Fabrication and characterisation of complex shaped Si-Al-O-N ceramic components via additively manufactured moulds

Munagala, Sai Priya (2020). Fabrication and characterisation of complex shaped Si-Al-O-N ceramic components via additively manufactured moulds. University of Birmingham. Ph.D.

Preview

Munagala2020PhD.pdf
Text
Available under License All rights reserved.
Download (7MB) | Preview

Abstract

This research programme was aimed at manufacturing complex-shaped components, such as gears and turbocharger rotors from SiAlON for use in engineering applications. SiAlONs are alloys of Si3N4 ceramics and offer the combination of good mechanical strength and chemical inertness together with the ability to perform at temperatures up to ~1400\(^o\)C. Although a lot of progress has been made for the fabrication of dense Si3N4 (and its alloys) components, research is still being conducted to find fabrication processes that are economical in terms of money and time and that yield high quality components.

The manufacturing route evaluated in the current study was based on the use of complex-shaped additively manufactured polymeric moulds that could then be used during the green forming of the ceramic components, followed by their densification by sintering. The goal was to reduce (ideally eliminate) the need for any machining, leading to faster and lower cost production of complex-shaped ceramic components. The heart of the project was thus the additive manufacturing machine (Solidscape, UK & Ireland). Although intended for use in jewellery making, its ability to 3D-print fine details using high-grade synthetic polymer was exploited. Once each batch of moulds was designed and produced, the process involved mixing the precursor SiAlON powder with additives, pressing the powder into the moulds and subsequent mould removal, debinding and then sintering at 1750\(^o\)C.

Dense (up to 99% of theoretical density) SiAlON ceramics were obtained. Components were characterised at every step of the process via visual inspection and physical and chemical analysis, including powder flowability, scanning electron microscopy, X-ray micro-computer tomography, X-ray diffraction, etc, for qualitative and quantitative analysis. Parameters affecting the resultant green and sintered body in terms of mechanical stability and surface finish were investigated. The study also included oxidation behaviour of the ceramic at temperatures from 1250 to 1450\(^o\)C.

An organic binder was chosen from a select few based on its adhesion property and flowability when mixed with the ceramic powders. The moulds’ examination revealed irregular surfaces leading to non uniform surface profiles on the sintered parts. An interfacial surface on the moulds was introduced to improve the surface finish of the green body. Phase analysis coupled with microscopic examination of the sintered ceramic revealed the presence of two phases namely α – SiAlON and β - SiAlON. The Vickers hardness was measured to be HV10 = 2545 and the flexural strength was measured to be 87 MPa. Oxidation study revealed the onset of degradation of the ceramic from 1100\(^o\)C. With an increase in the oxidising time and temperature, an increase in the prominence of cristobalite, grain growth and phase separation was observed.

Limitations of indirect additive manufacturing in the current study are identified. These include the part size limitation, the inefficiency of the printing unit to print uniform moulds without the presence of any debris, the strength of the moulds under stresses, failure in assessing the residual stresses in the sintered compacts and disadvantages of the powder compaction are discussed. The study concludes by suggesting future work to improve the fabrication process including the use of stronger mould material, pressureless green forming technique of consolidation such as gelcasting, development of simulations to predict the behaviour of the ceramic and its residual stresses.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):