Development of polymer derived ceramics for high-temperature applications

Younas, Muhammed (2025). Development of polymer derived ceramics for high-temperature applications. University of Birmingham. Ph.D.

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Abstract

Metal-modified polymer derived ceramics (PDCs) are a new and promising area of research to obtain advanced high-temperature capable ceramics. Incorporating these modified PDCs as matrices in fibre reinforced ceramic matrix composites (FRCMCs) could offer a cost-effective approach to the design and development of thermostructural components for use in aggressive, high-temperature environments. Of these modified PDCs, aluminium-modified SiCN is a promising example that been shown to improve oxidative resistance in both wet and dry environments up 1400°C. However, a Si(Al)CN ceramic has yet to be explored for use in FRCMCs and is the focus of the research within this thesis.

Si(Al)CN ceramic was successfully synthesised using a commercially available poly(silazane), Durazane1800, which was modified with aluminium isopropoxide; FT-IR and NMR analysis indicated that protonation occurred at the S-H and N-H bonds. The characterisation of the subsequent polymer-to-ceramic transformation with FT-IR showed the aluminium doping was retained during pyrolysis and that the resultant ceramic was amorphous in nature, as shown by XRD. EDX analysis of the Si(Al)CN ceramic also confirmed that the aluminium was homogenously distributed on an atomic scale.

To manufacture FRCMCs, a novel PDC prepreg was developed using a biomaterial-based hydrogel with poly(vinylpyrrolidone) (PVP) and poly(ethylene glycol) (PEG). The manufacturing process was designed to overcome many of the complex processing challenges associated with oxygen poor preceramic polymers such as Durazane1800. Two key processing steps were identified and addressed; the first step focused to the removal of hydroxyl groups during consolidation of the green body to make the process compatible with moisture sensitive preceramic polymers. The other processing step related to the binder burnout where a tailored heating profile was developed using TGA/DSC to analyse the thermal degradation of the PVP/PEG hydrogel. Furthermore, the tack properties of the prepregs were quantified using ASTM standard 1876. It was shown that prepreg tack could be retained even after prolonged storage at laboratory room temperature (average of 20°C), 30°C and 40°C for 14 days due to the humectant characteristics of the polymers.

The long-term oxidation of SiCf/SiCp/SiCN and SiCf/SiCp/Si(Al)CN CMCs were investigated in oxygen over the range 1300-1500°C for between 5-60 h where it was found that oxidation depended on both temperature and time. Measurements of oxide growth revealed that the oxidation reaction kinetics observed parabolic behaviour where oxygen diffusion though the oxide layer was the rate limiting step. The SiCf/SiCp/Si(Al)CNCMC showed better oxidation performance with a lower parabolic rate constant at all three oxidation temperatures and a higher activation energy than the SiCf/SiCp/SiCN CMC at 328.14 kJ mol-1 and 242.10 kJ mol-1 , respectively. It was determined that SiCf/SiCp/Si(Al)CN CMC was able to improve the oxidation resistance by being able to form a more structurally stable oxide in addition to the passivating effect with the inclusion of aluminium.

Furthermore, the short-term ablative behaviour of Cf/SiCp/SiCN and Cf/SiCp/Si(Al)CN CMCs were investigated above 1900°C for 60 seconds. It was found that both CMCs performed reasonably well within the ablative environment with the Cf/SiCp/Si(Al)CN CMC having a slightly improved ablation resistance of around 5%.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):