Material development and process optimisation for Digital Light Processing of translucent alumina

De Lisi, Michele ORCID: 0000-0003-3506-4453 (2024). Material development and process optimisation for Digital Light Processing of translucent alumina. University of Birmingham. Ph.D.

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Abstract

In the last decade, the urge to conduct additional research on viable production approaches for advanced ceramics has been arising in order to fully leverage the promise of ceramic materials and connect their distinctive thermomechanical capabilities with increased advanced features. It has been widely proven that Additive Manufacturing (AM) is a technology that is capable of integrating geometrical complexity and precision with fine microstructure and improved mechanical qualities while eliminating concerns about tool accessibility and reducing the costs associated with production assets. As a result, the ability to produce optimised components with extremely complicated geometries via AM, while simultaneously guaranteeing high standards of thermomechanical characteristics and delivering additional capabilities, has been a topic of significant interest among academics and businesses. In particular, Digital Light Processing (DLP) has been demonstrating the potential to rapidly manufacture intricate components suitable for a wide range of technological applications.
In this research, the production, formulation and optimisation of a slurry with high flowability as well as broad DLP printability were set as the first targets of the investigation. Subsequently, the main focus was shifted towards the determination of the optimal parameters for the fabrication of green ceramic samples that incorporated advanced characteristics. A photosensitive slurry with a high concentration of alumina particles and appropriate rheological properties was successfully formulated adopting a full-factorial design of experiments (DoE) and the influence of printing settings on the density and dimensions of the green samples was investigated. Afterwards, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed to determine the optimal thermal debinding parameters. Upon examination of various debinding regimes, the debinding heat rate of 0.2 °C/min and increased dwell times contributed to reducing defects and enhancing densification. Samples exhibiting a relative density higher than 99.0% and displaying translucent characteristics were fabricated by adopting a rapid high-temperature sintering cycle.
Furthermore, since the post-processing of AMed ceramic components has been demonstrated to be crucial to enhance the geometrical accuracy and mechanical properties of the finished product, especially the resulting density, numerical modelling was used to further optimise the sintering parameters and support experiments design. A thermo-viscoelastic model of the sintering process was formulated and validated against experimental data. The experimental findings and simulation results exhibited a discrepancy of less than 2% and, therefore, the numerical model demonstrated the potential to accurately forecast the characteristics of sintered alumina ceramics manufactured using DLP.
In addition, the effect of additives and different sintering regimes was investigated to further improve the translucency level of the produced samples. Magnesia and magnesium aluminate were incorporated into the ceramic slurry formulation and samples were sintered at different heating rates, dwelling times and sintering atmospheres. Finally, fine-grained translucent alumina discs sintered in air and exhibiting a maximum total transmittance of 90.4% and a maximum Real In-line Transmittance (RIT) of 16.9% in the visible and near-infrared regions were successfully fabricated.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):