two phases, one rich in Au and Ag and a second phase enriched in Pt and Cu. Hence, phase formation in these systems appears to be governed by the complex interplay between energetics and kinetic limitations rather than by configurational entropy.In the third part, 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 × 10-4 Pa). Optical microscopy, electron microscopy and EDX 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.In summary, this work restates that ab initio calculations, verified and complemented by experiments form the machinery to vindicate the phase formation of the binary and quinary noble metal-based coatings. The notion based on phase stabilization by configurational entropy is “debunked”, as kinetically limited, surface diffusion-mediated growth has been identified as the primary mechanism, leading to the formation of metastable phases in these noble metal systems. Moreover, the results presented here emphasize the importance of the surface chemistry of the applied noble metal coatings for precision glass molding.

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