Author: Akhtar, Awais
Title: Thin film coatings with ultra–high stability for precision glass molding
Advisors: Ruan, Haihui (ME)
Degree: Ph.D.
Year: 2023
Subject: Glass coatings
Thin films
Glass manufacture
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: vii, 146 pages : color illustrations
Language: English
Abstract: Precision glass molding (PGM) is an advanced glass manufacturing technology employed by the glass industry for the high-volume production of precision glass components. One of the key breakthroughs is the use of anti-sticking coatings that extend the tool life and render a molded glass surface polish-free. The improvement in oxidation resistance and anti-sticking performance of coatings is a major challenge, particularly when the PGM technology is to be adapted for thermoforming of glasses with high glass transition temperature (Tg). In this thesis, first, a systematic review was conducted, and the recent developments and progress of surface protective coatings for PGM applications were summarized. Specifically, the most important and recent breakthroughs in deposition processes, microstructural features, and anti-sticking performance were emphasized, as they establish the foundation for developing high-performance thin film protective coatings for thermoforming high-Tg glasses in PGM. Furthermore, based on these explorations, a perspective for further improving the reliability of PGM coatings was also provided.
Second, a new multilayer Ru-Pt coating design on graphite substrate was developed to evaluate its performance in PGM applications. Different from previous evaluations with other types of coatings for PGM temperatures below 650 °C, the anti-stick behavior of the developed coatings was studied by annealing them at 750 °C with aluminosilicate glass (ASG) coverage in a rough vacuum environment. Glass adhesion was not observed even after 40 h of annealing, suggesting the high chemical inertness of these coatings at high temperatures. In addition, Ru-Pt multilayer coatings exhibited a better anti-sticking property than monolayer (Ru or Pt) coatings. The microstructure, surface morphology, and hardness were further characterized. High-resolution transmission electron microscopy (HRTEM) analysis confirmed the preservation of the layered structure after annealing at 750 °C and the hindrance effect on dislocation motion and diffusion arising from the Ru-Pt interfaces. After long-term annealing, interdiffusion led to Pt-Ru solid solutions, which further enhanced the mechanical performance and stability of the noble metal coatings, which are beneficial for prolonging the lifespan of PGM molds and the optical quality of molded glass. The high anti-sticking and mechanical performances of the developed coatings were confirmed in industrial tests.
Finally, considering the high performance of Ru-based coatings, the concept was further extended, and another cost-effective coating design was prepared by deposition of Ru-Cr coatings on graphite substrates to examine its feasibility for the PGM industry. Ru-Cr coatings with varying Ru thicknesses were fabricated, revealing columnar morphology with preferred orientation and the effect of annealing on the microstructure and mechanical properties of the developed coatings at the desired molding temperature (750 °C) for different time durations (10-20 h) was studied. The outward diffusion of Cr and its preferential oxidation to the Cr-oxide scale resulted in the improvement of mechanical properties of the coatings. Furthermore, the nanostructure characterization revealed the expansion of the Cr coating layer and Ti interlayer and the disappearance of the Ti-Cr interface, indicating the formation of Ti-Cr alloys. In contrast, the Ru thickness remained unchanged after annealing, indicating stability. In addition, the TEM investigation demonstrated that Ru-Cr coatings with increased Ru thickness inhibited the outward movement of Cr and served as a diffusion barrier. Furthermore, phase field modeling data validated the experimental findings and showed that the varied Ru layer thickness impeded outward diffusion of Cr at different degrees and demonstrated external oxide scale stability after annealing.
Overall, the findings of this thesis provide insights into stable coating materials with high performance for PGM applications and could potentially benefit the glass industry.
Rights: All rights reserved
Access: open access

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